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POPHP: mcrypt mangles beginning of string to garbage
 

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    <p>I need medium to strong encryption on serverside, so I thought I would use mcrypt with PHP. If I use the functions below the beginning of my original string turns into binary garbage after decryption. (This is not the usual problem of getting appended <em>additional</em> garbage, instead my string is <em>altered</em>.) According to the documentation, mcrypt_encrypt() should have padded enough characters to match the block size of the selected algorithm but I suspect it does not work. </p> <p>However, if I pad it manually to the block size of 128 bit (16 bytes) of Rijndael, it doesn't work either. The only way I can get this to work is by prepending some string long enough to (likely) cover the garbaged block and add a known prefix like "DATA#" between that string and my data. After decryption that block has been partially mangled but my prefix and all data after that has been correctly decrypted.</p> <pre class="lang-php prettyprint-override"><code>$GLOBALS['encryptionmarker'] = 'DATA#'; function encrypt($plain, $key) { /* // workaround because beginning of decrypted string is being mangled // so we simply prefix with some text plus marker $prefix = str_pad('', 128, '#', STR_PAD_RIGHT).$GLOBALS['encryptionmarker']; $plain = $prefix.$plain; */ $encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $plain, MCRYPT_MODE_CFB, mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CFB), MCRYPT_DEV_URANDOM)); return $encrypted; } function decrypt($encrypted, $key) { $decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $encrypted, MCRYPT_MODE_CFB, mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CFB), MCRYPT_DEV_URANDOM)); /* // workaround: remove garbage $pos = strpos($decrypted, $GLOBALS['encryptionmarker']); $decrypted = trim(substr($decrypted, $pos + strlen($GLOBALS['encryptionmarker']))); */ return $decrypted; } </code></pre> <p>What's wrong with my functions? Why do I have to prefix my data like that (I consider it a dirty workaround, so I would like to fix it)?</p> <p>Storing the encrypted data is not the problem; decrypting it immediately after encryption without storing it to a database results in the same errors.</p>
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