What is the best way to compare hashed strings? (PHP)

Question

Should I use if(strcmp(md5($string),$hash)==0) or if(md5($string)==$hash)

Solution 1

You should be very careful when comparing hashes directly for things like authentification as you could be opening a window to a timing attack.

Although it sounds very counterintuitive, you should to a full comparison of the string, avoiding any optimizations (i.e. exiting early if a character is different).

Here are some links about the problem:

• https://wiki.php.net/rfc/timing_attack
• http://codahale.com/a-lesson-in-timing-attacks/
• http://carlos.bueno.org/2011/10/timing.html

And here are some ideas in order to fix it:

• The fastest and easiest: just use hash_equals() if available (PHP 5.6 and greater).
• https://github.com/symfony/polyfill-php56/blob/master/Php56.php (hash_equals() polyfill)
• https://github.com/zendframework/zend-crypt/blob/master/src/Utils.php (compareStrings())

Solution 2

If you compare strings, use strcmp or ===. People prefer === because strcmp can be confusing (it returns 0 on success, wat).

You must use ===, not ==. == will convert both operands to integers if they can be interpreted as such, and because an MD5 hash doesn’t fit in an integer, they will be truncated around the half. Therefore, only the first halves of the hashes have to be equal. See http://phpsadness.com/sad/47.

If you are hashing passwords, consider using a slow and strong hashing algorithm such as PBKDF2, rather than MD5.

Solution 3

If you're using anything newer than PHP 5.6 (and including) you should use timing attack safe string comparison function.

if (hash_equals($expected, $correct)) {

}

(If you're on PHP 5.5 or earlier, see here for equivalents.)

Comments

[Felix Kling]

Use strcmp in custom sort functions. For "normal" use, comparing the strings directly is easier and easier to read.

[ereOn]

For anyone reading this thread, you probably shouldn't do that at all.

[caw]

If you just want to compare hashes reliably, simply use ===. If you really care about security and potential timing attacks (even despite of network jitter), you should take a look at this or this discussion or use the hash_equals() function (PHP 5.6+).

[rugk]

No MD5, no MD5, no MD5!

[BoltClock]

Wow... that's truly sad indeed.

[Claudiu]

Look @MM answer. Second of all md5 is obsolete (proven that it can be hacked). Use sha (a better, not the best).

[lszrh]

Of course, you should use sha over md5. But the questioner asked which he should use. If someone asks you: "Do you want a strawberry ice cream or a chocolate ice cream?", you cannot answer "I take vanilla ice cream". Furthermore the question is not about using a hash algorithm, but about comparing values.

[rugk]

We do not talk about icecream here, but about security. So the answer is indeed "Do not use anything of what you proposed here!". And BTW: Also SHA-1 must not be used for passwords.

[David]

Who says this is about security? Maybe he just wants to hash contents of a file to see if its equal to another one.

[Fabiano Araujo]

I know this is an old answer but it's important to notice that this "sadness" was posted to PHP 5.4.4. 3v4l.org/EDcdY shows us that from versions 5.4.4 to 5.6.30 and 7+, the statement above is not true, though inferior versions will indeed result in a true statement comparing only half of the string.