Mohsen Afshin Mohsen Afshin - 12 days ago 5
Android Question

Java equivalent of .NET RSACryptoServiceProvider with SHA-1

I have the following data signing code in C#

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();

string PrivateKeyText = "<RSAKeyValue><Modulus>....</D></RSAKeyValue>";

rsa.FromXmlString(PrivateKeyText);

string data = "my data";

byte[] SignedByteData = rsa.SignData(Encoding.UTF8.GetBytes(data), new SHA1CryptoServiceProvider());


and I want reproduce the same code in Java (Android):

String modulusElem = "...";
String expElem = "...";

byte[] expBytes = Base64.decode(expElem, Base64.DEFAULT);
byte[] modulusBytes = Base64.decode(modulusElem, Base64.DEFAULT);

BigInteger modulus = new BigInteger(1, modulusBytes);
BigInteger exponent = new BigInteger(1, expBytes);

try {
KeyFactory factory = KeyFactory.getInstance("RSA");

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");

String data = "my data";

MessageDigest md = MessageDigest.getInstance("SHA-1");
byte[] hashedData = md.digest(data.getBytes("UTF-8"));

RSAPublicKeySpec pubSpec = new RSAPublicKeySpec(modulus, exponent);

PublicKey publicKey = factory.generatePublic(pubSpec);

cipher.init(Cipher.ENCRYPT_MODE, publicKey);

byte[] SignedByteData = cipher.doFinal(hashedData);

} catch (Exception e){

}


But I get mismatched output byte arrays. Where am I wrong and What should be the transformation used in
Cipher.getInstance(...)
?

Answer

Use Signature.getInstance("SHA1withRSA"). Encryption is not the same as signature generation. Different padding mechanisms for one.


Update by Afshin

Complete solution. Note the use of the private exponent, ie <D>, rather the public <Exponent>

String modulusElem = "...";
String dElem = "...";

byte[] modulusBytes = Base64.decode(modulusElem, Base64.DEFAULT);
byte[] dBytes = Base64.decode(dElem, Base64.DEFAULT);

BigInteger modulus = new BigInteger(1, modulusBytes);
BigInteger d = new BigInteger(1, dBytes);

String data = "my data";            

try {
        Signature signature = Signature.getInstance("SHA1withRSA");

        KeyFactory factory = KeyFactory.getInstance("RSA");

        RSAPrivateKeySpec privateKeySpec = new RSAPrivateKeySpec(modulus, d);

        PrivateKey privateKey = factory.generatePrivate(privateKeySpec);

        signature.initSign(privateKey);

        signature.update(data.getBytes("UTF-8"));

        byte[] SignedByteData = signature.sign();

} catch(Exception e) {
    e.printStackTrace();
}
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