Hexens Audit: Missing Constraints on Message Padding in Poseidon Decryption Circuit

[Faedon]

The application implements Poseidon encryption to secure balance and transfer amount data. The encryption algorithm is detailed in the following paper: Encryption with Poseidon by Dmitry Khovratovich

During the decryption process, one of the verification steps is as follows:

If 3 does not divide l, verify that the last 3 − (l mod 3) elements of M are zero. If this condition is not met, the ciphertext must be rejected.

However, the decryption circuit lacks constraints to enforce this condition. Specifically, when the message length (l) is not a multiple of 3, the required zero-padding is not adequately validated."

[furkan-ux]

Do we need to constraint remainder here?

[Faedon]

Do we need to constraint remainder here?

We are not constrainting the remainder, its just the name of the variable here. I am making sure the paddings are zeroes if the message length not a multiple of 3.

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