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RSAOAEPKeyExchangeFormatter Class

Updated: March 2011

Creates Optimal Asymmetric Encryption Padding (OAEP) key exchange data using RSA.

Namespace:  System.Security.Cryptography
Assembly:  mscorlib (in mscorlib.dll)

[ComVisibleAttribute(true)]
public class RSAOAEPKeyExchangeFormatter : AsymmetricKeyExchangeFormatter

Key exchange allows a sender to create secret information, for example, random data that can be used as a key in a symmetric encryption algorithm, and use encryption to send it to the intended recipient.

Use RSAOAEPKeyExchangeDeformatter to receive the key exchange and extract the secret information from it.

Caution noteCaution:

It is highly recommended that you not attempt to create your own key exchange method from the basic functionality provided, because many details of the operation must be performed carefully in order for the key exchange to be successful.

The following example shows how to use the RSAOAEPKeyExchangeFormatter class to create an exchange key for a message recipient.

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;


class Alice
{
    public static void Main(string[] args)
    {
        using (Bob bob = new Bob())
        {
            using (RSACryptoServiceProvider rsaKey = new RSACryptoServiceProvider())
            {
                // Get Bob's public key
                rsaKey.ImportCspBlob(bob.key);
                byte[] encryptedSessionKey = null;
                byte[] encryptedMessage = null;
                byte[] iv = null;
                Send(rsaKey, "Secret message", out iv, out encryptedSessionKey, out encryptedMessage);
                bob.Receive(iv, encryptedSessionKey, encryptedMessage);
            }
        }
    }

    private static void Send(RSA key, string secretMessage, out byte[] iv, out byte[] encryptedSessionKey, out byte[] encryptedMessage)
    {
        using (Aes aes = new AesCryptoServiceProvider())
        {
            iv = aes.IV;

            // Encrypt the session key
            RSAOAEPKeyExchangeFormatter keyFormatter = new RSAOAEPKeyExchangeFormatter(key);
            encryptedSessionKey = keyFormatter.CreateKeyExchange(aes.Key, typeof(Aes));

            // Encrypt the message 
            using (MemoryStream ciphertext = new MemoryStream())
            using (CryptoStream cs = new CryptoStream(ciphertext, aes.CreateEncryptor(), CryptoStreamMode.Write))
            {
                byte[] plaintextMessage = Encoding.UTF8.GetBytes(secretMessage);
                cs.Write(plaintextMessage, 0, plaintextMessage.Length);
                cs.Close();

                encryptedMessage = ciphertext.ToArray();
            }
        }
    }

}
public class Bob : IDisposable
{
    public byte[] key;
    private RSACryptoServiceProvider rsaKey = new RSACryptoServiceProvider();
    public Bob()
    {
        key = rsaKey.ExportCspBlob(false);
    }
    public void Receive(byte[] iv, byte[] encryptedSessionKey, byte[] encryptedMessage)
    {

        using (Aes aes = new AesCryptoServiceProvider())
        {
            aes.IV = iv;

            // Decrypt the session key
            RSAOAEPKeyExchangeDeformatter keyDeformatter = new RSAOAEPKeyExchangeDeformatter(rsaKey);
            aes.Key = keyDeformatter.DecryptKeyExchange(encryptedSessionKey);

            // Decrypt the message 
            using (MemoryStream plaintext = new MemoryStream())
            using (CryptoStream cs = new CryptoStream(plaintext, aes.CreateDecryptor(), CryptoStreamMode.Write))
            {
                cs.Write(encryptedMessage, 0, encryptedMessage.Length);
                cs.Close();

                string message = Encoding.UTF8.GetString(plaintext.ToArray());
                Console.WriteLine(message);
            }
        }
    }
    public void Dispose()
    {
        rsaKey.Dispose();
    }
}

System.Object
  System.Security.Cryptography.AsymmetricKeyExchangeFormatter
    System.Security.Cryptography.RSAOAEPKeyExchangeFormatter

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

Date

History

Reason

March 2011

Replaced the example.

Customer feedback.

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