solution

string hash(string a, string key)
{
    var ret = "";
    var add = "";
    a = a.ToLowerInvariant();
    key = key.ToLowerInvariant();
    var i = 0;
    while (i < a.Length && i < 20)
    {
        var _5 = a[i];
        var _6 = key[i];
        var _7 = (char)(((uint)_5 + (uint)_6) % 26 + 65);
        ret += _7;
        i++;
    }
    if (i < 20)
    {
        while (i < 20)
        {
            ret += "1";
            i++;
        }
    }
    ret += add;
    return ret;
}

void Main()
{
    //var ab = @"FMBTHGPCZ";
    var ab = @"FMBTHGPCZOJWETIHKDBF";
    var bc = @"smoothbozonjetmonies";
    hash(@"[email protected]", bc).Dump();
    hash(@"bobtcnccookbookhljll", bc).Dump();
    for (int i = 0; i < ab.Length; i++)
    {
        var mod = ((uint)ab[i] - 65);//.Dump("Mod");
        var tr = "";
        for (int j = 5; j < 12;j ++)
        {
            tr += ((char)(26*j+mod - (uint)bc[i]));
        }
        tr.Dump();
    }
}

// Define other methods and classes here