PHP - Decrypt Encrypted String From Node.js

Question

I want to decrypt a encrypted string (encrypted in Nodejs) using PHP passed to the server.

I have found the perfect Nodejs encrypt/decrypt library: Cryptr. I've created a connection in my JavaScript file sending a request to my server with the encrypted string in it.

Now basically I want to decrypt that string.

Taking a look at the Cryptr source, it seems they're using aes-256-ctr as algo method, and sha256 as encryption method.

My Nodejs: https://runkit.com/embed/keu82yjhwyxj

Encrypted string: 1d510024ad0a5da624b76a2be72022bff3aaadfe8ac5e0b6c178b00333

Then I do this in PHP:

<?php
$encrypted = "1d510024ad0a5da624b76a2be72022bff3aaadfe8ac5e0b6c178b00333";
$algorithm = "aes-256-ctr";
$secret_key = "myTotalySecretKey"; 
$iv = "";

$decrypted_data = openssl_decrypt(pack('H*', $encrypted), $algorithm, $secret_key, OPENSSL_RAW_DATA, $iv);
echo $decrypted_data;

But since the IV is randomly generated in Cryptr, how would I generate this in PHP?

How can I decrypt Cryptr encrypted strings using PHP, so it returns "I love pizza!"?

Edit:

Instead pack('H*', $encrypted) try with $encrypted only. Also, you need only data, method and key for decryption.

<?php
$encrypted = "1d510024ad0a5da624b76a2be72022bff3aaadfe8ac5e0b6c178b00333";
$algorithm = "aes-256-ctr";
$secret_key = "myTotalySecretKey";

$decrypted_data = openssl_decrypt($encrypted, $algorithm, $secret_key);
echo $decrypted_data;

@SilvioCro It does not work. The problem: I'm not sure how I can decrypt strings in PHP generated by the Cryptr dependency for Nodejs. — Appel Flap
– Appel Flap, Commented Feb 5, 2019 at 19:34

score 2 · Accepted Answer · 2019-02-06 00:25:54Z

2

DO NOT USE CRYPTR. It is insecure. I have opened an issue with the developer, although I'm not sure how it can be fixed without a complete rewrite of the module.

Cryptr uses the CTR encryption mode. This mode is designed for specific use cases, and is not resistant to malleability attacks. If the contents of any encrypted message are known, it is possible to transform that message into any other message. For example, given the encrypted string from the usage sample:

const cryptr = new Cryptr('myTotalySecretKey');

const encryptedString = cryptr.encrypt('bacon');
const decryptedString = cryptr.decrypt(encryptedString);

console.log(encryptedString); // "bcb23b81c4839d06644792878e569de4f251f08007"

(Note that the encrypted string isn't even the same length as what is shown in the module usage. This is an apparent error in their documentation.)

Without knowledge of the key, it is possible to modify this string to make it decrypt to "hello":

var tmp = Buffer.from(encryptedString, "hex");
var b1 = Buffer.from("bacon"), b2 = Buffer.from("hello");
for (var i = 0; i < b1.length; i++) {
    tmp[i + 16] ^= b1[i] ^ b2[i];
}
var ep = tmp.toString("hex");
console.log(ep); // "bcb23b81c4839d06644792878e569de4f855ff8306"

And indeed:

var dp = cryptr.decrypt(ep);
console.log(dp); // "hello"

This is a really big deal from a cryptographic perspective. An attacker has just modified an encrypted message in transit, and you have no way of detecting it.

Don't use this module. If you need portable encryption, use the Sodium library; there are bindings available for both Node and PHP.

edited Feb 6, 2019 at 0:25

answered Feb 5, 2019 at 20:01

user149341

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6 Comments

Appel Flap Over a year ago

I see, thanks for warning me. The encrypted data is basically a username which is required in the API, so the API can't be exploited. I've been looking for a good solution, but unable to find one. The library you mentioned, can I pass encrypted info from Nodejs and decrypt it in PHP?

user149341 Over a year ago

If you are implementing an API that runs over HTTP, you should use HTTPS (and only HTTPS) to protect communications. There's no reason to use anything else on top of that.

Appel Flap Over a year ago

So sending a GET request api.php?username=Test&removePoints=100 (which removes user balance) is 100% safe over HTTPS?

Pararera Over a year ago

No, POST request

Appel Flap Over a year ago

Got it. But if others have the API url, they can possibly send a fake POST request, and have the API, for example, remove points from other users, correct?

|

Pararera · Accepted Answer · 2019-02-05 20:19:22Z

0

You need to pass IV as you pass message and security key.

So I did little test

$encrypted = openssl_encrypt("test", "aes-256-ctr", "123", 0, "aaaaaaaaaaaaaaaa");
$algorithm = "aes-256-ctr";
$secret_key = "123"; 
$iv = "";

$decrypted_data = openssl_decrypt($encrypted, $algorithm, $secret_key, 0, "aaaaaaaaaaaaaaaa");
echo $decrypted_data;

And it Works. Yes, it's randomly generated, but you can and have to pass it(as encrypted message) to the server.

I'm not sure what is the purpose of IV parameter but PHP gives you warning if IV param is empty.

I just found this on GitHub page for Cryptr. It says:

The iv is randomly generated and prepended to the result

edited Feb 5, 2019 at 20:19

answered Feb 5, 2019 at 19:31

Pararera

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6 Comments

Appel Flap Over a year ago

I've edited my question, and have added the edited PHP code based on this answer. It does not seem to decrypt the string.

Appel Flap Over a year ago

Thanks for the example. I still can't really get it to work with the example in my question for some reason. Would you be able to give it a try with the code in the question?

Pararera Over a year ago

Where is the code that sends encrypted message and key from client to server?

Pararera Over a year ago

Without knowing IV when encrypted messaged is generated you can't decrypt it.

Appel Flap Over a year ago

That basically answers my question. It seems my code and this crypto library is not suitable for what I want to achieve.

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