mirror of
http://galexander.org/git/simplesshd.git
synced 2024-11-15 19:48:56 +00:00
74 lines
3.5 KiB
Plaintext
74 lines
3.5 KiB
Plaintext
Tech Note 0001
|
|
How to Gather Entropy on Embedded Systems
|
|
Tom St Denis
|
|
|
|
Introduction
|
|
------------
|
|
|
|
This tech note explains a relatively simple way to gather entropy for a PRNG (Yarrow in this case) in embedded systems
|
|
where there are few sources of entropy or physical sources.
|
|
|
|
When trying to setup a secure random number generator a fresh source of random data (entropy) is required to ensure the
|
|
deterministic state of the PRNG is not known or predetermined with respect to an attacker.
|
|
|
|
At the very least the system requires one timer and one source of un-timed interrupts. by "un-timed" I mean interrupts
|
|
that do not occur at regular intervals [e.g. joypad/keypad input, network packets, etc...].
|
|
|
|
First we shall begin by taking an overview of how the Yarrow PRNG works within libtomcrypt. At the heart of all
|
|
PRNGs is the "prng_state" data type. This is a union of structures that hold the PRNG state for the various prngs. The
|
|
first thing we require is a state...
|
|
|
|
prng_state myPrng;
|
|
|
|
Next we must initialize the state once to get the ball rolling
|
|
|
|
if (yarrow_start(&myPrng) != CRYPT_OK) {
|
|
// error should never happen!
|
|
}
|
|
|
|
At this point the PRNG is ready to accept fresh entropy which is added with
|
|
|
|
int yarrow_add_entropy(const unsigned char *buf, unsigned long len, prng_state *prng)
|
|
|
|
This function is **NOT** thread safe which will come under consideration later. To add entropy to our PRNG we must
|
|
call this function with fresh data as its sampled. Lets say we have a timer counter called "uTimer" which is a 32-bit
|
|
long and say a 32-bit joyPad state called "uPad". An example interrupt handler would look like
|
|
|
|
void joypad_interrupt(...) {
|
|
unsigned char buf[8];
|
|
|
|
STORE32L(uTimer, buf);
|
|
STORE32L(uPad, buf+4)
|
|
if (yarrow_add_entropy(buf, 8, &myPrng) != CRYPT_OK) {
|
|
// this should never occur either unless you didn't call yarrow_start
|
|
}
|
|
|
|
// handle interrupt
|
|
}
|
|
|
|
In this snippet the timer count and state of the joypad are added together into the entropy pool. The timer is important
|
|
because with respect to the joypad it is a good source of entropy (on its own its not). For example, the probability of
|
|
the user pushing the up arrow is fairly high, but at a specific time is not.
|
|
|
|
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
|
|
multiple sources. If you have a network adapter and other sources of events (keyboard, mouse, etc...) trapping their
|
|
data is ideal as well. Its important to gather the timer along with the event data.
|
|
|
|
As mentioned the "yarrow_add_entropy()" function is not thread safe. If your system allows interrupt handlers to be
|
|
interrupted themselves then you could have trouble. One simple way is to detect when an interrupt is in progress and
|
|
simply not add entropy during the call (jump over the yarrow_add_entropy() call)
|
|
|
|
Once you feel that there has been enough entropy added to the pool then within a single thread you can call
|
|
|
|
int yarrow_ready(prng_state *prng)
|
|
|
|
Now the PRNG is ready to read via the
|
|
|
|
unsigned long yarrow_read(unsigned char *buf, unsigned long len, prng_state *prng)
|
|
|
|
It is a very good idea that once you call the yarrow_ready() function that you stop harvesting entropy in your interrupt
|
|
functions. This will free up alot of CPU time. Also one more final note. The yarrow_read() function is not thread
|
|
safe either. This means if you have multiple threads or processes that read from it you will have to add your own semaphores
|
|
around calls to it.
|
|
|