public static class BasicAuthenticationUtil
{
    /// <summary>
    /// Authorization ヘッダーの内容をパースします。
    /// </summary>
    /// <returns>
    /// パースに成功した場合は true を、失敗した場合は false を返します。
    /// </returns>
    public static bool ParseAuthorizationHeader(string auth,
        out string username, out string password)
    {
        username = password = string.Empty;

        if (auth == null || auth.Substring(0, 5).ToLower() != "basic")
        { return false; }

        string base64Credentials = auth.Substring(6);

        byte[] bytesCredentials = Convert.FromBase64String(base64Credentials);
        string[] credentials = new UTF8Encoding().GetString(bytesCredentials).Split(':');
        username = credentials[0];
        password = credentials[1];

        // Windows Vista sends username in the form of DOMAIN\User
        int delimiterIndex = username.IndexOf('\\');
        if (delimiterIndex != -1)
        { username = username.Remove(0, delimiterIndex + 1); }

        return true;
    }
}