BlowfishEncoder.cs

using System;
using System.Linq;
using System.Text;

namespace Algorithms.Encoders;

/// <summary>
/// <para>
/// The Blowfish algorithm is a symmetric-key block cipher, which means it uses the same secret key to encrypt and
/// decrypt data. It was designed by Bruce Schneier in 1993.
/// </para>
/// <para>
/// The blowfish algorithm works on 64-bit blocks of data, which are divided into two 32-bit halves: left and right.
/// It uses a variable-length key, from 32 bits to 448 bits, to generate 18 subkeys and four S-boxes, which are arrays
/// of 256 32-bit words. The subkeys and the S-boxes are key-dependent, meaning that they change according to the secret key.
/// </para>
/// <para>
/// The blowfish algorithm performs 16 rounds of encryption or decryption on each block of data, using a Feistel network
/// structure. In each round, the left half is XORed with a subkey, then passed through a function F that applies four
/// S-box lookups and two XOR operations. The output of F is then XORed with the right half. The left and right halves
/// are swapped at the end of each round, except for the last one. The final output is XORed with two more subkeys to
/// produce the encrypted or decrypted block.
/// </para>
/// <see href="https://en.wikipedia.org/wiki/Blowfish_%28cipher%29">Blowfish on Wikipedia</see>.
/// </summary>
public class BlowfishEncoder
{
    // Initialize modVal to 2^32
    private const ulong ModVal = 4294967296L;

    // Initialize the substitution boxes
    private readonly string[][] s =
    {
        new[]
        {
            "d1310ba6", "98dfb5ac", "2ffd72db", "d01adfb7", "b8e1afed", "6a267e96", "ba7c9045", "f12c7f99",
            "24a19947", "b3916cf7", "0801f2e2", "858efc16", "636920d8", "71574e69", "a458fea3", "f4933d7e",
            "0d95748f", "728eb658", "718bcd58", "82154aee", "7b54a41d", "c25a59b5", "9c30d539", "2af26013",
            "c5d1b023", "286085f0", "ca417918", "b8db38ef", "8e79dcb0", "603a180e", "6c9e0e8b", "b01e8a3e",
            "d71577c1", "bd314b27", "78af2fda", "55605c60", "e65525f3", "aa55ab94", "57489862", "63e81440",
            "55ca396a", "2aab10b6", "b4cc5c34", "1141e8ce", "a15486af", "7c72e993", "b3ee1411", "636fbc2a",
            "2ba9c55d", "741831f6", "ce5c3e16", "9b87931e", "afd6ba33", "6c24cf5c", "7a325381", "28958677",
            "3b8f4898", "6b4bb9af", "c4bfe81b", "66282193", "61d809cc", "fb21a991", "487cac60", "5dec8032",
            "ef845d5d", "e98575b1", "dc262302", "eb651b88", "23893e81", "d396acc5", "0f6d6ff3", "83f44239",
            "2e0b4482", "a4842004", "69c8f04a", "9e1f9b5e", "21c66842", "f6e96c9a", "670c9c61", "abd388f0",
            "6a51a0d2", "d8542f68", "960fa728", "ab5133a3", "6eef0b6c", "137a3be4", "ba3bf050", "7efb2a98",
            "a1f1651d", "39af0176", "66ca593e", "82430e88", "8cee8619", "456f9fb4", "7d84a5c3", "3b8b5ebe",
            "e06f75d8", "85c12073", "401a449f", "56c16aa6", "4ed3aa62", "363f7706", "1bfedf72", "429b023d",
            "37d0d724", "d00a1248", "db0fead3", "49f1c09b", "075372c9", "80991b7b", "25d479d8", "f6e8def7",
            "e3fe501a", "b6794c3b", "976ce0bd", "04c006ba", "c1a94fb6", "409f60c4", "5e5c9ec2", "196a2463",
            "68fb6faf", "3e6c53b5", "1339b2eb", "3b52ec6f", "6dfc511f", "9b30952c", "cc814544", "af5ebd09",
            "bee3d004", "de334afd", "660f2807", "192e4bb3", "c0cba857", "45c8740f", "d20b5f39", "b9d3fbdb",
            "5579c0bd", "1a60320a", "d6a100c6", "402c7279", "679f25fe", "fb1fa3cc", "8ea5e9f8", "db3222f8",
            "3c7516df", "fd616b15", "2f501ec8", "ad0552ab", "323db5fa", "fd238760", "53317b48", "3e00df82",
            "9e5c57bb", "ca6f8ca0", "1a87562e", "df1769db", "d542a8f6", "287effc3", "ac6732c6", "8c4f5573",
            "695b27b0", "bbca58c8", "e1ffa35d", "b8f011a0", "10fa3d98", "fd2183b8", "4afcb56c", "2dd1d35b",
            "9a53e479", "b6f84565", "d28e49bc", "4bfb9790", "e1ddf2da", "a4cb7e33", "62fb1341", "cee4c6e8",
            "ef20cada", "36774c01", "d07e9efe", "2bf11fb4", "95dbda4d", "ae909198", "eaad8e71", "6b93d5a0",
            "d08ed1d0", "afc725e0", "8e3c5b2f", "8e7594b7", "8ff6e2fb", "f2122b64", "8888b812", "900df01c",
            "4fad5ea0", "688fc31c", "d1cff191", "b3a8c1ad", "2f2f2218", "be0e1777", "ea752dfe", "8b021fa1",
            "e5a0cc0f", "b56f74e8", "18acf3d6", "ce89e299", "b4a84fe0", "fd13e0b7", "7cc43b81", "d2ada8d9",
            "165fa266", "80957705", "93cc7314", "211a1477", "e6ad2065", "77b5fa86", "c75442f5", "fb9d35cf",
            "ebcdaf0c", "7b3e89a0", "d6411bd3", "ae1e7e49", "00250e2d", "2071b35e", "226800bb", "57b8e0af",
            "2464369b", "f009b91e", "5563911d", "59dfa6aa", "78c14389", "d95a537f", "207d5ba2", "02e5b9c5",
            "83260376", "6295cfa9", "11c81968", "4e734a41", "b3472dca", "7b14a94a", "1b510052", "9a532915",
            "d60f573f", "bc9bc6e4", "2b60a476", "81e67400", "08ba6fb5", "571be91f", "f296ec6b", "2a0dd915",
            "b6636521", "e7b9f9b6", "ff34052e", "c5855664", "53b02d5d", "a99f8fa1", "08ba4799", "6e85076a",
        },
        new[]
        {
            "4b7a70e9", "b5b32944", "db75092e", "c4192623", "ad6ea6b0", "49a7df7d", "9cee60b8", "8fedb266",
            "ecaa8c71", "699a17ff", "5664526c", "c2b19ee1", "193602a5", "75094c29", "a0591340", "e4183a3e",
            "3f54989a", "5b429d65", "6b8fe4d6", "99f73fd6", "a1d29c07", "efe830f5", "4d2d38e6", "f0255dc1",
            "4cdd2086", "8470eb26", "6382e9c6", "021ecc5e", "09686b3f", "3ebaefc9", "3c971814", "6b6a70a1",
            "687f3584", "52a0e286", "b79c5305", "aa500737", "3e07841c", "7fdeae5c", "8e7d44ec", "5716f2b8",
            "b03ada37", "f0500c0d", "f01c1f04", "0200b3ff", "ae0cf51a", "3cb574b2", "25837a58", "dc0921bd",
            "d19113f9", "7ca92ff6", "94324773", "22f54701", "3ae5e581", "37c2dadc", "c8b57634", "9af3dda7",
            "a9446146", "0fd0030e", "ecc8c73e", "a4751e41", "e238cd99", "3bea0e2f", "3280bba1", "183eb331",
            "4e548b38", "4f6db908", "6f420d03", "f60a04bf", "2cb81290", "24977c79", "5679b072", "bcaf89af",
            "de9a771f", "d9930810", "b38bae12", "dccf3f2e", "5512721f", "2e6b7124", "501adde6", "9f84cd87",
            "7a584718", "7408da17", "bc9f9abc", "e94b7d8c", "ec7aec3a", "db851dfa", "63094366", "c464c3d2",
            "ef1c1847", "3215d908", "dd433b37", "24c2ba16", "12a14d43", "2a65c451", "50940002", "133ae4dd",
            "71dff89e", "10314e55", "81ac77d6", "5f11199b", "043556f1", "d7a3c76b", "3c11183b", "5924a509",
            "f28fe6ed", "97f1fbfa", "9ebabf2c", "1e153c6e", "86e34570", "eae96fb1", "860e5e0a", "5a3e2ab3",
            "771fe71c", "4e3d06fa", "2965dcb9", "99e71d0f", "803e89d6", "5266c825", "2e4cc978", "9c10b36a",
            "c6150eba", "94e2ea78", "a5fc3c53", "1e0a2df4", "f2f74ea7", "361d2b3d", "1939260f", "19c27960",
            "5223a708", "f71312b6", "ebadfe6e", "eac31f66", "e3bc4595", "a67bc883", "b17f37d1", "018cff28",
            "c332ddef", "be6c5aa5", "65582185", "68ab9802", "eecea50f", "db2f953b", "2aef7dad", "5b6e2f84",
            "1521b628", "29076170", "ecdd4775", "619f1510", "13cca830", "eb61bd96", "0334fe1e", "aa0363cf",
            "b5735c90", "4c70a239", "d59e9e0b", "cbaade14", "eecc86bc", "60622ca7", "9cab5cab", "b2f3846e",
            "648b1eaf", "19bdf0ca", "a02369b9", "655abb50", "40685a32", "3c2ab4b3", "319ee9d5", "c021b8f7",
            "9b540b19", "875fa099", "95f7997e", "623d7da8", "f837889a", "97e32d77", "11ed935f", "16681281",
            "0e358829", "c7e61fd6", "96dedfa1", "7858ba99", "57f584a5", "1b227263", "9b83c3ff", "1ac24696",
            "cdb30aeb", "532e3054", "8fd948e4", "6dbc3128", "58ebf2ef", "34c6ffea", "fe28ed61", "ee7c3c73",
            "5d4a14d9", "e864b7e3", "42105d14", "203e13e0", "45eee2b6", "a3aaabea", "db6c4f15", "facb4fd0",
            "c742f442", "ef6abbb5", "654f3b1d", "41cd2105", "d81e799e", "86854dc7", "e44b476a", "3d816250",
            "cf62a1f2", "5b8d2646", "fc8883a0", "c1c7b6a3", "7f1524c3", "69cb7492", "47848a0b", "5692b285",
            "095bbf00", "ad19489d", "1462b174", "23820e00", "58428d2a", "0c55f5ea", "1dadf43e", "233f7061",
            "3372f092", "8d937e41", "d65fecf1", "6c223bdb", "7cde3759", "cbee7460", "4085f2a7", "ce77326e",
            "a6078084", "19f8509e", "e8efd855", "61d99735", "a969a7aa", "c50c06c2", "5a04abfc", "800bcadc",
            "9e447a2e", "c3453484", "fdd56705", "0e1e9ec9", "db73dbd3", "105588cd", "675fda79", "e3674340",
            "c5c43465", "713e38d8", "3d28f89e", "f16dff20", "153e21e7", "8fb03d4a", "e6e39f2b", "db83adf7",
        },
        new[]
        {
            "e93d5a68", "948140f7", "f64c261c", "94692934", "411520f7", "7602d4f7", "bcf46b2e", "d4a20068",
            "d4082471", "3320f46a", "43b7d4b7", "500061af", "1e39f62e", "97244546", "14214f74", "bf8b8840",
            "4d95fc1d", "96b591af", "70f4ddd3", "66a02f45", "bfbc09ec", "03bd9785", "7fac6dd0", "31cb8504",
            "96eb27b3", "55fd3941", "da2547e6", "abca0a9a", "28507825", "530429f4", "0a2c86da", "e9b66dfb",
            "68dc1462", "d7486900", "680ec0a4", "27a18dee", "4f3ffea2", "e887ad8c", "b58ce006", "7af4d6b6",
            "aace1e7c", "d3375fec", "ce78a399", "406b2a42", "20fe9e35", "d9f385b9", "ee39d7ab", "3b124e8b",
            "1dc9faf7", "4b6d1856", "26a36631", "eae397b2", "3a6efa74", "dd5b4332", "6841e7f7", "ca7820fb",
            "fb0af54e", "d8feb397", "454056ac", "ba489527", "55533a3a", "20838d87", "fe6ba9b7", "d096954b",
            "55a867bc", "a1159a58", "cca92963", "99e1db33", "a62a4a56", "3f3125f9", "5ef47e1c", "9029317c",
            "fdf8e802", "04272f70", "80bb155c", "05282ce3", "95c11548", "e4c66d22", "48c1133f", "c70f86dc",
            "07f9c9ee", "41041f0f", "404779a4", "5d886e17", "325f51eb", "d59bc0d1", "f2bcc18f", "41113564",
            "257b7834", "602a9c60", "dff8e8a3", "1f636c1b", "0e12b4c2", "02e1329e", "af664fd1", "cad18115",
            "6b2395e0", "333e92e1", "3b240b62", "eebeb922", "85b2a20e", "e6ba0d99", "de720c8c", "2da2f728",
            "d0127845", "95b794fd", "647d0862", "e7ccf5f0", "5449a36f", "877d48fa", "c39dfd27", "f33e8d1e",
            "0a476341", "992eff74", "3a6f6eab", "f4f8fd37", "a812dc60", "a1ebddf8", "991be14c", "db6e6b0d",
            "c67b5510", "6d672c37", "2765d43b", "dcd0e804", "f1290dc7", "cc00ffa3", "b5390f92", "690fed0b",
            "667b9ffb", "cedb7d9c", "a091cf0b", "d9155ea3", "bb132f88", "515bad24", "7b9479bf", "763bd6eb",
            "37392eb3", "cc115979", "8026e297", "f42e312d", "6842ada7", "c66a2b3b", "12754ccc", "782ef11c",
            "6a124237", "b79251e7", "06a1bbe6", "4bfb6350", "1a6b1018", "11caedfa", "3d25bdd8", "e2e1c3c9",
            "44421659", "0a121386", "d90cec6e", "d5abea2a", "64af674e", "da86a85f", "bebfe988", "64e4c3fe",
            "9dbc8057", "f0f7c086", "60787bf8", "6003604d", "d1fd8346", "f6381fb0", "7745ae04", "d736fccc",
            "83426b33", "f01eab71", "b0804187", "3c005e5f", "77a057be", "bde8ae24", "55464299", "bf582e61",
            "4e58f48f", "f2ddfda2", "f474ef38", "8789bdc2", "5366f9c3", "c8b38e74", "b475f255", "46fcd9b9",
            "7aeb2661", "8b1ddf84", "846a0e79", "915f95e2", "466e598e", "20b45770", "8cd55591", "c902de4c",
            "b90bace1", "bb8205d0", "11a86248", "7574a99e", "b77f19b6", "e0a9dc09", "662d09a1", "c4324633",
            "e85a1f02", "09f0be8c", "4a99a025", "1d6efe10", "1ab93d1d", "0ba5a4df", "a186f20f", "2868f169",
            "dcb7da83", "573906fe", "a1e2ce9b", "4fcd7f52", "50115e01", "a70683fa", "a002b5c4", "0de6d027",
            "9af88c27", "773f8641", "c3604c06", "61a806b5", "f0177a28", "c0f586e0", "006058aa", "30dc7d62",
            "11e69ed7", "2338ea63", "53c2dd94", "c2c21634", "bbcbee56", "90bcb6de", "ebfc7da1", "ce591d76",
            "6f05e409", "4b7c0188", "39720a3d", "7c927c24", "86e3725f", "724d9db9", "1ac15bb4", "d39eb8fc",
            "ed545578", "08fca5b5", "d83d7cd3", "4dad0fc4", "1e50ef5e", "b161e6f8", "a28514d9", "6c51133c",
            "6fd5c7e7", "56e14ec4", "362abfce", "ddc6c837", "d79a3234", "92638212", "670efa8e", "406000e0",
        },
        new[]
        {
            "3a39ce37", "d3faf5cf", "abc27737", "5ac52d1b", "5cb0679e", "4fa33742", "d3822740", "99bc9bbe",
            "d5118e9d", "bf0f7315", "d62d1c7e", "c700c47b", "b78c1b6b", "21a19045", "b26eb1be", "6a366eb4",
            "5748ab2f", "bc946e79", "c6a376d2", "6549c2c8", "530ff8ee", "468dde7d", "d5730a1d", "4cd04dc6",
            "2939bbdb", "a9ba4650", "ac9526e8", "be5ee304", "a1fad5f0", "6a2d519a", "63ef8ce2", "9a86ee22",
            "c089c2b8", "43242ef6", "a51e03aa", "9cf2d0a4", "83c061ba", "9be96a4d", "8fe51550", "ba645bd6",
            "2826a2f9", "a73a3ae1", "4ba99586", "ef5562e9", "c72fefd3", "f752f7da", "3f046f69", "77fa0a59",
            "80e4a915", "87b08601", "9b09e6ad", "3b3ee593", "e990fd5a", "9e34d797", "2cf0b7d9", "022b8b51",
            "96d5ac3a", "017da67d", "d1cf3ed6", "7c7d2d28", "1f9f25cf", "adf2b89b", "5ad6b472", "5a88f54c",
            "e029ac71", "e019a5e6", "47b0acfd", "ed93fa9b", "e8d3c48d", "283b57cc", "f8d56629", "79132e28",
            "785f0191", "ed756055", "f7960e44", "e3d35e8c", "15056dd4", "88f46dba", "03a16125", "0564f0bd",
            "c3eb9e15", "3c9057a2", "97271aec", "a93a072a", "1b3f6d9b", "1e6321f5", "f59c66fb", "26dcf319",
            "7533d928", "b155fdf5", "03563482", "8aba3cbb", "28517711", "c20ad9f8", "abcc5167", "ccad925f",
            "4de81751", "3830dc8e", "379d5862", "9320f991", "ea7a90c2", "fb3e7bce", "5121ce64", "774fbe32",
            "a8b6e37e", "c3293d46", "48de5369", "6413e680", "a2ae0810", "dd6db224", "69852dfd", "09072166",
            "b39a460a", "6445c0dd", "586cdecf", "1c20c8ae", "5bbef7dd", "1b588d40", "ccd2017f", "6bb4e3bb",
            "dda26a7e", "3a59ff45", "3e350a44", "bcb4cdd5", "72eacea8", "fa6484bb", "8d6612ae", "bf3c6f47",
            "d29be463", "542f5d9e", "aec2771b", "f64e6370", "740e0d8d", "e75b1357", "f8721671", "af537d5d",
            "4040cb08", "4eb4e2cc", "34d2466a", "0115af84", "e1b00428", "95983a1d", "06b89fb4", "ce6ea048",
            "6f3f3b82", "3520ab82", "011a1d4b", "277227f8", "611560b1", "e7933fdc", "bb3a792b", "344525bd",
            "a08839e1", "51ce794b", "2f32c9b7", "a01fbac9", "e01cc87e", "bcc7d1f6", "cf0111c3", "a1e8aac7",
            "1a908749", "d44fbd9a", "d0dadecb", "d50ada38", "0339c32a", "c6913667", "8df9317c", "e0b12b4f",
            "f79e59b7", "43f5bb3a", "f2d519ff", "27d9459c", "bf97222c", "15e6fc2a", "0f91fc71", "9b941525",
            "fae59361", "ceb69ceb", "c2a86459", "12baa8d1", "b6c1075e", "e3056a0c", "10d25065", "cb03a442",
            "e0ec6e0e", "1698db3b", "4c98a0be", "3278e964", "9f1f9532", "e0d392df", "d3a0342b", "8971f21e",
            "1b0a7441", "4ba3348c", "c5be7120", "c37632d8", "df359f8d", "9b992f2e", "e60b6f47", "0fe3f11d",
            "e54cda54", "1edad891", "ce6279cf", "cd3e7e6f", "1618b166", "fd2c1d05", "848fd2c5", "f6fb2299",
            "f523f357", "a6327623", "93a83531", "56cccd02", "acf08162", "5a75ebb5", "6e163697", "88d273cc",
            "de966292", "81b949d0", "4c50901b", "71c65614", "e6c6c7bd", "327a140a", "45e1d006", "c3f27b9a",
            "c9aa53fd", "62a80f00", "bb25bfe2", "35bdd2f6", "71126905", "b2040222", "b6cbcf7c", "cd769c2b",
            "53113ec0", "1640e3d3", "38abbd60", "2547adf0", "ba38209c", "f746ce76", "77afa1c5", "20756060",
            "85cbfe4e", "8ae88dd8", "7aaaf9b0", "4cf9aa7e", "1948c25c", "02fb8a8c", "01c36ae4", "d6ebe1f9",
            "90d4f869", "a65cdea0", "3f09252d", "c208e69f", "b74e6132", "ce77e25b", "578fdfe3", "3ac372e6",
        },
    };

    // Initialize the P-array sub-keys
    private readonly string[] p =
    {
        "243f6a88", "85a308d3", "13198a2e", "03707344", "a4093822", "299f31d0", "082efa98", "ec4e6c89", "452821e6",
        "38d01377", "be5466cf", "34e90c6c", "c0ac29b7", "c97c50dd", "3f84d5b5", "b5470917", "9216d5d9", "8979fb1b",
    };

    /// <summary>
    ///     Generate a key for the encryption algorithm based on the given string parameter.
    /// </summary>
    /// <param name="key">The key to generate the subkey from.</param>
    public void GenerateKey(string key)
    {
        var j = 0;
        for (var i = 0; i < p.Length; i++)
        {
            // Perform the key expansion
            var subKey = key.Substring(j % key.Length, 8);
            p[i] = Xor(p[i], subKey);

            j += 8;
        }
    }

    /// <summary>
    /// Encrypts a string using the blowfish algorithm.
    /// </summary>
    /// <param name="plainText">The string to be encrypted, represented as a hexadecimal string.</param>
    /// <returns>The encrypted string, represented as a hexadecimal string.</returns>
    public string Encrypt(string plainText)
    {
        // Perform the 16 rounds of the blowfish algorithm on the plainText.
        for (var i = 0; i < 16; i++)
        {
            plainText = Round(i, plainText);
        }

        // Swap the left and right parts of the plainText.
        var left = plainText.Substring(8, 8);
        var right = plainText[..8];

        // XOR the left half with the last subkey of the P-array.
        left = Xor(left, p[17]);

        // XOR the right half with the second to last subkey from the P-array.
        right = Xor(right, p[16]);

        // Return the encrypted string as a concatenated string.
        return left + right;
    }

    /// <summary>
    /// Decrypts a string using the blowfish algorithm.
    /// </summary>
    /// <param name="cipherText">The string to be decrypted, represented as a hexadecimal string.</param>
    /// <returns>The decrypted string, represented as a hexadecimal string.</returns>
    public string Decrypt(string cipherText)
    {
        // Perform 16 rounds of the blowfish algorithm on the cipherText in reverse order.
        for (var i = 17; i > 1; i--)
        {
            cipherText = Round(i, cipherText);
        }

        // Swap the left and right halves of the cipherText.
        var left = cipherText.Substring(8, 8);
        var right = cipherText.Substring(0, 8);

        // XOR the left half with the first subkey from the P-array.
        left = Xor(left, p[0]);

        // XOR the right half with the second subkey from the P-array.
        right = Xor(right, p[1]);

        // Return the decrypted string as a concatenated string.
        return left + right;
    }

    /// <summary>
    ///     Converts a hexadecimal string to a binary string.
    /// </summary>
    /// <param name="hex">The hexadecimal string to convert.</param>
    /// <returns>A multiple of 4 binary string representing the hexadecimal input.</returns>
    private string HexadecimalToBinary(string hex)
    {
        return hex.Select(t =>

                // Convert each character to an integer using base 16
                Convert.ToString(Convert.ToInt32(t.ToString(), 16), 2))

            // Pad each binary string with leading zeros to make it 4 bits long
            .Select(fourBitBinary => fourBitBinary.PadLeft(4, '0'))

            // Concatenate all the binary strings into one
            .Aggregate(string.Empty, (current, fourBitBinary) => current + fourBitBinary);
    }

    /// <summary>
    ///     Converts a binary string to a hexadecimal string.
    /// </summary>
    /// <param name="binaryInput">The multiple of 4 binary string to convert.</param>
    /// <returns>A hexadecimal string representing the binary input.</returns>
    private string BinaryToHexadecimal(string binaryInput)
    {
        return string.Concat(
            Enumerable.Range(0, binaryInput.Length / 4)

                // Select each group of 4 bits
                .Select(index => binaryInput.Substring(index * 4, 4))

                // Convert each group to an integer using base 2
                .Select(fourBitBinary => Convert.ToInt32(fourBitBinary, 2)

                    // Convert each integer to a hexadecimal character using base 16
                    .ToString("x")));
    }

    /// <summary>
    ///     Performs a bitwise XOR operation on two hexadecimal strings and returns the result.
    /// </summary>
    /// <param name="left">The first hexadecimal string to XOR.</param>
    /// <param name="right">The second hexadecimal string to XOR.</param>
    /// <returns>A hexadecimal string representing the XOR of the inputs.</returns>
    private string Xor(string left, string right)
    {
        // Convert the hexadecimal strings to binary strings using a helper method
        left = HexadecimalToBinary(left);
        right = HexadecimalToBinary(right);

        var xor = new StringBuilder();

        // Loop through each bit in the binary strings
        for (var i = 0; i < left.Length; i++)
        {
            // Perform a bitwise XOR operation on the corresponding bits and append the result to xor
            xor.Append((char)(((left[i] - '0') ^ (right[i] - '0')) + '0'));
        }

        // Convert the binary string to a hexadecimal string
        var result = BinaryToHexadecimal(xor.ToString());
        return result;
    }

    /// <summary>
    ///     Adds two hexadecimal strings and returns the result modulo _modVal.
    /// </summary>
    /// <param name="left">The first hexadecimal string to add.</param>
    /// <param name="right">The second hexadecimal string to add.</param>
    /// <returns>A hexadecimal string representing the sum of the inputs modulo _modVal.</returns>
    private string AddAndMod(string left, string right)
    {
        // Convert the hexadecimal strings to unsigned 64-bit integers using base 16
        var leftNumber = Convert.ToUInt64(left, 16);
        var rightNumber = Convert.ToUInt64(right, 16);

        // Add the two integers and calculate the remainder after dividing by _modVal
        var total = (leftNumber + rightNumber) % ModVal;

        // Convert the result to a hexadecimal string using base 16
        var result = total.ToString("x");

        // Pad the result with leading zeros to make it 8 characters long
        result = "00000000" + result;

        // Return the last 8 characters of the result
        return result[^8..];
    }

    /// <summary>
    ///     Performs the F function on a 32-bit input and returns a 32-bit output.
    /// </summary>
    /// <param name="plainText">The 32-bit hexadecimal input to the F function.</param>
    /// <returns>The 32-bit hexadecimal output of the F function.</returns>
    /// <remarks>
    ///     The F function is a non-linear function that operates on a 32-bit input and produces a 32-bit output. It is used
    ///     to generate the sub-keys and to perform the encryption and decryption of the data blocks.
    /// </remarks>
    private string F(string plainText)
    {
        var a = new string[4];

        for (var i = 0; i < 8; i += 2)
        {
            var col = Convert.ToUInt64(HexadecimalToBinary(plainText.Substring(i, 2)), 2);
            a[i / 2] = s[i / 2][col];
        }

        var answer = AddAndMod(a[0], a[1]);
        answer = Xor(answer, a[2]);
        answer = AddAndMod(answer, a[3]);
        return answer;
    }

    /// <summary>
    /// Performs one round of the blowfish encryption on a 64-bit block of data.
    /// </summary>
    /// <param name="feistelRound">The round number, from 0 to 15, indicating which subkey from the P-array to use.</param>
    /// <param name="plainText">The 64-bit block of data to be encrypted or decrypted, represented as a hexadecimal string.</param>
    /// <returns>The encrypted or decrypted block of data, represented as a hexadecimal string.</returns>
    private string Round(int feistelRound, string plainText)
    {
        // Split the plainText into two 32-bit halves.
        var left = plainText[..8];
        var right = plainText.Substring(8, 8);

        // XOR the left half with the subkey from the P-array.
        left = Xor(left, p[feistelRound]);

        // Apply the F function to the left half.
        var fOutput = F(left);

        // XOR the output of the F function with the right half.
        right = Xor(fOutput, right);

        // Swap the left and right halves and return them as a concatenated string.
        return right + left;
    }
}