Decryption & Encryption - Rot13 Cypher

Introduction:

This cryptographic tool allows you to encode and decode text that has been encrypted with the 'Rot 13' cypher. In modern times this does not provide the security of commercial systems like RSA or DES, nor even WW11 military systems like Enigma, Lorenz or more recent military code systems like BATCO, but it was commonly used in ancient times, and is still fun today, and sometimes appears in puzzle books and magazines.
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Caveats:

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Definitions:

Offset

The difference in position between each encrypted and un-encrypted character.

Rot

Short for 'Rotation'; referring to the fact that in the classical implementation of this cypher, the alphabet can be written in a circle, and larger numbers that would otherwise be beyond the end of the alphabet move round to the start of the alphabet.

Formulae:

The Rot13 Cypher works in the same manner as Caesar's Cypher, but uses a fixed offset of 13. The cypher works by converting each character into a corresponding number, like so: a=1, b=2 etc. Once this conversion is complete, the 'offset' is added to each number, then converted back to a character. This is the same mechanism as Caesar's Cypher, except that the offset is always the same. When used with the English alphabet, this has an important advantage: exactly the same mechanism is used to decrypt a message as is used to encrypt. This is because two 'rotations' by thirteen positions adds up to a single rotation by twenty-six positions, ie straight back to the original letter. Unfortunately, the whole basis of the Rot13 cypher is dependant on a single alphabet of 26 letters, so any punctuation or numbers in the input will remain un-encrypted.

Instructions

1. Type, or Copy-and-Paste the text to be encrypted / decrypted where indicated.
2. Click the 'Encrypt/Decrypt' button below the input field.
3. The encrypted text will be displayed below.
4. To do further encryptions / decryptions simply change the text in the input field, and then click the 'Encrypt/Decrypt' button again.