example implementation for Biscuit token cryptography

To aid in the implementation of Biscuit tokens in various languages, this repository contains an example implementation of its cryptographic algorithms using libsodium.

Biscuit uses Aggregated Gamma Signatures ("Aggregation of Gamma-Signatures and Applications to Bitcoin, Yunlei Zhao") for its attenuation scheme, and this algorithm is implemented over the Ristretto group ("Ristretto: prime order elliptic curve groups with non-malleable encodings"), of which libsodium proposes an implementation.

The libsodium implementation is available in src/sodium.rs.