# Question 334 of 530 from exam 400-251: CCIE Security written exam

### Question

Which two statements about the DES algorithm are true? (Choose two)

### Answers

### Explanations

CE.

The correct statements about the DES algorithm are:

C. The DES algorithm is based on symmetric cryptography: DES (Data Encryption Standard) is a symmetric key block cipher, meaning that the same key is used for both encryption and decryption. The sender and receiver must have the same key to communicate securely.

E. The DES algorithm uses a 56-bit key: The key size for DES is 56 bits. The actual key used by the algorithm is 64 bits, but 8 of those bits are used for error detection and do not contribute to the key. Thus, the effective key size is 56 bits.

A. Incorrect: The DES algorithm is based on symmetric cryptography, not asymmetric cryptography. Asymmetric cryptography uses a pair of keys, one public and one private, for encryption and decryption.

B. Incorrect: The DES algorithm is a block cipher, not a stream cipher. A stream cipher encrypts data one bit or byte at a time, whereas a block cipher encrypts a fixed-size block of data at a time.

D. Incorrect: The DES algorithm encrypts a block of 64 bits, not 128 bits. The input block is divided into two halves, each of 32 bits, and undergoes a series of permutations and substitutions to produce the output block of 64 bits.

In summary, DES is a symmetric key block cipher that uses a 56-bit key to encrypt and decrypt data in fixed-size blocks of 64 bits.