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Filename : SCRAM.php
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<?php
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// | Copyright (c) 2011 Jehan                                              |
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// +-----------------------------------------------------------------------+
// | Author: Jehan <jehan.marmottard@gmail.com                             |
// +-----------------------------------------------------------------------+
//
// $Id$

/**
* Implementation of SCRAM-* SASL mechanisms.
* SCRAM mechanisms have 3 main steps (initial response, response to the server challenge, then server signature
* verification) which keep state-awareness. Therefore a single class instanciation must be done and reused for the whole
* authentication process.
*
* @author  Jehan <jehan.marmottard@gmail.com>
* @access  public
* @version 1.0
* @package Auth_SASL
*/

require_once('Auth/SASL/Common.php');

class Auth_SASL_SCRAM extends Auth_SASL_Common
{
    /**
    * Construct a SCRAM-H client where 'H' is a cryptographic hash function.
    *
    * @param string $hash The name cryptographic hash function 'H' as registered by IANA in the "Hash Function Textual
    * Names" registry.
    * @link http://www.iana.org/assignments/hash-function-text-names/hash-function-text-names.xml "Hash Function Textual
    * Names"
    * format of core PHP hash function.
    * @access public
    */
    function __construct($hash)
    {
        // Though I could be strict, I will actually also accept the naming used in the PHP core hash framework.
        // For instance "sha1" is accepted, while the registered hash name should be "SHA-1".
        $hash = strtolower($hash);
        $hashes = array('md2' => 'md2',
            'md5' => 'md5',
            'sha-1' => 'sha1',
            'sha1' => 'sha1',
            'sha-224' > 'sha224',
            'sha224' > 'sha224',
            'sha-256' => 'sha256',
            'sha256' => 'sha256',
            'sha-384' => 'sha384',
            'sha384' => 'sha384',
            'sha-512' => 'sha512',
            'sha512' => 'sha512');
        if (function_exists('hash_hmac') && isset($hashes[$hash]))
        {
            $this->hash = create_function('$data', 'return hash("' . $hashes[$hash] . '", $data, TRUE);');
            $this->hmac = create_function('$key,$str,$raw', 'return hash_hmac("' . $hashes[$hash] . '", $str, $key, $raw);');
        }
        elseif ($hash == 'md5')
        {
            $this->hash = create_function('$data', 'return md5($data, true);');
            $this->hmac = array($this, '_HMAC_MD5');
        }
        elseif (in_array($hash, array('sha1', 'sha-1')))
        {
            $this->hash = create_function('$data', 'return sha1($data, true);');
            $this->hmac = array($this, '_HMAC_SHA1');
        }
        else
            return PEAR::raiseError('Invalid SASL mechanism type');
    }

    /**
    * Provides the (main) client response for SCRAM-H.
    *
    * @param  string $authcid   Authentication id (username)
    * @param  string $pass      Password
    * @param  string $challenge The challenge sent by the server.
    * If the challenge is NULL or an empty string, the result will be the "initial response".
    * @param  string $authzid   Authorization id (username to proxy as)
    * @return string|false      The response (binary, NOT base64 encoded)
    * @access public
    */
    public function getResponse($authcid, $pass, $challenge = NULL, $authzid = NULL)
    {
        $authcid = $this->_formatName($authcid);
        if (empty($authcid))
        {
            return false;
        }
        if (!empty($authzid))
        {
            $authzid = $this->_formatName($authzid);
            if (empty($authzid))
            {
                return false;
            }
        }

        if (empty($challenge))
        {
            return $this->_generateInitialResponse($authcid, $authzid);
        }
        else
        {
            return $this->_generateResponse($challenge, $pass);
        }

    }

    /**
    * Prepare a name for inclusion in a SCRAM response.
    *
    * @param string $username a name to be prepared.
    * @return string the reformated name.
    * @access private
    */
    private function _formatName($username)
    {
        // TODO: prepare through the SASLprep profile of the stringprep algorithm.
        // See RFC-4013.

        $username = str_replace('=', '=3D', $username);
        $username = str_replace(',', '=2C', $username);
        return $username;
    }

    /**
    * Generate the initial response which can be either sent directly in the first message or as a response to an empty
    * server challenge.
    *
    * @param string $authcid Prepared authentication identity.
    * @param string $authzid Prepared authorization identity.
    * @return string The SCRAM response to send.
    * @access private
    */
    private function _generateInitialResponse($authcid, $authzid)
    {
        $init_rep = '';
        $gs2_cbind_flag = 'n,'; // TODO: support channel binding.
        $this->gs2_header = $gs2_cbind_flag . (!empty($authzid)? 'a=' . $authzid : '') . ',';

        // I must generate a client nonce and "save" it for later comparison on second response.
        $this->cnonce = $this->_getCnonce();
        // XXX: in the future, when mandatory and/or optional extensions are defined in any updated RFC,
        // this message can be updated.
        $this->first_message_bare = 'n=' . $authcid . ',r=' . $this->cnonce;
        return $this->gs2_header . $this->first_message_bare;
    }

    /**
    * Parses and verifies a non-empty SCRAM challenge.
    *
    * @param  string $challenge The SCRAM challenge
    * @return string|false      The response to send; false in case of wrong challenge or if an initial response has not
    * been generated first.
    * @access private
    */
    private function _generateResponse($challenge, $password)
    {
        // XXX: as I don't support mandatory extension, I would fail on them.
        // And I simply ignore any optional extension.
        $server_message_regexp = "#^r=([\x21-\x2B\x2D-\x7E]+),s=((?:[A-Za-z0-9/+]{4})*(?:[A-Za-z0-9]{3}=|[A-Xa-z0-9]{2}==)?),i=([0-9]*)(,[A-Za-z]=[^,])*$#";
        if (!isset($this->cnonce, $this->gs2_header)
            || !preg_match($server_message_regexp, $challenge, $matches))
        {
            return false;
        }
        $nonce = $matches[1];
        $salt = base64_decode($matches[2]);
        if (!$salt)
        {
            // Invalid Base64.
            return false;
        }
        $i = intval($matches[3]);

        $cnonce = substr($nonce, 0, strlen($this->cnonce));
        if ($cnonce <> $this->cnonce)
        {
            // Invalid challenge! Are we under attack?
            return false;
        }

        $channel_binding = 'c=' . base64_encode($this->gs2_header); // TODO: support channel binding.
        $final_message = $channel_binding . ',r=' . $nonce; // XXX: no extension.

        // TODO: $password = $this->normalize($password); // SASLprep profile of stringprep.
        $saltedPassword = $this->hi($password, $salt, $i);
        $this->saltedPassword = $saltedPassword;
        $clientKey = call_user_func($this->hmac, $saltedPassword, "Client Key", TRUE);
        $storedKey = call_user_func($this->hash, $clientKey, TRUE);
        $authMessage = $this->first_message_bare . ',' . $challenge . ',' . $final_message;
        $this->authMessage = $authMessage;
        $clientSignature = call_user_func($this->hmac, $storedKey, $authMessage, TRUE);
        $clientProof = $clientKey ^ $clientSignature;
        $proof = ',p=' . base64_encode($clientProof);

        return $final_message . $proof;
    }

    /**
    * SCRAM has also a server verification step. On a successful outcome, it will send additional data which must
    * absolutely be checked against this function. If this fails, the entity which we are communicating with is probably
    * not the server as it has not access to your ServerKey.
    *
    * @param string $data The additional data sent along a successful outcome.
    * @return bool Whether the server has been authenticated.
    * If false, the client must close the connection and consider to be under a MITM attack.
    * @access public
    */
    public function processOutcome($data)
    {
        $verifier_regexp = '#^v=((?:[A-Za-z0-9/+]{4})*(?:[A-Za-z0-9]{3}=|[A-Xa-z0-9]{2}==)?)$#';
        if (!isset($this->saltedPassword, $this->authMessage)
            || !preg_match($verifier_regexp, $data, $matches))
        {
            // This cannot be an outcome, you never sent the challenge's response.
            return false;
        }

        $verifier = $matches[1];
        $proposed_serverSignature = base64_decode($verifier);
        $serverKey = call_user_func($this->hmac, $this->saltedPassword, "Server Key", true);
        $serverSignature = call_user_func($this->hmac, $serverKey, $this->authMessage, TRUE);
        return ($proposed_serverSignature === $serverSignature);
    }

    /**
    * Hi() call, which is essentially PBKDF2 (RFC-2898) with HMAC-H() as the pseudorandom function.
    *
    * @param string $str The string to hash.
    * @param string $hash The hash value.
    * @param int $i The iteration count.
    * @access private
    */
    private function hi($str, $salt, $i)
    {
        $int1 = "\0\0\0\1";
        $ui = call_user_func($this->hmac, $str, $salt . $int1, true);
        $result = $ui;
        for ($k = 1; $k < $i; $k++)
        {
            $ui = call_user_func($this->hmac, $str, $ui, true);
            $result = $result ^ $ui;
        }
        return $result;
    }


    /**
    * Creates the client nonce for the response
    *
    * @return string  The cnonce value
    * @access private
    * @author  Richard Heyes <richard@php.net>
    */
    private function _getCnonce()
    {
        // TODO: I reused the nonce function from the DigestMD5 class.
        // I should probably make this a protected function in Common.
        if (@file_exists('/dev/urandom') && $fd = @fopen('/dev/urandom', 'r')) {
            return base64_encode(fread($fd, 32));

        } elseif (@file_exists('/dev/random') && $fd = @fopen('/dev/random', 'r')) {
            return base64_encode(fread($fd, 32));

        } else {
            $str = '';
            for ($i=0; $i<32; $i++) {
                $str .= chr(mt_rand(0, 255));
            }

            return base64_encode($str);
        }
    }

}

?>