ch2_not_unique

ECDSA signatures can be different for the same message

Applying from last chapter, this chapter uses OpenZeppelin's ECDSA library for signature verification. However...

ECDSA uses a random number (aka nonce) to generate a signature. Read Nakov's description to understand more. Signature libraries hide this complexity from users, so we generally get the same signature when the private key signs a message multiple times.

This chapter shows how using different random numbers produces different signatures.

sign.js exposes the random number (aka nonce) argument in the signing function, generally hidden away. It then takes a message and signs it using two difference nonces. These signatures and the corresponding signing address is used in Verifier.t.sol for signature verification. To run this, use node sign.js.

These signatures are copied to Verifier.t.sol to show: It's possible to have two different signatures be produced on the same message and signer.

• Run FOUNDRY_PROFILE=ch2 forge test to see how 2 completely different signatures can recover the same signer address.

Be safe

If your smart contract depends on signature verification, and you don't want to verify for the same data twice - store the message instead of the signature to recognize that the data has been verified before.