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It has been determined by a large number of cryptographers that these algorithms are "very secure" as a result of their having been subjected to a reasonably in-depth investigation. Encrypting a lengthy series of bytes with zero values causes a stream cipher to behave as a pseudorandom number generator. This may be accomplished by using the stream cipher. Actually, the internal mechanism of many stream ciphers (but not all of them) is a PRNG, which generates a long sequence of key-dependent pseudorandom bytes (Mahdi, 2016). These bytes are then combined (by bitwise XOR) with the data in order to encrypt (or decrypt) it, and since encrypting zero bytes is the same as not using the XOR at all, encrypting zero bytes is the same as not using the XOR at as a result, stream ciphers are often used as a personal PRNG.

Stream ciphers are based on producing an "infinite" cryptographic keystream and encrypting one bit or byte at a time (equivalent to the one-time pad). However, block ciphers operate on more significant pieces of information (that is, blocks) at a time, sometimes merging blocks for enhanced protection, for example, AES in CBC mode.

Stream ciphers are often quicker than block ciphers, but this speed comes at a cost. Stream ciphers have very low memory needs since they simultaneously operate on just a few bits. However, block ciphers use more memory because they operate on bigger chunks of data and often "carry over" data from prior blocks. However, they are cheaper to execute in restricted scenarios such as embedded devices, firmware, and esp. hardware.

Stream ciphers are more challenging to use effectively and are susceptible to usage-based vulnerabilities; as the underlying concepts are similar to those of the one-time pad, the keystream has stringent constraints (Mahdi, 2016). On the other hand, this is often the most challenging aspect and may be delegated to an external box, for instance.

Due to the fact that block ciphers encrypt a whole component at once and also include "feedback" modes, which are highly recommended, they are more vulnerable to noise in communication; if you corrupt one portion of the data, it is likely that the remainder cannot be recovered. In contrast, stream ciphers encrypt bytes independently, with no link to other data blocks in the majority of ciphers/modes, and often tolerate line disruptions (Valea et al., 2019). Moreover, stream ciphers do not offer fiber reinforcement or verification, but (depending on the mode) some block ciphers might give integrity protection in combination with secrecy.

Consequently, stream ciphers are optimal for situations in which the quantity of data is either unpredictable or ongoing, such as network streams. Block ciphers, on the other hand, are more effective when the quantity of data is known beforehand, such as in the case of a file, data fields, or proposal protocols such as HTTP, where the length of the whole message is known from the start.

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

In some modes, the plaintext is XORed with the result of the encryption function rather than going through the encryption function itself. This is done to ensure the confidentiality of the plaintext (Malozemoff et al., July 2014). In some circumstances, the decryption function can only be performed with simultaneously using the encryption function.

Some modes of operation, such as CTR, are designed to function in such a way that only known values are ever encrypted. This results in the generation of a stream of pseudorandom data, which is then merged with the plaintext using a reversible keyless operation (often xor) to produce the cipher text. Other modes, such as Cipher Block Chaining, directly encrypt plaintext values, meaning decryption is necessary to discover the secret value.

One of the most significant benefits associated with a method that does not call for decryption is that it may be performed in hardware with a lower footprint (in other words, it is more compact). In addition, for block ciphers such as AES, it is often simpler to design effective encryption than it is to perform efficient decryption (Malozemoff et al., July 2014). This is because the internal coefficients have been optimized for the encryption direction rather than the decryption direction.

With block ciphers such as AES, it is only possible to encrypt one block at a time. This encryption is a random permutation with a key. This implies that each conceivable plaintext block corresponds to precisely one cipher text block (and vice versa; it is possible to employ a block cipher in the "wrong" manner). We need ciphers capable of encrypting communications of any length (Rogaway, 2011). Therefore, we must choose a strategy that allows the block cipher to handle both small and big messages. Therefore, a mode of operation will give this capability, but they often have a maximum message size (although one that is quite huge).

Another situation is that the mapping will always map identical plaintext to identical cipher text. This is problematic if you want to encrypt several communications since identical messages might be readily distinguished. Therefore, the majority of operational modes include an IV or nonce to ensure that similar (or substantially similar) messages cannot be identified.

Generally, we also want to preserve the message's integrity and originality. This can be accomplished by adding a MAC, but we typically use an authenticated mode of operation today. Neither message integrity nor authenticity is given by the block cipher (Rogaway, 2011). Another fascinating point is that a block cipher in counter mode only has to be utilized in one way. In addition, you may generate counter mode using a hash algorithm rather than a block cipher.

References

Ramkumar, M. (2014).  Symmetric Cryptographic Protocols. Springer.

Valea, E., Da Silva, M., Flottes, M. L., Di Natale, G., & Rouzeyre, B. (2019). Stream vs block ciphers for scan encryption.  Microelectronics Journal,  86, 65-76.

Mahdi, M. (2016). New Paradigm Design by Merging the Techniques of Stream Cipher with Block Cipher.  International Journal of Computer Science and Software Engineering,  5(1), 11.

Rogaway, P. (2011). Evaluation of some block cipher modes of operation.  Cryptography Research and Evaluation Committees (CRYPTREC) for the Government of Japan.

Malozemoff, A. J., Katz, J., & Green, M. D. (2014, July). Automated analysis and synthesis of block-cipher modes of operation. In  2014 IEEE 27th Computer Security Foundations Symposium (pp. 140-152). IEEE.