Classical Cryptosystems

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Classical Cryptosystems

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• Steganography
• Substitution Ciphers
• Transposition Techniques

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Steganography

• ???? Hidden or Covered writing
• Stegotext
• ??????????
• ???????? http//chemistry.about.com/cs/howtos/ht/i
  nvisibleink3.htm
• ??????????
• Digital watermarking

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Software

• Camouflages
• http//camouflage.unfiction.com/
• Jphide

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Encryption Techniques

• Substitution
• Transposition

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Substitution Cipher

• Monoalphabetic substitution
• every occurrence of a particular plaintext letter
  is replaced by a ciphertext letter
• Polyalphabetic substitution

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Caesar Cipher

• Replace A with D, B with E, ...
• meet me after the toga party
• phhw ph diwhu wkh wrjd sduwb
• cE(p)(p3) mod 26
• pD(c)(c-3) mod 26

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Brute force cryptanalysis

• 25 keys to try
• BZDRZQ'R VHED LTRS AD ZANUD RTROHBHNM
• http//www.simonsingh.net/The_Black_Chamber/caesar
  .html

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Monoalphabetic Substitution Cipher

• plain a b c d e ...
• cipher d z i g n ...
• 26! or 4x1026 possible keys

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Frequency Analysis

• e t a
• q x j
• http//www.simonsingh.net/The_Black_Chamber/freque
  ncyanalysis.html
• http//www.simonsingh.net/The_Black_Chamber/freque
  ncypuzzle.htm

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Cracking Substitution Cipher

• Identify Common Pairs Of Letters
• ss, ee, tt, ff, ll, mm and oo
• Identify The Smallest Words First
• a and I
• of, to, in, it, is, be, as, at, so, we, he, by,
  or, on, do, if, me, my, up, an, go, no, us, am

http//www.simonsingh.net/The_Black_Chamber/hintsa
ndtips.htm
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Cracking Substitution Cipher

• Tailor Made Frequency Tables
• If you know you are tackling a military message,
  you should use a frequency table generated from
  other military messages.
• T9
• 26 an - am - co
• 9653 wolf - woke - yoke
• Play The Guessing Game
• crib a section of an enciphered message that
  can easily be guessed
• 'In the name of God'
• Arab cryptanalyst vs Greek

http//www.simonsingh.net/The_Black_Chamber/hintsa
ndtips.htm
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Pigpen Cipher
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Pigpen Cipher
?????? Singh, The Code Book
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Mary Queen of Scots
Nomenclator
?????? Singh, The Code Book
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Mary Queen of Scots
?????? Singh, The Code Book
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Playfair Cipher

• Best known multiple-letter encryption
• Key
• Matrix
• i,j treated as one letter
• Plaintext
• Encryption
• Different row or column
• Same row (its right)
• Same column (below it)

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????????

• key
• playfair
• plaintext
• meet at the schoolhouse
• ciphertext
• eg mn fq qm kn bk sv vr gq xn ku

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National Treasure Book of Secrets

• ME IK QO TX CQ TE ZX CO MW QC TE HN FB IK ME HA
  KR QC UN GI KM AV
• DEATH

http//www.caesarcipher.com/2008/09/14/the-playfai
r-cipher-in-national-treasure-book-of-secrets/
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Relative Freq of Occurrence of Letters
?????? Stalling, Cryptography and Network Security
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Playfair Cipher Weakness

• digram frequency
• th he an in re es

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Polyalphabetic Substitution

• use different monoalphabetic substitutions as
  proceeds through the plaintext
• key
• choose monoalphabetic substitution
• Cipher
• Vigenère

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Vigenère table

• ABCDEFGHIJKLMNOPQRSTUVWXYZ BCDEFGHIJKLMNOPQRSTUV
  WXYZA CDEFGHIJKLMNOPQRSTUVWXYZAB
  DEFGHIJKLMNOPQRSTUVWXYZABC EFGHIJKLMNOPQRSTUVWXY
  ZABCD FGHIJKLMNOPQRSTUVWXYZABCDE
  GHIJKLMNOPQRSTUVWXYZABCDEF HIJKLMNOPQRSTUVWXYZAB
  CDEFG IJKLMNOPQRSTUVWXYZABCDEFGH
  JKLMNOPQRSTUVWXYZABCDEFGHI KLMNOPQRSTUVWXYZABCDE
  FGHIJ LMNOPQRSTUVWXYZABCDEFGHIJK
  MNOPQRSTUVWXYZABCDEFGHIJKL NOPQRSTUVWXYZABCDEFGH
  IJKLM OPQRSTUVWXYZABCDEFGHIJKLMN
  PQRSTUVWXYZABCDEFGHIJKLMNO QRSTUVWXYZABCDEFGHIJK
  LMNOP RSTUVWXYZABCDEFGHIJKLMNOPQ
  STUVWXYZABCDEFGHIJKLMNOPQR TUVWXYZABCDEFGHIJKLMN
  OPQRS UVWXYZABCDEFGHIJKLMNOPQRST
  VWXYZABCDEFGHIJKLMNOPQRSTU WXYZABCDEFGHIJKLMNOPQ
  RSTUV XYZABCDEFGHIJKLMNOPQRSTUVW
  YZABCDEFGHIJKLMNOPQRSTUVWX ZABCDEFGHIJKLMNOPQRST
  UVWXY

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Cryptanalysis of Vigenère Cipher

• Determine key length
• if keyword length is n, n monoalphabetic
  substitution ciphers
• Frequency

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One-Time Pad

• Vernam cipher
• Theoretically unbreakable method of encryption
• Has drawbacks in practice
• A random key as long as the message

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Rotor Machine

• Multiple stage
• ?????????? odometer
• Single rotor defines a polyalphabetic
  substitution with a period of 26
• Three rotor
• 26x26x2617,578 different substitution alphabets
  before the system repeat
• ??????? Enigma

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Rotor Machine
?????? Stalling, Cryptography and Network Security
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Transposition Technique

• ???????????
• Easy to recognize
• Same frequency
• Rail fence
• m e m a t r h t g p r y e t e f e t e o a a t
• Permute the order of column
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