Nullifiers

• Nullifiers are used to prevent things from happening more than once

• UniRep uses two different nullifiers

Epoch key nullifiers

• Epoch key nullifiers are used to prevent users from using the same epoch key twice and prevent users from double user state transition.

• If an epoch key nullifier is seen before, then drop the second user state transition proof.

• Nullifier of an epoch key is computed by

hash(
    EPOCH_KEY_NULLIFIER_DOMAIN, 
    identityNullifier, 
    epoch, 
    nonce
)

Reputation nullifiers

• Reputation nullifiers are used to prevent users from double spending the reputation if an action requires users to spend reputation.

• If a reputation nullifier is seen before, then the second reputation proof is recognized invalid.

• Nullifier of a reputation spent is computed by

hash(
    REPUTATION_NULLIFIER_DOMAIN, 
    identityNullifier, 
    epoch, 
    nonce, 
    attesterId
)

• The nonce can only be within 0 to posRep-negRep-1 with the given attesterID.

NOTE: EPOCH_KEY_NULLIFIER_DOMAIN and REPUTATION_NULLIFIER_DOMAIN are used to prevent mixed-up of epoch key nullifiers and reputation nullifiers.