Classical Cryptography

1
Classical Cryptography

• S V Raghavan

2
Overview of Presentation

• Introduction
• History of cryptography
• Classical Cryptography
• - Conventions
• - Types of Attacks
• - Shannons Formalism Confusion Diffusion
• - Conditions for secure communication
• - Classical Techniques like Substitution,
  Transposition

3
Overview (contd.)

• Steganography
• Ancient techniques
• Modern techniques
• Draw backs
• Machine ciphers
• Classical machines like Wheatstone disc.
• Enigma

4
What is Cryptography?

• Etymologically, it is secret( crypto) writing
  (graphy).
• Cryptography is a science encompassing the
  principles and methods of transforming an
  intelligible message to an unintelligible
  message, and then converting that message back to
  the original message.

5
Conventions

• Sender (Usually called Alice)
• Receiver (Usually called Bob)
• Senders message or plain text (x)
• Secret key (K)
• Cipher text (Y)
• Encryption (Ek) and Decryption (Dk)
• Plain text in small letters and cipher text in
  capitals

6
History of Cryptography

• Not a modern concept
• It is in practical use for a very long time
• Evidences show Spartans used secret coding as
  early as in 5th century B.C.
• Their cipher was called SKYTALE
• Working of skytale
• It can be termed as a crude form of Transposition.

7
Ancient ciphers

• The ancient Egyptians have enciphered some of
  their hieroglyphic writings on monuments. They
  were crude forms of data hiding.
• Julius Caesar used a simple substitution cipher
  which we now know as Caesar cipher.
• The Ancient Hebrews enciphered certain words in
  some scriptures. Their old scriptures use certain
  words which actually meant Jesus.

8
Ancient ciphers

• Roger Bacon developed several methods in 1200s
• Blaise de Vignere published a book on
  cryptography in 1585 A.D. and described the
  polyalphabetic substitution cipher.
• Many machine ciphers were in use like Jefferson
  cylinder.

9
Famous victims of Cryptography

• Mary Stuart( 1516- 1558 ) , Queen of Scotland was
  put to death by her cousin Queen Elizabeth for a
  crime of treason after spymaster Sir Francis
  Walsingham cracked the secret code she used to
  communicate with the conspirators.

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Ancient cryptography

• Thus we see that cryptography is not a modern
  concept
• Major developments during the period of 15th to
  18th centuries where Substitution and
  Transposition methods were extensively used
• Most of the techniques were used by Kings and the
  elite and the cryptographic techniques were kept
  very secret.

11
Developments Illustrations

• Polybius cipher
• Developed by a Greek called Polybius
• First to use numbers to encrypt letters of
  alphabet
• Encryption

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Polybius cipher Example

• Plain text top secret message
• Cipher text 443435 431513421544 32154343112215

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Bi-literarie Alphabet

• Described in Bacons The Advancement of
  Learning
• New development of using two different fonts to
  encrypt messages. One font is called a and
  other b
• Example in the book
• Cipher Text Manere Te volo donec venero
• aababb aa bbaa bbaaa baa

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Bi literarie Alphabet Decryption

• So, Manere Te volo donec venero
• aabab baabb aabba aabaa
• F U G E
• This was used in great extent during wars.
• This method came to light only after Bacon
  published his book.

15
Binary ciphers

• Bi-literarie cipher was nothing but a distortion
  of the present day binary code. Here a 0 and
  b 1
• The invention of Binary code is actually credited
  to Leibnitz who devised a calculating machine in
  1671 and found that binary was useful for his
  purposes.
• The binary code has been extended and being used
  as ASCII code in the present day computers and
  telecommunication systems.

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Bacons cipher

• Improvement over Bi-literarie cipher but uses the
  same principle
• Encryption
• A B C D E F G H I K L M N
• aa ab ac ad ae ba bb bc bd be ca
  cb cc
• O P Q R S T U W X Y Z
• cd ce da db dc dd de ea eb
  ec ed

17
Wilkins cipher

• Invented by Wilkins
• This is an example of encrypted using images or
  symbols
• Encryption

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Augmenting ciphers

• This is an example where redundancy was added to
  encrypt messages
• Encryption by Augmenting Words like by doubling
  the Vowels that make the Syllables and
  interposing G. or any other Consonant K, P, T, R,
  C or other Syllable.
• This was mostly used in speech cryptography
  rather than in written form as this is more
  suitable for secret discourse.

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Illustration

• Plain text our Plot is discovered
• Cipher text OUGUR PLOGOT IGIS
  DIGISCOGOVEGEREGED
• The key here is G

20
Wilkins Transposition cipher

• This is an example of transposition cipher
• According to Wilkins, the Meaning of any written
  message may be concealed by altering the order
  both of the Letters and the Lines together. The
  encryption is as seen. For example
• T e o l i r a e l m s f m s e s
  p l v o w e u t e l
• h s u d e s r a l o t a i h
  d u p y s r e m s y i d
• Then the Plain text
• The Souldiers are allmost famished Supply us or
  wee must yield.

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Main Issues of Cryptography

• Main issue in cryptography is secrecy i.e., the
  information in the message should be concealed
  from the eavesdropper .
• This should be possible even if the eavesdropper
  intercepts the message, as the channel is
  insecure, and reads it (Confidentiality).
• Also, if the enemy pretends to send the message
  pretending to come from the valid sender, it
  should be possible for the receiver to detect
  this (Authentication).
• Also the message should prevent either sender or
  receiver from denying a transmitted message
  (Nonrepudiation).

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Cryptography Terminology I
23
Cryptography Terminology II
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Types of Attacks

• Cipher text only
• Known plain text attack
• Chosen plain text
• Chosen plain text-cipher text

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Security

• Unconditional Security
• No matter how much computer power is available,
  the cipher cannot be broken
• Computational Security
• In this the assumption is made that the cipher
  cannot be broken in a finite amount of time.

26
Shannons Formalism

• First to formalize the principles of Cryptography
• Shannon valid assumption
• Principle of Confusion
• - Substitution
• Principle of Diffusion
• - Permutation or transposition

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Basic elements of Crypt systems

• Confusion - Substitution
• Diffusion - Transposition
• Variation - applying different substitution rules
  to different characters of the message like in
  Polyalphabetic ciphers
• Indirection - placing elements in a cipher behind
  other elements such that the analysis becomes
  harder.

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For Secure Communication

• The encryption function must be 1-1 function
• The reverse of the encryption function must
  result in the original message.
• If the domain of plain text and cipher text are
  the same, then each ek is the permutation of the
  plain text and each dk is the inverse permutation

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For Secure Communication (Contd.)

• It should be easy to encrypt and decrypt given a
  key. That is, Ek and Dk should be easily
  computable.
• Given a cipher text, the opponent should not be
  able to guess or find the key or plain text in
  polynomial time.

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Classical Techniques in Cryptography

• Substitution Methods
• Monoalphabetic ciphers
• Polyalphabetic ciphers
• Transposition Methods

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Substitution Mono alphabetic ciphers

• These ciphers follow the Shannons principle of
  confusion
• In this, each letter of the plain text is
  encrypted by substituting it with a single letter
  and the decryption is just the reverse.
• Some of the examples are Caesar cipher, Hill
  cipher, Playfair cipher etc

32
Caesar cipher

• The simplest substitution cipher
• Said to be used by Julius Caesar
• Encryption Substitute each letter of the
  alphabet by the letter three places down the
  sequence.
• E(x) y (p3) mod 26.
• Decryption is just the reverse.
• D(y) x (y-3)mod 26
• Later the three places were replaced by a random
  number and that was the key. Ek(x) (pk) mod
  26

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Illustration of Caeser cipher

• Plain text
• this is a secret message that
• need to be sent to India.
• Key 3
• Encryption
• WKLV LV D VHFUHW PHVVDJH WKDW
• QHHG WR VHQW WR LQGLD.

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Summary

• Cryptography Art of keeping messages secure
• Steps are Encryption and Decryption
• Ancient methods
• Polybius, Binary, Bi-literarie, Bacons, Wilkins
  and Augmenting
• Types of attacks
• Shannons formalism
• Confusion and Diffusion
• Classical techniques
• Substitution and Transposition methods