CYPHER INDEX

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CYPHER INDEX

• Introduction
• Background
• Demo

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INTRODUCTION

• Cypher is a software toolkit designed to aid in
  the decryption of standard (historical) ciphers
  by providing statistical data and algorithmic
  analysis on encrypted messages.
• This toolkit hopes to serve as a cryptographic
  learning tool and as a past time for amateur
  cryptographers.

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BACKGROUND

• Substitution ciphers are characterized by a
  substitution of a character in the original plain
  text message by the corresponding character in
  the cipher alphabet, often according to some
  protocol derived from a predetermined key.
• Transposition ciphers retain a characters true
  form but instead change its position.

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Substitution Ciphers

• Monoalphabetic Substitution Ciphers
• Polyalphabetic Substitution Ciphers

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Monoalphabetic Substitution

• In a monoalphabetic substitution cipher, a single
  character in the plain text alphabet is replaced
  by a single character in the cipher alphabet.
• Frequency Analysis is a key to solve
  monoalphabetic substitution cipher.

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Monoalphabetic Substitution

• Frequency example
• E - 12.3
• T - 9.6
• A - 8.0
• O - 7.94
• N - 7.19
• I - 7.18

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Monoalphabetic Substitution

• Plain alphabet abcdefghijklmnopqrstuvwxyz
• Cipher alphabet xzavoidbygerspcfhjklmnqtuw

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Polyalphabetic Substitution

• Multiple cipher alphabets are used in
  Polyalphabetic Substitution.
• Vigenère cipher is the simplest type of
  polyalphabetic cipher.

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Vigenère Cipher

• Keyword
• Plaintext
• Ciphertext

• W H I T E W H I T E W H I T E W H I T E W H I
• D I V E R T T R O O P S T O E A S T R I D G E
• Z P D X V P A Z H S L Z B H I W Z B K M Z N M

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Transposition Ciphers

• Transposition cipher rearranges the positions of
  the characters but not their true values, so that
  a frequency analysis of a message encrypted by
  transposition appears "normal".
• Rotating columns and/or rows of the message,
  rotating the entire message left or right, or
  along diagonals.

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Transposition Ciphers

• Plaintext The day is coming to an end, and
• CiphertextTDIOHASMEYCI NOE,GANATNDN

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Other Ciphers

• Ceaser shift cipher
• Plaintext abcdefghijklmnopqrstuvwxyz
• Ciphertext defghijklmnopqrstuvwxyzabc
• Encrypt a message is by using a key-text, such as
  the Declaration of Independence or a well-known
  poem, numbering the words in the said document
  and replacing letters in the plaintext by a
  number that represents the first character of
  that number word in the key-text.

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Demo