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CRC32 逆向算法

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CRC32 逆向算法

crc.h

//==========================================================================
//
//      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__

crc32.c

/*
 *  crc32.c
 *
 *  Gary S. Brown's 32 bit CRC
 *
 *  CopyRight (C) 2012 crazyleen ruishenglin@126.com
 */
#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;
}

结果

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 

标签:c++

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