Hybride Postquanten-Nachrichten

Hybrid, not replacement

hybridKey combines the KEM secret with the classic NIP-44 conversation key through a KDF, so the result is no weaker than either input. The post-quantum secret must never be used alone: a flaw in a comparatively young lattice scheme must not be able to make Nostr messaging worse than it is today.

The message envelope

A self-describing payload carrying an ML-KEM-1024 ciphertext and an XChaCha20-Poly1305-sealed message. It fits anywhere a string does — in practice as the content of a NIP-17 kind:14 rumor, sealed and gift-wrapped per NIP-59 with no change to either. Relays need no changes and clients that have not implemented it are unaffected.

import { encryptPq, decryptPq } from '@nostr-wot/pq';

const parties = { sender: myPubkey, recipient: theirPubkey };

const payload = encryptPq('hello', att.kem!, nip44ConversationKey, parties);
const text    = decryptPq(payload, myKem.secretKey, nip44ConversationKey, parties);

Two ways to look at a payload before opening it. isPqEnvelope(payload) is the boolean a cipher routes on. classifyEnvelope(payload) is the one a caller that reports failures should use, because a boolean cannot tell "somebody else's ciphertext" from "ours and broken":

The version and algorithm bytes are decoded from the first four base64 characters inline, without atob or Buffer, so a host that has neither still gets 'pq-unreadable' for a valid envelope rather than being told it is classic and sent to the wrong code. A payload claiming a version this package has never heard of cannot be distinguished from a classic ciphertext of that version, so it is called classic — the honest answer, not a guess.

Send a gift-wrapped direct message

Most callers want this rather than the raw envelope. It composes the envelope with NIP-17 and NIP-59 — the result is an ordinary kind:1059 gift wrap that today's relays accept and today's clients ignore.

import { createPqDirectMessage, openPqDirectMessage, inboxFilter } from '@nostr-wot/pq';

const wrap = createPqDirectMessage({
  content: 'hello',
  senderSecretKey: mySecretKey,
  recipientPubkey: theirPubkey,
  recipientKemKey: att.kem!,   // from their kind:10203 attestation
});
await pool.publish(theirInboxRelays, wrap);

// On the other side:
for (const w of await pool.querySync(myInboxRelays, inboxFilter(myPubkey))) {
  const msg = openPqDirectMessage({
    wrap: w,
    recipientSecretKey: mySecretKey,
    recipientKemSecretKey: myKem.secretKey,
  });
  if (msg) console.log(msg.sender, msg.content); // null = an ordinary classic message
}

openPqDirectMessage returns null for a classic gift wrap rather than throwing, so a post-quantum client does not choke on ordinary messages. It throws only when a message is malformed or fails authentication — including when a rumor claims an author the seal did not sign, which is the forgery a naive implementation would display as genuine.

Wire format

version   1 byte    0x01
alg       1 byte    0x01 = ML-KEM-1024 + NIP-44 conversation key, XChaCha20-Poly1305
kem_ct    1568      ML-KEM-1024 ciphertext
nonce     24        XChaCha20-Poly1305 nonce
sealed    variable  AEAD(padded plaintext), includes the 16-byte tag

base64-encoded for transport.

Why it looks like this

• Its own version byte, not NIP-44's. Overloading NIP-44's version registry would squat a number its authors own. This envelope is self-describing, so it can be adopted, renumbered or superseded without colliding with anyone.
• Hybrid, never bare. The KEM secret is combined with the NIP-44 conversation key through HKDF, so the result is no weaker than either input. Two tests assert this actually binds: a wrong conversation key fails, and a wrong ML-KEM key fails. If either passed, the construction would not be hybrid.
• The framing is authenticated. Version, algorithm and both pubkeys go into the AEAD's associated data, so a ciphertext cannot be replayed into another conversation, have its direction swapped, or have its algorithm byte downgraded.
• Length is padded using NIP-44's scheme, so ciphertext size does not leak message size on a public relay. Messages of 1, 2, 20 and 32 bytes all produce identical wire sizes.
• One generic error. Every decryption failure throws the same message. Distinguishing bad padding from a bad tag from a wrong key hands an attacker an oracle.

Gift-wrapped message size

Measured on the complete kind:1059 event, serialized as a relay sees it:

~3 KB constant overhead, about 2.4x on a typical chat message. At 100 messages a day that is +300 KB/day, or +107 MB/year per conversation — the number relay operators will care about, and the strongest argument anyone will make against adopting this.

Worth noting that NIP-59 is already expensive for its own reasons: a classic two-character "hi" costs 1,533 bytes. Gift wrap has a ~1.5 KB floor no matter what. Post-quantum raises that floor; it does not create it.

Where the layering saves 16-28%

NIP-59 base64-encodes at every layer, so anything placed in the rumor is expanded by 4/3 three times over. Putting the envelope at the seal layer instead of inside the rumor removes an entire expansion of the 1568-byte ML-KEM ciphertext:

This is a framing optimisation, not a cryptographic compromise — nothing is weakened, and it is arguably the more natural placement, since the seal is already where NIP-59 puts the rumor's confidentiality. The envelope replaces that layer's encryption rather than nesting inside it.

Speed

1.27 ms to encrypt, 1.60 ms to decrypt a 280-byte message (Node 20, Apple silicon, average of 100).

What this does not do

• It does not stop event forgery. Events are still signed with secp256k1. A quantum adversary can sign as any user, and can publish a replacement attestation carrying their own keys to intercept future messages. This makes past messages permanently confidential; it does not protect future messages once secp256k1 is broken.
• It does not hide metadata beyond what gift wrap already hides. Who talks to whom, and when, is a NIP-59 question, not this envelope's.

Parameter sets

ML-KEM-1024 and ML-DSA-87 (NIST Category 5) — the sets mandated by NSA CNSA 2.0. Australia's ISM withdraws approval for ML-KEM-768 and ML-DSA-65 after 2030, so the smaller sets would mean shipping parameters already scheduled for withdrawal.