J. Schaad, B. Kaliski, R. Housley. <a href="http://www.ietf.org/rfc/rfc4055.txt"><cite>Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile</cite></a>.  June 2005. IETF RFC 4055. <a href="http://www.ietf.org/rfc/rfc4055.txt">http://www.ietf.org/rfc/rfc4055.txt