Digital signature

How it works

A checksum detects change. A signature also names a key. The recipient needs the signer's certificate. A transfer can succeed and the signature still fail if the file was re-saved.

A 20 MB firmware image carries a signature. The download hash matches. The signature verifies against the vendor key. A mirror image with the same size and a broken signature is rejected. Size was never enough.

How it differs

A digital signature is not a mail signature block. One is math on the file. The other is a footer.

Re-encoding a PDF after signing breaks the signature without looking like a short download.

Sign last.

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 digital signature, 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 digital signature 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.