http://www.rfc-editor.org/rfc/rfc5035.txt
RFC 5035: Enhanced Security Services (ESS) Update: Adding CertID Algorithm Agility, J. Schaad2007-08-01T23:00:00-00:00In the original Enhanced Security Services for S/MIME document (RFC 2634), a structure for cryptographically linking the certificate to be used in validation with the signature was introduced; this structure was hardwired to use SHA-1. This document allows for the structure to have algorithm agility and defines a new attribute for this purpose. [STANDARDS-TRACK]