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Noir-Chacha20

This is a Noir implementation of ChaCha20 as defined by RFC7539.

[!IMPORTANT] This implementation works on 4-byte words, as opposed to the RFC working with 1-byte words.

Due to the above, the tests provided are adopted to 4-byte words

The crates/chacha20_example provides a small binary implementation to measure circuit size.

+------------------+------------------------+--------------+----------------------+
| Package          | Language               | ACIR Opcodes | Backend Circuit Size |
+------------------+------------------------+--------------+----------------------+
| chacha20_example | PLONKCSat { width: 3 } | 23969        | 34201               |
+------------------+------------------------+--------------+----------------------+

On a Ryzen 7 7700X, proving a single 32-byte message encryption took 42s.