Fix alt-bn128 multiplication syscall length

Summary

The alt_bn128_multiplication syscall takes in a byte slice as input, interprets the bytes as a bn128 elliptic curve point/scalar pair, and applies point-scalar multiplication. If the byte slice input has improper length then the function terminates early. Specifically, if the byte slice has length greater than 128, then the function terminates early with an error.

However, a bn128 curve point is 64 bytes and a scalar is 32 bytes. This means that the function should check if the byte slice is 96 bytes in length rather than 128 bytes.

This document proposes to fix this length check by checking for the correct length.

Motivation

The alt_bn128_multiplication function still works with the incorrect 128 length bound since a correct input of 96 bytes is still less than 128 bytes. However, there could be successful inputs that are greater than 96 bytes and smaller than 128 bytes in length. This could cause application logic harder to debug.

Alternatives Considered

Leave as is.

New Terminology

N/A

Detailed Design

The fix is simple.

Currently, the constant ALT_BN128_MULTIPLICATION_INPUT_LEN, which is set to 128 is used to sanity check the length of the input.

pub fn alt_bn128_multiplication(input: &[u8]) -> Result<Vec<u8>, AltBn128Error> {
    if input.len() > ALT_BN128_MULTIPLICATION_INPUT_LEN {
        return Err(AltBn128Error::InvalidInputData);

    // logic omitted...
}

A fix would entail updating the ALT_BN128_MULTIPLICATION_INPUT_LEN constant to the correct length of 96.

Impact

This fix will prevent accidental misuse of the alt_bn128_multiplication syscall function and make programs easier to debug.

Security Considerations

This does update the behavior of the syscall function and therefore, should be properly feature-gated.

Drawbacks (Optional)

None