A block cipher is an encryption algorithm that encrypts bits of data (known as a block) at one time. The usual sizes of each block are 64 bits, 128 bits or 256 bits. So, for example, a 64-bit block cipher will take in 64 bits of plaintext and encrypt it into 64 bits of ciphertext. The majority of the symmetric ciphers used today are block ciphers, including DES, Triple DES, AES, IDEA and Blowfish.
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