--- /dev/null
+<?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 CryptHKDF {
+
+ /**
+ * @var BagOStuff The persistent cache
+ */
+ protected $cache = null;
+
+ /**
+ * @var string Cache key we'll use for our salt
+ */
+ protected $cacheKey = null;
+
+ /**
+ * @var string The hash algorithm being used
+ */
+ protected $algorithm = null;
+
+ /**
+ * @var string binary string, the salt for the HKDF
+ * @see getSaltUsingCache
+ */
+ protected $salt = '';
+
+ /**
+ * @var string The pseudorandom key
+ */
+ private $prk = '';
+
+ /**
+ * The secret key material. This must be kept secret to preserve
+ * the security properties of this RNG.
+ *
+ * @var string
+ */
+ private $skm;
+
+ /**
+ * @var string 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
+ *
+ * @var array
+ */
+ protected $context = [];
+
+ /**
+ * Round count is computed based on the hash'es output length,
+ * which neither php nor openssl seem to provide easily.
+ *
+ * @var int[]
+ */
+ public static $hashLength = [
+ 'md5' => 16,
+ 'sha1' => 20,
+ 'sha224' => 28,
+ 'sha256' => 32,
+ 'sha384' => 48,
+ 'sha512' => 64,
+ 'ripemd128' => 16,
+ 'ripemd160' => 20,
+ 'ripemd256' => 32,
+ 'ripemd320' => 40,
+ 'whirlpool' => 64,
+ ];
+
+ /**
+ * @var CryptRand
+ */
+ private $cryptRand;
+
+ /**
+ * @param string $secretKeyMaterial
+ * @param string $algorithm Name of hashing algorithm
+ * @param BagOStuff $cache
+ * @param string|array $context Context to mix into HKDF context
+ * @param CryptRand $cryptRand
+ * @throws InvalidArgumentException if secret key material is too short
+ */
+ public function __construct( $secretKeyMaterial, $algorithm, BagOStuff $cache, $context,
+ CryptRand $cryptRand
+ ) {
+ if ( strlen( $secretKeyMaterial ) < 16 ) {
+ throw new InvalidArgumentException( "secret was too short." );
+ }
+ $this->skm = $secretKeyMaterial;
+ $this->algorithm = $algorithm;
+ $this->cache = $cache;
+ $this->context = is_array( $context ) ? $context : [ $context ];
+ $this->cryptRand = $cryptRand;
+
+ // 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 = $cache->makeKey( '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 CryptRand, 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 = $this->cryptRand->generate( 16 );
+ }
+ // Get a binary string that is hashLen long
+ $this->salt = hash( $this->algorithm, $lastSalt, true );
+ }
+ return $this->salt;
+ }
+
+ /**
+ * 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
+ */
+ public function generate( $bytes, $context = '' ) {
+ if ( $this->prk === '' ) {
+ $salt = $this->getSaltUsingCache();
+ $this->prk = self::HKDFExtract(
+ $this->algorithm,
+ $salt,
+ $this->skm
+ );
+ }
+
+ $CTXinfo = implode( ':', array_merge( $this->context, [ $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 InvalidArgumentException
+ */
+ private static function HKDFExpand( $hash, $prk, $info, $bytes, &$lastK = '' ) {
+ $hashLen = self::$hashLength[$hash];
+ $rounds = ceil( $bytes / $hashLen );
+ $output = '';
+
+ if ( $bytes > 255 * $hashLen ) {
+ throw new InvalidArgumentException( '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 );
+ }
+}
scripts.mit.edu / autoinstallsdev / mediawiki.git / blobdiff