[Medium-1] Verifier panics on short hashes vector

[this-vishalsingh]

Context: src/signature/generalized_xmss.rs

Description:

verify() indexes sig.hashes[chain_index] without first checking sig.hashes.len() == IE::DIMENSION.

A malicious signature (e.g., decoded from SSZ/serde) with an empty/short hashes vector will trigger an out-of-bounds panic, crashing the verifier (remote DoS in any service that verifies untrusted signatures).

Impact

• Bug type: Resource management
• Code:

// ...
let chain_length = IE::BASE;
let num_chains = IE::DIMENSION;
// ...
for (chain_index, xi) in x.iter().enumerate() {
    let steps = (chain_length - 1) as u8 - xi;
    let start_pos_in_chain = *xi;
    let start = &sig.hashes[chain_index];
    let end = chain::<TH>(
        &pk.parameter,
        epoch,
        chain_index as u8,
        start_pos_in_chain,
        steps as usize,
        start,
    );
    chain_ends.push(end);
}
// ...

Recommendation

Before the loop, validate sig.hashes.len() == IE::DIMENSION (and ideally reject any other length early in SSZ/serde decoding).

Return false on mismatch instead of panicking.

[b-wagn]

Closing this, addressed in #36