Overview

The Nullmask protocol enables privacy-preserving transactions on EVM chains using zero-knowledge proofs. This section provides the formal cryptographic specification.

Core Concepts

Concept	Description
Notes	Private UTXOs — encrypted value containers. Note commitments are stored in an on-chain Merkle tree; encrypted note data is emitted in contract events
Nullifiers	Unique spend identifiers that prevent double-spending without revealing which note was spent
Viewing Key	Enables decryption of incoming and outgoing notes; grants view access to transaction history
Receiving Key	Public key registered on-chain that enables others to send shielded funds to an address
Transaction Nullifier	Prevents replay of signed transactions by a malicious proxy

Cryptographic Stack

Primitive	Purpose
Poseidon2 T4	Hash function for note commitments, nullifiers, key derivation, and Merkle tree
LeanIMT	Incremental Merkle tree for note storage (depth 16, ~65K leaves)
Grumpkin	Embedded curve for key exchange and encryption
secp256k1 ECDSA	Transaction signature verification (standard Ethereum signatures)
UltraHonk	ZK proof system (Barretenberg prover)

Protocol Flow

Notation

Throughout this specification:

• $\mathbb{E}$ denotes the Grumpkin embedded curve

• $\mathbb{F}$ denotes the base field of $\mathbb{E}$ (scalar field of BN254)

• $\mathsf{G}$ denotes the generator point of $\mathbb{E}$

• $\operatorname{Poseidon2T4}(x)$ denotes the Poseidon2 hash function with state width 4