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Chapter 2 CONVENTIONAL ENCRYPTION CLASSICAL TECHNIQUES Outline

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					                                                                                Outline

                                                                                     Conventional Encryption Model
                              Chapter 2
 CONVENTIONAL ENCRYPTION:                                                            Steganography

         CLASSICAL TECHNIQUES                                                        Classical Encryption Techniques


                           Yeuan-Kuen Lee
                             September

                                                                          Ch 2 Conventional Encryption: Classical Techniques          2




2.1 Conventional Encryption Model                                         2.1 Conventional Encryption Model
                                                                                Plaintext
                                                                                    Original intelligible message
                                                                                Ciphertext
                                                                                    Apparently random nonsense message
                                                                                Encryption process
                                                                                    An algorithm - produce a different output depending
                                                                                    on the specific key being used at the time.
                                                                                    A key – a value independent of plaintext, shared by
                                                                                    sender and recipient.
             Figure 2.1 Simplified Model of Conventional Encryption

Ch 2 Conventional Encryption: Classical Techniques                    3   Ch 2 Conventional Encryption: Classical Techniques          4
2.1 Conventional Encryption Model                                     2.1 Conventional Encryption Model
     The ciphertext can be transformed back to the original                                                                      ˆ
                                                                                                                                 X
                                                                       Encryption algorithm ( E )        Cryptanalyst
     plaintext by using a decryption algorithm and the same                                                                      ˆ
                                                                                                                                 K
                                                                               Y = EK(X)
     key that was used for encryption.
     The security of conventional encryption depends on the           Message      X        Encryption
                                                                                           Encryption     Y               Decryption
                                                                                                                          Decryption   X
                                                                                            Algorithm                      Algorithm       Destination
     secrecy of the key, not the secrecy of the algorithm.             source              Algorithm     Ciphertext       Algorithm
                                                                                                      Y = [Y1, Y2, …, YN]
     It is impractical to decrypt a message based on the             Plaintext                    K
     ciphertext plus knowledge of the encryption/decryption          X = [X1, X2, …, XM]
     algorithm.                                                                                          Secure channel
                                                                                                                               Decryption algorithm ( D )
                                                                                              Key
     The principal security problem is maintaining the secrecy                               source
                                                                                                                                      X = DK(Y)
                                                                                                         Key
     of the key.                                                                                         K = [K1, K2, …, KJ]
                                                                                       Figure 2.2 Model of Conventional Cryptosystem

Ch 2 Conventional Encryption: Classical Techniques               5   Ch 2 Conventional Encryption: Classical Techniques                           6




2.1 Conventional Encryption Model                                     2.1 Conventional Encryption Model
      An opponent                                                          Cryptography - the art of secret writing.
                                                                                Classified along three independent dimensions:
          Observing Y, but not having access to K and X, may
                                                                                1. The type of operations used for transforming
          attempt to recover X or K, or both X and K.                              plaintext to ciphertext.
                                                                                       Substitution
          Assumed that the opponent knows E and D
                                                                                       Transposition
          If only the message is interested, then an estimated                  2. The number of keys used.
                     ˆ
          plaintext X is generated.                                                    Symmetric, single-key, secret-key encryption
                                                                                       Asymmetric, two-key, public-key encryption
          If future messages are interested, then an estimated
                                                                                3. The way in which the plaintext is processed.
               ˆ
          key K is generated.
                                                                                       Block cipher
                                                                                       Stream cipher

Ch 2 Conventional Encryption: Classical Techniques               7   Ch 2 Conventional Encryption: Classical Techniques                           8
2.1 Conventional Encryption Model                                          2.1 Conventional Encryption Model
      Cryptanalysis                                                              Cryptanalysis (Conti.)
          The process of attempting to discover X or K or both.                      Known plaintext attack –
          Table 2.1 summarizes the various types of                                       Known to cryptography
          cryptanalytic attacks based on the amount of                                   1. Encryption algorithm
          information known to the cryptanalyst.                                         2. Ciphertext to be decoded
          Ciphertext only attack -                                                       3. One or more plaintext-ciphertext pairs formed with the
                                                                                            secret key
               Known to cryptography
                                                                                          Probable-word attack – may have little knowledge of
              1.   Encryption algorithm
                                                                                          what is in the message
              2. Ciphertext to be decoded
                                                                                             Accounting file: placement of certain key words
               Brute-force approach of trying all possible keys
                                                                                             Copyright statement in some standardized position
               Statistical tests: type of plaintext
Ch 2 Conventional Encryption: Classical Techniques                   9     Ch 2 Conventional Encryption: Classical Techniques                    10




2.1 Conventional Encryption Model                                          2.1 Conventional Encryption Model
      Cryptanalysis (Conti.)                                                     Cryptanalysis (Conti.)
          Chosen-plaintext attack –                                                  Chosen-ciphertext attack –
               Known to cryptography                                                      Known to cryptography
              1. Encryption algorithm                                                    1. Encryption algorithm
              2. Ciphertext to be decoded                                                2. Ciphertext to be decoded
              3. Plaintext message chosen by cryptanalyst, together with                 3. Purported ciphertext chosen by cryptanalyst, together
                 its corresponding ciphertext generated with the secret                     with its corresponding decrypted plaintext generated
                 key                                                                        with the secret key
               Example: password file                                                Chosen-text attack – chosen-plaintext or chosen-
               Differential cryptanalysis (explored Ch3)                             ciphertext attack



Ch 2 Conventional Encryption: Classical Techniques                  11     Ch 2 Conventional Encryption: Classical Techniques                    12
2.1 Conventional Encryption Model                                                      2.1 Conventional Encryption Model
       Cryptanalysis (Conti.)                                                                Unconditionally secure
             Only relative weak algorithms fail to withstand a                                    If the ciphertext generated by an encryption scheme does not
             ciphertext-only attack.                                                              contain enough information to determine uniquely the
                                                                                                  corresponding plaintext, no matter how much ciphertext is
             Generally, an encryption algorithm is designed to                                    available and how much time an opponent has.
             withstand a know-plaintext attack.
                                                                                                  No encryption algorithm is unconditionally secure, except the
                                                                                                  one-time pad scheme

                                                                                             Conditionally secure
                                                                                             1.   The cost of breaking the cipher exceeds the value of the
                                                                                                  encrypted information
                                                                                             2. The time required to break the cipher exceeds the useful
                                                                                                lifetime of the information

Ch 2 Conventional Encryption: Classical Techniques                            13       Ch 2 Conventional Encryption: Classical Techniques                    14




2.1 Conventional Encryption Model                                                      2.2 Steganography
                                                                                              Cryptography
  Table 2.2 Average Time Required for Exhaustive Key Search                                       crypto – graphy : secret – writing
                                                                                                  Conceal the meaning of message
                      Number of           Time required at         Time required at
 Key Size (bits)
                   alternative keys        1 encryption/us         106 encryption/us          Steganography
        32           232 = 4.3*109         231 us = 35.8 min           2.15 ms                    stegano – graphy : covered – writing
        56           256 = 7.2*1016      255 us = 1142 years           10.01 hrs                  Conceal the existence of message
       128           2128 = 3.4*1038   2127 us = 5.4*1024 years     5.4*1018 years

 26 char perm.        26! = 4*1026     2*1026 us = 6.4*1012years     6.4*106 years




Ch 2 Conventional Encryption: Classical Techniques                            15       Ch 2 Conventional Encryption: Classical Techniques                    16
2.2 Steganography                                         2.2 Steganography
       Stegosaur (Roof Lizard)                                     Dear George,
                                                                   Greetings to all at Oxford. Many thanks for your
                                                                   Letter and for the summer examination package.
                                                                   All Entry Forms and Fees Forms should be ready
                                                                   for final despatch to the syndicate by Friday
                                                                   20th or at the very latest, I’m told, by the 21st.
                                                                   Admin has improved here, though there’s room
                                                                   for improvement still; just give us all two or three
                                                                   more years and we’ll really show you! Please
                                                                   don’t let these wretched 16 + proposals destroy
                                                                   your basic O and A pattern. Certainly this
                                                                   sort of change, if implemented immediately,
                                                                   would bring chaos.
                                                                                          Sincerely yours,
Ch 2 Conventional Encryption: Classical Techniques   17   Ch 2 Conventional Encryption: Classical Techniques               18




2.2 Steganography                                         2.2 Steganography
      Historical steganographic techniques
          Character marking                                      Cryptography
                                                                 Conceal the meaning of message
          Invisible ink
          Pin punctures
          Typewriter correction ribbon


                                                                                  Conceal the existence of message
                                                                                                           Steganography

Ch 2 Conventional Encryption: Classical Techniques   19   Ch 2 Conventional Encryption: Classical Techniques               20
2.2 Steganography                                                         2.2 Steganography
      General Steganographic Model                                              Requirements of a Steganographic System
           Sender    Message                           Message Receiver
                                                                                     Imperceptible (image fidelity)
                    Compressing
                    Compressing                      Decompressing
                                                     Decompressing                   Undetectable (Steganalysis)
 image,
 text
                                                      Decrypting
                                                                                     Security
 audio,             Encrypting
                    Encrypting                        Decrypting
 video                                                                               Payload
  Cover-                                 Stego-
  media
                    Embedding
                    Embedding
                                         media
                                                       Extracting
                                                       Extracting                    Limited Robustness

                     Stego-key                        Stego-key
                              Warden                   (Blindness)

Ch 2 Conventional Encryption: Classical Techniques                   21   Ch 2 Conventional Encryption: Classical Techniques                      22




2.2 Steganography                                                         2.2 Steganography
      Steganalysis                                                               Specific Pattern of S-Tools palette in cover-image
           The art of detecting any hidden message on
           the communication channel.
           If the existence of the hidden message is
           revealed, the goal of steganography is
           defeated.
           Two types of steganalytic techniques
                Visual attack                                                  Result of the Airfield image embedded in the
                Statistical attack                                             8-bit Renoir with S-Tools. (the cover image
                                                                               was reduced from 248 to 32 unique colors)
                                                                                                            luminance-ordered palette in stego-image
Ch 2 Conventional Encryption: Classical Techniques                   23   Ch 2 Conventional Encryption: Classical Techniques                      24
2.3 Classical Encryption Techniques                                2.3 Classical Encryption Techniques
      Two basic building blocks                                          Caesar cipher
          Substitution techniques - the letters of plaintext                 Replacing each letter of the alphabet with the letter
          are replaced by other letters or by numbers of symbols
                                                                             standing three places further down the alphabet
          - Caeser cipher
                                                                             Transformation
          - Monoalphabetic cipher
                                                                         Plain:      a b c d e f g h i      j k l m n o p q r s t u v w x y z
          - Playfair cipher                                              Cipher:     D E F G H I J K L M N O P Q R S T U V WX Y Z A B C
          - Hill cipher
                                                                          <Example>
          Transposition techniques - performing some sort
                                                                          Plain:  me e t       me      a f t e r        t h e   t o g a   p a r t y
          of permutation on the plaintext letters
                                                                          Cipher: P H H W      P H     D I WH U         WK H    WR J D    S D U WB
      Rotor machines - multiple stages of encryption
Ch 2 Conventional Encryption: Classical Techniques            25   Ch 2 Conventional Encryption: Classical Techniques                         26




2.3 Classical Encryption Techniques                                2.3 Classical Encryption Techniques
      Caesar cipher                (Conti.)                              Caesar cipher                (Conti.)
          If we assign a numerical equivalent to each letter                 Brute-force cryptanalysis
          (a=0, b=1, c=2…etc), then for each plaintext letter p,              Fig.2.4
          substitute the ciphertext letter C :
                                                                             Why? Three important characteristics:
          C = E(p) = (p + 3) mod 26
                                                                              1. The encryption and decryption algorithms are known.
          General Caesar algorithm                                            2. There are only 25 keys to try.
          C = E(p) = (p + k) mod 26                                           3. The language of the plaintext is known and easily
                    where 1 ≤ k ≤ 25                                             recognizable.
          Decryption algorithm
                                                                                   < Fig.2.5 >
          p = D(C) = (C - k) mod 26                                                Using ZIP algorithm to Compress the plaintext before encryption


Ch 2 Conventional Encryption: Classical Techniques            27   Ch 2 Conventional Encryption: Classical Techniques                         28
2.3 Classical Encryption Techniques                                       2.3 Classical Encryption Techniques
      Monoalphabetic cipher
          An arbitrary substitution is used
          26! ( ≈ 4×1026 ) possible keys:
          to eliminate brute-force attack (table 2.2)
          If the cryptanalyst knows the nature of the
          plaintext (e.g., noncompressed English text), then
          the analyst can exploit the regularities of the
          language.
          < Fig.2.6 >
            Relative frequency of letters in English text
                                                                                         Fig. 2.6 Relative frequency of letters in English text

Ch 2 Conventional Encryption: Classical Techniques                   29   Ch 2 Conventional Encryption: Classical Techniques                             30




2.3 Classical Encryption Techniques                                       2.3 Classical Encryption Techniques
      Monoalphabetic cipher                          (Conti.)                   Playfair cipher
          Digram – two-letter combination
                                                                                    The best-known multiple-letter encryption cipher
          Frequency of diagrams is a powerful regularity.
                                                                                    Treat digrams in the plaintext as single units and
               The most common digram is ‘th’. (ZW)
                                                                                    translates these units into ciphertext digrams.
          Trigram – three-letter combination
               The most frequent trigram is ‘the’. (ZWP)                            5*5 matrix of letters                           M
                                                                                                                                    M    O
                                                                                                                                         O    N
                                                                                                                                              N   A
                                                                                                                                                  A      R
                                                                                                                                                         R
                                                                                    constructed using a keyword.                    C    H    Y   B      D
          Homophone –                                                                                                               C    H    Y   B      D

               Provide multiple substitutes for a single letter                                                                     E
                                                                                                                                    E    F
                                                                                                                                         F    G
                                                                                                                                              G   I/J
                                                                                                                                                   I/J   K
                                                                                                                                                         K
               Multiple-letter patterns (e.g., digram frequencies)                                                                  L
                                                                                                                                    L    P
                                                                                                                                         P    Q
                                                                                                                                              Q   S
                                                                                                                                                  S      T
                                                                                                                                                         T
               still survive in the ciphertext
                                                                                                                                    U
                                                                                                                                    U    V
                                                                                                                                         V   W
                                                                                                                                             W    X
                                                                                                                                                  X      Z
                                                                                                                                                         Z

Ch 2 Conventional Encryption: Classical Techniques                   31   Ch 2 Conventional Encryption: Classical Techniques                             32
2.3 Classical Encryption Techniques                                          2.3 Classical Encryption Techniques
      Playfair cipher               (Conti.)                                       Playfair cipher               (Conti.)

          Plaintext is encrypted two letters at a time, according to               3. Plaintext letters that fall in the same column of the
          the following rules:                                                        matrix are replaced by the letter beneath, with the top
      1. Repeating plaintext letter that would fall in the same pair                  element of the column circularity following the last.
         are separated with a filler letter (such as x)                                [ mu ]       [ CM ]
          [ balloon ]        [ ba lx lo on ]         M
                                                     M   O
                                                         O   N
                                                             N   A
                                                                 A      R
                                                                        R          4. Otherwise, each plaintext letter              M
                                                                                                                                    M   O
                                                                                                                                        O    N
                                                                                                                                             N    A
                                                                                                                                                  A      R
                                                                                                                                                         R
                                                                                      is replaced by the letter that lies
      2. Plaintext letters that fall in the          C   H   Y   B      D                                                           C   H    Y    B      D
                                                     C   H   Y   B      D             in its own row and the column                 C   H    Y    B      D
         same row of the matrix are
                                                     E
                                                     E   F
                                                         F   G
                                                             G   I/J
                                                                  I/J   K
                                                                        K             occupied by the other plaintext               E
                                                                                                                                    E    F
                                                                                                                                         F   G
                                                                                                                                             G    I/J
                                                                                                                                                   I/J   K
                                                                                                                                                         K
         replaced by the letter to the                                                letter.
         right in a circular fashion                 L
                                                     L   P
                                                         P   Q
                                                             Q   S
                                                                 S      T
                                                                        T              [ hs ]     [ BP ],                           L
                                                                                                                                    L   P
                                                                                                                                        P    Q
                                                                                                                                             Q    S
                                                                                                                                                  S      T
                                                                                                                                                         T
          [ ar ]     [ RM ]                          U
                                                     U   V
                                                         V   W
                                                             W   X
                                                                 X      Z
                                                                        Z              [ ea ]     [ IM ] ( or [ JM ] )              U
                                                                                                                                    U   V
                                                                                                                                        V    W
                                                                                                                                             W    X
                                                                                                                                                  X      Z
                                                                                                                                                         Z

Ch 2 Conventional Encryption: Classical Techniques                      33   Ch 2 Conventional Encryption: Classical Techniques                          34




2.3 Classical Encryption Techniques                                          2.3 Classical Encryption Techniques
      Playfair cipher                (Conti.)
          There are 26*26=676 digrams, so that identification of
          individual digrams is more difficult.
          The relative frequencies of individual letters exhibit a
          much greater range than that of diagrams, making
          frequency analysis much more difficult.
          Standard field system by the British Army in WWI
          Considerable use by the U.S. Army and other allied
          forces during WWII.
          However, it still leaves much of the structure of the
          plaintext language intact.
                                                                                           Fig.2.7 Relative Frequency of Occurrence of Letters.

Ch 2 Conventional Encryption: Classical Techniques                      35   Ch 2 Conventional Encryption: Classical Techniques                          36
2.3 Classical Encryption Techniques                                    2.3 Classical Encryption Techniques
      Hill cipher                                                            Hill cipher             (Conti.)
          Lester Hill, 1929                                                      Matrix-vector form
          Take m successive plaintext letters and substitutes for
          them m ciphertext letters
                                                                                               c1   k11 k12 k13   p1 
                                                                                              c  = k             
          The substitution is determined by m linear                                           2   21 k22 k23   p2 
          transformation.                                                                     c3  k31 k32 k33   p3 
                                                                                                                 
          For m = 3,                                                             C = KP
               C1 = (k11p1+k12p2+k13p3) mod 26                                   where C and P are column vectors of length 3,
               C2 = (k21p2+k22p2+k23p3) mod 26                                   representing the plaintext and ciphertext, and K is a
               C3 = (k31p3+k32p2+k33p3) mod 26                                   3*3 matrix, representing the encryption key.
                                                                                 Operation are performed mod 26.


Ch 2 Conventional Encryption: Classical Techniques                37   Ch 2 Conventional Encryption: Classical Techniques             38




2.3 Classical Encryption Techniques                                    2.3 Classical Encryption Techniques
      Hill cipher             (Conti.)                                       Hill cipher             (Conti.)
          Example:                                                               Decryption requires using K-1 , the inverse of the
               Plaintext “paymoremoney”                                          matrix K,
                                                                                                          4 9 15
               Key              17 17 5                                                         K -1 = 15 17 6 
                                                                                                                 
                            K = 21 18 21
                                                                                                       24 0 17
                                                                                                                 
                                
                                 2 2 19                                       KK-1 = K-1K=I
               The first three letters is “pay” = (15, 0, 24) t                  General Expressions
               C = KP mod 26 = (375, 819, 486) t mod 26                          C = EK(P) = KP
               = (11, 13, 18) t = “LNS”
                                                                                 P = DK(C) = K-1C = K-1KP = P
               Ciphertext “LNSHDLEWMTRW”


Ch 2 Conventional Encryption: Classical Techniques                39   Ch 2 Conventional Encryption: Classical Techniques             40
2.3 Classical Encryption Techniques                                   2.3 Classical Encryption Techniques
      Hill cipher             (Conti.)                                      Hill cipher             (Conti.)
          As with Playfair, the strength of the Hill cipher is that             For an m*m Hill cipher,
          it completely hides single-letter frequencies.                        suppose we have m plaintext-ciphertext pairs,
          A 3*3 Hill cipher hides not only single-letter but two-               each of length m.
          letter frequency information.                                         Pj = ( p1j, p2j, p3j, p4j . . ., pmj )
          Use a larger matrix hides more frequency information                  Cj = ( c1j, c2j, c3j, c4j . . ., cmj )
          Strong against a ciphertext-only attack                               Cj = KPj for 1≤ j ≤ m and for some unknown key
          Easily broken with a known plaintext attack.                          matrix K.
                                                                                Define X = (pij) , Y = (cij).            Y = XK
                                                                                If X has an inverse, K =X-1Y

Ch 2 Conventional Encryption: Classical Techniques               41   Ch 2 Conventional Encryption: Classical Techniques           42




2.3 Classical Encryption Techniques                                   2.3 Classical Encryption Techniques
      Polyalphabetic ciphers
          Use different monoalphabetic substitutions as one
          proceeds through the plaintext message
      1. A set of related monoalphabetic substitution rules is
         used.
      2. A key determines which particular rule is chosen for a
         given transformation.

      Vigenere cipher
          26 Caesar ciphers are used, with shifts of 0 through 25
          Each cipher is denoted by a key letter (from a to z)
                                                                                           Table 2.4 The Modern Vigenere Tablean

Ch 2 Conventional Encryption: Classical Techniques               43   Ch 2 Conventional Encryption: Classical Techniques           44
 2.3 Classical Encryption Techniques                                           2.3 Classical Encryption Techniques
       Vigenere cipher (Conti.)                                                      Vigenere cipher                (Conti.)
           Given a key letter x and a plaintext letter y, the                            Not all knowledge of the plaintext structure is lost.
           ciphertext letter is at the intersection of the row                           Example: Fig. 2.7.
           labeled x and the column labeled y                                            Attack:
key:       d e c e p t i v e d e c e p t i v e d e c e p t i v e                          1. Either monoalphabetic substitution or a Vigenere
plaintext: w e a r e d i s c o v e r e d s a v e y o u r s e l f                             cipher?
ciphertext: Z I C V T W Q N G R Z G V T W A V Z H C Q Y G L M G J                             If a monoalphabetic substitution is used, then the
                                                                                              statistical properties of the ciphertext should be the
           The strength is that there are multiple ciphertext                                 same as that of the language of the plaintext.
           letters for each plaintext letter, one for each unique
                                                                                              Referring to Fig. 2.6
           letter of the keyword.


 Ch 2 Conventional Encryption: Classical Techniques                    45      Ch 2 Conventional Encryption: Classical Techniques                      46




 2.3 Classical Encryption Techniques                                           2.3 Classical Encryption Techniques
       Vigenere cipher                (Conti.)                                       Vigenere cipher                 (Conti.)
           Attack (Conti.)                                                               Attack (Conti.)
            2. How to determine the keyword length?                                       3. If the keyword length is N, then the cipher consists
                   If two identical sequences of plaintext letters occur at                  of N monoalphabetic substitution ciphers.
                   a distance that is an integer multiple of the keyword                         The letters at positions 1, N+1, 2N+1, and so on will be
                   length, they will generate identical ciphertext sequences                     encrypted with the same monoalphabetic ciphers.
                   An analyst looking at only the ciphertext can detect the               4. Each monoalphabetic ciphers can be attacked using
                   repeated sequences, e.g., VTW at a displacement of 9.                     frequency characteristics
                   Assume that the keyword either 3 or 9 in length
                   By looking for common factors in the displacements of
                                                                                         Using a non-repeating keyword can eliminate the
                   the various sequences, the analyst will make a good guess             periodic nature
                   of the keyword length.


 Ch 2 Conventional Encryption: Classical Techniques                    47      Ch 2 Conventional Encryption: Classical Techniques                      48
 2.3 Classical Encryption Techniques                                                               2.3 Classical Encryption Techniques
         Vigenere cipher                 (Conti.)                                                        Vigenere cipher                 (Conti.)

             Autokey system – a keyword is concatenated with the                                             Ultimate defense - To choose a keyword that is as
             plaintext itself to provide a running key                                                       long as the plaintext and has no statistical
                                                                                                             relationship to it
key:         d e   c e p t   i   v e w e a r e          d i   s   c   o v e r e   d s      a v
                                                                                                             Vernam cipher: 1918, AT&T engineer, Gilbert Vernam
plaintext:   w e   a r e   d i   s   c o v e   r e      d s   a v e y o u r       s   e    l   f
                                                                                                                  binary data
ciphertext: Z I    C V T W Q N G K Z E I            I   G A S X S T S L V V W L A
                                                                                                                  C i = p i ⊕ ki
             Statistical techniques can be applied to cryptanalysis                                                   pi = ith binary digit of plaintext
             since the key and the plaintext share the same                                                           ki = ith binary digit of key
             frequency distribution of letters                                                                        Ci = ith binary digit of ciphertext
             Example: e enciphered by e can be expeated to occur                                                      ⊕ = exclusive-or (XOR) operation
             with a frequency of (0.1275)2=0.0163                                                                 pi = Ci ⊕ ki
 Ch 2 Conventional Encryption: Classical Techniques                                   49           Ch 2 Conventional Encryption: Classical Techniques                   50




 2.3 Classical Encryption Techniques                                                               2.3 Classical Encryption Techniques
         Vigenere cipher                 (Conti.)                                                        Vigenere cipher                 (Conti.)
             Vernam cipher (Conti.)                                                                          One-time pad
                   The essence of this technique is the mean of                                                   Army Signal Corp officer, Joseph Mauborgne
                   construction of the key.                                                                       Using a random key that was truly as long as the message
                   Use a running loop of tape as keyword : a very long but
                                                                                                                  Unbreakable
                   repeating keyword
                   Can be broken with sufficient ciphertext, the use of                                           Produce random output that bears no statistical
                   known or probable plaintext sequences, or both.                                                relationship to the plaintext
                                                                                                                  The practical difficult – sender and receiver must be
                                                                                                                  in possession of, and protect, the random key.




 Ch 2 Conventional Encryption: Classical Techniques                                   51           Ch 2 Conventional Encryption: Classical Techniques                   52
2.3 Classical Encryption Techniques                                                                             2.3 Classical Encryption Techniques
      Transposition Techniques                                                                                        Transposition Techniques                       (Conti.)

          Performs some sort of permutation on the plaintext                                                              A more complex scheme
          letters                                                                                                              to write the message in a rectangle, row by row, and
                                                                                                                               read the message off, column by column, but permute
          Rail fence technique                                                                                                 the order of the columns.
               The plaintext is written down as a sequence of diagonals                                                        The order of the columns then becomes the key.
               and then read off as a sequence of rows                                                                         Plaintext “attack postponed until two am xyz”
               Plaintext “meet me after the toga party”                                                                        Key:        4 3 1 2 5 6 7
               m       e       m       a       t       r       h       t       g       p       r       y                       plaintext: a t t a c k p
                                                                                                                                           o s t p o n e
                   e       t       e       f       e       t       e       o       a       a       t                                       d u n t i l t
               Ciphertext ”MEMATRHTGPRYETEFETEOAAT                                                                                         w o a m x y z
                                                                                                                               Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ

Ch 2 Conventional Encryption: Classical Techniques                                                         53   Ch 2 Conventional Encryption: Classical Techniques                54




2.3 Classical Encryption Techniques                                                                             2.3 Classical Encryption Techniques
      Transposition Techniques                                                 (Conti.)                               Transposition Techniques                       (Conti.)

          Perform more than one stage of transposition                                                                    Perform more than one stage of transposition (Conti.)
               Key:       4 3 1 2 5 6 7                                                                                        The original sequence of letters is
                                                                                                                               01 02 03 04 05 06 07 08 09 10 11 12 13 14
               plaintext: t t n a a p t                                                                                        15 16 17 18 19 20 21 22 23 24 25 26 27 28
                          m t s u o a o
                                                                                                                               After the first transposition:
                          d w c o i x k                                                                                        03 10 17 24 04 11 18 25 02 09 16 23 01 08
                          n l y p e t z                                                                                        15 22 05 12 19 26 06 13 20 27 07 14 21 28
               Ciphertext: NSCYAUOPTTWLTMDNAOIEPAXTTOKZ                                                                        After the second transposition:
                                                                                                                               17 09 05 27 24 16 12 07 10 02 22 20 03 25
                                                                                                                               15 13 04 23 19 14 11 01 26 21 18 08 06 28
                                                                                                                               This is a much less structured permutation and is much
                                                                                                                               more difficult to cryptanalysis.

Ch 2 Conventional Encryption: Classical Techniques                                                         55   Ch 2 Conventional Encryption: Classical Techniques                56
2.3 Classical Encryption Techniques                                        2.3 Classical Encryption Techniques
      Rotor machines                                   Rotors are 75a-e.         Rotor machines                  (Conti.)
                                                                                     Consists of a set of independently rotating cylinders
                                                                                     A single cylinder defines a monoalphabetic
                                                                                     substitution
                                                                                     After each input key is depressed, the cylinder rotates
                                                                                     one position, so that the internal connections are
                                                                                     shifted accordingly. Thus, a different monoalphabetic
                                                                                     substitution cipher is defined.
                                                                                     A polyalphabetic substitution algorithm with a period
                                                                                     of 26.
               Edward Hebern’s “Electric Code Machine,” 1921
                          U.S. Patent 1683072.
Ch 2 Conventional Encryption: Classical Techniques                   57    Ch 2 Conventional Encryption: Classical Techniques                      58




2.3 Classical Encryption Techniques                                        2.3 Classical Encryption Techniques
      Rotor machines                  (Conti.)
          Multiple cylinders
               The output pins of one cylinder are connected to the
               input pins of the next
               The cylinder farthest from the operator input
               rotates one pin position with each keystroke
               For every complete rotation of the outer cylinder, the
               middle cylinder rotates one pin position
               For every complete rotation of the middle cylinder,
               the inner cylinder rotates one pin position
               26*26*26=17576 different substitution algorithms
               Point to the way to DES
                                                                            Fig. 2.8 Three-Rotor Machine with wiring represented by numbered contacts.

Ch 2 Conventional Encryption: Classical Techniques                   59    Ch 2 Conventional Encryption: Classical Techniques                      60

				
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