CRC-32

How it works

ZIP members carry CRC-32. A mismatch means the member is corrupt. It does not name a sender. It is fast. Attackers can forge it. Use it as the archive's internal check and SHA-256 as the delivery check.

An unzip reports a CRC error on file 40 of a 3 GB ZIP. The archive hash already failed. They refetch. A CRC-only workflow would have caught this member and not a swapped archive with rebuilt CRCs.

How it differs

CRC-32 is not SHA-256. One is for noise. The other is for adversaries.

A tool that skips CRC on extract will accept a truncated member.

Leave the check on.

On the ticket

• The practical close is a log line: time, actor, byte count, result.
• Without that line the transfer is a story.
• With it, the next person can see whether this door did what the ticket claimed.
• If the path is shared, say so in the partner profile so a later change does not silently pick a different limit, key, or region.
• On a real ticket, write down the door, the byte count, and the clock.
• For crc32, that means naming the host or bucket, the expected size, and the time the other side must have a complete file.
• A progress bar is not that record.
• A 200 response that arrives before the complete call is not that record.
• If a retry is allowed, say how many and whether it resumes.
• If a person must approve the send, name the person.
• Partners who receive crc32 files should match on hash or size before they import.
• A same-length corrupt file passes a size check and fails a hash.
• Keep the published hash off the only channel an attacker can edit, or treat it as a corruption check rather than a substitution check.
• When the path changes, new key, new region, new cap, update the profile the same day so the next run does not use a stale limit.