Hexens Audit: Proof Reuse and Malleability in privateMint Function Bypasses Auditor Controls in privateMint #30
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The privateMint function in the EncryptedERC contract, despite being restricted to the owner, allows the reuse of zero-knowledge proofs which affects the intended auditor control mechanism. While the mint circuit (mint_circuit.go) verifies the auditor's encryption and approval of the mint amount, the current implementation does not prevent the same proof from being used multiple times.
This implementation allows the owner to:
Mint multiple times using a single auditor approval
Exceed the auditor-approved mint amount
Operate beyond the scope of auditor oversight
The issue is compounded by proof malleability, where manipulating the y-coordinates of the proof points can create multiple valid variations of the same proof.