C#3des加密时候需要密钥

/// <summary>
/// 3des加密字符串
/// </summary>
/// <param name="a_strString">要加密的字符串</param>
/// <param name="a_strKey">密钥</param>
/// <returns>加密后并经base64编码的字符串</returns>
/// <remarks>静态方法，采用默认ascii编码</remarks>
public static string Encrypt3DES(string a_strString, string a_strKey)
{
TripleDESCryptoServiceProvider DES = new
TripleDESCryptoServiceProvider();
MD5CryptoServiceProvider hashMD5 = new MD5CryptoServiceProvider();

DES.Key = hashMD5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(a_strKey));
DES.Mode = CipherMode.ECB;

ICryptoTransform DESEncrypt = DES.CreateEncryptor();

byte[] Buffer = ASCIIEncoding.ASCII.GetBytes(a_strString);
return Convert.ToBase64String(DESEncrypt.TransformFinalBlock
(Buffer, 0, Buffer.Length));
}
要传2个参数，第2个参数需要什么密钥。。这密钥是什么。