Privacy-Enhanced Mail

Privacy-Enhanced Mail is a set of protocols and mechanisms providing confidentiality and authentication for Internet electronic mail. Relevant mechanisms include the following (see RFC 1423 for details):

PKCS #1 RSA key pair generation (508-1024 bits)

PKCS #1 RSA (508-1024 bits)

DES key generation

DES-CBC

DES-ECB

double-length DES key generation

triple-DES-ECB

MD2
MD5

Variations on this set are certainly possible. For instance, PEM applications which make use only of asymmetric key management do not need the DES-ECB or triple-DES-ECB mechanisms, or the double-length DES key generation mechanism. Similarly, those which make use only of symmetric key management do not need the PKCS #1 RSA or RSA key pair generation mechanisms.

An "authentication-only" version of PEM with asymmetric key management would not need DES-CBC or DES key generation.

It is also possible to consider "exportable" variants of PEM which replace DES-CBC with RC2-CBC, perhaps limited to 40 bits, and limit the RSA key size to 512 bits.

RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v200