Padding error when using RSA Encryption in C# and Decryption in Java

Question

Currently I am receiving the following error when using Java to decrypt a Base64 encoded RSA encrypted string that was made in C#:

javax.crypto.BadPaddingException: Not PKCS#1 block type 2 or Zero padding

The setup process between the exchange from .NET and Java is done by creating a private key in the .NET key store then from the PEM file extracted, created use keytool to create a JKS version with the private key. Java loads the already created JKS and decodes the Base64 string into a byte array and then uses the private key to decrypt.

Here is the code that I have in C# that creates the encrypted string:

public string Encrypt(string value) {
    byte[] baIn = null;
    byte[] baRet = null;
    string keyContainerName = "test";

    CspParameters cp = new CspParameters();
    cp.Flags = CspProviderFlags.UseMachineKeyStore;
    cp.KeyContainerName = keyContainerName;
    RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(cp);

    // Convert the input string to a byte array 
    baIn = UnicodeEncoding.Unicode.GetBytes(value);

    // Encrypt
    baRet = rsa.Encrypt(baIn, false);

    // Convert the encrypted byte array to a base64 string
    return Convert.ToBase64String(baRet);
}

Here is the code that I have in Java that decrypts the inputted string:

public void decrypt(String base64String) {
    String keyStorePath = "C:\Key.keystore";
    String storepass = "1234";
    String keypass = "abcd";
    byte[] data = Base64.decode(base64String);
    byte[] cipherData = null;

    keystore = KeyStore.getInstance("JKS");
    keystore.load(new FileInputStream(keyStorePath), storepass.toCharArray());

    RSAPrivateKey privateRSAKey = (RSAPrivateKey) keystore.getKey(alias, keypass.toCharArray());

    Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
    cipher.init(Cipher.DECRYPT_MODE, privateRSAKey);
    cipherData = cipher.doFinal(data);

    System.out.println(new String(cipherData));
}

Does anyone see a step missing or where the padding or item needs to be changed? I have done hours of reading on this site and others but haven't really found a concrete solution.

You're help is vastly appreciated.

Thanks. -Matt

Answer 1

I had exactely the same problem and I finally find the solution!

I was stubborn using PKCS1Padding but I didn't manage to make it work.

The best result I got using "rsa.Encrypt(baIn, false)" on the C# side and "RSA/NONE/NoPadding" on the Java side was this kind of string : "☻?o+_>??5?l0Q*???*?R▲???♀7..." followed by my decrypted string. So in a way it got decrypted but since there is no padding specified, the data is shifted. So I tried all the paddings available in bouncycastle but I would alway get errors such as "block incorrect size" or "data hash wrong".

So I decided to start trying OAEP paddings and I finally managed to get it working by using "rsa.Encrypt(baIn, true)" on the C# side and "RSA/NONE/OAEPWithSHA1AndMGF1Padding" on the java side!

It worked for me, I hope it will work for you too! If it doesn't work make sure you're using the right key, very often the problem comes from the key.