perf: Stop validating JSON for stored state (#2845)

Removes some validation checks when storing and retrieving state from
`snap_manageState`. Since we have input sanitization on the Snap side,
we can improve performance significantly when retrieving state by
parsing it without additional validation.

Also improves performance slightly by not using `isVaultUpdated` as the
sole check for caching (as that function can be somewhat expensive to
call).

packages/snaps-controllers/coverage.json

@@ -1,6 +1,6 @@
 {
-  "branches": 92.67,
+  "branches": 92.73,
   "functions": 96.65,
-  "lines": 97.98,
-  "statements": 97.68
+  "lines": 97.99,
+  "statements": 97.69
 }

packages/snaps-controllers/src/snaps/SnapController.ts

@@ -87,7 +87,6 @@ import {
   NpmSnapFileNames,
   OnNameLookupResponseStruct,
   getLocalizedSnapManifest,
-  parseJson,
   MAX_FILE_SIZE,
 } from '@metamask/snaps-utils';
 import type { Json, NonEmptyArray, SemVerRange } from '@metamask/utils';
@@ -101,7 +100,6 @@ import {
   hasProperty,
   inMilliseconds,
   isNonEmptyArray,
-  isValidJson,
   isValidSemVerRange,
   satisfiesVersionRange,
   timeSince,
@@ -1749,6 +1747,17 @@ export class SnapController extends BaseController<
     return { key: encryptionKey, salt };
   }
 
+  /**
+   * Check if a given Snap has a cached encryption key stored in the runtime.
+   *
+   * @param snapId - The Snap ID.
+   * @returns True if the Snap has a cached encryption key, otherwise false.
+   */
+  #hasCachedEncryptionKey(snapId: SnapId) {
+    const runtime = this.#getRuntimeExpect(snapId);
+    return runtime.encryptionKey !== null && runtime.encryptionSalt !== null;
+  }
+
   /**
    * Decrypt the encrypted state for a given Snap.
    *
@@ -1759,9 +1768,15 @@ export class SnapController extends BaseController<
    */
   async #decryptSnapState(snapId: SnapId, state: string) {
     try {
-      const parsed = parseJson<EncryptionResult>(state);
+      // We assume that the state string here is valid JSON since we control serialization.
+      // This lets us skip JSON validation.
+      const parsed = JSON.parse(state) as EncryptionResult;
       const { salt, keyMetadata } = parsed;
-      const useCache = this.#encryptor.isVaultUpdated(state);
+
+      // We only cache encryption keys if they are already cached or if the encryption key is using the latest key derivation params.
+      const useCache =
+        this.#hasCachedEncryptionKey(snapId) ||
+        this.#encryptor.isVaultUpdated(state);
       const { key } = await this.#getSnapEncryptionKey({
         snapId,
         salt,
@@ -1772,8 +1787,7 @@ export class SnapController extends BaseController<
       });
       const decryptedState = await this.#encryptor.decryptWithKey(key, parsed);
 
-      assert(isValidJson(decryptedState));
-
+      // We assume this to be valid JSON, since all RPC requests from a Snap are validated and sanitized.
       return decryptedState as Record<string, Json>;
     } catch {
       throw rpcErrors.internal({
@@ -1864,7 +1878,8 @@ export class SnapController extends BaseController<
     }
 
     if (!encrypted) {
-      return parseJson(state);
+      // For performance reasons, we do not validate that the state is JSON, since we control serialization.
+      return JSON.parse(state);
     }
 
     const decrypted = await this.#decryptSnapState(snapId, state);