hard20 pts · first answer countscryptographysmart-contract-security

Using a signature as a nonce

A contract prevents replay by storing used[keccak256(signature)] = true. Why can an attacker still reuse a valid signature?

A.keccak256 of a signature can collide easilyB.Signatures expire after one blockC.ECDSA signatures are malleable: (r, s) can be changed to (r, n - s) with a flipped v, which is a different byte string that recovers the same signerD.The contract cannot read the signature bytes

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