In RSA Encrypting working in Java but not working with encrypting from c#?

Question

RSA encryption and decryption work well in java side with modulus and exponent as below:

Java RSA Modulus and Exponent:

String nModulusPublic = "AJ+L/dVL9jnRX6IM87H8x2fR24t6wpzBDV7bcgPWblegR0LNK91z/OSX+lSLUgHSKJ9to/Eo8OMsREpNoJlEzI0=";
String eExponentPublic = "AQAB";
String eExponentPrivate = "AIpmE5C9TiAlgYG/Hn5dOlTS9FFv8fWseX65eZPepOUY4ivxN0lOZ+MsugZd03wmKvnxBuCGu5nv2qrUBTPzjcE=";

Java Public and Private Key Generators:

static PublicKey GetPublicKey(String publicKString, String publicExponentStr) throws Exception {
    byte[] modulusBytes = Base64.getDecoder().decode(publicKString);
    byte[] exponentBytes = Base64.getDecoder().decode(publicExponentStr);
    BigInteger modulus = new BigInteger(1, modulusBytes);
    BigInteger exponent = new BigInteger(1, exponentBytes);

    RSAPublicKeySpec rsaPubKey = new RSAPublicKeySpec(modulus, exponent);
    KeyFactory fact = KeyFactory.getInstance("RSA");
    PublicKey pubKey = fact.generatePublic(rsaPubKey);
    return pubKey;
}

static PrivateKey GetPrivateKey(String nModulusPublic, String eExponentPrivate) throws Exception {
    byte[] modulusBytes = Base64.getDecoder().decode(nModulusPublic);
    byte[] exponentBytes = Base64.getDecoder().decode(eExponentPrivate);
    BigInteger modulus = new BigInteger(1, modulusBytes);
    BigInteger exponent = new BigInteger(1, exponentBytes);

    RSAPrivateKeySpec privSpec = new RSAPrivateKeySpec(modulus, exponent);
    KeyFactory fact = KeyFactory.getInstance("RSA");
    PrivateKey privKey = fact.generatePrivate(privSpec);
    return privKey;
}

I use nModulusPublic and eExponentPublic in c# to encrypting and decrypting in Java but doesn't work. Worked on RSA.Encrypt(textBytes, true); parameters in c# function change it to false and RSAEncryptionPadding.Pkcs1 but doesn't' work. When I use the result of the c# Encrypt function in java to decrypt it always encounter with this error :

javax.crypto.IllegalBlockSizeException: Data must not be longer than 64 bytes

C# encrypt function:

static string Encrypt(string text)
    {
        string outputB64 = string.Empty;
        byte[] textBytes = Encoding.UTF8.GetBytes(text);

        RSAParameters result = new RSAParameters()
        {
            Modulus = Convert.FromBase64String(nModulusPublic),
            Exponent = Convert.FromBase64String(eExponentPublic)
        };

        using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
        {
            RSA.ImportParameters(result);
            byte[] encryptedData = RSA.Encrypt(textBytes, true);
            outputB64 = Convert.ToBase64String(encryptedData);
        }          
        
        return outputB64;
    }

Extra Information, Java Encrypt and Decrypt main functions:

 static String Decrypt(String encodedString,PrivateKey privKey) {
    try {
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.DECRYPT_MODE, privKey);
        byte[] decrypted = cipher.doFinal(Base64.getDecoder().decode(encodedString));
        return new String(decrypted, "UTF-8");
    } catch (Exception err) {
        return err.fillInStackTrace().toString();
    }
}
 static String Encrypt(String encodedString,PublicKey pubKey) {
    try {
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.ENCRYPT_MODE, pubKey);
        byte[] plainBytes = new String(encodedString).getBytes("UTF-8");
        byte[] cipherData = cipher.doFinal(plainBytes);
        String encryptedString = Base64.getEncoder().encodeToString(cipherData);
        return encryptedString;
    } catch (Exception err) {
        return err.fillInStackTrace().toString();
    }
}

Update:

I was working on it and found that java encrypts function and c# have two different types, Java result always ended with "==" but c# function have one "=" at the end, It seems this is the problem.

C# Encrypt function result:

AJiltxqa1/8HU20XZlKJsJvclQ8PyQetpWdbCOpbqrXVg0q
/v4x5tXLxbzGKbO5InvKkib7tDQp+9BU0SYbZLv0=

Java Encrypt function result:

RlarFQBo2mcCWjidQ5l7ho2EOG6KGQWpR3ByXXHsGo6+HRQzmO4v7
TUTMdfB9wjI3aO6quruSReitrWu7QF9Vw==

Michael Fehr · Accepted Answer · 2020-08-29 19:46:22Z

2

On C# encrypt function you give the parameter 'true':

byte[] encryptedData = RSA.Encrypt(textBytes, true);

This means that C# is NOT using the PKCS1Padding but the OEAPPadding.

Simply change on Java-side in your Decrypt-method (and in your ENCRYPT-method as well :-) the line

// change:
//Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
// to
Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-1ANDMGF1PADDING");

I tested it with your keypair and it works like expected.

Edit: On C#-side I run this code:

string plaintext = "The quick brown fox";
string encryptedDataBase64So = Encrypt(plaintext);
Console.WriteLine("encrypted SO : " + encryptedDataBase64So);

Console output:
encrypted SO : ZLylMsqcqbuDM7DprrmqIU8c8Q79fPXHudOY4INCNAo+iU7Oor3mZ8i+PP5PjtDkifqAXKYT8ON/ia9WjEFqRQ==

On Java-side I took the base64-String as input:

String fromCsharp = "Ew3nTEQuOX1tWfRNJEERa75A1o2bn6+HurVPYzGzA7kt+HAZAMdXKNACY2emvU6Bf42i8zpBO89lqvzuxNmRIw==";
String decryptedtext = DecryptWorking(fromCsharp, privateKey);
System.out.println("\ndecrypted from c#: " + decryptedtext);

Console output:
decrypted from c#: The quick brown fox

BTW: this is the PublicKey on C#-side that I generated from the PublicKey on Java-side and used it like this as source for the RSA-Encryption:

var publicKey = "<RSAKeyValue><Modulus>n4v91Uv2OdFfogzzsfzHZ9Hbi3rCnMENXttyA9ZuV6BHQs0r3XP85Jf6VItSAdIon22j8Sjw4yxESk2gmUTMjQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";
RSA.FromXmlString(publicKey);

edited Aug 29, 2020 at 19:46

answered Aug 29, 2020 at 19:09

Michael Fehr

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4 Comments

Davood Over a year ago

Tested but doesn't work, Can you please share me output of c# encrypt function.

Michael Fehr Over a year ago

@motevalizadeh kindly see my edited answer with test data.

Davood Over a year ago

Thanks for your time but I could not handle it, there is something in your codes that I have not If it's possible to share your full source code on Java side, Did you used my Java function to create public and private keys?

Michael Fehr Over a year ago

I used your keys and your GetPrivate/PublicKey methods to encrypt and decrypt on Java side. Only on C#-side I changed the code to get the PublicKey loaded to RSA, as I rebuild your PublicKey to a XML-string (the onliner is shown above. To get it working you need to change the Java-padding as well.

Topaco · Accepted Answer · 2020-08-30 20:11:13Z

Besides the padding error described in detail in Michael Fehr's answer, there is another issue which is actually responsible for the error message IllegalBlockSizeException: Data must not be longer than 64 bytes. The inconsistent padding would throw a BadPaddingException if the IllegalBlockSizeException would not be thrown first.

The posted ciphertext has a leading 0 byte and is therefore 65 bytes in size:

Base64: AJiltxqa1/8HU20XZlKJsJvclQ8PyQetpWdbCOpbqrXVg0q/v4x5tXLxbzGKbO5InvKkib7tDQp+9BU0SYbZLv0=
Hex:    0098a5b71a9ad7ff07536d17665289b09bdc950f0fc907ada5675b08ea5baab5d5834abfbf8c79b572f16f318a6cee489ef2a489beed0d0a7ef415344986d92efd

If you try to decrypt this ciphertext on the Java side, you will get the posted error: IllegalBlockSizeException: Data must not be longer than 64 bytes.

Why does the C# code produce a too long ciphertext? This is because of the modulus, which also has a leading 0 byte and therefore a length of 65 bytes:

Base64: AJ+L/dVL9jnRX6IM87H8x2fR24t6wpzBDV7bcgPWblegR0LNK91z/OSX+lSLUgHSKJ9to/Eo8OMsREpNoJlEzI0=
Hex:    009f8bfdd54bf639d15fa20cf3b1fcc767d1db8b7ac29cc10d5edb7203d66e57a04742cd2bdd73fce497fa548b5201d2289f6da3f128f0e32c444a4da09944cc8d

The modulus was derived with BigInteger.toByteArray() (see this question, Update section), which returns the two's-complement representation and places a leading 0 byte in front if the frontmost byte has a value larger than 0x7f.

The leading 0 byte in the modulus results in a ciphertext generated by the C# code, which also has a leading 0 byte and thus an invalid length of 65 bytes. This does not make much sense and might be a bug.

To solve the problem the 0 byte in the modulus should be removed for the C# code, resulting in the following Base64 encoded modulus (which will produce 64 bytes ciphertexts):

n4v91Uv2OdFfogzzsfzHZ9Hbi3rCnMENXttyA9ZuV6BHQs0r3XP85Jf6VItSAdIon22j8Sjw4yxESk2gmUTMjQ==

Alternatively the 0 byte in the ciphertext can be removed, resulting in the following ciphertext (Base64 encoded):

mKW3GprX/wdTbRdmUomwm9yVDw/JB62lZ1sI6luqtdWDSr+/jHm1cvFvMYps7kie8qSJvu0NCn70FTRJhtku/Q==

which can now be successfully decrypted by the Java code to the plaintext (if a consistent padding is applied, see Michael Fehr's answer):

Davood

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