conventional crypto - noack conventional crypto diffusion and confusion how mary queen of scots lost...

Conventional crypto - Noack

Conventional crypto

Diffusion and confusionHow Mary Queen of Scots lost her head

Various hand operable ciphersVarious Enigmas


Confusion and diffusion

ConfusionReplacing one element by another of the same size, seemingly

randomly chosen

The substitution must be one-to-one so it can be undone

DiffusionPermuting the elements from place to place in a seemingly random

fashion

Permutations rather than random scrambles must be used so the rearrangement can be undone

Expansion and hidingCiphers that don’t expand the text rely on confusion and diffusion

Concealment ciphers bury the content in noise or nondata


Confusion methods

Monoalphabetic substitutionTrivial example – Caesar cipher

He replaced each letter of the plaintext by one three letters before

Weakness is the fixed scheme – once diagnosed, the Gauls win –

Fairly trivial example – randomly chosen permuted alphabet26! Of these alphabets exist – a very large numberFrequency table is used to break this one – plaintext has

nonuniform distribution of characters and diphthongs

Polyalphabetic substitutionA sequence of permutation alphabets is used Methods include

Vigenere table (very simplistic)Code strips and such (bulky)Rotor machine


Comments on permutations

A permutation isA one-to-one mapping of a set onto itself

With the underlying operation it forms a group (more later)

A permutation of a permutation is still a permutation

An substitution alphabet is a permutation, but the resulting cipher is not a permutation

Permutations have unique inverses

The simple transposition ciphers are permutations


Diffusion methods

Transposition ciphersGeneral idea is to rearrange the characters without changing them to

produce a random-appearing text.

Example – Playfair cipher – named for its inventor


Monoalphabetic cipher example

Note the use of the frequency tableA bit easier – Excel wasn’t available in those daysThese often have nonstandard letter frequenciesAlso they have blanks

U Z C B R Y A O L D E Z P U V N A Y A L D V R E F U S I N G - T O - B E - D R A W N - I N T O - A - -

T Z A O L M X V U O B S Z A L L M Z E V E X R Y - L E N G T H Y - A R G U M E N T - T H E - B A B Y S I - -

L L Z U N Y R Z T X N Y L M P U Z N C U D S L M ZT T E R - W I S E L Y - W I T H D R E W - F R O M - T H E

E Z P U D D S B E D R O O M - - - - - - - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - - - - - - - - - - - - - - - -

' - A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ' - N U F O B T H W G D S M L R A Y I E

5 2 2 5 4 0 0 0 0 0 0 9 4 4 3 3 0 3 3 2 7 4 0 3 5 10

Typical newspaper monoalphabetic crypto puzzle


How Mary Queen of Scots lost her head

From Singh, Simon, The Code Book

Note: This is nothing but a monoalphabetic cipher with some word substitution


The 4-rotor Enigma, with wiringpictures from Budiansky, Stephen, Battle of Wits

Uses the polyalphabetic principleRepositioning the rotors gives a new alphabetThe rotors are stepped at each characterIt was broken at least partly because of operator carelessness


The Bombe , used to break Enigma messages

CommentsThis is actually a copy of the machine

conceived by Turing

It still used a plugboard approach rather than a strictly electronic stored program

Material captured from ships and submarines was also used

This was a combination of known plaintext and brute force cryptanalysis

It is not a Turing machine in the computer science sense

picture from Budiansky, Stephen, Battle of Wits


The Vigenere table and an example

WeaknessesOnly one simple shifted

translation alphabet

Relatively short period

Can be broken by frequency analysis of spaced groups

Could be strengthened somewhat with a longer keyword and different alphabets

A B C D E F G H I J K L M N O P Q R S T U V W X Y ZA B C D E F G H I J K L M N O P Q R S T U V W X Y Z AB C D E F G H I J K L M N O P Q R S T U V W X Y Z A BC D E F G H I J K L M N O P Q R S T U V W X Y Z A B CD E F G H I J K L M N O P Q R S T U V W X Y Z A B C DE F G H I J K L M N O P Q R S T U V W X Y Z A B C D EF G H I J K L M N O P Q R S T U V W X Y Z A B C D E FG H I J K L M N O P Q R S T U V W X Y Z A B C D E F GH I J K L M N O P Q R S T U V W X Y Z A B C D E F G HI J K L M N O P Q R S T U V W X Y Z A B C D E F G H IJ K L M N O P Q R S T U V W X Y Z A B C D E F G H I JK L M N O P Q R S T U V W X Y Z A B C D E F G H I J KL M N O P Q R S T U V W X Y Z A B C D E F G H I J K LM N O P Q R S T U V W X Y Z A B C D E F G H I J K L MN O P Q R S T U V W X Y Z A B C D E F G H I J K L M NO P Q R S T U V W X Y Z A B C D E F G H I J K L M N OP Q R S T U V W X Y Z A B C D E F G H I J K L M N O PQ R S T U V W X Y Z A B C D E F G H I J K L M N O P QR S T U V W X Y Z A B C D E F G H I J K L M N O P Q RS T U V W X Y Z A B C D E F G H I J K L M N O P Q R ST U V W X Y Z A B C D E F G H I J K L M N O P Q R S TU V W X Y Z A B C D E F G H I J K L M N O P Q R S T UV W X Y Z A B C D E F G H I J K L M N O P Q R S T U VW X Y Z A B C D E F G H I J K L M N O P Q R S T U V WX Y Z A B C D E F G H I J K L M N O P Q R S T U V W XY Z A B C D E F G H I J K L M N O P Q R S T U V W X YZ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

The Vigenere table

H O W D O Y O U U S E T H I S T H IC R Y P T O C R Y P T O C R Y P T OK F V T I N R M T I Y I K A R J B X

How do you use this thing

conventional crypto - noack conventional crypto diffusion and confusion how mary queen of scots lost...

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