User State Transition Proof

The user state transition proof is used to process attestations from the latest epoch the user transitioned and then compute the latest global state tree leaf.

The user state transition circuit checks that

1. The user has registered and performed user state transition in the previous epoch.

2. The user state tree is updated correctly by the attestations, and the same attestations are chained by hash functions.

   • For example, the original user state tree root is r_1, and the original user state tree leaf has 5 positive reputation

   user_state_tree_leaf = hash(5, 0, 0, 0, 0)
   • An incoming attestation has
     • attester_id = 1
     • pos_rep = 3
   • compute the hash of reputation

   hash_reputation = hash(5 + 3, 0, 0, 0, 0)
   • compute the updated user state tree root r_2 with user state tree leaf hash(8, 0, 0, 0, 0) in the leaf index 1
   • compute the hash of attestation

   hash_attestation = hash(1, 3, 0, 0, 0)
   • compute the hash chain

   hash_chain = hash(hash_attestation, hash_chain)

3. After all attestations of all epoch keys are processed, the circuit seals all hash chains and computes the epoch tree. (If the output epoch tree root mismatches others' epoch tree roots, then the user state transition proof is invalid because the user process attestations in a wrong way.)

4. Compute the updated global state tree leaf by

hash(identity_commitment, updated_user_state_root)

Ideally, the user state transition proof should be in one circuit. But the circuit size is too big to generate a proof in a browser, we separate the proof into three different parts: start transition proof, process attestations proof, and user state transition proof.

The idea of chaining these three proofs but without revealing user's sensitive information is to use blinded_user_state and blinded_hash_chain

const blinded_user_state = hash(
    identity_nullifier, 
    user_tree_root, 
    epoch, 
    from_epoch_key_nonce
)
const blinded_hash_chain = hash(
    identity_nullifier, 
    current_hash_chain, 
    epoch, 
    from_epoch_key_nonce
)

In the start transition proof, the circuit will compute the initial blinded_user_state and blinded_hash_chain, the user_tree_root is the latest transitioned user state tree and the hash(identity_commitment, updated_user_state_root) should be in a global state tree. The current_hash_chain is start with 0 as the definition of hash chain.

After blinded_user_state and blinded_hash_chain are submitted, the user can take them as public inputs and start to process attestations according to the user_tree_root and current_hash_chain. When the attestations limit reaches (e.g. a processAttestations circuit can process only 10 attestations per proof) or all attestations to the epoch key are processed, the circuit will output another blinded_user_state and blinded_hash_chain to continue processing attestations.

The epoch_key_nonce is used in the blinded user state and blinded hash chain to indicate the attestations of which epoch key is processed. In the final userStateTransition proof, it checks all epoch key with different epoch_key_nonce are processed and the hash chain result matches the epoch tree.

There are only one user state tree result after all attestations are processed, so in the final proof it only takes the initial blinded_user_state and the final one and computes the new global state tree leaf. On the other hand, there are numEpochKeyNoncePerEpoch hash chains after processing attestations, so the final circuit will take numEpochKeyNoncePerEpoch blinded_hash_chain to check the epoch tree root.

The user state tree root is continuously updated: the output should be the input of another proof, so the processAttestation proof takes blinded_user_state as public input and output another blinded_user_state. The hash chain results might not be continuously. When all attestations of one epoch key is processed, the hash chain of the next epoch key should be 0 but not the previous hash chain. Therefore, processAttestation proof does not take blinded_hash_chain as input.

While verifying all of the proofs, there are the following things to check to make sure it follows the rules:

1. startTransitionProof takes a valid global state tree root as a public input.
2. processAttestationsProof takes blinded_user_state as public input and blinded_hash_chain as private input from either startTransitionProof or another processAttestationsProof.
3. userStateTransitionProof takes two blinded_user_state, one is from startTransitionProof and the other is from the latest processAttestationsProof.
4. userStateTransitionProof takes numEpochKeyNoncePerEpoch blinded_hash_chain from processAttestationsProof.

1. Start Transition proof

Public outputs

• GST_root
• blinded_user_state
• blinded_hash_chain_result

Private inputs

• epoch
• epoch_key_nonce
• user_tree_root
• identity_nullifier
• identity_trapdoor
• GST_path_elements
• GST_path_index

Contraints

1. Check if user exists in the Global State Tree

Check if hash(identity_commitment, UST_root) is one of the leaves in the global state tree of root GST_root.

2. Compute blinded public output

const blinded_user_state = hash(
    identity_nullifier, 
    user_tree_root, 
    epoch, 
    from_epoch_key_nonce
)
const blinded_hash_chain = hash(
    identity_nullifier, 
    current_hash_chain, 
    epoch, 
    from_epoch_key_nonce
)

info

See the whole circuit in circuits/startTransition.circom

2. Process attestations proof

Public inputs

• input_blinded_user_state

Public outputs

• blinded_user_state
• blinded_hash_chain_result

Private inputs

• epoch
• from_nonce
• to_nonce
• identity_nullifier
• intermediate_user_state_tree_roots
• old_pos_reps
• old_neg_reps
• old_graffities
• old_sign_ups
• path_elements
• attester_ids
• pos_reps
• neg_reps
• graffities
• overwrite_graffities
• sign_ups
• selectors
• hash_chain_starter

Contraints

1. Verify blinded input user state

Check if

const input_blinded_user_state = hash(
    identity_nullifier, 
    intermediate_user_state_tree_roots[0], 
    epoch, 
    from_nonce
)

2. Verify attestation hash chain

Check if

const current_hash_chain = hash(
    hash(
        attester_ids[i]
        pos_reps[i],
        neg_reps[i],
        graffities[i],
        sign_ups[i]
    ),
    prev_hash_chain
)

3. Process attestations and update user state tree

Check intermediate_user_state_tree_roots[i] has leaves

hash(
    old_pos_reps[i],
    old_neg_reps[i],
    old_graffities[i],
    old_sign_ups[i]
)

with index attester_ids.

And update the leaf by

new_pos_reps[i] = pos_reps[i] + old_pos_reps[i]
new_neg_reps[i] = neg_reps[i] + old_neg_reps[i]
if (overwrite_graffities[i]) {
    new_graffities[i] = graffities[i]
} else {
    new_graffities[i] = old_graffities[i]
}
    
new_sign_ups[i] = sign_ups[i] || old_sign_ups[i]

And the new leaf

hash(
    new_pos_reps[i],
    new_neg_reps[i],
    new_graffities[i],
    new_sign_ups[i]
)

Should be one of the leaves in intermediate_user_state_tree_roots[i+1] with index attester_ids[i].

4. Compute blinded public output

const output_blinded_user_state = hash(
    identity_nullifier, 
    intermediate_user_state_tree_roots[n+1], 
    epoch, 
    to_nonce
)
const output_blinded_hash_chain = hash(
    identity_nullifier, 
    current_hash_chain, 
    epoch, 
    to_nonce
)

info

See the whole circuit in circuits/processAttestations.circom

3. User State Transition proof

Public inputs

• epoch
• blinded_user_state
• blinded_hash_chain_results
• epoch_tree_root

Public outputs

• GST_root
• new_GST_leaf
• epoch_key_nullifier

Private inputs

• intermediate_user_state_tree_roots
• start_epoch_key_nonce
• end_epoch_key_nonce
• identity_nullifier
• identity_trapdoor
• GST_path_elements
• GST_path_index
• epk_path_elements
• hash_chain_results

Contraints

1. Check if user exists in the Global State Tree

Check if hash(identity_commitment, UST_root) is one of the leaves in the global state tree of root GST_root.

2. Process the hashchain of the epoch key specified by nonce n

Get all blinded hash chains with nonce iterates from 0 to n-1

const output_blinded_hash_chain = hash(
    identity_nullifier, 
    hash_chains[i], 
    epoch, 
    i
)

Seal all hash chains

const sealed_hash_chain = hash(1, hash_chains[i])

Check the epoch tree root with epoch_keys[i] and hash_chains[i].

3. Check if blinded user state matches

Output two blinded user states with start_epoch_key_nonce and end_epoch_key_nonce.

const output_blinded_user_state[0] = hash(
    identity_nullifier, 
    intermediate_user_state_tree_roots[0], 
    epoch, 
    start_epoch_key_nonce
)
const output_blinded_user_state[1] = hash(
    identity_nullifier, 
    intermediate_user_state_tree_roots[1], 
    epoch, 
    end_epoch_key_nonce
)

4. Compute and output nullifiers and new GST leaf

const new_GST_leaf = hash(
    id_commitment, 
    intermediate_user_state_tree_roots[1]
)

info

See the whole circuit in circuits/userStateTransition.circom