ROT13

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ROT13 table with example.svg

ROT13 (Rotate13, "rotate by 13 places", sometimes hyphenated ROT-13) is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the Latin alphabet. ROT13 is a special case of the Caesar cipher which was developed in ancient Rome.

Contents

Because there are 26 letters (2×13) in the basic Latin alphabet, 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 virtually no cryptographic security, and is often cited as a canonical example of weak encryption. [1]

ROT13 was used in online forums as a means of hiding spoilers, punchlines, puzzle solutions, and offensive materials from the casual glance. ROT13 has inspired a variety of letter and word games online, and is frequently mentioned in newsgroup conversations.

Description

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 continues at the beginning of the alphabet: 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, punctuation, 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]

for any basic Latin-alphabet 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 ).

The transformation can be done using a lookup table, such as the following:

InputABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
OutputNOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm

For example, in the following joke, the punchline has been obscured by ROT13:

Why did the chicken cross the road?
Gb trg gb gur bgure fvqr!

Transforming the entire text via ROT13 form, the answer to the joke is revealed:

Jul qvq gur puvpxra pebff gur ebnq?
To get to the other side!

A second application of ROT13 would restore the original.

Usage

ROT13 is a special case of the encryption algorithm known as a Caesar cipher, used by Julius Caesar in the 1st century BC. [3]

Johann Ernst Elias Bessler, an 18th-century clockmaker and constructor of perpetual motion machines, pointed out that ROT13 encodes his surname as Orffyre. He used its latinised form, Orffyreus, as his pseudonym. [4]

ROT13 was in use in the net.jokes newsgroup by the early 1980s. [lower-alpha 1] It is used to hide potentially offensive jokes, or to obscure an answer to a puzzle or other spoiler. [2] [7] [ unreliable source? ] 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. [7] ROT13 is typically supported as a built-in feature to news reading software. [7] Email addresses are also sometimes encoded with ROT13 to hide them from less sophisticated spam bots. [8] [ dubious – discuss ] It is also used to circumvent email screening and spam filtering. By obscuring an email's content, the screening algorithm is unable to identify the email as, for instance, a security risk, and allows it into the recipient's in-box.

In encrypted, normal, English-language text of any significant size, ROT13 is recognizable from some letter/word patterns. The words "n", "V" (capitalized only), and "gur" (ROT13 for "a", "I", and "the"), and words ending in "yl" ("ly") are examples.

ROT13 is not intended to be used where secrecy is of any concern—the use of a constant shift means that the encryption effectively has no key, and decryption requires no more knowledge than the fact that ROT13 is in use. Even without this knowledge, the algorithm is easily broken through frequency analysis. [2] Because of its utter unsuitability for real secrecy, ROT13 has become a catchphrase to refer to any conspicuously weak encryption scheme; a critic might claim that "56-bit DES is little better than ROT13 these days". Also, in a play on real terms like "double DES", the terms "double ROT13", "ROT26", or "2ROT13" crop up with humorous intent (due to the fact that, since applying ROT13 to an already ROT13-encrypted text restores the original plaintext, ROT26 is equivalent to no encryption at all), including a spoof academic paper entitled "On the 2ROT13 Encryption Algorithm". [9] By extension, triple-ROT13 (used in joking analogy with 3DES) is equivalent to regular ROT13.

In December 1999, it was found that Netscape Communicator used ROT13 as part of an insecure scheme to store email passwords. [10] In 2001, Russian programmer Dimitry Sklyarov demonstrated that an eBook vendor, New Paradigm Research Group (NPRG), used ROT13 to encrypt their documents; it has been speculated that NPRG may have mistaken the ROT13 toy example—provided with the Adobe eBook software development kit—for a serious encryption scheme. [11] Windows XP uses ROT13 on some of its registry keys. [12] ROT13 is also used in the Unix fortune program to conceal potentially offensive dicta.

Letter games and net culture

abcdefghijklmnopqrstuvwxyz
NOPQRSTUVWXYZABCDEFGHIJKLM
aha ↔ nunant ↔ nag
balk ↔ onyxbar ↔ one
barf ↔ onesbe ↔ or
bin ↔ ovaebbs ↔ roof
envy ↔ railer ↔ re
errs ↔ reefflap ↔ sync
fur ↔ shegel ↔ try
gnat ↔ tangirk ↔ vex
clerk ↔ pyrexpurely ↔ cheryl
PNG ↔ catSHA ↔ fun
furby ↔ sheolterra ↔ green
what ↔ JungURL ↔ hey
purpura ↔ Chechenshone ↔ FUBAR
Ares ↔ Nerfabjurer ↔ nowhere

ROT13 provides an opportunity for letter games. Some words will, when transformed with ROT13, produce another word. Examples of 7-letter pairs in the English language are abjurer and nowhere, and Chechen and purpura . Other examples of words like these are shown in the table. [13] The pair gnat and tang is an example of words that are both ROT13 reciprocals and reversals.

The 1989 International Obfuscated C Code Contest (IOCCC) included an entry by Brian Westley. Westley's computer program can be encoded in ROT13 or reversed and still compiles correctly. Its operation, when executed, is either to perform ROT13 encoding on, or to reverse its input. [14]

The newsgroup alt.folklore.urban coined a word—furrfu—that was the ROT13 encoding of the frequently encoded utterance "sheesh". "Furrfu" evolved in mid-1992 as a response to postings repeating urban myths on alt.folklore.urban, after some posters complained that "Sheesh!" as a response to newcomers was being overused. [15]

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, sometimes called ROT18 (18 = 13 + 5) or ROT13.5.

ROT47 is a derivative of ROT13 which, in addition to scrambling the basic letters, 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; for example, a telephone number such as +1-415-839-6885 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 encoded.

Example:

The Quick Brown Fox Jumps Over The Lazy Dog.

enciphers to

%96 "F:4< qC@H? u@I yF>AD ~G6C %96 {2KJ s@8]

The GNU C library, a set of standard routines available for use in computer programming, contains a function—memfrob() [16] —which has a similar purpose to ROT13, although it is intended for use with arbitrary binary data. The function operates by combining each byte with the binary pattern 00101010 (42) using the exclusive or (XOR) operation. This effects a simple XOR cipher. Like ROT13, XOR (and therefore memfrob()) is self-reciprocal, and provides a similar, virtually absent, level of security.

Implementation

tr

The ROT13 and ROT47 are fairly easy to implement using the Unix terminal application tr ; to encrypt the string "The Quick Brown Fox Jumps Over The Lazy Dog" in ROT13:

$ # Map upper case A-Z to N-ZA-M and lower case a-z to n-za-m$ tr'A-Za-z''N-ZA-Mn-za-m'<<<"The Quick Brown Fox Jumps Over The Lazy Dog"Gur Dhvpx Oebja Sbk Whzcf Bire Gur Ynml Qbt

and the same string for ROT47:

$ echo"The Quick Brown Fox Jumps Over The Lazy Dog"|tr'\!-~''P-~\!-O'%96"F:4< qC@H? u@I yF>AD ~G6C %96 {2KJ s@8

Emacs and Vim

In Emacs, one can ROT13 the buffer or a selection with the commands [17] M-x toggle-rot13-mode, M-x rot13-other-window, or M-x rot13-region.

In the Vim text editor, one can ROT13 a buffer with the command [18] ggg?G.

Python

The module codecs provides 'rot13' text transform. [19] 

>>> importcodecs>>> print(codecs.encode('The Quick Brown Fox Jumps Over The Lazy Dog','rot13'))Gur Dhvpx Oebja Sbk Whzcf Bire Gur Ynml Qbt

Without importing any libraries, it can be done in a two-line list comprehension:

string="Quartz glyph job vext cwm porshrop finks?!"forabcdin["abcdefghijklmnopqrstuvwxyz","ABCDEFGHIJKLMNOPQRSTUVWXYZ"]:string=''.join([abcd[(abcd.index(char)+13)%26]ifcharinabcdelsecharforcharinstring])print(string)# Dhnegm tylcu wbo irkg pjz cbefuebc svaxf?!

See also

Related Research Articles

In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication channel or storage in a storage medium. An early example is an invention of language, which enabled a person, through speech, to communicate what they thought, saw, heard, or felt to others. But speech limits the range of communication to the distance a voice can carry and limits the audience to those present when the speech is uttered. The invention of writing, which converted spoken language into visual symbols, extended the range of communication across space and time.

<span class="mw-page-title-main">Cipher</span> Algorithm for encrypting and decrypting information

In cryptography, a cipher is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.

<span class="mw-page-title-main">Encryption</span> Process of converting plaintext to ciphertext

In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decipher a ciphertext back to plaintext and access the original information. Encryption does not itself prevent interference but denies the intelligible content to a would-be interceptor.

<span class="mw-page-title-main">One-time pad</span> Encryption technique

In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is larger than or equal to the size of the message being sent. In this technique, a plaintext is paired with a random secret key. Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition.

Atbash is a monoalphabetic substitution cipher originally used to encrypt the Hebrew alphabet. It can be modified for use with any known writing system with a standard collating order.

In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters, pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution process to extract the original message.

<span class="mw-page-title-main">Caesar cipher</span> Simple and widely known encryption technique

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.

<span class="mw-page-title-main">Vigenère cipher</span> Simple type of polyalphabetic encryption system

The Vigenère cipher is a method of encrypting alphabetic text where each letter of the plaintext is encoded with a different Caesar cipher, whose increment is determined by the corresponding letter of another text, the key.

<span class="mw-page-title-main">Tabula recta</span> Fundamental tool in cryptography

In cryptography, the tabula recta is a square table of alphabets, each row of which is made by shifting the previous one to the left. The term was invented by the German author and monk Johannes Trithemius in 1508, and used in his Trithemius cipher.

In cryptography, coincidence counting is the technique of putting two texts side-by-side and counting the number of times that identical letters appear in the same position in both texts. This count, either as a ratio of the total or normalized by dividing by the expected count for a random source model, is known as the index of coincidence, or IC for short.

<span class="mw-page-title-main">Ciphertext</span> Encrypted information

In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it. This process prevents the loss of sensitive information via hacking. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. Ciphertext is not to be confused with codetext because the latter is a result of a code, not a cipher.

The affine cipher is a type of monoalphabetic substitution cipher, where each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which. As such, it has the weaknesses of all substitution ciphers. Each letter is enciphered with the function (ax + b) mod 26, where b is the magnitude of the shift.

In cryptography, a cryptosystem is a suite of cryptographic algorithms needed to implement a particular security service, such as confidentiality (encryption).

<span class="mw-page-title-main">Polybius square</span> Type of code

The Polybius square, also known as the Polybius checkerboard, is a device invented by the ancient Greeks Cleoxenus and Democleitus, and made famous by the historian and scholar Polybius. The device is used for fractionating plaintext characters so that they can be represented by a smaller set of symbols, which is useful for telegraphy, steganography, and cryptography. The device was originally used for fire signalling, allowing for the coded transmission of any message, not just a finite number of predetermined options as was the convention before.

In cryptography, a classical cipher is a type of cipher that was used historically but for the most part, has fallen into disuse. In contrast to modern cryptographic algorithms, most classical ciphers can be practically computed and solved by hand. However, they are also usually very simple to break with modern technology. The term includes the simple systems used since Greek and Roman times, the elaborate Renaissance ciphers, World War II cryptography such as the Enigma machine and beyond.

<span class="mw-page-title-main">M-209</span> Mechanical cipher machine

In cryptography, the M-209, designated CSP-1500 by the United States Navy is a portable, mechanical cipher machine used by the US military primarily in World War II, though it remained in active use through the Korean War. The M-209 was designed by Swedish cryptographer Boris Hagelin in response to a request for such a portable cipher machine, and was an improvement of an earlier machine, the C-36.

The four-square cipher is a manual symmetric encryption technique. It was invented by the French cryptographer Felix Delastelle.

The Two-square cipher, also called double Playfair, is a manual symmetric encryption technique. It was developed to ease the cumbersome nature of the large encryption/decryption matrix used in the four-square cipher while still being slightly stronger than the single-square Playfair cipher.

The Beaufort cipher, invented by some Giovanni Sestri in early 18th century but widely attributed to Sir Francis Beaufort, is a substitution cipher similar to the Vigenère cipher, with a slightly modified enciphering mechanism and tableau. Its most famous application was in a rotor-based cipher machine, the Hagelin M-209. The Beaufort cipher is based on the Beaufort square which is essentially the same as a Vigenère square but in reverse order starting with the letter "Z" in the first row, where the first row and the last column serve the same purpose.

<span class="mw-page-title-main">Cryptography</span> Practice and study of secure communication techniques

Cryptography, or cryptology, is the practice and study of techniques for secure communication in the presence of adversarial behavior. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, information security, electrical engineering, digital signal processing, physics, and others. Core concepts related to information security are also central to cryptography. Practical applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications.

References

  1. ↑ Early uses of ROT13 found in the Google USENET archive date back to 8 October 1982, posted to the net.jokes newsgroup. [5] [6]
  1. ↑ Christopher Swenson (17 March 2008). Modern Cryptanalysis: Techniques for Advanced Code Breaking. John Wiley & Sons. p. 5. ISBN   978-0-470-13593-8. Archived from the original on 24 June 2016. Retrieved 5 October 2015.
  2. 1 2 3 4 Schneier, Bruce (1996). Applied Cryptography (Second ed.). John Wiley & Sons. pp.  11. ISBN   0-471-11709-9.
  3. ↑ Kahn, David. The Codebreakers: The Story of Secret Writing. New York: Macmillan. ISBN   0-684-83130-9.
  4. ↑ Simanek, Donald E. (2012). "Perpetual Futility: A short history of the search for perpetual motion". The Museum of Unworkable Devices. Archived from the original on 10 October 2020. Retrieved 28 October 2020.
  5. ↑ desoto!gog (8 October 1982). "very mildly obscence joke, shift=13". Newsgroup:  net.jokes.
  6. ↑ utcsrgv!dave (8 October 1982). "encrypted jokes". Newsgroup:  net.jokes.
  7. 1 2 3 Raymond, Eric S., ed. (29 December 2003). "ROT13". The Jargon File, 4.4.7. Archived from the original on 13 January 2012. Retrieved 19 September 2007.
  8. ↑ Ferner, Matt (9 December 2010). "How to Hide Email Addresses From Spam Bots". PracticalEcommerce. Archived from the original on 8 May 2016. Retrieved 12 June 2014.
  9. ↑ "On the 2ROT13 Encryption Algorithm" (PDF). Prüfziffernberechnung in der Praxis. 25 September 2004. Archived (PDF) from the original on 15 April 2012. Retrieved 20 September 2007.
  10. ↑ Hollebeek, Tim; Viega, John. "Bad Cryptography in the Netscape Browser: A Case Study". CiteSeerX   10.1.1.15.9271 .
  11. ↑ Perens, Bruce (1 September 2001). "Dimitry Sklyarov: Enemy or friend?". ZDNet News. Archived from the original on 17 October 2014. Retrieved 3 February 2011.
  12. ↑ "ROT13 is used in Windows". 24 July 2006. Archived from the original on 20 December 2016. Retrieved 15 December 2016.
  13. ↑ De Mulder, Tom. "ROT13 Words". Furrfu!. Archived from the original on 2 April 2012. Retrieved 19 September 2007.
  14. ↑ Westley, Brian (1989). "westley.c". IOCCC . Archived from the original on 9 June 2012. Retrieved 13 August 2007.
  15. ↑ "Furrfu". Foldoc. 25 October 1995. Archived from the original on 14 July 2014. Retrieved 3 October 2016.
  16. ↑ "5.13 Obfuscating Data". The GNU C Library Reference Manual. Free Software Foundation. 3 December 2006. Archived from the original on 2 August 2019. Retrieved 2 August 2019.
  17. ↑ "Rmail Rot13 – GNU Emacs Manual" Archived 24 March 2016 at the Wayback Machine . www.gnu.org.
  18. ↑ "Best of VIM Tips, gVIM's Key Features zzapper". rayninfo.co.uk. 3 March 2016. Archived from the original on 3 March 2016.
  19. ↑ "codecs – Codec registry and base classes – Python 3.9.6 documentation". ww6.python.org. 2 November 2023. Archived from the original on 6 October 2020. Retrieved 7 October 2020.
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