Bagaimana mengimplementasikan sebuah algoritma cipher menggunakan basis pemrograman tertentu, dalam hal ini saya menggunakan bahasa php untuk mengimplementasikah caesar dan vigenere cipher untuk meng-encrypt sebuah plantext menjadi cipher text.

• Pengertian Vinegar Cipher Dan Source Code Matlab File

This cipher produces encrypted ciphertext from an input plaintext message using a key and a matrix of substitution alphabets. Recovery of the plaintext from the ciphertext requires the key. By modern standards, this isn't a terribly secure cryptosystem, but it is easily implemented, and difficult enough that encrypted messages can't be casually deciphered.

The Vigenere cipher encrypts its inputs by using the key and the plaintext as indices into a fixed lookup table: the Vigenere square. For ease of computation, the algorithm first maps letters to numbers: A=1, B=2, .. SPACE=27. As shown in the matrix below, each row in the square is derived from the row above by circularly shifting it one place to the left:

To encrypt, replicate the letters in the key so the key and plaintext are the same length. Then, derive each ciphertext letter by lookup in the Vigenere square: use the key letter as the row index and the plaintext letter as the column index. If the key K and the plaintext P are n letters long, form the ciphertext result C by indexing into the Vingere square V, as follows:

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Decryption simply reverses the processs, using the key letter to determine the row index and the ciphertext letter to determine the colum index, which is the plaintext letter. For example:

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Implementing the Vigenere Cipher in MATLAB

To implement the Vigenere cipher in MATLAB, I wrote three functions:

• vigenere - Generates the Vigenere square.
• encrypt - Transforms a plaintext message into ciphertext.
• decrypt - Recovers the plaintext message from the ciphertext.

The vigenere function uses three additional MATLAB functions: circshift - Creates each row of the Vigenere square. arrayfun - Iterates over the row indices, calling circshift on each; returns a cell array of the rows. reshape - Builds a matrix from the cell array.

Pengertian Vinegar Cipher Dan Source Code Matlab File

encrypt creates a Vigenere square and transforms the key and plaintext from strings into vectors of numbers. It encrypts the plaintext and then translates the encrypted message from a vector of numbers into a character string. encrypt calls arrayfun to perform the encryption lookup instead of using a for-loop. arrayfun is faster and it won't ever get the length of the plaintext wrong, as a hand-written for-loop might.

decrypt reverses the encryption process by searching for the ciphertext letter in the key letter row. The column index of the ciphertext letter maps back (1=A, 2=B, .., 27=SPACE) to the plaintext letter. Like encrypt, decrypt loops over the cipertext using arrayfun instead of a for-loop.

Most of the complexity in both encrypt and decrypt stems from extending the algorithm to allow spaces in the plaintext and the key. Without spaces, reading decrypted messages is difficult and error-prone.

Published with MATLAB® 7.10