ETag

How it works

RFC 9110 defines validators. On a simple S3 PUT the ETag may be an MD5. On a multipart upload it is not. If-Match on a download detects a change. Comparing a multipart ETag to a local MD5 fails a good file.

A client checks the ETag of a 6 GB multipart object against an MD5 and rejects the download. The SHA-256 published beside it matches. They stop using ETag as the contract hash.

How it differs

An ETag is not a checksum you can always recompute locally. Know how it was built.

Weak ETags can match bytes that are not identical.

Use a published SHA-256 for delivery.

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 etag, 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 etag 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.