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Poseidon is a ZK-friendly hash function designed to be efficient inside R1CS and Plonk circuits. Unlike general-purpose hashes, Poseidon is defined natively over prime fields, making it orders of magnitude cheaper to evaluate inside a zero-knowledge proof circuit than SHA-256 or Keccak. Umbra uses the BN254 (alt-bn128) variant - the same curve as Ethereum’s ecPairing precompile and the Groth16 verifier on Solana. Poseidon operates over the BN254 scalar field, whose prime order is approximately 2^254. See the Poseidon Cipher and Poseidon as a PRF: Security Proofs pages for the full cryptographic treatment.

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Uses in Umbra

UTXO commitments: Each UTXO inserts a Poseidon commitment into the Indexed Merkle Tree. The commitment hides the UTXO inputs on-chain while binding the creator to them for the ZK proof. Nullifiers: Each UTXO has a corresponding Poseidon nullifier derived from the holder’s private key. Burning the nullifier on-chain prevents double-spending, and only the key holder can produce the correct nullifier for a given commitment. User commitment: Each registered user has a Poseidon commitment stored on-chain that links their ZK identity to their encryption key without revealing either in plaintext.

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References