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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
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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.