I understand the reasons for and against ROT13, but I'm wondering why specifically people have chosen 13 places to shift the alphabet? I understand it's halfway around, but is there an elegant reason to go -that- far, but not 12 or 14 spots?
It seems to me like making each letter "as far away" as possible from its starting position only is meaningful to a human who might recognize "close" characters (although I doubt this is possible/probable).
Anyone know the answer to this?
Because it has the nice property of being involutive, that is to say, ROT13(ROT13(alphaOnlyString)) = alphaOnlyString.
According to Wikipedia:
A shift of thirteen was chosen over other values, such as three as in the original Caesar cipher, because thirteen is the value for which encoding and decoding are equivalent, thereby allowing the convenience of a single command for both.
Probably cause it is its own inverse. The same algorithm can be used for "encryption" as well as "decryption".
Because shifting by 13 moves the characters half way around the alphabet (which has 26 places). So, to get back to plaintext you only need to shift it 13 moves again. This way, you don't have to have separate functions for encoding or decoding because the same operation will be encode or decode.