问Base-64字符数组或字符串的长度无效

EN

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Security.Cryptography;
 using System.IO;
 namespace AccountSystem.Class{
class ClEncrDecr
{
    private TripleDESCryptoServiceProvider tripleDESCryptoServiceProvider = new TripleDESCryptoServiceProvider();

    private byte[] TruncateHash(string key, int length)
    {
        SHA1CryptoServiceProvider sha1 = new SHA1CryptoServiceProvider();
        //Hash the Key
        byte[] keyBytes = System.Text.Encoding.Unicode.GetBytes(key);
        byte[] hash = sha1.ComputeHash(keyBytes);

        // truncate or pad the hash
        Array.Resize(ref hash, length);
        return hash;
    }

    public ClEncrDecr()
    {
        string key = "ABCD";
        tripleDESCryptoServiceProvider.Key = TruncateHash(key, tripleDESCryptoServiceProvider.KeySize / 8 );
        tripleDESCryptoServiceProvider.IV = TruncateHash("", tripleDESCryptoServiceProvider.BlockSize / 8 );
    }

    public string EncryptData(string plainText)
    {
        byte[] plaintextBytes = System.Text.Encoding.Unicode.GetBytes(plainText);
        MemoryStream ms = new MemoryStream();
        CryptoStream encStream = new CryptoStream(ms, tripleDESCryptoServiceProvider.CreateEncryptor(), System.Security.Cryptography.CryptoStreamMode.Write);
        encStream.Write(plaintextBytes, 0, plaintextBytes.Length);
        encStream.FlushFinalBlock();
        return Convert.ToBase64String(ms.ToArray());
    }

    public string DecryptData(string encryptedtext)
    {
        byte[] encryptedBytes = Convert.FromBase64String(encryptedtext);
        MemoryStream ms = new MemoryStream();
        CryptoStream decStream = new CryptoStream(ms, tripleDESCryptoServiceProvider.CreateDecryptor(), CryptoStreamMode.Write);
        decStream.Write(encryptedBytes, 0, encryptedBytes.Length);
        decStream.FlushFinalBlock();
        return System.Text.Encoding.Unicode.GetString(ms.ToArray());
    }
}
}

MessageBox.Show(crypto.DecryptData(obj.password))