This program's approach is to divide the alphabet into two groups:
- letters under 13 (a-m) can have 13 added to get the cipher (n-z)
- letters over 12 (n-z) can have 13 subtracted to get the cipher (a-m)
The alphabet is 0-25 characters. All other characters are ignored but appended anyway (numbers, spaces, special characters).
Maybe partly derived from a Ceaser Cipher. The ceaser cipher replaced each letter with different letter by a fixed number of places. The rot13, or "rotate by 13 places" cipher replaces each letter with the 13th letter afterwards.
rot13 is considered a very weak security cipher. Sometimes used in combination with another cipher.