Are you saying that given a size and a target CRC checksum other than zero, it's easy to compose a message of length size that produces the target checksum?
Yes.

Also, if you know the CRC of some data, you can calculate the CRC of "data xor something", even if you don't know what the data was! 

use String::CRC32;
# given $crc == crc32($data)
$crc2 = $crc ^ crc32($diff) ^ crc32("\0" x length($diff));
# now, $crc2 == crc32($data ^ $pad.$diff)
# where $pad = "\0" x (length($data) - length($diff))

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In reply to Tweaking CRCs by no_slogan
in thread How safe is truncating an MD5 digest string? by lemmett

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