feat: add method to get conversation HMAC keys

package.json

@@ -106,7 +106,7 @@
   },
   "dependencies": {
     "@noble/secp256k1": "^1.5.2",
-    "@xmtp/proto": "^3.37.0-beta.1",
+    "@xmtp/proto": "^3.37.0-beta.2",
     "@xmtp/user-preferences-bindings-wasm": "^0.3.4",
     "async-mutex": "^0.4.0",
     "elliptic": "^6.5.4",

src/crypto/encryption.ts

@@ -84,7 +84,7 @@ async function hkdf(secret: Uint8Array, salt: Uint8Array): Promise<CryptoKey> {
   )
 }
 
-async function hkdfHmacKey(
+export async function hkdfHmacKey(
   secret: Uint8Array,
   salt: Uint8Array
 ): Promise<CryptoKey> {

src/keystore/InMemoryKeystore.ts

@@ -34,7 +34,7 @@ import {
   userPreferencesEncrypt,
   generateUserPreferencesTopic,
 } from '../crypto/selfEncryption'
-import { generateHmacSignature } from '../crypto/encryption'
+import { generateHmacSignature, hkdfHmacKey } from '../crypto/encryption'
 
 const { ErrorCode } = keystore
 
@@ -602,7 +602,43 @@ export default class InMemoryKeystore implements Keystore {
     return this.v2Store.lookup(topic)
   }
 
-  getTopicHmacs() {
-    const topics = this.v2Store.topics.map((topic) => topic.invitation?.topic)
+  async getV2ConversationHmacKeys(): Promise<keystore.GetConversationHmacKeysResponse> {
+    const thirtyDayPeriodsSinceEpoch = Math.floor(
+      Date.now() / 1000 / 60 / 60 / 24 / 30
+    )
+
+    const hmacKeys: keystore.GetConversationHmacKeysResponse['hmacKeys'] = {}
+
+    this.v2Store.topics.forEach(async (topicData) => {
+      if (topicData.invitation?.topic) {
+        const keyMaterial = getKeyMaterial(topicData.invitation)
+        const values = await Promise.all(
+          [
+            thirtyDayPeriodsSinceEpoch - 1,
+            thirtyDayPeriodsSinceEpoch,
+            thirtyDayPeriodsSinceEpoch + 1,
+          ].map(async (value) => {
+            const salt = `${value}-${this.accountAddress}`
+            const hmacKey = await hkdfHmacKey(
+              keyMaterial,
+              new TextEncoder().encode(salt)
+            )
+            return {
+              thirtyDayPeriodsSinceEpoch: value,
+              // convert CryptoKey to Uint8Array to match the proto
+              hmacKey: new Uint8Array(
+                await crypto.subtle.exportKey('raw', hmacKey)
+              ),
+            }
+          })
+        )
+
+        hmacKeys[topicData.invitation.topic] = {
+          values,
+        }
+      }
+    })
+
+    return { hmacKeys }
   }
 }

src/keystore/interfaces.ts

@@ -97,6 +97,11 @@ export interface Keystore {
    * Get the private preferences topic identifier
    */
   getPrivatePreferencesTopicIdentifier(): Promise<keystore.GetPrivatePreferencesTopicIdentifierResponse>
+  /**
+   * Returns the conversation HMAC keys for the current, previous, and next
+   * 30 day periods since the epoch
+   */
+  getV2ConversationHmacKeys(): Promise<keystore.GetConversationHmacKeysResponse>
 }
 
 export type TopicData = WithoutUndefined<keystore.TopicMap_TopicData>

src/keystore/rpcDefinitions.ts

@@ -85,4 +85,8 @@ export const apiDefs: ApiDefs = {
     req: null,
     res: keystore.GetPrivatePreferencesTopicIdentifierResponse,
   },
+  getV2ConversationHmacKeys: {
+    req: null,
+    res: keystore.GetConversationHmacKeysResponse,
+  },
 } as const