Differences between Stream and Block Cipher and DES and AES

Block Cipher is a type of cryptographic algorithm used for encryption. Block Cipher processes fixed-size blocks simultaneously as opposed to a stream cipher, which encrypts data one bit at a time. Block Ciphers normally encrypt data in fixed-size blocks of either 64 or 128 bits where Stream Cipher utilizes 8 bits. The more blocks used equals to an increased amount of security. However, the trade-off is that the more blocks used equals to an increased amount of time to process or execute the encryption. Additionally, Block Cipher has multiple operations modes, which is a technique used for applying the cipher by mixing its outputs to hide cipher text patterns. These mode techniques include Electronic codebook (ECB) that has no mixing at all, Cipher block chaining (CBC) that’s block oriented, Cipher feedback (CFB) that blocks and streams, Output feedback (OFB) that creates a keystream, and Counter (CTR) that creates a keystream.

ECB is one of the weakest operation modes because it encrypts and decrypts data blocks independently in the same exact way, making it easy for hackers to detect patterns. The CBC operation mode works by connecting plaintext blocks with previously encrypted plaintext blocks in which, can be slow and susceptible to manipulation. The CFB operates similarly to a stream cipher that generates a keystream to encrypt the plaintext block using XOR. However, its drawback is that an error in one blockchain can be carried into the next one. The OFB mode works similarly to CFB by transforming a block cipher into a stream cipher. However, it does that without processing the plaintext in fixed-size blocks before encrypting it. The CTR mode transforms a block cipher into a synchronous stream cipher. It was designed to encrypt plaintext data of various lengths in situations when the streamed data is of variable length, for example, in disk and network encryption.

Stream Cipher is another type of cryptographic algorithm used for encryption that does not encrypt a block of characters but rather, one bit at a time. Stream Cipher typically involves combining plaintext with a pseudorandom keystream using a bitwise operation such as XOR. Stream Cipher usually uses smaller keys whereas Block Cipher usually uses larger keys. There are basically 2 types of Stream Ciphers, synchronous and asynchronous. Synchronous stream ciphers don’t rely on the previous plaintext or ciphertext but require both the sender and the receiver of the encrypted data to be in sync. Asynchronous stream ciphers doesn’t rely on synchronization and usually generate their keystream based on the plaintext and previous ciphertext bits.

Data Encryption Standard (DES) is an older standard cryptographic algorithm used for encryption that uses a 56-bit key and operates on 64-bit blocks of plaintext. DES has a smaller key size and is known to be more susceptible to brute force attacks. DES uses a Feistel Cipher structure and is kept secret in its selection process and secret from the public; unlike AES, where the selection process is accepted for the open public to comment. 

Advanced Encryption Standard (AES) is a newer cryptographic algorithm standard used for encryption that supports key sizes of 128, 192, or 256 bits and operates on 128-bit blocks of plaintext. Some of the advantages AES have over DES is the fact that it's known to be faster and more secure. AES uses a larger key size and uses a substitution-permutation network (SPN) structure, which involves multiple rounds of substitution, permutation, and mixing operations. AES is considered to be much more secure, faster, and flexible than DES and is the most widely used today. 

To Summarize, Block Cipher and Stream Cipher are two cryptographic algorithms used for encryption. Block Cipher processes fixed-size blocks simultaneously, offering increased security with more blocks but slower processing times. Its operation modes, like ECB, CBC, CFB, OFB, and CTR, offer varying levels of security and efficiency. Stream Cipher encrypts one bit at a time and comes in synchronous and asynchronous types. DES, an older standard, operates on 64-bit blocks with a 56-bit key and is susceptible to brute force attacks. AES, a newer standard, supports key sizes of 128, 192, or 256 bits, operates on 128-bit blocks, and offers faster and more secure encryption.