What is the difference between a block cipher and a stream cipher?

A block cipher and a stream cipher are two different types of symmetric encryption algorithms used in cryptography. The main difference between them lies in how they process data and encrypt information.

1. Block Cipher:
- A block cipher operates on fixed-size blocks of data, typically 64 or 128 bits in length.
- It divides the plaintext into blocks and encrypts each block separately.
- The same key is used to encrypt and decrypt each block.
- The encryption and decryption processes are typically more complex and involve multiple rounds of substitution and permutation operations.
- Examples of block ciphers include Advanced Encryption Standard (AES), Data Encryption Standard (DES), and Triple Data Encryption Standard (3DES).

2. Stream Cipher:
- A stream cipher encrypts data bit by bit or byte by byte.
- It generates a continuous stream of pseudorandom key bits, often called a keystream.
- The keystream is combined with the plaintext using a bitwise XOR operation to produce the ciphertext.
- The same keystream is used to encrypt and decrypt the data.
- Stream ciphers are generally faster and more efficient for encrypting large amounts of data in real-time or for communication channels.
- Examples of stream ciphers include RC4, Salsa20, and ChaCha20.

In summary, the main difference between a block cipher and a stream cipher is the way they process data. Block ciphers encrypt fixed-size blocks of data separately, while stream ciphers encrypt data bit by bit or byte by byte using a continuous keystream.