Poseidon V1 variable-length input collision via implicit zero-padding
Poseidon V1 (PoseidonSponge) accepts variable-length inputs without injective padding. When a caller provides fewer inputs than the sponge rate (inputs.len() < T - 1), unused rate positions are implicitly zero-filled. This allows trivial hash collisions: for any input vector [m1, …, mk] hashed with a sponge of rate > k, hash([m1, …, mk]) equals hash([m1, …, mk, 0]) because both produce identical pre-permutation states. This affects any use of PoseidonSponge …