js-encrypt-decrypt.html

<script>
    ///////////////////
    // DECRYPT
    //////////////////

    function decrypt(password, text) {

        // convert the string to decrypt into an array
        const arr = text.split('');

        // let's also create an array from our password
        const arrPass = password.split('');

        // keep control about which letter from the password we use
        let lastPassLetter = 0;

        // this is the final decrypted string
        let decrypted = '';

        // let's start...
        for (let i=0; i < arr.length; i++) {

            // next letter from the string to decrypt
            const letter = arr[ i ];

            // get the next letter from the password
            const passwordLetter = arrPass[ lastPassLetter ];

            // get the decrypted letter according to the password
            const temp = getInvertedLetterFromAlphabetForLetter( passwordLetter, letter );

            if (temp) {
                // concat the response
                decrypted += temp;
            } else {
                // if any error, return null
                return null;
            }		

            // if our password is too short, let's start again from the first letter
            if (lastPassLetter == (arrPass.length - 1) ) {
                lastPassLetter = 0;
            } else {
                lastPassLetter ++;
            }		
        }

        // return the decrypted string and converted from base64 to plain text
        return atob( decrypted );
    }


    function getInvertedLetterFromAlphabetForLetter(letter, letterToChange) {

        const abc = 'abcdefghijklmnopqrstuvwxyz0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ';

        const posLetter = abc.indexOf( letter );

        if (posLetter == -1) {
            console.log('Password letter ' + letter + ' not allowed.');
            return null;
        }

        const part1 = abc.substring(posLetter, abc.length);
        const part2 = abc.substring(0, posLetter);

        const newABC = '' + part1 + '' + part2;	

        const posLetterToChange = newABC.indexOf( letterToChange );

        if (posLetterToChange == -1) {
            console.log('Password letter ' + letter + ' not allowed.');
            return null;
        }

        const letterAccordingToAbc = abc.split('')[ posLetterToChange ];

        return letterAccordingToAbc;	
    }



    ///////////////////
    // ENCRYPT
    //////////////////

    function encrypt(password, text) {

        // move text to base64
        const base64 = btoa( text );

        // text string to array
        const arr = base64.split('');

        // arr of password
        const arrPass = password.split('');
        let lastPassLetter = 0;

        // encrypted string
        let encrypted = '';

        // encrypt
        for (let i=0; i < arr.length; i++) {

            const letter = arr[ i ];

            const passwordLetter = arrPass[ lastPassLetter ];

            const temp = getLetterFromAlphabetForLetter( passwordLetter, letter );

            if (temp) {
                // concat to the final response encrypted string
                encrypted += temp;
            } else {
                // if any error, return null
                return null;
            }		

            /*
                This is important: if we're out of letters in our 
                password, we need to start from the begining.
            */
            if (lastPassLetter == (arrPass.length - 1) ) {
                lastPassLetter = 0;
            } else {
                lastPassLetter ++;
            }		
        }

        // We finally return the encrypted string
        return encrypted;
    }


    function getLetterFromAlphabetForLetter(letter, letterToChange) {

        // this is the alphabet we know, plus numbers and the = sign 
        const abc = 'abcdefghijklmnopqrstuvwxyz0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ';

        // get the position of the given letter, according to our abc
        const posLetter = abc.indexOf( letter );

        // if we cannot get it, then we can't continue
        if (posLetter == -1) {
            console.log('Password letter ' + letter + ' not allowed.');
            return null;
        }

        // according to our abc, get the position of the letter to encrypt
        const posLetterToChange = abc.indexOf( letterToChange );

        // again, if any error, we cannot continue...
        if (posLetterToChange == -1) {
            console.log('Password letter ' + letter + ' not allowed.');
            return null;
        }

        // let's build the new abc. this is the important part
        const part1 = abc.substring(posLetter, abc.length);
        const part2 = abc.substring(0, posLetter);
        const newABC = '' + part1 + '' + part2;

        // we get the encrypted letter
        const letterAccordingToAbc = newABC.split('')[ posLetterToChange ];

        // and return to the routine...
        return letterAccordingToAbc;	
    }

    
    const text = 'Hello World';
    const password = "pass";
    
    const e = encrypt(password, text);
    console.log('encrypted: ' + e);

    const d = decrypt('pass', e);
    console.log('decrypted: ' + d);

</script>