Question

I want to ensure message privacy between an Android app and C# ASP.NET server, in a situation where HTTPS can’t be assumed to be available.

I want to use RSA to encrypt a symmetric key which is transferred from the Android device when it first contacts the server.

The RSA key pair has been generated on the server and the private key is held on the server.
The key pair was generated in C# using:

// Create a new instance of RSACryptoServiceProvider
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(2048);
// Ensure that the key doesn't get persisted
rsa.PersistKeyInCsp = false;
RSAParameters parameters = rsa.ExportParameters(false);
string modulus = Convert.ToBase64String(parameters.Modulus);
string exponent = Convert.ToBase64String(parameters.Exponent);
string xmlKeys = rsa.ToXmlString(true);

Attempting to embed the public key by hard-coding (copying from Visual Studio to Eclipse) doesn’t work. The code throws an org.bouncycastle.crypto.DataLengthException: input too large for RSA cipher on the rsaCipher.doFinal() method call.

// Generate a new AES key
byte[] key = null;
try {
    KeyGenerator keygen = KeyGenerator.getInstance("AES");
    keygen.init(128);            
    key = keygen.generateKey().getEncoded();
}
catch (NoSuchAlgorithmException e) {}

// Set up modulus and exponent
String mod = "qgx5606ADkXRxndzurIRa5GDxzDYg5Xajeym7I8BXG1HBSzaaGmX+rjQfZK1h4JtQU+Xaowsc81mgJU8+gwneQa56r1bl6/5jFue4FsdXKfpau5az8rY2SAHKcOeyHAOsT9ZqcNa1x6cL/jl9P3cBtOzMk51Hk/w6VNoQ5JJo/0m/eAJzlhVKr2xbOYFhd0xp3qUgRuK8TN4TsSvfc+R1LOWc8+3H22Zj3vhBxSqSgeXxdxi7ThiGiAl6HUwMf8ph7FHNJvoUQq+QPL6dx77pu6xVFiHv1JOfpbKcOubn0VSPLYKY3QPKCzNMYQ6pxUDqzpGtydHR1xaX5K0FGTraw==";

String ex = "AQAB";
BigInteger modulus = new BigInteger(Base64.decode(mod, Base64.DEFAULT));
BigInteger exponent = new BigInteger(Base64.decode(ex, Base64.DEFAULT));

// Encrypt the AES key
PublicKey pubKey;
byte[] cipherData;
try {
    pubKey = KeyFactory.getInstance("RSA").generatePublic(new RSAPublicKeySpe(modulus, exponent));
    Cipher rsaCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");     
    rsaCipher.init(Cipher.ENCRYPT_MODE, pubKey);
    // The following line fails with:
    // org.bouncycastle.crypto.DataLengthException
    cipherData = rsaCipher.doFinal(key);     
}
catch (InvalidKeySpecException e) {}
catch (NoSuchAlgorithmException e) {}
catch (InvalidKeyException e) {}
catch (NoSuchPaddingException e) {}
catch (BadPaddingException e) {}
catch (IllegalBlockSizeException e) {}

I suspect that I have decoded the modulus string incorrectly, because generating the public key in Android encrypts the key successfully. I used this code:

KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");     
kpg.initialize(1024);     
KeyPair kpa = kpg.genKeyPair();     
pubKey = kpa.getPublic();   

So, what have I done wrong?

Answer 1

Try using new BigInteger(1, modulus). BigIntegers are signed and as the modulus starts with the first bit set to 1, it will always be interpreted as a negative number.