Context:

For the first release of Unison Cloud JIT, we need to implement primitives for cryptographic hash functions in chez scheme.
Which ones?

• sha1
• sha2(256/512)
• sha3(256/512)
• blake2b(256/512)
• blake2s(512)

Existing implementations

Scheme

There exist pure scheme implementations of sha1 and sha2¹², and some C bindings to sha1³⁴ and sha2⁵⁶. There's a set of C bindings for OpenSSL as well⁷, which would provide all of the hashing functions we need.

Other languages

The haskell-based unison runtime is using the library cryptonite, which has the hash functions implemented in C⁸

Nodejs, on the other hand, links to openssl for its hashing functions.

Recommendation

Given the anemic nature of the scheme community, I don't think it wise to depend on a pure scheme implementation for something as critical as cryptographic hashing, unless it were to be vetted by a security expert.

This leaves us with C FFI, and I see two options:

1. link to openssl, which would give us all of the hashing we need, as well as x509 certificates & TLS stuff, which we'll need for the Cloud JIT anyway
2. copy over the C implementations from cryptonite

Option 1 has the benefit of giving us TLS support & certificates and such, but it's obviously a much bigger library to include. Give the ubiquity of openssl, it's likely to be available on any cloud infrastructure we're running.
Option 2 is much leaner, and there's something nice about having the exact same C backing the haskell runtime and the cloud jit runtime, but then we'd have to figure out something else for TLS & x509 certificates.