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

Question

In Chapter 6, Question 6.8, of his book “Cryptography and Network Security Principles and Practices”, William Stalling asks:

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

ECB and CBC use encryption function and decryption function while CFB, OFB and CTR use encryption function for decryption as well. What is the logic behind it? Is it because ECB and CBC generates output of Block Cipher type – while CFB, OFB and CTR generates output of Stream Cipher type?

Answer 1

That is the general idea of it yes.

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

I'm not sure if your understanding is correct. However, to explain it a bit to be sure you’re on the right track…

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