AesCtr::decrypt()

Complexity

Conditions	11
Paths	73

Size

Total Lines	64
Code Lines	34

Duplication

Lines	0
Ratio	0 %

Importance

Changes	1
Bugs	0	Features	0

<?php
    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));

$pwBytes of type array is incompatible with the type cipher expected by parameter $key of Aes::keyExpansion().

$pwBytes of type array is incompatible with the type message expected by parameter $input of Aes::cipher().

    $key = array_merge($key, array_slice($key, 0, $nBytes - 16)); // expand key to 16/24/32 bytes long

$nBytes - 16 of type double is incompatible with the type integer|null expected by parameter $length of array_slice().

$key of type ciphertext is incompatible with the type array expected by parameter $arrays of array_merge().

$key of type ciphertext is incompatible with the type array expected by parameter $array of array_slice().

    // 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);

$key of type array is incompatible with the type cipher expected by parameter $key of Aes::keyExpansion().

    // 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;
    }