Herewe discuss some modern ciphers, modern ciphers are bit oriented. Some simplemodern ciphers are XOR, Rotation, Substitution cipher (s-box), Permutationcipher (p-box).
XOR Cipher
XORciphers use logical XOR operation to produce ciphertext. Two inputs are neededfor XOR cipher algorithm, one input from plaintext and other input is the keyto encrypt. The inputs are just XORed to produce ciphertext.
Rotation Cipher
Rotationciphers rotate the bits in the input plaintext to produce the ciphertext. Rotationciphers can be either keyed or keyless. The number of rotation must be inbetween 1 to N-1 for an N bit input. After Nth rotation, we get the originalstring.
Substitution cipher: S-box
It isa parallel version of traditional substitution cipher. The input to an S-box isa group of bits with length N and the output is another group of bits withlength M (M and N need not to same).
Transposition Cipher: P-box
The P-box (permutation box), produce cipher text bytransposition at bit level in input string. There exist three types of P-boxciphers: the straightpermutation, expansion permutation, andcompression permutation.
In straight P-box, the number of inputs andoutputs are same.
In expansion P-box, number of outputs isgreater than number of inputs.
In compression P-box cipher number of outputsare smaller than number of inputs.
Modern Ciphers
Herewe discuss some modern ciphers, modern ciphers are bit oriented. Some simplemodern ciphers are XOR, Rotation, Substitution cipher (s-box), Permutationcipher (p-box).
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| P-Boxes |
Modernciphers are some times called round ciphers, because they contain multiplerounds operations for encryption and decryption. DES and AES are two modernsymmetric ciphers. Both AES and DES divide plaintext to different blocks anduse same key for encrypt and decrypt the blocks.
Data Encryption Standard (DES)
DES wasoriginally designed by IBM and adopted by US government for secure exchange ofdata. The algorithm divides the plaintext to 64-bit blocks and produce 64-bitciphertext for each block. Outline of DES is shown below;
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| DES |
DES use two transposition blocks (p-box), {they arestraight and inverse of each other and keyless}, 16 round ciphers (each rounduse different keys derived from original key), and the stage prior to the finalpermutation, exchanges the leftmost 32 bits with the rightmost 32 bits.
The followingfigure shows on round in DES.
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| DES Encryption |
TheDES function apply 48-bit key (Ki) to right most 32-bits of the input (Ri) toproduce 32-bit output. This function contains four operations: an XORoperation, an expansion permutation, a group of s-boxes and a straightpermutation as shown below,
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| DES function |
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