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copied!<p>This is the steps from the wikipedia <a href="http://en.wikipedia.org/wiki/Ciphertext_stealing#CBC_ciphertext_stealing_decryption_using_a_standard_CBC_interface" rel="nofollow">article</a> interjected with my comments explaining the </p> <ol> <li><p>LDn = Decrypt (K, Cn−1). Decrypt the second to last ciphertext block(the second to last 128bit/16 byte chunk chunk), using zeros as IV.</p> <p>You can do this with the standard PHP mcrypt function, just pass </p> <p>$second_to_last_cipher=array_slice($your_cipher_text_byte_array,count($your_cipher_text_byte_array)-32,16)</p> <p>to mcrypt_decrypt with a null Iv</p> <p>$second_to_last_clear = mcrypt_decrypt"MCRYPT_RIJNDAEL_128",$key,$second_to_last_ciphe)</p></li> <li><p>Cn = Cn || Tail (Dn, B−M). Pad the ciphertext to the nearest multiple of the block size using the last B−M bits of block cipher decryption of the second-to-last ciphertext block.</p> <p>Copy the last n bytes of the value you just decrypted into the last block of ciphertext.</p> <p>$n = 16 - ($second_to_last_clear % 16) </p> <p>Then use array copy to copy the data </p></li> <li><p>Swap the last two ciphertext blocks.</p> <p>Simply switch the swap the contents of the last and second two last cells in your ciphertext array</p></li> <li><p>Decrypt the ciphertext using the standard CBC mode up to the last block.</p> <p>make the standard decryption call.</p></li> <li><p>Exclusive-OR the last ciphertext (was already decrypted in step 1) with the second last ciphertext. </p> <p>Self explanatory. </p></li> <li>Truncate the plaintext to the length of the original ciphertext.</li> </ol>

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