Cryptography 7
DES (Data Encryption Standered)
DES :
DES adopted in 1977 by (NIST) National Institute of Slandered and Technology).
In DES data are encrypted in 64 bit block with a key length 56 bits and with output ciphertext 64 bits.
DES has the exact structure of Feistel Cipher but without Initial Permutation (IP) and Inverse Initial Permutation (IP ^-1).
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Key generator algorithm :The key generator passes through many steps to produces subkeys.
1- Key generator algorithm take 64 bits key as input,the input key numbered in a table from 1 to 64 as the following:
2- Every eighth bit is ignored and produce 56 bits.
3- 56 bits passes through a permutation Choice one(PC-1)as the following:
4- The output is separated into two 28 bits C and D , the first 28 bits are called C0(left part) and the last 28 bits are called D0.
5- At each round a circular left shift is preformed on Ci-1 and Di-1 by 1 or 2 bits by the next table.
6- Then Ci-1 and Di-1 in each round passes through permutation choice two (PC-2) to produces 48 bits.
7- The permutation Choice Two output in each round is use as input to encryption algorithm.
Encryption Algorithm :
There are two inputs to encryption algorithm 1- Plaintext 64 bits2- Encryption key 48 bits.
Encryption algorithm also passes through many steps to produce a ciphertext, The next figure
1- The plaintext block 64 bits pass through an initial permutation (IP) that rearranged bits and produces the permuted input.
Initial Permutation :
A- Initial Permutation takes the plaintext as input table consists of 64 bits numbered from 1 to 64
B- Then the initial permutation will be permuted input 64 bits as the following :
C- The Inverse Initial Permutation is:
2- The permuted input block split into two halves each is 32 bits, the first 32 bits are called L and the last 32 bits are called R.
Now The F function will start by the rest of all steps.
3- Expand R 32 bits to 48 bits to fit the subkey by preform Expansion permutation(E) as the following:
4- Preform Exclusive-OR between the subkey and Expansion Permutation (E) on R .
E(Ri-1)⊕ Ki.
5- The result of E(Ri-1) ⊕ Ki pass through a substitution function and produces 32 bits output
Substitution Function :
Substitution Function is rolled by S-Box , S-Box consists of 8 boxes each of which accepts 6 bits as input and produces 4 bits output by the following:
A- Break the result of E(Ri-1)⊕Ki into 8 blocks each contain 6 bits , These blocks are numbered from 1 to 8.
B- Each block will preform a substitution with S-Box with the same number by the following roles :
A- The first and the last bits of each block together as 2 bit value to indicate the number of row in the same number S-Box.
B- The middle four bits of each block together as-bit value to indicate the number of column in the same number S-Box.
C- The decimal value which selected by the row and the column converted to-bit value in all S-Boxes.
For Example :
Suppose the first 6 bits of the result of E(Ri-1)⊕Ki = 010101.
So, the input to S1 = 010101.
The row value = 0 1 = 1 (decimal).
The column value = 1010 = 10 (decimal).
The decimal value will be 12 = 1100 (4-bit value).
D- Combine results of each S-Box together 32 bits.
6- The result of the substitution operation(output of S-Boxes) passes through a Permutation Function (P)
At this point the function F is finished.
7- Preform Exclusive-OR between the output of the Permutation Function(P) and Li-1 , and then put the result in Ri , and put Ri-1 in Li.
The overall formulas for DES Encryption Algorithm.
Li = Ri-1.
Ri = Li-1 ⊕ F(Ri-1,Ki).
8- Preform 32-bit swap on the result of the final round , then preform Inverse Initial Permutation(IP^-1) on the swapped data to produces the ciphertext 64 bits.
Decryption Algorithm :
The inputs to decryption algorithm are ciphertext and subkey Ki but in reverse order, start with Kn then K(n-1) and so on until K1 in the last round.
Note: You can use DES-Calculator https://github.com/Hamza-Megahed/des-calculator to study each round in detail.