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Diffstat (limited to 'includes/utils/MWCryptHKDF.php')
| -rw-r--r-- | includes/utils/MWCryptHKDF.php | 332 |
1 files changed, 332 insertions, 0 deletions
diff --git a/includes/utils/MWCryptHKDF.php b/includes/utils/MWCryptHKDF.php new file mode 100644 index 00000000..cc136793 --- /dev/null +++ b/includes/utils/MWCryptHKDF.php @@ -0,0 +1,332 @@ +<?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 + */ + 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 + */ + 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 + */ + 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 ); + } + +} |