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<?php namespace MediaWiki\User\TempUser; use LogicException; use OutOfBoundsException; use RuntimeException; /** * A mapping which converts sequential input into an output sequence that looks * pseudo-random, but preserves the base-10 length of the input number. * * Take a sequence generated by multiplying the previous element of the * sequence by a fixed number "g", then applying the modulus "p": * * X(0) = 1 * X(i) = ( g X(i-1) ) mod p * * If g is a primitive root modulo p, then this sequence will cover all values * from 1 to p-1 before it repeats. X(i) is a modular exponential function * (g^i mod p) and algorithms are available to calculate it efficiently. * * Loosely speaking, we choose a sequence based on the number of digits N in the * input, with the period being approximately 10^N, so that the number of digits * in the output will be approximately the same. * * More precisely, after offsetting the subsequent sequences to avoid colliding * with the previous sequences, the period ends up being about 0.9 * 10^N * * The modulo p is always a prime number because that makes the maths easier. * We use a value for g close to p/sqrt(3) since that seems to stir the digits * better than the largest or smallest primitive root. * * @internal */ class ScrambleMapping implements SerialMapping { /** * Appropriately sized prime moduli and primitive roots. Generated with * this GP/PARI script: * s=0; \ * for(q = 2, 10, \ * p=precprime(10^q - s); \ * s = s + p; \ * forstep(i = floor(p/sqrt(3)), 1, -1, \ * if(znorder(Mod(i, p)) == p-1, \ * print("[ ", i, ", ", p, " ],"); \ * break ))) */ private const GENERATORS = [ [ 56, 97 ], [ 511, 887 ], [ 5203, 9013 ], [ 51947, 90001 ], [ 519612, 900001 ], [ 5196144, 8999993 ], [ 51961523, 89999999 ], [ 519615218, 899999963 ], [ 5196152444, 9000000043 ], ]; /** @var int */ private $offset; /** @var bool */ private $hasGmp; /** @var bool */ private $hasBcm; public function __construct( $config ) { $this->offset = $config['offset'] ?? 0; $this->hasGmp = extension_loaded( 'gmp' ); $this->hasBcm = extension_loaded( 'bcmath' ); if ( !$this->hasGmp && !$this->hasBcm ) { throw new RuntimeException( __CLASS__ . ' requires the bcmath or gmp extension' ); } } public function getSerialIdForIndex( int $index ): string { if ( $index <= 0 ) { return (string)$index; } $offset = $this->offset; if ( $index - $offset < 0 ) { throw new OutOfBoundsException( __METHOD__ . ": The configured offset $offset is too large." ); } foreach ( self::GENERATORS as [ $g, $p ] ) { if ( $index - $offset < $p ) { return (string)( $offset + $this->powmod( $g, $index - $offset, $p ) ); } $offset += $p - 1; } throw new RuntimeException( __METHOD__ . ": The index $index is too large" ); } private function powmod( $num, $exponent, $modulus ) { if ( $this->hasGmp ) { return \gmp_intval( \gmp_powm( $num, $exponent, $modulus ) ); } elseif ( $this->hasBcm ) { return (int)\bcpowmod( (string)$num, (string)$exponent, (string)$modulus ); } else { throw new LogicException( __CLASS__ . ' requires the bcmath or gmp extension' ); } } }
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