C# – how to use RSA to encrypt files (huge data) in C#


I'm new to encryption. I need to implement asymmetric encryption algorithm, which i think it uses private/public key. I started using a sample of RSACryptoServiceProvider. it was ok with small data to encrypt. But when using it on relatively larger data "2 lines", i get the exception CryptographicException "Bad Length"!

//Create a new instance of RSACryptoServiceProvider.
using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())

    //Import the RSA Key information. This only needs
    //toinclude the public key information.
    byte[] keyValue = Convert.FromBase64String(publicKey);

    //Encrypt the passed byte array and specify OAEP padding.  
    //OAEP padding is only available on Microsoft Windows XP or
    encryptedData = RSA.Encrypt(DataToEncrypt, DoOAEPPadding);

Then I found some samples of encrypting large data (or files) by using CryptoStream, and only use symmetric algorithms like DES or 3DES, which have the function CreateEncryptor to return ICryptoTransform as one of the input to the constructor of CryptoStream!!!

CryptoStream cStream = new CryptoStream(fStream,
                new TripleDESCryptoServiceProvider().CreateEncryptor(Key, IV),

What is the way to encrypt files using RSA?

Best Solution

RSA can only encrypt data blocks that are shorter than the key length so what you normally do is

  1. Generate a random key of the correct length required for AES (or similar).
  2. Encrypt your data using AES or similar using that key
  3. Encrypt the random key using your RSA key

Then you publish both the outputs from 2 and 3

To decrypt

  1. Decrypt the AES key using your RSA key.
  2. Decrypt the data using that AES key
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