BLOCK CIPHER SERIES
The permuted block is then fed into the actual encryption process, which consists of 16 “rounds” (i.e., series of actions) that all perform identical operations. Before any encryption takes place, the input block’s bits are permuted in a pseudorandom way-that is, they’re shuffled around to mask their original order. Internally, DES is composed of a series of mathematical structures known as Feistel networks, and it’s these constructs that perform the actual encryption.Įncryption and Decryption with Feistel NetworksĮncryption with DES is a fairly straightforward process. These blocks are fed through the cipher with a 56-bit key, which produces 64-bit blocks of ciphertext. We’ll see why that’s the case, but let’s first explore how the DES cipher actually works.Īt the highest level, DES is a symmetric block cipher that operates on 64-bit blocks. The DES cipher was the de facto standard for encryption for 20 years, but despite that impressive history, DES has since been retired. In 1977, the cipher became publicly available and saw widespread use in many nongovernmental applications. The cipher was originally designed by IBM and the NSA in the 1970s to secure government communications. The Data Encryption Standard (DES) is the classical example of a block cipher. These blocks are then fed one-by-one into the cipher to produce the blocks that comprise the final ciphertext.
How is a block cipher different from a stream cipher? Instead of encrypting each plaintext character individually, the plaintext’s characters are divided into a series of equal-sized blocks.
These characteristics make block ciphers popular choices for encryption. Stream ciphers like RC4 aren’t great at this, but block ciphers are really good at confusion and diffusion. In other words, a single bit change in the plaintext may change many bits in the ciphertext. The former obscures the relationship between plaintext and ciphertext, and the latter spreads the influence of each plaintext bit over many ciphertext bits. Information Theory pioneer Claude Shannon described two important properties that make ciphers secure: Confusion and diffusion. We can solve this problem by using block ciphers.
For example, if Eve intercepts Bob’s RC4-encypted message, she can use mathematical analysis to break the encryption. That is, the one-to-one correspondence between plaintext characters and ciphertext characters makes stream ciphers susceptible to certain attacks. This seems intuitive, but this can actually make a cipher insecure. We’ve seen that stream ciphers like RC4 encrypt each character of a message individually.
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