The Alphabet Cipher

Last updated

"The Alphabet Cipher" was a brief study published by Lewis Carroll in 1868, describing how to use the alphabet to send encrypted codes. [1] It was one of four ciphers he invented between 1858 and 1868, and one of two polyalphabetic ciphers he devised during that period and used to write letters to his friends. [2]

Contents

It describes what is known as a Vigenère cipher, a well-known scheme in cryptography. While Carroll calls this cipher "unbreakable", Friedrich Kasiski had already published in 1863 a volume describing how to break such ciphers and Charles Babbage had secretly found ways to break polyalphabetic ciphers in the previous decade during the Crimean War.

The piece begins with a tabula recta.

"The Alphabet-Cipher", Lewis Carroll, 1868

   ABCDEFGHIJKLMNOPQRSTUVWXYZ  A abcdefghijklmnopqrstuvwxyz A  B bcdefghijklmnopqrstuvwxyza B  C cdefghijklmnopqrstuvwxyzab C  D defghijklmnopqrstuvwxyzabc D  E efghijklmnopqrstuvwxyzabcd E  F fghijklmnopqrstuvwxyzabcde F  G ghijklmnopqrstuvwxyzabcdef G  H hijklmnopqrstuvwxyzabcdefg H  I ijklmnopqrstuvwxyzabcdefgh I  J jklmnopqrstuvwxyzabcdefghi J  K klmnopqrstuvwxyzabcdefghij K  L lmnopqrstuvwxyzabcdefghijk L  M mnopqrstuvwxyzabcdefghijkl M  N nopqrstuvwxyzabcdefghijklm N  O opqrstuvwxyzabcdefghijklmn O  P pqrstuvwxyzabcdefghijklmno P  Q qrstuvwxyzabcdefghijklmnop Q  R rstuvwxyzabcdefghijklmnopq R  S stuvwxyzabcdefghijklmnopqr S  T tuvwxyzabcdefghijklmnopqrs T  U uvwxyzabcdefghijklmnopqrst U  V vwxyzabcdefghijklmnopqrstu V  W wxyzabcdefghijklmnopqrstuv W  X xyzabcdefghijklmnopqrstuvw X  Y yzabcdefghijklmnopqrstuvwx Y  Z zabcdefghijklmnopqrstuvwxy Z    ABCDEFGHIJKLMNOPQRSTUVWXYZ

Explanation

Each column of this table forms a dictionary of symbols representing the alphabet: thus, in the A column, the symbol is the same as the letter represented; in the B column, A is represented by B, B by C, and so on.

To use the table, some word or sentence should be agreed on by two correspondents. This may be called the 'key-word', or 'key-sentence', and should be carried in the memory only.

In sending a message, write the key-word over it, letter for letter, repeating it as often as may be necessary: the letters of the key-word will indicate which column is to be used in translating each letter of the message, the symbols for which should be written underneath: then copy out the symbols only, and destroy the first paper. It will now be impossible for anyone, ignorant of the key-word, to decipher the message, even with the help of the table.

For example, let the key-word be vigilance, and the message 'meet me on Tuesday evening at seven', the first paper will read as follows—

v i g i l a n c e v i g i l a n c e v i g i l a n c e v i m e e t m e o n t u e s d a y e v e n i n g a t s e v e n h m k b x e b p x p m y l l y r x i i q t o l t f g z z v

The second will contain only 'h m k b x e b p x p m y l l y r x i i q t o l t f g z z v'.

The receiver of the message can, by the same process, retranslate it into English.

If this table is lost, it can easily be written out from memory, by observing that the first symbol in each column is the same as the letter naming the column, and that they are continued downwards in alphabetical order. It would only be necessary to write out the particular columns required by the key-word, but such a paper would afford an adversary the means for discovering the key-word.

Related Research Articles

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">Transposition cipher</span> Method of encryption

In cryptography, a transposition cipher is a method of encryption which scrambles the positions of characters (transposition) without changing the characters themselves. Transposition ciphers reorder units of plaintext according to a regular system to produce a ciphertext which is a permutation of the plaintext. They differ from substitution ciphers, which do not change the position of units of plaintext but instead change the units themselves. Despite the difference between transposition and substitution operations, they are often combined, as in historical ciphers like the ADFGVX cipher or complex high-quality encryption methods like the modern Advanced Encryption Standard (AES).

<span class="mw-page-title-main">ROT13</span> Simple encryption method

ROT13 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.

<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">English alphabet</span> Latin-script alphabet consisting of 26 letters

Modern English is written with a Latin-script alphabet consisting of 26 letters, with each having both uppercase and lowercase forms. The word alphabet is a compound of alpha and beta, the names of the first two letters in the Greek alphabet. Old English was first written down using the Latin alphabet during the 7th century. During the centuries that followed, various letters entered or fell out of use. By the 16th century, the present set of 26 letters had largely stabilised:

Alphabetical order is a system whereby character strings are placed in order based on the position of the characters in the conventional ordering of an alphabet. It is one of the methods of collation. In mathematics, a lexicographical order is the generalization of the alphabetical order to other data types, such as sequences of numbers or other ordered mathematical objects.

When used as a diacritic mark, the term dot refers to the glyphs "combining dot above", and "combining dot below" which may be combined with some letters of the extended Latin alphabets in use in a variety of languages. Similar marks are used with other scripts.

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.

The trifid cipher is a classical cipher invented by Félix Delastelle and described in 1902. Extending the principles of Delastelle's earlier bifid cipher, it combines the techniques of fractionation and transposition to achieve a certain amount of confusion and diffusion: each letter of the ciphertext depends on three letters of the plaintext and up to three letters of the key.

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.

Banburismus was a cryptanalytic process developed by Alan Turing at Bletchley Park in Britain during the Second World War. It was used by Bletchley Park's Hut 8 to help break German Kriegsmarine (naval) messages enciphered on Enigma machines. The process used sequential conditional probability to infer information about the likely settings of the Enigma machine. It gave rise to Turing's invention of the ban as a measure of the weight of evidence in favour of a hypothesis. This concept was later applied in Turingery and all the other methods used for breaking the Lorenz cipher.

The grill method, in cryptology, was a method used chiefly early on, before the advent of the cyclometer, by the mathematician-cryptologists of the Polish Cipher Bureau in decrypting German Enigma machine ciphers. The Enigma rotor cipher machine changes plaintext characters into cipher text using a different permutation for each character, and so implements a polyalphabetic substitution cipher.

<span class="mw-page-title-main">Cipher disk</span> Encryption and decryption tool consisting of two metal plates with alphabets

A cipher disk is an enciphering and deciphering tool developed in 1470 by the Italian architect and author Leon Battista Alberti. He constructed a device, consisting of two concentric circular plates mounted one on top of the other. The larger plate is called the "stationary" and the smaller one the "moveable" since the smaller one could move on top of the "stationary".

John William Jamieson Herivel was a British science historian and World War II codebreaker at Bletchley Park.

The Smithy code is a series of letters embedded, as a private amusement, within the April 2006 approved judgement of Mr Justice Peter Smith on The Da Vinci Code copyright case. It was first broken, in the same month, by Dan Tench, a lawyer who writes on media issues for The Guardian, after he received a series of email clues about it from Justice Smith.

In computer science, more specifically in automata and formal language theory, nested words are a concept proposed by Alur and Madhusudan as a joint generalization of words, as traditionally used for modelling linearly ordered structures, and of ordered unranked trees, as traditionally used for modelling hierarchical structures. Finite-state acceptors for nested words, so-called nested word automata, then give a more expressive generalization of finite automata on words. The linear encodings of languages accepted by finite nested word automata gives the class of visibly pushdown languages. The latter language class lies properly between the regular languages and the deterministic context-free languages. Since their introduction in 2004, these concepts have triggered much research in that area.

General der Nachrichtenaufklärung Training Referat was the training organization within the General der Nachrichtenaufklärung (GDNA), the military signals intelligence agency of the Wehrmacht during World War II. Initially established from May 1941, it continued teaching until September 1944. Until 1942, the work of the Training Referat was not fully exploited and only a small beginners course was taught.

References

  1. Azcárate, Asunción López-Varela (2019). "Beyond Analogy: the Semiosis of Lewis Carroll's Fantasy Worlds". ESSE Messenger. 28 (1): 75–97 via EBSCOHost.
  2. Dale, Ashley (2013). "The Alice Cipher". Word Ways: The Journal of Recreational Linguistics. 46 (2): 142–148 via EBSCOHost.