ROT13 cipher

The ROT13 cipher is a substitution cipher with a specific key where the letters of the alphabet are offset 13 places. Example: all 'A's are replaced with 'N's, all 'B's are replaced with 'O's, and so on. It can also be thought of as a Caesar cipher with a shift of 13.

Security The ROT13 cipher offers almost no security, and can be broken very easily. Even if an adversary doesn't know a piece of ciphertext has been enciphered with the ROT13 cipher, they can still break it by assuming it is a substitution cipher and determining the key using hill-climbing. The ROT13 cipher is also an Caesar cipher with a key of 13, so breaking it as a Caesar cipher also works.

The Algorithm The ROT13 cipher is essentially a substitution cipher with a fixed key, if you know the cipher is ROT13, then no additional information is needed to decrypt the message. The substitution key is: ABCDEFGHIJKLMNOPQRSTUVWXYZ NOPQRSTUVWXYZABCDEFGHIJKLM

Encipher To encipher a message, find the letter you wish to encipher in the top row, then replace it with the letter in the bottom row. In the example below, we encipher the message 'ATTACK AT DAWN'. The first letter we wish to encipher is 'A', which is above 'N', so the first ciphertext letter is 'N'. The next letter is 'T', which is above 'G', so that comes next. The whole message is enciphered: ATTACK AT DAWN NGGNPX NG QNJA

Decipher To decipher a message, the exact same procedure is followed. Find 'N' in the top row, which is 'A' in the bottom row. Continue until the whole message is deciphered.

Cryptanalysis The ROT13 cipher is trivial to break since there is no key, as soon as you know it is an ROT13 cipher you can simply decrypt it. If you didn't know it was a ROT13 cipher, you could break it by assuming the ciphertext is a substitution cipher, which can still be easily broken. Alternatively, it can be broken if it is assumed to be a Caesar cipher.

Variants ROT5 is a practice similar to ROT13 that applies to numeric digits (0 to 9). ROT13 and ROT5 can be used together in the same message. ROT47 is a derivative of ROT13 which, in addition to scrambling the basic letters, also treats numbers and common symbols. Instead of using the sequence A–Z as the alphabet, ROT47 uses a larger set of characters from the common character encoding known as ASCII. Specifically, the 7-bit printable characters, excluding space, from decimal 33 '!' through 126 '~', 94 in total, taken in the order of the numerical values of their ASCII codes, are rotated by 47 positions, without special consideration of case. For example, the character A is mapped to p, while a is mapped to 2. The use of a larger alphabet produces a more thorough obfuscation than that of ROT13; Example - a telephone number such as is not obvious at first sight from the scrambled result Z`\c`d\gbh\eggd. On the other hand, because ROT47 introduces numbers and symbols into the mix without discrimination, it is more immediately obvious that the text has been enciphered. Example: The Quick Brown Fox Jumps Over The Lazy Dog. enciphers to %96 "F:4 AD ~G6C %96 {2KJ