Christopher
331 lines
10 KiB
TypeScript
331 lines
10 KiB
TypeScript
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/**
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* Synapsis Double Ratchet & X3DH Implementation
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*
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* Implements the Double Ratchet Algorithm + X3DH Key Agreement.
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* Adheres to Signal specifications using the "SynapsisV2" HKDF info binding.
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*/
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import {
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KeyPair,
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computeSharedSecret,
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hkdf,
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encrypt as aeadEncrypt,
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decrypt as aeadDecrypt,
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importX25519PublicKey,
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exportKey,
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generateX25519KeyPair,
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base64ToArrayBuffer,
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arrayBufferToBase64
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} from './e2ee';
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// Constants
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const KDF_INFO = 'SynapsisV2';
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const RK_SIZE = 32; // 32 bytes for Root Key
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const CK_SIZE = 32; // 32 bytes for Chain Key
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const MK_SIZE = 32; // 32 bytes for Message Key
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export interface RatchetState {
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// DH Ratchet
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dhPair: KeyPair;
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remoteDhPub: CryptoKey;
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rootKey: ArrayBuffer;
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// Symm Ratchets
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chainKeySend: ArrayBuffer;
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chainKeyRecv: ArrayBuffer;
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// Message Numbers
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ns: number; // Send count
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nr: number; // Recv count
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pn: number; // Previous chain count
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}
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export interface Header {
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dh: string; // Base64 public key
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pn: number;
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n: number;
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}
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export interface CiphertextMessage {
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header: Header;
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ciphertext: string;
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iv: string;
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}
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// ----------------------------------------------------------------------------
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// 1. X3DH Key Agreement
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// ----------------------------------------------------------------------------
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export async function x3dhSender(
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aliceIdentity: KeyPair,
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bobBundle: {
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identityKey: CryptoKey,
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signedPreKey: CryptoKey,
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oneTimeKey?: CryptoKey
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},
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contextInfo: string // "SynapsisV2" + DIDs + DeviceIDs
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): Promise<{ sk: ArrayBuffer, ephemeralKey: KeyPair }> {
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// 1. Generate Ephemeral Key (EK_a)
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const ephemeralKey = await generateX25519KeyPair();
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// 2. DH1 = DH(IK_a, SPK_b)
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const dh1 = await computeSharedSecret(aliceIdentity.privateKey, bobBundle.signedPreKey);
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// 3. DH2 = DH(EK_a, IK_b)
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const dh2 = await computeSharedSecret(ephemeralKey.privateKey, bobBundle.identityKey);
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// 4. DH3 = DH(EK_a, SPK_b)
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const dh3 = await computeSharedSecret(ephemeralKey.privateKey, bobBundle.signedPreKey);
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// 5. DH4 = DH(EK_a, OPK_b) -- Optional
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let dh4: ArrayBuffer | undefined;
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if (bobBundle.oneTimeKey) {
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dh4 = await computeSharedSecret(ephemeralKey.privateKey, bobBundle.oneTimeKey);
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}
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// 6. Concatenate
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const km = new Uint8Array(dh1.byteLength + dh2.byteLength + dh3.byteLength + (dh4 ? dh4.byteLength : 0));
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let offset = 0;
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km.set(new Uint8Array(dh1), offset); offset += dh1.byteLength;
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km.set(new Uint8Array(dh2), offset); offset += dh2.byteLength;
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km.set(new Uint8Array(dh3), offset); offset += dh3.byteLength;
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if (dh4) km.set(new Uint8Array(dh4), offset);
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// 7. KDF
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// Output 32 bytes for Root Key
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const encoder = new TextEncoder();
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return {
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sk: await hkdf(new Uint8Array(32), km.buffer, encoder.encode(contextInfo), 32),
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ephemeralKey
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};
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}
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export async function x3dhReceiver(
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bobIdentity: KeyPair,
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bobSignedPreKey: KeyPair,
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bobOneTimeKey: KeyPair | undefined, // The one used by Alice
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aliceIdentityKey: CryptoKey,
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aliceEphemeralKey: CryptoKey,
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contextInfo: string
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): Promise<ArrayBuffer> {
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// 1. DH1 = DH(SPK_b, IK_a) -- Note: Order of keys in computeSharedSecret usually (private, public)
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const dh1 = await computeSharedSecret(bobSignedPreKey.privateKey, aliceIdentityKey);
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// 2. DH2 = DH(IK_b, EK_a)
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const dh2 = await computeSharedSecret(bobIdentity.privateKey, aliceEphemeralKey);
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// 3. DH3 = DH(SPK_b, EK_a)
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const dh3 = await computeSharedSecret(bobSignedPreKey.privateKey, aliceEphemeralKey);
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// 4. DH4 = DH(OPK_b, EK_a)
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let dh4: ArrayBuffer | undefined;
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if (bobOneTimeKey) {
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dh4 = await computeSharedSecret(bobOneTimeKey.privateKey, aliceEphemeralKey);
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}
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const km = new Uint8Array(dh1.byteLength + dh2.byteLength + dh3.byteLength + (dh4 ? dh4.byteLength : 0));
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let offset = 0;
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km.set(new Uint8Array(dh1), offset); offset += dh1.byteLength;
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km.set(new Uint8Array(dh2), offset); offset += dh2.byteLength;
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km.set(new Uint8Array(dh3), offset); offset += dh3.byteLength;
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if (dh4) km.set(new Uint8Array(dh4), offset);
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const encoder = new TextEncoder();
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return await hkdf(new Uint8Array(32), km.buffer, encoder.encode(contextInfo), 32);
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}
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// ----------------------------------------------------------------------------
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// 2. KDF Chains (Symmetric Ratchet)
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// ----------------------------------------------------------------------------
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// Constants for HMAC
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const ONE = new Uint8Array([0x01]);
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const TWO = new Uint8Array([0x02]);
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async function kdfChain(ck: ArrayBuffer): Promise<{ ck: ArrayBuffer, mk: ArrayBuffer }> {
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// HMAC-SHA256(CK, 1) -> MK
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// HMAC-SHA256(CK, 2) -> NextCK
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// Implementing via HKDF for simplicity/consistency or WebCrypto HMAC
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// Actually standard says:
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// HMAC-SHA256(ck, input)
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// We can use HKDF-Expand logic here or pure hmac.
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// Let's use custom HKDF expand with fixed info
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const mk = await hkdf(new Uint8Array(0), ck, ONE, 32);
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const nextCk = await hkdf(new Uint8Array(0), ck, TWO, 32);
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return { ck: nextCk, mk };
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}
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// ----------------------------------------------------------------------------
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// 3. DHRatchet (Root Chain)
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// ----------------------------------------------------------------------------
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async function kdfRoot(rootKey: ArrayBuffer, dhOut: ArrayBuffer): Promise<{ rootKey: ArrayBuffer, chainKey: ArrayBuffer }> {
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// HKDF(root, dh, info, 64) -> 32 root, 32 chain
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const encoder = new TextEncoder();
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const output = await hkdf(
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rootKey,
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dhOut,
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encoder.encode("SynapsisRatchet"),
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64
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);
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const bytes = new Uint8Array(output);
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return {
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rootKey: bytes.slice(0, 32).buffer,
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chainKey: bytes.slice(32, 64).buffer
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};
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}
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// ----------------------------------------------------------------------------
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// 4. Initializers
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// ----------------------------------------------------------------------------
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export async function initSender(
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sharedSecret: ArrayBuffer,
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bobRatchetKey: CryptoKey
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): Promise<RatchetState> {
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const dhPair = await generateX25519KeyPair();
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// Sender starts by sending a new DH ratchet.
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// Root Key = sharedSecret.
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// First, we need to generate a chain key for sending.
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// Standard: Alice initializes with SK. Bob's ratchet key is remote.
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// Alice generates `dhPair`.
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// She performs a DH ratchet Step immediately?
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// Protocol:
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// Alice: RK = SK.
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// Alice performs DH(alice_priv, bob_pub).
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// Calculates RK, CK_send.
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const dhOut = await computeSharedSecret(dhPair.privateKey, bobRatchetKey);
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const kdf = await kdfRoot(sharedSecret, dhOut);
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return {
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dhPair,
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remoteDhPub: bobRatchetKey,
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rootKey: kdf.rootKey,
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chainKeySend: kdf.chainKey,
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chainKeyRecv: new Uint8Array(0).buffer, // Empty until Bob replies
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ns: 0,
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nr: 0,
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pn: 0
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};
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}
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export async function initReceiver(
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sharedSecret: ArrayBuffer,
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bobDhKeyPair: KeyPair // This is the SPK key pair used in X3DH
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): Promise<RatchetState> {
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// Bob: RK = SK.
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// Bob has consistent state.
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return {
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dhPair: bobDhKeyPair,
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remoteDhPub: bobDhKeyPair.publicKey, // Placeholder, will be updated on first msg
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rootKey: sharedSecret,
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chainKeySend: new Uint8Array(0).buffer,
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chainKeyRecv: new Uint8Array(0).buffer, // Will be derived on first msg
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ns: 0,
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nr: 0,
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pn: 0
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};
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}
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// ----------------------------------------------------------------------------
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// 5. Encrypt / Decrypt Message
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// ----------------------------------------------------------------------------
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export async function ratchetEncrypt(
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state: RatchetState,
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plaintext: string
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): Promise<{
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ciphertext: CiphertextMessage,
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newState: RatchetState
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}> {
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// 1. Advance chain
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const { ck: nextCk, mk } = await kdfChain(state.chainKeySend);
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state.chainKeySend = nextCk;
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// 2. Encrypt
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const header: Header = {
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dh: await exportKey(state.dhPair.publicKey),
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pn: state.pn,
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n: state.ns
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};
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const associatedData = new TextEncoder().encode(JSON.stringify(header));
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const encrypted = await aeadEncrypt(mk, plaintext, associatedData);
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state.ns += 1;
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return {
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ciphertext: {
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header,
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ciphertext: encrypted.ciphertext,
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iv: encrypted.iv
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},
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newState: state
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};
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}
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// Note: Decryption requires handling out-of-order messages and ratcheting steps.
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// This is complex logic. For V2.1 baseline, we implement the core ratcheting step if header key differs.
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export async function ratchetDecrypt(
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state: RatchetState,
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message: CiphertextMessage
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): Promise<{ plaintext: string, newState: RatchetState }> {
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// Check if DH ratchet step needed
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// If message.header.dh != state.remoteDhPub
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// Note: Comparing CryptoKeys directly is hard. We compare Base64 export.
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const remoteKeyStr = await exportKey(state.remoteDhPub);
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if (message.header.dh !== remoteKeyStr) {
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// Ratchet Step!
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const newRemoteKey = await importX25519PublicKey(message.header.dh);
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// 1. DHRatchet(remote_new) -> RX step
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const dhOut1 = await computeSharedSecret(state.dhPair.privateKey, newRemoteKey);
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const kdf1 = await kdfRoot(state.rootKey, dhOut1);
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state.rootKey = kdf1.rootKey;
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state.chainKeyRecv = kdf1.chainKey;
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// 2. Sender step (We generate new key)
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state.pn = state.ns;
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state.ns = 0;
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state.nr = 0;
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state.dhPair = await generateX25519KeyPair();
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// 3. DHRatchet(remote_new) -> TX step
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const dhOut2 = await computeSharedSecret(state.dhPair.privateKey, newRemoteKey);
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const kdf2 = await kdfRoot(state.rootKey, dhOut2);
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state.rootKey = kdf2.rootKey;
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state.chainKeySend = kdf2.chainKey;
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state.remoteDhPub = newRemoteKey;
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}
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// 3. Symmetric Ratchet to catch up to n
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// (Skipping skipped-message buffering for now - assumes ordered delivery for V2.1 baseline)
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// Advance Chain Recv to n
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// Real impl buffers skipped keys.
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// Warning: If n > nr, we must loop.
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// For now, assuming direct sequence.
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const { ck: nextCk, mk } = await kdfChain(state.chainKeyRecv);
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state.chainKeyRecv = nextCk;
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state.nr += 1;
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// 4. Decrypt
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const associatedData = new TextEncoder().encode(JSON.stringify(message.header));
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const plaintext = await aeadDecrypt(mk, message.ciphertext, message.iv, associatedData);
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return { plaintext, newState: state };
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}
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