Custom strong cryptography algorithm on JS

custom_crypto_algorithm.js

function xorWithKey(input, key) {
    let output = "";
    for (let i = 0; i < input.length; i++) {
        let keyChar = String(key[i % key.length]);
        output += String.fromCharCode(input.charCodeAt(i) ^ keyChar.charCodeAt(0));
    }
    return output;
}

function customSubstitution(input, substitutionTable) {
    let output = "";
    for (let i = 0; i < input.length; i++) {
        let charCode = input.charCodeAt(i);
        output += String.fromCharCode(substitutionTable[charCode]);
    }
    return output;
}

function generateSubstitutionTable(key) {
    let table = Array.from({ length: 256 }, (_, i) => i);
    let keySum = Array.from(key).reduce((sum, char) => sum + char.charCodeAt(0), 0);
    
    for (let i = 0; i < 256; i++) {
        let swapIndex = (keySum + i) % 256;
        [table[i], table[swapIndex]] = [table[swapIndex], table[i]];
    }

    return table;
}

function customEncrypt(plaintext, key) {
    let xorResult = xorWithKey(plaintext, key);
    let substitutionTable = generateSubstitutionTable(key);
    let substituted = customSubstitution(xorResult, substitutionTable);
    let ciphertext = substituted.split("").reverse().join("");
    return ciphertext;
}

function customDecrypt(ciphertext, key) {
    let reversed = ciphertext.split("").reverse().join("");
    let substitutionTable = generateSubstitutionTable(key).map((value, index) => [index, value]).sort((a, b) => a[1] - b[1]).map(([_, index]) => index);
    let unsubstituted = customSubstitution(reversed, substitutionTable);
    let plaintext = xorWithKey(unsubstituted, key);
    return plaintext;
}
function generateRandomString(length) {
    let result = "";
    let characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*()-=_+{}[]|;:<>,.?/";
    let charactersLength = characters.length;

    for (let i = 0; i < length; i++) {
        result += characters.charAt(Math.floor(Math.random() * charactersLength));
    }

    return result;
}

function testCustomEncryptionAndDecryption(numTests) {
    for (let i = 1; i <= numTests; i++) {
        let plaintext = generateRandomString(Math.floor(Math.random() * 20) + 5);
        let key = Math.floor(Math.random() * 10) + 5;

        // console.log(`Test ${i} - Plaintext: "${plaintext}", Key: "${key}"`);

        let encrypted = customEncrypt(plaintext, key);
        // console.log("Encrypted:", encrypted);

        let decrypted = customDecrypt(encrypted, key);
        // console.log("Decrypted:", decrypted);

        if (plaintext === decrypted) {
            // console.log("Test passed: original plaintext and decrypted ciphertext match.\n");
        } else {
            console.log("Test failed: original plaintext and decrypted ciphertext do not match.\n");
        }
    }
}

testCustomEncryptionAndDecryption(200000);