cyclic redundancy check. crc32 is supposed to be faster than shasum, you should only use shasum if the output of crc32 for two files is equal, otherwise, you are safe to say the two files are not equal.
crc32 /path/to/file
md5 file.iso
md5 /path/to/file
shasum -a 1 file.iso
shasum -a 1 /path/to/file
shasum -a 256 file.dmg
shasum -a 256 /path/to/file
echo "secret-string" | shasum -a 256