feat: Actually compute selectors

[sirasistant]

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• If the pull request requires a cryptography review (e.g. cryptographic algorithm implementations) I have added the 'crypto' tag.
• I have reviewed my diff in github, line by line and removed unexpected formatting changes, testing logs, or commented-out code.
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[Maddiaa0]

proving a keccak scares me

[sirasistant]

One selector compute:

fn main(signature: str<10>) -> pub Field {
    compute_selector(signature)
}

is

+---------+------------------------+--------------+----------------------+
| Package | Language               | ACIR Opcodes | Backend Circuit Size |
+---------+------------------------+--------------+----------------------+
| test    | PLONKCSat { width: 3 } | 17           | 54830                |
+---------+------------------------+--------------+----------------------+
%                                                                                      

[sirasistant]

It's not great, maybe we should switch to a different hash for selectors? But the computation needs to be proven, else a malicious PXE could call other function in the target

[sirasistant]

Note: this will only be executed in-circuit for non-constant signatures. For constant signatures the keccak gets simplified away:

fn main() -> pub Field {
    compute_selector("foo and bar")
}

+---------+------------------------+--------------+----------------------+
| Package | Language               | ACIR Opcodes | Backend Circuit Size |
+---------+------------------------+--------------+----------------------+
| test    | PLONKCSat { width: 3 } | 1            | 7                    |
+---------+------------------------+--------------+----------------------+

[LeilaWang]

LGTM!