Two Signatures Added to a File Received

A contract often arrives as a finished file somebody else wrote, with nowhere to sign it. Two people still have to sign it — one approves, one reviews — and the second signature must not break the first. Writing the file again would do exactly that: a signature is worked out from the bytes of the file, and a single byte moved breaks it.

This example never writes again the file it receives. It adds the two framed places to sign in a section written after the file's last byte, then signs the approval, then the review, each in a section of its own again. The review covers the whole file; the approval still covers exactly the bytes it covered, and still checks out. Both signatures use a certificate made only for this example, and the page prints the OpenSSL command that checks each of them without this library.

Last, the certificate both signatures rest on is kept inside the file, filed under each of them, in a section of its own. Years later, once the certificate has expired, the file still holds what shows that both signatures were valid on the day they were made.

What This Example Shows

  • A file with no form given two framed places to sign, not one of its bytes moved.
  • Two signatures one after the other, the first still valid once the second is made.
  • The OpenSSL command that checks both signatures without this library.
  • The certificate kept in the file, so both signatures stay checkable after it expires.

The Source Code, Complete

Nothing is left out and nothing is simplified: this is the program exactly as it runs, for the developer who uses this library.

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The Document Itself

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