Privacy

Zcash (Zooko Wilcox-O'Hearn, Eli Ben-Sasson) launched October 2016 with zk-SNARKs proving shielded transactions are valid without revealing sender, recipient, or amount. The Powers of Tau ceremony — a multi-party trusted setup where each participant contributes randomness and destroys their entropy — bootstrapped the proving system. If any single participant kept their toxic waste, they could counterfeit. The ceremony is the cryptographic equivalent of a bonfire of hard drives. Halo 2 (2022) eliminated the trusted setup. The reference implementation of usable zero-knowledge.

Monero (Riccardo Spagni and contributors, forked from Bytecoin in 2014) hides the sender by signing transactions with a ring of plausible signers — any of the ring members could have produced the signature. RingCT (Ring Confidential Transactions, 2017) extended this to hide amounts. Stealth addresses hide recipients. The combination — sender, receiver, amount all obscured by default, no opt-in — is why Monero remains the privacy reference for adversarial threat models. Delisted from many regulated exchanges; thrives in mining and circular use.

Aztec (Zac Williamson, Joe Andrews) is a privacy-first ZK rollup on Ethereum. Accounts are split into public and private state; users prove transitions over private state with client-side ZK proofs and post the proof to L1. The PLONK proving system was developed in-house and adopted across the industry. Aztec Connect (2022-2024) bridged DeFi privately into mainnet. The Network testnet shipped in 2024 with a focus on programmable privacy — private smart contracts, not just private payments. Privacy as a developer primitive, not a single application.

Tornado Cash deposits ETH into a fixed-denomination smart contract; later, an unrelated address withdraws using a ZK proof of deposit without revealing which deposit. Roman Semenov and Alexey Pertsev shipped it in 2019. August 2022, OFAC sanctioned the contract addresses themselves — the first time the U.S. Treasury sanctioned code rather than a person or entity. Pertsev was convicted in the Netherlands in 2024. The Fifth Circuit ruled in 2024 that immutable smart contracts aren't 'property' OFAC can sanction. The legal frontier of code-as-speech.

Nym (Harry Halpin) ships a Loopix-derived mixnet — Sphinx-formatted packets routed through layered mix nodes that add timing delays and cover traffic. Network-layer privacy is the part most chains punt on: a perfectly anonymous ZK transaction sent from your home IP defeats itself. Nym sits below the application, anonymizing the transport. NYM token incentivizes mix-node operation. Useful for wallets, messaging apps, and any application where revealing the IP defeats the on-chain privacy. The missing layer for most of the privacy stack.

Railgun is a privacy smart-contract system on Ethereum, BSC, Polygon, and Arbitrum. Users deposit into shielded balances and transact privately within the system — transfers between shielded addresses don't leak amounts or counterparties. The Private Proofs of Innocence opt-in lets users cryptographically demonstrate their funds didn't pass through known sanctioned addresses, threading the regulatory needle Tornado couldn't. Vitalik Buterin publicly used Railgun and posted the proof of innocence. The compliance-aware shielded pool design.