How to generate unguessable "tiny url" based on an id?

Question

Hi there,

I'm interested in creating tiny url like links. My idea was to simply store an incrementing identifier for every long url posted and then convert this id to it's base 36 variant, like the following in PHP:

$tinyurl = base_convert($id, 10, 36)

The problem here is that the result is guessable, while it has to be hard to guess what the next url is going to be, while still being short (tiny). Eg. atm if my last tinyurl was a1, the next one will be a2. This is a bad thing for me.

So, how would I make sure that the resulting tiny url is not as guessable but still short?

Answer 1

Generate a random key every time, but remember to check that it's not already in use. Some sample code, generating a key consisting of 5 characters in the a-z range:

$key = "";
$key_length = 5;
$chars = range("a", "z");
$char_count = count($chars);
for ($i = 0; $i < $key_length; $i++) {
   $index = rand(0, $char_count - 1);
   $key .= $chars[$index];
}

Answer 2

I would simply crc32 url

$url = 'http://www.google.com';
$tinyurl = hash('crc32', $url ); // db85f073

cons: constant 8 character long identifier

Answer 3

This is really cheap, but if the user doesn't know it's happening then it's not as guessable, but prefix and postfix the actual id with 2 or 3 random numbers/letters.

If I saw 9d2a1me3 I wouldn't guess that dm2a2dq2 was the next in the series.

Answer 4

Try Xor'ing the $id with some value, e.g. $id ^ 46418 - and to convert back to your original id you just perform the same Xor again i.e. $mungedId ^ 46418. Stack this together with your base_convert and perhaps some swapping of chars in the resultant string and it'll get quite tricky to guess a URL.

Answer 5

What you are asking for is a balance between reduction of information (URLs to their indexes in your database), and artificial increase of information (to create holes in your sequence).

You have to decide how important both is for you. Another question is whether you just do not want sequential URLs to be guessable, or have them sufficiently random to make guessing any valid URL difficult.

Basically, you want to declare n out of N valid ids. Choose N smaller to make the URLs shorter, and make n smaller to generate URLs that are difficult to guess. Make n and N larger to generate more URLs when the shorter ones are taken.

To assign the ids, you can just take any kind of random generator or hash function and cap this to your target range N. If you detect a collision, choose the next random value. If you have reached a count of n unique ids, you must increase the range of your ID set (n and N).

Answer 6

You can pre-define the 4-character codes in advance (all possible combinations), then randomize that list and store it in this random order in a data table. When you want a new value, just grab the first one off the top and remove it from the list. It's fast, no on-the-fly calculation, and guarantees pseudo-randomness to the end-user.

Answer 7

If you want an injective function, you can use any form of encryption. For instance:

<?php
$key = "my secret";
$enc = mcrypt_ecb (MCRYPT_3DES, $key, "42", MCRYPT_ENCRYPT);
$f = unpack("H*", $enc);
$value = reset($f);
var_dump($value); //string(16) "1399e6a37a6e9870"

To reverse:

$rf = pack("H*", $value);
$dec = rtrim(mcrypt_ecb (MCRYPT_3DES, $key, $rf, MCRYPT_DECRYPT), "\x00");
var_dump($dec); //string(2) "42"

This will not give you a number in base 32; it will give you the encrypted data with each byte converted to base 16 (i.e., the conversion is global). If you really need, you can trivially convert this to base 10 and then to base 32 with any library that supports big integers.

Answer 8

Another way would be to set the maximum number of characters for the URL (let's say it's n). You could then choose a random number between 1 and n!, which would be your permutation number.

On which new URL, you would increment the id and use the permutation number to associate the actual id that would be used. Finally, you would base 32 (or whatever) encode your URL. This would be perfectly random and perfectly reversible.

Answer 9

I ended up creating a md5 sum of the identifier, use the first 4 alphanumerics of it and if this is a duplicate simply increment the length until it is no longer a duplicate.

function idToTinyurl($id) {
    $md5 = md5($id);
    for ($i = 4; $i < strlen($md5); $i++) {
        $possibleTinyurl = substr($md5, 0, $i);
        $res = mysql_query("SELECT id FROM tabke WHERE tinyurl='".$possibleTinyurl."' LIMIT 1");
        if (mysql_num_rows($res) == 0) return $possibleTinyurl;
    }
    return $md5;
}

Accepted relet's answer as it's lead me to this strategy.