Glossary
Key terms used throughout the Senddy documentation.
A
Attestation
A cryptographic signature from the attestor that certifies a zero-knowledge proof is valid. Used in place of on-chain proof verification to reduce gas costs.
Attestor
A service running inside a hardware-isolated trusted execution environment (TEE) that verifies zero-knowledge proofs off-chain and signs attestations. The attestor cannot access funds or user data.
Association Proof
An optional zero-knowledge proof that demonstrates funds trace back to legitimate deposit sources, enabling compliance without breaking privacy.
B
Base
A Layer 2 blockchain built on Ethereum by Coinbase. Senddy's smart contracts are deployed on Base for fast finality and low transaction costs.
C
CCTP (Cross-Chain Transfer Protocol)
Circle's native USDC bridge protocol used for moving USDC between blockchains (e.g., Solana to Base). More secure than third-party bridges because it burns and mints native USDC.
Commitment
A cryptographic hash (Poseidon2) of a note's contents. Commitments are stored in the Merkle tree on-chain. Only the note owner can link a commitment to its underlying note.
D
DeFi (Decentralized Finance)
Financial services built on blockchain smart contracts. Senddy uses trusted DeFi lending protocols to generate yield on deposited funds.
E
EIP-3009
An Ethereum standard that allows token transfers with a signature instead of an on-chain approval transaction. Enables gasless deposits.
G
Gasless
A transaction model where the user doesn't need to hold ETH to pay for gas. Senddy embeds fees in the ZK circuit, and relayers submit transactions on behalf of users.
H
HSM (Hardware Security Module)
A dedicated hardware device used to manage the attestor's signing key. The key is restricted to the trusted execution environment and cannot be accessed externally.
M
Merkle Tree
A binary tree data structure where each leaf is a commitment and each node is the hash of its children. Allows efficient membership proofs — proving a commitment exists without revealing which one.
N
Noir
A domain-specific language for writing zero-knowledge circuits. Senddy's shield, spend, and association circuits are written in Noir.
Note
A private unit of value in Senddy's UTXO system. Each note has a value, owner, and randomness. Notes are never stored in plaintext — only their commitments are on-chain.
Nullifier
A unique value derived from a note and its owner's secret key. Publishing a nullifier on-chain marks a note as spent, preventing double-spending. Nullifiers cannot be linked to their commitments without the secret key.
P
Permit2
A token approval mechanism that allows more flexible and gas-efficient token permissions. Used for gasless deposits.
Poseidon2
A ZK-friendly hash function optimized for use inside zero-knowledge circuits. Used to compute note commitments and nullifiers.
Privacy Pool
The smart contract that holds all deposited funds. Called a "pool" because all deposits are commingled, making it impossible to trace specific funds.
R
Relayer
A service that submits blockchain transactions on behalf of users. Enables gasless transactions — users pay fees from their private balance via the ZK circuit.
S
Shield
The process of converting a public deposit into private notes. After shielding, the connection between the public deposit and the private notes is cryptographically broken.
Smart Wallet
An on-chain wallet created automatically for each Senddy user. Managed via account abstraction — no seed phrase or browser extension required.
Spend
The process of using private notes to transfer value or withdraw funds. Requires a zero-knowledge proof that demonstrates ownership, existence, and value conservation.
Spend Key
The secret key required to spend your notes. Derived from your login credentials and never leaves your device.
Sync Engine
The client-side component that scans the blockchain for new commitments and nullifiers relevant to your keys, keeping your note state up to date.
T
TEE (Trusted Execution Environment)
A hardware-isolated enclave with no persistent storage, no network access, and cryptographic attestation of its code. Used to run the attestor securely.
U
UltraHonk
The zero-knowledge proving system used by Senddy. Uses a universal trusted setup, meaning no per-circuit ceremony is required. Optimized for efficient client-side proof generation in the browser.
Universal Setup
A trusted setup ceremony that works for any circuit. Unlike circuit-specific setups, a universal setup doesn't need to be repeated when circuits are added or upgraded.
USDC
A stablecoin issued by Circle, pegged 1:1 to the US dollar. Senddy uses USDC as its underlying currency.
UTXO (Unspent Transaction Output)
A transaction model (used by Bitcoin and Zcash) where value is tracked as discrete "notes" rather than account balances. Each note can be spent exactly once.
V
View Key
A key that allows scanning the blockchain to identify which notes belong to you. Can be shared with auditors for voluntary transparency without granting spend access.
Z
Zero-Knowledge Proof (ZK Proof)
A cryptographic technique that proves a statement is true without revealing any information beyond the truth of the statement itself. Senddy uses ZK proofs to verify transactions are valid without revealing amounts, senders, or recipients.