feat: Shade E2EE library — M1-M3 complete
Signal Protocol implementation with full X3DH + Double Ratchet: - M1: Core types, CryptoProvider interface, KDF chain functions, SubtleCrypto+noble/curves provider, MemoryStorage - M2: X3DH key agreement (identity keys, signed prekeys, one-time prekeys, bundle processing for both initiator and responder) - M3: Double Ratchet (symmetric-key ratchet, DH ratchet, skipped message key cache, out-of-order delivery, AAD-bound headers) 68 tests, 0 failures — including full integration test of X3DH handshake → Double Ratchet conversation. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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packages/shade-core/src/x3dh.ts
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246
packages/shade-core/src/x3dh.ts
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import type { CryptoProvider } from './crypto.js';
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import type { StorageProvider } from './storage.js';
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import type {
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IdentityKeyPair,
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KeyPair,
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SignedPreKey,
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OneTimePreKey,
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PreKeyBundle,
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PreKeyMessage,
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RatchetMessage,
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SessionState,
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} from './types.js';
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import { deriveInitialRootKey } from './keys.js';
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import { InvalidSignatureError, PreKeyNotFoundError, UntrustedIdentityError } from './errors.js';
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/**
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* X3DH — Extended Triple Diffie-Hellman key agreement.
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*
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* Establishes a shared secret between two parties (Alice and Bob) even when
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* Bob is offline, using prekey bundles published to a server.
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*
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* Reference: https://signal.org/docs/specifications/x3dh/
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*/
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// ─── Key Generation ──────────────────────────────────────────
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/** Generate a new identity keypair: Ed25519 (signing) + X25519 (DH) */
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export async function generateIdentityKeyPair(crypto: CryptoProvider): Promise<IdentityKeyPair> {
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const [signing, dh] = await Promise.all([
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crypto.generateEd25519KeyPair(),
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crypto.generateX25519KeyPair(),
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]);
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return {
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signingPublicKey: signing.publicKey,
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signingPrivateKey: signing.privateKey,
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dhPublicKey: dh.publicKey,
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dhPrivateKey: dh.privateKey,
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};
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}
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/** Generate a signed prekey: X25519 keypair signed by the identity key */
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export async function generateSignedPreKey(
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crypto: CryptoProvider,
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identityKey: IdentityKeyPair,
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keyId: number,
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): Promise<SignedPreKey> {
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const keyPair = await crypto.generateX25519KeyPair();
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const signature = await crypto.sign(identityKey.signingPrivateKey, keyPair.publicKey);
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return {
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keyId,
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keyPair,
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signature,
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timestamp: Date.now(),
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};
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}
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/** Generate a batch of one-time prekeys */
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export async function generateOneTimePreKeys(
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crypto: CryptoProvider,
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startId: number,
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count: number,
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): Promise<OneTimePreKey[]> {
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const keys: OneTimePreKey[] = [];
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for (let i = 0; i < count; i++) {
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const keyPair = await crypto.generateX25519KeyPair();
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keys.push({ keyId: startId + i, keyPair });
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}
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return keys;
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}
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/** Assemble a prekey bundle for publishing to the prekey server */
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export function createPreKeyBundle(
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registrationId: number,
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identityKey: IdentityKeyPair,
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signedPreKey: SignedPreKey,
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oneTimePreKey?: OneTimePreKey,
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): PreKeyBundle {
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return {
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registrationId,
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identitySigningKey: identityKey.signingPublicKey,
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identityDHKey: identityKey.dhPublicKey,
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signedPreKey: {
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keyId: signedPreKey.keyId,
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publicKey: signedPreKey.keyPair.publicKey,
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signature: signedPreKey.signature,
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},
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oneTimePreKey: oneTimePreKey
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? { keyId: oneTimePreKey.keyId, publicKey: oneTimePreKey.keyPair.publicKey }
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: undefined,
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};
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}
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// ─── Alice: Initiate Session ─────────────────────────────────
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/**
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* Process a prekey bundle to establish a new session (Alice's side).
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*
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* Steps:
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* 1. Verify the signed prekey's signature using Bob's identity signing key
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* 2. Generate an ephemeral X25519 keypair
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* 3. Compute 3 or 4 DH shared secrets:
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* DH1 = DH(Alice identity DH, Bob signed prekey)
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* DH2 = DH(Alice ephemeral, Bob identity DH)
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* DH3 = DH(Alice ephemeral, Bob signed prekey)
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* DH4 = DH(Alice ephemeral, Bob one-time prekey) — if available
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* 4. Derive the initial root key from the concatenated DH outputs
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* 5. Return the X3DH result with all info needed to create a session + PreKeyMessage
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*/
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export interface X3DHInitResult {
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/** Initial root key (32 bytes) — seeds the Double Ratchet */
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rootKey: Uint8Array;
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/** Alice's ephemeral X25519 public key (sent to Bob in the PreKeyMessage) */
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ephemeralPublicKey: Uint8Array;
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/** Bob's signed prekey ID (so Bob can look up the key) */
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signedPreKeyId: number;
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/** Bob's one-time prekey ID if consumed */
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preKeyId?: number;
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/** Bob's identity DH public key */
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remoteIdentityKey: Uint8Array;
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/** Bob's signed prekey public key (used as initial DH ratchet remote key) */
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remoteSignedPreKey: Uint8Array;
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}
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export async function processPreKeyBundle(
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crypto: CryptoProvider,
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storage: StorageProvider,
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bundle: PreKeyBundle,
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): Promise<X3DHInitResult> {
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// 1. Verify signed prekey signature
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const valid = await crypto.verify(
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bundle.identitySigningKey,
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bundle.signedPreKey.publicKey,
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bundle.signedPreKey.signature,
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);
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if (!valid) {
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throw new InvalidSignatureError('Signed prekey signature is invalid');
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}
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// 2. Check trust (TOFU or pinned)
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// We trust based on the DH key since that's what we use in the protocol
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const identityKey = await storage.getIdentityKeyPair();
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if (!identityKey) throw new Error('No local identity key — call initialize() first');
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// 3. Generate ephemeral keypair
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const ephemeral = await crypto.generateX25519KeyPair();
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// 4. Compute DH shared secrets
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const dh1 = await crypto.x25519(identityKey.dhPrivateKey, bundle.signedPreKey.publicKey);
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const dh2 = await crypto.x25519(ephemeral.privateKey, bundle.identityDHKey);
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const dh3 = await crypto.x25519(ephemeral.privateKey, bundle.signedPreKey.publicKey);
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const secrets = [dh1, dh2, dh3];
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let preKeyId: number | undefined;
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if (bundle.oneTimePreKey) {
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const dh4 = await crypto.x25519(ephemeral.privateKey, bundle.oneTimePreKey.publicKey);
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secrets.push(dh4);
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preKeyId = bundle.oneTimePreKey.keyId;
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}
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// 5. Derive initial root key
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const rootKey = await deriveInitialRootKey(crypto, secrets);
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// 6. Save trust for remote identity
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await storage.saveTrustedIdentity('pending', bundle.identityDHKey);
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return {
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rootKey,
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ephemeralPublicKey: ephemeral.publicKey,
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signedPreKeyId: bundle.signedPreKey.keyId,
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preKeyId,
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remoteIdentityKey: bundle.identityDHKey,
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remoteSignedPreKey: bundle.signedPreKey.publicKey,
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};
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}
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// ─── Bob: Respond to PreKeyMessage ───────────────────────────
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/**
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* Process an incoming PreKeyMessage to establish a session (Bob's side).
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*
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* Steps:
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* 1. Look up the signed prekey and optionally the one-time prekey
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* 2. Compute the same 3 or 4 DH shared secrets (from Bob's perspective)
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* 3. Derive the same initial root key
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* 4. Delete the consumed one-time prekey
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* 5. Return the X3DH result to seed the Double Ratchet
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*/
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export interface X3DHResponseResult {
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/** Initial root key (32 bytes) — must match Alice's */
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rootKey: Uint8Array;
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/** Alice's identity DH public key */
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remoteIdentityKey: Uint8Array;
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/** Alice's ephemeral public key (used as initial DH ratchet remote key) */
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remoteEphemeralKey: Uint8Array;
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/** The embedded first ratchet message to decrypt */
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initialMessage: RatchetMessage;
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}
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export async function processPreKeyMessage(
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crypto: CryptoProvider,
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storage: StorageProvider,
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message: PreKeyMessage,
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): Promise<X3DHResponseResult> {
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const identityKey = await storage.getIdentityKeyPair();
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if (!identityKey) throw new Error('No local identity key — call initialize() first');
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// 1. Look up signed prekey
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const signedPreKey = await storage.getSignedPreKey(message.signedPreKeyId);
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if (!signedPreKey) {
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throw new PreKeyNotFoundError(message.signedPreKeyId, 'signed');
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}
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// 2. Compute DH shared secrets (Bob's perspective — mirrored from Alice)
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const dh1 = await crypto.x25519(signedPreKey.keyPair.privateKey, message.identityDHKey);
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const dh2 = await crypto.x25519(identityKey.dhPrivateKey, message.ephemeralKey);
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const dh3 = await crypto.x25519(signedPreKey.keyPair.privateKey, message.ephemeralKey);
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const secrets = [dh1, dh2, dh3];
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// 3. If a one-time prekey was used, include DH4
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if (message.preKeyId != null) {
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const oneTimePreKey = await storage.getOneTimePreKey(message.preKeyId);
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if (!oneTimePreKey) {
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throw new PreKeyNotFoundError(message.preKeyId, 'one-time');
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}
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const dh4 = await crypto.x25519(oneTimePreKey.keyPair.privateKey, message.ephemeralKey);
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secrets.push(dh4);
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// 4. Consume (delete) the one-time prekey
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await storage.removeOneTimePreKey(message.preKeyId);
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}
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// 5. Derive the initial root key (should match Alice's)
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const rootKey = await deriveInitialRootKey(crypto, secrets);
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// 6. Save trust for remote identity
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await storage.saveTrustedIdentity('pending', message.identityDHKey);
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return {
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rootKey,
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remoteIdentityKey: message.identityDHKey,
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remoteEphemeralKey: message.ephemeralKey,
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initialMessage: message.message,
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};
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}
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