Vigenete Cipher

encrypt-decrypt.js

function isUpperCase(letter){
    let l = letter.charCodeAt();

    return l >= 65 && l <= 90;
}

function isLowerCase(letter){
    let l = letter.charCodeAt();

    return l >= 97 && l <= 122;
}

function encrypt(plainMsg, key){
    let cipher = "";

    for(let i = 0, j = 0; i < plainMsg.length; i++){
        let currentLetter = plainMsg[i];

        switch (true) {
            case isUpperCase(currentLetter):
                let upperLetter = ((currentLetter.charCodeAt() - 65)
                    + (key[j%key.length].toUpperCase().charCodeAt() - 65))
                    % 26;

                cipher += String.fromCharCode(upperLetter+65);
                j++;
                break;
            case isLowerCase(currentLetter):
                let lowerLetter = ((currentLetter.charCodeAt() - 97)
                    + (key[j%key.length].toLowerCase().charCodeAt() - 97))
                    % 26;

                cipher += String.fromCharCode(lowerLetter+97);
                j++;
                break;
            default:
                cipher += currentLetter;
                break;
        }
    }

    return cipher;
}

function decrypt(cipher, key) {
    let plainMsg = '';

    for(let i = 0, j = 0; i < cipher.length; i++){
        let currentLetter = cipher[i];

        switch (true) {
            case isUpperCase(currentLetter):
                let upperLetter = ((currentLetter.charCodeAt() - 65)
                    - (key[j%key.length].toUpperCase().charCodeAt() - 65)
                    + 26)
                    % 26;

                plainMsg += String.fromCharCode(upperLetter + 65);
                j++;
                break;
            case isLowerCase(currentLetter):
                let lowerLetter = ((currentLetter.charCodeAt() - 97)
                    - (key[j%key.length].toLowerCase().charCodeAt() - 97)
                    + 26)
                    % 26;

                plainMsg += String.fromCharCode(lowerLetter + 97);
                j++;
                break;
            default:
                plainMsg += currentLetter;
                break;
        }
    }

    return plainMsg;

}

console.log(encrypt('pawn', 'secret'))
console.log(decrypt('heye', 'secret'))