Our password or data is secure in every website due to Data Encryption Standard.
But how lets we know about it....
Data Encryption Standard (DES):-
Data Encryption Standard (DES) was issued in 1977 as Federal Information Processing Standard (FIPS) by the National Institute of Standards and Technology (NIST).
It contained 1,536 microprocessor running at 40MHz, which performed 60 million test decryption per second per chip.
DES use symetric system means both sender and reciever was used same key for encryption and Decryption.
_ The most widely used encryption scheme
– The plaintext is processed in 64-bit blocks
– The key is 56-bits in length
– 8 bits are used for parity.
– Sixteen rounds of transposition and substitution are performed in order to achieve DES.
– DES is a block cipher
Block Cipher:-
A block cipher processes plaintext input in fixed-sized blocks and produces a block of cipher text of equal size for each plaintext block.Block ciphers process messages into blocks, each of which is then encrypted/decrypted.
The two users share a common encryption and decryption key. DES is an example like a substitution on very big characters.
DES Encryption:-
Data are encrypted in 64-bit blocks using a 56-bit key. The algorithm transforms 64-bit input in a series of steps into a 64-bit output. There are two inputs to the encryption function: the plaintext to be encrypted and the key. The function expects a 64-bit key out of which only 56 are used; other 8 bits can be set arbitrarily.
Before any rounds, the plaintext bits are permuted using an initial permutation. Hence, at the end of the 16 rounds the inverse permutation is applied. The initial permutation is public knowledge.
This is the substitution part of the cipher. Each S block has a different functionality as defined by the corresponding tables.
Three Phases Of Plaintext:-
Plaintext proceeds in three phases.
• 1st phase, the 64-bit plaintext passes through an initial permutation (IP) that rearranges the bits to produce the permuted input.
• 2nd phase consists of 16 rounds of the same function, which involves both permutation and substitution functions.
• The output of the last round consists of 64 bits that are a function of the input plaintext and the key.
• The left and right halves of the output are swapped to produce pre-output.
• 3rd phase, the pre-output is passed through a permutation that is the inverse of the initial permutation function, to produce the 64-bit cipher text.
Sub-Key Generation:-
Initially, the key is passed through a permutation function. Then, for each of the 16 rounds, a sub-key (Ki) is produced by the combination of a left circular shift and a permutation. The permutation function is the same for each round, but a different sub-key is produced because of the repeated shifts of the key bits.
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:
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-bitand round key is a 48-bit, we first need to expand right input to 48 bits. Permutation logic is graphically depicted in the following illustrious.
The graphically depicted permutation logic is generally described as table in DES specification illustrated.
Cipher Design Principles:-
1. Block size
– increasing size improves security, but slows cipher
2. Key size
– increasing size improves security, makes exhaustive key searching harder, but may slow cipher
3. Number of rounds
– increasing number improves security, but slows cipher
4. Sub-key generation
– greater complexity can make analysis harder, but slows cipher
5. Round function
– greater complexity can make analysis harder, but slows cipher
6. Fast software en/decryption & ease of analysis
– are more recent concerns for practical use and testing
.
DES Decryption:-
• It uses the same algorithm as encryption, except that the application of the sub-keys in reverse order (SK16 … SK1)
• Also, the initial and final permutations are reversed.
• Thus recovering original data value.
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