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CS Special Topics on Network Security

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									          Cryptography Overview
                  Cryptography is a strong tool
             against many kinds of security threats.

             Encryption
             Message Authentication Code
             Digital Signature




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Cryptography vs. Cryptanalysis
          Cryptography
              Studies the encryption and decryption schemes.
          Cryptanalysis
              studies how to “break the code”, i.e., how to decrypt
               the ciphertext without the knowledge of the encryption
               details.




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Attacking Encryption Scheme
          Brute-force attack tires every possible key on a
          piece of cihphertext until an intelligible translation
          into plaintext is obtained.
          Cryptanalysis exploits the characteristics of the
          algorithm and the traces of structure or pattern in
          the plaintext that survive encryption
              break a single message
              deduce the key in order to break the subsequent
               messages.
              How ? -- Using statistical tools and properties of
               languages.
@Yuan Xue (yuan.xue@vanderbilt.edu)
          Cryptanalytic Attack Classification
          Ciphertext only attack
              encryption algorithm and ciphertext
          Known plaintext attack
              encryption algorithm, ciphertext, and one or more plaintext-ciphertext
               pairs formed with the secret key.
          Chosen plaintext attack
              encryption algorithm, ciphertext, and chosen plaintext and its
               corresponding ciphertext generated with the secret key.
          Chosen ciphertext attack
              encryption algorithm, ciphertext, and chosen ciphertext and its
               corresponding decrypted plaintext with the secret key.
          Chosen text attack
              chosen plaintext attack + chosen ciphertext attack
            Encryption algorithm is designed to withstand
                      a known plaintext attack.
@Yuan Xue (yuan.xue@vanderbilt.edu)
          Cryptanalytic Attack Classification
          Unconditionally secure
              If the ciphertext generated by the algorithm does not contain
               enough information to determine uniquely the corresponding
               plaintext, no matter how much ciphertext is available, and how
               much time an opponents has.
          Computationally secure
              The cost of breaking the cipher exceeds the value of the encrypted
               information
              The time required to break the cipher exceeds the useful lifetime of
               the information




@Yuan Xue (yuan.xue@vanderbilt.edu)
                      CS 285 Network Security

              Classical Encryption Algorithm

                                      Fall 2008




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Mono-Alphabetic Ciphers
          Caeser cipher
              How many possible substitutions?
              Bruce-force attack
                   Enumerate all possible keys

          Allow arbitrary substitution
              How many possible substitutions?
              How to represent the key?




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Frequency Analysis




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Multiple-letter Ciphers
          Key idea: Treat consecutive letters in the plaintext
          as a single unit (block) and translate the unit into
          ciphertext.
          Benefit: Lessen the extent to which the structure
          of the plaintext survives in the ciphertext, thus are
          stronger against frequency analysis.
          Issue: How to specify a mapping (substitution)?




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Playfair Cipher

             - Use a table




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Playfair Cipher
    Playfair cipher was invented in 1854 by Charles
    Wheatstone, but bears the name of Lord Playfair
    who promoted the use of the cipher.
    Playfair cipher takes two-letter combinations
    (digrams) as single units for encryption.
    The encryption algorithm takes a 5*5 matrix of
    letters as a key table to translate digrams.
    The key table is constructed by a keyword.




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Playfair Cipher
  To encrypt a message,
  1. If both letters are the same (or only one letter is left), add an
  ``x'' (any uncommon letter will do) after the first letter. For example,
  ``balloon'' would be treated as ``ba lx lo on''.

  2. If the letters appear on the same row of the table, replace them with
  the letters to their immediate right respectively (the table wraps around).
  For example, ar is encrypted as RM.

  3. If the letters appear on the same column of the table, replace them
  with the letters immediately below respectively. For example, mu is
  encrypted as CM.

  4. If the letters are not on the same row or column, replace them with
  the letters on the same row respectively but at the other pair of corners
  of the rectangle defined by the original pair. For example, hs is encrypted
  as BP, and ea is encrypted as IM (or JM).




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Hill Cipher
          Use a mathematical function to specify a mapping




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Hill Cipher
          Hill cipher was invented by Lester Hill in 1929. In the
          encryption algorithm, n successive letters in plaintext are
          considered as a n-dimension vector P.
          The algorithm takes a n*n matrix K as a key.
          The ciphertext C of P is also a n-dimension vector derived
          by multiplying P by K, modulo 26.




          Issue
              Known plaintext attack



@Yuan Xue (yuan.xue@vanderbilt.edu)
          Hill Cipher Example
                                      Key




                         P: pay




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Multiple-letter Ciphers
          Benefit: Reduce the extent to which the structure
          of the plaintext survives in the ciphertext, thus are
          stronger against frequency analysis.
          How to design a stronger cipher?

          Larger block size is stronger




@Yuan Xue (yuan.xue@vanderbilt.edu)
          Polyalphabetic Ciphers
          Polyalphabetic cipher uses different
          monoalphabetic substitution as it proceeds
          through the plaintext.
              A set of monoalphabetic substitution rules;
              A key that determines which particular rule is chosen for
               a given transformation.
          Vigenere cipher




          One-time pad
@Yuan Xue (yuan.xue@vanderbilt.edu)
          Transposition
          Transposition
              Transposition technique changes the order of the letters
               in a message.
              Columnar Transposition Cipher




              Permutation Cipher

@Yuan Xue (yuan.xue@vanderbilt.edu)
          Summary
          Two types of ciphers
              Stream cipher
                   E.g. Vigenere cipher
                   encrypts one letter at a time.
              Block cipher
                   E.g. Hill cipher
                   treats a n-letter block of plaintext as a whole and produce a ciphertext
                    block of equal length.

          Two basic building components for block ciphers
              Substitution
              Transposition



@Yuan Xue (yuan.xue@vanderbilt.edu)

								
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