Implement Java Decryption AES Algorithm in C#

Question

We are implementing some kind of auto sign on in our application and got struck at decrypting something in C#, that was encrypted in java.

Basically, a Java app generates some kind of URL. When the users clicks on the link, I need to validate the query strings and if they match, let the user sign in.

Someone provided the java code. I need to convert the same code to C# because my app is in C#. I get errors when I implement it exactly in C#.

Here is the java decryption code :

String vParameter= "ksyR31QsRcbeJoysNOsAGBHajLKWsT00wavt9LJYGOMRC8zc_vqrNOeOlGHKJHIt3sLmFhDVw_JZKr4JT0H3Jj7_Di9bKNw99qCzMOKCXYM=";  //The string that nees to be decoded.
byte[] encryptedV = Base64.decodeBase64(vParameter);
String salt = “jkjkyt4”; // the i parameter - user’s id
String password = “^hjkh673!v@!a89mz+%5rT”; // application specific
MessageDigest digester = MessageDigest.getInstance("SHA-1");
digester.update((salt + password).getBytes("UTF-8"));
byte[] key = digester.digest();
SecretKeySpec secretKey = new SecretKeySpec(key, 2, 16, “AES”);
String appIV = "SampleIV"// application specific
IvParameterSpec iv= new IvParameterSpec(appIV.getBytes(“UTF-8”));
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, secretKey, iv);
byte[] decryptedV = cipher.doFinal(encryptedV, 0, encryptedV.length);
String v = new String(decryptedV, “UTF-8”);

Here is the corresponding C# code

        string vParameter = "ksyR31QsRcbeJoysNOsAGBHajLKWsT00wavt9LJYGOMRC8zc_vqrNOeOlGHKJHIt3sLmFhDVw_JZKr4JT0H3Jj7_Di9bKNw99qCzMOKCXYM="; //v parameter
        byte[] encryptedV = Encoding.UTF8.GetBytes(vParameter);
        String salt = "jkjkyt4"; // the i parameter - user’s id
        String password = "^hjkh673!v@!a89mz+%5rT"; // application specific
        var sha1 = SHA1Managed.Create();

        byte[] keyBytes = Encoding.UTF8.GetBytes(salt + password); //salt + password
        byte[] key = sha1.ComputeHash(keyBytes);
        byte[] finalKey = { 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8 };
        String appIV = "SampleIV";
        byte[] iv = Encoding.UTF8.GetBytes(appIV); //iv
        Array.Copy(key, 2, finalKey, 0, 16); //key 2, 16
        AesManaged tdes = new AesManaged();
        tdes.Key = finalKey;
        tdes.Mode = CipherMode.CBC;
        tdes.Padding = PaddingMode.PKCS7;
        tdes.IV = iv;
        ICryptoTransform crypt = tdes.CreateDecryptor();
        byte[] cipher = crypt.TransformFinalBlock(encryptedV, 0, encryptedV.Length);
        string decryptedText = Convert.ToBase64String(cipher);
        return decryptedText;

What am I doing wrong? Can anyone point out the mistake?

EDIT : I've update the V Parameter... Note - The keys, password and IV are not real. I had to change them since I didn't want my companies keys to be public.

EDIT 2 : Hi, I've updated the vParameter.. Now they are the same. The Java code is working... It's been implemented in another app. Now, I have to create a similar version for my C# app. Can you guys point out any issues in C# code?

There are at least two mistakes in the Java code: 1) the IV is not 16 bytes (let alone random bytes) and 2) the cipher text is directly converted into a String, and UTF-8 encoding that does not encode a valid character is lost. — Maarten Bodewes
– Maarten Bodewes, Commented Dec 17, 2012 at 23:07
Instead of Encoding.UTF8.GetBytes() try, Convert.FromBase64String() for the first Base64.decodeBase64() anyway — Dominic Zukiewicz
– Dominic Zukiewicz, Commented Dec 17, 2012 at 23:08
Why are the vParameter codes different, if the password and IV isn't? You would expect a different plain text, what use is that? — Maarten Bodewes
– Maarten Bodewes, Commented Dec 17, 2012 at 23:08

Rasmus Faber · Accepted Answer · 2012-12-18 12:44:13Z

3

You are mixing up the UTF8 en/decodings and the base64 en/decodings. Also the code as posted does not work, since the ciphertext and the IV have the wrong lengths. I suspect that is just because you changed them to avoid revealing your real data.

Anyway, here is a Java-snippet and a C#-snippet that returns the same result:

Java (I only changed vParameter and appIV into something that works):

String vParameter= "Lq4aURUiyvKvEZBWMWpUr2wRSMu96E+J1UeHLTOhKEM=";  //The string that needs to be decoded.
byte[] encryptedV = Base64.decodeBase64(vParameter.getBytes("ASCII"));
String salt = "jkjkyt4"; // the i parameter - user’s id
String password = "^hjkh673!v@!a89mz+%5rT"; // application specific
MessageDigest digester = MessageDigest.getInstance("SHA-1");
digester.update((salt + password).getBytes("UTF-8"));
byte[] key = digester.digest();
SecretKeySpec secretKey = new SecretKeySpec(key, 2, 16, "AES");
String appIV = "SampleIV12345678";// application specific
IvParameterSpec iv= new IvParameterSpec(appIV.getBytes("UTF-8"));
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, secretKey, iv);
byte[] decryptedV = cipher.doFinal(encryptedV, 0, encryptedV.length);
String v = new String(decryptedV, "UTF-8");
System.out.println(v); // foobarfoobarfoobarfoobarfoobar

C# (decode vParameter using Base64 and decode the decrypted data as UTF-8. Also rename the AES object to aes instead of tdes.):

string vParameter = "Lq4aURUiyvKvEZBWMWpUr2wRSMu96E+J1UeHLTOhKEM="; //v parameter
byte[] encryptedV = Convert.FromBase64String(vParameter);
string salt = "jkjkyt4"; // the i parameter - user’s id
string password = "^hjkh673!v@!a89mz+%5rT"; // application specific
var sha1 = SHA1Managed.Create();
byte[] keyBytes = Encoding.UTF8.GetBytes(salt + password); //salt + password
byte[] key = sha1.ComputeHash(keyBytes);
byte[] finalKey = { 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8 };
string appIV = "SampleIV12345678";
byte[] iv = Encoding.UTF8.GetBytes(appIV); //iv
Array.Copy(key, 2, finalKey, 0, 16); //key 2, 16
AesManaged aes = new AesManaged();
aes.Key = finalKey;
aes.Mode = CipherMode.CBC;
aes.Padding = PaddingMode.PKCS7;
aes.IV = iv;
ICryptoTransform crypt = aes.CreateDecryptor();
byte[] cipher = crypt.TransformFinalBlock(encryptedV, 0, encryptedV.Length);
string decryptedText = Encoding.UTF8.GetString(cipher);
Console.WriteLine(decryptedText); // foobarfoobarfoobarfoobarfoobar

answered Dec 18, 2012 at 12:44

Rasmus Faber

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3 Comments

Preetham Reddy Over a year ago

Hi Rasmus Faber, Thank you for the help. I was able to get further along but I get the error message 'Padding is invalid and cannot be removed.' on the crypt.TransformFinalBlock function...

Rasmus Faber Over a year ago

@InvisibleDev: Did you get the padding error on the exact code I wrote above or when you substitute your own values?

Preetham Reddy Over a year ago

Hi Rasmus, It works now. The IV and Key was wrong. When I replaced it with the correct values, it works now. Thanks a lot for your help on this.

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