Data Encryption
Standard
Reference: Google and You Tube Videos
Data Encryption
Standard
What is DES?
• Data Encryption Standard (DES) is a block cipher with a 56-bit
key length that has played a significant role in data security.
• Data encryption standard (DES) has been found vulnerable to
very powerful attacks therefore, the popularity of DES has been
found slightly on the decline.
• DES is a block cipher and encrypts data in blocks of size of 64
bits each, which means 64 bits of plain text go as the input to
DES, which produces 64 bits of ciphertext.
• The same algorithm and key are used for encryption and
decryption, with minor differences.
• The key length is 56 bits.
Initial Permutation (IP)
• As we have noted, the initial permutation (IP) happens only once and it
happens before the first round.
• It suggests how the transposition in IP should proceed, as shown in the
figure.
• For example, it says that the IP replaces the first bit of the original plain
text block with the 58th bit of the original plain text, the second bit with the
50th bit of the original plain text block, and so on.
Initial and Final Permutation
The initial and final permutations are straight Permutation boxes (P-boxes) that are inverses
of each other.
They have no cryptography significance in DES. The initial and final permutations are shown
as follows −
DES is based on the Feistel Cipher, all that is required to specify DES is −
•Round function
•Key schedule
•Any additional processing − Initial and final permutation
Round Function
• The heart of this cipher is the DES function, f.
• The DES function applies a 48-bit key to the rightmost 32 bits to produce a
32-bit output.
Expansion Permutation Box −
• Since right input is 32-bit and round key is a 48-bit, we first need to expand right input to
48 bits.
• Permutation logic is graphically depicted in the following illustration −
The graphically depicted permutation logic is generally described as table in DES
specification illustrated as shown
•XOR (Whitener). − After the expansion permutation, DES does XOR operation on the
expanded right section and the round key. The round key is used only in this operation.
•Substitution Boxes. − The S-boxes carry out the real mixing (confusion). DES uses 8 S-
boxes, each with a 6-bit input and a 4-bit output. Refer the following illustration −
•The S-box rule is illustrated below −
Key Generation
• The round-key
generator creates
sixteen 48-bit keys
out of a 56-bit
cipher key.
• The process of key
generation is
depicted in the
following
illustration −
(Repeat)
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.
• Strength of DES -- 56-bit keys have 256 = 7.2 x 1016 values
Brute force search looks hard.
A machine performing one DES encryption per microsecond would take more than a
thousand year to break the cipher.
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.
Avalanche Effect
key desirable property of encryption alg
where a change of one input or key bit results in
changing approx. half output bits
making attempts to “home-in” by guessing keys
impossible
DES exhibits strong avalanche
Avalanche in DES
References:
[1] William Stallings, Cryptography and Network Security, 1999.
[2] Google and You Tube Videos.
