A block cipher encrypts data in blocks using a deterministic algorithm and a symmetric key.
As in the case of stream ciphers, most encryption methods encrypt bits one by one (stream ciphers). Block ciphers, on the other hand, encrypt 128 bit blocks with a key of predetermined length: 128, 192, or 256 bits.
A 128-bit block cipher brings 128 bits of plaintext and encrypts it into 128 bits of ciphertext. Where the amount of plaintext is less than 128 bits, in this example, the cipher will employ methods to reconcile the difference (padding schemes).
Block ciphers have the advantage of high diffusion and strong tamper resistance without detection. They have the disadvantage of slower encryption speed since the entire block must be captured for encryption/decryption. Block ciphers also breed errors since a mistake in just one symbol could alter the whole block.
The dominant example of a symmetric block cipher is the Advanced Encryption Standard (AES).
“AES is the leading block cipher used for encrypting closely held data. Another well-known block cipher is Blowfish.”