BlowfishEncoder.cs

usingSystem;
usingSystem.Linq;
usingSystem.Text;

namespaceAlgorithms.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>
publicclassBlowfishEncoder
{
// Initialize modVal to 2^32
privateconstulongModVal=4294967296L;

// Initialize the substitution boxes
privatereadonlystring[][]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
privatereadonlystring[]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>
publicvoidGenerateKey(stringkey)
{
varj=0;
for(vari=0;i<p.Length;i++)
{
// Perform the key expansion
varsubKey=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>
publicstringEncrypt(stringplainText)
{
// Perform the 16 rounds of the blowfish algorithm on the plainText.
for(vari=0;i<16;i++)
{
plainText=Round(i,plainText);
}

// Swap the left and right parts of the plainText.
varleft=plainText.Substring(8,8);
varright=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.
returnleft+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>
publicstringDecrypt(stringcipherText)
{
// Perform 16 rounds of the blowfish algorithm on the cipherText in reverse order.
for(vari=17;i>1;i--)
{
cipherText=Round(i,cipherText);
}

// Swap the left and right halves of the cipherText.
varleft=cipherText.Substring(8,8);
varright=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.
returnleft+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>
privatestringHexadecimalToBinary(stringhex)
{
returnhex.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>
privatestringBinaryToHexadecimal(stringbinaryInput)
{
returnstring.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>
privatestringXor(stringleft,stringright)
{
// Convert the hexadecimal strings to binary strings using a helper method
left=HexadecimalToBinary(left);
right=HexadecimalToBinary(right);

varxor=newStringBuilder();

// Loop through each bit in the binary strings
for(vari=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
varresult=BinaryToHexadecimal(xor.ToString());
returnresult;
}

/// <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>
privatestringAddAndMod(stringleft,stringright)
{
// Convert the hexadecimal strings to unsigned 64-bit integers using base 16
varleftNumber=Convert.ToUInt64(left,16);
varrightNumber=Convert.ToUInt64(right,16);

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

// Convert the result to a hexadecimal string using base 16
varresult=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
returnresult[^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>
privatestringF(stringplainText)
{
vara=newstring[4];

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

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

/// <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>
privatestringRound(intfeistelRound,stringplainText)
{
// Split the plainText into two 32-bit halves.
varleft=plainText[..8];
varright=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.
varfOutput=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.
returnright+left;
}
}