The document discusses the Data Encryption Standard (DES), including: - DES is a block cipher that encrypts data in 64-bit blocks using a 56-bit key and 16 rounds of encryption. - DES has a block size of 64 bits, a key size of 56 bits (though the key specification is 64 bits with 8 bits used for error checking), and uses 16 intermediate 48-bit keys over its 16 rounds. - The DES encryption process uses substitution boxes (S-boxes) to encrypt each block, and has 4 known weak keys that should be avoided during key generation.

1. DATA ENCRYPTION STANDARD(DES)
Md Fazle Rabbi
ID:16CSE057

2. CONTENTS
 What is DES?
 DES Features
 DES Rounds
 Key length in DES
 Festal Rounds
 DES “f(•)” Function
 S-boxes
 DES weak keys
 DES Analysis
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3. WHAT IS DES?
 The Data Encryption Standard is a block cipher,
meaning a cryptographic key and algorithm are
applied to a block of data simultaneously rather
than one bit at a time.
 DES uses a 64-bit key, but eight of those bits are
used for parity checks, effectively limiting the key to
56-bits.

4. DES FEATURES
 Block size = 64 bits
 Key size = 56 bits (in reality, 64 bits, but 8 are used
as parity-check bits for error control, see next slide)
 Number of rounds = 16
 16 intermediary keys, each 48 bits

5. KEY LENGTH IN DES
 In the DES specification, the key length is 64 bit:
 8 bytes; in each byte, the 8th bit is a parity-check bit

6. DES ROUNDS

7. WORKFLOW OF FESTAL ROUNDS

8. DES “f(•)” Function

9. S-BOXES
• S = matrix 4x16, values from 0 to 15
• B (6 bit long) = b1b2b3b4b5b6
– b1b6 r = row of the matrix (2 bits: 0,1,2,3)
– b2b3b4b5 c = column of the matrix(4bits:0,1,…15)
• C (4 bit long) = Binary representation of S(r, c)

11. DES WEAK KEYS
• DES uses 16 48-bits keys generated from a master
56- bit key (64 bits if we consider also parity bits)
• Weak keys: keys make the same sub-key to be
generated in more than one round.
• Result: reduce cipher complexity • Weak keys can be
avoided at key generation.
• DES has 4 weak keys
– 01010101 01010101
– FEFEFEFE FEFEFEFE
– E0E0E0E0 F1F1F1F1
– 1F1F1F1F 0E0E0E0E

12. DES ANALYSIS
The DES satisfies both the desired properties of block cipher.
These two properties make cipher very strong.
Avalanche effect − A small change in plaintext results in
the very great change in the ciphertext.
Completeness − Each bit of ciphertext depends on many
bits of plaintext.
During the last few years, cryptanalysis have found some
weaknesses in DES when key selected are weak keys. These
keys shall be avoided.
DES has proved to be a very well designed block cipher.
There have been no significant cryptanalytic attacks on DES
other than exhaustive key search

13. THE END