mirror of
https://github.com/pascallanger/DIY-Multiprotocol-TX-Module.git
synced 2026-10-30 21:44:24 +00:00
Compute xn297 whitening instead of using an hardcoded array (#1201)
* Compute xn297 whitening instead of using an hardcoded array * Add function to scramble byte for XN297 interface * Refactor XN297 scrambling logic to use a global LFSR and improve byte generation * Add technical specification and implementation details for XN297 whitening and CRC
This commit is contained in:
goebish
+81
-106
@@ -6,22 +6,49 @@ bool xn297_scramble_enabled, xn297_crc, xn297_bitrate, xn297_rf;
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uint8_t xn297_addr_len, xn297_rx_packet_len;
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uint8_t xn297_tx_addr[5], xn297_rx_addr[5];
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// xn297 address / pcf / payload scramble table
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const uint8_t xn297_scramble[] = {
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0xE3, 0xB1, 0x4B, 0xEA, 0x85, 0xBC, 0xE5, 0x66,
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0x0D, 0xAE, 0x8C, 0x88, 0x12, 0x69, 0xEE, 0x1F,
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0xC7, 0x62, 0x97, 0xD5, 0x0B, 0x79, 0xCA, 0xCC,
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0x1B, 0x5D, 0x19, 0x10, 0x24, 0xD3, 0xDC, 0x3F,
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0x8E, 0xC5, 0x2F, 0xAA, 0x16, 0xF3, 0x95 };
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// XN297 whitening: x^7 + x^3 + 1, seed 0x71, MSB first (E3 B1 4B EA 85 ...).
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// Generated a byte at a time with (x^7 + x^3 + 1)^4 = x^28 + x^12 + 1: s[i] = s[i-28] ^ s[i-12].
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// xn297_lfsr holds the next 28 bits in bits 31..4, 0 when scrambling is off.
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uint32_t xn297_lfsr;
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static uint8_t __attribute__((unused)) XN297_scramble_byte()
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{
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uint8_t s = xn297_lfsr >> 24;
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xn297_lfsr = (xn297_lfsr << 8) | (uint8_t)(s ^ (xn297_lfsr >> 8)) << 4;
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return s;
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}
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static void __attribute__((unused)) XN297_scramble_init()
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{
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xn297_lfsr = xn297_scramble_enabled ? XN297_SCRAMBLE_SEED : 0;
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}
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// XN297 CRC: CRC-16/CCITT (init 0xFFFF, xorout 0xFFFF) over the unscrambled
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// address, pcf and payload, sent scrambled. The emulation computes it from 0xB5D2
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// over the bits as sent: the xorout is the difference for a CRC over 'bits' bits,
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// the CRC of the scramble bits plus the scramble of the CRC bits.
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// address, pcf and payload, sent scrambled: XORed with the scramble bits that follow,
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// (xn297_lfsr >> 16) once the last byte has been scrambled.
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// Returns the scrambled byte, crc updated with the unscrambled one.
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static uint8_t __attribute__((unused)) XN297_crc_scramble(uint8_t b)
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{
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crc16_update(b, 8);
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return b ^ XN297_scramble_byte();
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}
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// Returns the unscrambled byte, crc updated with it.
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static uint8_t __attribute__((unused)) XN297_unscramble_crc(uint8_t b)
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{
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b ^= XN297_scramble_byte();
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crc16_update(b, 8);
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return b;
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}
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#ifdef XN297DUMP_NRF24L01_INO
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// XN297Dump computes the CRC from 0xB5D2 over the bits as sent: the xorout is the
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// difference for a CRC over 'bits' bits, the CRC of the scramble bits plus the
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// scramble of the CRC bits.
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static uint16_t __attribute__((unused)) XN297_crc_xorout(bool scramble, uint16_t bits)
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{
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uint16_t xorout = 0xFFFF ^ 0xB5D2;
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uint8_t lfsr = 0x71, s; // scramble bits (xn297_scramble[]): 1110001, then s[i] = s[i-7] ^ s[i-3]
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uint8_t lfsr = 0x71, s; // scramble bits: 1110001, then s[i] = s[i-7] ^ s[i-3]
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for(uint16_t i = 0; i < bits + 16; i++)
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{
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s = scramble ? lfsr >> 6 : 0;
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@@ -38,6 +65,7 @@ static uint16_t __attribute__((unused)) XN297_crc_xorout(bool scramble, uint16_t
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}
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return ~xorout;
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}
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#endif
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static bool __attribute__((unused)) XN297_Configure(bool crc_en, bool scramble_en, bool bitrate, bool force_nrf)
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{
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@@ -103,13 +131,10 @@ static void __attribute__((unused)) XN297_SetTXAddr(const uint8_t* addr, uint8_t
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static void __attribute__((unused)) XN297_SetRXAddr(const uint8_t* addr, uint8_t rx_packet_len)
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{
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//Scramble address
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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{
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xn297_rx_addr[i] = addr[i];
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if(xn297_scramble_enabled)
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xn297_rx_addr[i] ^= xn297_scramble[xn297_addr_len-i-1];
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}
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//Scramble address in transmission order
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XN297_scramble_init();
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for (uint8_t i = xn297_addr_len; i--; )
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xn297_rx_addr[i] = addr[i] ^ XN297_scramble_byte();
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if(xn297_crc)
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rx_packet_len += 2; // Include CRC
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@@ -257,32 +282,19 @@ static void __attribute__((unused)) XN297_WritePayload(uint8_t* msg, uint8_t len
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}
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// address
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crc = 0xFFFF;
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XN297_scramble_init();
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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{
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buf[last] = xn297_tx_addr[xn297_addr_len-i-1];
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[i];
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last++;
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}
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buf[last++] = XN297_crc_scramble(xn297_tx_addr[xn297_addr_len-i-1]);
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// payload
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for (uint8_t i = 0; i < len; ++i)
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{
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// bit-reverse bytes in packet
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buf[last] = bit_reverse(msg[i]);
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[xn297_addr_len+i];
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last++;
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}
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buf[last++] = XN297_crc_scramble(bit_reverse(msg[i])); // bit-reverse bytes in packet
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// crc
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if (xn297_crc)
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{
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uint8_t offset = (xn297_addr_len < 4 && xn297_rf == XN297_NRF) ? 1 : 0;
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crc = 0xb5d2;
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for (uint8_t i = offset; i < last; ++i)
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crc16_update( buf[i], 8);
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crc ^= XN297_crc_xorout(xn297_scramble_enabled, 8 * (xn297_addr_len + len));
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crc = ~crc ^ (xn297_lfsr >> 16);
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buf[last++] = crc >> 8;
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buf[last++] = crc & 0xff;
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}
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@@ -294,7 +306,6 @@ static void __attribute__((unused)) XN297_WritePayload(uint8_t* msg, uint8_t len
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static void __attribute__((unused)) XN297_WriteEnhancedPayload(uint8_t* msg, uint8_t len, uint8_t noack)
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{
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uint8_t buf[32];
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uint8_t scramble_index=0;
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uint8_t last = 0;
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static uint8_t pid=0;
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@@ -304,54 +315,37 @@ static void __attribute__((unused)) XN297_WriteEnhancedPayload(uint8_t* msg, uin
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}
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// address
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crc = xn297_scramble_enabled ? 0xFFFF : 0xB5D2; // unscrambled: no xorout
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XN297_scramble_init();
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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{
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buf[last] = xn297_tx_addr[xn297_addr_len-i-1];
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[scramble_index++];
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last++;
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}
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buf[last++] = XN297_crc_scramble(xn297_tx_addr[xn297_addr_len-i-1]);
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// pcf
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buf[last] = (len << 1) | (pid>>1);
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[scramble_index++];
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last++;
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buf[last++] = XN297_crc_scramble((len << 1) | (pid>>1));
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buf[last] = (pid << 7) | (noack << 6);
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// payload
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if(len)
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{
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buf[last]|= bit_reverse(msg[0]) >> 2; // first 6 bit of payload
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[scramble_index++];
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buf[last] = XN297_crc_scramble(buf[last] | (bit_reverse(msg[0]) >> 2)); // first 6 bit of payload
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for (uint8_t i = 0; i < len-1; ++i)
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{
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last++;
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buf[last] = (bit_reverse(msg[i]) << 6) | (bit_reverse(msg[i+1]) >> 2);
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[scramble_index++];
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buf[last] = XN297_crc_scramble((bit_reverse(msg[i]) << 6) | (bit_reverse(msg[i+1]) >> 2));
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}
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last++;
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buf[last] = bit_reverse(msg[len-1]) << 6; // last 2 bit of payload
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}
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if(xn297_scramble_enabled)
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buf[last] ^= xn297_scramble[scramble_index++] & 0xc0;
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crc16_update( buf[last], 2);
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buf[last] ^= (xn297_lfsr >> 24) & 0xc0;
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// crc
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if (xn297_crc)
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{
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uint8_t offset = (xn297_addr_len < 4 && xn297_rf == XN297_NRF) ? 1 : 0;
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crc = 0xb5d2;
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for (uint8_t i = offset; i < last; ++i)
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crc16_update( buf[i], 8);
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crc16_update( buf[last] & 0xc0, 2);
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if (xn297_scramble_enabled)
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crc ^= XN297_crc_xorout(true, 8 * (xn297_addr_len + len) + 10);
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//else
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// crc ^= XN297_crc_xorout(false, 8 * (xn297_addr_len + len) + 10);
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crc = ~crc ^ (xn297_lfsr >> 14);
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buf[last++] |= (crc >> 8) >> 2;
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buf[last++] = ((crc >> 8) << 6) | ((crc & 0xff) >> 2);
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@@ -408,28 +402,18 @@ static bool __attribute__((unused)) XN297_ReadPayload(uint8_t* msg, uint8_t len)
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// Read payload
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XN297_ReceivePayload(buf, len);
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// Decode payload
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// Decode payload, calculate CRC
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crc = 0xFFFF;
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XN297_scramble_init();
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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XN297_unscramble_crc(xn297_rx_addr[xn297_addr_len-i-1]);
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for(uint8_t i=0; i<len; i++)
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{
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uint8_t b_in=buf[i];
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if(xn297_scramble_enabled)
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b_in ^= xn297_scramble[i+xn297_addr_len];
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msg[i] = bit_reverse(b_in);
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}
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msg[i] = bit_reverse(XN297_unscramble_crc(buf[i]));
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if (!xn297_crc)
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return true; // No CRC so OK by default...
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// Calculate CRC
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crc = 0xb5d2;
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//process address
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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crc16_update( xn297_rx_addr[xn297_addr_len-i-1], 8);
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//process payload
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for (uint8_t i = 0; i < len; ++i)
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crc16_update( buf[i], 8);
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//xorout
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crc ^= XN297_crc_xorout(xn297_scramble_enabled, 8 * (xn297_addr_len + len));
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crc = ~crc ^ (xn297_lfsr >> 16);
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//test
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if( (crc >> 8) == buf[len] && (crc & 0xff) == buf[len+1])
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return true; // CRC OK
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@@ -443,40 +427,31 @@ static uint8_t __attribute__((unused)) XN297_ReadEnhancedPayload(uint8_t* msg, u
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// Read payload
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XN297_ReceivePayload(buffer, len+2); // Read pcf + payload + CRC
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// Decode payload
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pcf_size = buffer[0];
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if(xn297_scramble_enabled)
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pcf_size ^= xn297_scramble[xn297_addr_len];
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pcf_size = pcf_size >> 1;
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// Decode payload, calculate CRC
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#ifdef XN297DUMP_NRF24L01_INO
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bool xorout = true;
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#else
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bool xorout = xn297_scramble_enabled; // unscrambled: no xorout
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#endif
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crc = xorout ? 0xFFFF : 0xB5D2;
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XN297_scramble_init();
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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XN297_unscramble_crc(xn297_rx_addr[xn297_addr_len-i-1]);
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pcf_size = XN297_unscramble_crc(buffer[0]) >> 1;
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if(pcf_size>32)
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return 255; // Error
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for(uint8_t i=0; i< pcf_size; i++)
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{
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msg[i] = bit_reverse((buffer[i+1] << 2) | (buffer[i+2] >> 6));
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if(xn297_scramble_enabled)
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msg[i] ^= bit_reverse((xn297_scramble[xn297_addr_len+i+1] << 2) |
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(xn297_scramble[xn297_addr_len+i+2] >> 6));
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{ // payload is shifted by 2 bits: unscramble with bits 29..22 of the next scramble bits
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msg[i] = bit_reverse(((buffer[i+1] << 2) | (buffer[i+2] >> 6)) ^ (xn297_lfsr >> 22));
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XN297_unscramble_crc(buffer[i+1]);
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}
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crc16_update( (buffer[pcf_size+1] ^ (xn297_lfsr >> 24)) & 0xc0, 2);
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if (!xn297_crc)
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return pcf_size; // No CRC so OK by default...
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// Calculate CRC
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crc = 0xb5d2;
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//process address
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for (uint8_t i = 0; i < xn297_addr_len; ++i)
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crc16_update( xn297_rx_addr[xn297_addr_len-i-1], 8);
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//process payload
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for (uint8_t i = 0; i < pcf_size+1; ++i)
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crc16_update( buffer[i], 8);
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crc16_update( buffer[pcf_size+1] & 0xc0, 2);
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//xorout
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if (xn297_scramble_enabled)
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crc ^= XN297_crc_xorout(true, 8 * (xn297_addr_len + pcf_size) + 10);
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#ifdef XN297DUMP_NRF24L01_INO
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else
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crc ^= XN297_crc_xorout(false, 8 * (xn297_addr_len + pcf_size) + 10);
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#endif
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if (xorout)
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crc = ~crc ^ (xn297_lfsr >> 14);
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uint16_t crcxored=(buffer[pcf_size+1]<<10)|(buffer[pcf_size+2]<<2)|(buffer[pcf_size+3]>>6) ;
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if( crc == crcxored)
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return pcf_size; // CRC OK
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