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

A block cipher encrypts data in fixed-size blocks, typically 128 bits for AES. If the data is not an exact multiple of the block size, padding is added. Block ciphers process each block independently or chained together depending on the mode of operation, such as CBC or GCM. A stream cipher encrypts data one bit or byte at a time, generating a continuous keystream that is combined with plaintext using XOR. Stream ciphers like ChaCha20 are faster and suitable for real-time communications. Block ciphers are generally considered more secure and versatile.