1. SYMMETRIC KEY ALGORITHMS

2. Comparison of Symmetric and Asymmetric Encryption
Decryption
Ciphertext
Original Plaintext
Plaintext
Secret Key
Symmetric (Single Key) Cryptography
Encryption
Decryption
Ciphertext
Original Plaintext
Private Key
Public Key
Plaintext
Asymmetric (Two Key) Cryptography

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 Shannon’s 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

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) = (P0,1 P0,2) = (10) = 2
(P1,0 P1,3) = (11) = (P1,1 P1,2) = (00) = 0

19. Concatenate S0 (e. g. 3=11) and S1 (e. g. 2=10) into a 4-bit value (e
Concatenate S0 (e.g. 3=11) and S1 (e.g. 2=10) 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 E/P K 1 S0 S1 P4 SW 4 8 8 4 4 42 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.