conventional crypto - noack conventional crypto diffusion and confusion how mary queen of scots lost...
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Conventional crypto - Noack
Conventional crypto
Diffusion and confusionHow Mary Queen of Scots lost her head
Various hand operable ciphersVarious Enigmas
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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
Conventional crypto - Noack
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