| Shared Secret Algorithms |
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In our second call to str_irepace() we're reversing the process by using the values of the codebook as search terms and the keys of the codebook as the replacement values. Because we're using the same codebook and the same operation (albeit in a different order) for both the encryption and decryption phases, we'd call this a shared secret encryption method.
Character Substitution
Although phrase substitution is handy for speaking in code, it's mechanically ill-suited to computer-based cryptography because of some fundamental principles of language. First off, not only must a massive translation dictionary be maintained by both parties, but factors such as pluralization and dialect localization have to be taken into account, causing the size and complexity of the dictionary to grow even larger when placed in a computational context. A much simpler substitution dictionary for a computer to understand is one that operates only on single characters. For single-byte encodings, that means no more than 256 possible search/replace pairs; further, if we concern ourselves only with translating English word characters with no concern for case, we're reduced to only 26 search/replace pairs.
PHP provides a faster and simpler replacement function for single-character substitutions such as this. Let's take a quick look at an encryption algorithm for making a message look like it was touch-typed on a Dvorak keyboard.
<?php
$qwerty = 'qwertyuiopasdfghjklzxcvbnm' . 'QWERTYUIOPASDFGHJKLZXCVBNM';
$dvorak = 'abcdefghijklmnopqrstuvwxyz' . 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$message = "Start the attack when the sun rises.";
$encoded = strtr($message, $qwerty, $dvorak);
$decoded = strtr($encoded, $dvorak, $qwerty);
?>
As with our phrase substitution algorithm, we start by defining a translation dictionary. The way strtr() works, the first character in the source map (the first parameter to strtr()), will map to the first character in the destination map (the second parameter to strtr()).
$encoded = strtr($message, $qwerty, $dvorak);
In our first call, the characters in $message get remapped to "Lekde epc keekvr bpcy epc lgy dhlcl". Notice that the spaces and period were not remapped and appear in their original form. The next call to strtr() maps the message back to its original form.
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