C Symmetric Encryption (AES Encryption) produces different results each time

Idea: Use random vectors, put random vectors into ciphertext, and intercept the first 16 bits from ciphertext every time when decrypting, which is actually the random vectors we encrypted before.

Code:

public static string Encrypt(string plainText, string AESKey)
{
  RijndaelManaged rijndaelCipher = new RijndaelManaged();
  byte[] inputByteArray = Encoding.UTF8.GetBytes(plainText);// Get the byte array to be encrypted 
  rijndaelCipher.Key = Convert.FromBase64String(AESKey);// Encryption and decryption parties agree on the key: AESKey
  rijndaelCipher.GenerateIV();
  byte[] keyIv = rijndaelCipher.IV;
  byte[] cipherBytes = null;
  using (MemoryStream ms = new MemoryStream())
  {
    using (CryptoStream cs = new CryptoStream(ms, rijndaelCipher.CreateEncryptor(), CryptoStreamMode.Write))
    {
      cs.Write(inputByteArray, 0, inputByteArray.Length);
      cs.FlushFinalBlock();
      cipherBytes = ms.ToArray();// Get the encrypted byte array 
      cs.Close();
      ms.Close();
    }
  }
  var allEncrypt = new byte[keyIv.Length + cipherBytes.Length];
  Buffer.BlockCopy(keyIv, 0, allEncrypt, 0, keyIv.Length);
  Buffer.BlockCopy(cipherBytes, 0, allEncrypt, keyIv.Length * sizeof(byte), cipherBytes.Length);
  return Convert.ToBase64String(allEncrypt);
}
 
public static string Decrypt(string showText, string AESKey)
{
  string result = string.Empty;
  try
  {
    byte[] cipherText = Convert.FromBase64String(showText);
    int length = cipherText.Length;
    SymmetricAlgorithm rijndaelCipher = Rijndael.Create();
    rijndaelCipher.Key = Convert.FromBase64String(AESKey);// Encrypt and decrypt the key agreed by both parties 
    byte[] iv = new byte[16];
    Buffer.BlockCopy(cipherText, 0, iv, 0, 16);
    rijndaelCipher.IV = iv;
    byte[] decryptBytes = new byte[length - 16];
    byte[] passwdText = new byte[length - 16];
    Buffer.BlockCopy(cipherText, 16, passwdText, 0, length - 16);
    using (MemoryStream ms = new MemoryStream(passwdText))
    {
      using (CryptoStream cs = new CryptoStream(ms, rijndaelCipher.CreateDecryptor(), CryptoStreamMode.Read))
      {
        cs.Read(decryptBytes, 0, decryptBytes.Length);
        cs.Close();
        ms.Close();
      }
    }
    result = Encoding.UTF8.GetString(decryptBytes).Replace("\0", "");  /// End of the string '\0' Remove 
  }
  catch { }
  return result;
}

Call:

string jiaMi = MyAESTools.Encrypt(textBox1.Text, "abcdefgh12345678abcdefgh12345678");
 
string jieMi = MyAESTools.Decrypt(textBox3.Text, "abcdefgh12345678abcdefgh12345678");