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Version: V3

Glossary

Semaphore identity

The identity of a user in the Semaphore protocol. An identity contains the following three values:

  • Identity commitment: the public value.
  • Identity trapdoor and identity nullifier: secret values known only by the user.

Identity commitment

The public Semaphore identity value used in Semaphore groups.

Semaphore uses the Poseidon hash function to create the identity commitment from the Semaphore identity secret values.

Semaphore group

A group is a binary incremental Merkle tree in which each leaf contains an identity commitment for a user. The identity commitment proves that the user is a group member without revealing the Semaphore identity of the user.

Semaphore uses the Poseidon hash function to create Merkle trees. For more information, see the Poseidon website.

Merkle tree

A tree in which every leaf (i.e., a node that doesn't have children) is labelled with the cryptographic hash of a data block, and every node that isn't a leaf is labelled with the cryptographic hash of its child node labels. In zero-knowledge protocols, Merkle trees can be used to efficiently summarize and validate large data sets. To validate that a tree contains a specific leaf, a verifier only needs a portion of the complete data structure.

For more information, see Merkle tree in Wikipedia.

Nullifier

A value used to prevent double entry or double signalling.

See Circuit nullifier hash.

Relay

A third-party who receives a fee for including relayed transactions in the blockchain (McMenamin, Daza, and Fitz. https://eprint.iacr.org/2022/155.pdf, p.3). To preserve the anonymity of the user broadcasting a signal with Semaphore, an application may use a relayer to post the signal transaction to Ethereum on behalf of the user.

Applications may provide rewards for relayers and implement front-running prevention mechanisms, such as requiring the signals to include the relayer’s address, binding the signal to that specific address (https://semaphore.pse.dev/whitepaper-v1.pdf, p.6).

Trusted setup files

The secure, verifiable parameters generated by Semaphore's trusted setup ceremony. Semaphore uses the trusted setup files to generate and verify valid zero-knowledge proofs. To generate or verify valid zero-knowledge proofs with Semaphore, applications must include the following Semaphore trusted setup files:

  • semaphore.zkey
  • semaphore.wasm
  • semaphore.json

For a complete list of ready-to-use files, see trusted-setup-pse.org. To learn more, see the trusted setup ceremony.

Signals

The term "signals" in Semaphore refers to the values the user broadcasts when voting, confirming, sending a message and so on. On the other hand, "signals" in Circom refers to data that contain elements within the field of Z/pZ. In Circom, "signals" can be defined as input or output, and are considered intermediate signals otherwise.