A direct translation of TweetNaCl into Common Lisp
Common Lisp
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README.org Mark as dormant experiment Dec 18, 2015
naclcl.asd
naclcl.lisp
package.lisp

README.org

Status

This experiment is currently abandoned. If you want to use NaCl from Common Lisp, there are bindings for libsodium at https://github.com/Harleqin/foreign-sodium.

I might return to native implementation later, but that may or may not be based on the code in this repository.

Overview

This is a more-or-less direct translation of TweetNaCl, a version of the nacl.cr.yp.to library that was published in 100 tweets on Twitter in July 2013 by the NaCl main author, Daniel J. Bernstein. I watched his talk “The year in crypto” at the 30th Chaos Communication Congress in Hamburg, together with some fellow Lispers, and we half-jokingly agreed that this needed to be translated to a more useful language. ;-)

Note that this is not just a wrapper, but a complete reimplementation. I know that a wrapper is likely to be more useful, but I am doing this rather as a comparative linguistics experiment.

TweetNaCl is written in C, and condensed in almost code golf style to make it fit the 100 tweets. The C original is included in comments in the naclcl.lisp file (formatted to my own, not so fashionable tastes).

The translation is to Common Lisp. Current status: first pass, trying to get the bit fiddling translated one-to-one. This was mostly an exercise in Common Lisp’s bit manipulation features. The code compiles, but is otherwise untested. Current goal: Correct remaining mistakes, and a deeper understanding should lead to clearer Common Lisp code.

Some first impressions

  • Common Lisp is just as useful for bit fiddling as C, with its elaborate set of bit manipulation functions.
  • C-style “return parameters”, prevalent in the original, can be replaced by multiple-value semantics (although I currently have not yet refactored in this direction).
  • C-style return of status codes can be replaced by conditions or additional return values. The original uses them frequently, and it is often not clear what meaning -1 or 0 are supposed to convey.
  • The original often has pointers as parameters, which makes it unclear how big the underlying arrays/streams are supposed to be. Replacing these by Lisp vectors is useful to make these assumptions more explicit, and it also adds bounds checking. I am quite sure that I have made mistakes when extracting the intent from the C code.

Roadmap

  • Add unit tests, understand each function for itself, add documentation to each, improve translation
  • Add integration tests, understand the exported functions, separate into more files, add general documentation
  • Understand the features outlined at http://nacl.cr.yp.to/features.html and try to support them as well
  • Refactor! Fun! I especially want to get better parameter and return value handling and convert the stream operations to real stream handling.
  • Optimize! More fun! Avoid consing, but keep an eye on constant running time to prevent side channel attacks (this should also be automatically tested).
  • Compare the final result with both TweetNaCl and NaCl from nacl.cr.yp.to, by speed, clarity of code, and clarity of the API that a library user gets to see. Also include the C++ and Python wrapper APIs in this survey.
  • Perhaps I will decide along the way to start afresh, this time based on the exported functions of the NaCl library from nacl.cr.yp.to and building from there.
  • Brag a lot