User:Texnic/Cryptology: Introduction, history and cracking

Links
 * Cryptanalysis
 * Kerckhoffs%27_principle
 * Topics_in_Cryptography

Outline

 * Layout of the talk: introduction, history, classical ciphers and cracking; modern cryptography principles; applications
 * What cryptology is about, cryptography and cryptanalysis
 * Requirements to cryptography
 * Approaches: secret method, steganography
 * Classical - modern - quantum, symmetrical and asymmetrical encryption, a few words about modern cryptography
 * Classical cryptography (some methods)
 * Cryptanalysis, methods, applications to described above
 * Cracking Enigma, current cracking activity

What cryptology is about

 * Cryptology: cryptography + cryptanalysis

Tasks of cryptology:
 * Protection of information exchange
 * Authentication check: connection between a text and an author to 1) prove that the text has come from a particular person and 2) that a particular person is an author of the text
 * Integrity check: that the text has not been changed since written by the author

Отслеживание проколов в передаче информации или шифра к классической криптологии не относится, однако стало частью криптологии современной, т.к. является неотъемлемой составляющей квантовой связи.

Terminology

 * Plaintext: information to be transmitted
 * Cleartext is the text "immediately comprehensible to a human being without additional processing"


 * Ciphertext: plaintext in a form, that cannot be easily understood by unauthorized people
 * Encryption: the process of conversion of a plaintext into a ciphertext
 * Decryption: the reverse process
 * Сipher (or cypher): an algorithm for performing encryption and decryption
 * The word code is often used in daily life in the same meaning as cipher, but in cryptology a code is a different matter: it is a form of text transmission (e.g. Morse code)

Steganography

 * We are talking about cryptology, but just for fun
 * Steganography---hiding the very fact of secret data transmission
 * Steganography is the art and science of writing hidden messages in such a way that no-one apart from the sender and intended recipient even realizes there is a hidden message, a form of security through obscurity
 * Classical historical examples: 440 BC: Herodotus mentions two examples of steganography in The Histories of Herodotus. 1) Demaratus sent a warning about a forthcoming attack to Greece by writing it on a wooden panel and covering it in wax. Wax tablets were in common use then as re-usable writing surfaces. 2) Another ancient example is that of Histiaeus, who shaved the head of his most trusted slave and tattooed a message on it. After his hair had grown the message was hidden.
 * Invisible ink.
 * Modern example: start with an ordinary-looking image file, then adjust the colour of every 100th pixel to correspond to a letter in the alphabet—a change so subtle that someone who isn't actively looking for it is unlikely to notice it.
 * More obscurely, during World War II, a spy for the Japanese in New York City, Velvalee Dickinson, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed how many of this or that doll to ship. The stegotext in this case was the doll orders; the 'plaintext' being concealed was itself a codetext giving information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.

Requirements to cryptography
Kerckhoffs' principle:

a cryptosystem should be secure even if everything about the system, except the key, is public knowledge.

Claude Shannon: "The enemy knows the system."

Kerckhoffs design principles for military ciphers :
 * 1) The system must be practically, if not mathematically, indecipherable;
 * 2) It must not be required to be secret, and it must be able to fall into the hands of the enemy without inconvenience;
 * 3) Its key must be communicable and retainable without the help of written notes, and changeable or modifiable at the will of the correspondents;
 * 4) It must be applicable to telegraphic correspondence;
 * 5) It must be portable, and its usage and function must not require the concourse of several people;
 * 6) Finally, it is necessary, given the circumstances that command its application, that the system be easy to use, requiring neither mental strain nor the knowledge of a long series of rules to observe.

The opposite principle: Security through obscurity: hiding the method or even the very fact of information exchange.

Classical - modern - quantum

 * Classical ciphers operate on an alphabet of letters
 * Modern ciphers operate on bits and bytes.
 * Classical schemes are often susceptible to ciphertext-only attacks, sometimes even without knowledge of the system itself, using tools such as frequency analysis.

Transposition ciphers and substitution ciphers

 * Transposition: each letter (or bit) is kept, but repositioned in the text
 * Substitution: the order is not changed, but each letter (or bit) is replaced by another one according to some rule

Classical implementations
Ломается методом частотного анализа
 * Caesar cipher (substitution)

Ломается поиском повторов: повторы в шифротексте с большей вероятностью вызваны повторами в исходном тексте, закодированными с одним и тем же сдвигом.
 * Перестановок с изменяемым шагом

Не меняет распределения букв, но применим частотный анализ по сочетаниям букв.
 * Scytale (Sparta cipher) (transposition cipher) Scytale
 * Поворотный квадрат: turning grille cipher, transposition


 * ADFGVX_cipher, сочетание секретного алфавита и секретного слова-ключа. В лоб без компьютера не ломается, был взломан при налачии большого числа шифрограмм. Расшифровавший серьёзно заболел.


 * Enigma, substitution c изменяемым шагом (не ломается частостным анализом): http://enigmaco.de/_fs/index-enigma.html Сочетание четырёх-пяти (3-4 ротора, отражатель) секретных алфавитов и ключа из 3-4 букв (456976) и штекертной разводки.

Modern implementations

 * Block cipher: When encrypting, a block cipher might take (for example) a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext. The exact transformation is controlled using a second input — the secret key.
 * Stream cipher:

Cryptanalysis

 * Frequency analysis
 * Brute-force attack: Aristotel (for the Sparta cipher), bombe, distributed network, quantum computers for factorizing large numbers

= Questions for the meeting =
 * Which terms Armita and Thomas need?
 * Do we cover everything by now?
 * Has Kotya said enough for people to understand Armita?
 * Have Armita and Kotya said enough for people to understand Thomas?
 * Have we all together said enough to proceed to quantum communication?
 * Template to use (space on the slides, structuring, colours)
 * Timing
 * Do we make a single file?
 * Do we want to exchange the files before Tuesday meeting to check them? Or: to meet on Tuesday earlier to correct possible problems / join the files?

= References =