conventional security protocols and example.ppt

conventional security protocols and example

1. Conventional crypto - Noack
Conventional crypto
Diffusion and confusion
How Mary Queen of Scots lost her head
Various hand operable ciphers
Various Enigmas

2. Conventional crypto - Noack
Confusion and diffusion
Confusion
Replacing one element by another of the same size, seemingly
randomly chosen
The substitution must be one-to-one so it can be undone
Diffusion
Permuting 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 hiding
Ciphers that don’t expand the text rely on confusion and diffusion
Concealment ciphers bury the content in noise or nondata

3. Conventional crypto - Noack
Confusion methods
Monoalphabetic substitution
Trivial 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 alphabet
26! Of these alphabets exist – a very large number
Frequency table is used to break this one – plaintext has
nonuniform distribution of characters and diphthongs
Polyalphabetic substitution
A sequence of permutation alphabets is used
Methods include
Vigenere table (very simplistic)
Code strips and such (bulky)
Rotor machine

4. Conventional crypto - Noack
Comments on permutations
A permutation is
A 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

5. Conventional crypto - Noack
Diffusion methods
Transposition ciphers
General idea is to rearrange the characters without changing them to
produce a random-appearing text.
Example – Playfair cipher – named for its inventor

6. Conventional crypto - Noack
Monoalphabetic cipher example
Note the use of the frequency table
A bit easier – Excel wasn’t available in those days
These often have nonstandard letter frequencies
Also they have blanks
UZCBRYAO LD EZ PUVNA YALD V
REFUSING-TO-BE-DRAWN-INTO-A--
TZAOLMX VUOBSZAL LMZ EVEXRY-
LENGTHY-ARGUMENT-THE-BABYSI--
LLZU NYRZTX NYLMPUZN CUDS LMZ
TTER-WISELY-WITHDREW-FROM-THE
EZPUDDS
BEDROOM----------------------

-----------------------------
'-ABCDEFGHIJKLMNOPQRSTUVWXYZ
'-NUFOB THWGD SMLRA YIE
522540000009443303327403510
Typical newspaper monoalphabetic crypto puzzle

7. 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

8. Conventional crypto - Noack
The 4-rotor Enigma, with wiring
pictures from Budiansky, Stephen, Battle of Wits
Uses the polyalphabetic principle
Repositioning the rotors gives a new alphabet
The rotors are stepped at each character
It was broken at least partly because of operator carelessness

9. Conventional crypto - Noack
The Bombe , used to break Enigma
messages
Comments
This 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

10. Conventional crypto - Noack
The Vigenere table and an example
Weaknesses
Only 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
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZA
BCDEFGHIJKLMNOPQRSTUVWXYZAB
CDEFGHIJKLMNOPQRSTUVWXYZABC
DEFGHIJKLMNOPQRSTUVWXYZABCD
EFGHIJKLMNOPQRSTUVWXYZABCDE
FGHIJKLMNOPQRSTUVWXYZABCDEF
GHIJKLMNOPQRSTUVWXYZABCDEFG
HIJKLMNOPQRSTUVWXYZABCDEFGH
IJKLMNOPQRSTUVWXYZABCDEFGHI
JKLMNOPQRSTUVWXYZABCDEFGHIJ
KLMNOPQRSTUVWXYZABCDEFGHIJK
LMNOPQRSTUVWXYZABCDEFGHIJKL
MNOPQRSTUVWXYZABCDEFGHIJKLM
NOPQRSTUVWXYZABCDEFGHIJKLMN
OPQRSTUVWXYZABCDEFGHIJKLMNO
PQRSTUVWXYZABCDEFGHIJKLMNOP
QRSTUVWXYZABCDEFGHIJKLMNOPQ
RSTUVWXYZABCDEFGHIJKLMNOPQR
STUVWXYZABCDEFGHIJKLMNOPQRS
TUVWXYZABCDEFGHIJKLMNOPQRST
UVWXYZABCDEFGHIJKLMNOPQRSTU
VWXYZABCDEFGHIJKLMNOPQRSTUV
WXYZABCDEFGHIJKLMNOPQRSTUVW
XYZABCDEFGHIJKLMNOPQRSTUVWX
YZABCDEFGHIJKLMNOPQRSTUVWXY
ZABCDEFGHIJKLMNOPQRSTUVWXYZ
The Vigenere table
HOWDOYOUUSETHISTHI
CRYPTOCRYPTOCRYPTO
KFVTINRMTIYIKARJBX
How do you use this thing