Trying to decrypt AES, bad decrypt

[Thijs Koerselman]

Hi,

I'm trying to encrypt some string in C#, send it to a Node server and decrypt it there. I am having a hard time because I keep getting this error:

TypeError: error:06065064:digital envelope routines:EVP_DecryptFinal_ex:bad decrypt

    at Decipher.Cipher.final (crypto.js:257:27)

I have tried different AES modes and key sizes but it didn't help. Just for testing I'm sending a json post to the server containing base64 encoded versions of key, iv and the encrypted string (fingerprint in this example).

In the post request handler I do this:

        var crypto = require('crypto');

var key = req.body.key;

var iv = req.body.iv;

var binkey = new Buffer(key, 'base64');

var biniv = new Buffer(iv, 'base64');

var crypted = req.body.fingerprint;

var bincrypted = new Buffer(crypted, 'base64');

var decipher = crypto.createDecipher('aes-256-cbc',binkey, biniv);

var dec = decipher.update(bincrypted,'binary','utf8');

dec += decipher.final('utf8');

console.log("dec", dec);

To be sure I'm using the same Aes mode and padding on the encryption this is what's reported by my C# Aes, including the JSON being sent.

Keysize 256

Mode CBC

Padding PKCS7

Key: YJAG4xYTTQ0Ke3FBIDgmobERgbi/Tl/LYt9cNmt5w0g=

IV :VUdInSdIlCLS/D3AbXhhnQ==

json: {

   "fingerprint" : "a7pNFC3Bnac7Y/k7/b+b4jHdH5CE/nbu23Mmj9pAhZw=",

   "key" : "YJAG4xYTTQ0Ke3FBIDgmobERgbi/Tl/LYt9cNmt5w0g=",

   "iv" : "VUdInSdIlCLS/D3AbXhhnQ=="

}

Am I overlooking something maybe? 

[Kelsey Dawes]

Hi,

>> var decipher = crypto.createDecipher('aes-256-cbc',binkey, biniv);

I think you want crypto.createDecipheriv here, instead of crypto.createDecipher.

crypto.createDecipheriv(algorithm, key, iv)

creates and returns a decipher object, with the given algorithm, key and iv. This is the mirror of thecreateCipheriv() above

-Kelsey

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[Thijs Koerselman]

On Thu, Jun 27, 2013 at 4:13 PM, Kelsey Dawes <kda...@gmail.com> wrote:

I think you want crypto.createDecipheriv here, instead of crypto.createDecipher.

Ouch! I must have read over that about 30 times. I was assuming the two flavors were based on the number of arguments you supplied to the same create function. Thanks for pointing it out. It's working now!

I would prefer to use a password to derive the key from that, and append IV before the data. That way I can just have one shared password between my apps. How do I know in what way Node crypto derives that key from the password so that I can match it in C#? Or the other way around?