Encryption Algorithms

using System.Text;

namespace EncryptionAlgos {
    public class CaesarCipher {
        public string Text { get; set; } = String.Empty;
        public long Key;
        public CaesarCipher(long key) {
            Key = key % 26;
        }
        public CaesarCipher(string text, long key) {
            if (text == null) throw new ArgumentNullException("text");
            if (key < 1) throw new ArgumentOutOfRangeException("key should be a positive integer");
            Text = text;
            Key = key % 26;
        }
        public string Encrypt() {
            var encryptedText = new StringBuilder();
            foreach (char t in Text) {
                if (!Char.IsAsciiLetter(t)) {
                    encryptedText.Append(t);
                    continue;
                }
                char offset = Char.IsUpper(t) ? 'A' : 'a';
                char c = (char)((t - offset + Key) % 26 + offset);
                encryptedText.Append(c);
            }
            return encryptedText.ToString();
        }

        public static string Encrypt(string text, int key) {
            if (text == null) throw new ArgumentNullException("text");
            if (key < 1) throw new ArgumentOutOfRangeException("key should be a positive integer");
            key %= 26;
            var encryptedText = new StringBuilder();
            foreach (char t in text) {
                if (!Char.IsAsciiLetter(t)) {
                    encryptedText.Append(t);
                    continue;
                }
                char offset = Char.IsUpper(t) ? 'A' : 'a';
                char c = (char)((t - offset + key) % 26 + offset);
                encryptedText.Append(c);
            }
            return encryptedText.ToString();
        }

        public static string Decrypt(string encryptedText, long key) {
            if (encryptedText == null) throw new ArgumentNullException("encryptedText");
            if (key < 1) throw new ArgumentOutOfRangeException("key should be a positive integer");
            key %= 26;
            var decryptedText = new StringBuilder();
            foreach (char c in encryptedText) {
                if (!Char.IsAsciiLetter(c)) {
                    decryptedText.Append(c);
                    continue;
                }
                char offset = Char.IsUpper(c) ? 'A' : 'a';
                char t = (char)((c - offset - key + 26) % 26 + offset);
                decryptedText.Append(t);
            }
            return decryptedText.ToString();
        }

        public static List<string> Decrypt(string encryptedText) {
            if (encryptedText == null) throw new ArgumentNullException("encryptedText");
            var list = new List<string>();
            for (int key = 1; key <= 25; ++key)
                list.Add(Decrypt(encryptedText, key));
            return list;
        }
    }

    public class MonoAlphabeticCipher {
        public string Text { get; set; }

        public string Key { get; set; }
        public Dictionary<char, char> dict;

        public MonoAlphabeticCipher(string text) {
            if (text == null) throw new ArgumentNullException("text");
            Text = text;
            var key = new StringBuilder();
            for (char c = 'a'; c <= 'z'; ++c)
                key.Append(c);
            for (char c = 'A'; c <= 'Z'; ++c)
                key.Append(c);
            var rnd = new Random();
            for (int i = 0; i < key.Length; ++i) {
                int j = rnd.Next(0, key.Length);
                (key[i], key[j]) = (key[j], key[i]);
            }
            dict = new Dictionary<char, char>();
            int idx = 0;
            for (char c = 'a'; c <= 'z'; ++c)
                dict.Add(c, key[idx++]);
            for (char c = 'A'; c <= 'Z'; ++c)
                dict.Add(c, key[idx++]);
            Key = key.ToString();
        }
        public string Encrypt() {
            var encryptedText = new StringBuilder();
            foreach (char t in Text) {
                if (!Char.IsAsciiLetter(t)) {
                    encryptedText.Append(t);
                    continue;
                }
                encryptedText.Append(dict[t]);
            }
            return encryptedText.ToString();
        }

        public static string Encrypt(string text, string key) {
            if (text == null) throw new ArgumentNullException("text");
            if (key == null) throw new ArgumentNullException("key");

            if (!ValidKey(key))
                throw new Exception("invalid key");
            var d = new Dictionary<char, char>();
            int idx = 0;
            for (char c = 'a'; c <= 'z'; ++c)
                d.Add(c, key[idx++]);
            for (char c = 'A'; c <= 'Z'; ++c)
                d.Add(c, key[idx++]);
            var encryptedText = new StringBuilder();
            foreach (char t in text) {
                if (!Char.IsAsciiLetter(t)) {
                    encryptedText.Append(t);
                    continue;
                }
                encryptedText.Append(d[t]);
            }
            return encryptedText.ToString();
        }

        public static string Decrypt(string encryptedText, string key) {
            if (encryptedText == null) throw new ArgumentNullException("encryptedText");
            if (key == null) throw new ArgumentNullException("key");
            if (!ValidKey(key))
                throw new Exception("invalid key");
            var d = new Dictionary<char, char>();
            int idx = 0;
            for (char c = 'a'; c <= 'z'; ++c)
                d.Add(key[idx++], c);
            for (char c = 'A'; c <= 'Z'; ++c)
                d.Add(key[idx++], c);
            var decryptedText = new StringBuilder();
            foreach (char c in encryptedText) {
                if (!Char.IsAsciiLetter(c)) {
                    decryptedText.Append(c);
                    continue;
                }
                decryptedText.Append(d[c]);
            }
            return decryptedText.ToString();
        }

        static bool ValidKey(string key) {
            if (key.Length != 52) return false;
            var st = new HashSet<char>();
            foreach (char ch in key)
                st.Add(ch);
            for (char c = 'a'; c <= 'z'; ++c)
                if (!st.Contains(c)) return false;
            for (char c = 'A'; c <= 'Z'; ++c)
                if (!st.Contains(c)) return false;
            foreach (char c in key)
                if (!Char.IsAsciiLetter(c)) return false;
            return true;
        }
    }

    public class PolyAlphabeticCipher {
        public string Text { get; set; }
        public string Key { get; set; }
        public PolyAlphabeticCipher(string text) {
            if (text == null) throw new ArgumentNullException("text");
            Text = text.ToUpper();
            var rnd = new Random();
            var key = new StringBuilder();
            for (int i = 0; i < text.Length; ++i) {
                char c = (char)('A' + rnd.Next(0, 26));
                key.Append(c);
            }
            Key = key.ToString();
        }

        public PolyAlphabeticCipher(string text, string key) {
            if (text == null) throw new ArgumentNullException("text");
            if (String.IsNullOrEmpty(key)) throw new ArgumentException("key cannot be empty");
            Text = text.ToUpper();
            Key = key.ToUpper();
        }

        static int ToNumber(char c) => c - 'A';

        public string Encrypt() {
            var encryptedText = new StringBuilder();
            int idx = 0;
            foreach (char t in Text) {
                if (!Char.IsAsciiLetter(t)) {
                    encryptedText.Append(t);
                    continue;
                }
                char c = (char)((ToNumber(t) + ToNumber(Key[idx++])) % 26 + 'A');
                idx %= Key.Length;
                encryptedText.Append(c);
            }
            return encryptedText.ToString();
        }

        public static string Encrypt(string text, string key) {
            if (text == null) throw new ArgumentNullException("text");
            if (string.IsNullOrEmpty(key)) throw new ArgumentException("key cannot be empty");
            var encryptedText = new StringBuilder();
            int idx = 0;
            foreach (char t in text) {
                if (!Char.IsAsciiLetter(t)) {
                    encryptedText.Append(t);
                    continue;
                }
                char c = (char)((ToNumber(t) + ToNumber(key[idx++])) % 26 + 'A');
                idx %= key.Length;
                encryptedText.Append(c);
            }
            return encryptedText.ToString();
        }

        public static string Decrypt(string encryptedText, string key) {
            if (encryptedText == null) throw new ArgumentNullException("encryptedText");
            if (string.IsNullOrEmpty(key)) throw new ArgumentException("key cannot be empty");
            var decryptedText = new StringBuilder();
            int idx = 0;
            foreach (char c in encryptedText) {
                if (!Char.IsAsciiLetter(c)) {
                    decryptedText.Append(c);
                    continue;
                }
                char t = (char)((ToNumber(c) - ToNumber(key[idx++]) + 26) % 26 + 'A');
                idx %= key.Length;
                decryptedText.Append(t);
            }
            return decryptedText.ToString();
        }
    }
    internal class Program {
        static void Main(string[] args) {
            string s = Console.ReadLine()!;
            var cipher = new MonoAlphabeticCipher(s);
            Console.WriteLine("Key = " + cipher.Key);
            string encryption = cipher.Encrypt();
            Console.WriteLine("encrypted text = " + encryption);
            Console.WriteLine("decrypted text = " + MonoAlphabeticCipher.Decrypt(encryption, cipher.Key));
        }
    }
}