The Base RSA Digital Signature Profile outlines the use of RSA encryption of a MAC to generate a Digital Signature. This Profile does not specify any particular set of Data Elements to sign. Other Digital Signature profiles may refer to this profile, adding specifications of which Data Elements to sign or other customizations.
The creator of a digital signature shall use one of the RIPEMD-160, MD5, SHA-1 or SHA-2 family (SHA256, SHA384, SHA512) of hashing functions to generate a MAC, which is then encrypted using a private RSA key. All validators of digital signatures shall be capable of using a MAC generated by any of the hashing functions specified (RIPEMD-160, MD5, SHA-1 or SHA256, SHA384, SHA512).
The use of MD5 is not recommended by its inventors, RSA. See:ftp://ftp.rsasecurity.com/pub/pdfs/bulletn4.pdf
The MAC to be signed shall be padded to a block size matching the RSA key size, as directed in RFC 2437 (PKCS #1). The Value of MAC Algorithm (0400,0015) shall be set to either "RIPEMD160", "MD5", "SHA1", "SHA256", "SHA384" or "SHA512". The public key associated with the private key as well as the identity of the Application Entity or equipment manufacturer that owns the RSA key pair shall be transmitted in an X.509 (1993) signature certificate. The Value of the Certificate Type (0400,0110) Attribute shall be set to "X509_1993_SIG". A site-specific policy determines how the X.509 certificates are generated, authenticated, and distributed. A site may issue and distribute X.509 certificates directly, may utilize the services of a Certificate Authority, or use any reasonable method for certificate generation and verification.
If an implementation utilizes timestamps, it shall use a Certified Timestamp Type (0400,0305) of "CMS_TSP". The Certified Timestamp (0400,0310) shall be generated as described in "Internet X.509 Public Key Infrastructure; Time Stamp Protocols; March 2000".