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From the Schneier on Security blog

Interesting summary of various ways to derive the public key from digitally signed files.

Normally, with a signature scheme, you have the public key and want to know whether a given signature is valid. But what if we instead have a message and a signature, assume the signature is valid, and want to know which public key signed it? A rather delightful property if you want to attack anonymity in some proposed “everybody just uses cryptographic signatures for everything” scheme.