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A block cipher is a symmetric cryptographic algorithm that operates on a fixed-size block of data using a shared, secret key. Plaintext is used during the encryption, and the resulting encrypted text is called a ciphertext. The same key is used for both the encryption of the plaintext and the decryption of the ciphertext.
Block cipher encrypts/decrypts its input one block at a time instead of one bit at a time using a shared, secret key. The block is fixed in size; otherwise, padding is necessary. This algorithm is symmetric. During encryption, it uses the shared key to transform its plaintext input into a cyphertext (encrypted text). During decryption, it uses the same key to transform the cyphertext back to the original plaintext. The length of the output is the same as the input.
Well-known implementations of the block cipher algorithm are the Data Encryption Standard (DES), TripleDES and the Advanced Encryption standard (AES).
The counterpart of block cypher is the stream cypher, which operates on its input one bit at a time, also using a shared key.
An alternative to the block cipher algorithm is public-key cryptography or asymmetric cryptography. This algorithm uses a public key to encrypt plaintext and a private key to decrypt the resulting ciphertext.