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aesctr.class.php
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aesctr.class.php
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<?php
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
/* AES counter (CTR) mode implementation in PHP (c) Chris Veness 2005-2010. Right of free use is */
/* granted for all commercial or non-commercial use under CC-BY licence. No warranty of any */
/* form is offered. */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
class AesCtr extends Aes
{
/**
* Encrypt a text using AES encryption in Counter mode of operation
* - see http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
*
* Unicode multi-byte character safe
*
* @param plaintext source text to be encrypted
* @param password the password to use to generate a key
* @param nBits number of bits to be used in the key (128, 192, or 256)
* @return string text
*/
public static function encrypt($plaintext, $password, $nBits)
{
$blockSize = 16; // block size fixed at 16 bytes / 128 bits (Nb=4) for AES
if (!($nBits == 128 || $nBits == 192 || $nBits == 256)) {
return '';
}
// standard allows 128/192/256 bit keys
// note PHP (5) gives us plaintext and password in UTF8 encoding!
// use AES itself to encrypt password to get cipher key (using plain password as source for
// key expansion) - gives us well encrypted key
$nBytes = $nBits / 8; // no bytes in key
$pwBytes = array();
for ($i = 0; $i < $nBytes; $i++) {
$pwBytes[$i] = ord(substr($password, $i, 1)) & 0xff;
}
$key = Aes::cipher($pwBytes, Aes::keyExpansion($pwBytes));
$key = array_merge($key, array_slice($key, 0, $nBytes - 16)); // expand key to 16/24/32 bytes long
// initialise counter block (NIST SP800-38A §B.2): millisecond time-stamp for nonce in
// 1st 8 bytes, block counter in 2nd 8 bytes
$counterBlock = array();
$nonce = floor(microtime(true) * 1000); // timestamp: milliseconds since 1-Jan-1970
$nonceSec = floor($nonce / 1000);
$nonceMs = $nonce % 1000;
// encode nonce with seconds in 1st 4 bytes, and (repeated) ms part filling 2nd 4 bytes
for ($i = 0; $i < 4; $i++) {
$counterBlock[$i] = self::urs($nonceSec, $i * 8) & 0xff;
}
for ($i = 0; $i < 4; $i++) {
$counterBlock[$i + 4] = $nonceMs & 0xff;
}
// and convert it to a string to go on the front of the ciphertext
$ctrTxt = '';
for ($i = 0; $i < 8; $i++) {
$ctrTxt .= chr($counterBlock[$i]);
}
// generate key schedule - an expansion of the key into distinct Key Rounds for each round
$keySchedule = Aes::keyExpansion($key);
//print_r($keySchedule);
$blockCount = ceil(strlen($plaintext) / $blockSize);
$ciphertxt = array(); // ciphertext as array of strings
for ($b = 0; $b < $blockCount; $b++) {
// set counter (block #) in last 8 bytes of counter block (leaving nonce in 1st 8 bytes)
// done in two stages for 32-bit ops: using two words allows us to go past 2^32 blocks (68GB)
for ($c = 0; $c < 4; $c++) {
$counterBlock[15 - $c] = self::urs($b, $c * 8) & 0xff;
}
for ($c = 0; $c < 4; $c++) {
$counterBlock[15 - $c - 4] = self::urs($b / 0x100000000, $c * 8);
}
$cipherCntr = Aes::cipher($counterBlock, $keySchedule); // -- encrypt counter block --
// block size is reduced on final block
$blockLength = $b < $blockCount - 1 ? $blockSize : (strlen($plaintext) - 1) % $blockSize + 1;
$cipherByte = array();
for ($i = 0; $i < $blockLength; $i++) { // -- xor plaintext with ciphered counter byte-by-byte --
$cipherByte[$i] = $cipherCntr[$i] ^ ord(substr($plaintext, $b * $blockSize + $i, 1));
$cipherByte[$i] = chr($cipherByte[$i]);
}
$ciphertxt[$b] = implode('', $cipherByte); // escape troublesome characters in ciphertext
}
// implode is more efficient than repeated string concatenation
$ciphertext = $ctrTxt.implode('', $ciphertxt);
$ciphertext = base64_encode($ciphertext);
return $ciphertext;
}
/**
* Decrypt a text encrypted by AES in counter mode of operation
*
* @param ciphertext source text to be decrypted
* @param password the password to use to generate a key
* @param nBits number of bits to be used in the key (128, 192, or 256)
* @return string text
*/
public static function decrypt($ciphertext, $password, $nBits)
{
$blockSize = 16; // block size fixed at 16 bytes / 128 bits (Nb=4) for AES
if (!($nBits == 128 || $nBits == 192 || $nBits == 256)) {
return '';
}
// standard allows 128/192/256 bit keys
$ciphertext = base64_decode($ciphertext);
// use AES to encrypt password (mirroring encrypt routine)
$nBytes = $nBits / 8; // no bytes in key
$pwBytes = array();
for ($i = 0; $i < $nBytes; $i++) {
$pwBytes[$i] = ord(substr($password, $i, 1)) & 0xff;
}
$key = Aes::cipher($pwBytes, Aes::keyExpansion($pwBytes));
$key = array_merge($key, array_slice($key, 0, $nBytes - 16)); // expand key to 16/24/32 bytes long
// recover nonce from 1st element of ciphertext
$counterBlock = array();
$ctrTxt = substr($ciphertext, 0, 8);
for ($i = 0; $i < 8; $i++) {
$counterBlock[$i] = ord(substr($ctrTxt, $i, 1));
}
// generate key schedule
$keySchedule = Aes::keyExpansion($key);
// separate ciphertext into blocks (skipping past initial 8 bytes)
$nBlocks = ceil((strlen($ciphertext) - 8) / $blockSize);
$ct = array();
for ($b = 0; $b < $nBlocks; $b++) {
$ct[$b] = substr($ciphertext, 8 + $b * $blockSize, 16);
}
$ciphertext = $ct; // ciphertext is now array of block-length strings
// plaintext will get generated block-by-block into array of block-length strings
$plaintxt = array();
for ($b = 0; $b < $nBlocks; $b++) {
// set counter (block #) in last 8 bytes of counter block (leaving nonce in 1st 8 bytes)
for ($c = 0; $c < 4; $c++) {
$counterBlock[15 - $c] = self::urs($b, $c * 8) & 0xff;
}
for ($c = 0; $c < 4; $c++) {
$counterBlock[15 - $c - 4] = self::urs(($b + 1) / 0x100000000 - 1, $c * 8) & 0xff;
}
$cipherCntr = Aes::cipher($counterBlock, $keySchedule); // encrypt counter block
$plaintxtByte = array();
for ($i = 0; $i < strlen($ciphertext[$b]); $i++) {
// -- xor plaintext with ciphered counter byte-by-byte --
$plaintxtByte[$i] = $cipherCntr[$i] ^ ord(substr($ciphertext[$b], $i, 1));
$plaintxtByte[$i] = chr($plaintxtByte[$i]);
}
$plaintxt[$b] = implode('', $plaintxtByte);
}
// join array of blocks into single plaintext string
$plaintext = implode('', $plaintxt);
return $plaintext;
}
/*
* Unsigned right shift function, since PHP has neither >>> operator nor unsigned ints
*
* @param a number to be shifted (32-bit integer)
* @param b number of bits to shift a to the right (0..31)
* @return a right-shifted and zero-filled by b bits
*/
private static function urs($a, $b)
{
$a &= 0xffffffff; $b &= 0x1f; // (bounds check)
if ($a & 0x80000000 && $b > 0) { // if left-most bit set
$a = ($a >> 1) & 0x7fffffff; // right-shift one bit & clear left-most bit
$a = $a >> ($b - 1); // remaining right-shifts
} else { // otherwise
$a = ($a >> $b); // use normal right-shift
}
return $a;
}
}
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */