Question

is this password check algorithm safe to use in low security environment like local network

 static string keys = "qwertyuiopüõasdfghjklöäzxcvbnmQWERTYUIOPÜÄÖLKJHGFDSAZXCVBNM";
    static Int64 key = 0;
    public static bool check(string input)
    {
        string tmp = "";
        string encAnswer = "ỬốỒởỐỚ";
        for (int i = 0; i < input.Length; i++)
        {
            tmp += keys.IndexOf(input[i]).ToString();
        }
        key = Int64.Parse(tmp);
        string res = "";
        char c;
        for (int i = 0; i < input.Length; i++)
        {
            c = input[i];
            c = (char)(c ^ key);
            res += c;
        }
        if (res == encAnswer)
            return true;
        return false;
    }

Answer 1

You should stick with hash algorithms, even in low security environments. Take a look on SHA256 Class

Note you will not be able to recover original password by using hash algorithms.

EDIT: As OP noted, just using a hash algorithm isn’t enough. You must to add a salt to make it harder to break. Some examples can be found here: How To: Hash Data with Salt (C#/VB.NET)