diff libtomcrypt/notes/tech0002.txt @ 285:1b9e69c058d2

propagate from branch 'au.asn.ucc.matt.ltc.dropbear' (head 20dccfc09627970a312d77fb41dc2970b62689c3) to branch 'au.asn.ucc.matt.dropbear' (head fdf4a7a3b97ae5046139915de7e40399cceb2c01)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:58 +0000
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+Tech Note 0002
+How to avoid non-intrusive timing attacks with online computations
+Tom St Denis
+A timing attack is when an attacker can observe a side channel of the device (in this case time).  In this tech note
+we consider only non-intrusive timing attacks with respect to online computations.  That is an attacker can
+determine when a computation (such as a public key encryption) begins and ends but cannot observe the device 
+directly.  This is specifically important for applications which transmit data via a public network.
+Consider a Diffie-Hellman encryption which requires the sender to make up a public key "y = g^x mod p".  Libtomcrypt
+uses the MPI bignum library to perform the operation.  The time it takes to compute y is controlled by the number
+of 1 bits in the exponent 'x'.  To a large extent there will be the same number of squaring operations.  "1" bits in
+the exponent require the sender to perform a multiplication.  This means to a certain extent an attacker can 
+determine not only the magnitude of 'x' but the number of one bits.  With this information the attacker cannot directly
+learn the key used.  However, good cryptography mandates the close scrutiny of any practical side channel.
+Similar logic applies to the other various routines.  Fortunately for this case there is a simple solution.  First, 
+determine the maximum time the particular operation can require.  For instance, on an Athlon 1.53Ghz XP processor a
+DH-768 encryption requires roughly 50 milliseconds.  Take that time and round it up.  Now place a delay after the call.
+For example, 
+void demo(void) {
+   clock_t t1;
+   // get initial clock
+   t1 = clock();
+   // some PK function
+   // now delay 
+   while (clock() < (t1 + 100));
+   // transmit data...
+This code has the effect of taking at least 100 ms always.  In effect someone analyzing the traffic will see that the
+operations always take a fixed amount of time.  Since no two platforms are the same this type of fix has not been 
+incorporated into libtomcrypt (nor is it desired for many platforms).  This requires on the developers part to profile
+the code to determine the delays required.
+Note that this "quick" fix has no effect against an intrusive attacker.  For example, power consumption will drop
+significantly in the loop after the operation.  However, this type of fix is more important to secure the user of the 
+application/device.  For example, a user placing an order online won't try to cheat themselves by cracking open their
+device and performing side-channel cryptanalysis.  An attacker over a network might try to use the timing information
+against the user.