Signal Begins Enforcing Its Post-Quantum Triple Ratchet as the First Messaging App to Offer Continuous Quantum-Resistant Encryption
Signal's SPQR protocol adds a quantum-safe ratchet alongside the Double Ratchet, with server-side enforcement now rolling out to new accounts.
Overview
Signal has begun server-side enforcement of the Sparse Post-Quantum Ratchet (SPQR), the cryptographic upgrade that makes it the first major messaging platform to provide continuous quantum-resistant encryption throughout entire conversations. The protocol, announced in October 2025 and now entering its enforcement phase for new accounts, pairs a quantum-safe ratchet with Signal’s existing Double Ratchet to form what the organization calls the Triple Ratchet.
The move addresses the growing “harvest-now-decrypt-later” threat, in which adversaries record encrypted traffic today with the expectation that future quantum computers will be able to break the underlying elliptic-curve cryptography.
What We Know
SPQR operates as a parallel encryption system alongside Signal’s established Double Ratchet. When a message is sent, both ratchets independently generate encryption keys that are mixed together through a Key Derivation Function, producing a hybrid key that requires an attacker to break both classical elliptic-curve and post-quantum ML-KEM cryptography to compromise the session, according to Bleeping Computer.
The upgrade builds on Signal’s earlier PQXDH (Post-Quantum Extended Diffie-Hellman) protocol, which added quantum resistance only during the initial key exchange when a conversation was established. SPQR extends that protection to every message in a conversation’s lifetime, providing what cryptographers call continuous forward secrecy and post-compromise security against quantum adversaries, as CSO Online reported.
One of the protocol’s key engineering challenges was bandwidth. ML-KEM-768 public keys are 1,184 bytes and ciphertexts are 1,088 bytes, roughly 35 times larger than the 32-byte elliptic-curve keys used in the Double Ratchet. Signal solved this through erasure-coded chunking that breaks quantum key material into 42-byte fragments sent alongside regular messages, according to Bleeping Computer. This approach allows the receiving client to reconstruct the full key material after accumulating enough chunks, even if some messages are lost or arrive out of order.
The protocol was designed in collaboration with PQShield, Japan’s AIST, and New York University. Research underlying the design was presented at Eurocrypt 2025 and USENIX Security 2025, and the implementation undergoes continuous formal verification using ProVerif and the hax tool on every code commit, as noted by Bruce Schneier.
As Schneier highlighted, the Triple Ratchet provides a security bonus even for users unconcerned about quantum threats: “even if one ratchet is fully broken, the Signal message will still be protected by the second ratchet,” effectively doubling the cryptographic redundancy of every conversation.
What We Don’t Know
Signal has not published a specific timeline for completing enforcement across all existing sessions. The current phase requires SPQR capability at registration for new accounts and prevents capability downgrades, but existing conversations between users on older app versions continue without quantum protection. Signal has stated it will eventually archive all sessions that do not support SPQR, but no date has been set for that step.
It also remains unclear how quickly the broader messaging industry will follow Signal’s lead. Apple added post-quantum protections to iMessage with its PQ3 protocol in 2024, but that system operates differently and does not use the same continuous ratcheting approach. WhatsApp, which uses the Signal Protocol for its end-to-end encryption, has not announced plans to adopt SPQR.
The practical timeline for large-scale quantum computers capable of breaking current encryption remains a matter of debate among researchers, with estimates ranging from a decade to several decades. The urgency of harvest-now-decrypt-later attacks depends on how long intercepted data retains its value.
Analysis
Signal’s Triple Ratchet represents the most comprehensive post-quantum upgrade yet deployed in consumer messaging. While PQXDH protected only the initial handshake, SPQR closes the gap by ensuring quantum resistance is maintained as conversations evolve over time. The chunking approach that distributes large post-quantum key material across ordinary messages is a practical solution to a problem that could have made quantum-safe messaging noticeably slower or more bandwidth-intensive.
The broader post-quantum migration is accelerating across the technology industry. As previously reported, Broadcom shipped the first PQC-safe network encryption hardware in March 2026, and both Ubuntu 26.04 and Java 26 have incorporated post-quantum cryptographic support. Signal’s SPQR enforcement adds consumer messaging to the growing list of systems preparing for a post-quantum future, setting a benchmark that other messaging platforms will face pressure to match.