117 lines
3.7 KiB
C#
117 lines
3.7 KiB
C#
using System.Diagnostics;
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const string cypher = "ADFGVX";
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var N = cypher.Length;
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var size = N * N;
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const string symbols = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
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Debug.Assert(size == symbols.Length);
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var square = new int[size];
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for (var i = 0; i < size; i++)
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{
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var j = Random.Shared.Next(i + 1);
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square[i] = square[j];
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square[j] = i;
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}
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Console.WriteLine($"{N} x {N} Polybius square:");
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Console.WriteLine();
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Console.WriteLine($" | {string.Join(' ', cypher.AsEnumerable())}");
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Console.WriteLine(new string('-', 2 * N + 3));
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for (var row = 0; row < N; row++)
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{
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Console.Write($"{cypher[row]} |");
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for (var col = 0; col < N; col++)
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{
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Console.Write($" {symbols[square[col + row * N]]}");
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}
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Console.WriteLine();
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}
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static bool IsSuitableTranspositionKey(string s) =>
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s.Length >= 7 && s.Length <= 12 && s.Length == s.Distinct().Count() && s.All(symbols.ToLowerInvariant().Contains);
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var keys = File.ReadAllLines("unixdict.txt").Where(IsSuitableTranspositionKey).ToArray();
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var key = keys[Random.Shared.Next(keys.Length)].ToUpperInvariant();
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Console.WriteLine();
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Console.WriteLine($"The key is {key}");
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var plaintext = "ATTACKAT1200AM";
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Console.WriteLine($"Plaintext: {plaintext}");
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string Encrypt(string plaintext, string key, int[] square)
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{
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// Create a lookup table from symbol to fragment
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var map = (from i in Enumerable.Range(0, size)
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let k = square[i]
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let code = cypher[i / N] + "" + cypher[i % N]
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select (k, code))
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.ToDictionary(kc => symbols[kc.k], kc => kc.code);
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// Map the plaintext to fragments
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var e = string.Join("", plaintext.Select(c => map[c]));
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// Determine number of rows
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var K = key.Length;
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var rows = (e.Length + K - 1) / K;
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// Pad with spaces
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e += new string(' ', rows * K - e.Length);
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// Sort the key
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var order = string.Join("", key.OrderBy(c => c)).Select(c => key.IndexOf(c)).ToArray();
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// Reorder the fragments.
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e = string.Join("", Enumerable.Range(0, K * rows).Select(i => e[(i / K) * K + order[i % K]]));
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// Transpose
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e = new([.. from col in Enumerable.Range(0, K)
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from row in Enumerable.Range(0, rows)
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select e[row * K + col]]);
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// Read off each column.
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return string.Join(" ", Enumerable.Range(0, K).Select(col => e.Substring(col * rows, rows).Trim()));
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}
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var encrypted = Encrypt(plaintext, key, square);
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Console.WriteLine($"Encrypted: {encrypted}");
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string Decrypt(string encrypted, string key, int[] square)
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{
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// Create a lookup table from fragment to symbol
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var map = (from i in Enumerable.Range(0, size)
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let k = square[i]
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let code = cypher[i / N] + "" + cypher[i % N]
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select (k, code))
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.ToDictionary(kc => kc.code, kc => symbols[kc.k]);
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// Split into columns
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var cols = encrypted.Split(' ');
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// Determine number of rows
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var K = key.Length;
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var rows = cols.Max(c => c.Length);
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// Pad each column
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cols = [.. cols.Select(c => c.PadRight(rows))];
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// Transpose
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string e = new([.. from row in Enumerable.Range(0, rows)
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from col in Enumerable.Range(0, K)
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select cols[col][row]]);
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// Sort the key
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var sorted = string.Join("", key.OrderBy(c => c));
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var order = key.Select(c => sorted.IndexOf(c)).ToArray();
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// Reorder the fragments
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e = string.Join("", Enumerable.Range(0, K * rows).Select(i => e[(i / K) * K + order[i % K]])).Trim();
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// Map the fragments to plaintext
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return string.Join("", Enumerable.Range(0, e.Length / 2).Select(i => map[e.Substring(2 * i, 2)]));
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}
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var decrypted = Decrypt(encrypted, key, square);
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Console.WriteLine($"Decrypted: {decrypted}");
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