Ubuntu Manpage: String::Compare::ConstantTime - Timing side-channel protected string compare

Provided by: libstring-compare-constanttime-perl_0.300-1build1_amd64

NAME

       String::Compare::ConstantTime - Timing side-channel protected string compare

SYNOPSIS

           use String::Compare::ConstantTime;
           use Digest::HMAC_SHA1; ## or whatever

           my $hmac_ctx = Digest::HMAC_SHA1->new($key);
           $hmac_ctx->add($data);
           my $digest = $hmac_ctx->digest;

           if (String::Compare::ConstantTime::equals($digest, $candidate_digest)) {
             ## The candidate digest is valid
           }

DESCRIPTION

       This module provides one function, "equals" (not exported by default).

       You should pass this function two strings of the same length. It will return true if they
       are string-wise identical and false otherwise, just like "eq". However, comparing any two
       differing strings will take a fixed amount of time, unlike "eq".

       NOTE: If the lengths of the strings are different, "equals" will return false right away.
       Also, comparing two identical strings will take a different amount of time than comparing
       two differing strings.

TIMING SIDE-CHANNEL

       Some programs take different amounts of time to run depending on the input values provided
       to them. When untrusted parties control input, they might be able to learn information you
       might otherwise not want them to know. This is called a "timing side-channel".

       Most routines that compare strings (like perl's "eq" and "cmp" and C's "strcmp" and
       "memcmp") start scanning from the start and terminate as soon as they determine the
       strings won't match. This is good for efficiency but bad because it opens a timing side-
       channel. If one of the strings being compared is a secret and the other is controlled by
       some untrusted party, it is sometimes possible for this untrusted party to learn the
       secret using a timing side-channel.

HMAC

HMACs are "Message Authentication Codes" built on top of cryptographic hashes. The HMAC
algorithm produces digests that are included along with a message in order to verify that
whoever created the message knows a particular secret password, and that this message
hasn't been tampered with since.

To verify a candidate digest included with a message, you re-compute the digest using the
message and the secret password. If this computed digest is is the same as the candidate
digest then the message is considered authenticated.

A very common side-channel attack against services that verify unlimited numbers of
messages automatically is to create a forged message and then just send some random junk
as the candidate digest. Continue sending this message and the junk digest, varying the
first character in the digest. Repeat many times. If you find a particular digest that
statistically takes a longer time to be rejected than the other digests, it is probably
because this particular digest has the first character correct and the service's final
string comparison is running a little longer.

At this point, you keep this first character fixed and start varying the second character.
Repeat until all the characters are solved or until the amount of remaining possibilities
are so small you can brute force it. At this point, your candidate digest is considered
valid and you have forged a message.

Note that this particular attack doesn't allow the attacker to recover the secret input
key, but the attacker can still use the service itself to produce a valid digest for any
message.

LOCK-PICKING ANALOGY

       Pin tumbler locks are susceptible to being picked in a similar way to an attacker forging
       HMAC digests using a timing side-channel.

       The most common way to pick cheap pin tumbler locks is to apply torque to the lock
       cylinder so that the pins are pressed against the cylinder. However, because of slight
       manufacturing discrepancies one particular pin will be the widest by a slight margin and
       will actually be the only pin pressed against the cylinder (the cheaper the lock, the
       higher the manufacturing "tolerances"). The attacker lifts this pin until the cylinder
       gives a little bit, indicating that this pin has been solved and the next widest pin is
       now the one being pressed against the cylinder. This process is repeated until all the
       pins are solved and the lock opens.

       Just like an attacker trying to solve HMAC digests can work on one character at a time, a
       lock pick can work on each pin in isolation. To protect against this, quality locks force
       all pins to be fixed into place before the cylinder rotation can begin just as secure HMAC
       verifiers force attackers to guess the entire digest on each attempt.

AUTHOR

       Doug Hoyte, "<doug@hcsw.org>"

COPYRIGHT & LICENSE

       Copyright 2012 Doug Hoyte.

       This module is licensed under the same terms as perl itself.