What is urcrypt?
----------------
urcrypt is a library of cryptography routines used by urbit jets.

Why is urcrypt?
---------------
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.

What is a cryptography routine?
-------------------------------
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.

A word on OpenSSL
-----------------
Urcrypt depends on OpenSSL's libcrypto, which has global state. In order
to avoid dealing with this state, urcrypt refuses to build with an internal
libcrypto. Either build statically (pass `--disable-shared` to `./configure`)
or provide a shared libcrypto for urcrypt to link against. It is the library
user's responsibility to initialize openssl, set custom memory functions, etc.