md5
md5 is a cryptographic Message-Digest algorithm from RSA Data Security, Inc. It is defined in RFC 1321: The MD5 Message-Digest Algorithm, by Ron L. Rivest. An implementation is included in the Ironclad package.

An implementation by Fare Rideau in portable CL is available from his CVS site.

Another portable implementation, by Pierre R. Mai, which has been highly bummed for CMUCL with copious input from other developers and users on the cmucl-help mailing list, is available from http://www.pmsf.de/resources/lisp/MD5.html.

and also (adjusted for SBCL compatibility) in SBCL contrib as sb-md5 - just (require :asdf) and (require :sb-md5) to load it.

and also as a Debian package(stale) thanks to Kevin Rosenberg.

See Cryptography.

ASDF-install package (obsolete) http://files.b9.com/md5/md5-1.8.5.tar.gz

Alternate download: http://ftp.de.debian.org/debian/pool/main/c/cl-md5/cl-md5_1.8.5.orig.tar.gz (debian to the rescue once again)

(package link stale)

Note that MD5 collisions have been found: Xiaoyun Wang and Dengguo Feng and Xuejia Lai and Hongbo Yu: Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD, so you should consider SHA1 instead.