Hash Functions
PHP Manual

hash_equals

Timing attack safe string comparison

Description

bool hash_equals ( string $known_string , string $user_string )

Compares two strings using the same time whether they're equal or not.

This function should be used to mitigate timing attacks; for instance, when testing crypt password hashes.

Parameters

known_string

The string of known length to compare against

user_string

The user-supplied string

Return Values

Returns TRUE when the two strings are equal, FALSE otherwise.

Errors/Exceptions

Emits an E_WARNING message when either of the supplied parameters is not a string.

Examples

Example #1 example

<?php
$expected  = crypt('12345', '$2a$07$usesomesillystringforsalt$');
$correct   = crypt('12345', '$2a$07$usesomesillystringforsalt$');
$incorrect = crypt('apple',  '$2a$07$usesomesillystringforsalt$');

var_dump(hash_equals($expected, $correct));
var_dump(hash_equals($expected, $incorrect));
?>

The above example will output:

bool(true)
bool(false)

Notes

Note:

Both arguments must be of the same length to be compared successfully. When arguments of differing length are supplied, FALSE is returned immediately and the length of the known string may be leaked in case of a timing attack.

Note:

It is important to provide the user-supplied string as the second parameter, rather than the first.