ADR-108: Kyber post-quantum key exchange — privacy chain quantum-resistant#731
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…ation Closes the quantum-resistance gap explicitly deferred from ADR-107. Final ADR in the privacy + federation chain. Replaces DH key exchange in ADR-107's Layer 4 secure aggregation with Kyber-768 KEM (NIST FIPS 203, CNSA 2.0 default). Migration timeline: - Phase 0 (NOW 2026): Classical X25519 (ADR-107 default) - Phase 1 (2026-Q4 -> 2027): Kyber-768 opt-in via --enable-pqc flag - Phase 2 (2027-Q2 -> 2028): Hybrid (X25519 + Kyber-768) becomes default - Phase 3 (2030+): Pure Kyber-768 (classical retired) Why hybrid for Phase 2 (belt-and-braces): - Protects against future Kyber breaks (Kyber is ~5 years old) - Protects against classical breaks (X25519 backup) - Protects against implementation bugs in either primitive - Cost: ~3 kB/round/installation extra (negligible) Why now (record-now-decrypt-later): Adversaries can record federated updates today and decrypt them in 2035 when quantum capabilities arrive. Without ADR-108, the (epsilon, delta) guarantees of ADR-106 silently expire when quantum computers arrive. Proactive migration is cheap insurance. Why Kyber-768 (not 512 or 1024): - NIST FIPS 203 (2024); ~AES-192 equivalent - CNSA 2.0 recommended default - Used by Cloudflare, Google, AWS in 2024-2026 rollouts - Public key 1184 B, ciphertext 1088 B, secret 32 B - 512 lacks CNSA 2.0 sign-off; 1024 doubles bandwidth without benefit LOC: +220 on top of ADR-107. Total federation budget ADR-105+106+107+108: ~1,550 LOC. Threat model: 8 threats, every row has mitigation. Hybrid mode is the belt-and-braces against both Kyber breaks AND classical breaks. ADR CHAIN COMPLETE: 7 ADRs in the privacy + federation chain: ADR-100 (cog packaging) -> ADR-103 (cog example) -> ADR-104 (MCP/CLI) -> ADR-105 (within-installation federation) -> ADR-106 (DP + isolation) -> ADR-107 (cross-installation + SA) -> ADR-108 (PQC key exchange). No remaining unspecified privacy gap at any threat horizon (classical or quantum). Future ADRs catalogued: - ADR-109: PQC signatures (Dilithium replaces Ed25519 in ADR-100) - ADR-110: PQC hardware acceleration on Cognitum-v0 - ADR-111: PQC for cog-store distribution Composes: - R3 / R14 / R15 / R7 / R12 PABS: privacy chain intact through quantum transition - R10 / R11 (long-deployment): benefit most from forward secrecy as data ages Honest scope: - Kyber ~5 years old; hybrid mitigates uncertainty - 'When do we need this?' uncertain (2030 aggressive / 2050+ conservative) - ESP32-S3 timing ~10 ms per handshake estimated negligible; needs measurement - Phase 3 retirement of classical needs future decision Coordination: ticks/tick-28.md, no PROGRESS.md edit.
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Twenty-eighth tick. Final ADR in the privacy + federation chain. Closes the quantum-resistance gap deferred from ADR-107.
Migration timeline
Why now
Record-now-decrypt-later: adversaries can record federated updates today and decrypt in 2035 with quantum hardware. Without ADR-108, ADR-106's (ε, δ) guarantees silently expire.
Why hybrid
Belt-and-braces against future Kyber breaks (Kyber is ~5 years old) OR classical breaks OR implementation bugs. Cost: ~3 kB/round/installation (negligible).
ADR chain complete
7 ADRs: 100 → 103 → 104 → 105 → 106 → 107 → 108. No remaining unspecified privacy gap at any threat horizon (classical or quantum).
Total federation budget
ADR-105 (500) + ADR-106 (300) + ADR-107 (530) + ADR-108 (220) = ~1,550 LOC.
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