Consider changing PoseidonPermutation::set_from_slice

[sdwoodbury]

potential security issue

relevant code
Duplex Sponge Paper

It appears that this function will

• overwrite the capacity portion of the state if the slice is large enough
• not pad the rate portion of the state when the slice is less than rate field elements.

If the sponge ingested elements by adding them into the state, and the padding scheme is to pad with zeros, then ignoring the unset portion of the rate is ok (adding zero-padding is a no-op). But here, overwrite mode is being used. I think that this would require the unset portion of the rate to be overwritten with padding.

[sdwoodbury]

for reference, this is what plonky2 does:

• challenger.rs
• poseidon.rs

related PRs

• feat: poseidon port #180
• feat: integrate poseidon port into mpcs + transcript #325

[sdwoodbury]

based on this it looks like Plonky2 waits until the rate portion is full before permuting. This eliminates the need for padding.