How Do Cross-Chain Bridges Work?
A Case on Wormhole (Part 2)
January 11, 2023

Following Part 1, in this article we focus on guardian signatures verification in Wormhole on both Solana and Ethereum.

How Are the Guardian Signatures Verified (To Prevent Fake VAAs)?

On Solana, Wormhole uses the verify_signatures function to verify all the signatures in a VAA. Each VAA may contain multiple signatures (at least 2/3 of 19 verified signatures to reach a quorum). Because of the compute limit, it splits the signature verification into multiple steps (i.e., calling verify_signatures multiple times), with each call verifying a subset (e.g., six or seven) of the guardian signatures:

The input accounts in VerifySignatures are defined below:

The two PDA accounts guardian_set and signature_set are important. The guardian_set account must have been initialized (AccountState::Initialized) and it stores a set of the verified guardians (including their keys):

The signature_set stores the verified status (true or false) of each guardian signature (signatures: Vec<bool>), the message hash, and the guardian_set_index:

The signature_set account is created and initialized in the first call to verify_signatures:

In the subsequent calls, the same signature_set is validated with its corresponding message hash and guardian_set_index to ensure it cannot be faked (lines 188 and 192 above).

Once a signature is verified, the corresponding signer_index of signature_set.signatures will be set true:

How is Each Guardian Signature Verified?

Wormhole (on Solana) uses the Precompiled Secp256k1 SigVerify Program and each call to verify_signatures is prepended with an instruction to he SigVerify program, which verifies an input sequence of signatures:

Wormhole uses sysvar_instructions to load the current_instruction corresponding to verify_signatures and it checks that its previous instruction must be a secp verification instruction (i.e., a call to SigVerify):

The program instruction logs illustrate the flow clearly:

How Are Signatures Verified on Ethereum?

Different from Solana, Wormhole on Ethereum verifies all signatures in a single transaction by calling the verifyVM function (vm is a VAA byte array):

It ensures quorum by checking the signatures length:

It calls verifySignatures (similar to Solana, taking also message hash and guardianSet as input):

The verifySignatures verifies the input signatures on by one by using ecrecover (line 93) :

The ecrecover(message, v, r, s) API computes the key that produced a signature (v, r, s) on message. See here for a technical description of ecrecover. If verification passes if the returned key matches with the corresponding guardian’s public key (guardianSet.keys[sig.guardianIndex]).

What if a (Malicious) Guardian Submits Multiple Repeated Signatures in a VAA?

Wormhole prevents this attack surfaces by ensuring that the signature indices in the VAA must be ascending (on Ethereum):

On Solana, Wormhole maintains a pair (sig_index, signer_index) for each signature, and matches the key of the verified signature with the corresponding guardian public key (line 209):

Even if an attacker were able to control a guardian and place 19 repeated signatures in an VAA, only one guardian's signer_index would be set to true in signature_set.signature, thus the attack would fail.

Wormhole also checks validity of the guardian_set and integrity of the signature_set:

The quorum check is done in the PostVAA instruction:

In the next article, we will continue to discuss the security of cross-chain token mint/burn in Wormhole.

About sec3 (Formerly Soteria)

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