OK, so I said I'd get some notes on the environment within which IoT crypto
has to function, here's what the peanut gallery came up with. A lot of this
isn't my own work and I don't claim it to be, it's a collaboration created by
people who for various business/legal reasons can't attach their names to
public comments. Note that every one of the for-instances given below are
actual real-life examples, not something someone invented to make a good
story.
Peter.
IoT Crypto, What you Need to Know
---------------------------------
The problem we have is not how to get stronger crypto in place, it's how to
get more crypto in place.
-- Ian Grigg, 28 August 2016.
... and to raise the level of security of the rest of the system so that
attackers are actually forced to target the crypto rather than just
strolling around it.
-- Peter Gutmann, in corollary.
The device may be operating under severe power constraints. There are IoT
devices that need to run for several years on a single battery pack. If
you're lucky, it's a bundle of 18650s. If you're less lucky, it's a CR2032.
Renegotiating protocol state on every wake event is incompatible with low
power consumption. The crypto should be able to resume from a pause an
arbitrary amount of time later.
Even if the device is constantly powered, many components will be powered only
when needed, and will lose state when powered off (they work by warm-starting
very quickly rather than saving state across restarts).
Some IoT chips can't cost more than a few cents each. If you're lucky,
they're allowed to cost tens of cents. Any fancy crypto hardware will break
the budget.
When crypto hardware support is available, it's universally AES, occasionally
SHA-1 and/or DES, and very rarely RSA and/or DH and/or ECDSA (there are also
oddballs like ones that do SHA-1 but not AES, but they're pretty special
cases, and AES in software is very efficient in any case). Any crypto had
therefore better be based mostly, or exclusively, around AES. As a convenient
side-effect of this, you won't have to worry about which flavour of PKC will
be in fashion in ten years' time, or what keysize they're wearing in Paris
that year.
Even if the device includes crypto hardware, the HAL or vendor-supplied
firmware may not make it available. In addition the crypto engine will be in
a separate IP core that's effectively an external peripheral, and accessing it
is so painful that it's quicker to do it in software (you never get AES
instructions, you get something that you talk to via PIO). As a result you
have to run your crypto in software while the crypto hardware sits idle.
Devices have a design lifetime of ten to twenty years, possibly more. There
is hardware deployed today that was designed when the people now maintaining
it were in kindergarten.
Firmware is never updated, and frequently *can* never be updated. This is
typically because it's not writeable, or there's no room (the code already
occupies 120% of available storage, brought down to 100% by replacing the
certificate-handling code with a memcpy() for encoding and a seek + read of {
n, e } for decoding, see below), leaving 0% available for firmware updates.
Alternatively, there's no connectivity to anything to provide updates, either
of firmware or anything else (for example in one globally-deployed system the
CRL arrives once every 6-12 months via sneakernet, although I'm not sure why
they use CRLs since they can just disable the certificate or device
centrally). Or the device, once approved and operational, can't ever be
changed. Like children, make one mistake here and you have to live with it
for the next 15-20 years.
Even if the hardware and/or firmware could be updated, the rest of the
infrastructure often can't. Some firmware needs to be built with a guaranteed
correspondence between the source code and the binary. This means not only
using approved compilers from the late 1990s that cleanly translate the code
without using any tricks or fancy optimisations, but also scouring eBay for
the appropriate late-1990s hardware because it's not guaranteed that the
compiler running on current CPUs will produce the same result.
Don't bother asking "have you thought about using $shiny_new_thing from
$vendor" (or its closely-related overgeneralisation "Moore's Law means that
real soon now infinite CPU/memory/crypto will be available to anyone for
free"). They're already aware of $shiny_new_thing, $shiny_other_thing, and
$shiny_thing_you_havent_even_heard_of_yet, but aren't about to redo their
entire hardware design, software toolchain, BSP, system firmware,
certification, licensing, and product roadmap for any of them, no matter how
shiny they are.
The device may have no or only inadequate entropy sources. Alternatively, if
there is an entropy source, it may lose state when it's powered off (see the
earlier comment on power management), requiring it to perform a time-consuming
entropy collection step before it can be used. Since this can trigger the
watchdog (see the comment further down), it'll end up not being used. Any
crypto protocol should therefore allow the entropy used in it to be injected
by both parties like TLS' client and server random values, because one party
may not have any entropy to inject. In addition, it's best to prefer
algorithms that aren't dependent on high-quality randomness (ECDSA is a prime
example of something that fails catastrophically when there are problems with
randomness).
Many SoCs have different portions developed by different vendors, and the only
way to communicate between them is via predefined APIs. If you need entropy
for your crypto and the entropy source is on a separate piece of IP that
doesn't provide an entropy-extraction interface, you either need to spend
twelve months negotiating access to the source, and pay handsomely for the
privilege, or do without.
(The following is a special case that only applies to very constrained
devices: As a variant of the above, there may be no accessible writeable non-
volatile memory on your section of the device. Storing a seed for crypto keys
may work when you bake it into the firmware, but you can't update it once the
firmware is installed because there's no access to writeable nonvolatile
memory, unless you negotiate it with one of the vendors whose IP has access to
it).
Fuses are expensive and per-device provisioning is prohibitively expensive for
low-cost IoT chips.
As mentioned previously, certificates are handled by memcpy()ing a pre-encoded
certificate blob to the output and seeking to the appropriate location in an
incoming certificate and extracting { n, e } (note that that's { n, e }, not
{ p, q, g, y } or { p, a, b, G, n, ... }). If you've ever wondered why you
can feed a device an expired, digital-signature-only certificate and use it
for encryption, this is why (but see also the point on error handling below).
This is precisely what you get when you take a hardware spec targeted at
Cortex M0s, Coldfire's, AVRs, and MSP430s, and write a security spec that
requires the use of a PKI.
The whole device may be implemented as a single event-driven loop, with no
processes, threads, or different address spaces. In addition there are hard
real-time constraints on processing. You can't go off and do ECC or RSA or DH
and halt all other processing while you do so because the system watchdog will
hard-reset the CPU if you spend too long on something. While it is possible,
with some effort, to write a manually-timesliced modmult implementation, the
result is horribly inefficient and a goldmine of timing channels. It's also
painful to implement, and a specific implementation is completely tied to a
particular CPU architecture and clock speed.
MSP 430s. Apologies to all the embedded devs who have just gone into
anaphylactic shock at the mention of that name. There are billions of these
things in active use. For a very recent one (August 2016), look at
http://www.ti.com/lit/ds/slase78a/slase78a.pdf.
Hardware-wise, a Raspberry Pi is a desktop PC, not an embedded device. So is
an Alix APU, a BeagleBoard, a SheevaPlug, a CI20, a CubieBoard, a C.H.I.P, and
any number of similar things that people like to cite as examples of IoT
devices.
In general terms, errors are divided into two classes, recoverable and
nonrecoverable (this really is generalising a lot in order to avoid writing a
small essay). Recoverable errors are typically handled by trying to find a
way to continue, possibly in slightly degraded form. Non-recoverable errors
are typically handled by forcing a hard-fault, which restarts the system in a
known-good state. For example one system that uses the event-loop model has,
sprinkled throughout the code, "if ( errType == FATAL ) while ( 55 );" (the 55
has no special significance, it's just a way to force an endless loop, which
causes the watchdog to reset the system). An expired certificate or incorrect
key usage is a soft error for which the appropriate handling action is to
continue, so there's no point in even checking for it (see a previous point on
the lack of checking for this sort of thing).
(The equivalent in standard PCs - which includes tablets, phones, and other
devices - is to dump responsibility on the user, popping up a dialog that they
have to click past in order to continue, but at least now it's the user's
fault and not the developer's. Embedded systems developers don't have the
luxury of doing this but have to explicitly manage these types of error
conditions themselves. So when a protocol spec says SHOULD NOT or MUST NOT
then for standard PCs it means "throw up a warning/error dialog and blame the
user if they continue" and for embedded devices it means "continue if
possible". Have you ever seen a security spec of any kind that tells you what
step to take next when a problem occurs?).
Development will be done by embedded systems engineers who are good at making
things work in tough environments but aren't crypto experts. In addition,
portions of the device won't always work as they should. Any crypto used had
better be able to take a huge amount of of abuse without failing. AES-CBC,
even in the worst-case scenario of a constant, all-zero IV, at worst degrades
to AES-ECB. AES-GCM (and related modes like AES-CTR), on the other hand, fail
completely for both confidentiality and integrity protection. And you don't
even want to think about all the ways ECDSA can fail (see, for example, the
issues with entropy, and timing issues, above).
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