The first 16 bits of the message digest in a PGP signature are translated in the clear. As we know, sending something in clear text does not sound like a very good idea from a security point of view. But PGP does this. Why clear text is used in this way in PGP?
closed as unclear what you're asking by Adi, Xander, scuzzy-delta, TildalWave, Manishearth Oct 15 '13 at 16:10
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RFC 4880, the standard for the format of PGP messages, says:
However, you are thinking it wrong. Signatures are not encryption, and signatures are not encrypted. In fact, given the value of the public key (which is public) and the signature itself, one can recompute the complete hash value of the message which is signed (at least for RSA, which is technically known as a signature algorithm with recovery). The first 16 bits are just a helper so that software can avoid many modular exponentiations when it is looking for the "correct" public key among a set of candidates; they save a few milliseconds worth of computation, that's all.
A generic note is that signatures can leak information on that which is signed; so if you sign and encrypt a confidential message, then you should, conceptually, either sign the encrypted message, or encrypt the signature along with the message contents as well. OpenPGP uses the latter method.
When a message is just signed, not encrypted, then it makes no sense to hide the message hash, since the message itself, by definition, is transmitted as cleartext.