String::Compare::ConstantTime(3pm)

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NAME

6       String::Compare::ConstantTime - Timing side-channel protected string
7       compare
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SYNOPSIS

10           use String::Compare::ConstantTime;
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12           if (String::Compare::ConstantTime::equals($secret_data, $user_supplied_data)) {
13             ## The strings are eq
14           }
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16       An example with HMACs:
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18           use String::Compare::ConstantTime;
19           use Digest::HMAC_SHA1; ## or whatever
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21           my $hmac_ctx = Digest::HMAC_SHA1->new($key);
22           $hmac_ctx->add($data);
23           my $digest = $hmac_ctx->digest;
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25           if (String::Compare::ConstantTime::equals($digest, $candidate_digest)) {
26             ## The candidate digest is valid
27           }
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DESCRIPTION

30       This module provides one function, "equals" (not exported by default).
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32       You should pass this function two strings of the same length. Just like
33       perl's "eq", it will return true if they are string-wise identical and
34       false otherwise. However, comparing any two differing strings of the
35       same length will take a fixed amount of time. If the lengths of the
36       strings are different, "equals" will return false right away.
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38       NOTE: This does byte-wise comparison of the underlying string storage,
39       meaning that comparing strings with non-ASCII data with different
40       states of the internal UTF-8 flag is not reliable.  You should always
41       encode your data to bytes before comparing.
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TIMING SIDE-CHANNEL

44       Some programs take different amounts of time to run depending on the
45       input values provided to them. Untrusted parties can sometimes learn
46       information you might not want them to know by measuring this time.
47       This is called a "timing side-channel".
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49       Most routines that compare strings (like perl's "eq" and "cmp" and C's
50       "strcmp" and "memcmp") start scanning from the start of the strings and
51       terminate as soon as they determine the strings won't match. This is
52       good for efficiency but bad because it opens a timing side-channel. If
53       one of the strings being compared is a secret and the other is
54       controlled by some untrusted party, it is sometimes possible for this
55       untrusted party to learn the secret using a timing side-channel.
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57       If the lengths of the strings are different, because "equals" returns
58       false right away the size of the secret string may be leaked (but not
59       its contents).
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HMAC

62       HMACs are "Message Authentication Codes" built on top of cryptographic
63       hashes. The HMAC algorithm produces digests that are included along
64       with a message in order to verify that whoever created the message
65       knows a particular secret password, and that this message hasn't been
66       tampered with since.
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68       To verify a candidate digest included with a message, you re-compute
69       the digest using the message and the secret password. If this computed
70       digest is is the same as the candidate digest then the message is
71       considered authenticated.
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73       A common side-channel attack against services that verify unlimited
74       numbers of messages automatically is to create a forged message and
75       then just send some random junk as the candidate digest. Continue
76       sending this message and junk digests that vary by the first character.
77       Repeat many times. If you find a particular digest that statistically
78       takes a longer time to be rejected than the other digests, it is
79       probably because this particular digest has the first character correct
80       and the service's final string comparison is running slightly longer.
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82       At this point, you keep this first character fixed and start varying
83       the second character until it is solved. Repeat until all the
84       characters are solved or until the amount of remaining possibilities
85       are so small you can brute force it. At this point, your candidate
86       digest is considered valid and you have forged a message.
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88       Note that this particular attack doesn't allow the attacker to recover
89       the secret input key to the HMAC but nevertheless can produce a valid
90       digest for any message given enough time because the service that
91       validates the HMAC is acting as an "oracle".
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93       NOTE: Although this module protects against a common attack against
94       applications that store state in browser cookies, it is in no way an
95       endorsement of this practise.
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LOCK-PICKING ANALOGY

98       Pin tumbler locks are susceptible to being picked in a similar way to
99       an attacker forging HMAC digests using a timing side-channel.
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101       The traditional way to pick cheap pin tumbler locks is to apply torque
102       to the lock cylinder so that the pins are pressed against the cylinder.
103       However, because of slight manufacturing discrepancies one particular
104       pin will be the widest by a slight margin and will be pressed against
105       the cylinder tighter than the others (the cheaper the lock, the higher
106       the manufacturing tolerances). The attacker lifts this pin until the
107       cylinder gives a little bit, indicating that this pin has been solved
108       and the next widest pin is now the one being pressed against the
109       cylinder the tightest. This process is repeated until all the pins are
110       solved and the lock opens.
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112       Just like an attacker trying to solve HMAC digests can work on one
113       character at a time, a lock pick can work on each pin in isolation. To
114       protect against this, quality locks force all pins to be fixed into
115       place before the cylinder rotation can be attempted, just as secure
116       HMAC verifiers force attackers to guess the entire digest on each
117       attempt.
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AUTHOR

141       Doug Hoyte, "<doug@hcsw.org>"
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COPYRIGHT & LICENSE

144       Copyright 2012-2018 Doug Hoyte.
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146       Contributions from Paul Cochrane.
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148       This module is licensed under the same terms as perl itself.
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152perl v5.34.0                      2022-01-21  String::Compare::ConstantTime(3)