urcrypt is a library of cryptography routines used by urbit jets.
Urbit's C runtime (long the only urbit runtime) has accumulated a collection of cryptography dependencies, some with custom additions or patches. These libraries have different conventions and have been managed by u3 in an ad-hoc manner. Reproducing that arrangement in other runtimes is tricky and error-prone. The (sometimes inconsistent) logic must be reproduced and suitable cryptography primitives must be found (or worse, written) for the new environment.
To ease these burdens, urcrypt isolates the quirks behind a consistent calling convention. Everything is a little-endian byte array, and each jetted operation has a corresponding function in the library. Jets simply unpack their nouns, call urcrypt, and pack the results.
This is more of a subjective question than it might appear. Any of the following conditions are sufficient, but not necessary, for a function to be included in urcrypt:
- The routine is sensitive to side-channel attacks (encryption, etc)
- Some property of the routine is cryptographically useful (SHA, RIPE, etc)
- The routine typically lives in a crypto library, for whatever reason.
It is the library user's responsibility to initialize openssl, set custom memory functions, etc.