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Version: V2

Semaphore identities

In order to join a Semaphore group, a user must first create a Semaphore identity. A Semaphore identity contains two values generated with the identity:

  • Identity trapdoor
  • identity nullifier

To use and verify the identity, the identity owner (user) must know the trapdoor and nullifier values. To prevent fraud, the owner should keep both values secret.

Create identities

In your code, use the @semaphore-protocol/identity library to create a Semaphore identity deterministically (from the hash of a message) or randomly.

Create random identities

To create a random identity, instantiate Identity without any parameters--for example:

import { Identity } from "@semaphore-protocol/identity"

const { trapdoor, nullifier, commitment } = new Identity()

The new identity contains two random secret values: trapdoor and nullifier, and one public value: commitment.

The Poseidon hash of the identity nullifier and trapdoor is called the identity secret, and its hash is the identity commitment.

An identity commitment, similarly to Ethereum addresses, is a public value used in Semaphore groups to represent the identity of a group member. The secret values are similar to Ethereum private keys and are used to generate Semaphore zero-knowledge proofs and authenticate signals.

Create deterministic identities

If you pass a message as a parameter, Semaphore generates trapdoor and nullifier from the SHA256 hash of the message. The message might be a password or a message that the user cryptographically signs with a private key.

When using deterministic identities, you should always keep the message secret. Given that the hash is deterministic, anyone with the same message can recreate the same identity.

const identity = new Identity("secret-message")
tip

Building a system to save or recover secret values of Semaphore identities is nontrivial. You may choose to delegate such functionality to existing wallets such as Metamask--for example:

  1. In Metamask, a user signs a message with the private key of their Ethereum account.
  2. In your application, the user creates a deterministic identity with the signed message.
  3. The user can now recreate their Semaphore identity whenever they want by signing the same message with their Ethereum account in Metamask.

Save your identities

You can output an identity as a JSON string that you can save and reuse later. The Identity.toString() method generates a JSON array from an identity--for example:

console.log(identity.toString()) // View the identity trapdoor and nullifier.

// '["8255d...", "62c41..."]'

The array contains the trapdoor and nullifier.

To reuse the saved identity, pass the JSON to the Identity() constructor.

const identity2 = new Identity(identity.toString())