74 lines
3.5 KiB
Plaintext
74 lines
3.5 KiB
Plaintext
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Tech Note 0001
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How to Gather Entropy on Embedded Systems
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Tom St Denis
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Introduction
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------------
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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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where there are few sources of entropy or physical sources.
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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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deterministic state of the PRNG is not known or predetermined with respect to an attacker.
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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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that do not occur at regular intervals [e.g. joypad/keypad input, network packets, etc...].
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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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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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first thing we require is a state...
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prng_state myPrng;
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Next we must initialize the state once to get the ball rolling
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if (yarrow_start(&myPrng) != CRYPT_OK) {
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// error should never happen!
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}
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At this point the PRNG is ready to accept fresh entropy which is added with
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int yarrow_add_entropy(const unsigned char *buf, unsigned long len, prng_state *prng)
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This function is **NOT** thread safe which will come under consideration later. To add entropy to our PRNG we must
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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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long and say a 32-bit joyPad state called "uPad". An example interrupt handler would look like
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void joypad_interrupt(...) {
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unsigned char buf[8];
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STORE32L(uTimer, buf);
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STORE32L(uPad, buf+4)
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if (yarrow_add_entropy(buf, 8, &myPrng) != CRYPT_OK) {
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// this should never occur either unless you didn't call yarrow_start
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}
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// handle interrupt
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}
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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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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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the user pushing the up arrow is fairly high, but at a specific time is not.
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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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multiple sources. If you have a network adapter and other sources of events (keyboard, mouse, etc...) trapping their
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data is ideal as well. Its important to gather the timer along with the event data.
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As mentioned the "yarrow_add_entropy()" function is not thread safe. If your system allows interrupt handlers to be
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interrupted themselves then you could have trouble. One simple way is to detect when an interrupt is in progress and
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simply not add entropy during the call (jump over the yarrow_add_entropy() call)
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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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int yarrow_ready(prng_state *prng)
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Now the PRNG is ready to read via the
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unsigned long yarrow_read(unsigned char *buf, unsigned long len, prng_state *prng)
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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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functions. This will free up alot of CPU time. Also one more final note. The yarrow_read() function is not thread
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safe either. This means if you have multiple threads or processes that read from it you will have to add your own semaphores
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around calls to it.
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