decrypt attachment code

src/matrix/e2ee/attachment.js

@@ -0,0 +1,58 @@
+/*
+Copyright 2020 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 base64 from "../../../lib/base64-arraybuffer/index.js";
+
+/**
+ * Decrypt an attachment.
+ * @param {ArrayBuffer} ciphertextBuffer The encrypted attachment data buffer.
+ * @param {Object} info The information needed to decrypt the attachment.
+ * @param {Object} info.key AES-CTR JWK key object.
+ * @param {string} info.iv Base64 encoded 16 byte AES-CTR IV.
+ * @param {string} info.hashes.sha256 Base64 encoded SHA-256 hash of the ciphertext.
+ * @return {Promise} A promise that resolves with an ArrayBuffer when the attachment is decrypted.
+ */
+export async function decryptAttachment(cryptoDriver, ciphertextBuffer, info) {
+    if (info === undefined || info.key === undefined || info.iv === undefined
+        || info.hashes === undefined || info.hashes.sha256 === undefined) {
+       throw new Error("Invalid info. Missing info.key, info.iv or info.hashes.sha256 key");
+    }
+
+    var ivArray = base64.decode(info.iv);
+    // re-encode to not deal with padded vs unpadded
+    var expectedSha256base64 = base64.encode(base64.decode(info.hashes.sha256));
+    // Check the sha256 hash
+    const digestResult = await cryptoDriver.digest("SHA-256", ciphertextBuffer);
+    if (base64.encode(new Uint8Array(digestResult)) != expectedSha256base64) {
+        throw new Error("Mismatched SHA-256 digest");
+    }
+    var counterLength;
+    if (info.v == "v1" || info.v == "v2") {
+        // Version 1 and 2 use a 64 bit counter.
+        counterLength = 64;
+    } else {
+        // Version 0 uses a 128 bit counter.
+        counterLength = 128;
+    }
+
+    const decryptedBuffer = await cryptoDriver.aes.decryptCTR({
+        jwkKey: info.key,
+        iv: ivArray,
+        data: ciphertextBuffer,
+        counterLength
+    });
+    return decryptedBuffer;
+}