Title: BLS12_381_Pairing_Consensus_Issue
Category: Protocol_Calls___happenings

Author: MariusVanDerWijden
Date: 2025-02-28T17:40:45.869Z

Incident Response
Yesterday 27 Feb. 2025 , during the Pectra issues happening on Holesky,
EF security got a bug report at ~3:45pm CET via the Pectra Audit Competition which described a consensus issue between Geth and Nethermind when calling the BLS12-381 Pairing precompile. Magicians thread for EIP-2537 here.
Because I was attending a funeral, I could only start looking into the issue at 4:40pm CET and quickly confirmed that geth might be deviating from the specification described in EIP-2537.
The bug report contained three test cases that showed the issue. Nethermind quickly confirmed that they were passing all three test cases, while geth only passed the first one, thus confirming the consensus issue at 5pm CET, shoutout to Luk from Nethermind for responding so quickly.
All other client teams were quickly informed about the ongoing incident and a group was assembled with Antonio, Gotti, Justin and Kev from the EF cryptography and research teams.
After discussion with the cryptography team, we came to the conclusion at ~5:45pm that the specification is misleading and the spec and the Nethermind implementation should be changed.
The EEST (testing) team meanwhile prepared test vectors to confirm that no other client than Nethermind was affected.
In parallel to these discussions an impact analysis was done to confirm that no L2s or Ethereum forks were already running the BLS precompiles. At 5:50pm we confirmed that the issue was not live on any other network than Holesky.
At 5:50pm CET Nethermind also published their fix.
At 6:30pm Pawel from the Ypsilon team notified us that this edge case can also happen on the BN256 pairing precompile, but fortunately we had test vectors in ethereum/tests that covered that edge case for the BN curve.
In the aftermath more tests were added to ethereum/bls12-381, evmone, the EIP, EEST and Nethermind cut a new release.
And finally the misleading sentence was removed from the EIP.
Overall it took one hour and ten minutes from when we first confirmed the issue until the fix was implemented and shipped in a client.
Big thanks to everyone who was involved in helping to triage, debug, confirm and fix this issue!
If you are running Nethermind on Holesky, please update to version v1.31.2.
Root cause analysis
The BLS12-381 pairing check precompile does the following:
On an input of list of points {P,Q} ∈ G1 x G2
it computes the equation e(P1, Q1) * e(P2, Q2) * ... * e(Pk, Qk) == 1.
The precompile outputs 1 if the equation holds and 0 otherwise.
The specification for the EIP stated:
Note:

If any input is the infinity point, pairing result will be 1. 
Protocols may want to check and reject infinity points prior to calling the precompile.

This explanation can be interpreted in two different ways:
If a pair {P, Q} where either P or Q is infinity on their respective curves, either
(1) the pair may be ignored, but the multi-pairing should be computed¹
(2) the precompile should return 1 and the multi-pairing should not be computed
We confirmed later that the original author intended (1). The note was added to make smart contract developers aware that some use cases of pairing checks such as signature verification may require the caller to check that none of the inputs is the point at infinity prior to calling this ABI. The misleading sentence was later removed.
¹ Since e(P, Q) will always be 1, thus eliminating e(P, Q) from e(P, Q) * ... * e(Pk, Qk) == 1 will result in 1 * .... * e(Pk, Qk) == 1 which is equivalent.