Breaking the Code

‘I am fairly familiar with all forms of secret writings, and am myself the author of a trifling monograph upon the subject, in which I analyze one
hundred and sixty separate ciphers.’
‘THE ADVENTURE OF THE DANCING MEN’
Cryptography (from the Greek for ‘hidden writing’) is the science of code- and cipher-creating, while cryptanalysis is concerned with breaking them.



In Holmes’s world, filled with deceits and intrigues, a thorough comprehension of cryptanalysis was an invaluable way to discover useful information.
Some basics to begin with – a code and a cipher are not the same:
In a code, each word or phrase is replaced by another word, phrase or symbol. Both the message sender and recipient must know what the
coded words, phrases and symbols mean either by prior agreement or through use of tools such as code books. A code is a secret language.
In a cipher, each letter is either replaced by an alternative letter, number or symbol (a substitution cipher) or the letters are shuffled about (a
transposition cipher). Plaintext is the name given to the information you wish to communicate, while ciphertext is what is actually sent.
Here are a few of history’s more famous codes and ciphers:
The Caesar cipher
(also known as the Caesar shift)
This was supposedly devised by Julius Caesar for use by his troops. It involves shifting letters along the alphabet by a set number of positions.
If you move the alphabet by one position, for instance, a is signified by b and b is signified by c. So the word cat would be communicated as
dbu. Shift two positions and a is signified by c and b is signified by d (with cat becoming ecv). There are a potential twenty-five Caesar shift
alphabets.
The transposition code
In its simplest form, this involves writing your message into a grid of suitable size, which is then read from a predetermined start point and in a
particular direction. Can you decode the following message?
Start at the bottom right-hand corner and read each column from bottom to top. The message is ‘The enemy will attack in one hour.’
Pig Latin
This technique was probably developed in the early Victorian era. Its basic rules are simple. If a word starts with a consonant (or consonant
cluster), move that to the end of the word and add the letters ‘ay’ after it. If a word begins with a vowel (or a silent consonant), add ‘way’ to the
end. Try deciphering this:
ewarebay ethay angryway anmay inway ethay oldway athay.
The message reads: ‘Beware the angry man in the old hat.’
The pigpen cipher
A centuries-old cipher that uses four simple grids containing all the letters of the alphabet:
When writing their message, the sender replaces each letter with the relevant fragment of grid. Hence ‘Beware’ becomes:
Jargon code
Here a seemingly arbitrary phrase is used in place of a known word or phrase. So, for instance, ‘The spider has spun his web from the queen
to the Eiffel Tower’ might easily be read by a recipient in the know as ‘Professor Moriarty (the spider) has taken a train (has spun his web) from
Victoria station (the queen) to Paris (the Eiffel Tower)’.
Morse Code
A remarkable code that may be rendered by long and short bursts of sound, flashes of lights, or written dots and dashes. Devised by Samuel
Morse in the 1830s, it is still widely used today.
Cryptanalysis works by trying to establish a pattern in coded material. Once a pattern is recognised, breaking a cipher or code becomes
achievable. One of the greatest weapons in the arsenal of the cryptanalyst is ‘frequency analysis’. This relies on the fact that in almost all languages,
certain letters are used more frequently than others. As an example, e is the most common letter in English so there is a reasonable possibility
(though not a certainty) that a symbol appearing most often in a ciphertext represents e.
Cryptanalysis is harder today than it has ever been, with new technologies allowing for more complex codes and ciphers. One of the most
legendarily difficult to break was the Enigma code employed by German forces during the Second World War. The Enigma machine, a typewriterlike
device, relied on a series of rotors that could be set in billions of combinations, allowing for plaintext to be delivered as seemingly impenetrable
ciphertext. However, even this machine had weaknesses. A team of brilliant minds housed at Bletchley Park eventually broke the code. They were
helped in part by frequency analysis, as they knew certain phrases (such as the names of German generals) recurred often in the encrypted
messages. This gave the Allies a huge advantage in the war.
Given the complex challenges of cryptanalysis, it is little surprise that Holmes trained himself to excel at the art. As well as the monograph alluded
to in the quote at the beginning of this section, he is witnessed breaking one word puzzle (in ‘The Musgrave Ritual’), three codes (in ‘The Adventure
of the “Gloria Scott”’, ‘The Adventure of the Red Circle’ and The Valley of