Public key support, analogous to that provided by RSAREF. Suggested by
Dan Egnor. Signatures are done now, but RSA encryption is still
missing. References:

  http://download.gale.org/rsaref20.tar.Z
  http://www.openssl.org/docs/crypto/evp.html
  http://www.openssl.org/docs/crypto/rsa.html

More feedback modes, in order of decreasing priority:  CBC-MAC, OFB,
and CFB. Suggested by Rafael 'Dido' Sevilla. References:

  http://csrc.nist.gov/encryption/modes/Recommendation/Modes01.pdf

Valgrind reports errors on the des-compat test program. Investigate.

The make rules for building position independent *_p.o files doesn't
get dependencies right.