Rijndael encoding with base 64 in C #

I'm doing a Rijndael encryption and I imagine I'm close to finishing it, but when decrypting I get the error:

  

Invalid length of data to be decrypted.

using System.Text;

using static Array;

class EncoderRijndael
{

    public static Rijndael InstanciaRijndael()
    {


        Rijndael cripto = Rijndael.Create();

        cripto.KeySize = 128;
        cripto.Mode = CipherMode.CBC;
        cripto.Padding = PaddingMode.PKCS7;
        //cripto.GenerateKey(); //= Encoding.ASCII.GetBytes(key); //associação à propriedade Key
        cripto.GenerateIV();  //=  Encoding.ASCII.GetBytes(iv); //associação à propriedade do vetor de inicialização IV.
        cripto.Key = CriptoMD5.EncoderMD5.EncodeMD5(md5generate: "123");



        return cripto;
    }

    public static string Encripta(string plaintext)
    {
       using (Rijndael cripto = InstanciaRijndael()){ //instanciada a classe Rijndael
            ICryptoTransform encrypt = cripto.CreateEncryptor(cripto.Key, cripto.IV); //objeto encrypt para a execução das operações de transformação de criptografia.
            using (MemoryStream streamResultado = new MemoryStream()) //instancia a memória para armazenamento em memory stream.
          {
                using (CryptoStream cStream = new CryptoStream(streamResultado, encrypt, CryptoStreamMode.Write)) //instanciado a classe crypto stream para referenciar que a
                                                                                            //será feita fluxo Crypto Transform e memory stream
                {
                    using (StreamWriter writer = //instanciado o stream writer para indicar que será feito o processo de escrita de dados criptografados no objeto memory stream
                       new StreamWriter(cStream))
                    {
                        writer.Write(plaintext); //associando o texto em que será criptografado.
                    }
                }
              String concat = System.Text.Encoding.UTF8.GetString(cripto.IV);
              String auxMStream = System.Text.Encoding.UTF8.GetString(streamResultado.ToArray());
              String.Concat(concat, auxMStream);
              concat.Trim();
              var debyte64 = Encoding.UTF8.GetBytes(concat);
              concat = Convert.ToBase64String(debyte64);

              return concat;
          }
        }



    }

    public static string Decripta(string concat, string textdecrypted)

    {
        var concaat = Convert.FromBase64String(concat);
        concaat = Encoding.UTF8.GetBytes(concat);
        using (var cripto = InstanciaRijndael())
        {
            int btLength = concat.Length - 16;
            Debugger.Break();
            byte[] decryBT = new byte[btLength];

            Copy(concaat, 1, decryBT, 0, btLength);
            Debugger.Break();

            //System.Text.Encoding.UTF8.GetBytes(newIV);
            byte[] newIV = new byte[16];
            Copy(concaat, newIV, 16);

            cripto.IV = newIV;                
            var decrypt = cripto.CreateDecryptor(cripto.Key, cripto.IV);

            using (var streamCryptText = new MemoryStream(decryBT))
            {
                using (var cStream = new CryptoStream(streamCryptText, decrypt, CryptoStreamMode.Read))

                {

                    using (var reader = new StreamReader(cStream))
                    {
                      textdecrypted = reader.ReadLine();
                        Debugger.Break();
                    }
                }
            }
        }
        return textdecrypted;
    } 
}
}

Main method for running it.

class Executa
{
    class Program
    {
        public static void Main(string[] args)
        {
            var keyRij = CriptoRijndael.EncoderRijndael.Encripta(plaintext: "teste cripto");
            String textdecrypted = "";
            CriptoRijndael.EncoderRijndael.Decripta(concat: keyRij, textdecrypted: textdecrypted);

            Console.WriteLine(textdecrypted);
            Console.ReadLine();
        }
    }
}
}

Code for generating the MD5 Hash key being used key "123"

class EncoderMD5
{


public static byte[] EncodeMD5(String md5generate)
    {
        MD5 md5Hash = MD5.Create();
        byte[] data = md5Hash.ComputeHash(Encoding.UTF8.GetBytes(md5generate)); // converte para array de bytes
        StringBuilder sb = new StringBuilder();            
        foreach (byte t in data) //formatar byte para string decimal
        {
            sb.Append(t.ToString("x2"));
        }
        var key = Encoding.ASCII.GetBytes(sb.ToString());

        return key;
    }
}
}

Thanks, if anyone can give me a light on why this error is occurring, I'm running in console application in case just for testing.