I am reading over a slide that I found online regarding the DES algorithm for encryption and I am a little confused about the per round key generation. From the slide below, I understand that each per round key is obtained by shifting left either 1 or 2 bits depending on the round. Apr 12, 2020 For the Love of Physics - Walter Lewin - May 16, 2011 - Duration: 1:01:26. Lectures by Walter Lewin. They will make you ♥ Physics. Recommended for you. Jun 10, 2016 The simplified DES (S-DES) is a modified version of the data encryption standard DES algorithm. Another modified version of the DES algorithm is famously known as Triple DES. The key generator method creates 16 48-bit keys. Note: This implementation of simplified data encryption standard in C programming. Let's move on to the DES subkey generation, which takes a 56 bit key and then outputs 16 subkeys, each of which are 48 bits long. The diagram illustrates how the subkey generation works. PC blocks stand for permuted choice, and is a permutation and a compression block, where the outputs are shorter than the inputs. Asymmetric key. Depending on the type of cryptographic system used, the public key is obtained from an encryption of the private key or vice versa. For example, the public key that you use to transfer your bitcoins is created from the private key by applying a series of a cryptographic hash.
The Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST).
DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure. The block size is 64-bit. Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits of the key are not used by the encryption algorithm (function as check bits only). General Structure of DES is depicted in the following illustration −
Since DES is based on the Feistel Cipher, all that is required to specify DES is −
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 −
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 −
There are a total of eight S-box tables. The output of all eight s-boxes is then combined in to 32 bit section.
Straight Permutation − The 32 bit output of S-boxes is then subjected to the straight permutation with rule shown in the following illustration:
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 −
The logic for Parity drop, shifting, and Compression P-box is given in the DES description.
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.
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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.
Java provides KeyGenerator class this class is used to generate secret keys and objects of this class are reusable.
To generate keys using the KeyGenerator class follow the steps given below.
The KeyGenerator class provides getInstance() method which accepts a String variable representing the required key-generating algorithm and returns a KeyGenerator object that generates secret keys.
Create KeyGenerator object using the getInstance() method as shown below.
The SecureRandom class of the java.Security package provides a strong random number generator which is used to generate random numbers in Java. Instantiate this class as shown below.
The KeyGenerator class provides a method named init() this method accepts the SecureRandom object and initializes the current KeyGenerator.
Initialize the KeyGenerator object created in the previous step using the init() method.
Following example demonstrates the key generation of the secret key using the KeyGenerator class of the javax.crypto package.
The above program generates the following output −