How to compute and verify a digest

rustruct.digest() computes a checksum or hash during pack and optionally verifies it during unpack:

from rustruct import Struct, U8, U32, digest, slice


class Chunk(Struct):
    size: U8
    data: bytes = slice(len="size")
    crc: U32 = digest("crc32", over=("data",))


chunk = Chunk(data=b"abc")
wire = chunk.pack()
assert wire == bytes.fromhex("03 616263 352441c2")
assert Chunk.unpack(wire).crc == 0x352441C2

over=("data",) names a specific sibling span rather than "*" the whole structure, so the checksum covers only data, not size:

Offset	Bytes	Field	Value
0	1	size	0x03
1	3	data	b"abc"
4	4	crc	0x352441c2

Built-in algorithms include CRC presets, IPv4’s Internet checksum, MD5, SHA-1, and SHA-256. over="*" covers the enclosing scope with the digest’s own bytes treated as zero. A tuple covers named sibling spans.

A hash algorithm works the same way as a CRC – only the algorithm name and the resulting field’s width change. A hash digest is a bytes field, not an integer: its width is whatever the algorithm produces (16 bytes for MD5, 32 for SHA-256), not something declared separately:

import hashlib


class HashedChunk(Struct):
    size: U8
    data: bytes = slice(len="size")
    checksum: bytes = digest("sha256", over=("data",))


hashed = HashedChunk.unpack(HashedChunk(data=b"abc").pack())
assert hashed.checksum == hashlib.sha256(b"abc").digest()

"md5" and "sha1" work the same way, each with their own fixed digest width.

Verification failures raise rustruct.InvalidDataError(kind=”checksum”).