diff --git a/backend-php/include/config-sample.php b/backend-php/include/config-sample.php
index ca09537..104a151 100644
--- a/backend-php/include/config-sample.php
+++ b/backend-php/include/config-sample.php
@@ -132,6 +132,51 @@
// not be honored.
"reserve_whitelist" => false,
+// The type of links to generate when making new links for shares. Can be any
+// of the following:
+//
+// | Link style | Example | No. of combinations | Avg. bruteforce time |
+// +----------------------------+---------------------------------------+-----------------------+-------------------------------+
+// | LINK_4_PLUS_4_UPPER_CASE | EIRG-0CYE | 2.82 * 10^12 (36^8) | 44.7 years |
+// | LINK_4_PLUS_4_LOWER_CASE | qae3-ulna | 2.82 * 10^12 (36^8) | 44.7 years |
+// | LINK_4_PLUS_4_MIXED_CASE | WRho-uHLG | 1.68 * 10^14 (60^8) | 2663 years |
+// | LINK_UUID_V4 | 09c8a3b1-e78f-48b1-a604-0da49e99cb5d | 5.32 * 10^36 (2^122) | 84.2 septillion years |
+// | LINK_16_HEX | 6cde14c4c6551b41 | 1.84 * 10^19 (2^64) | 292 million years |
+// | LINK_16_UPPER_CASE | 49OFGRK6SGPU93KV | 7.95 * 10^24 (36^16) | 126 trillion years |
+// | LINK_16_LOWER_CASE | bdyslxszs14cj359 | 7.95 * 10^24 (36^16) | 126 trillion years |
+// | LINK_16_MIXED_CASE | NTHX2HDsTn0kS3aj | 2.82 * 10^28 (60^16) | 447 quadrillion years |
+// | LINK_32_HEX | 22adf21f11491ae8f3ae128e23a6782f | 3.40 * 10^38 (2^128) | 5.39 octillion years |
+// | LINK_32_UPPER_CASE | MG42MW2DKIMHM87B4AO0WAB2PIY26TR1 | 6.33 * 10^49 (36^32) | 1 duodecillion years |
+// | LINK_32_LOWER_CASE | itgbolrbq1c02eot5o46c5wixhdrdb5m | 6.33 * 10^49 (36^32) | 1 duodecillion years |
+// | LINK_32_MIXED_CASE | cTK82MJ7rUOP138WNVznQR0Ck3BwZp6b | 7.96 * 10^57 (60^32) | 12.6 quattuordecillion years |
+//
+// For any MIXED_CASE variants, upper-case I and lower-case L will not appear
+// because they are visually very similar and are easily confused.
+//
+// The default value is LINK_4_PLUS_4_UPPER_CASE, which is still considered very
+// secure. The bruteforce times in the table below are the average time it would
+// take to find a valid sharing link, when there is one link active, at 1000
+// guesses per second. For the default setting, this means it would take almost
+// 45 years to find the link.
+//
+// This is assuming that the link is active 24/7 for that entire time. If you
+// only have a link active 2% of the time, it would take over 2200 years.
+//
+// At 1000 guesses per second, you will likely notice that your server is
+// noticeably slower and rapidly filling up with access logs.
+//
+// Very long links are also time-consuming to type, should you find yourself
+// in need of typing in a link manually on another computer. This is the reason
+// that short links are default.
+//
+// ---- PLEASE NOTE ----
+// This option is provided to you only because several people have requested it
+// as a convenience. You are free to change it, but you should know that
+// changing the default here gives you, for all intents and purposes, no
+// security advantage in practice.
+//
+"link_style" => LINK_4_PLUS_4_UPPER_CASE,
+
// Leaflet tile URI template for the map frontend. Here are some examples:
//
// - OpenStreetMap directly:
diff --git a/backend-php/include/inc.php b/backend-php/include/inc.php
index 308ef7c..8ef49e5 100644
--- a/backend-php/include/inc.php
+++ b/backend-php/include/inc.php
@@ -29,6 +29,20 @@ const REDIS = 1;
const PASSWORD = 0;
const HTPASSWD = 1;
+// Share link types.
+const LINK_4_PLUS_4_UPPER_CASE = 0;
+const LINK_4_PLUS_4_LOWER_CASE = 1;
+const LINK_4_PLUS_4_MIXED_CASE = 2;
+const LINK_UUID_V4 = 3;
+const LINK_16_HEX = 4;
+const LINK_16_UPPER_CASE = 5;
+const LINK_16_LOWER_CASE = 6;
+const LINK_16_MIXED_CASE = 7;
+const LINK_32_HEX = 8;
+const LINK_32_UPPER_CASE = 9;
+const LINK_32_LOWER_CASE = 10;
+const LINK_32_MIXED_CASE = 11;
+
const KILOMETERS_PER_HOUR = array(
// Relative distance per second multiplied by number of seconds per hour.
"mpsMultiplier" => 3.6,
@@ -114,6 +128,7 @@ const DEFAULTS = array(
"allow_link_req" => true,
"reserved_links" => [],
"reserve_whitelist" => false,
+ "link_style" => LINK_4_PLUS_4_UPPER_CASE,
"map_tile_uri" => 'https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png',
"map_attribution" => 'Map data © OpenStreetMap contributors, CC-BY-SA',
"default_zoom" => 14,
@@ -300,8 +315,74 @@ class Share {
protected function generateLinkID() {
$s = "";
do {
- $s = strtoupper(base_convert(hash("sha256", openssl_random_pseudo_bytes(LINK_ID_RAND_BYTES)), 16, 36));
- $s = substr($s, 0, 4)."-".substr($s, -4);
+ switch (getConfig("link_style")) {
+ case LINK_UUID_V4:
+ // UUID version 4.
+ $uuid = openssl_random_pseudo_bytes(16);
+ $uuid[6] = chr(ord($uuid[6]) & 0x0f | 0x40);
+ $uuid[8] = chr(ord($uuid[8]) & 0x3f | 0x80);
+ $s = vsprintf("%s%s-%s-%s-%s-%s%s%s", str_split(bin2hex($uuid), 4));
+ break;
+ case LINK_16_HEX:
+ // 16-char (8-byte) hexadecimal string.
+ $s = bin2hex(openssl_random_pseudo_bytes(8));
+ break;
+ case LINK_16_LOWER_CASE:
+ // 16-char lower-case alphanumeric string.
+ $alpha = "0123456789abcdefghijklmnopqrstuvwxyz";
+ for ($i = 0; $i < 16; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ break;
+ case LINK_16_MIXED_CASE:
+ // 16-char mixed-case alphanumeric string.
+ $alpha = "0123456789ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
+ for ($i = 0; $i < 16; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ break;
+ case LINK_16_UPPER_CASE:
+ // 16-char upper-case alphanumeric string.
+ $alpha = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+ for ($i = 0; $i < 16; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ break;
+ case LINK_32_HEX:
+ // 32-char (16-byte) hexadecimal string.
+ $s = bin2hex(openssl_random_pseudo_bytes(16));
+ break;
+ case LINK_32_LOWER_CASE:
+ // 32-char lower-case alphanumeric string.
+ $alpha = "0123456789abcdefghijklmnopqrstuvwxyz";
+ for ($i = 0; $i < 32; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ break;
+ case LINK_32_MIXED_CASE:
+ // 32-char mixed-case alphanumeric string.
+ // 'l' and 'I' not included because of visual similarity.
+ $alpha = "0123456789ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
+ for ($i = 0; $i < 32; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ break;
+ case LINK_32_UPPER_CASE:
+ // 32-char upper-case alphanumeric string.
+ $alpha = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+ for ($i = 0; $i < 32; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ break;
+ case LINK_4_PLUS_4_LOWER_CASE:
+ // 4+4-char lower-case alphanumeric string.
+ $alpha = "0123456789abcdefghijklmnopqrstuvwxyz";
+ for ($i = 0; $i < 8; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ $s = substr($s, 0, 4)."-".substr($s, -4);
+ break;
+ case LINK_4_PLUS_4_MIXED_CASE:
+ // 4+4-char mixed-case alphanumeric string.
+ // 'l' and 'I' not included because of visual similarity.
+ $alpha = "0123456789ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
+ for ($i = 0; $i < 8; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ $s = substr($s, 0, 4)."-".substr($s, -4);
+ break;
+ case LINK_4_PLUS_4_UPPER_CASE:
+ default:
+ // 4+4-char upper-case alphanumeric string.
+ $alpha = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+ for ($i = 0; $i < 8; $i++) $s .= $alpha[random_int(0, strlen($alpha)-1)];
+ $s = substr($s, 0, 4)."-".substr($s, -4);
+ break;
+ }
} while ($this->memcache->get(PREFIX_LOCDATA.$s) !== false);
return $s;
}