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copied!<h2>Answer Background and Explanation</h2> <p>To understand this question, you must first understand what SHA256 is. SHA256 is a <strong>Cryptographic Hash Function</strong>. A Cryptographic Hash Function is a one-way function, whose output is cryptographically secure. This means it is easy to compute a hash (equivalent to encrypting data), but hard to get the original input using the hash (equivalent to decrypting the data). Since using a Cryptographic hash function means decrypting is computationally infeasible, so therefore you cannot perform decryption with SHA256. </p> <p>What you want to use is a two-way function, but more specifically, a <strong>Block Cipher</strong>. A function that allows for both encryption and decryption of data. The functions <code>mcrypt_encrypt</code> and <code>mcrypt_decrypt</code> by default use the Blowfish algorithm. PHP's use of mcrypt can be found in this <a href="http://www.php.net/manual/en/intro.mcrypt.php">manual</a>. A list of <a href="http://www.php.net/manual/en/mcrypt.ciphers.php">cipher definitions</a> to select the cipher mcrypt uses also exists. A wiki on Blowfish can be found at <a href="http://en.wikipedia.org/wiki/Blowfish_%28cipher%29">Wikipedia</a>. A block cipher encrypts the input in blocks of known size and position with a known key, so that the data can later be decrypted using the key. This is what SHA256 cannot provide you.</p> <h2>Code</h2> <pre><code>$key = 'ThisIsTheCipherKey'; $ciphertext = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, 'This is plaintext.', MCRYPT_MODE_CFB); $plaintext = mcrypt_decrypt(MCRYPT_BLOWFISH, $key, $encrypted, MCRYPT_MODE_CFB); </code></pre>

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