The checksum module

sysl.checksum holds checksums that catch accidental corruption. A CRC is not a digest: a second input with the same CRC-32 can be built on purpose, so where an attacker gains from a collision use sysl.crypto.

CRC-32

The polynomial is IEEE 802.3’s, reflected (0xedb88320), with initial value and final xor both 0xffffffff — the CRC zip, gzip, PNG and Ethernet carry. Its 256-entry table is data laid down at compile time, so the module costs no start-up work and builds for a freestanding target.

import sysl.checksum.{crc32, new_crc32}

print(crc32("123456789".bytes))

var c = new_crc32()

c.update("1234".bytes)
c.update("56789".bytes)
print(c.finish())
print(crc32("".bytes))

3421780262
3421780262
0

0xcbf43926, which is 3421780262, is the standard check value for these nine bytes. Crc32 is an ordinary struct, so copying one starts another from the same point, and finish does not end it: more input may follow.