Why RSA encryption can return different results with C# and Java?

Question

I using:

• c#: RSACryptoServiceProvider
• JAVA: KeyFactory.getInstance("RSA")+Cipher

I sending public key (exponent + modulus) as byte array from java to c#. It's ok, there is the same bytes. But when i try to encrypt some data with one key in Java and c# - there is different results.

Java Key Generation:

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize( Config.CRYPTO_KEY_NUM_BITS );

m_KeyPair = keyGen.genKeyPair();

m_PublicKey = KeyFactory.getInstance("RSA").generatePublic(
 newX509EncodedKeySpec(m_KeyPair.getPublic().getEncoded()));

byte[] exponent = m_PublicKey.getPublicExponent().toByteArray();
byte[] modulus  = m_PublicKey.getModulus().toByteArray(); // then sending...

C# Key Recieve:

// Recieved...
m_ExternKey = new RSAParameters();
m_ExternKey.Exponent    = exponent;
m_ExternKey.Modulus     = modulus;

m_RsaExtern = new RSACryptoServiceProvider();
m_RsaExtern.ImportParameters(m_ExternKey);

byte[] test = m_RsaExtern.Encrypt(bytesToEncrypt, true);

and problem is that encrypted bytes is different.

Thank you.

Answer 1

RSA Encription mustn't return diffferent values with simular keys - its standardized algorithm. Check your keys.

Answer 2

RSA Parameters contains more parameters than modulus and exponent if i remember correctly. You need fully initialized rsa parameters to get the encryption correct (in .net).

Moreover, your private and private key is not even set in .net

Answer 3

RSA encryption is randomized. For a given public key and a given message, each attempt at encryption yields a distinct sequence of bytes. This is normal and expected; random bytes are injected as part of the padding phase, and not injecting random bytes would result in a weak encryption system. During decryption, the padding bytes are located and removed, and the original message is recovered unscathed.

Hence it is expected that you will get distinct encrypted messages with Java and C#, but also if you run your Java or C# code twice.