Kata

For encrypting strings this region of chars is given (in this order!):

• all letters (ascending, first all UpperCase, then all LowerCase)
• all digits (ascending)
• the following chars: .,:;-?! '()$%&"

These are 77 chars! (This region is zero-based.)

Write two methods:

string Encrypt(string text)
string Decrypt(string encryptedText)

function encrypt(text)
function decrypt(encryptedText)

export function encrypt(text:string):string
export function decrypt(encryptedText:string):string

def encrypt(text)
def decrypt(encrypted_text)

std::string encrypt(std::string text)
std::string decrypt(std::string text)

(encrypt (text))
(decrypt (text))

Prechecks:

1. If the input-string has chars, that are not in the region, throw an Exception(C#, Python) or Error(JavaScript).
   
2. If the input-string is null or empty return exactly this value!
   

For building the encrypted string:

1. For every second char do a switch of the case.
   
2. For every char take the index from the region. Take the difference from the region-index of the char before (from the input text! Not from the fresh encrypted char before!). (Char2 = Char1-Char2)
   Replace the original char by the char of the difference-value from the region. In this step the first letter of the text is unchanged.
   
3. Replace the first char by the mirror in the given region. ('A' -> '"', 'B' -> '&', ...)

Simple example:

• Input: "Business"
• Step 1: "BUsInEsS"
• Step 2: "B61kujla"
  • B -> U
    • B (1) - U (20) = -19
    • -19 + 77 = 58
    • Region[58] = "6"
  • U -> s
    • U (20) - s (44) = -24
    • -24 + 77 = 53
    • Region[53] = "1"
• Step 3: "&61kujla"

This kata is part of the Simple Encryption Series:
Simple Encryption #1 - Alternating Split
Simple Encryption #2 - Index-Difference
Simple Encryption #3 - Turn The Bits Around
Simple Encryption #4 - Qwerty

Have fun coding it and please don't forget to vote and rank this kata! :-)