Translating C# RSACryptoServiceProvider code to Java

Question

I need to encrypt String for project related purpose and was given the below code for the same by vendor.

public static string EncryptString(string StringToEncrypt)
{
    RSACryptoServiceProvider provider = new RSACryptoServiceProvider();
    string xmlString = "<RSAKeyValue><Modulus>qqoWhMwGrrEBRr92VYud3j+iIEm7652Fs20HvNckH3tRDJIL465TLy7Cil8VYxJre69zwny1aUAPYItybg5pSbSORmP+hMp6Jhs+mg3qRPvHfNIl23zynb4kAi4Mx/yEkGwsa6L946lZKY8f9UjDkLJY7yXevMML1LT+h/a0a38=</Modulus><Exponent>AQAB</Exponent><P>20PwC7nSsfrfA9pzwSOnRYdbhOYivFSuERxvXHvNjCll5XdmFYYp1d2evXcXbyj3E1k8azce1avQ9njH85NMNQ==</P><Q>x0G0lWcQ13NDhEcWbA7R2W5LPUmRqcjQXo8qFIaHk7LZ7ps9fAk/kOxaCR6hvfczgut1xSpXv6rnQ5IGvxaHYw==</Q><DP>lyybF2qSEvYVxvFZt8MeM/jkJ5gIQPLdZJzHRutwx39PastMjfCHbZW0OYsflBuZZjSzTHSfhNBGbXjO22gmNQ==</DP><DQ>NJVLYa4MTL83Tx4vdZ7HlFi99FOI5ESBcKLZWQdTmg+14XkIVcZfBxDIheWWi3pEFsWqk7ij5Ynlc/iCXUVFvw==</DQ><InverseQ>X5Aw9YSQLSfTSXEykTt7QZe6SUA0QwGph3mUae6A2SaSTmIZTcmSUsJwhL7PLNZKbMKSWXfWoemj0EVUpZbZ3Q==</InverseQ><D>jQL4lEUYCGNMUK6GEezIRgiB5vfFg8ql3DjsOcXxnOmBcEeD913kcYnLSBWEUFW55Xp0xW/RXOOHURgnNnRF3Ty5UR73jPN3/8QgMSxV8OXFo3+QvX+KHNHzf2cjKQDVObJTKxHsHKy+L2qjfULA4e+1cSDNn5zIln2ov51Ou3E=</D></RSAKeyValue>";
    provider.FromXmlString(xmlString);
    return Convert.ToBase64String(provider.Encrypt(Encoding.ASCII.GetBytes(StringToEncrypt), false));
}

However I need to modify or translate it to JAVA. I have wrote the below method for the same purpose.

public static String EncryptString(String strToBeEncrypted) throws NoSuchAlgorithmException, InvalidKeySpecException, NoSuchPaddingException, InvalidKeyException, UnsupportedEncodingException, IllegalBlockSizeException, BadPaddingException
{
    String modulusString = "qqoWhMwGrrEBRr92VYud3j+iIEm7652Fs20HvNckH3tRDJIL465TLy7Cil8VYxJre69zwny1aUAPYItybg5pSbSORmP+hMp6Jhs+mg3qRPvHfNIl23zynb4kAi4Mx/yEkGwsa6L946lZKY8f9UjDkLJY7yXevMML1LT+h/a0a38=";
    String publicExponentString = "AQAB";
    byte[] modulusBytes = Base64.decodeBase64(modulusString);
    byte[] exponentBytes = Base64.decodeBase64(publicExponentString);
    BigInteger modulus = new BigInteger(1, modulusBytes);
    BigInteger publicExponent = new BigInteger(1, exponentBytes);
    RSAPublicKeySpec rsaPubKey = new RSAPublicKeySpec(modulus, publicExponent);
    KeyFactory fact = KeyFactory.getInstance("RSA");
    PublicKey pubKey = fact.generatePublic(rsaPubKey);
    Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
    cipher.init(Cipher.ENCRYPT_MODE, pubKey);
    byte[] plainBytes = strToBeEncrypted.getBytes("US-ASCII");
    byte[] cipherData = cipher.doFinal(plainBytes);
    String encryptedStringBase64 = Base64.encodeBase64String(cipherData);

    return encryptedStringBase64;
}

But the sample results do not match.

String is "4111111111111111" and encrypted result should be:

PfU31ai9dSwWX4Im19TlikfO9JetkJbUE+btuvpBuNHTnnfrt4XdM4PmGA19z8rF+lPUC/kcOEXciUSxFrAPyuRJHifIDqWFbbJvPhatbf269BXUiAW31UBX3X5bBOqNWjh4LDitYY0BtarlTU4xzOFyb7vLpLJe9aHGWhzs6q0=

But the result from Java code is

Cxp5AIzTHEkrU6YWwYo5yYvpED2qg9IC/0ct+tRgDZi9fJb8LAk+E1l9ljEt7MFQ2KB/exo4NYwijnBKYPeLStXyfVO1Bj6S76zMeKygAlCtDukq1UhJaJKaCXY94wi9Kel09VTmj+VByIYvAGUFqZGaK1CyLnd8QXMcdcWi3sA=

Answer 1

Every encryption algorithm needs to be randomized in order to provide semantic security. Otherwise, an attacker might notice that you've sent the same message again, just by observing ciphertexts. In symmetric ciphers, this property is achieved by a random IV. In RSA, this is achieved by a randomized padding (PKCS#1 v1.5 type 2 and PKCS#1 v2.x OAEP are randomized).

You can check whether the padding is randomized by running the encryption again with the same key and plaintext, and comparing the ciphertexts to previous ciphertexts. If the ciphertexts change in either C# or Java between executions, then you will not be able to tell whether the encryption is compatible, just by looking at the ciphertexts.

The proper way to check this, would be to encrypt something in one language and then decrypt in the other. For full compatibility, you should also try it the other way around.

Looking at your code, both seem equivalent, because false is passed as the second parameter into RSACryptoServiceProvider#Encrypt to use PKCS#1 v1.5 padding, and Cipher.getInstance("RSA/ECB/PKCS1PADDING") requests the same padding. The input/output encodings also seem equivalent. So, yes this code will be equivalent.

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PKCS#1 v1.5 padding should not be used nowadays, because it is vulnerable against a Bleichenbacher attack (reference). You should use OAEP for encryption and PSS for signing, which are considered secure. C# and Java both support OAEP, but there may be differences in the default hash functions that are used (hash and MGF1).