README.md

Match the CRC32 value of files

To make my Almost-Narcissist program, I needed to be able to match the CRC32 value of a given file, the source of the Almost-Narcissist itself. I could only find a PHP CRC32-matcher, and it didn't seem to work.

So I wrote a CRC32 value matcher.

I used math from:

Reversing CRC - Theory and Practice
HU Berlin Public Report SAR-PR-2006-05
Martin Stigge, Henryk Plotz, Wolf Muller, Jens-Peter Redlich

I pretty much followed their appendices to get to the final matcher.

Files

• stdfile: 9 bytes, "123456789", CRC32 cbf43926
• stdfile2: 4 bytes, "asd\n", CRC32 152ddece
• bitoriented.go - Code from Appendix A.2, transliterated into Go
• tabledriven.go - Appendix A.3, transliterated from C to Go
• crc32.go - calculate CRC32 using standard package hash/crc32
• matchfile.go - give back bytes to append to a file to match another file's CRC32. Appendix A.5
• crc32.php - calculate CRC32 using PHP's library routine, just to double check.

To create a file with the same CRC32 value as another file:

$ go build matchfile.go
$ ./matchfile file_to_match some_file > bytes
$ cat bytes >> some_file
$ go build crc32.go
$ ./crc32 file_to_match some_file

To test the code:

$ make test

It should build a file with a CRC32 that matches that of stdfile.