Signal
Signal is a privacy-focused messaging service associated with the Signal Technology Foundation. Its public protocol documentation has strongly influenced modern end-to-end encrypted messaging.
Model
Signal provides E2EE by default through a centralized delivery service. Its published PQXDH and Double Ratchet specifications provide asynchronous key agreement, forward secrecy, and PCS. PQXDH adds protection against passive “harvest now, decrypt later” attacks, while authentication against an active quantum attacker remains an open problem in the current specification. Signal announced SPQR in 2025 and combines it with the classical Double Ratchet as a Triple Ratchet intended to add continuing post-quantum refresh, not merely a post-quantum initial handshake. The rollout announcement said conversations would migrate gradually, so deployment coverage should not be inferred from the specification alone.
Signal is not federated. The service remains the registration, directory, abuse-control, and availability authority. A phone number is still required to register, although optional usernames and phone-number privacy settings let users initiate chats without revealing the number to each other. Sealed sender removes the sender identity from the normal delivery envelope when applicable. Signal identifies timing and network-address correlation as residual metadata problems.
Marmot Protocol uses a different cryptographic architecture: MLS supplies native group epochs and tree-based membership changes, while Signal’s strongest documented pairwise path uses PQXDH and ratcheting sessions. Marmot currently has no required post-quantum ciphersuite; Signal has the stronger post-quantum design and deployment history, while Marmot has a standards-based native group-state construction.
The service also shows the power of a simple, consumer-friendly privacy product that does not depend on advertising.
Lesson for the venture
Do not build a new messenger first. Help customers correctly use established tools: verify contacts, understand new-device warnings, decide on backups, manage group membership, and recover after device loss.
If a future proprietary communication product is justified, reuse mature audited protocols and libraries. Do not invent cryptography. Privacy product architecture lists the wider system requirements.
Limit
End-to-end encryption protects message content between uncompromised endpoints. It does not make all metadata disappear, make a compromised endpoint safe, or eliminate business, legal, or recovery decisions. Secure and decentralized communication stacks compares these limits with metadata-minimizing, federated, peer-to-peer, and public-social protocols. EU private communications law explains why the product’s legal claims must be as precise as its cryptographic claims.