GnuPG: www.gnupg.org

RFC 2440: www.rfc-editor.org/rfc/rfc2440.txt

Philip Zimmermann: www.philzimmermann.com

Diceware Passphrase Home Page: www.diceware.com

Wikipedia: Passphrase Article: www.wikipedia.org/wiki/Passphrase

GnuPG HOWTOs: www.gnupg.org/documentation/howtos.html