A block cipher is a method of encrypting data that processes fixed-size blocks of plaintext to generate ciphertext. This technique uses keys to secure the information, ensuring that unauthorized parties cannot easily decipher the data. In block ciphers, data is divided into uniform chunks, typically 64 or 128 bits.

Each block is then transformed through a series of mathematical operations and substitutions, guided by a secret key. The same key can be used to both encrypt and decrypt, though some systems employ different keys for each process. Block ciphers are essential for securing transactions and sensitive information.

They provide confidentiality and integrity, allowing users to trust the systems that rely on them. Examples of widely-used block cipher algorithms include AES (Advanced Encryption Standard) and DES (Data Encryption Standard). The reliable encryption provided by block ciphers helps maintain data security and privacy, which are crucial in financial applications and personal data protection.