CRC32 逆向算法的C语言实现
CRC32:
CRC32校验应用很广泛。本文提供一种算法添加4个字节实现任意的crc32校验值转换，逆向计算crc32的值。可以随意修改文件任意4个字节实现任意的crc32校验值。
原理：
下面是一组CRC32计算过程：
添加了四个字节00ba ff 30, 原crc32值2be0dd1d变成eee8a9a6。
src32val:2be0dd1d ^ 00 -->x=1d
src32val:002be0dd ^
T(x) : 63066cd9
src32val:632d8c04 （现在计算的CRC值）
src32val:632d8c04 ^ 00-->x=04 x[4]
src32val:00632d8c ^
T(x) : 076dc419
src32val:070ee995
src32val:070ee995 ^ ba-->x=2f x[3]
src32val:00070ee9 ^
T(x) : abd13d59
src32val:abd633b0
src32val:abd633b0 ^ ff-->x=4f x[1]
src32val:00abd633 ^
T(x) : e6635c01
src32val:e6c88a32
src32val:e6c88a32 ^ 30-->x=02 x[0]
src32val:00e6c88a ^
T(x) : ee0e612c tx
src32val:eee8a9a6 （添加4个字节后的目标CRC值）
事实上，crc32的table里256个元素的最高位是唯一的。
可以通过crc32值倒推前四个crctable值，如图绿色部分。
1.由最终的crc值推导出前4个T(x)值，即：T（x） x 已知。
2.由原CRC32值结合x值，推导出4个所要添加的未知字节。
以下是C语言的一个实现：
程序输出：
dlrc@crazyleen-desktop:~/sniffer/fw_tool$ ./a.out crc32: reverse 4 bytes crc32: enter 2 CRC value: src dst 632d8c04 eee8a9a6 Add those 4 bytes: ( 00 ba ff 30 ) to change CRC 632d8c04 to eee8a9a6 change crc: 632d8c04 ==> eee8a9a6 reserve crc: eee8a9a6 ==> 632d8c04
头文件：
//========================================================================== // // crc.h // // Interface for the CRC algorithms. //========================================================================== #ifndef __CRC32_H__ #define __CRC32_H__ // Gary S. Brown's 32 bit CRC unsigned int cyg_crc32(unsigned char *s, int len); // Gary S. Brown's 32 bit CRC, but accumulate the result from a // previous CRC calculation unsigned int cyg_crc32_accumulate(unsigned int crc, unsigned char *s, int len); // Ethernet FCS Algorithm unsigned int cyg_ether_crc32(unsigned char *s, int len); // Ethernet FCS algorithm, but accumulate the result from a previous // CRC calculation. unsigned int cyg_ether_crc32_accumulate(unsigned int crc, unsigned char *s, int len); /** * add 4 bytes num[4] to change crc32 value from crc_src to crc_dst * @return: 0 on success, -1 on error. */ int cyg_crc32_change(unsigned int crc_dst, unsigned int crc_src, uint8_t num[4]); /** * cyg_crc32_reserve - reserve CRC32 value by dropping data[len] * @return: return the CRC value before data[len] */ unsigned int cyg_crc32_reserve(unsigned int crc, void *data, int len); #endif // __CRC32_H__
C文件：
/* * crc32.c * * Gary S. Brown's 32 bit CRC * * CopyRight (C) 2012 crazyleen [email protected] */ #include "crc32.h" #include <stdio.h> #include <stdint.h> /* ====================================================================== */ /* COPYRIGHT (C) 1986 Gary S. Brown. You may use this program, or */ /* code or tables extracted from it, as desired without restriction. */ /* */ /* First, the polynomial itself and its table of feedback terms. The */ /* polynomial is */ /* X^32+X^26+X^23+X^22+X^16+X^12+X^11+X^10+X^8+X^7+X^5+X^4+X^2+X^1+X^0 */ /* */ /* ====================================================================== */ static const unsigned int crc32_tab[] = { 0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 0x076dc419L, 0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0x0edb8832L, 0x79dcb8a4L, 0xe0d5e91eL, 0x97d2d988L, 0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L, 0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL, 0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L, 0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L, 0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L, 0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL, 0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L, 0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL, 0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L, 0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L, 0x01db7106L, 0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x06b6b51fL, 0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0x0f00f934L, 0x9609a88eL, 0xe10e9818L, 0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L, 0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL, 0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L, 0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L, 0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L, 0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL, 0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L, 0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L, 0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL, 0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L, 0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L, 0x03b6e20cL, 0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x04db2615L, 0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L, 0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 0xf00f9344L, 0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL, 0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL, 0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L, 0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L, 0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL, 0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L, 0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL, 0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L, 0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL, 0x026d930aL, 0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L, 0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 0x92d28e9bL, 0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 0x86d3d2d4L, 0xf1d4e242L, 0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L, 0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL, 0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L, 0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L, 0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L, 0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L, 0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L, 0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL, 0x2d02ef8dL }; /** * This is the standard Gary S. Brown's 32 bit CRC algorithm, but * accumulate the CRC into the result of a previous CRC. */ unsigned int cyg_crc32_accumulate(unsigned int crc32val, unsigned char *s, int len) { int i; for (i = 0; i < len; i++) { crc32val = crc32_tab[(crc32val ^ s[i]) & 0xff] ^ (crc32val >> 8); } return crc32val; } /** * This is the standard Gary S. Brown's 32 bit CRC algorithm */ unsigned int cyg_crc32(unsigned char *s, int len) { return (cyg_crc32_accumulate(0, s, len)); } /** * Return a 32-bit CRC of the contents of the buffer accumulating the * result from a previous CRC calculation. This uses the Ethernet FCS * algorithm. */ unsigned int cyg_ether_crc32_accumulate(unsigned int crc32val, unsigned char *s, int len) { int i; if (s == 0) return 0L; crc32val = crc32val ^ 0xffffffff; for (i = 0; i < len; i++) { crc32val = crc32_tab[(crc32val ^ s[i]) & 0xff] ^ (crc32val >> 8); } return crc32val ^ 0xffffffff; } /** * Return a 32-bit CRC of the contents of the buffer, using the * Ethernet FCS algorithm. */ unsigned int cyg_ether_crc32(unsigned char *s, int len) { return cyg_ether_crc32_accumulate(0, s, len); } /** * cyg_crc32_in_table - src32_tab[i] = value; return i * @return: return i, or -1 for no such value in tabale */ static int cyg_crc32_in_table(unsigned int value) { int i; for (i = 0; (crc32_tab[i] >> 24) != (value >> 24) && i < sizeof(crc32_tab); i++) { ; //nothing } if (i == sizeof(crc32_tab)) { return -1; } return i; } /** * add 4 bytes num[4] to change crc32 value from crc_src to crc_dst * @return: 0 on success, -1 on error. */ int cyg_crc32_change(unsigned int crc_dst, unsigned int crc_src, uint8_t num[4]) { int i = 0; int tx[4]; uint8_t value_x[4]; //printf("CRC32: %08x --> %08x\n", crc_src, crc_dst); //crc_dst ==> tx[4] (get previous 4 CRC table codes) for (i = 0; i < 4; i++) { //previous CRC table codes tx[i] = cyg_crc32_in_table(crc_dst); if (tx[i] == -1) { printf("no such value in CRC32 table"); return -1; } //printf("T(%d): %08x -->%02x\n", i, crc32_tab[tx[i]], tx[i]); //previous CRC value crc_dst = (crc_dst ^ crc32_tab[tx[i]]) << 8; } //calculate last 4 unknown bytes for (i = 3; i >= 0; i--) { //unknown value x value_x[i] = (crc_src & 0xff) ^ tx[i]; //printf("value X = %02x\n", value_x[i]); //calculate next CRC32: add byte value_x[i] crc_src = cyg_crc32_accumulate(crc_src, value_x + i, 1); } //output 4 bytes for (i = 0; i < 4; i++) { num[i] = value_x[3 - i]; } return 0; } /** * cyg_crc32_reserve - reserve CRC32 value by dropping data[len] * @return: return the CRC value before data[len] */ unsigned int cyg_crc32_reserve(unsigned int crc, void *data, int len) { int i; int x; char *p = (char *) data; unsigned int tx = 0; //tx(x) = crcTable[x] for (i = len - 1; i >= 0; i--) { //tx=crc32_tab[x] x = cyg_crc32_in_table(crc); tx = crc32_tab[x]; crc = crc ^ tx; crc = ((crc << 8) & 0xffffff00) | ((x ^ p[i]) & 0xff); } return crc; } /* * Demo main * This demo shows you how to add 4 bytes to change CRC32 value to * whatever you want. And how to reverse CRC32 value by dropping * bytes from target CRC32 value. */ #include <stdio.h> #include <string.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <stdlib.h> int main(int argc, char **argv) { int i; uint8_t num[4] = { 0, 0, 0, 0 }; unsigned int crc_dst = 0; unsigned int crc_src = 0; printf("crc32: reverse 4 bytes\n"); printf("crc32: enter 2 CRC value: src dst\n"); scanf("%08x %08x", &crc_src, &crc_dst); cyg_crc32_change(crc_dst, crc_src, num); printf("Add those 4 bytes: ( "); for (i = 0; i < 4; i++) { printf("%02hhx ", num[i]); } printf(") to change CRC %08x to %08x\n", crc_src, crc_dst); printf("change crc: %08x ==> %08x\n", crc_src, cyg_crc32_accumulate(crc_src, num, 4)); printf("reserve crc: %08x ==> %08x\n", crc_dst, cyg_crc32_reserve(crc_dst, num, 4)); return 0; }