1.) When are digital signatures and footer stamping incompatible?
A digital signature is something stamped onto a message by the sender. The signature is a cryptographic operation, usually a hash, across the message content, which is then locked with the private key of the sender. The recipient can use the sender's public key to compare a hash they compute with the transmitted hash to see if the message has been tampered with, or isn't from the purported sender at all.
Injecting a footer into a message after it is signed by the sender will invalidate the digital signature. As a result, all recipients of the message think that the message has been tampered with or was forged. If you want to digitally sign messages and have footers, then you need to put the footer into the message before the sender adds their digital signature.
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