RSA Encryption and Decryption in C#.NET

Question

I have below code to encrypt and decrypt the message in c#. when i am trying to run it is giving an exception ie "The data to be decrypted exceeds the maximum for this modulus of 256 bytes"

 public static void Main(string[] args)
    {
        X509Certificate2 cert = new X509Certificate2(@"C:\Data\ABC-rsa-public-key-certificate.cer");
        string encryptedText = EncrypIt("Hello", cert);
        string decryptedText = DecrptIt(encryptedText, cert);
        System.Console.WriteLine(decryptedText);


    }

    public static string EncrypIt(string inputString, X509Certificate2 cert)
    {
        RSACryptoServiceProvider publicKey = (RSACryptoServiceProvider)cert.PublicKey.Key;
        byte[] plainBytes = Encoding.UTF8.GetBytes(inputString);
        byte[] encryptedBytes = publicKey.Encrypt(plainBytes, false);
        string encryptedText = Encoding.UTF8.GetString(encryptedBytes);
        return encryptedText;      
    }

    public static string DecrptIt(string encryptedText, X509Certificate2 cert)
   {
       RSACryptoServiceProvider privateKey = (RSACryptoServiceProvider)cert.PublicKey.Key;
       byte[] encryptedBytes = Encoding.UTF8.GetBytes(encryptedText);
       byte[] decryptedBytes = privateKey.Decrypt(encryptedBytes, false);
       string decryptedText = Encoding.UTF8.GetString(decryptedBytes);
       return decryptedText;
   }

If you had searched google with that error message, you would have found this as your first result: stackoverflow.com/questions/10362409/… — Pete
– Pete, Commented Sep 20, 2013 at 15:41

CodesInChaos · Accepted Answer · 2013-09-20 15:45:14Z

Several problems:

RSA by default only encrypts one block. It's not suitable for long messages. You shouldn't encrypt the message itself with RSA. Generate a random AES key and encrypt the key with RSA and the actual message with AES.
You must use a binary safe encoding such as Hex or Base64 for the ciphertext. Using UTF-8 corrupts the data since it doesn't allow arbitrary byte sequences.

UTF-8 is designed to encode text, so it's fine for your plaintext.
Use OAEP, the old 1.5 padding mode is not secure. i.e. pass true as second parameter to Encrypt/Decrypt. (Technically it's possible to use it securely, but it's tricky and I wouldn't recommend it)

As a further note, once you use AES, there are some more pitfalls: 1) Use a MAC in an encrypt-then-mac scheme, else active attacks including padding-oracles will break your code 2) Use a random IV that's different for each message

3) MAC the IV too. And if you go this far @user2791316, why don't you just use a tried and tested library instead of implementing your own protocol? Use TLS for data in transit and PGP for data at rest. Even the pros don't always get cryptography right; it's unreasonable to expect a newcomer to do that.

vcsjones · Accepted Answer · 2013-09-20 15:41:19Z

3

RSA should not be used to encrypt this kind of data. You should be encrypting your data with a symmetric key like AES, then encrypting the symmetric key with RSA.

answered Sep 20, 2013 at 15:41

vcsjones

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