Modern Ciphers

Here we discuss some modern ciphers, modern ciphers are bit oriented. Some simple modern ciphers are XOR, Rotation, Substitution cipher (s-box), Permutation cipher (p-box).

XOR Cipher

XOR ciphers use logical XOR operation to produce ciphertext. Two inputs are needed for XOR cipher algorithm, one input from plaintext and other input is the key to encrypt. The inputs are just XORed to produce ciphertext.

Rotation Cipher

Rotation ciphers rotate the bits in the input plaintext to produce the ciphertext. Rotation ciphers can be either keyed or keyless. The number of rotation must be in between 1 to N-1 for an N bit input. After Nth rotation, we get the original string.

Substitution cipher: S-box

It is a parallel version of traditional substitution cipher. The input to an S-box is a group of bits with length N and the output is another group of bits with length M (M and N need not to same).

Transposition Cipher: P-box

The P-box (permutation box), produce cipher text by transposition at bit level in input string. There exist three types of P-box ciphers:  the straight permutation, expansion permutation, and compression permutation.

In straight P-box, the number of inputs and outputs are same.

In expansion P-box, number of outputs is greater than number of inputs.

In compression P-box cipher number of outputs are smaller than number of inputs.

Modern Ciphers

Here we discuss some modern ciphers, modern ciphers are bit oriented. Some simple modern ciphers are XOR, Rotation, Substitution cipher (s-box), Permutation cipher (p-box).

P-Boxes

Modern ciphers are some times called round ciphers, because they contain multiple rounds operations for encryption and decryption. DES and AES are two modern symmetric ciphers. Both AES and DES divide plaintext to different blocks and use same key for encrypt and decrypt the blocks.

Data Encryption Standard (DES)

DES was originally designed by IBM and adopted by US government for secure exchange of data. The algorithm divides the plaintext to 64-bit blocks and produce 64-bit ciphertext for each block. Outline of DES is shown below;

DES

DES use two transposition blocks (p-box), {they are straight and inverse of each other and keyless}, 16 round ciphers (each round use different keys derived from original key), and the stage prior to the final permutation, exchanges the leftmost 32 bits with the rightmost 32 bits.

The following figure shows on round in DES.

DES Encryption 

The DES function apply 48-bit key (Ki) to right most 32-bits of the input (Ri) to produce 32-bit output. This function contains four operations: an XOR operation, an expansion permutation, a group of s-boxes and a straight permutation as shown below,

DES function

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