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What is a Block Cipher?

By Michael Linn
Updated May 16, 2024
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A block cipher is a symmetric cryptographic algorithm used to scramble sensitive data. Programs or algorithms that actually perform the task of hiding data are called ciphers. Scrambling, or encrypting, data provides other benefits besides secrecy. Encryption can also ensure that a message has not been altered, as well as verify the identity of the sender.

Cryptography is the science of turning readable unencrypted data, called plaintext, into encrypted data, called ciphertext. Block ciphers differ from the other major category of symmetric algorithms, stream ciphers, in that they encrypt data in chunks, or blocks, instead of one character at a time. A block cipher is generally considered to be more secure than a stream cipher because it is more random, while a stream cipher works faster when the plaintext is short.

The concept of a block cipher being symmetric comes from the actual process of encrypting data. Symmetric cryptography, also called private key cryptography, uses a single key to encrypt and decrypt data. It is essential that the key remains private because anyone who has it can read any message that is sent. This is in contrast to asymmetric cryptography, which uses two keys, a private key that is secured and a public key that is available to everyone.

Use of cryptography dates back many centuries and was present in ancient Egypt and Rome. Julius Caesar used what is now widely known as the Caesar cipher to pass secret messages to his generals. Cryptographic systems are constantly under attack, and as old algorithms are broken, cryptographers develop new programs to hide messages. Some popular versions of block ciphers include Data Encryption Standard (DES), Advanced Encryption Standard (AES), and International Data Encryption Algorithm (IDEA).

DES is a well-known conventional block cipher that uses a 56-bit symmetric key. Data is encrypted in 64-bit blocks. Each time DES encrypts a 64-bit plaintext block, it runs it through the algorithm 16 times or rounds. Due to the short 56-bit key, DES is no longer considered to be secure for many applications.

AES was selected in October 2000 by the National Institute of Standards and Technology (NIST) as a more secure replacement for DES. AES uses three different keys to perform multiple rounds of encryption on plaintext blocks of 128-bits. The key sizes are 128, 192, and 256-bits in length.

Another block cipher is IDEA, which is widely used in Europe and utilizes a 128-bit key to perform eight rounds of encryption on 64-bit blocks of plaintext. IDEA was created in 1992 by James Massey and Xuejia Lai under a research contract with a Swedish technology foundation. Even though IDEA was developed with private grants, it is free for non-commercial use.

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