我找到了一个反转CRC32的代码，但我不知道它是如何工作的，因为我在编程方面不是那么好，我刚开始。我只想比较2个文件，旧的和新的，然后在新的修复中CRC32在文件的末尾添加4个字节，因此2个文件将具有相同的CRC32。这是代码，在C＃中：
public class Crc32
{
public const uint poly = 0xedb88320;
public const uint startxor = 0xffffffff;
static uint[] table = null;
static uint[] revtable = null;
public void FixChecksum(byte[] bytes, int length, int fixpos, uint wantcrc)
{
if (fixpos + 4 > length) return;
uint crc = startxor;
for (int i = 0; i < fixpos; i++) {
crc = (crc >> 8) ^ table[(crc ^ bytes[i]) & 0xff];
}
Array.Copy(BitConverter.GetBytes(crc), 0, bytes, fixpos, 4);
crc = wantcrc ^ startxor;
for (int i = length - 1; i >= fixpos; i--) {
crc = (crc << 8) ^ revtable[crc >> (3 * 8)] ^ bytes[i];
}
Array.Copy(BitConverter.GetBytes(crc), 0, bytes, fixpos, 4);
}
public Crc32()
{
if (Crc32.table == null) {
uint[] table = new uint[256];
uint[] revtable = new uint[256];
uint fwd, rev;
for (int i = 0; i < table.Length; i++) {
fwd = (uint)i;
rev = (uint)(i) << (3 * 8);
for (int j = 8; j > 0; j--) {
if ((fwd & 1) == 1) {
fwd = (uint)((fwd >> 1) ^ poly);
} else {
fwd >>= 1;
}
if ((rev & 0x80000000) != 0) {
rev = ((rev ^ poly) << 1) | 1;
} else {
rev <<= 1;
}
}
table[i] = fwd;
revtable[i] = rev;
}
Crc32.table = table;
Crc32.revtable = revtable;
}
}
}
答案 0 :(得分：7)
这里有一些PHP代码，它们也可以满足您的需求 我花了很多时间试图为合法目的这样做 所以我希望它有助于某人
这样的功能可用于：
要在线测试crc32字符串，您可以使用：http://www.lammertbies.nl/comm/info/crc-calculation.html
请参阅改编自的http://www.reversing.be/article.php?story=20061209172953555 因为我不是原作者，原作在他的文章上花了很多时间，而且它 帮助不小“
这是PHP代码，可用于修改给定文件，如下所示：
<?php
//Calculate the original file's CRC32
$DesiredCRC32 = hexdec(hash_file('crc32b','originalcleanfile.ext'));
//Calculate the newfile's current CRC
$CurrentIncorrectCRC32 = hexdec(hash_file('crc32b','newupdatedfile.ext'));
//Generate a 4 byte string that, if appended to the newfile will result in the original CRC32
echo modCRC($DesiredCRC32,$CurrentIncorrectCRC32);
// I should note that the original guy that posted this has some incorrect calcs on his page
// but if you read carefully and understand the math, you can pick up where it's incorrect
// and learn a thing or two (I beleive last time I tested this code it was accurate)
// if you wanted to do something via the CLI you could use something like:
parse_str(implode('&', array_slice($argv, 1)), $_GET);
// and grab the files w/
$file1 = $_GET['a'];
//... etc.
//then you could run it via something like this:
//c:\php\php -f somefile.php a=c:\original.txt b=c:\new.txt c=c:\newwithcrc.txt
public static function modCRC($ExistingCRC, $DesiredCRC){
$crc32lookup = array(
0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F, 0xE963A535, 0x9E6495A3,
0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91,
0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5,
0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B,
0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F,
0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924, 0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D,
0x76DC4190, 0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01,
0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457,
0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB,
0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9,
0x5005713C, 0x270241AA, 0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD,
0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A, 0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683,
0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 0x196C3671, 0x6E6B06E7,
0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5,
0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79,
0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236, 0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F,
0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713,
0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21,
0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45,
0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB,
0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF,
0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D
);
//More Description:
//Want to append bytes: X Y Z W
//Take for register a3 a2 a1 a0 (a is the 'already calculated' crc for string)
//Note that a3 is the most significant byte and a0 the least.
// f is the wanted new CRC byte string, and xyzw is the modification pad
/*
I'll show it a little different way:
a0 + X =(1) points to b3 b2 b1 b0 in table
a1 + b0 + Y =(2) points to c3 c2 c1 c0 in table
a2 + b1 + c0 + Z =(3) points to d3 d2 d1 d0 in table
a3 + b2 + c1 + d0 + W =(4) points to e4 e3 e2 e1 in table
b3 + c2 + d1 + e0 =f0
c3 + d2 + e1 =f1
d3 + e2 =f2
e3 =f3
(1) (2) (3) (4)
(figure 4)
This is reversed in the same way as the 16bit version. I shall give an example with real values. For the table values use the CRC-32 table in the appendix.
Take for CRC register before, a3 a2 a1 a0 -> AB CD EF 66
Take for CRC register after, f3 f2 f1 f0 -> 56 33 14 78 (wanted value)
Here we go:
First byte of entries entry value
e3=f3 =56 -> 35h=(4) 56B3C423 for e3 e2 e1 e0
d3=f2+e2 =33+B3 =E6 -> 4Fh=(3) E6635C01 for d3 d2 d1 d0
c3=f1+e1+d2 =14+C4+63 =B3 -> F8h=(2) B3667A2E for c3 c2 c1 c0
b3=f0+e0+d1+c2=78+23+5C+66=61 -> DEh=(1) 616BFFD3 for b3 b2 b1 b0
Now we have all needed values, then
X=(1)+ a0= DE+66=B8
Y=(2)+ b0+a1= F8+D3+EF=C4
Z=(3)+ c0+b1+a2= 4F+2E+FF+CD=53
W=(4)+d0+c1+b2+a3=35+01+7A+6B+AB=8E
(final computation)
*/
$ExistingCRC = ~$ExistingCRC;
$DesiredCRC = ~$DesiredCRC;
//a3a2a1a0
$a3 = (($ExistingCRC & 0xFF000000) >> 24) & 0x000000FF;
$a2 = ($ExistingCRC & 0x00FF0000) >> 16;
$a1 = ($ExistingCRC & 0x0000FF00) >> 8;
$a0 = ($ExistingCRC & 0x000000FF);
//f3f2f1f0
$f3 = (($DesiredCRC & 0xFF000000) >> 24) & 0x000000FF;
$f2 = ($DesiredCRC & 0x00FF0000) >> 16;
$f1 = ($DesiredCRC & 0x0000FF00) >> 8;
$f0 = ($DesiredCRC & 0x000000FF);
//echo "Existing CRC:" . dechex($ExistingCRC) . " DesiredCRC:" . dechex($DesiredCRC) . "<BR>";
//echo "a3:" . dechex($a3) . " a2:" . dechex($a2) . " a1:" . dechex($a1) . " a0:" . dechex($a0) . "<br>";
//echo "f3:" . dechex($f3) . " f2:" . dechex($f2) . " f1:" . dechex($f1) . " f0:" . dechex($f0) . "<br>";
//capture e3/e2/e1/e0/(4) values
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($DesiredCRC & 0xFF000000)){
$e3 = (($crc32lookup[$i] & 0xFF000000) >> 24) & 0x000000FF;
$e2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$e1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$e0 = ($crc32lookup[$i] & 0x000000FF);
$four = $i;
break;
}
}
//echo "e3:" . dechex($e3) . " e2:" . dechex($e2) . " e1:" . dechex($e1) . " e0:" . dechex($e0) . "<br>";
//$d3=f2+e2 =33+B3 =E6 -> 4Fh=(3) E6635C01 for d3 d2 d1 d0
$d3 = $f2^$e2; //lookup $d3 and assign the values for d2/d1/d0/(3)
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($d3 << 24)){
$d2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$d1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$d0 = ($crc32lookup[$i] & 0x000000FF);
$three = $i;
break;
}
}
//echo "d3:" . dechex($d3) . " d2:" . dechex($d2) . " d1:" . dechex($d1) . " d0:" . dechex($d0) . "<br>";
//c3=f1+e1+d2 =14+C4+63 =B3 -> F8h=(2) B3667A2E for c3 c2 c1 c0
$c3 = $f1^$e1^$d2;
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($c3 << 24)){
$c2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$c1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$c0 = ($crc32lookup[$i] & 0x000000FF);
$two = $i;
break;
}
}
//echo "c3:" . dechex($c3) . " c2:" . dechex($c2) . " c1:" . dechex($c1) . " c0:" . dechex($c0) . "<br>";
//b3=f0+e0+d1+c2=78+23+5C+66=61 -> DEh=(1) 616BFFD3 for b3 b2 b1 b0
$b3 = $f0^$e0^$d1^$c2;
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($b3 << 24)){
$b2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$b1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$b0 = ($crc32lookup[$i] & 0x000000FF);
$one = $i;
break;
}
}
/*
Now we have all needed values, then
X=(1)+ a0= DE+66=B8
Y=(2)+ b0+a1= F8+D3+EF=C4
Z=(3)+ c0+b1+a2= 4F+2E+FF+CD=53
W=(4)+d0+c1+b2+a3=35+01+7A+6B+AB=8E
(final computation)
*/
$X = $one ^ $a0;
$Y = $two ^ $b0^$a1;
$Z = $three ^ $c0^$b1^$a2;
$W = $four ^ $d0 ^ $c1 ^ $b2^$a3;
//echo "Four:" . dechex($four) . " Three:" .dechex($three)." Two:".dechex($two)."One:" .dechex($one) . "<br>";
//echo "X:" . dechex($X) . " Y:" . dechex($Y) . " Z:" . dechex($Z) . " W:" . dechex($W) . "<br>";
return chr($X) . chr($Y) . chr($Z) . chr($W);
}
?>
答案 1 :(得分：1)
您想知道它是如何工作的，或者如何使用它？
如果它是后者，而不是代码签名：
public void FixChecksum(byte[] bytes, int length, int fixpos, uint wantcrc)
似乎您将第二个文件的内容放到一个数组中（最后还有4个字节用于修复），并将其作为bytes参数传递。您将此数组的长度作为length参数传递，将偏移量传递到插入修复的位置（在这种情况下长度为4）作为fixpos参数，然后放置所需的CRC为wantcrc参数，可以通过计算第一个文件的CRC来获得该值。
FixChecksum方法似乎在您提供的偏移处提供的数组中写入4字节修复。在您拨打FixChecksum之后，您只需将结果写入第二个文件。