One-Pass Signature Packets (Tag 4)
The One-Pass Signature packet precedes the signed data and contains enough information to allow the receiver to begin calculating any hashes needed to verify the signature. It allows the Signature Packet to be placed at the end of the message, so that the signer can compute the entire signed message in one pass.
A One-Pass Signature does not interoperate with PGP 2.6.x or earlier.
The body of this packet consists of:
Note that if a message contains more than one one-pass signature, then the signature packets bracket the message; that is, the first signature packet after the message corresponds to the last one-pass packet and the final signature packet corresponds to the first one- pass packet.