At the suggestion of commenter Rodolfo, I have submitted the Mulkrypt cryptographic library for Racket on PLaneT.

Currently, Mulkrypt implements a sufficient number of advanced cryptographic algorithms (a stream cipher, HMAC, and two hashing functions) to securely send messages over a network, which is the use case it is geared towards. What it lacks is a good block cipher—while the library does contain something that likes to think of itself as an implementation of Threefish, that implementation is apparently buggy. One problem I encountered when writing it was that I was unable to locate sufficient test data, the only set I could test against being the example vectors contained in PySkein.

If you happen to run across other implementations of the Threefish cipher and get around to generating a couple of non-PySkein test vectors, I would be thankful for a comment on this blog post.