Accounts Registry Tree

Accounts Registry Trees are used to structure data and store it onchain in the form of Data Groups. As a result, users can participate in proving schemes and prove facts about data aggregated in their Data Vault.

Key-Value Merkle tree

A KV Merkle tree is a key-value database enhanced with a Merkle tree. The Merkle tree stores the following data: hash(key, value).

Accounts Tree

An Accounts Tree is a KV Merkle tree where the keys and values are accounts (e.g. keys = account identifiers, values = accounts values).

This makes it easy and cheap for a verifier with access to the root of the tree (e.g. a smart contract) to verify that a prover (a user) owns an account in the KV database. This prover just has to send the Merkle proof to the verifier.

An Accounts Tree can store a group of accounts (i.e. a Data Group).

For example:

• Account identifiers — all addresses that voted in the ENS DAO

• Account values — the number of votes submitted for each address

• (Group identifier): 3

A user (owner of 0x123..def) can easily prove to a smart contract with access to the root) that they voted 5 times.

They just need to provide the following information:

• Proof of 0x123..def ownership: signature of a message with their private key

• 5: the account value

• Merkle proof (data needed for the smart contract to reconstruct the Accounts Tree root)

The attester will then just need to:

• Verify the signature

• Compute the leaf = hash(0x123..def,5)

• Verify the Merkle proof (hashing the leaf against the Merkle proof elements to check whether it corresponds to the Accounts Tree root)

Registry tree

The registry tree is another KV Merkle tree where the key is an accounts tree root, and the value is a specific value linked to the accounts tree (e.g. for Hydra-S1, we choose the groupIndex as the accounts tree value).

Registry tree structure

Taking the previous example.

A group of ENS Voters has a group identifier = 3

We can register the previous accounts tree of ENS Voters in the Registry tree with the value 3.

Users will then be able to prove to a verifier (that only has access to the registry tree root) that:

• They have an account that's part of one of the accounts trees registered

• The accounts tree they are a part of has a value of 3

=> They will have effectively proved they are part of group 3!

Registry tree structure

Hydra Proving Schemes

Hydra proving schemes use Account Registry Trees to enable users to prove they are part of a Data Group. Generating proof of ownership is more complex than simply signing a message, but the general flow is exactly the same. As the scheme is done in a zk-SNARK, it is impossible for anyone to know which account was used.