<?php
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
/*  AES counter (CTR) mode implementation in PHP (c) Chris Veness 2005-2011. Right of free use is */
/*    granted for all commercial or non-commercial use under CC-BY licence. No warranty of any    */
/*    form is offered.                                                                            */
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
namespace Classes\Crypt;

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 string source text to be encrypted
   * @param password  string 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 encrypted 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 1st 8 bytes of counter block with nonce (NIST SP800-38A �B.2): [0-1] = millisec,
      // [2-3] = random, [4-7] = seconds, giving guaranteed sub-ms uniqueness up to Feb 2106
        $counterBlock = array();
        $nonce = floor(microtime(true)*1000);   // timestamp: milliseconds since 1-Jan-1970
        $nonceMs = $nonce%1000;
        $nonceSec = floor($nonce/1000);
        $nonceRnd = floor(rand(0, 0xffff));

        for ($i = 0; $i<2;
        $i++) {
            $counterBlock[$i]   = self::urs($nonceMs, $i*8) & 0xff;
        }
        for ($i = 0; $i<2;
        $i++) {
            $counterBlock[$i+2] = self::urs($nonceRnd, $i*8) & 0xff;
        }
        for ($i = 0; $i<4;
        $i++) {
            $counterBlock[$i+4] = self::urs($nonceSec, $i*8) & 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 string source text to be decrypted
   * @param password   string 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 decrypted 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;
    }
}
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
