Brute Force Attack- 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.

Classical Cipher- a cryptographic algorithms that have been used in the past (pre WWII). Some of them have only ever been used by amateurs, while others have been used by armies to secure their top level communications.

Cipher- a secret or disguised way of writing; a code.

Ciphertext- 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.

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.

Cryptography- the art of writing or solving codes.

Decipher- convert (a text written in code, or a coded signal) into normal language.

Decrypt- make (a coded or unclear message) intelligible.

Digraph Substitution Cipher- a cipher that works similar to monoalphabetic substitution, but it replaces pairs of letters with other pairs of letters (or digraph).

Encipher- convert (a message or piece of text) into a coded form; encrypt.

Encrypt- convert (information or data) into a cipher or code, especially to prevent unauthorized access.

Encryption- the method by which information is converted into secret code that hides the information's true meaning.

Key- a piece of information (a parameter) that determines the functional output of a cryptographic algorithm.

Monoalphabetic Substitution Cipher- a type of substitution cipher, relies on a fixed replacement structure. That is, the substitution is fixed for each letter of the alphabet. 

Plaintext- unencrypted information pending input into cryptographic algorithms, usually encryption algorithms.

Polybius Square- an ancient Greek invention, discovered by a scholar named Polybius. For the Greek alphabet of 24 letters, it consisted of a 5 by 5 grid where each square of the grid was filled by a single letter. In the English Alphabet of 26 letters, we have one too many letters. To get round this we combine two letters, traditionally "i" and "j". It is also possible to combine other pairs, such as "v" and "u".

Public Key- a key that is public and open to anyone in the system. The public key is used to encrypt data.

Private Key- a key that is private. It is only ever stored on user’s device. The private key is used to decrypt data. A message encrypted with the public key cannot be decrypted without using the corresponding private key.

Substitution Cipher- probably the most common form of cipher. They work by replacing each letter of the plaintext (and sometimes punctuation  marks and spaces) with another letter (or possibly even a random symbol). 

Transpose- cause (two or more things) to change places with each other.

Transposition Cipher- a cipher where the letters are moved around to create ciphertext.