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SYMMETRIC KEY ALGORITHMS

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SYMMETRIC KEY ALGORITHMS * Comparison of Symmetric and Asymmetric Encryption Encryption Decryption Ciphertext Original Plaintext Plaintext Secret Key Symmetric ... – PowerPoint PPT presentation

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Title: SYMMETRIC KEY ALGORITHMS


1
SYMMETRIC KEY ALGORITHMS
2
Comparison of Symmetric and Asymmetric Encryption
3
BLOCK CIPHER DESIGN PRINCIPLES
  • Two properties of operation of secure cipher
  • Confusion make the relationship between the
    statistics of the ciphertext and the value of the
    encryption key as complex as possible. Confusion
    is achieved through a complex substitution.
  • Diffusion dissipates the redundancies of the
    plaintext by distributing over the ciphertext.
    Diffusion is achieved through permutations.
  • Claude Shannons Papers of 1948/1949
  • A Mathematical Theory of Communication
  • Communication Theory of Secrecy Systems
  • To thwart cryptanalysis based on statistical
    analysis

4
SIMPLIFIED DES
  • Developed 1996 as a teaching tool
  • Santa Clara University
  • Prof. Edward Schaefer
  • Takes an 8-bit block plaintext, a 10 bit key and
    produces an 8-bit block of ciphertext
  • Decryption takes the 8-bit block of ciphertext,
    the same 10-bit key and produces the original
    8-bit block of plaintext

5
Simplified DES scheme
  • Five Functions to Encrypt
  • IP an initial permutation
  • fk - a complex, 2-input function
  • SW a simple permutation that swaps the two
    halves of data
  • fk - a complex, 2-input function again
  • IP inverse permutation of the initial
    permutation

6
(No Transcript)
7
S-DES KEY GENERATION
8
S-DES KEY GENERATION
  • 10-bit key be designated as
  • (k1, k2,k3, k4, k5, k6, k7, k8, k9, k10)
  • Then the permutation P10 is defined as
  • P10(k1, k2, k3, k4, k5, k6, k7, k8, k9, k10)
  • (k3, k5, k2, k7, k4, k10, k1,
    k9, k8, k6)

9
S-DES KEY GENERATION
  • Perform a circular shift (LS-1), or rotation,
    separately on the 1st 5 bits and the 2nd 5 bits.
  • Next, we apply P8,permute 8 of the 10 bits as
  • Result is subkey 1, K1 , of 8 bits

10
S-DES KEY GENERATION
  • Go back to the pair of 5-bit strings produced by
    the two LS-1 functions,
  • and perform a circular left shift of 2 bit
    positions, LS-2 ,on each string
  • Finally, P8 is applied again to produce K2 ,the
    subkey 2

11
S-DES ENCRYPTION
12
S-DES Encryption
  • 8-bit block of plaintext (eg. 10111101)
  • First permute using the IP(Initial Permutation)
    function as
  • At the end, apply inverse permutation IP-1

13
Encryption Detail
14
S-DES Encryption
  • The Function F and fk
  • Divide the value after IP into two parts L,
    R
  • fk(L,R) (L F(R,SK),R)
  • where SK is a subkey and is the bit-by-bit
    XOR operation

15
S-DES Encryption- F(R,SK)
  • Rightmost 4 bits(n1,n2,n3,n4) as input to E/P
  • Expand the 4-bit value and concatenate it twice
    into an 8-bit value . Then permute it.
  • Create a matrix based on the result
  • Row 1
  • Row 2

16
S-DES Encryption- F(R,SK)
  • 8-bit subkey K1 (k11, k12, k13, k14, k15, k16,
    k17,k18) and perform an exclusive-OR function on
    the matrix in prev. step

17
  • Rename the resultant matrix as
  • The first 4 bits (first row of the preceding
    matrix) are fed into the S-box S0 to produce a
    2-bit output, and the remaining 4 bits (second
    row) are fed into S1 to produce another 2-bit
    output

18
  • The S-boxes are
  • The first and fourth input bits are treated as a
    2-bit number that specify a row of the S-box
  • and the second and third input bits specify a
    column of the S-box
  • (P0,0 P0,3) (11) 3 (P0,1 P0,2)
    (10) 2
  • (P1,0 P1,3) (11) 3 (P1,1 P1,2)
    (00) 0

19
  • Concatenate S0 (e.g. 311) and S1 (e.g. 210)
    into a 4-bit value (e.g. 1110)
  • Permute 4 bit value as P4
  • Output of Function F
  • fk(L,R) (L F(R,SK),R)
  • 4 bits of L F(R,SK) and R are given to
    SW
  • SW interchanges the L and R bits to next function
    fk(L,R)

20
Encryption Detail
8 Bit Plaintext
I P
4
E/P
8
8
K 1
4
4
4
S1
S0
2
2
P4
4
SW
4
21
  • 8 bits passes through next function fk
  • Key used is K2
  • Finally , apply inverse permutation IP-1

22
S-DES Decryption
  • Reverse process of encryption
  • Input is the ciphertext
  • Key K2 is applied before K1
  • Output is the plaintext.
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