# Glossary

> Key terms used throughout the Senddy documentation.

Source: https://www.senddy.com/docs/glossary
Last modified: 2026-08-02T02:33:05.000Z

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## 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. Senddy uses receiver-bound `ReceiveWithAuthorization` signatures for gas-sponsored Base deposits.

### Ephemeral Deposit Address
A one-time EOA derived locally for a single Base deposit. It replaces reuse of a stable smart-wallet deposit address, reducing trivial cross-deposit clustering. It is retired after its deposit is shielded.

## 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 deposited funds and records private note commitments. Funds are commingled, while zero-knowledge proofs hide which notes are consumed by later private spends.

## 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. Later private spends do not reveal which shielded commitment they consume.

### 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.