Introduction to Cryptology

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Introduction to Cryptology

• Fall 2006

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Definitions

• Digital encryption techniques are used to protect
  data in two ways to maintain privacy and to
  prove integrity 1
• Cryptography comes from the Greek words
• Kryptos, meaning hidden,
• Graphen, meaning to write.
• Cryptography is the art of communicating
  confidentially through an insecure channel
• Cryptanalysis is the art of deciphering those
  communications when one is not the legitimate
  receiver.
• Cryptology is the union of these two domains.
• Cryptology is actually the study of codes and
  ciphers.

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Definitions

• Plaintext the original message that is to be
  changed into a secret form
• Cipher, Cryptosystem, Encryption Method is a
  procedure for converting the plaintext message
  into encrypted message ciphertext
• The word cipher originates from the Hebrew word
  "Saphar," meaning "to number."
• Decryption - the process that conducted by the
  intended receiver to recover original message
  (plaintext) from the encrypted message
  (ciphertext)
• Key - keeps the message secret

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Encryption-Decryption
Plaintext
Ciphertext
Encryption key
Encryption Method
Ciphertext
Plaintext
Decryption Key
Decryption method
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Symmetric and Asymmetric Ciphers

• Symmetric Cipher knowledge of encryption key is
  equivalent to knowledge of the decryption key, or
  the same key is used for encryption and
  decryption.
• Asymmetric Cipher (Public-Key Cryptosystem) the
  knowledge of encryption key give little
  information about the decryption key and vice
  versa

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Early History of Cryptology

• Polybius Checkboard
• 205 123 B.C.
• Plaintext POLYBIUS produces the ciphertext
• 3534315412244543

1 2 3 4 5
1 A B C D E
2 F G H IJ K
3 L M N O P
4 Q R S T U
5 V W X Y Z
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Julius Caesar Cipher

• Caesar enciphered messages by replacing each
  letter by the third letter after that one
• A ? D, B ? E, C ? F, D ? G, Z ? C
• Plaintext ET TU, BRUTE?'' becomes the ciphertext
  HW WX, EUXWH?''
• Ciphertext may be deciphered by replacing each
  letter by the third previous letter.
• We think of each letter as corresponding to a
  number from 0 to 25
• To encipher, we map numbers according to
• C ? ( P 3 )(mod 26)
• To decipher, we map numbers according to
• P ? (C 3) (mod 26)