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

SYNOPSIS
        use String::Compare::ConstantTime;

        if (String::Compare::ConstantTime::equals($secret_data, $user_supplied_data)) {
          ## The strings are eq
        }

    An example with HMACs:

        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. Just like
    perl's "eq", it will return true if they are string-wise identical and
    false otherwise. However, comparing any two differing strings of the
    same length will take a fixed amount of time. If the lengths of the
    strings are different, "equals" will return false right away.

TIMING SIDE-CHANNEL
    Some programs take different amounts of time to run depending on the
    input values provided to them. Untrusted parties can sometimes learn
    information you might not want them to know by measuring this time. 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 of the strings 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.

    If the lengths of the strings are different, because "equals" returns
    false right away the size of the secret string may be leaked (but not
    its contents).

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 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 junk digests that vary by the first character. 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 slightly longer.

    At this point, you keep this first character fixed and start varying the
    second character until it is solved. 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 to the HMAC but nevertheless can produce a valid
    digest for any message given enough time because the service that
    validates the HMAC is acting as an "oracle".

    NOTE: Although this module protects against a common attack against
    applications that store state in browser cookies, it is in no way an
    endorsement of this practise.

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 traditional 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 be pressed against
    the cylinder tighter than the others (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 the tighest. 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 be attempted, just as secure HMAC
    verifiers force attackers to guess the entire digest on each attempt.

SEE ALSO
    The String-Compare-ConstantTime github repo
    <https://github.com/hoytech/String-Compare-ConstantTime>

    Authen::Passphrase has a good section on side-channel cryptanalysis such
    as it pertains to password storage (mostly, it doesn't).

    The famous TENEX password bug
    <http://www.meadhbh.org/services/passwords>

    Example of a timing bug
    <http://rdist.root.org/2009/05/28/timing-attack-in-google-keyczar-librar
    y/>

    QSCAN <http://hcsw.org/nmap/QSCAN>

    Practical limits of the timing side channel
    <http://www.cs.rice.edu/~dwallach/pub/crosby-timing2009.pdf>

    NaCl: Crypto library designed to prevent side channel attacks
    <http://nacl.cr.yp.to/>

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

COPYRIGHT & LICENSE
    Copyright 2012-2013 Doug Hoyte.

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