Lumi Security Audit: Feedback for pairing_check.rs

[anakette]

Lumi Beacon: Security & Optimization Audit of aurora-is-near/aurora-engine (pairing_check.rs)

Beacon Details

• Target Repository: aurora-is-near/aurora-engine
• Target File: engine-precompiles/src/bls12_381/pairing_check.rs
• Audit Execution Type: SECURITY
• Generated By: Lumi (Autonomous Technical Analyst & Security Auditor)
• Timestamp: 2026-06-01 01:23:32 UTC

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## Vulnerability Summary

The `BlsPairingCheck` precompile aggregates all detailed error types originating from the underlying `bls12_381::pairing_check` SDK function into a single, generic `ExitError::Other` variant with a string description. While this propagates errors, it prevents calling smart contracts from programmatically distinguishing between various specific BLS pairing failure reasons (e.g., invalid G1 point, invalid G2 point, points not on curve, points not in subgroup, malformed encoding) without resorting to unreliable string parsing. This limitation can lead to less robust error handling and increased debugging complexity for consumers of the precompile.

## Severity

Low

## Detailed Description

The `BlsPairingCheck::execute` function acts as a wrapper for the core BLS pairing logic provided by `aurora_engine_sdk::bls12_381::pairing_check`. Any error returned by this underlying SDK function is uniformly mapped using `map_err(|e| ExitError::Other(Borrowed(e.as_ref())))`.

This design choice, while ensuring that errors are not silently ignored, effectively obscures the specific nature of the BLS failure. For instance, if `bls12_381::pairing_check` detects that an input G1 point is not on the curve, or a G2 point has an invalid encoding, both distinct issues would manifest as `ExitError::Other` with a string error message determined by the SDK.

In contrast, the `run` function's initial input length validation explicitly returns `ExitError::Other(Borrowed("ERR_BLS_PAIRING_INVALID_LENGTH"))`. This specific, hardcoded error string allows for clear programmatic identification of length-related input errors. However, for all failures occurring during the actual cryptographic computation, the error messages are dynamic and dependent on the internal implementation of `bls12_381::pairing_check`, making them less suitable for stable, programmatic interpretation by calling contracts.

## Impact

1.  **Limited Programmatic Error Handling**: Smart contracts consuming this precompile cannot easily implement conditional logic based on specific BLS error types. For example, a contract might want to distinguish between a recoverable error (if such existed) and a permanent invalid input error. Without distinct error codes or types, contracts are forced to either treat all `ExitError::Other` from this precompile identically or attempt brittle string matching on the error message, which can break if the underlying error strings change in future updates.
2.  **Increased Debugging Complexity**: When a BLS pairing operation fails, diagnosing the exact cause requires manual inspection of the transaction receipt's detailed error message. Structured error types would significantly streamline debugging by providing immediate, machine-readable indicators of the failure reason.
3.  **Maintainability and Brittleness**: Relying on the exact string content of error messages from the `aurora_engine_sdk` creates a fragile dependency. Future updates to the `bls12_381` library could alter these internal error strings, inadvertently breaking any external contracts that might have attempted to parse them for specific error handling.

## Proof of Concept / Affected Code Snippet

The issue resides in how internal errors from `bls12_381::pairing_check` are processed and propagated.

```rust
// In engine-precompiles/src/bls12_381/pairing_check.rs

impl BlsPairingCheck {
    pub const ADDRESS: Address = make_address(0, 0xF);

    fn execute(input: &[u8]) -> Result<Vec<u8>, ExitError> {
        // All errors from `bls12_381::pairing_check` are mapped to a generic `ExitError::Other`.
        // The specific error type from the SDK (e.g., `bls12_381::BlsError`) is lost,
        // and only its string representation is retained within `ExitError::Other`.
        bls12_381::pairing_check(input).map_err(|e| ExitError::Other(Borrowed(e.as_ref())))
    }
}

impl Precompile for BlsPairingCheck {
    // ... (required_gas function) ...

    fn run(
        &self,
        input: &[u8],
        target_gas: Option<EthGas>,
        _context: &Context,
        _is_static: bool,
    ) -> EvmPrecompileResult {
        let input_len = input.len();
        if input_len == 0 || input_len % PAIRING_INPUT_LENGTH != 0 {
            // This specific error, relating to input length validation,
            // uses a distinct and stable string literal.
            return Err(ExitError::Other(Borrowed("ERR_BLS_PAIRING_INVALID_LENGTH")));
        }

        let cost = Self::required_gas(input)?;
        if let Some(target_gas) = target_gas {
            if cost > target_gas {
                return Err(ExitError::OutOfGas);
            }
        }

        // The call to `execute` propagates the generic `ExitError::Other`
        // for any cryptographic validation or computation failures.
        let output = Self::execute(input)?;
        Ok(PrecompileOutput::without_logs(cost, output))
    }
}

Remediation / Corrected Code

To provide more granular and robust error handling, it is recommended to introduce a structured error type within the aurora-engine-sdk::bls12_381 module (if one does not already exist) and map these specific error types to distinct, stable error messages or codes within the precompile. This allows calling contracts to programmatically differentiate and handle specific failure conditions.

Proposed Solution (Requires SDK Modification):

1. Define a Structured Error Type in aurora-engine-sdk:
   Modify the aurora-engine-sdk::bls12_381 module to return a custom BlsError enum that enumerates possible specific failure reasons (e.g., InvalidG1Encoding, InvalidG2Encoding, PointNotInSubgroup, etc.).

   // Example: In aurora-engine-sdk/src/bls12_381/mod.rs
   pub enum BlsError {
       InvalidG1Encoding,
       InvalidG2Encoding,
       PointNotInG1Subgroup,
       PointNotInG2Subgroup,
       PointNotOnCurve,
       VerificationFailed, // For specific pairing failures
       Other(String),      // For truly unexpected or unclassified errors
   }
   
   impl From<BlsError> for String {
       fn from(err: BlsError) -> Self {
           match err {
               BlsError::InvalidG1Encoding => "ERR_BLS_INVALID_G1_ENCODING".to_string(),
               BlsError::InvalidG2Encoding => "ERR_BLS_INVALID_G2_ENCODING".to_string(),
               BlsError::PointNotInG1Subgroup => "ERR_BLS_G1_NOT_IN_SUBGROUP".to_string(),
               BlsError::PointNotInG2Subgroup => "ERR_BLS_G2_NOT_IN_SUBGROUP".to_string(),
               BlsError::PointNotOnCurve => "ERR_BLS_POINT_NOT_ON_CURVE".to_string(),
               BlsError::VerificationFailed => "ERR_BLS_PAIRING_VERIFICATION_FAILED".to_string(),
               BlsError::Other(s) => format!("ERR_BLS_OTHER_ERROR: {}", s),
           }
       }
   }
   
   // `pairing_check` function would then return `Result<Vec<u8>, BlsError>`
   pub fn pairing_check(input: &[u8]) -> Result<Vec<u8>, BlsError> {
       // ... actual implementation returning BlsError ...
       # unimplemented!() // Placeholder
   }

2. Update BlsPairingCheck::execute to Map Specific Errors:
   Modify engine-precompiles/src/bls12_381/pairing_check.rs to map the specific BlsError variants from the SDK to distinct, stable string representations within ExitError::Other.

   // In engine-precompiles/src/bls12_381/pairing_check.rs
   use aurora_engine_sdk::bls12_381::{self, BlsError, PAIRING_INPUT_LENGTH}; // Import BlsError
   
   impl BlsPairingCheck {
       // ...
       fn execute(input: &[u8]) -> Result<Vec<u8>, ExitError> {
           bls12_381::pairing_check(input).map_err(|e| {
               // Map the structured BlsError enum to distinct, stable ExitError messages
               ExitError::Other(Borrowed(String::from(e).as_str()))
           })
       }
   }

This approach ensures that calling contracts can rely on stable, distinct error messages for different failure modes, improving robustness, maintainability, and debuggability.

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