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Optional decryption of communication using keystream
[proxmark3-svn] / common / crc16.c
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bd20f8f4 1//-----------------------------------------------------------------------------
2// This code is licensed to you under the terms of the GNU GPL, version 2 or,
3// at your option, any later version. See the LICENSE.txt file for the text of
4// the license.
5//-----------------------------------------------------------------------------
6// CRC16
7//-----------------------------------------------------------------------------
8
fdcc61d3 9#include "crc16.h"
10
23b598ee 11
fdcc61d3 12unsigned short update_crc16( unsigned short crc, unsigned char c )
13{
14 unsigned short i, v, tcrc = 0;
15
16 v = (crc ^ c) & 0xff;
17 for (i = 0; i < 8; i++) {
18 tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
19 v >>= 1;
20 }
21
22 return ((crc >> 8) ^ tcrc)&0xffff;
23}
23b598ee 24
25uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial) {
26
27 if (length == 0)
28 return (~remainder);
29
30 for (int byte = 0; byte < length; ++byte) {
31 remainder ^= (message[byte] << 8);
32 for (uint8_t bit = 8; bit > 0; --bit) {
33 if (remainder & 0x8000) {
34 remainder = (remainder << 1) ^ polynomial;
35 } else {
36 remainder = (remainder << 1);
37 }
38 }
39 }
40 return remainder;
41}
42
43uint16_t crc16_ccitt(uint8_t const *message, int length) {
44 return crc16(message, length, 0xffff, 0x1021);
45}
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