Which of the following statements pertaining to block ciphers is NOT true?

A.
It operates on fixed-size blocks of plaintext.

B.
It is more suitable for software than hardware implementations.

C.
Plain text is encrypted with a public key and decrypted with a private key.

D.
Some Block ciphers can operate internally as a stream.

Explanation:
It is not true that plain text is encrypted with a public key and decrypted with a private key with a block cipher.
Block ciphers use symmetric keys.
In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called
blocks, with an unvarying transformation that is specified by a symmetric key. Block ciphers are importantelementary components in the design of many cryptographic protocols, and are widely used to implement
encryption of bulk data.
Stream ciphers represent a different approach to symmetric encryption from block ciphers. Block ciphers
operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut:
in some modes of operation, a block cipher primitive is used in such a way that it acts effectively as a stream
cipher.
Incorrect Answers:
A: It is true that a block cipher operates on fixed-size blocks of plaintext.
B: Stream ciphers require a lot of randomness and encrypt individual bits at a time. This requires more
processing power than block ciphers require, which is why stream ciphers are better suited to be implemented
at the hardware level. Because block ciphers do not require as much processing power, they can be easily
implemented at the software level.
D: It is true that some Block ciphers can operate internally as a stream.

https://en.wikipedia.org/wiki/Block_cipher
https://en.wikipedia.org/wiki/Stream_cipher