AES encryption in Node.js to match expected decryption in Python

Question

I have the following Python script for decryption:

from Crypto.Cipher import AES

shared_secret = raw_input('Enter crypted_shared_secret: ').strip()
cipher = AES.new(shared_secret.decode('base64'), AES.MODE_ECB)

blob = raw_input('Enter crypted_blob: ').strip()
plain = cipher.decrypt(blob.decode('base64'))

print(plain)

I'm trying to generate the values that would produce the original blob using that script, using Node. Here is my attempt:

const Crypto = require('crypto');

var shared_secret = Crypto.randomBytes(32);
var cipher = Crypto.createCipher('aes-256-ecb', shared_secret);
crypted_blob = cipher.update(blob, 'utf8', 'base64') + cipher.final('base64');

I can only modify the Node.js script, but I'm not sure where it's going wrong.

Community · Accepted Answer · 2017-04-13 12:48:18Z

2

You need to encode the shared secret key to Base64 only after you use it for encryption:

var shared_secret = Crypto.randomBytes(32);
var cipher = Crypto.createCipheriv('aes-256-ecb', shared_secret, "");
crypted_blob = cipher.update(blob, 'utf8', 'base64') + cipher.final('base64');
// send `shared_secret.toString('base64')`

Other problems:

crypto.createCipher assumes the shared secret is a password and not a key, which is why it will use a bad key derivation (OpenSSL-compatible).
Node.js' crypto module automatically applies PKCS#7 padding (same as PKCS#5 padding), but PyCrypto doesn't apply any padding on its own. So you either need to use the same unpadding in Python or you can disable padding in node.js with Cipher.setAutoPadding(false);, but then you will have to provide plaintexts that are a multiple of the block size (16 byte for AES).

Security considerations:

Never use ECB mode. It's deterministic and therefore not semantically secure. You should at the very least use a randomized mode like CBC or CTR. It is better to authenticate your ciphertexts so that attacks like a padding oracle attack are not possible. This can be done with authenticated modes like GCM or EAX, or with an encrypt-then-MAC scheme.

edited Apr 13, 2017 at 12:48

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answered Jun 20, 2016 at 9:47

Artjom B.

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3 Comments

wizplum Over a year ago

I still can't get this to work. I have cut the RSA code which was working, the AES is the part that doesn't seem to work. If I open up node and start with blob = 'Hello world' and then run the 4 lines of code, I get e.g. FH1hDHh49uGiWZhNCtg5Dw== as crypted_blob. Then I can run shared_secret.toString('base64') and get e.g. bUQXpYLhZnR+9vKTU8txkRo7O6j7nrks1MJnxI/YIGo=. Then I open up Python, enter the latter as shared_secret and the former as blob, but I still get a scrambled string (wrong decryption).

Artjom B. Over a year ago

Yes, I missed that you've used the wrong method in node.js. See my edit.

wizplum Over a year ago

That worked! Will look into replacing ECB with CBC, thanks for the tip!

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