Encryption\Crypt\aesctr::encrypt()

Complexity

Conditions	13
Paths	273

Size

Total Lines	71
Code Lines	38

Duplication

Lines	0
Ratio	0 %

Importance

Changes	1
Bugs	0	Features	0

<?php
    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);

        $blockCount = ceil(strlen($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;
    }