What is the length of cryptographic key in Digital Encryption Standard DES )?

Data Encryption Standard uses a single key for encrypting and decrypting a message. This means that the sender and receiver both must have the same key to access the message. At one point in time, DES was the go-to encryption technology. However, over a period, DES was overpowered by the more sophisticated AES (Advanced Encryption Standard).

Here are some of the important features that impact the working of the Data Encryption Standard.

Block cipher This means that the entire Data Encryption Standard is a cryptographic key, which is applied to a block of data and not on a single bit.  For instance, to encrypt a plain text message, DES will put the message into blocks of 64 bits and then encrypt it.

Multiple rounds of encryption The DES methodology is a process of encryption that is done 16 times. This is done in four different modes, by encrypting blocks individually or creating a relationship of each cipher block with all previous blocks. Decryption is just the opposite of encryption, where you must follow the same steps but in reverse order.

64-bit Key DES actually uses a 64-bit key, however, eight of those bits are utilized for checks making the effective length to only 56 bits. The algorithm for encryption products 16 different subkeys of 48 bits each. Each of these subkeys is used for 16 encryption rounds.

Replacement & Permutation The algorithm also helps in defining the sequence of replacement and permutation that the cipher undergoes during the process of encryption.

 Backward Compatibility DES also provides this compatibility in some cases.

The following diagram explains how encryption converts a plain text message into an encrypted message

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 −

• Round function
• Key schedule
• Any additional processing − Initial and final permutation

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-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:

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 −

The logic for Parity drop, shifting, and Compression P-box is given in the DES description.

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.

What is the encryption key length in DES?

Why does DES use 56

What is the length of the crypto variable used in DES?