Block ciphers are a type of symmetric cryptographic algorithm that operates on fixed-size blocks of plaintext data, typically consisting of bits or bytes. Unlike stream ciphers, which encrypt data bit by bit, block ciphers process data in discrete blocks of a predefined size.

Here’s how block ciphers typically work:

**Block Size**: Block ciphers have a fixed block size, which is the number of bits or bytes of plaintext data that they operate on in each encryption or decryption operation. Common block sizes include 64 bits (e.g., DES), 128 bits (e.g., AES), and 256 bits (e.g., AES).
**Key Initialization**: A secret cryptographic key is used to initialize the block cipher. The key is typically of the same length as the block size or a multiple thereof. The key is kept confidential and shared between the sender and the recipient.
**Encryption**: To encrypt plaintext data, the block cipher takes a block of plaintext as input and applies a series of mathematical transformations, known as rounds, using the cryptographic key. Each round typically involves substitution, permutation, and mixing operations to obscure the relationship between the plaintext and the ciphertext. The result is a block of ciphertext, which appears random and unintelligible without knowledge of the key.
**Decryption**: To decrypt the ciphertext and recover the original plaintext, the same block cipher algorithm is applied in reverse, using the same cryptographic key. The ciphertext block is processed through a series of decryption rounds, which undo the transformations applied during encryption and recover the original plaintext block.

Block ciphers are widely used in various cryptographic applications, including data encryption, secure communication protocols (e.g., TLS/SSL), disk encryption, and digital signatures. They offer a high level of security and efficiency, especially for encrypting large volumes of data. However, block ciphers must be carefully designed and analyzed to ensure resistance against cryptanalysis and security vulnerabilities. Additionally, block cipher modes of operation, such as ECB, CBC, and GCM, define how blocks are combined and processed to encrypt data larger than a single block size.

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