Caesars Cipher in Navision

Lately I have been reading some books about Cryptology and there I stumbled upon Caesars Cipher.

Caesars Cipher is a very simple form of Cryptation. Its method is to replace each plaintext letter with one a fixed number of places down the alphabet. The fixed number is the Key.

Ex. with a key 3 – the letter A will become D. B will become E etc.

So the word SECRET will be come VHFUHW.

This form for cryptology can be represented by the following formular:

c = p + k mod n

where c = cipher, p = plaintext, k = key, n = number of letters in the alphabet

Letters (p) will be represented as numbers, where a = 0, b = 1 etc.

If the cipher is greater than the number of letters in the alphabet, we have to start from the begin of the alphabet – this is solved by mod.

Ex. the letter K & letter Z:

K is the 10’te letter in the alphabet. We use the key 3 and there is 26 letters in the used alphabet.

c = p + k mod n = 10 + 3 mod 26 = 13, which gives us the letter N.

Z is the 26’te letter in the alphabet.

c = 26 + 3 mod 26 = 29 mod 26 = 3, which gives us the letter C.

Now that the theory is on place – we can look on some functions for Navision.

To Encrypt you can use this function:

Key := 3;
NoLetters := 26;

IF TextToCrypt = '' THEN
   EXIT;

// Remove spaces
ClearText := UPPERCASE(DELCHR(TextToCrypt,'=',' '));

// Encrypt letter by letter
FOR i := 1 TO STRLEN(ClearText) DO BEGIN

   // Char to Int - where A = 0
   ClearChar := ClearText[i] - 65;

   //Encrypt char - +65 to get a letter
   CryptChar := ((ClearChar + Key) MOD NoLetters) + 65;

   // Add char to crypt text
   CryptText += FORMAT(CryptChar);
END;

TextToCrypt := UPPERCASE(TextToCrypt);

Ok now you can encrypt – but what about decryption. Decryption is actually the same as encryption – all you have to do, is to use a different key 🙂

So which key to use? The key to encrypt was 3 and the number of letters was 26. To decrypt you have to deduct 3 letters. This gives us the decrypt key equals 23 (number of letters – key = 23).

Decrypt example:

Key := 3;
NoLetters := 26;
DecryptKey := NoLetters - Key;

IF CryptText = '' THEN
   EXIT;

//DeCrypt letter by letter
FOR i := 1 TO STRLEN(CryptText) DO BEGIN

   //Char to Int - where A = 0
   CryptChar := CryptText[i] - 65;

   //Encrypt char - +65 to get a letter
   ClearChar := ((CryptChar + DecryptKey) MOD NoLetters) + 65;

   // Add char to crypt text
   DeCryptText += FORMAT(ClearChar);
END;

If you compare the two functions, you will see that the encryption and decryptions functions are pretty much the same – they only use different keys.

Now you can encrypt and decrypt with Caesars Cipher in Navision 🙂