feat: support caching of verification kys (#81)

## Summary

Closes #9 

Previously, clients who fetched verification keys from the server had no
way to cache those keys and reuse them for other requests.

This PR proposes a change using the GitHub API's [conditional
requests](https://docs.github.com/en/rest/using-the-rest-api/best-practices-for-using-the-rest-api?apiVersion=2022-11-28#use-conditional-requests-if-appropriate)
feature: clients can optionally specify a cache for their keys and only
fetch new ones if the cache is outdated.

BREAKING CHANGE: `verifyRequestByKeyId() now returns an object with a
`isValid` property and a`cache` property.

Before

```js
const isValid = await verifyRequestByKeyId();
```

After

```js
const { isValid, cache } = await verifyRequestByKeyId();
```

BREAKING CHANGE: `fetchVerificationKeys()` now returns an object with a
`keys` property and a`cache` property.

Before

```js
const keys = await fetchVerificationKeys();
```

After

```js
const { keys, cache } = await fetchVerificationKeys();
```

---------

Co-authored-by: Gregor Martynus <[email protected]>

Author: francisfuzz

Diff for: README.md

@@ -23,15 +23,16 @@ We consider this SDK alpha software in terms of API stability, but we adhere to
 ```js
 import { verifyRequestByKeyId } from "@copilot-extensions/preview-sdk";
 
-const payloadIsVerified = await verifyRequestByKeyId(
+const { isValid, cache } = await verifyRequestByKeyId(
   request.body,
   signature,
   keyId,
   {
     token: process.env.GITHUB_TOKEN,
   },
 );
-// true or false
+// isValid: true or false
+// cache: { id, keys }
 ```
 
 ### Build a response
@@ -76,54 +77,82 @@ try {
 
 Verify the request payload using the provided signature and key ID. The method will request the public key from GitHub's API for the given keyId and then verify the payload.
 
-The `options` argument is optional. It can contain a `token` to authenticate the request to GitHub's API, or a custom `request` instance to use for the request.
+The `requestOptions` argument is optional. It can contain:
+
+- a `token` to authenticate the request to GitHub's API
+- a custom [octokit `request`](https://github.com/octokit/request.js) instance to use for the request
+- a `cache` to use cached keys
 
 ```js
 import { verifyRequestByKeyId } from "@copilot-extensions/preview-sdk";
 
-const payloadIsVerified = await verifyRequestByKeyId(
+const { isValid, cache } = await verifyRequestByKeyId(
   request.body,
   signature,
-  key,
+  keyId,
 );
 
 // with token
-await verifyRequestByKeyId(request.body, signature, key, { token: "ghp_1234" });
+const { isValid, cache } = await verifyRequestByKeyId(
+  request.body,
+  signature,
+  keyId,
+  { token: "ghp_1234" },
+);
 
 // with custom octokit request instance
-await verifyRequestByKeyId(request.body, signature, key, { request });
+const { isValid, cache } = await verifyRequestByKeyId(
+  request.body,
+  signature,
+  keyId,
+  { request },
+);
+
+// with cache
+const previousCache = {
+  id: "etag_value",
+  keys: [{ key_identifier: "key1", key: "public_key1", is_current: true }],
+};
+const { isValid, cache } = await verifyRequestByKeyId(
+  request.body,
+  signature,
+  keyId,
+  { cache: previousCache },
+);
 ```
 
 #### `async fetchVerificationKeys(options)`
 
-Fetches public keys for verifying copilot extension requests [from GitHub's API](https://api.github.com/meta/public_keys/copilot_api)
-and returns them as an array. The request can be made without authentication, with a token, or with a custom [octokit request](https://github.com/octokit/request.js) instance.
+Fetches public keys for verifying copilot extension requests [from GitHub's API](https://api.github.com/meta/public_keys/copilot_api) and returns them as an array. The request can be made without authentication, with a token, with a custom [octokit request](https://github.com/octokit/request.js) instance, or with a cache.
 
 ```js
 import { fetchVerificationKeys } from "@copilot-extensions/preview-sdk";
 
 // fetch without authentication
-const [current] = await fetchVerificationKeys();
+const { id, keys } = await fetchVerificationKeys();
 
 // with token
-const [current] = await fetchVerificationKeys({ token: "ghp_1234" });
+const { id, keys } = await fetchVerificationKeys({ token: "ghp_1234" });
 
 // with custom octokit request instance
-const [current] = await fetchVerificationKeys({ request });)
+const { id, keys } = await fetchVerificationKeys({ request });
+
+// with cache
+const cache = {
+  id: "etag_value",
+  keys: [{ key_identifier: "key1", key: "public_key1" }],
+};
+const { id, keys } = await fetchVerificationKeys({ cache });
 ```
 
-#### `async verifyRequestPayload(rawBody, signature, keyId)`
+#### `async verifyRequest(rawBody, signature, keyId)`
 
 Verify the request payload using the provided signature and key. Note that the raw body as received by GitHub must be passed, before any parsing.
 
 ```js
 import { verify } from "@copilot-extensions/preview-sdk";
 
-const payloadIsVerified = await verifyRequestPayload(
-  request.body,
-  signature,
-  key,
-);
+const payloadIsVerified = await verifyRequest(request.body, signature, key);
 // true or false
 ```
 
@@ -274,17 +303,22 @@ Convenience method to verify and parse a request in one go. It calls [`verifyReq
 ```js
 import { verifyAndParseRequest } from "@copilot-extensions/preview-sdk";
 
-const { isValidRequest, payload } = await verifyAndParseRequest(
-  request,
+const { isValidRequest, payload, cache } = await verifyAndParseRequest(
+  request.body,
   signature,
-  key,
+  keyId,
+  {
+    token: process.env.GITHUB_TOKEN,
+  },
 );
 
 if (!isValidRequest) {
   throw new Error("Request could not be verified");
 }
 
+// `isValidRequest` is a boolean.
 // `payload` has type support.
+// `cache` contains the id and keys used for verification.
 ```
 
 #### `getUserMessage()`

Diff for: index.d.ts

@@ -3,9 +3,14 @@ import { request } from "@octokit/request";
 // verification types
 
 type RequestInterface = typeof request;
+export type VerificationKeysCache = {
+  id: string;
+  keys: VerificationPublicKey[];
+};
 type RequestOptions = {
   request?: RequestInterface;
   token?: string;
+  cache?: VerificationKeysCache;
 };
 export type VerificationPublicKey = {
   key_identifier: string;
@@ -18,7 +23,7 @@ interface VerifyRequestInterface {
 }
 
 interface FetchVerificationKeysInterface {
-  (requestOptions?: RequestOptions): Promise<VerificationPublicKey[]>;
+  (requestOptions?: RequestOptions): Promise<VerificationKeysCache>;
 }
 
 interface VerifyRequestByKeyIdInterface {
@@ -27,7 +32,10 @@ interface VerifyRequestByKeyIdInterface {
     signature: string,
     keyId: string,
     requestOptions?: RequestOptions,
-  ): Promise<boolean>;
+  ): Promise<{
+    isValid: boolean;
+    cache: VerificationKeysCache;
+  }>;
 }
 
 // response types
@@ -188,7 +196,11 @@ export interface VerifyAndParseRequestInterface {
     signature: string,
     keyID: string,
     requestOptions?: RequestOptions,
-  ): Promise<{ isValidRequest: boolean; payload: CopilotRequestPayload }>;
+  ): Promise<{
+    isValidRequest: boolean;
+    payload: CopilotRequestPayload;
+    cache: VerificationKeysCache;
+  }>;
 }
 
 export interface GetUserMessageInterface {

Diff for: index.test-d.ts

@@ -22,6 +22,7 @@ import {
   prompt,
   PromptResult,
   getFunctionCalls,
+  VerificationKeysCache,
 } from "./index.js";
 
 const token = "";
@@ -32,7 +33,10 @@ export async function verifyRequestByKeyIdTest(
   keyId: string,
 ) {
   const result = await verifyRequestByKeyId(rawBody, signature, keyId);
-  expectType<boolean>(result);
+  expectType<{
+    isValid: boolean;
+    cache: { id: string; keys: VerificationPublicKey[] };
+  }>(result);
 
   // @ts-expect-error - first 3 arguments are required
   verifyRequestByKeyId(rawBody, signature);
@@ -51,6 +55,11 @@ export async function verifyRequestByKeyIdTest(
 
   // accepts a request argument
   await verifyRequestByKeyId(rawBody, signature, keyId, { request });
+
+  // accepts a cache argument
+  await verifyRequestByKeyId(rawBody, signature, keyId, {
+    cache: { id: "test", keys: [] },
+  });
 }
 
 export async function verifyRequestTest(
@@ -76,13 +85,16 @@ export async function verifyRequestTest(
 
 export async function fetchVerificationKeysTest() {
   const result = await fetchVerificationKeys();
-  expectType<VerificationPublicKey[]>(result);
+  expectType<{ id: string; keys: VerificationPublicKey[] }>(result);
 
   // accepts a token argument
   await fetchVerificationKeys({ token });
 
   // accepts a request argument
   await fetchVerificationKeys({ request });
+
+  // accepts a cache argument
+  await fetchVerificationKeys({ cache: { id: "test", keys: [] } });
 }
 
 export function createAckEventTest() {
@@ -181,9 +193,11 @@ export async function verifyAndParseRequestTest(
   keyId: string,
 ) {
   const result = await verifyAndParseRequest(rawBody, signature, keyId);
-  expectType<{ isValidRequest: boolean; payload: CopilotRequestPayload }>(
-    result,
-  );
+  expectType<{
+    isValidRequest: boolean;
+    payload: CopilotRequestPayload;
+    cache: { id: string; keys: VerificationPublicKey[] };
+  }>(result);
 }
 
 export function getUserMessageTest(payload: CopilotRequestPayload) {

Diff for: lib/parse.js

@@ -22,16 +22,17 @@ export function transformPayloadForOpenAICompatibility(payload) {
 
 /** @type {import('..').VerifyAndParseRequestInterface} */
 export async function verifyAndParseRequest(body, signature, keyID, options) {
-  const isValidRequest = await verifyRequestByKeyId(
+  const { isValid, cache } = await verifyRequestByKeyId(
     body,
     signature,
     keyID,
     options
   );
 
   return {
-    isValidRequest,
+    isValidRequest: isValid,
     payload: parseRequestBody(body),
+    cache,
   };
 }
 

Diff for: lib/verification.js

@@ -4,7 +4,7 @@ import { createVerify } from "node:crypto";
 
 import { request as defaultRequest } from "@octokit/request";
 
-/** @type {import('..').VerifyRequestByKeyIdInterface} */
+/** @type {import('..').VerifyRequestInterface} */
 export async function verifyRequest(rawBody, signature, key) {
   // verify arguments
   assertValidString(rawBody, "Invalid payload");
@@ -23,51 +23,67 @@ export async function verifyRequest(rawBody, signature, key) {
 
 /** @type {import('..').FetchVerificationKeysInterface} */
 export async function fetchVerificationKeys(
-  { token = "", request = defaultRequest } = { request: defaultRequest }
+  { token = "", request = defaultRequest, cache = { id: "", keys: [] } } = {
+    request: defaultRequest,
+  },
 ) {
-  const { data } = await request("GET /meta/public_keys/copilot_api", {
-    headers: token
+  try {
+    const headers = token
       ? {
           Authorization: `token ${token}`,
         }
-      : {},
-  });
+      : {};
+
+    if (cache.id) headers["if-none-match"] = cache.id;
 
-  return data.public_keys;
+    const response = await request("GET /meta/public_keys/copilot_api", {
+      headers,
+    });
+
+    const cacheId = response.headers.etag || "";
+    return { id: cacheId, keys: response.data.public_keys };
+  } catch (error) {
+    if (error.status === 304) {
+      return cache;
+    }
+
+    throw error;
+  }
 }
 
 /** @type {import('..').VerifyRequestByKeyIdInterface} */
 export async function verifyRequestByKeyId(
   rawBody,
   signature,
   keyId,
-  requestOptions
+  requestOptions,
 ) {
   // verify arguments
   assertValidString(rawBody, "Invalid payload");
   assertValidString(signature, "Invalid signature");
   assertValidString(keyId, "Invalid keyId");
 
   // receive valid public keys from GitHub
-  const keys = await fetchVerificationKeys(requestOptions);
+  const { id, keys } = await fetchVerificationKeys(requestOptions);
 
   // verify provided key Id
   const publicKey = keys.find((key) => key.key_identifier === keyId);
 
   if (!publicKey) {
     const keyNotFoundError = Object.assign(
       new Error(
-        "[@copilot-extensions/preview-sdk] No public key found matching key identifier"
+        "[@copilot-extensions/preview-sdk] No public key found matching key identifier",
       ),
       {
         keyId,
         keys,
-      }
+      },
     );
     throw keyNotFoundError;
   }
 
-  return verifyRequest(rawBody, signature, publicKey.key);
+  const isValid = await verifyRequest(rawBody, signature, publicKey.key);
+  return { isValid, cache: { id: id, keys } };
 }
 
 function assertValidString(value, message) {