References

   [1] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part
       I: Message Encryption and Authentication Procedures", RFC 1421,
       DEC, February 1993.

   [2] Kent, S., "Privacy Enhancement for Internet Electronic Mail: Part
       II: Certificate-Based Key Management", RFC 1422, BBN, February
       1993.

   [3] Balenson, D., "Privacy Enhancement for Internet Electronic Mail:
       Part III: Algorithms, Modes, and Identifiers", RFC 1423, TIS,
       February 1993.