A hash based cipher built using a Feistel Network

HashCipher.php

<?php

class HashCipher {
    protected $hash = 'sha512';
    protected $hashLen = 64;
    protected $rounds = 16;

    public function __construct($hash, $rounds = 16) {
        $this->hash = $hash;
        // run a test hash, to get the proper hash length
        // Our block size will be double the output of the underlying hash function
        $this->hashLen = strlen(hash($hash, '', true)) * 2;
        $this->rounds = $rounds;
    }

    public function decrypt($key, $macKey, $cipherText) {
        $cipherText = base64_decode($cipherText);
        $iv = substr($cipherText, 0, $this->hashLen);
        $mac = substr($cipherText, -1 * ($this->hashLen / 2));
        $cipherText = substr($cipherText, $this->hashLen, -1 * ($this->hashLen / 2));

        // verify MAC
        $ourMac = hash_hmac($this->hash, $iv . $cipherText, $macKey, true);
        if (!$this->compare($mac, $ourMac)) {
            return false; // mac verification failure
        }

        // decrypt:
        $i = 0;
        $n = strlen($cipherText);
        $plainText = "";
        while ($i < $n) {
            $block = substr($cipherText, $i, $this->hashLen);
            $plainText .= ($this->decryptBlock($block, $key) ^ $iv);
            $iv = $block;
            $i += $this->hashLen;
        }
        // Strip padding
        $len = ord(substr($plainText, -1, 1));
        $padding = str_repeat(chr($len), $len);
        if (substr($plainText, -1 * $len) !== $padding) {
            return false; // invalid decrypt, as has invalid padding
        }
        return substr($plainText, 0, -1 * $len);
    }

    public function encrypt($key, $macKey, $plainText) {
        // pad the text using PKCS7:
        $padSize = $this->hashLen - (strlen($plainText) % $this->hashLen);
        $plainText .= str_repeat(chr($padSize), $padSize);

        // generate IV:
        $iv = mcrypt_create_iv($this->hashLen, MCRYPT_DEV_URANDOM);
        $cipherText = $iv;

        $i = 0;
        $n = strlen($plainText);
        while ($i < $n) {
            // XOR the plaintext block with the IV
            $block = (substr($plainText, $i, $this->hashLen) ^ $iv);
            $cipherBlock = $this->encryptBlock($block, $key);
            $cipherText .= $cipherBlock;
            $iv = $cipherBlock;
            $i += $this->hashLen;
        }
        $cipherText .= hash_hmac($this->hash, $cipherText, $macKey, true);
        return base64_encode($cipherText);
    }

    protected function decryptBlock($block, $key) {
        $keys = array_reverse($this->deriveKeys($key));
        return $this->feistelNetwork($keys, $block);
    }

    protected function encryptBlock($block, $key) {
        $keys = $this->deriveKeys($key);
        return $this->feistelNetwork($keys, $block);
    }

    protected function feistelNetwork(array $keys, $block) {
        $left = substr($block, 0, $this->hashLen / 2);
        $right = substr($block, strlen($left));
        for ($i = 0; $i < $this->rounds; $i++) {
            $temp = $right;
            $right = ($left ^ hash_hmac($this->hash, $right, $keys[$i], true));
            $left = $temp;
        }
        return $right . $left;
    }

    protected function deriveKeys($key) {
        $keys = [];
        for ($i = 0; $i < $this->rounds; $i++) {
            $keys[] = hash_hmac($this->hash, $key, chr($i), true);
        }
        return $keys;
    }

    protected function compare($a, $b) {
        if (strlen($a) !== strlen($b)) {
            return false;
        }
        $r = 0;
        for ($i = 0, $n = strlen($a); $i < $n; $i++) {
            $r |= (ord($a[$i]) ^ ord($b[$i]));
        }
        return $r === 0;
    }

}

test.php

<?php

require_once "HashCipher.php";

$hash = "sha512";

// Keys are twice as long as our underlying hash function
$key = hash($hash, "a", true) . hash($hash, "b", true);
$macKey = hash($hash, "c", true) . hash($hash, "d", true);

// More rounds == more secure
$cipher = new HashCipher($hash, 64);

$e = $cipher->encrypt($key, $macKey, "foobarbazbizbuzsomethingelsereally long in here, because our block size is huge, so we'd want to make this really long to compensate. Something like this should do for now.foobarbazbizbuzsomethingelsereally long in here, because our block size is huge, so we'd want to make this really long to compensate. Something like this should do for now.foobarbazbizbuzsomethingelsereally long in here, because our block size is huge, so we'd want to make this really long to compensate. Something like this should do for now.foobarbazbizbuzsomethingelsereally long in here, because our block size is huge, so we'd want to make this really long to compensate. Something like this should do for now.");

$d = $cipher->decrypt($key, $macKey, $e);

var_dump($e, $d);