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<?php
/**
* Extract-and-Expand Key Derivation Function (HKDF). A cryptographicly
* secure key expansion function based on RFC 5869.
*
* This relies on the secrecy of $wgSecretKey (by default), or $wgHKDFSecret.
* By default, sha256 is used as the underlying hashing algorithm, but any other
* algorithm can be used. Finding the secret key from the output would require
* an attacker to discover the input key (the PRK) to the hmac that generated
* the output, and discover the particular data, hmac'ed with an evolving key
* (salt), to produce the PRK. Even with md5, no publicly known attacks make
* this currently feasible.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @author Chris Steipp
* @file
*/
class MWCryptHKDF {
/**
* Singleton instance for public use
*/
protected static $singleton = null;
/**
* The persistant cache
*/
protected $cache = null;
/**
* Cache key we'll use for our salt
*/
protected $cacheKey = null;
/**
* The hash algorithm being used
*/
protected $algorithm = null;
/**
* binary string, the salt for the HKDF
*/
protected $salt;
/**
* The pseudorandom key
*/
private $prk;
/**
* The secret key material. This must be kept secret to preserve
* the security properties of this RNG.
*/
private $skm;
/**
* The last block (K(i)) of the most recent expanded key
*/
protected $lastK;
/**
* a "context information" string CTXinfo (which may be null)
* See http://eprint.iacr.org/2010/264.pdf Section 4.1
*/
protected $context = array();
/**
* Round count is computed based on the hash'es output length,
* which neither php nor openssl seem to provide easily.
*/
public static $hashLength = array(
'md5' => 16,
'sha1' => 20,
'sha224' => 28,
'sha256' => 32,
'sha384' => 48,
'sha512' => 64,
'ripemd128' => 16,
'ripemd160' => 20,
'ripemd256' => 32,
'ripemd320' => 40,
'whirlpool' => 64,
);
/**
* @param string $secretKeyMaterial
* @param string $algorithm Name of hashing algorithm
* @param BagOStuff $cache
* @param string|array $context Context to mix into HKDF context
* @throws MWException
*/
public function __construct( $secretKeyMaterial, $algorithm, $cache, $context ) {
if ( strlen( $secretKeyMaterial ) < 16 ) {
throw new MWException( "MWCryptHKDF secret was too short." );
}
$this->skm = $secretKeyMaterial;
$this->algorithm = $algorithm;
$this->cache = $cache;
$this->salt = ''; // Initialize a blank salt, see getSaltUsingCache()
$this->prk = '';
$this->context = is_array( $context ) ? $context : array( $context );
// To prevent every call from hitting the same memcache server, pick
// from a set of keys to use. mt_rand is only use to pick a random
// server, and does not affect the security of the process.
$this->cacheKey = wfMemcKey( 'HKDF', mt_rand( 0, 16 ) );
}
/**
* Save the last block generated, so the next user will compute a different PRK
* from the same SKM. This should keep things unpredictable even if an attacker
* is able to influence CTXinfo.
*/
function __destruct() {
if ( $this->lastK ) {
$this->cache->set( $this->cacheKey, $this->lastK );
}
}
/**
* MW specific salt, cached from last run
* @return string Binary string
*/
protected function getSaltUsingCache() {
if ( $this->salt == '' ) {
$lastSalt = $this->cache->get( $this->cacheKey );
if ( $lastSalt === false ) {
// If we don't have a previous value to use as our salt, we use
// 16 bytes from MWCryptRand, which will use a small amount of
// entropy from our pool. Note, "XTR may be deterministic or keyed
// via an optional “salt value” (i.e., a non-secret random
// value)..." - http://eprint.iacr.org/2010/264.pdf. However, we
// use a strongly random value since we can.
$lastSalt = MWCryptRand::generate( 16 );
}
// Get a binary string that is hashLen long
$this->salt = hash( $this->algorithm, $lastSalt, true );
}
return $this->salt;
}
/**
* Return a singleton instance, based on the global configs.
* @return HKDF
* @throws MWException
*/
protected static function singleton() {
global $wgHKDFAlgorithm, $wgHKDFSecret, $wgSecretKey;
$secret = $wgHKDFSecret ?: $wgSecretKey;
if ( !$secret ) {
throw new MWException( "Cannot use MWCryptHKDF without a secret." );
}
// In HKDF, the context can be known to the attacker, but this will
// keep simultaneous runs from producing the same output.
$context = array();
$context[] = microtime();
$context[] = getmypid();
$context[] = gethostname();
// Setup salt cache. Use APC, or fallback to the main cache if it isn't setup
try {
$cache = ObjectCache::newAccelerator( array() );
} catch ( Exception $e ) {
$cache = wfGetMainCache();
}
if ( is_null( self::$singleton ) ) {
self::$singleton = new self( $secret, $wgHKDFAlgorithm, $cache, $context );
}
return self::$singleton;
}
/**
* Produce $bytes of secure random data. As a side-effect,
* $this->lastK is set to the last hashLen block of key material.
* @param int $bytes Number of bytes of data
* @param string $context Context to mix into CTXinfo
* @return string Binary string of length $bytes
*/
protected function realGenerate( $bytes, $context = '' ) {
if ( $this->prk === '' ) {
$salt = $this->getSaltUsingCache();
$this->prk = self::HKDFExtract(
$this->algorithm,
$salt,
$this->skm
);
}
$CTXinfo = implode( ':', array_merge( $this->context, array( $context ) ) );
return self::HKDFExpand(
$this->algorithm,
$this->prk,
$CTXinfo,
$bytes,
$this->lastK
);
}
/**
* RFC5869 defines HKDF in 2 steps, extraction and expansion.
* From http://eprint.iacr.org/2010/264.pdf:
*
* The scheme HKDF is specifed as:
* HKDF(XTS, SKM, CTXinfo, L) = K(1) || K(2) || ... || K(t)
* where the values K(i) are defined as follows:
* PRK = HMAC(XTS, SKM)
* K(1) = HMAC(PRK, CTXinfo || 0);
* K(i+1) = HMAC(PRK, K(i) || CTXinfo || i), 1 <= i < t;
* where t = [L/k] and the value K(t) is truncated to its first d = L mod k bits;
* the counter i is non-wrapping and of a given fixed size, e.g., a single byte.
* Note that the length of the HMAC output is the same as its key length and therefore
* the scheme is well defined.
*
* XTS is the "extractor salt"
* SKM is the "secret keying material"
*
* N.B. http://eprint.iacr.org/2010/264.pdf seems to differ from RFC 5869 in that the test
* vectors from RFC 5869 only work if K(0) = '' and K(1) = HMAC(PRK, K(0) || CTXinfo || 1)
*
* @param string $hash The hashing function to use (e.g., sha256)
* @param string $ikm The input keying material
* @param string $salt The salt to add to the ikm, to get the prk
* @param string $info Optional context (change the output without affecting
* the randomness properties of the output)
* @param int $L Number of bytes to return
* @return string Cryptographically secure pseudorandom binary string
*/
public static function HKDF( $hash, $ikm, $salt, $info, $L ) {
$prk = self::HKDFExtract( $hash, $salt, $ikm );
$okm = self::HKDFExpand( $hash, $prk, $info, $L );
return $okm;
}
/**
* Extract the PRK, PRK = HMAC(XTS, SKM)
* Note that the hmac is keyed with XTS (the salt),
* and the SKM (source key material) is the "data".
*
* @param string $hash The hashing function to use (e.g., sha256)
* @param string $salt The salt to add to the ikm, to get the prk
* @param string $ikm The input keying material
* @return string Binary string (pseudorandm key) used as input to HKDFExpand
*/
private static function HKDFExtract( $hash, $salt, $ikm ) {
return hash_hmac( $hash, $ikm, $salt, true );
}
/**
* Expand the key with the given context
*
* @param string $hash Hashing Algorithm
* @param string $prk A pseudorandom key of at least HashLen octets
* (usually, the output from the extract step)
* @param string $info Optional context and application specific information
* (can be a zero-length string)
* @param int $bytes Length of output keying material in bytes
* (<= 255*HashLen)
* @param string &$lastK Set by this function to the last block of the expansion.
* In MediaWiki, this is used to seed future Extractions.
* @return string Cryptographically secure random string $bytes long
* @throws MWException
*/
private static function HKDFExpand( $hash, $prk, $info, $bytes, &$lastK = '' ) {
$hashLen = MWCryptHKDF::$hashLength[$hash];
$rounds = ceil( $bytes / $hashLen );
$output = '';
if ( $bytes > 255 * $hashLen ) {
throw new MWException( "Too many bytes requested from HDKFExpand" );
}
// K(1) = HMAC(PRK, CTXinfo || 1);
// K(i) = HMAC(PRK, K(i-1) || CTXinfo || i); 1 < i <= t;
for ( $counter = 1; $counter <= $rounds; ++$counter ) {
$lastK = hash_hmac(
$hash,
$lastK . $info . chr( $counter ),
$prk,
true
);
$output .= $lastK;
}
return substr( $output, 0, $bytes );
}
/**
* Generate cryptographically random data and return it in raw binary form.
*
* @param int $bytes The number of bytes of random data to generate
* @param string $context String to mix into HMAC context
* @return string Binary string of length $bytes
*/
public static function generate( $bytes, $context ) {
return self::singleton()->realGenerate( $bytes, $context );
}
/**
* Generate cryptographically random data and return it in hexadecimal string format.
* See MWCryptRand::realGenerateHex for details of the char-to-byte conversion logic.
*
* @param int $chars The number of hex chars of random data to generate
* @param string $context String to mix into HMAC context
* @return string Random hex characters, $chars long
*/
public static function generateHex( $chars, $context = '' ) {
$bytes = ceil( $chars / 2 );
$hex = bin2hex( self::singleton()->realGenerate( $bytes, $context ) );
return substr( $hex, 0, $chars );
}
}
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