random_key.php

<?php
/*
 * Copyright (c) 2017 Tim Kuijsten
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 *
 * Generate a random string of base 56 characters. Either provide a desired
 * string length or bit length, not both.
 *
 * When a bit length is provided, the value is ceiled to the closest factor of
 * log2(56). Thus you can get at most 5.8 more bits than requested.
 *
 * @param int  [$strlen]  minimum of 1
 * @param int  [$bitlen]  minimum of 1
 * @param int  [$alphabet]  the alphabet must contain at least 2 symbols,
 *   defaults to base 56 which is optimized for human inspection.
 * @return string          base 56 string
 */
function random_key($strlen, $bitlen = 0, $alphabet = '23456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz')
{
    $base = strlen($alphabet); // php strlen runs in constant time
    // bits per character log2(56)
    $bpc = log($base, 2);
    // int in php is unsigned
    $maxintbits = (PHP_INT_SIZE / 2) * 8;
    if ($strlen && $bitlen) {
        return false;
    }
    if ($strlen) {
        /* truncate to ensure string length */
        $bitlen = intval($strlen * $bpc);
    } else {
        $strlen = $bitlen / $bpc;
    }
    if ($bitlen == 0) {
        return false;
    }
    $result = '';
    while ($bitlen > $bpc) {
        $fetch = ($bitlen > $maxintbits) ? $maxintbits : $bitlen;
        $number = random_int(0, 2 ** $fetch);
        if ($number === 0) {
            $numbits = 0;
        } else {
            $numbits = log($number, 2);
        }
        $chars = 0;
        while ($number && (($chars + 1) * $bpc) <= $numbits) {
            $remainder = $number % $base;
            $number = ($number - $remainder) / $base;
            $result .= $alphabet[$remainder];
            $chars++;
        }
        $bitlen -= $chars * $bpc;
    }
    if ($bitlen > 0) {
        $result .= $alphabet[random_int(0, $base - 1)];
    }
    return $result;
}