ROT13

ROT13 CODING
Rot13 is a coding method of the alphabet.As its name suggest,rotate the alphabet by thirteen place.

eg.
code:             cypher

ab                   no
unpx                 hack

ROT13 has been described as the "Usenet equivalent of a magazine printing the answer to a quiz upside down".[1] ROT13 is a variation of the Caesar cipher, developed in ancient Rome.
ROT13 is its own inverse; that is, to undo ROT13, the same algorithm is applied, so the same action can be used for encoding and decoding. The algorithm provides no real cryptographic security and is not normally used for such. It is often cited as a canonical example of weak encryption. ROT13 has inspired a variety of letter and word games on-line, and is frequently mentioned in newsgroup conversations. Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary.[2] A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence reverses: N becomes A, O becomes B, and so on to Z, which becomes M. Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English alphabet and 26 = 2 × 13, the ROT13 function is its own inverse:[2]
ROT13(ROT13(x)) = ROT26(x) = x for any text x. In other words, two successive applications of ROT13 restore the original text (in mathematics, this is sometimes called an involution; in cryptography, a reciprocal cipher).