feat(crypto): sender-key per-conversation multi-device E2EE

packages/shared/src/crypto/index.ts

@@ -1,3 +1,4 @@
 export * from './backend.js';
 export * from './box.js';
 export * from './keys.js';
+export * from './sessionKeys.js';

packages/shared/src/crypto/sessionKeys.ts

@@ -0,0 +1,58 @@
+import { getCryptoBackend } from './backend.js';
+import { decryptFrom, encryptFor, type EncryptedEnvelope } from './box.js';
+
+// Sender-Key (Signal-style) helpers — one symmetric XSalsa20-Poly1305 key per
+// conversation, wrapped with `crypto_box` for each recipient device's pubkey.
+//
+// Flow:
+//   - generateConvKey() produces 32 random bytes
+//   - wrapConvKeyForRecipient() encrypts the conv-key with sender's private key
+//     and recipient's pubkey -> stored in `conversation_keys` table
+//   - unwrapConvKey() reverses it on the receiving side
+//   - encryptWithConvKey()/decryptWithConvKey() do the message-payload work
+
+export function generateConvKey(): Uint8Array {
+  const backend = getCryptoBackend();
+  return backend.randomBytes(backend.secretboxKeyLength);
+}
+
+export async function wrapConvKeyForRecipient(
+  convKey: Uint8Array,
+  recipientPublicKey: Uint8Array,
+  senderPrivateKey: Uint8Array,
+): Promise<EncryptedEnvelope> {
+  return encryptFor(convKey, recipientPublicKey, senderPrivateKey);
+}
+
+export async function unwrapConvKey(
+  encryptedKey: Uint8Array,
+  nonce: Uint8Array,
+  senderPublicKey: Uint8Array,
+  recipientPrivateKey: Uint8Array,
+): Promise<Uint8Array> {
+  return decryptFrom(encryptedKey, nonce, senderPublicKey, recipientPrivateKey);
+}
+
+export interface ConvCipher {
+  ciphertext: Uint8Array;
+  nonce: Uint8Array;
+}
+
+export function encryptWithConvKey(
+  plaintext: Uint8Array,
+  convKey: Uint8Array,
+): ConvCipher {
+  const backend = getCryptoBackend();
+  const nonce = backend.randomBytes(backend.secretboxNonceLength);
+  const ciphertext = backend.secretbox(plaintext, nonce, convKey);
+  return { ciphertext, nonce };
+}
+
+export function decryptWithConvKey(
+  ciphertext: Uint8Array,
+  nonce: Uint8Array,
+  convKey: Uint8Array,
+): Uint8Array {
+  const backend = getCryptoBackend();
+  return backend.secretboxOpen(ciphertext, nonce, convKey);
+}