Fix alt-bn128-multiplication syscall length check

Author: samkim-crypto

proposals/0222-fix-alt-bn128-multiplication-length-check.md

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+---
+simd: "0222"
+title: Fix alt-bn128 multiplication syscall length
+authors:
+  - Sam Kim
+category: Standard
+type: Core
+status: Review
+created: 2025-01-10
+feature:
+supersedes:
+superseded-by:
+extends:
+---
+
+## 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.
+
+```rust
+
+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