CryptRand.php

Go to the documentation of this file.

<?php
use Psr\Log\LoggerInterface;
 
class CryptRand {
    const MIN_ITERATIONS = 1000;
 
    const MSEC_PER_BYTE = 0.5;
 
    protected $strong = null;
 
    protected $randomFuncs = [];
 
    protected $randomFiles = [];
 
    protected $logger;
 
    public function __construct( array $randomFuncs, array $randomFiles, LoggerInterface $logger ) {
        $this->randomFuncs = $randomFuncs;
        $this->randomFiles = $randomFiles;
        $this->logger = $logger;
    }
 
    protected function initialRandomState() {
        // $_SERVER contains a variety of unstable user and system specific information
        // It'll vary a little with each page, and vary even more with separate users
        // It'll also vary slightly across different machines
        $state = serialize( $_SERVER );
 
        // Try to gather a little entropy from the different php rand sources
        $state .= rand() . uniqid( mt_rand(), true );
 
        // Include some information about the filesystem's current state in the random state
        $files = $this->randomFiles;
 
        // We know this file is here so grab some info about ourselves
        $files[] = __FILE__;
 
        // We must also have a parent folder, and with the usual file structure, a grandparent
        $files[] = __DIR__;
        $files[] = dirname( __DIR__ );
 
        foreach ( $files as $file ) {
            MediaWiki\suppressWarnings();
            $stat = stat( $file );
            MediaWiki\restoreWarnings();
            if ( $stat ) {
                // stat() duplicates data into numeric and string keys so kill off all the numeric ones
                foreach ( $stat as $k => $v ) {
                    if ( is_numeric( $k ) ) {
                        unset( $k );
                    }
                }
                // The absolute filename itself will differ from install to install so don't leave it out
                $path = realpath( $file );
                if ( $path !== false ) {
                    $state .= $path;
                } else {
                    $state .= $file;
                }
                $state .= implode( '', $stat );
            } else {
                // The fact that the file isn't there is worth at least a
                // minuscule amount of entropy.
                $state .= '0';
            }
        }
 
        // Try and make this a little more unstable by including the varying process
        // id of the php process we are running inside of if we are able to access it
        if ( function_exists( 'getmypid' ) ) {
            $state .= getmypid();
        }
 
        // If available try to increase the instability of the data by throwing in
        // the precise amount of memory that we happen to be using at the moment.
        if ( function_exists( 'memory_get_usage' ) ) {
            $state .= memory_get_usage( true );
        }
 
        foreach ( $this->randomFuncs as $randomFunc ) {
            $state .= call_user_func( $randomFunc );
        }
 
        return $state;
    }
 
    protected function driftHash( $data ) {
        // Minimum number of iterations (to avoid slow operations causing the
        // loop to gather little entropy)
        $minIterations = self::MIN_ITERATIONS;
        // Duration of time to spend doing calculations (in seconds)
        $duration = ( self::MSEC_PER_BYTE / 1000 ) * MWCryptHash::hashLength();
        // Create a buffer to use to trigger memory operations
        $bufLength = 10000000;
        $buffer = str_repeat( ' ', $bufLength );
        $bufPos = 0;
 
        // Iterate for $duration seconds or at least $minIterations number of iterations
        $iterations = 0;
        $startTime = microtime( true );
        $currentTime = $startTime;
        while ( $iterations < $minIterations || $currentTime - $startTime < $duration ) {
            // Trigger some memory writing to trigger some bus activity
            // This may create variance in the time between iterations
            $bufPos = ( $bufPos + 13 ) % $bufLength;
            $buffer[$bufPos] = ' ';
            // Add the drift between this iteration and the last in as entropy
            $nextTime = microtime( true );
            $delta = (int)( ( $nextTime - $currentTime ) * 1000000 );
            $data .= $delta;
            // Every 100 iterations hash the data and entropy
            if ( $iterations % 100 === 0 ) {
                $data = sha1( $data );
            }
            $currentTime = $nextTime;
            $iterations++;
        }
        $timeTaken = $currentTime - $startTime;
        $data = MWCryptHash::hash( $data );
 
        $this->logger->debug( "Clock drift calculation " .
            "(time-taken=" . ( $timeTaken * 1000 ) . "ms, " .
            "iterations=$iterations, " .
            "time-per-iteration=" . ( $timeTaken / $iterations * 1e6 ) . "us)" );
 
        return $data;
    }
 
    protected function randomState() {
        static $state = null;
        if ( is_null( $state ) ) {
            // Initialize the state with whatever unstable data we can find
            // It's important that this data is hashed right afterwards to prevent
            // it from being leaked into the output stream
            $state = MWCryptHash::hash( $this->initialRandomState() );
        }
        // Generate a new random state based on the initial random state or previous
        // random state by combining it with clock drift
        $state = $this->driftHash( $state );
 
        return $state;
    }
 
    public function wasStrong() {
        if ( is_null( $this->strong ) ) {
            throw new RuntimeException( __METHOD__ . ' called before generation of random data' );
        }
 
        return $this->strong;
    }
 
    public function generate( $bytes, $forceStrong = false ) {
 
        $bytes = floor( $bytes );
        static $buffer = '';
        if ( is_null( $this->strong ) ) {
            // Set strength to false initially until we know what source data is coming from
            $this->strong = true;
        }
 
        if ( strlen( $buffer ) < $bytes ) {
            // If available make use of PHP 7's random_bytes
            // On Linux, getrandom syscall will be used if available.
            // On Windows CryptGenRandom will always be used
            // On other platforms, /dev/urandom will be used.
            // Avoids polyfills from before php 7.0
            // All error situations will throw Exceptions and or Errors
            if ( PHP_VERSION_ID >= 70000
                || ( defined( 'HHVM_VERSION_ID' ) && HHVM_VERSION_ID >= 31101 )
            ) {
                $rem = $bytes - strlen( $buffer );
                $buffer .= random_bytes( $rem );
            }
            if ( strlen( $buffer ) >= $bytes ) {
                $this->strong = true;
            }
        }
 
        if ( strlen( $buffer ) < $bytes ) {
            // If available make use of mcrypt_create_iv URANDOM source to generate randomness
            // On unix-like systems this reads from /dev/urandom but does it without any buffering
            // and bypasses openbasedir restrictions, so it's preferable to reading directly
            // On Windows starting in PHP 5.3.0 Windows' native CryptGenRandom is used to generate
            // entropy so this is also preferable to just trying to read urandom because it may work
            // on Windows systems as well.
            if ( function_exists( 'mcrypt_create_iv' ) ) {
                $rem = $bytes - strlen( $buffer );
                $iv = mcrypt_create_iv( $rem, MCRYPT_DEV_URANDOM );
                if ( $iv === false ) {
                    $this->logger->debug( "mcrypt_create_iv returned false." );
                } else {
                    $buffer .= $iv;
                    $this->logger->debug( "mcrypt_create_iv generated " . strlen( $iv ) .
                        " bytes of randomness." );
                }
            }
        }
 
        if ( strlen( $buffer ) < $bytes ) {
            if ( function_exists( 'openssl_random_pseudo_bytes' ) ) {
                $rem = $bytes - strlen( $buffer );
                $openssl_bytes = openssl_random_pseudo_bytes( $rem, $openssl_strong );
                if ( $openssl_bytes === false ) {
                    $this->logger->debug( "openssl_random_pseudo_bytes returned false." );
                } else {
                    $buffer .= $openssl_bytes;
                    $this->logger->debug( "openssl_random_pseudo_bytes generated " .
                        strlen( $openssl_bytes ) . " bytes of " .
                        ( $openssl_strong ? "strong" : "weak" ) . " randomness." );
                }
                if ( strlen( $buffer ) >= $bytes ) {
                    // openssl tells us if the random source was strong, if some of our data was generated
                    // using it use it's say on whether the randomness is strong
                    $this->strong = !!$openssl_strong;
                }
            }
        }
 
        // Only read from urandom if we can control the buffer size or were passed forceStrong
        if ( strlen( $buffer ) < $bytes &&
            ( function_exists( 'stream_set_read_buffer' ) || $forceStrong )
        ) {
            $rem = $bytes - strlen( $buffer );
            if ( !function_exists( 'stream_set_read_buffer' ) && $forceStrong ) {
                $this->logger->debug( "Was forced to read from /dev/urandom " .
                    "without control over the buffer size." );
            }
            // /dev/urandom is generally considered the best possible commonly
            // available random source, and is available on most *nix systems.
            MediaWiki\suppressWarnings();
            $urandom = fopen( "/dev/urandom", "rb" );
            MediaWiki\restoreWarnings();
 
            // Attempt to read all our random data from urandom
            // php's fread always does buffered reads based on the stream's chunk_size
            // so in reality it will usually read more than the amount of data we're
            // asked for and not storing that risks depleting the system's random pool.
            // If stream_set_read_buffer is available set the chunk_size to the amount
            // of data we need. Otherwise read 8k, php's default chunk_size.
            if ( $urandom ) {
                // php's default chunk_size is 8k
                $chunk_size = 1024 * 8;
                if ( function_exists( 'stream_set_read_buffer' ) ) {
                    // If possible set the chunk_size to the amount of data we need
                    stream_set_read_buffer( $urandom, $rem );
                    $chunk_size = $rem;
                }
                $random_bytes = fread( $urandom, max( $chunk_size, $rem ) );
                $buffer .= $random_bytes;
                fclose( $urandom );
                $this->logger->debug( "/dev/urandom generated " . strlen( $random_bytes ) .
                    " bytes of randomness." );
 
                if ( strlen( $buffer ) >= $bytes ) {
                    // urandom is always strong, set to true if all our data was generated using it
                    $this->strong = true;
                }
            } else {
                $this->logger->debug( "/dev/urandom could not be opened." );
            }
        }
 
        // If we cannot use or generate enough data from a secure source
        // use this loop to generate a good set of pseudo random data.
        // This works by initializing a random state using a pile of unstable data
        // and continually shoving it through a hash along with a variable salt.
        // We hash the random state with more salt to avoid the state from leaking
        // out and being used to predict the /randomness/ that follows.
        if ( strlen( $buffer ) < $bytes ) {
            $this->logger->debug( __METHOD__ .
                ": Falling back to using a pseudo random state to generate randomness." );
        }
        while ( strlen( $buffer ) < $bytes ) {
            $buffer .= MWCryptHash::hmac( $this->randomState(), strval( mt_rand() ) );
            // This code is never really cryptographically strong, if we use it
            // at all, then set strong to false.
            $this->strong = false;
        }
 
        // Once the buffer has been filled up with enough random data to fulfill
        // the request shift off enough data to handle the request and leave the
        // unused portion left inside the buffer for the next request for random data
        $generated = substr( $buffer, 0, $bytes );
        $buffer = substr( $buffer, $bytes );
 
        $this->logger->debug( strlen( $buffer ) .
            " bytes of randomness leftover in the buffer." );
 
        return $generated;
    }
 
    public function generateHex( $chars, $forceStrong = false ) {
        // hex strings are 2x the length of raw binary so we divide the length in half
        // odd numbers will result in a .5 that leads the generate() being 1 character
        // short, so we use ceil() to ensure that we always have enough bytes
        $bytes = ceil( $chars / 2 );
        // Generate the data and then convert it to a hex string
        $hex = bin2hex( $this->generate( $bytes, $forceStrong ) );
 
        // A bit of paranoia here, the caller asked for a specific length of string
        // here, and it's possible (eg when given an odd number) that we may actually
        // have at least 1 char more than they asked for. Just in case they made this
        // call intending to insert it into a database that does truncation we don't
        // want to give them too much and end up with their database and their live
        // code having two different values because part of what we gave them is truncated
        // hence, we strip out any run of characters longer than what we were asked for.
        return substr( $hex, 0, $chars );
    }
}