Double ratchet NIP — Signal style DM encryption on Nostr. Deployed on https://iris.to

NIP-117

The Double Ratchet Algorithm

The Double Ratchet is a key rotation algorithm for secure private messaging.

It allows us to 1) communicate on Nostr without revealing metadata (who you are communicating with and when), and 2) keep your message history and future messages safe even if your main Nostr key is compromised.

Additionally, it enables disappearing messages that become undecryptable when past message decryption keys are discarded after use.

See also: NIP-118: Nostr Double Ratchet Invites

Overview

“Double ratchet” means we use 2 “ratchets”: cryptographic functions that can be rotated forward, but not backward: current keys can be used to derive next keys, but not the other way around.

Ratchet 1 uses Diffie-Hellman (DH) shared secrets and is rotated each time the other participant acknowledges a new key we have sent along with a previous message.

Ratchet 2 generates encryption keys for each message. It rotates after every message, using the previous message’s key as input (and the Ratchet 1 key when it rotates). This process ensures forward secrecy for consecutive messages from the same sender in between Ratchet 1 rotations.

Nostr implementation

We implement the Double Ratchet Algorithm on Nostr similarly to Signal’s Double Ratchet with header encryption, but encrypting the message headers with NIP-44 conversation keys instead of symmetric header keys.

Ratchet 1 keys are standard Nostr keys. In addition to encryption, they are also used for publishing and subscribing to messages on Nostr. As they are rotated and not linked to public Nostr identities, metadata privacy is preserved.

Nostr event format

Message

Outer event

{
  kind: 1060,
  content: encryptedInnerEvent,
  tags: [["header", encryptedHeader]],
  pubkey: ratchetPublicKey,
  created_at,
  id,
  sig
}

We subscribe to Double Ratchet events based on author public keys which are ephemeral — not used for other purposes than the Double Ratchet session. We use the regular event kind 1060 to differentiate it from other DM kinds, retrieval of which may be restricted by relays.

The encrypted header contains our next nostr public key, our previous sending chain length and the current message number.

Inner event

Inner events must be NIP-59 Rumors (unsigned Nostr events) allowing plausible deniability.

With established Nostr event kinds, clients can implement all kinds of features, such as replies, reactions, and encrypted file sharing in private messages.

Direct message and encrypted file messages are defined in NIP-17.

Algorithm

Signal’s Double Ratchet with header encryption document is a comprehensive description and explanation of the algorithm.

In this NIP, the algorithm is only described in code, in order to highlight differences to the Signal implementation.

External functions

We use the following Nostr functions (NIP-01):

generateSecretKey() for creating Nostr private keys
finalizeEvent(partialEvent, secretKey) for creating valid Nostr events with pubkey, id and signature

We use NIP-44 functions for encryption:

nip44.encrypt
nip44.decrypt
nip44.getConversationKey

NIP-59:

createRumor

Key derivation function:

export function kdf(
  input1: Uint8Array,
  input2: Uint8Array = new Uint8Array(32),
  numOutputs: number = 1
): Uint8Array[] {
  const prk = hkdf_extract(sha256, input1, input2);

  const outputs: Uint8Array[] = [];
  for (let i = 1; i <= numOutputs; i++) {
    outputs.push(hkdf_expand(sha256, prk, new Uint8Array([i]), 32));
  }
  return outputs;
}

Session state

With this information you can start or continue a Double Ratchet session. Save it locally after each sent and received message.

interface SessionState {
  theirCurrentNostrPublicKey?: string;
  theirNextNostrPublicKey: string;

  ourCurrentNostrKey?: KeyPair;
  ourNextNostrKey: KeyPair;

  rootKey: Uint8Array;
  receivingChainKey?: Uint8Array;
  sendingChainKey?: Uint8Array;

  sendingChainMessageNumber: number;
  receivingChainMessageNumber: number;
  previousSendingChainMessageCount: number;

  // Cache of message & header keys for handling out-of-order messages
  // Indexed by Nostr public key, which you can use to resubscribe to unreceived messages
  skippedKeys: {
    [pubKey: string]: {
      headerKeys: Uint8Array[];
      messageKeys: { [msgIndex: number]: Uint8Array };
    };
  };
}

Initialization

Alice is the chat initiator and Bob is the recipient. Ephemeral keys were exchanged earlier.

static initAlice(
  theirEphemeralPublicKey: string,
  ourEphemeralNostrKey: KeyPair,
  sharedSecret: Uint8Array
) {
  // Generate ephemeral key for the next ratchet step
  const ourNextNostrKey = generateSecretKey();

  // Use ephemeral ECDH to derive rootKey and sendingChainKey
  const [rootKey, sendingChainKey] = kdf(
    sharedSecret,
    nip44.getConversationKey(ourEphemeralNostrKey.private, theirEphemeralPublicKey),
    2
  );

  return {
    rootKey,
    theirNextNostrPublicKey: theirEphemeralPublicKey,
    ourCurrentNostrKey: ourEphemeralNostrKey,
    ourNextNostrKey,
    receivingChainKey: undefined,
    sendingChainKey,
    sendingChainMessageNumber: 0,
    receivingChainMessageNumber: 0,
    previousSendingChainMessageCount: 0,
    skippedKeys: {},
  };
}

static initBob(
  theirEphemeralPublicKey: string,
  ourEphemeralNostrKey: KeyPair,
  sharedSecret: Uint8Array
) {
  return {
    rootKey: sharedSecret,
    theirNextNostrPublicKey: theirEphemeralPublicKey,
    // Bob has no ‘current’ key at init time — Alice will send to next and trigger a ratchet step
    ourCurrentNostrKey: undefined,
    ourNextNostrKey: ourEphemeralNostrKey,
    receivingChainKey: undefined,
    sendingChainKey: undefined,
    sendingChainMessageNumber: 0,
    receivingChainMessageNumber: 0,
    previousSendingChainMessageCount: 0,
    skippedKeys: {},
  };
}

Sending messages

sendEvent(event: Partial<UnsignedEvent>) {
  const innerEvent = nip59.createRumor(event)
  const [header, encryptedData] = this.ratchetEncrypt(JSON.stringify(innerEvent));

  const conversationKey = nip44.getConversationKey(this.state.ourCurrentNostrKey.privateKey, this.state.theirNextNostrPublicKey);
  const encryptedHeader = nip44.encrypt(JSON.stringify(header), conversationKey);

  const outerEvent = finalizeEvent({
    content: encryptedData,
    kind: MESSAGE_EVENT_KIND,
    tags: [["header", encryptedHeader]],
    created_at: Math.floor(now / 1000)
  }, this.state.ourCurrentNostrKey.privateKey);

  // Publish outerEvent on Nostr, store inner locally if needed
  return {outerEvent, innerEvent};
}

ratchetEncrypt(plaintext: string): [Header, string] {
  // Rotate sending chain key
  const [newSendingChainKey, messageKey] = kdf(this.state.sendingChainKey!, new Uint8Array([1]), 2);
  this.state.sendingChainKey = newSendingChainKey;
  const header: Header = {
    number: this.state.sendingChainMessageNumber++,
    nextPublicKey: this.state.ourNextNostrKey.publicKey,
    previousChainLength: this.state.previousSendingChainMessageCount
  };
  return [header, nip44.encrypt(plaintext, messageKey)];
}

Receiving messages

handleNostrEvent(e: NostrEvent) {
  const [header, shouldRatchet, isSkipped] = this.decryptHeader(e);

  if (!isSkipped) {
    if (this.state.theirNextNostrPublicKey !== header.nextPublicKey) {
      // Received a new key from them
      this.state.theirCurrentNostrPublicKey = this.state.theirNextNostrPublicKey;
      this.state.theirNextNostrPublicKey = header.nextPublicKey;
      this.updateNostrSubscriptions()
    }

    if (shouldRatchet) {
      this.skipMessageKeys(header.previousChainLength, e.pubkey);
      this.ratchetStep(header.nextPublicKey);
    }
  }

decryptHeader(event: any): [Header, boolean, boolean] {
  const encryptedHeader = event.tags[0][1];
  if (this.state.ourCurrentNostrKey) {
    const conversationKey = nip44.getConversationKey(this.state.ourCurrentNostrKey.privateKey, event.pubkey);
    try {
      const header = JSON.parse(nip44.decrypt(encryptedHeader, conversationKey)) as Header;
      return [header, false, false];
    } catch (error) {
      // Decryption with currentSecret failed, try with nextSecret
    }
  }

  const nextConversationKey = nip44.getConversationKey(this.state.ourNextNostrKey.privateKey, event.pubkey);
  try {
    const header = JSON.parse(nip44.decrypt(encryptedHeader, nextConversationKey)) as Header;
    return [header, true, false];
  } catch (error) {
    // Decryption with nextSecret also failed
  }

  const skippedKeys = this.state.skippedKeys[event.pubkey];
  if (skippedKeys?.headerKeys) {
    // Try skipped header keys
    for (const key of skippedKeys.headerKeys) {
      try {
        const header = JSON.parse(nip44.decrypt(encryptedHeader, key)) as Header;
        return [header, false, true];
      } catch (error) {
        // Decryption failed, try next secret
      }
    }
  }

  throw new Error("Failed to decrypt header with current and skipped header keys");
}

ratchetDecrypt(header: Header, ciphertext: string, nostrSender: string): string {
  const plaintext = this.trySkippedMessageKeys(header, ciphertext, nostrSender);
  if (plaintext) return plaintext;

  this.skipMessageKeys(header.number, nostrSender);

  // Rotate receiving key
  const [newReceivingChainKey, messageKey] = kdf(this.state.receivingChainKey!, new Uint8Array([1]), 2);
  this.state.receivingChainKey = newReceivingChainKey;
  this.state.receivingChainMessageNumber++;

  return nip44.decrypt(ciphertext, messageKey);
}

ratchetStep(theirNextNostrPublicKey: string) {
  this.state.previousSendingChainMessageCount = this.state.sendingChainMessageNumber;
  this.state.sendingChainMessageNumber = 0;
  this.state.receivingChainMessageNumber = 0;
  this.state.theirNextNostrPublicKey = theirNextNostrPublicKey;

  // 1st step yields the new conversation key they used
  const conversationKey1 = nip44.getConversationKey(this.state.ourNextNostrKey.privateKey, this.state.theirNextNostrPublicKey!);
  // and our corresponding receiving chain key
  const [theirRootKey, receivingChainKey] = kdf(this.state.rootKey, conversationKey1, 2);
  this.state.receivingChainKey = receivingChainKey;

  // Rotate our Nostr key
  this.state.ourCurrentNostrKey = this.state.ourNextNostrKey;
  const ourNextSecretKey = generateSecretKey();
  this.state.ourNextNostrKey = {
    publicKey: getPublicKey(ourNextSecretKey),
    privateKey: ourNextSecretKey
  };

  // 2nd step yields the new conversation key we'll use
  const conversationKey2 = nip44.getConversationKey(this.state.ourNextNostrKey.privateKey, this.state.theirNextNostrPublicKey!);
  // And our corresponding sending chain key
  const [rootKey, sendingChainKey] = kdf(theirRootKey, conversationKey2, 2);
  this.state.rootKey = rootKey;
  this.state.sendingChainKey = sendingChainKey;
}

skipMessageKeys(until: number, nostrSender: string) {
  if (this.state.receivingChainMessageNumber + MAX_SKIP < until) {
    throw new Error("Too many skipped messages");
  }

  if (!this.state.skippedKeys[nostrSender]) {
    this.state.skippedKeys[nostrSender] = {
      headerKeys: [],
      messageKeys: {}
    };

    if (this.state.ourCurrentNostrKey) {
      const currentSecret = nip44.getConversationKey(this.state.ourCurrentNostrKey.privateKey, nostrSender);
      this.state.skippedKeys[nostrSender].headerKeys.push(currentSecret);
    }
    const nextSecret = nip44.getConversationKey(this.state.ourNextNostrKey.privateKey, nostrSender);
    this.state.skippedKeys[nostrSender].headerKeys.push(nextSecret);
  }

  while (this.state.receivingChainMessageNumber < until) {
    const [newReceivingChainKey, messageKey] = kdf(this.state.receivingChainKey!, new Uint8Array([1]), 2);
    this.state.receivingChainKey = newReceivingChainKey;
    this.state.skippedKeys[nostrSender].messageKeys[this.state.receivingChainMessageNumber] = messageKey;
    this.state.receivingChainMessageNumber++;
  }
}

trySkippedMessageKeys(header: Header, ciphertext: string, nostrSender: string): string | null {
  const skippedKeys = this.state.skippedKeys[nostrSender];
  if (!skippedKeys) return null;

  const messageKey = skippedKeys.messageKeys[header.number];
  if (!messageKey) return null;

  delete skippedKeys.messageKeys[header.number];

  if (Object.keys(skippedKeys.messageKeys).length === 0) {
    delete this.state.skippedKeys[nostrSender];
  }

  return nip44.decrypt(ciphertext, messageKey);
}

Double ratchet invites:

quoting
naddr1qv…daur

NIP-118

Nostr Double Ratchet Invites

In order to start a NIP-117 Nostr Double Ratchet session, Alice and Bob need to exchange ephemeral public keys and a shared secret.

In a Nostr Double Ratchet invite, Alice gives Bob her

ephemeral public key, which is also used to respond to the invite on Nostr,

shared secret, and

Nostr identity public key, which is used to authenticate the exchange.

Nostr event

Publishing an invite event on Nostr allows other users to start a Double Ratchet session with you.
{
  pubkey: inviterIdentityKey,
  content: '',
  tags: [
    ['d', `double-ratchet/invites/${uid}`], // uid: unique name for the invitation, e.g. "public"
    ['l', 'double-ratchet/invites'], // label, for listing all your invites
    ['ephemeralKey', inviterEphemeralKey],
    ['sharedSecret', sharedSecret]
  ],
  kind: 30078,
  created_at,
  id,
  sig
}
l is a NIP-32 label tag that can be used to list your invites. We use this approach with NIP-78 application specific data kind 30078 in order to not bloat the event kind space and instead have a human-readable label.

URL

Invites can be also shared privately, without requiring a Nostr event. This improves privacy, removing the public association between the inviter and response.

URL and QR code are often convenient ways to share privately, especially when the other user is not yet on Nostr. Format:
const invite = { inviter, ephemeralKey, sharedSecret };
const json = JSON.stringify(invite);
const url = `https://example.com/#${encodeURIComponent(json)}`;
Encoding the invite into the URL hash ensures it’s not sent to the server and logged by default.

nostr: URI scheme is another possible way to share invites, when we have native clients supporting the feature.

Invite response

Outer event

Invite response outer event is a NIP-59 gift wrap event of kind 1059, sent from a random, one-time-use pubkey, hiding the responder’s identity from the public.

It is addressed to the ephemeral key in the invite. If the invite was publicly shared, responses can be publicly associated to the inviter.

Inner event

NIP-59 Rumor.
const conversationKey = nip44.getConversationKey(
  inviteeKeyPair,
  inviterPublicKey
);
const encrypted1 = encrypt(inviteeEphemeralKey, conversationKey);
const encrypted2 = encrypt(encrypted1, sharedSecret);

const rumor = {
  pubkey: inviteePublicKey,
  kind: 1060,
  content: encrypted2,
  tags: [],
  createdAt,
  id,
};
After receiving the invite response, both parties have what they need to start a double ratchet session: each others’ ephemeral public keys and a shared secret.

Both parties have authenticated by encrypting or decrypting using their nip44 conversation key.

The shared secret from the invite is used in the response to ensure that only actual recipients of the invite can follow it. Otherwise, attackers could initiate double ratchet sessions by sending invite responses to all addresses that received gift wraps, some of which are private invite addresses.

Github PR https://github.com/nostr-protocol/nips/pull/1813

Martti Malmi on Nostr: Double ratchet NIP — Signal style DM encryption on Nostr. Deployed on ...

NIP-117

The Double Ratchet Algorithm

Overview

Nostr implementation

Nostr event format

Message

Outer event

Inner event

Algorithm

External functions

Session state

Initialization

Sending messages

Receiving messages

NIP-118

Nostr Double Ratchet Invites

Nostr event

URL

Invite response

Outer event

Inner event