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(php section 3)
man page for hash_equals

HASH_EQUALS(3)								 1							    HASH_EQUALS(3)

hash_equals - Timing attack safe string comparison

SYNOPSIS
bool hash_equals (string $known_string, string $user_string) DESCRIPTION
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(3) password hashes. PARAMETERS
o $known_string - The string of known length to compare against o $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. PHP Documentation Group HASH_EQUALS(3)

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