using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Security.Cryptography;
using System.IO;
using System.Text;  

/// <summary>
/// Summary description for Encryption
/// </summary>
public class Encryption
{
    public TripleDES CreateDES(string key)
    {
        MD5 md5 = new MD5CryptoServiceProvider();
        TripleDES des = new TripleDESCryptoServiceProvider();
        des.Key = md5.ComputeHash(Encoding.Unicode.GetBytes(key));
        des.IV = new byte[des.BlockSize / 8];
        return des;
    }
    public  byte[] Encryptiondata(string PlainText)
    {
        TripleDES des = CreateDES("DreamMLMKey");
        ICryptoTransform ct = des.CreateEncryptor();
        byte[] input = Encoding.Unicode.GetBytes(PlainText);
        return ct.TransformFinalBlock(input, 0, input.Length);
    }

    public string Decryptiondata(string CypherText)
    {
        string stringToDecrypt = CypherText.Replace(" ", "+");
        int len = stringToDecrypt.Length;
        byte[] inputByteArray = Convert.FromBase64String(stringToDecrypt); 

        byte[] b = Convert.FromBase64String(CypherText);
        TripleDES des = CreateDES("DreamMLMKey");
        ICryptoTransform ct = des.CreateDecryptor();
        byte[] output = ct.TransformFinalBlock(b, 0, b.Length);
        return Encoding.Unicode.GetString(output);
    }
    public string Decryptiondataurl(string CypherText)
    {
        string newcyperttext=CypherText.Replace(' ', '+');
        byte[] b = Convert.FromBase64String(newcyperttext);
        TripleDES des = CreateDES("DreamMLMKey");
        ICryptoTransform ct = des.CreateDecryptor();
        byte[] output = ct.TransformFinalBlock(b, 0, b.Length);
        return Encoding.Unicode.GetString(output);
    }


    #region  encryption & Decription
    public  string Encrypt(string input, string key)
    {
        byte[] inputArray = UTF8Encoding.UTF8.GetBytes(input);
        TripleDESCryptoServiceProvider tripleDES = new TripleDESCryptoServiceProvider();
        tripleDES.Key = UTF8Encoding.UTF8.GetBytes(key);
        tripleDES.Mode = CipherMode.ECB;
        tripleDES.Padding = PaddingMode.PKCS7;
        ICryptoTransform cTransform = tripleDES.CreateEncryptor();
        byte[] resultArray = cTransform.TransformFinalBlock(inputArray, 0, inputArray.Length);
        tripleDES.Clear();
        return Convert.ToBase64String(resultArray, 0, resultArray.Length);
    }
    public  string Decrypt(string input, string key)
    {
        byte[] inputArray = Convert.FromBase64String(input);
        TripleDESCryptoServiceProvider tripleDES = new TripleDESCryptoServiceProvider();
        tripleDES.Key = UTF8Encoding.UTF8.GetBytes(key);
        tripleDES.Mode = CipherMode.ECB;
        tripleDES.Padding = PaddingMode.PKCS7;
        ICryptoTransform cTransform = tripleDES.CreateDecryptor();
        byte[] resultArray = cTransform.TransformFinalBlock(inputArray, 0, inputArray.Length);
        tripleDES.Clear();
        return UTF8Encoding.UTF8.GetString(resultArray);
    }

    public string encrypt(string encryptString)
    {
        string EncryptionKey = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        byte[] clearBytes = Encoding.Unicode.GetBytes(encryptString);
        using (Aes encryptor = Aes.Create())
        {
            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] {
                0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76
            });
            encryptor.Key = pdb.GetBytes(32);
            encryptor.IV = pdb.GetBytes(16);
            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
                {
                    cs.Write(clearBytes, 0, clearBytes.Length);
                    cs.Close();
                }
                encryptString = Convert.ToBase64String(ms.ToArray());
            }
        }
        return encryptString;
    }

    public string Decrypt(string cipherText)
    {
        string EncryptionKey = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        cipherText = cipherText.Replace(" ", "+");
        byte[] cipherBytes = Convert.FromBase64String(cipherText);
        using (Aes encryptor = Aes.Create())
        {
            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] {
                0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76
            });
            encryptor.Key = pdb.GetBytes(32);
            encryptor.IV = pdb.GetBytes(16);
            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    cs.Write(cipherBytes, 0, cipherBytes.Length);
                    cs.Close();
                }
                cipherText = Encoding.Unicode.GetString(ms.ToArray());
            }
        }
        return cipherText;
    }

    #endregion
}

EDIT 2013- 10月:虽然我随着时间的推移编辑了这个答案以解决缺点，但请参阅jbtule的答案以获得更健壮、更明智的解决方案。

https://stackoverflow.com/a/10366194/188474

最初的回答:

下面是一个从“RijndaelManaged Class”文档和MCTS训练工具包中派生出来的工作示例。

EDIT 2012- 4月:根据jbtule的建议，这个答案被编辑为IV之前的答案，如下图所示:

http://msdn.microsoft.com/en-us/library/system.security.cryptography.aesmanaged%28v=vs.95%29.aspx

好运！

public class Crypto
{

    //While an app specific salt is not the best practice for
    //password based encryption, it's probably safe enough as long as
    //it is truly uncommon. Also too much work to alter this answer otherwise.
    private static byte[] _salt = __To_Do__("Add a app specific salt here");

    /// <summary>
    /// Encrypt the given string using AES.  The string can be decrypted using 
    /// DecryptStringAES().  The sharedSecret parameters must match.
    /// </summary>
    /// <param name="plainText">The text to encrypt.</param>
    /// <param name="sharedSecret">A password used to generate a key for encryption.</param>
    public static string EncryptStringAES(string plainText, string sharedSecret)
    {
        if (string.IsNullOrEmpty(plainText))
            throw new ArgumentNullException("plainText");
        if (string.IsNullOrEmpty(sharedSecret))
            throw new ArgumentNullException("sharedSecret");

        string outStr = null;                       // Encrypted string to return
        RijndaelManaged aesAlg = null;              // RijndaelManaged object used to encrypt the data.

        try
        {
            // generate the key from the shared secret and the salt
            Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);

            // Create a RijndaelManaged object
            aesAlg = new RijndaelManaged();
            aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);

            // Create a decryptor to perform the stream transform.
            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

            // Create the streams used for encryption.
            using (MemoryStream msEncrypt = new MemoryStream())
            {
                // prepend the IV
                msEncrypt.Write(BitConverter.GetBytes(aesAlg.IV.Length), 0, sizeof(int));
                msEncrypt.Write(aesAlg.IV, 0, aesAlg.IV.Length);
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                    {
                        //Write all data to the stream.
                        swEncrypt.Write(plainText);
                    }
                }
                outStr = Convert.ToBase64String(msEncrypt.ToArray());
            }
        }
        finally
        {
            // Clear the RijndaelManaged object.
            if (aesAlg != null)
                aesAlg.Clear();
        }

        // Return the encrypted bytes from the memory stream.
        return outStr;
    }

    /// <summary>
    /// Decrypt the given string.  Assumes the string was encrypted using 
    /// EncryptStringAES(), using an identical sharedSecret.
    /// </summary>
    /// <param name="cipherText">The text to decrypt.</param>
    /// <param name="sharedSecret">A password used to generate a key for decryption.</param>
    public static string DecryptStringAES(string cipherText, string sharedSecret)
    {
        if (string.IsNullOrEmpty(cipherText))
            throw new ArgumentNullException("cipherText");
        if (string.IsNullOrEmpty(sharedSecret))
            throw new ArgumentNullException("sharedSecret");

        // Declare the RijndaelManaged object
        // used to decrypt the data.
        RijndaelManaged aesAlg = null;

        // Declare the string used to hold
        // the decrypted text.
        string plaintext = null;

        try
        {
            // generate the key from the shared secret and the salt
            Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);

            // Create the streams used for decryption.                
            byte[] bytes = Convert.FromBase64String(cipherText);
            using (MemoryStream msDecrypt = new MemoryStream(bytes))
            {
                // Create a RijndaelManaged object
                // with the specified key and IV.
                aesAlg = new RijndaelManaged();
                aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);
                // Get the initialization vector from the encrypted stream
                aesAlg.IV = ReadByteArray(msDecrypt);
                // Create a decrytor to perform the stream transform.
                ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))

                        // Read the decrypted bytes from the decrypting stream
                        // and place them in a string.
                        plaintext = srDecrypt.ReadToEnd();
                }
            }
        }
        finally
        {
            // Clear the RijndaelManaged object.
            if (aesAlg != null)
                aesAlg.Clear();
        }

        return plaintext;
    }

    private static byte[] ReadByteArray(Stream s)
    {
        byte[] rawLength = new byte[sizeof(int)];
        if (s.Read(rawLength, 0, rawLength.Length) != rawLength.Length)
        {
            throw new SystemException("Stream did not contain properly formatted byte array");
        }

        byte[] buffer = new byte[BitConverter.ToInt32(rawLength, 0)];
        if (s.Read(buffer, 0, buffer.Length) != buffer.Length)
        {
            throw new SystemException("Did not read byte array properly");
        }

        return buffer;
    }
}

下面是一个简单的例子，在c#中使用AES CBC模式加密字符串，并使用随机IV和HMAC和密码派生密钥，以显示基本的移动部分:

private byte[] EncryptBytes(byte[] key, byte[] plaintext)
{
    using (var cipher = new RijndaelManaged { Key = key })
    {
        using (var encryptor = cipher.CreateEncryptor())
        {
            var ciphertext = encryptor.TransformFinalBlock(plaintext, 0, plaintext.Length);

            // IV is prepended to ciphertext
            return cipher.IV.Concat(ciphertext).ToArray();
        }
    }
}

private byte[] DecryptBytes(byte[] key, byte[] packed)
{
    using (var cipher = new RijndaelManaged { Key = key })
    {
        int ivSize = cipher.BlockSize / 8;

        cipher.IV = packed.Take(ivSize).ToArray();

        using (var encryptor = cipher.CreateDecryptor())
        {
            return encryptor.TransformFinalBlock(packed, ivSize, packed.Length - ivSize);
        }
    }
}

private byte[] AddMac(byte[] key, byte[] data)
{
    using (var hmac = new HMACSHA256(key))
    {
        var macBytes = hmac.ComputeHash(data);

        // HMAC is appended to data
        return data.Concat(macBytes).ToArray();
    }
}

private bool BadMac(byte[] found, byte[] computed)
{
    int mismatch = 0;

    // Aim for consistent timing regardless of inputs
    for (int i = 0; i < found.Length; i++)
    {
        mismatch += found[i] == computed[i] ? 0 : 1;
    }

    return mismatch != 0;
}

private byte[] RemoveMac(byte[] key, byte[] data)
{
    using (var hmac = new HMACSHA256(key))
    {
        int macSize = hmac.HashSize / 8;

        var packed = data.Take(data.Length - macSize).ToArray();

        var foundMac = data.Skip(packed.Length).ToArray();

        var computedMac = hmac.ComputeHash(packed);

        if (this.BadMac(foundMac, computedMac))
        {
            throw new Exception("Bad MAC");
        }

        return packed;
    }            
}

private List<byte[]> DeriveTwoKeys(string password)
{
    var salt = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 };

    var kdf = new Rfc2898DeriveBytes(password, salt, 10000);

    var bytes = kdf.GetBytes(32); // Two keys 128 bits each

    return new List<byte[]> { bytes.Take(16).ToArray(), bytes.Skip(16).ToArray() };
}

public byte[] EncryptString(string password, String message)
{
    var keys = this.DeriveTwoKeys(password);

    var plaintext = Encoding.UTF8.GetBytes(message);

    var packed = this.EncryptBytes(keys[0], plaintext);

    return this.AddMac(keys[1], packed);
}

public String DecryptString(string password, byte[] secret)
{
    var keys = this.DeriveTwoKeys(password);

    var packed = this.RemoveMac(keys[1], secret);

    var plaintext = this.DecryptBytes(keys[0], packed);

    return Encoding.UTF8.GetString(plaintext);
}

public void Example()
{
    var password = "correcthorsebatterystaple";

    var secret = this.EncryptString(password, "Hello World");

    Console.WriteLine("secret: " + BitConverter.ToString(secret));

    var recovered = this.DecryptString(password, secret);

    Console.WriteLine(recovered);
}

用于AES-GCM加密的BouncyCastle的替代方案是libsodium-net。它包装了libsodium C库。一个很好的优点是它在cpu中使用AES-NI扩展进行非常快速的加密。缺点是，如果CPU没有扩展，它根本无法工作。软件没有退路。

下面是一个使用RSA的例子。

重要:使用RSA加密KeySize - MinimumPadding加密的数据大小是有限制的。例如256字节(假设2048位密钥)- 42字节(最小OEAP填充)= 214字节(最大明文大小)

用RSA密钥替换your_rsa_key。

var provider = new System.Security.Cryptography.RSACryptoServiceProvider();
provider.ImportParameters(your_rsa_key);

var encryptedBytes = provider.Encrypt(
    System.Text.Encoding.UTF8.GetBytes("Hello World!"), true);

string decryptedTest = System.Text.Encoding.UTF8.GetString(
    provider.Decrypt(encryptedBytes, true));

更多信息，请访问MSDN - RSACryptoServiceProvider

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

public class Program
{
    public static void Main()
    {
        var key = Encoding.UTF8.GetBytes("SUkbqO2ycDo7QwpR25kfgmC7f8CoyrZy");
        var data = Encoding.UTF8.GetBytes("testData");

        //Encrypt data
        var encrypted = CryptoHelper.EncryptData(data,key);

        //Decrypt data
        var decrypted = CryptoHelper.DecryptData(encrypted,key);

        //Display result
        Console.WriteLine(Encoding.UTF8.GetString(decrypted));
    }
}

public static class CryptoHelper
{
    public static byte[] EncryptData(byte[] data, byte[] key)
    {
        using (var aesAlg = Aes.Create())
        {
            aesAlg.Mode = CipherMode.CBC;
            using (var encryptor = aesAlg.CreateEncryptor(key, aesAlg.IV))
            {
                using (var msEncrypt = new MemoryStream())
                {
                    msEncrypt.Write(aesAlg.IV, 0, aesAlg.IV.Length);

                    using (var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                        csEncrypt.Write(data, 0, data.Length);

                    return msEncrypt.ToArray();
                }
            }
        }

    }

    public static byte[] DecryptData(byte[] encrypted, byte[] key)
    {
        var iv = new byte[16];
        Buffer.BlockCopy(encrypted, 0, iv, 0, iv.Length);
        using (var aesAlg = Aes.Create())
        {
            aesAlg.Mode = CipherMode.CBC;
            using (var decryptor = aesAlg.CreateDecryptor(key, iv))
            {
                using (var msDecrypt = new MemoryStream(encrypted, iv.Length, encrypted.Length - iv.Length))
                {
                    using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                    {
                        using (var resultStream = new MemoryStream())
                        {
                            csDecrypt.CopyTo(resultStream);
                            return resultStream.ToArray();
                        }
                    }
                }
            }
        }
    }
}