Reference

Draft - Multiple Signatures using Security Multiparts
Describes how the Security Multiparts defined in RFC 1847 can be used to transport multiple digital signatures.
http://www.ietf.org/proceedings/01mar/I-D/openpgp-multsig-02.txt

PGP Key signing policy of Juergen Nieveler
PGP Key and Key signing policy of Juergen Nieveler.
http://www.nieveler.org/PGP/pgp.htm

PGP related RFC
PGP query results in HTML format with tables of contents and crosslinked references
http://zvon.org/tmRFC/RFC_share/PHP/titleSearch.php?request=pgp&searchType=1&matchType=1

RFC 2015 - MIME Security with Pretty Good Privacy (PGP)
This document describes how Pretty Good Privacy (PGP) can be used to provide privacy and authentication using the Multipurpose Internet Mail Extensions (MIME) security content types described in RFC1847. (M. Elkins / Aerospace Corporation)
http://tools.ietf.org/html/rfc2015

RFC 2440 - OpenPGP Message Format
This document describes information needed to develop interoperable applications based on the OpenPGP format.
http://tools.ietf.org/html/rfc2440

RFC 1991 - PGP Message Exchange Formats
This document describes the format of PGP 2.x files, i.e., messages that have been encrypted and/or signed with PGP. Replaced by RFC 2440.
http://tools.ietf.org/html/rfc1991