Introduction to Cryptology

What is Cryptology?

Cryptology is the science and technology of rendering language or writing illegible to another, then converting it back to legibility by use of a secret. It is also the science of attempting to read the illegible information without the use of the secret.

• Creating and using cryptology to secure information is called Cryptography
• Attempting to read the information without the secret is called Cryptanalysis

History

Cryptology has been used for at least 2,500 years in one form or another, the Ancient Greeks were familiar with it as were Roman generals. Cryptology has been seen in Egyptian Hieroglyphics, Arabic and Sanskrit writings and certainly has been a function of political and military life since records began.

The science of cryptology was developed to a higher degree with the advent in Europe of the Renaissance, and though cryptanalysis has been used for at least a thousand years, the majority of developments in the field have been much more recent.

Glossary

Autokey A form of cipher in which the plaintext acts as its own key.

Blocklength The number of characters in a plaintext message simultaneously encrypted using a cipher.

Cipher A type of encryption where individual letters are replaced or moved in order to render the plaintext illegible.

Ciphertext The illegible output from a code or cipher.

Code A type of encryption where entire words or groups of letters are substituted for others using a code book.

Cryptanalysis Attempting to render an encrypted message legible without knowing the secret, or attempting to calculate the secret without knowing it initially.

Cryptography Encryption and Decryption.

Decryption Rendering a ciphertext legible by using the secret.

Encryption Rendering a message illegible using a secret. The message should be easy to understand if the secret is known, ideally impossible to understand if the secret is unknown.

Key The secret used to encrypt and decrypt.

Keyspace The maximum number of possible keys that could be used to encrypt a message.

Null A letter added to a plaintext message for some technical reason, often to make the number of characters divisible by a certain number.

Plaintext The information to be hidden when it is in legible form.

Steganography Hiding the plaintext in a way that makes it difficult to find, but if it could be found it would be easy to read.¹

Substitution Encryption by replacing an item of plaintext with an item of ciphertext.

Transposition Encryption by moving elements of a plaintext, anagramming.

Further Reading

Stephen Budiansky Battle of Wits An examination of the cryptology of the Second World War, makes extensive use of recently declassified documents.

Helen Fouche Gaines Cryptanalysis A good overview of the procedures for breaking paper-based ciphers.

Paul B Garrett Making, Breaking Codes An excellent cryptology textbook, heavily mathematical.

David Kahn The Codebreakers The one book that every professional cryptologist has on their desk. A detailed and fascinating history of cryptology up to the Second World War, rather outdated for information after 1939.

Tony Sale The Colossus Computer 1943-1996 A technical and historical description of the first stored-program computer.

Bruce Schneier Applied Cryptography The definitive guide to modern encryption by computer. Written by the author of the Blowfish and Twofish algorithms.

Hugh Sebag-Montefiore Enigma: The Battle for the Code A detailed history of the attempts made to capture Enigma machines and documents.

Michael Smith The Emperor's Codes The story of how Japan's communications were compromised by the allies during the Second World War. Much more recent than the account in 'The Codebreakers'.

Gordon Welchman The Hut Six Story An insider's history of Bletchley Park during much of the Second World War.

A Cryptographic Compendium A good basic resource covering many historical cipher systems.