Ron Rivest's message digest algorithm MD5 is described in RFC 1321. Implementing the algorithm killed a boring afternoon and were as such not meant to be used. Later the implementation were improved Lars T. Hansen, who replaced bignum arithmetic with arithmetic of pairs of fixnums. Subsequently it were included in Larceny and PLT Scheme.
The source md5.ss is available in the PLT Scheme Source Browser.Jens Axel Søgaard