System.Security.Cryptography.CryptographicException : Bad length in RSACryptoserviceProvider

Question

I want encrypt and decrypt data using RSACryptoServiceProvider in c# in wp8 project. I am creating asymmetric keys as :

CspParameters parameters = new CspParameters();
parameters.KeyContainerName = "MyContainer";

RSACryptoServiceProvider provider = new RSACryptoServiceProvider(parameters);  

Now I want do encrypt data. I am doing:

CspParameters parameters = new CspParameters();

parameters.KeyContainerName = "MyContainer";
RSACryptoServiceProvider obj = new RSACryptoServiceProvider(parameters);
byte[] a = Generic.RSAEncrypt(ByteConverter.GetBytes(s[0]),
                              obj.ExportParameters(false), false); 

public static byte[] RSAEncrypt(byte[] DataToEncrypt, RSAParameters RSAKeyInfo,
                                bool DoOAEPPadding)
{
    try {
        byte[] encryptedData;
        //Create a new instance of RSACryptoServiceProvider. 
        CspParameters parameters = new CspParameters();
        parameters.KeyContainerName = "TCSContainer";
        using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider(parameters))
        {
            //Import the RSA Key information. This only needs 
            //to include the public key information.

            RSA.ImportParameters(RSAKeyInfo);

            //Encrypt the passed byte array and specify OAEP padding.   
            //OAEP padding is only available on Microsoft Windows XP or 
            //later.  
            encryptedData = RSA.Encrypt(DataToEncrypt, DoOAEPPadding);
        }
        return encryptedData;
    } catch (CryptographicException e) {
        //Catch and display a CryptographicException   
        //to the console. 
        //Console.WriteLine(e.Message);
        return null;
    }
}

Now I am getting exception while encypting:

RSA.EncryptSystem.Security.Cryptography.CryptographicException : Bad length in RSACryptoserviceProvider. 

Stacktrace is:

at System.Security.Cryptography.CryptographicException.ThrowCryptographicException(Int32 hr)
at System.Security.Cryptography.RSACryptoServiceProvider.EncryptKey(SafeKeyHandle pKeyContext, Byte[] pbKey, Int32 cbKey, Boolean fOAEP, ObjectHandleOnStack ohRetEncryptedKey)
at System.Security.Cryptography.RSACryptoServiceProvider.Encrypt(Byte[] rgb, Boolean fOAEP)
at WindowsAppmart.Generic.RSAEncrypt(Byte[] DataToEncrypt, RSAParameters RSAKeyInfo, Boolean DoOAEPPadding)

and message is Bad Length.

I am not getting where can I go wrong?

Solution 1

RSA is only meant to be used for encrypting small amounts of data. The exact amount you can encrypt depends on the key length + the amount used by the padding. A 1024 bit key would allow for a bit above 100 bytes.

Since RSA is quite slow, the usual way to encrypt large messages is using hybrid encryption. In hybrid encryption you use a fast symmetric encryption algorithm (like AES) for encrypting the data with a random key. The random key is then encrypted with RSA and send along with the symmetric key encrypted data.

Solution 2

This indicates that the amound of data you are trying to encrypt is too long. You should encrypt it in smaller bulks.

Comments

[Vaibhav]

In login request I have to send public key to server.In response I get token as "eJzFVVlzo0YQf..." around 1300-1400 characters I want to sign this token and send back to server in next request.As server has public key,it will validate token and send me new token in reposnse.How can I achieve this in c# in windows phone 8 project?

[Ebbe M. Pedersen]

Signing don't need to encrypt the full message, but just a one-way hash of the message. This hash are then encrypted with the private key, and can be verified with the public key. Take a look here

[Vaibhav]

In given link, some of the methods were not cleared.Also he has 1st encrypted and then signed. I am totally confused now as there is no good documentation on how to sign data using private key at device end(c#) and how to verify signed data using corresponding public key at server end(java).I am desperately waiting for right solution.

[Ebbe M. Pedersen]

We are moving away from the original question about why you get a "Bad Length" execption. The answer is, that the RSA methode is not designed for encrypting big messages. If you really need to use RSA for signing instead, then that is a new question.