Index-Difference Encryptor/Decryptor

index-difference-encryptor-ecryptor.js

function encrypt(text) {
  if (!text) { return text; }
  const region = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.,:;-?! \'()$%&"';
  let output = '';
  let encPhase = ''; // output copy without char replacement - needed to calculate diff for replacement
  for (let i = 0, max = text.length; i < max; i++) {
    // throw error if input has characters that are out of range
    if (region.indexOf(text[i]) === -1) { throw 'Error'; }
    // enc phase 1 - every second char to upper case
    if (i % 2 === 0) {
      output += text[i];
      encPhase += text[i];
    } else {
      output += (text[i] === text[i].toUpperCase()) ? text[i].toLowerCase() : text[i].toUpperCase();
      encPhase += (text[i] === text[i].toUpperCase()) ? text[i].toLowerCase() : text[i].toUpperCase();
    }
    if (i > 0) {
      // enc phase 2 - replace every char with char at index = diff in region: original[i] - encrypted[i]
      let diff = region.indexOf(encPhase[i - 1]) - region.indexOf(output[i]);
      diff = (diff < 0) ? region.length + diff : diff;
      //console.log('>> diff:', diff, ':', region[diff]);
      output = output.substring(0, output.length - 1) + region[diff];
      //console.log('output:', output);
    }
  }
  // enc phase 3 - replace first character with char at mirror index from the range ending
  output = region[region.length - 1 - region.indexOf(output.slice(0,1))] + output.slice(1);
  return output;
}

function decrypt(encryptedText) {
  if (!encryptedText) { return encryptedText; }
  const region = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.,:;-?! \'()$%&"';
  let output = encryptedText;
  // decr phase 1 - replace first character with char at mirror index from the range start
  output = region[region.length - 1 - region.indexOf(output.slice(0,1))] + output.slice(1);
  let encPhase = output; // output copy without char replacement - needed to calculate diff for replacement
  for (let i = 1, max = output.length; i < max; i++) {
    // throw error if input has characters that are out of range
    if (region.indexOf(encryptedText[i]) === -1) { throw 'Error'; }
    // decr phase 2 - replace every char with char at index = diff in region: original[i] - encrypted[i]
    let diff = region.indexOf(encPhase[i - 1]) - region.indexOf(output[i]);
    diff = (diff < 0) ? region.length + diff : diff;
    //console.log('>> decription diff:', diff, ':', region[diff]);
    const replacement = (i % 2 === 0) ? region[diff] : (region[diff] === region[diff].toLowerCase()) ? region[diff].toUpperCase() : region[diff].toLowerCase(); // decr phase 3 - convert to lower case every 2nd char
    output = output.substring(0, i) + replacement + output.substring(i + 1, output.length);
    //console.log('output:', output);
    encPhase = encPhase.substring(0, i) + region[diff] + encPhase.substring(i + 1, encPhase.length);
    //console.log('encPhase:', encPhase);
  }
  return output;
}

encrypt("This is a test!"); // "5MyQa9p0riYplZc"
decrypt("5MyQa9p0riYplZc"); // "This is a test!"

Index-DifferenceEncryptorDecryptor.markdown

Index-Difference Encryptor/Decryptor

Function encrypt encrypts string with characters from range ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.,:;-?! \'()$%&".

Function encrypt returns null or empty string if input is empty.

Function encrypt throws Error in characters in input string are out of given range.

Function decrypt decrypts previously encrypted string with function encrypt.

A script by V.

License.