includes/libs/CryptHKDF.php

   1 <?php
   2 /**
   3  * Extract-and-Expand Key Derivation Function (HKDF). A cryptographicly
   4  * secure key expansion function based on RFC 5869.
   5  *
   6  * This relies on the secrecy of $wgSecretKey (by default), or $wgHKDFSecret.
   7  * By default, sha256 is used as the underlying hashing algorithm, but any other
   8  * algorithm can be used. Finding the secret key from the output would require
   9  * an attacker to discover the input key (the PRK) to the hmac that generated
  10  * the output, and discover the particular data, hmac'ed with an evolving key
  11  * (salt), to produce the PRK. Even with md5, no publicly known attacks make
  12  * this currently feasible.
  13  *
  14  * This program is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU General Public License as published by
  16  * the Free Software Foundation; either version 2 of the License, or
  17  * (at your option) any later version.
  18  *
  19  * This program is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  22  * GNU General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU General Public License along
  25  * with this program; if not, write to the Free Software Foundation, Inc.,
  26  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  27  * http://www.gnu.org/copyleft/gpl.html
  28  *
  29  * @author Chris Steipp
  30  * @file
  31  */
  32
  33 class CryptHKDF {
  34
  35         /**
  36          * @var BagOStuff The persistent cache
  37          */
  38         protected $cache = null;
  39
  40         /**
  41          * @var string Cache key we'll use for our salt
  42          */
  43         protected $cacheKey = null;
  44
  45         /**
  46          * @var string The hash algorithm being used
  47          */
  48         protected $algorithm = null;
  49
  50         /**
  51          * @var string binary string, the salt for the HKDF
  52          * @see getSaltUsingCache
  53          */
  54         protected $salt = '';
  55
  56         /**
  57          * @var string The pseudorandom key
  58          */
  59         private $prk = '';
  60
  61         /**
  62          * The secret key material. This must be kept secret to preserve
  63          * the security properties of this RNG.
  64          *
  65          * @var string
  66          */
  67         private $skm;
  68
  69         /**
  70          * @var string The last block (K(i)) of the most recent expanded key
  71          */
  72         protected $lastK;
  73
  74         /**
  75          * a "context information" string CTXinfo (which may be null)
  76          * See http://eprint.iacr.org/2010/264.pdf Section 4.1
  77          *
  78          * @var array
  79          */
  80         protected $context = [];
  81
  82         /**
  83          * Round count is computed based on the hash'es output length,
  84          * which neither php nor openssl seem to provide easily.
  85          *
  86          * @var int[]
  87          */
  88         public static $hashLength = [
  89                 'md5' => 16,
  90                 'sha1' => 20,
  91                 'sha224' => 28,
  92                 'sha256' => 32,
  93                 'sha384' => 48,
  94                 'sha512' => 64,
  95                 'ripemd128' => 16,
  96                 'ripemd160' => 20,
  97                 'ripemd256' => 32,
  98                 'ripemd320' => 40,
  99                 'whirlpool' => 64,
 100         ];
 101
 102         /**
 103          * @var CryptRand
 104          */
 105         private $cryptRand;
 106
 107         /**
 108          * @param string $secretKeyMaterial
 109          * @param string $algorithm Name of hashing algorithm
 110          * @param BagOStuff $cache
 111          * @param string|array $context Context to mix into HKDF context
 112          * @param CryptRand $cryptRand
 113          * @throws InvalidArgumentException if secret key material is too short
 114          */
 115         public function __construct( $secretKeyMaterial, $algorithm, BagOStuff $cache, $context,
 116                 CryptRand $cryptRand
 117         ) {
 118                 if ( strlen( $secretKeyMaterial ) < 16 ) {
 119                         throw new InvalidArgumentException( "secret was too short." );
 120                 }
 121                 $this->skm = $secretKeyMaterial;
 122                 $this->algorithm = $algorithm;
 123                 $this->cache = $cache;
 124                 $this->context = is_array( $context ) ? $context : [ $context ];
 125                 $this->cryptRand = $cryptRand;
 126
 127                 // To prevent every call from hitting the same memcache server, pick
 128                 // from a set of keys to use. mt_rand is only use to pick a random
 129                 // server, and does not affect the security of the process.
 130                 $this->cacheKey = $cache->makeKey( 'HKDF', mt_rand( 0, 16 ) );
 131         }
 132
 133         /**
 134          * Save the last block generated, so the next user will compute a different PRK
 135          * from the same SKM. This should keep things unpredictable even if an attacker
 136          * is able to influence CTXinfo.
 137          */
 138         function __destruct() {
 139                 if ( $this->lastK ) {
 140                         $this->cache->set( $this->cacheKey, $this->lastK );
 141                 }
 142         }
 143
 144         /**
 145          * MW specific salt, cached from last run
 146          * @return string Binary string
 147          */
 148         protected function getSaltUsingCache() {
 149                 if ( $this->salt == '' ) {
 150                         $lastSalt = $this->cache->get( $this->cacheKey );
 151                         if ( $lastSalt === false ) {
 152                                 // If we don't have a previous value to use as our salt, we use
 153                                 // 16 bytes from CryptRand, which will use a small amount of
 154                                 // entropy from our pool. Note, "XTR may be deterministic or keyed
 155                                 // via an optional “salt value”  (i.e., a non-secret random
 156                                 // value)..." - http://eprint.iacr.org/2010/264.pdf. However, we
 157                                 // use a strongly random value since we can.
 158                                 $lastSalt = $this->cryptRand->generate( 16 );
 159                         }
 160                         // Get a binary string that is hashLen long
 161                         $this->salt = hash( $this->algorithm, $lastSalt, true );
 162                 }
 163                 return $this->salt;
 164         }
 165
 166         /**
 167          * Produce $bytes of secure random data. As a side-effect,
 168          * $this->lastK is set to the last hashLen block of key material.
 169          *
 170          * @param int $bytes Number of bytes of data
 171          * @param string $context Context to mix into CTXinfo
 172          * @return string Binary string of length $bytes
 173          */
 174         public function generate( $bytes, $context = '' ) {
 175                 if ( $this->prk === '' ) {
 176                         $salt = $this->getSaltUsingCache();
 177                         $this->prk = self::HKDFExtract(
 178                                 $this->algorithm,
 179                                 $salt,
 180                                 $this->skm
 181                         );
 182                 }
 183
 184                 $CTXinfo = implode( ':', array_merge( $this->context, [ $context ] ) );
 185
 186                 return self::HKDFExpand(
 187                         $this->algorithm,
 188                         $this->prk,
 189                         $CTXinfo,
 190                         $bytes,
 191                         $this->lastK
 192                 );
 193         }
 194
 195         /**
 196          * RFC5869 defines HKDF in 2 steps, extraction and expansion.
 197          * From http://eprint.iacr.org/2010/264.pdf:
 198          *
 199          * The scheme HKDF is specifed as:
 200          *   HKDF(XTS, SKM, CTXinfo, L) = K(1) || K(2) || ... || K(t)
 201          * where the values K(i) are defined as follows:
 202          *   PRK = HMAC(XTS, SKM)
 203          *   K(1) = HMAC(PRK, CTXinfo || 0);
 204          *   K(i+1) = HMAC(PRK, K(i) || CTXinfo || i), 1 <= i < t;
 205          * where t = [L/k] and the value K(t) is truncated to its first d = L mod k bits;
 206          * the counter i is non-wrapping and of a given fixed size, e.g., a single byte.
 207          * Note that the length of the HMAC output is the same as its key length and therefore
 208          * the scheme is well defined.
 209          *
 210          * XTS is the "extractor salt"
 211          * SKM is the "secret keying material"
 212          *
 213          * N.B. http://eprint.iacr.org/2010/264.pdf seems to differ from RFC 5869 in that the test
 214          * vectors from RFC 5869 only work if K(0) = '' and K(1) = HMAC(PRK, K(0) || CTXinfo || 1)
 215          *
 216          * @param string $hash The hashing function to use (e.g., sha256)
 217          * @param string $ikm The input keying material
 218          * @param string $salt The salt to add to the ikm, to get the prk
 219          * @param string $info Optional context (change the output without affecting
 220          *      the randomness properties of the output)
 221          * @param int $L Number of bytes to return
 222          * @return string Cryptographically secure pseudorandom binary string
 223          */
 224         public static function HKDF( $hash, $ikm, $salt, $info, $L ) {
 225                 $prk = self::HKDFExtract( $hash, $salt, $ikm );
 226                 $okm = self::HKDFExpand( $hash, $prk, $info, $L );
 227                 return $okm;
 228         }
 229
 230         /**
 231          * Extract the PRK, PRK = HMAC(XTS, SKM)
 232          * Note that the hmac is keyed with XTS (the salt),
 233          * and the SKM (source key material) is the "data".
 234          *
 235          * @param string $hash The hashing function to use (e.g., sha256)
 236          * @param string $salt The salt to add to the ikm, to get the prk
 237          * @param string $ikm The input keying material
 238          * @return string Binary string (pseudorandm key) used as input to HKDFExpand
 239          */
 240         private static function HKDFExtract( $hash, $salt, $ikm ) {
 241                 return hash_hmac( $hash, $ikm, $salt, true );
 242         }
 243
 244         /**
 245          * Expand the key with the given context
 246          *
 247          * @param string $hash Hashing Algorithm
 248          * @param string $prk A pseudorandom key of at least HashLen octets
 249          *    (usually, the output from the extract step)
 250          * @param string $info Optional context and application specific information
 251          *    (can be a zero-length string)
 252          * @param int $bytes Length of output keying material in bytes
 253          *    (<= 255*HashLen)
 254          * @param string &$lastK Set by this function to the last block of the expansion.
 255          *    In MediaWiki, this is used to seed future Extractions.
 256          * @return string Cryptographically secure random string $bytes long
 257          * @throws InvalidArgumentException
 258          */
 259         private static function HKDFExpand( $hash, $prk, $info, $bytes, &$lastK = '' ) {
 260                 $hashLen = self::$hashLength[$hash];
 261                 $rounds = ceil( $bytes / $hashLen );
 262                 $output = '';
 263
 264                 if ( $bytes > 255 * $hashLen ) {
 265                         throw new InvalidArgumentException( 'Too many bytes requested from HDKFExpand' );
 266                 }
 267
 268                 // K(1) = HMAC(PRK, CTXinfo || 1);
 269                 // K(i) = HMAC(PRK, K(i-1) || CTXinfo || i); 1 < i <= t;
 270                 for ( $counter = 1; $counter <= $rounds; ++$counter ) {
 271                         $lastK = hash_hmac(
 272                                 $hash,
 273                                 $lastK . $info . chr( $counter ),
 274                                 $prk,
 275                                 true
 276                         );
 277                         $output .= $lastK;
 278                 }
 279
 280                 return substr( $output, 0, $bytes );
 281         }
 282 }