PKCS #7: Cryptographic Message Syntax Standard
This standard describes general syntax for data that may have cryptography applied to it, such as digital signatures and digital envelopes.
Version 1.6 Bulletin
In support of the SET protocol for secure electronic transactions, RSA Laboratories has produced a bulletin detailing changes to be made to PKCS #7 version 1.5 to update it to version 1.6 (which has not yet been published). Version 1.5 is still recommended for general use, and is the basis for the S/MIME specification.