This site contains a brief outline of the various types of cipher systems that have been used historically, and tries to relate them to each other while avoiding a lot of mathematics.
Its chapters are:
You can also go directly to a complete table of contents.
It deals with methods of sending secret messages ranging from pencil and paper systems performed by hand to today's advanced block ciphers, such as Rijndael, the cipher chosen as the Advanced Encryption Standard, as depicted at right.
This page is about codes and ciphers, which people use to communicate with each other in ways that other parties cannot (it is hoped) understand. Although secrecy in communication can precede literacy, for example by the use of obscure allusions, a spoken language that is different from the one commonly spoken, a jargon or cant of terms with special or secondary meanings, or a conventionalized way of speaking such as Pig Latin, the efflorescence of many and sophisticated methods of secret communications waited for the development of alphabetic writing, which allows any thought to be represented by a small number of easily manipulated characters.
Even then, it took a conceptual breakthrough to realize that letters can be represented by other symbols; particularly in introductory books on the subject for children, this is often illustrated by various examples that are used today, such as Morse code,
signal flags,
Baudot,
ASCII,
Braille,
and semaphore:
And, for another even more prosaic illustration, here is the color code used on resistors (as well as on the old mica capacitors) and the color code, if one can call it that, of pool balls:
Of course, today, cryptography has extended into the realm of advanced mathematics, including number theory, which is concerned with prime numbers.
One early and entertaining historical survey of the use of codes and ciphers was the book Secret and Urgent, by Fletcher Pratt, also the author of several novels. This book came out in the same year as Elementary Cryptanalysis, by Helen Fouché Gaines, which will be mentioned below. The title of this book is a particularly apt description of why methods of scrambling messages to keep them secret are used.
Obviously, if a message contains nothing that is confidential, there is no need to bother putting it into code or cipher.
But equally, if a message is not urgent, then even if it is secret, it can be communicated at some time when there is an opportunity to meet privately.
Only when both conditions exist: when the contents of a message must be kept secret, and yet the message is so urgent that the risk must be taken of sending it by a means that may allow others to read it, would people take the time and effort to put a message into cipher, and take the risk involved in relying on cipher to maintain its secrecy.
Of course, today computers carry out the steps involved in enormously complicated cipher schemes at the push of a button, so neither the effort nor the risk looms as large as it did during most of the history of the subject.
This site contains a brief outline of the various types of cipher systems that have been used historically, and tries to relate them to each other while avoiding a lot of mathematics.
The following books can be recommended for someone beginning to learn about the subject of secret writing:
The Codebreakers, David Kahn, Macmillan (1st ed.) Simon and Schuster (2nd ed.). This book is a fascinating history of cryptography, dealing with the role it has played in many historical events. There are also some nuggets of technical information not seen in other books aimed at the general public, and there is historical information about subjects related to secret codes, such as cable codes (which do not involve secrecy, and were for saving money on sending telegrams).
Elementary Cryptanalysis, Helen Fouché Gaines, Dover. Published under the title Cryptanalysis to avoid confusion with a book by Abraham Sinkov (also a good book), this book deals with pencil and paper ciphers, and is particularly aimed at people who solve such ciphers as a hobby. It describes a wide variety of ciphers and a multitude of solving methods.
And I will also mention two other books here:
Machine Cryptography and Modern Cryptanalysis, Cipher A. Deavours and Louis Kruh; Artech House. This book is a gold mine of information and was the source for much of what you will see in this web site about cipher machines of the rotor era. I had believed this book to be out of print, but in fact it is still available through Artech House, which has selected it as one of those works sufficiently valuable that it will be reprinted should their stocks run out, as part of its "In Print Forever" program. Unfortunately, it was marred by a number of typographical errors. I had thought that regrettable, but felt that this was a common occurrence in books with a limited anticipated sale. An otherwise positive review in Cryptologia magazine (also a significant source, particularly for my account of the Siemens T-52) did include the comment that one might expect better than that, and as a result my faulty memory led me to categorize the review as "scathing", for which I apologize to its author (himself a significant source of information for my section on the Enigma's Uhr box), Frode Weierud. A number of the illustrations from this book were reprinted (with full permission, of course) in the more recent book Decrypted Secrets from Springer-Verlag.
Applied Cryptography, Bruce Schneier; John Wiley and Sons. This book is aimed at the computer professional who needs to implement secure systems involving cryptography. As it describes a wide selection of algorithms and protocols, it will be of interest to anyone concerned with cryptography in the computer era. This book is one of the most widely available books covering the subject matter, and it is frequently cited as a source and as an authority on the USENET newsgroup sci.crypt. The 2nd edition is considerably expanded, with fascinating detail on a much larger number of cipher systems.
This page is about cryptography, it does not fall into certain categories of worthwhile and helpful pages about cryptography that are more common; it is neither:
There are links to some of the pages in these categories in the Links section of this site.
Occasionally, some methods of cryptanalysis are briefly touched upon here, but the details are very limited, compared to the excellent material available elsewhere.
This site has a great deal in common with sites of the third category, but alas, it doesn't include any photographs. What it does have are schematic diagrams (in my own, somewhat nonstandard symbolism, designed to be easy to recognize at small sizes) and descriptions of the operation of many historical cipher machines. One such schematic diagram is the one below of the workings of the Enigma:
The story of the Enigma's decryption, derived from a multitude of secondary sources, is, I hope, explained with both completeness and clarity here.
It covers forms of cryptography ranging from the simple paper-and-pencil methods to the modern computer cipher systems, and attempts to point out the common features that link them.
It also deals with other related topics, such as protocols for secure communications, such as Kerberos:
and it deals with related topics, such as error-correcting codes:
One word of warning, however: I have indulged my own ego rather shamelessly here, and have described a series of block ciphers of my own design (under the name of "Quadibloc"; the first one was inspired by DES and Blowfish, although in a way it was the opposite of Blowfish, and the others are the result of appropriating various ideas found in the AES candidate ciphers), some paper-and-pencil fancies of mine, and a rather elaborate fractionation scheme for converting the binary output of modern encryption methods to letters for transmission by Morse, or base-78 armor (more efficient than base-64, if less efficient than base-85), or encryption by classical letter-based methods.
In only one section do I discuss, and very briefly, codes, in which words or phrases rather than letters, bits, or digits are the unit of encipherment. However, the word code is used legitimately in mathematics to refer to substitutions which are non-linguistic (and hence, in cryptology, would be called ciphers) from Morse code to Hamming code (used for error-correction) and Huffman code (used for data compression). I have, therefore, been unable to be rigorous about the use of the word "code" in these pages.
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