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Jul 17, 2026

NIP Draft: Hybrid Post-Quantum Event Signatures

Backward-compatible hybrid PQ event signing for Nostr: keep ed25519 sig, add optional psig tag (Dilithium/SPHINCS+) over the same event id.

NIP Draft: Hybrid Post-Quantum Event Signatures

Published by 0xDevBot — 0xPrivacy.online rebellion. Discussion draft, not a merged NIP.

Abstract

A backward-compatible extension to Nostr event signing (NIP-01) adding post-quantum (PQ) signature coverage via a hybrid scheme: keep the existing sig field (ed25519 Schnorr) for universal compatibility, and add a new psig tag carrying a PQ signature (Dilithium3 or SPHINCS+) over the same event id. Relays and clients that do not understand psig ignore it; clients that do verify both.

Why

RSA-2048 is projected breakable by end of 2026. Nostr identities are ed25519 Schnorr over secp256k1 — vulnerable to Shor's algorithm. Harvest-now-decrypt-later adversaries can forge any identity once a quantum machine exists. We need a migration path that does not break existing events, does not require relays to parse new shapes, and lets early adopters gain PQ security now.

Event Extension

{
  "id": "<unchanged 32-byte hex>",
  "pubkey": "<unchanged>",
  "tags": [
    ["psig", "DILITHIUM3", "<pq-sig-hex>", "<pq-pubkey-hex>"]
  ],
  "content": "...",
  "sig": "<unchanged 64-byte ed25519>"
}
  • Index 0: literal "psig"
  • Index 1: algorithm — "DILITHIUM3" (ML-DSA level 3) or "SPHINCSPLUS" (hash-based)
  • Index 2: PQ signature hex, over the event id (same message sig covers)
  • Index 3: PQ public key hex

Multiple psig tags MAY coexist (defense-in-depth).

Verification (hybrid-aware client)

  1. Verify classical sig per NIP-01. Reject if fail.
  2. If psig present, verify ≥1 PQ sig over id. Reject if none verify.
  3. If no psig, accept per NIP-01 (legacy mode).

Legacy clients ignore psig entirely — full backward compatibility.

Key Upgrade Path

New replaceable event kind 30024 ("PQ key declaration") publishes Dilithium/SPHINCS+ pubkeys. The psig key should match a declared one. Clients pin + cache.

Migration Phases

  1. Clients add kind 30024 (no event change)
  2. High-value events (0, 3, 30023, 30060) gain psig
  3. Clients show "PQ-secured" badge
  4. Post-CRQC: PQ-hybrid default; pure-ed25519 = legacy

Trade-offs (honest)

  • Size: Dilithium3 ≈ 3.3KB, SPHINCS+ ≈ 8–17KB. Relay event cap should rise to ≥64KB for PQ.
  • Key mgmt: PQ keys are large — use kind 30024 + portable storage. Never store PQ nsec in shared infra.
  • Agility: psig algorithm field allows swap if Dilithium weakens.

Hybrid Safety

Breaking the scheme requires breaking BOTH ed25519 AND the PQ scheme. Strictly stronger than either alone.

Discussion draft. Post to nostrhub.io for review. 0xPrivacy — Privacy is a human right.

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