Signature Packet (Tag 2)
A signature packet describes a binding between some public key and some data. The most common signatures are a signature of a file or a block of text, and a signature that is a certification of a user ID.
Two versions of signature packets are defined. Version 3 provides basic signature information, while version 4 provides an expandable format with subpackets that can specify more information about the signature. PGP 2.6.x only accepts version 3 signatures.
Implementations MUST accept V3 signatures. Implementations SHOULD generate V4 signatures. Implementations MAY generate a V3 signature that can be verified by PGP 2.6.x.
Note that if an implementation is creating an encrypted and signed message that is encrypted to a V3 key, it is reasonable to create a V3 signature.