Merge pull request #660 from matrix-org/rav/megolm_export_fixes

Two megolm export fixes:

src/utils/MegolmExportEncryption.js

@@ -50,7 +50,7 @@ export function decryptMegolmKeyFile(data, password) {
     }
 
     const ciphertextLength = body.length-(1+16+16+4+32);
-    if (body.length < 0) {
+    if (ciphertextLength < 0) {
         throw new Error('Invalid file: too short');
     }
 
@@ -107,14 +107,14 @@ export function encryptMegolmKeyFile(data, password, options) {
     const salt = new Uint8Array(16);
     window.crypto.getRandomValues(salt);
 
-    // clear bit 63 of the salt to stop us hitting the 64-bit counter boundary
-    // (which would mean we wouldn't be able to decrypt on Android). The loss
-    // of a single bit of salt is a price we have to pay.
-    salt[9] &= 0x7f;
-
     const iv = new Uint8Array(16);
     window.crypto.getRandomValues(iv);
 
+    // clear bit 63 of the IV to stop us hitting the 64-bit counter boundary
+    // (which would mean we wouldn't be able to decrypt on Android). The loss
+    // of a single bit of iv is a price we have to pay.
+    iv[9] &= 0x7f;
+
     return deriveKeys(salt, kdf_rounds, password).then((keys) => {
         const [aes_key, hmac_key] = keys;