关于c＃：Base-64 char数组或字符串的长度无效

Invalid length for a Base-64 char array or string

这是我的加密和解密方法。我有两个数据库，我将加密密码从一个数据库复制到另一个数据库。代码是用VB编写的，但我把它转换成了C。

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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());
}
}
}

登录代码：

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

当我们调用DecryptData(string encryptedtext)时，它抛出了一个例外，说Invalid length for a Base-64 char array or string。我能做什么？