Glossary
This glossary gives a brief description of some of the most important words used in the site. It is a place you can refer to if you have forgotten what a word means from a previous section, without having to look through all the previous bits.
Alphabet

The alphabet is the set of characters which will be affected by the encryption process. This is usually the set of 26 letters, but could also be enlarged to contain punctuation and numbers to make a cipher more secure.

Break

To break a code is to work out what the original message of an intercepted text is. This is done using a variety of ways and a large amount of human input is required.

Brute Force Attack

A brute force attack is a method of breaking a code by using all possible keys for a given cipher. Although this was very time consuming in the past, with computers these can be completed in a matter of seconds on simple substitution ciphers.

Cipher

A cipher is an algorithm which somehow changes the letters within a message.

Cipher Disc

A cipher disc is a tool used to aid in encrypting and decrypting messages that use a Caesar Shift. This includes many polyalphabetic ciphers. It consists of two concentric discs, each split into 26 equal parts, and labelled with the letters of the alphabet. The inner disc can be twisted in relation to the outer disc, so that different letters pair up.

Ciphertext

The ciphertext is the encrypted message that is sent to the recipient. It is always written in uppercase, and surounded by quotation marks.

Ciphertext Alphabet

A ciphertext alphabet is a way to quickly manually encrypt a message using a monoalphabetic substitution. It can be represented as a table where the top row shows the plaintext letter, and the bottom row shows the ciphertext letter which is used to replace it.

Code

A code is a system where whole words and phrases are replaced by other words and phrases to hide their meaning.

Codebook

A codebook is an extensive book that contains all the words and phrases to be used in a code system. It is a bit like a foreign language dictionary, as it displays the codeword for each normal word to encrypt the message, and the normal word for each codeword for when decrypting.

Crib

A crib is a portion of ciphertext which is known or assumed to be an encryption of a specific phrase of plaintext. It is a very useful technique to use in cryptanalysis, and was fundamental to the breaking of many ciphers in periods of war. Examples might by the inclusion of the date at the beginning of a message, starting with "Dear ..." in a letter, or finishing a message with a standard ending such as "Heil Hitler".

Cryptanalysis

Cryptanalysis is the study of revealing the hidden meaning of an encrypted text, when you do not know what method has been used to encrypt it. We often talk about "breaking the code".

Cryptography

Cryptography is the study of hidding the meaning of a message by changing the content of the message by some rule. This includes ciphers and codes.

Cryptology

Cryptology is the study of hidden writtings, or writtings with hidden meanings. It is the use of any means to send a secret message. This includes cryptography and steganography

Decrypt/Decipher

To decrypt (or decipher) a message is to use revert the ciphertext back into the plaintext using the cipher and key.

Digraph

A digraph is a pair of letters or characters. It is useful to look at common digraphs in frequency analysis, and they appear regularly in ciphers that use a fractionation.

Encrypt/Encipher

To encrypt (or encipher) a message is to take the original message and perform the necessary changes to the text to create the ciphertext according to the cipher and key chosen.

Fractionation

A fractionation method is one that represents single characters by a digraph (or pair of characters). Usually this involves using the digits 09, but not always. It also allows us to use a substantially smaller set of characters than were in the original message.

Frequency Analysis

A method used by cryptanalysts to break substitution ciphers. It relies on the fact that in any given language, each letter has its own "personality", and in particular, certain letters appear more frequently than others. By analysing which letters appear frequently in an intercept, you can make educated guesses as to which letter they might represent.

Homophones

Using homophones is a way to disrupt the natural frequency of letters in a piece of text, and thus make frequency analysis more complex. It involves assigning multiple symbols to the higher frequency letters to even out the distribution.

Indicator Block

An indicator block is a block of ciphertext that is used to hide a part of the key within the message itself. This is used in the Running Key Cipher to identify the page and line number to use within the key text as the start of the keystream.

Intercept

An intercept is a piece of encrypted text for which you do not know the cipher or key used in the encryption process.

Interceptor

An interceptor is any person who comes across an encrypted message who was not the intended recipient of the message. An interceptor would need to break the code, as they would not know the key or cipher used.

Iterations

Iterations are how many times a particular cipher is applied. Some ciphers can be applied multiple times to make the encryption more secure (transposition ciphers), whereas for others it has no effect (monoalphabetic substitution ciphers).

Kerckhoffs's Principle

In 1883, Auguste Kerckhoffs wrote an article where he stated six principles for military ciphers. With the advent of computers, many are now redundant, but the second is one of the most important ideas in cryptography. It states that a cryptosystem must remain secure if the algorithm of the cipher is known publicly, and that the strength of a cipher relies on keeping the key secret.

Key

A key is a piece of information which makes the encryption process unique and more secure.

Keyword/Keyphrase

A keyword is a key which takes the form of a word. A keyphrase is similar, but consists of a phrase instead of a word. They are written in lowercase and in italics.

Keystream

A keystream is a string of letters that is as long as the plaintext. Each letter is used to identify which column of a tabula recta should be used in the encryption process of a polyalphabetic cipher.

Modulo

Working modulo is a mathematical idea that is key to many ciphers. The basic principal is that items repeat after you reach the modulo number. In ciphers, this is usually the length of the alphabet (normally 26). So when you are counting, and you get to 24 then 25 and finally 26, you then start counting again (so instead of 27, we go back to 1, or alphabetically, after "z" we go back to "a"). To take a number modulo another number is to take the remainder when you divide the first number by the second. For example, to work out 19 modulo 8, we work out that 2 x 8 = 16, and we need 3 more to get from 16 to 19. So 19 modulo 8 is 3 (the remainder). For a more in depth discussion see Wolfram Alpha.

Monoalphabetic Substitution Cipher

A monoalphabetic substitution cipher is a cipher where each letter in the plaintext is replaced by the same letter or symbol everytime.

Null

A null is a letter or symbol used as a placeholder in a message. It has no meaning, and is easily recognised by the recipient as a letter of no meaning. Usually the letter "X" is used. It is often needed to make a message the right length for a particular cipher algorithm to be performed.

Numeric Escape Character

A numeric escape character is really useful in ciphers which convert letters to numbers. This character, usually # or \, is used to indicate that the next number is actually a number, and not a piece of enciphered text.

Perfect Secrecy

An idea in cryptography that all codes and ciphers aim for, perfect secrecy means that the cipher is truly unbreakable. Claude Shannon devised the idea that a piece of cipher text is perfectly secret if it provides no information about the original plaintext. The only known example of a cipher that has perfect secrecy is the OneTime Pad, but there are considerable practical issues with using this cipher.

Plaintext

The plaintext is the original message before encryption. It is what will be retrieved from the ciphertext after a successful decryption. It is written in lowercase, and surounded by quotation marks.

Polyalphabetic Substitution Cipher

A polyalphabetic substitution cipher is one where the ciphertext alphabet changes according to some rule. That means that the same plaintext letter will not always encrypt to the same ciphertext letter, and also that the same ciphertext letter could represent different plaintext letters. This is a way to overcome frequency analysis.

Polygraphic Substitution Cipher

A polygraphic substitution cipher replaces groups of letters or characters at a time. for example, in a Digraph Substitution Cipher, the plaintext "at" could be enciphered to "GD". This encryption is the same everytime, but it is different to Monoalphabetic Substituiton Ciphers in that it doesn't act on individual letters, but rather groups of letters of the same length.

Recipient

The recipient of a message is the intended recipient when the message was sent. This person will know both the cipher and key used, and will be able to decrypt the ciphertext using these.

Secure/Security

The security of cipher is a measure of how hard it is to break the code. We do not have a strict scale of security, but we talk in terms of ciphers previously encountered.

Sender

The sender of a message is the person who writes the original message and encrypts this for security before sending the information to the recipient.

Shannon's Maxim

This is a reformulation of Kerckhoffs's Principle which states "the enemy knows the system". That is, assume that an enemy will immediately gain a full working familiarity with any cryptographic system you employ.

Steganography

Steganography is the study of physical means to hide a message, such as invisible inks and microdots. There is no actual change made to the message, it is simply hid in a (usually) inventive way.

Substitution Cipher

A cipher where each letter (or groups of letters) of the plaintext is replaced by another letter or symbol in some predetermined way.

Tabula Recta

A tabula recta is a table which shows all the possible shifts of a given alphabet in one place. You choose the letter from the keystream across the top, and the plaintext letter down the left hand side, and the ciphertext letter is the column and row that represents these two pieces of information.

Transparency

A transparency is when a character (or group of characters) encipher to themselves. When a cipher does not allow for transparencies, its strength is compromised as this allows cryptanalysts to ascertain information about what it is not. However, a system that produces a lot of transparencies is also less secure. Ideally they can and do happen, but not regularly.

Transposition Cipher

A cipher where the order of the letters in the message is changed. The same letters appear, but they are mixed up in a nonsensical way.
