SIMD-0334: Fix alt_bn128_pairing Syscall Length Check (#334)

* Fix alt_bn128_pairing syscall length check

* fix error

* fix error

proposals/0334-fix-alt-bn128-pairing-length-check.md

@@ -0,0 +1,90 @@
+---
+simd: "0334"
+title: Fix alt_bn128_pairing syscall length check
+authors:
+  - Stanislav Ladyzhenskiy
+category: Standard
+type: Core
+status: Review
+created: 2025-08-11
+feature:
+supersedes:
+superseded-by:
+extends:
+---
+
+## Summary
+
+The `alt_bn128_pairing` syscall takes a byte slice as input,
+interprets the bytes as an array of pairs of g1 and g2 points on bn128 elliptic
+curve, and applies a pairing operation. If the byte slice input has an improper
+length, the function should terminate early. Specifically, if the byte slice
+length is not a multiple of 192 (the sum of the lengths of g1 and g2 points),
+the function should terminate early with an error.
+
+However, the current code does not perform this check correctly.
+
+This document proposes to fix this length check by checking for the correct
+length.
+
+## Motivation
+
+The `alt_bn128_pairing` function still works with the incorrect length check
+because it only processes multiples of 192 bytes and discards the rest.
+However, there could be successful inputs that are not multiples of 192.
+This could make the application logic harder to debug.
+
+## Alternatives Considered
+
+Leave as is.
+
+## New Terminology
+
+N/A
+
+## Detailed Design
+
+Currently, the code checks than `checked_rem` of the input length and
+`ALT_BN128_PAIRING_ELEMENT_LEN` (which is 192) is not `None`.
+However, `checked_rem` returns `None` when the rhs is 0,
+which never happens in this context.
+
+```rust
+pub fn alt_bn128_pairing(input: &[u8]) -> Result<Vec<u8>, AltBn128Error> {
+    if input
+        .len()
+        .checked_rem(consts::ALT_BN128_PAIRING_ELEMENT_LEN)
+        .is_none()
+    {
+        return Err(AltBn128Error::InvalidInputData);
+    }
+
+    // logic omitted...
+}
+```
+
+The correct logic should check that the reminder is 0.
+
+```rust
+pub fn alt_bn128_pairing(input: &[u8]) -> Result<Vec<u8>, AltBn128Error> {
+    if input.len() % ALT_BN128_PAIRING_ELEMENT_LEN != 0 {
+        return Err(AltBn128Error::InvalidInputData);
+    }
+
+    // logic omitted...
+}
+```
+
+## Impact
+
+This fix will prevent accidental misuse of the `alt_bn128_pairing`
+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