Subject | Computer and Network Security |
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NU Year | Set: 2.(a) Marks: 5 Year: 2017 |

The Data Encryption
Standard (DES) is an outdated symmetric-key method of data encryption.

DES works by using the
same key to encrypt and decrypt a message, so both the sender and the
receiver must know and use the same private key. Once the go-to,
symmetric-key algorithm for the encryption of electronic data, DES has been
superseded by the more secure Advanced Encryption Standard (AES)
algorithm.

The Data Encryption
Standard is a block cipher, meaning a cryptographic key and algorithm are
applied to a block of data simultaneously rather than one bit at a
time. To encrypt a plaintext message, DES groups it into 64-bit
blocks. Each block is enciphered using the secret key into a 64-bit cipher
text by means of permutation and substitution. The process involves 16
rounds and can run in four different modes, encrypting blocks individually or
making each cipher block dependent on all the previous blocks. Decryption is
simply the inverse of encryption, following the same steps but reversing the
order in which the keys are applied. For any cipher, the most basic method of
attack is brute force, which involves trying each key until you find
the right one. The length of the key determines the number of possible keys --
and hence the feasibility -- of this type of attack. DES uses a 64-bit key, but
eight of those bits are used for parity checks, effectively limiting the key to
56-bits. Hence, it would take a maximum of 2^56, or 72,057,594,037,927,936,
attempts to find the correct key.