Bài giảng Mã hóa DES Data Encryption Standard - Huỳnh Trọng Thưa

Mã hóa DES
Data Encryption Standard
Huỳnh Trọng Thưa
htthua@ptithcm.edu.vn
Part 1 - Encryption of DES
• Feistel structure of DES
• S-boxes
• Key Schedule
2
Data Encryption Standard (DES) 
and Alternatives
• Basic design ideas of block ciphers, including 
confusion (xáo trộn) and diffusion (khuếch 
tán), which are important properties of all 
modern block ciphers
• The internal structure of DES, including Feistel 
networks, S-boxes and the key schedule.
• Alternatives to DES, including 3DES
3
Confusion and Diffusion
• Confusion: the relationship between key 
and ciphertext is obscured.
– for achieving confusion: substitution, which 
is found in both DES and AES.
• Diffusion: the influence of one plaintext 
symbol is spread over many ciphertext 
symbols with the goal of hiding 
statistical properties of the plaintext.
– A simple diffusion element is the bit 
permutation, which is used frequently 
within DES.
4
Principle of an N round 
product cipher, where 
each round performs a 
confusion and diffusion 
operation
Modern block ciphers
• Changing of one bit of plaintext results on 
average in the change of half the output bits, 
i.e., the second ciphertext looks statistically 
independent of the first one. 
5
Principle of diffusion of a block cipher
DES block cipher
• DES is a cipher which encrypts blocks of length 
of 64 bits with a key of size of 56 bits
• DES is a symmetric cipher.
• An iterative algorithm.
6
Round structure of DES
• For each block of plaintext, 
encryption is handled in 16
rounds which all perform the 
identical operation.
• In every round a different 
subkey is used and all subkeys ki
are derived from the main key k.
7
The Feistel structure of DES
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The Feistel structure of DES (cont.)
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Internal Structure of DES
• Initial and Final Permutation
• f – function
• Key Schedule
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Initial and Final Permutation
• are bitwise permutations
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bit swaps of the initial permutation bit swaps of the final permutation
read from left to right, top to bottom
f - function
12
Bit swaps of the expansion 
function E
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S-boxes
• Each S-box contains 26 =64 entries.
• Each entry is a 4-bit value.
14
Decoding of the input 
1001012 by S-box 1
• Ex: The S-box input b =(100101)2 indicates the row 
112 = 3 (i.e., fourth row, numbering starts with 002) 
and the column 00102 = 2 (i.e., the third column). If 
the input b is fed into S-box 1, the output is S1(37 = 
1001012)= 8 = 10002.
S-boxes table for Ref.
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The permutation P within the f -
function
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Key Schedule
• PC-1: ignoring every eighth bit 
(64-bit key -> 56 bits ) 
• 56-bit key is split into two 
halves C0 and D0
• The two 28-bit halves are 
cyclically shifted, i.e., rotated, 
left by one or two bit positions 
depending on the round i.
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Initial key permutation PC−1
In rounds i = 1,2,9,16, the two halves are rotated left by one bit.
In the other rounds i 1,2,9,16, the two halves are rotated left by two bits.
Key schedule for DES encryption
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Round key permutation PC−2
Part 2 - Descryption of DES
• Descryption of DES
• Security of DES
• DES Alternatives
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Block diagram for DES decryption
20
y
Block diagram for DES decryption (cont.)
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Reversed Key Schedule
• k16 can be directly derived after PC−1.
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• Round 1, the key is not rotated.
• Rounds 2, 9, and 16 the two halves are rotated right by one bit.
• Other rounds 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14 and 15 the two 
halves are rotated right by two bits.
Reversed key schedule for decryption of DES
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Why is the decryption function essentially 
the same as the encryption function?
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Why is the decryption function essentially the 
same as the encryption function? (cont.)
25
where i = 0,1,...,16. In particular, after the last decryption round:
Finally, at the end of the decryption process, we have to reverse the 
initial permutation:
Security of DES
• The key space is too small, i.e., the algorithm 
is vulnerable against brute-force attacks.
• The design criteria of the S-boxes was kept 
secret and there might have existed an 
analytical attack that exploits mathematical 
properties of the S-boxes, but which is only 
known to the DES designers.
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DES Alternatives
• Advanced Encryption Standard (AES) and the 
AES Finalist Ciphers
• Triple DES (3DES) and DESX
• Lightweight Cipher PRESENT
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Advanced Encryption Standard 
(AES) and the AES Finalist Ciphers
• AES is with its three key lengths of 128, 192 
and 256 bit secure
• Against brute-force attacks for several decades
• There are no analytical attacks with any 
reasonable chance of success known.
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Triple DES (3DES) and DESX
• 3DES consists of three subsequent DES encryptions 
with different keys
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Another version of 3DES is
A different approach for strengthening DES is to use key whitening
Lightweight Cipher PRESENT
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Next class
• Advanced Encryption Standard (AES)
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