Convert Java.Security.KeyPair to .NET RSACryptoServiceProvider

Question

How can I encode a Java-produced RSA private (and public) key such that it can be decoded in .NET for use within RSACryptoServiceProvider?

I have tried

Answer 1

C# has a load from xml functionm for RSA Crypto Provider. Try the approach below:

JAVA

                KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
    keyGen.initialize(1024);
    KeyPair keyPair = keyGen.generateKeyPair();
    RSAPrivateKey privateKey = (RSAPrivateKey) keyPair.getPrivate();
    RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();
    BigInteger mod_Int = publicKey.getModulus();
    BigInteger exp_Int = publicKey.getPublicExponent();
    byte[] mod_Bytes_Extra = mod_Int.toByteArray();
    byte[] mod_Bytes = new byte[128];
    System.arraycopy(mod_Bytes_Extra, 1, mod_Bytes, 0, 128);
    byte[] exp_Bytes = exp_Int.toByteArray();
    String modulus = Base64.encodeToString(mod_Bytes, Base64.DEFAULT);
    String exponent = Base64.encodeToString(exp_Bytes, Base64.DEFAULT);
    System.out.println(modulus);
    System.out.println(exponent);
    String public_Xml = "<BitStrength>0124</BitStrength><RSAKeyValue><Modulus>"+modulus+"</Modulus><Exponent>"+exponent+"</Exponent></RSAKeyValue>";

C#

class KeyBuilder
    {
        private RSACryptoServiceProvider rsa;

        public RSACryptoServiceProvider CreateKey(string rsaKeyPair)
        {
            rsa = new RSACryptoServiceProvider();
            StringBuilder output = new StringBuilder();
            string xmlString = "<Key>" + rsaKeyPair + "</Key>";
            rsa.FromXmlString(xmlString);
            return rsa;
        }
    }

Always make you read/write byte arrays with UTF-8 encoding on java side because Java by default uses Unicode encoding whereas C# uses UTF-8.

Also byte arrays in java are signed whereas in C# they are unsigned. So if you have to parse binary data sent from Java code in C# side read it in a sbyte array.

Answer 2

I found the answer in the PvkConvert.java file, which I've rewritten (in the relevant parts) as C# for use in Xamarin.Android apps as PvkConvert.cs.

I learned that RSACryptoServiceProvider's native file format (when you use ExportCspBlob) is called PRIVATEKEYBLOB or PUBLICKEYBLOB (depending on whether the private key is included). Encoding a Java IPrivateKey as a PRIVATEKEYBLOB requires a few fields that evidently are only available on an originally generated IPrivateKey instance (one deserialized from storage does not cast to the necessary IRSAPrivateCrtKey interface.)