Mercurial > dropbear
annotate libtomcrypt/notes/tech0002.txt @ 1369:ddfcadca3c4c fuzz
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author | Matt Johnston <matt@ucc.asn.au> |
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date | Tue, 23 May 2017 22:43:34 +0800 |
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1 Tech Note 0002 |
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2 How to avoid non-intrusive timing attacks with online computations |
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3 Tom St Denis |
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4 |
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5 Introduction |
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6 ------------ |
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7 |
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8 A timing attack is when an attacker can observe a side channel of the device (in this case time). In this tech note |
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9 we consider only non-intrusive timing attacks with respect to online computations. That is an attacker can |
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10 determine when a computation (such as a public key encryption) begins and ends but cannot observe the device |
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11 directly. This is specifically important for applications which transmit data via a public network. |
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12 |
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13 Consider a Diffie-Hellman encryption which requires the sender to make up a public key "y = g^x mod p". Libtomcrypt |
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14 uses the MPI bignum library to perform the operation. The time it takes to compute y is controlled by the number |
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15 of 1 bits in the exponent 'x'. To a large extent there will be the same number of squaring operations. "1" bits in |
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16 the exponent require the sender to perform a multiplication. This means to a certain extent an attacker can |
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17 determine not only the magnitude of 'x' but the number of one bits. With this information the attacker cannot directly |
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18 learn the key used. However, good cryptography mandates the close scrutiny of any practical side channel. |
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19 |
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20 Similar logic applies to the other various routines. Fortunately for this case there is a simple solution. First, |
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21 determine the maximum time the particular operation can require. For instance, on an Athlon 1.53Ghz XP processor a |
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22 DH-768 encryption requires roughly 50 milliseconds. Take that time and round it up. Now place a delay after the call. |
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23 |
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24 For example, |
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25 |
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26 void demo(void) { |
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27 clock_t t1; |
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28 |
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29 // get initial clock |
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30 t1 = clock(); |
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31 |
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32 // some PK function |
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33 |
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34 // now delay |
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35 while (clock() < (t1 + 100)); |
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36 |
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37 // transmit data... |
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38 |
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39 } |
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40 |
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41 This code has the effect of taking at least 100 ms always. In effect someone analyzing the traffic will see that the |
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42 operations always take a fixed amount of time. Since no two platforms are the same this type of fix has not been |
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43 incorporated into libtomcrypt (nor is it desired for many platforms). This requires on the developers part to profile |
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44 the code to determine the delays required. |
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45 |
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46 Note that this "quick" fix has no effect against an intrusive attacker. For example, power consumption will drop |
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47 significantly in the loop after the operation. However, this type of fix is more important to secure the user of the |
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48 application/device. For example, a user placing an order online won't try to cheat themselves by cracking open their |
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49 device and performing side-channel cryptanalysis. An attacker over a network might try to use the timing information |
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50 against the user. |
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51 |
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52 |