Mercurial > dropbear
annotate libtomcrypt/notes/tech0001.txt @ 1306:34e6127ef02e
merge fixes from PuTTY import.c
toint() from misc.c
(revids are from hggit conversion)
changeset: 4620:60a336a6c85c
user: Simon Tatham <[email protected]>
date: Thu Feb 25 20:26:33 2016 +0000
files: import.c
description:
Fix potential segfaults in reading OpenSSH's ASN.1 key format.
The length coming back from ber_read_id_len might have overflowed, so
treat it as potentially negative. Also, while I'm here, accumulate it
inside ber_read_id_len as an unsigned, so as to avoid undefined
behaviour on integer overflow, and toint() it before return.
Thanks to Hanno Böck for spotting this, with the aid of AFL.
(cherry picked from commit 5b7833cd474a24ec098654dcba8cb9509f3bf2c1)
Conflicts:
import.c
(cherry-picker's note: resolving the conflict involved removing an
entire section of the original commit which fixed ECDSA code not
present on this branch)
changeset: 4619:9c6c638d98d8
user: Simon Tatham <[email protected]>
date: Sun Jul 14 10:45:54 2013 +0000
files: import.c ssh.c sshdss.c sshpubk.c sshrsa.c
description:
Tighten up a lot of casts from unsigned to int which are read by one
of the GET_32BIT macros and then used as length fields. Missing bounds
checks against zero have been added, and also I've introduced a helper
function toint() which casts from unsigned to int in such a way as to
avoid C undefined behaviour, since I'm not sure I trust compilers any
more to do the obviously sensible thing.
[originally from svn r9918]
changeset: 4618:3957829f24d3
user: Simon Tatham <[email protected]>
date: Mon Jul 08 22:36:04 2013 +0000
files: import.c sshdss.c sshrsa.c
description:
Add an assortment of extra safety checks.
[originally from svn r9896]
changeset: 4617:2cddee0bce12
user: Jacob Nevins <[email protected]>
date: Wed Dec 07 00:24:45 2005 +0000
files: import.c
description:
Institutional failure to memset() things pointed at rather than pointers.
Things should now be zeroed and memory not leaked. Spotted by Brant Thomsen.
[originally from svn r6476]
changeset: 4616:24ac78a9c71d
user: Simon Tatham <[email protected]>
date: Wed Feb 11 13:58:27 2004 +0000
files: import.c
description:
Jacob's last-minute testing found a couple of trivial bugs in
import.c, and my attempts to reproduce them in cmdgen found another
one there :-)
[originally from svn r3847]
changeset: 4615:088d39a73db0
user: Simon Tatham <[email protected]>
date: Thu Jan 22 18:52:49 2004 +0000
files: import.c
description:
Placate some gcc warnings.
[originally from svn r3761]
changeset: 4614:e4288bad4d93
parent: 1758:108b8924593d
user: Simon Tatham <[email protected]>
date: Fri Oct 03 21:21:23 2003 +0000
files: import.c
description:
My ASN.1 decoder returned wrong IDs for anything above 0x1E! Good
job it's never had to yet. Ahem.
[originally from svn r3479]
author | Matt Johnston <matt@ucc.asn.au> |
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date | Tue, 12 Jul 2016 23:00:01 +0800 |
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1 Tech Note 0001 |
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2 How to Gather Entropy on Embedded Systems |
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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 This tech note explains a relatively simple way to gather entropy for a PRNG (Yarrow in this case) in embedded systems |
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9 where there are few sources of entropy or physical sources. |
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10 |
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11 When trying to setup a secure random number generator a fresh source of random data (entropy) is required to ensure the |
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12 deterministic state of the PRNG is not known or predetermined with respect to an attacker. |
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13 |
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14 At the very least the system requires one timer and one source of un-timed interrupts. by "un-timed" I mean interrupts |
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15 that do not occur at regular intervals [e.g. joypad/keypad input, network packets, etc...]. |
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16 |
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17 First we shall begin by taking an overview of how the Yarrow PRNG works within libtomcrypt. At the heart of all |
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18 PRNGs is the "prng_state" data type. This is a union of structures that hold the PRNG state for the various prngs. The |
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19 first thing we require is a state... |
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20 |
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21 prng_state myPrng; |
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22 |
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23 Next we must initialize the state once to get the ball rolling |
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24 |
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25 if (yarrow_start(&myPrng) != CRYPT_OK) { |
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26 // error should never happen! |
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27 } |
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28 |
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29 At this point the PRNG is ready to accept fresh entropy which is added with |
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30 |
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31 int yarrow_add_entropy(const unsigned char *buf, unsigned long len, prng_state *prng) |
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32 |
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33 This function is **NOT** thread safe which will come under consideration later. To add entropy to our PRNG we must |
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34 call this function with fresh data as its sampled. Lets say we have a timer counter called "uTimer" which is a 32-bit |
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35 long and say a 32-bit joyPad state called "uPad". An example interrupt handler would look like |
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36 |
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37 void joypad_interrupt(...) { |
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38 unsigned char buf[8]; |
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39 |
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40 STORE32L(uTimer, buf); |
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41 STORE32L(uPad, buf+4) |
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42 if (yarrow_add_entropy(buf, 8, &myPrng) != CRYPT_OK) { |
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43 // this should never occur either unless you didn't call yarrow_start |
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44 } |
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45 |
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46 // handle interrupt |
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47 } |
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48 |
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49 In this snippet the timer count and state of the joypad are added together into the entropy pool. The timer is important |
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50 because with respect to the joypad it is a good source of entropy (on its own its not). For example, the probability of |
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51 the user pushing the up arrow is fairly high, but at a specific time is not. |
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52 |
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53 This method doesn't gather alot of entropy and has to be used to for quite a while. One way to speed it up is to tap |
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54 multiple sources. If you have a network adapter and other sources of events (keyboard, mouse, etc...) trapping their |
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55 data is ideal as well. Its important to gather the timer along with the event data. |
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56 |
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57 As mentioned the "yarrow_add_entropy()" function is not thread safe. If your system allows interrupt handlers to be |
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58 interrupted themselves then you could have trouble. One simple way is to detect when an interrupt is in progress and |
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59 simply not add entropy during the call (jump over the yarrow_add_entropy() call) |
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60 |
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61 Once you feel that there has been enough entropy added to the pool then within a single thread you can call |
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62 |
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63 int yarrow_ready(prng_state *prng) |
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64 |
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65 Now the PRNG is ready to read via the |
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66 |
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67 unsigned long yarrow_read(unsigned char *buf, unsigned long len, prng_state *prng) |
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68 |
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69 It is a very good idea that once you call the yarrow_ready() function that you stop harvesting entropy in your interrupt |
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70 functions. This will free up alot of CPU time. Also one more final note. The yarrow_read() function is not thread |
1b9e69c058d2
propagate from branch 'au.asn.ucc.matt.ltc.dropbear' (head 20dccfc09627970a312d77fb41dc2970b62689c3)
Matt Johnston <matt@ucc.asn.au>
parents:
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71 safe either. This means if you have multiple threads or processes that read from it you will have to add your own semaphores |
1b9e69c058d2
propagate from branch 'au.asn.ucc.matt.ltc.dropbear' (head 20dccfc09627970a312d77fb41dc2970b62689c3)
Matt Johnston <matt@ucc.asn.au>
parents:
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72 around calls to it. |
1b9e69c058d2
propagate from branch 'au.asn.ucc.matt.ltc.dropbear' (head 20dccfc09627970a312d77fb41dc2970b62689c3)
Matt Johnston <matt@ucc.asn.au>
parents:
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73 |