Move out crypto/aes

src/utils/decryptAES.ts

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2024 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { decodeBase64 } from "../base64.ts";
+import { deriveKeys } from "../rust-crypto/deriveKeys.ts";
+import { SecretEncryptedPayload } from "./@types/SecretEncryptedPayload.ts";
+
+/**
+ * Decrypt an AES-encrypted string.
+ *
+ * @param data - the encrypted data, returned by {@link encryptAES}.
+ * @param key - the encryption key to use as an input to the HKDF function which is used to derive the AES key. Must
+ *    be the same as provided to {@link encryptAES}.
+ * @param name - the name of the secret. Also used as an input to the HKDF operation which is used to derive the AES
+ *    key, so again must be the same as provided to {@link encryptAES}.
+ */
+export async function decryptAES(data: SecretEncryptedPayload, key: Uint8Array, name: string): Promise<string> {
+    const [aesKey, hmacKey] = await deriveKeys(key, name);
+
+    const ciphertext = decodeBase64(data.ciphertext);
+
+    if (!(await globalThis.crypto.subtle.verify({ name: "HMAC" }, hmacKey, decodeBase64(data.mac), ciphertext))) {
+        throw new Error(`Error decrypting secret ${name}: bad MAC`);
+    }
+
+    const plaintext = await globalThis.crypto.subtle.decrypt(
+        {
+            name: "AES-CTR",
+            counter: decodeBase64(data.iv),
+            length: 64,
+        },
+        aesKey,
+        ciphertext,
+    );
+
+    return new TextDecoder().decode(new Uint8Array(plaintext));
+}