Question

Why do some block cipher modes of operation only use encryption while others use both encryption and decryption?

Answer 1

Answer:

Some modes of operation (eg CTR) work in such a way that only known values are ever encrypted, forming a stream of pseudo-random data that is then combined with the plaintext by a keyless reversible operation (often xor) to form the ciphertext.

Other modes (eg CBC) directly encrypt secret (ie plaintext) values, meaning decryption is required to find out what the secret value was.

One of the biggest advantages of a scheme that does not require decryption is that it can be implemented in hardware with reduced footprint (ie it's smaller). Moreover, for block ciphers such as AES it can often be easier to implement efficient encryption than decryption because the internal coefficients have been optimized for this direction.

Answer 2

Explanation:

Some mode of operation only use an encryption function because it is used to generate something to XOR with the plain text. There is no point decrypt the generated bytes. To decrypt the ciphertext, u just need the same stream of bytes.