using System;
using System.Text;
using System.IO;
using System.Security.Cryptography;
  
namespace CI
{
    class CI_Encrypt
    {
        private string key;
        private string hash_type = "sha1";
  
        public CI_Encrypt(string key = "")
        {
            if(key.Length > 0)
            {
                this.key = key;
            }
        }
        public string encode(string data)
        {
            RijndaelManaged aes = new RijndaelManaged();
            aes.KeySize = 256;
            aes.BlockSize = 256;
            aes.Padding = PaddingMode.Zeros;
            aes.Mode = CipherMode.CBC;
  
            aes.Key = this.get_key(key);
            aes.GenerateIV();
            var _IV = aes.IV;
  
            byte[] buffer;
              
            ICryptoTransform AESEncrypt = aes.CreateEncryptor(aes.Key, aes.IV);
            using (MemoryStream msEncrypt = new MemoryStream())
            {
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, AESEncrypt, CryptoStreamMode.Write))
                {
                    using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                    {
  
                        //Write all data to the stream.
                        swEncrypt.Write(data);
                    }
                    buffer = msEncrypt.ToArray();
                }
            }
  
            var enc = new byte [ _IV.Length + buffer.Length ];
  
            Array.Copy(_IV,enc ,_IV.Length);
            Array.Copy(buffer,0,enc,_IV.Length,buffer.Length);
  
            return this.add_cipher_noise(enc);
        }
  
        public string decode(string data)
        {
            string plainText;
            byte[] _enc_data = this.remove_cipher_noise(Convert.FromBase64String(data));
            using (RijndaelManaged aes = new RijndaelManaged())
            {
                aes.KeySize = 256;
                aes.BlockSize = 256;
                aes.Padding = PaddingMode.Zeros;
                aes.Mode = CipherMode.CBC;
                aes.Key = this.get_key(key);
                var _IV = new byte[aes.IV.Length];
                Array.Copy(_enc_data, _IV, _IV.Length);
                aes.IV = _IV;
                var endata = new byte[_enc_data.Length - aes.IV.Length];
                Array.Copy(_enc_data, aes.IV.Length, endata, 0, endata.Length);
  
                // Create a decrytor to perform the stream transform.
                ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
  
                // Create the streams used for decryption. 
                using (MemoryStream msDecrypt = new MemoryStream(endata))
                {
                    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();
                        }
                    }
                }
  
            }
            return plainText;
        }
  
        public string add_cipher_noise(byte[] data)
        {
            string hashkey = this.hash(System.Text.Encoding.Default.GetString(this.get_key(this.key)));
            char[] keyChar = hashkey.ToCharArray();
            byte[] encrpytData = new byte[data.Length];
            for (int i = 0, j = 0; i < data.Length; i++, j++)
            {
                if (j >= hashkey.Length)
                {
                    j = 0;
                }
                var temp = (((int)data[i] + (int)keyChar[j]) % 256);
                encrpytData[i] = (byte)temp;
            }
            return System.Convert.ToBase64String(encrpytData);
        }
  
        public byte[] remove_cipher_noise(byte[] data)
        {
            string hashkey = this.hash(System.Text.Encoding.Default.GetString(this.get_key(this.key)));
            char[] keyChar = hashkey.ToCharArray();
            byte[] encrpytData = new byte[data.Length];
            int temp;
  
            for (int i = 0, j = 0; i < data.Length; i++, j++)
            {
                if (j >= hashkey.Length)
                {
                    j = 0;
                }
                temp = (int)data[i] - (int)keyChar[j];
                if(temp < 0)
                {
                    temp += 256;
                }
                encrpytData[i] = (byte)temp;
            }
            return encrpytData;
        }
  
        private string hash(string key)
        {
            byte[] hashBytes;
            if (this.hash_type == "sha1")
            {
                SHA1 sha = new SHA1CryptoServiceProvider();
                hashBytes = (sha.ComputeHash(System.Text.Encoding.Default.GetBytes(key)));
            }
            else
            {
                MD5 md5 = new MD5CryptoServiceProvider();
                hashBytes = (md5.ComputeHash(System.Text.Encoding.Default.GetBytes(key)));
            }
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < hashBytes.Length; i++)
            {
                sb.Append(hashBytes[i].ToString("X2"));
            }
            return sb.ToString().ToLower();
        }
  
        private byte[] get_key(string key)
        {
            MD5 md5 = new MD5CryptoServiceProvider();
            byte[] hashBytes = (md5.ComputeHash(System.Text.Encoding.UTF8.GetBytes(key)));
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < hashBytes.Length; i++)
            {
                sb.Append(hashBytes[i].ToString("X2"));
            }
            return System.Text.Encoding.Default.GetBytes(sb.ToString().ToLower());
        }
    }
}

用C#实现跟CI里面的加密解密