# Question 88 of 530 from exam 400-251: CCIE Security written exam

### Question

Which three statements are true about DES? (Choose three.)

### Answers

### Explanations

BCE.

DES (Data Encryption Standard) is a symmetric-key block cipher that uses a 56-bit key to encrypt 64-bit blocks of plaintext. Here are the correct statements about DES:

A. False. A 56-bit key is used to encrypt 64-bit blocks of plaintext, not 56-bit blocks.

B. True. A 56-bit key is used to encrypt 64-bit blocks of plaintext.

C. True. Each block of plaintext is processed through 16 rounds of identical operations, which include permutation, substitution, and XOR operations.

D. False. Each block of plaintext is processed through 16 rounds of identical operations, not 64 rounds.

E. True. ECB (Electronic Codebook), CBC (Cipher Block Chaining), and CFB (Cipher Feedback) are modes of DES. ECB encrypts each block of plaintext separately, while CBC and CFB use the output of the previous block to encrypt the current block, resulting in a more secure encryption.

F. False. Each block of plaintext is processed through 16 rounds of identical operations, not 8 rounds.

G. False. CTR (Counter) and OFB (Output Feedback) are modes of DES, but not CBC. CBC was mentioned in statement E.

In summary, the correct statements about DES are B, C, and E.