Which DES modes can best be used for authentication?

A.
Cipher Block Chaining and Electronic Code Book.

B.
Cipher Block Chaining and Output Feedback.

C.
Cipher Block Chaining and Cipher Feedback.

D.
Output Feedback and Electronic Code Book.

Explanation:
Cipher Block Chaining (CBC) uses feedback to feed the result of encryption back into the encryption
of the next block. The plain-text is XOR’ed with the previous cipher-text block before it is encrypted.
The encryption of each block depends on all the previous blocks. This requires that the decryption
side processes all encrypted blocks sequentially. This mode requires a random initialization vector
which is XOR’ed with the first data block before it is encrypted. The initialization vector does not
have to be kept secret. The initialization vector should be a random number (or a serial number), to
ensure that each message is encrypted uniquely. In the Cipher Feedback Mode (CFB) is data
encrypted in units smaller than the block size. This mode can be used to encrypt any number of bits
e.g. single bits or single characters (bytes) before sending across an insecure data link. Both of those
method can be best used to provide user authentication capabilities.