Which of the following types of ciphers operates on a group of bits rather than an individual
character or bit of a message?

A.
Block cipher

B.
Classical cipher

C.
Substitution cipher

D.
Stream cipher

Explanation:
Block cipher is a symmetric key cipher that operates on fixed-length blocks (groups of
bits) with an unvarying transformation. During the
encryption process, a block cipher might take an n bit block of plaintext as input, and output a
corresponding n bit block of the ciphertext. The
exact transformation is controlled using a secret key. While decrypting, the decryption algorithm
takes the n bit block of the ciphertext
together with the secret key, and yields the original n bit block of the plaintext.
Answer options B, C, and D are incorrect. All these three types of ciphers operate on an individual
character or bit of a message.