Now ntru.c contains just the NTRU business, and kex-hybrid.c contains
the system for running a post-quantum and a classical KEX and hashing
together the results. In between them is a new small vtable API for
the key encapsulation mechanisms that the post-quantum standardisation
effort seems to be settling on.
This fixes a vulnerability that compromises NIST P521 ECDSA keys when
they are used with PuTTY's existing DSA nonce generation code. The
vulnerability has been assigned the identifier CVE-2024-31497.
PuTTY has been doing its DSA signing deterministically for literally
as long as it's been doing it at all, because I didn't trust Windows's
entropy generation. Deterministic nonce generation was introduced in
commit d345ebc2a5, as part of the initial version of our DSA
signing routine. At the time, there was no standard for how to do it,
so we had to think up the details of our system ourselves, with some
help from the Cambridge University computer security group.
More than ten years later, RFC 6979 was published, recommending a
similar system for general use, naturally with all the details
different. We didn't switch over to doing it that way, because we had
a scheme in place already, and as far as I could see, the differences
were not security-critical - just the normal sort of variation you
expect when any two people design a protocol component of this kind
independently.
As far as I know, the _structure_ of our scheme is still perfectly
fine, in terms of what data gets hashed, how many times, and how the
hash output is converted into a nonce. But the weak spot is the choice
of hash function: inside our dsa_gen_k() function, we generate 512
bits of random data using SHA-512, and then reduce that to the output
range by modular reduction, regardless of what signature algorithm
we're generating a nonce for.
In the original use case, this introduced a theoretical bias (the
output size is an odd prime, which doesn't evenly divide the space of
2^512 possible inputs to the reduction), but the theory was that since
integer DSA uses a modulus prime only 160 bits long (being based on
SHA-1, at least in the form that SSH uses it), the bias would be too
small to be detectable, let alone exploitable.
Then we reused the same function for NIST-style ECDSA, when it
arrived. This is fine for the P256 curve, and even P384. But in P521,
the order of the base point is _greater_ than 2^512, so when we
generate a 512-bit number and reduce it, the reduction never makes any
difference, and our output nonces are all in the first 2^512 elements
of the range of about 2^521. So this _does_ introduce a significant
bias in the nonces, compared to the ideal of uniformly random
distribution over the whole range. And it's been recently discovered
that a bias of this kind is sufficient to expose private keys, given a
manageably small number of signatures to work from.
(Incidentally, none of this affects Ed25519. The spec for that system
includes its own idea of how you should do deterministic nonce
generation - completely different again, naturally - and we did it
that way rather than our way, so that we could use the existing test
vectors.)
The simplest fix would be to patch our existing nonce generator to use
a longer hash, or concatenate a couple of SHA-512 hashes, or something
similar. But I think a more robust approach is to switch it out
completely for what is now the standard system. The main reason why I
prefer that is that the standard system comes with test vectors, which
adds a lot of confidence that I haven't made some other mistake in
following my own design.
So here's a commit that adds an implementation of RFC 6979, and
removes the old dsa_gen_k() function. Tests are added based on the
RFC's appendix of test vectors (as many as are compatible with the
more limited API of PuTTY's crypto code, e.g. we lack support for the
NIST P192 curve, or for doing integer DSA with many different hash
functions). One existing test changes its expected outputs, namely the
one that has a sample key pair and signature for every key algorithm
we support.
In some compilers (I'm told clang 10, in particular), the NEON
intrinsic vaddq_p128 is missing, even though its input type poly128_t
is provided.
vaddq_p128 is just an XOR of two vector registers, so that's easy to
work around by casting to a more mundane type and back. Added a
configure-time test for that intrinsic, and a workaround to be used in
its absence.
I only recently found out that OpenSSH defined their own protocol IDs
for AES-GCM, defined to work the same as the standard ones except that
they fixed the semantics for how you select the linked cipher+MAC pair
during key exchange.
(RFC 5647 defines protocol ids for AES-GCM in both the cipher and MAC
namespaces, and requires that you MUST select both or neither - but
this contradicts the selection policy set out in the base SSH RFCs,
and there's no discussion of how you resolve a conflict between them!
OpenSSH's answer is to do it the same way ChaCha20-Poly1305 works,
because that will ensure the two suites don't fight.)
People do occasionally ask us for this linked cipher/MAC pair, and now
I know it's actually feasible, I've implemented it, including a pair
of vector implementations for x86 and Arm using their respective
architecture extensions for multiplying polynomials over GF(2).
Unlike ChaCha20-Poly1305, I've kept the cipher and MAC implementations
in separate objects, with an arm's-length link between them that the
MAC uses when it needs to encrypt single cipher blocks to use as the
inputs to the MAC algorithm. That enables the cipher and the MAC to be
independently selected from their hardware-accelerated versions, just
in case someone runs on a system that has polynomial multiplication
instructions but not AES acceleration, or vice versa.
There's a fourth implementation of the GCM MAC, which is a pure
software implementation of the same algorithm used in the vectorised
versions. It's too slow to use live, but I've kept it in the code for
future testing needs, and because it's a convenient place to dump my
design comments.
The vectorised implementations are fairly crude as far as optimisation
goes. I'm sure serious x86 _or_ Arm optimisation engineers would look
at them and laugh. But GCM is a fast MAC compared to HMAC-SHA-256
(indeed compared to HMAC-anything-at-all), so it should at least be
good enough to use. And we've got a working version with some tests
now, so if someone else wants to improve them, they can.
This commit is groundwork for full certificate support, but doesn't
complete the job by itself. It introduces the new key types, and adds
a test in cryptsuite ensuring they work as expected, but nothing else.
If you manually construct a PPK file for one of the new key types, so
that it has a certificate in the public key field, then this commit
enables PuTTY to present that key to a server for user authentication,
either directly or via Pageant storing and using it. But I haven't yet
provided any mechanism for making such a PPK, so by itself, this isn't
much use.
Also, these new key types are not yet included in the KEXINIT host
keys list, because if they were, they'd just be treated as normal host
keys, in that you'd be asked to manually confirm the SSH fingerprint
of the certificate. I'll enable them for host keys once I add the
missing pieces.
This consists of DJB's 'Streamlined NTRU Prime' quantum-resistant
cryptosystem, currently in round 3 of the NIST post-quantum key
exchange competition; it's run in parallel with ordinary Curve25519,
and generates a shared secret combining the output of both systems.
(Hence, even if you don't trust this newfangled NTRU Prime thing at
all, it's at least no _less_ secure than the kex you were using
already.)
As the OpenSSH developers point out, key exchange is the most urgent
thing to make quantum-resistant, even before working quantum computers
big enough to break crypto become available, because a break of the
kex algorithm can be applied retroactively to recordings of your past
sessions. By contrast, authentication is a real-time protocol, and can
only be broken by a quantum computer if there's one available to
attack you _already_.
I've implemented both sides of the mechanism, so that PuTTY and Uppity
both support it. In my initial testing, the two sides can both
interoperate with the appropriate half of OpenSSH, and also (of
course, but it would be embarrassing to mess it up) with each other.
After this change, the cmake setup now works even on Debian stretch
(oldoldstable), which runs cmake 3.7.
In order to support a version that early I had to:
- write a fallback implementation of 'add_compile_definitions' for
older cmakes, which is easy, because add_compile_definitions(FOO)
is basically just add_compile_options(-DFOO)
- stop using list(TRANSFORM) and string(JOIN), of which I had one
case each, and they were easily replaced with simple foreach loops
- stop putting OBJECT libraries in the target_link_libraries command
for executable targets, in favour of adding $<TARGET_OBJECTS:foo>
to the main sources list for the same target. That matches what I
do with library targets, so it's probably more sensible anyway.
I tried going back by another Debian release and getting this cmake
setup to work on jessie, but that runs CMake 3.0.1, and in _that_
version of cmake the target_sources command is missing, and I didn't
find any alternative way to add extra sources to a target after having
first declared it. Reorganising to cope with _that_ omission would be
too much upheaval without a very good reason.
When preparing commit fca13a17b1, I redesigned the cmake test
function at the last minute, and apparently didn't quite get all the
call sites correctly rewritten. This one still omitted some of the
argument-type keywords, and had an obsolete parameter giving an
explicit name for a sub-library, which I later decided wasn't needed.
This applies to all of AES, SHA-1, SHA-256 and SHA-512. All those
source files previously contained multiple implementations of the
algorithm, enabled or disabled by ifdefs detecting whether they would
work on a given compiler. And in order to get advanced machine
instructions like AES-NI or NEON crypto into the output file when the
compile flags hadn't enabled them, we had to do nasty stuff with
compiler-specific pragmas or attributes.
Now we can do the detection at cmake time, and enable advanced
instructions in the more sensible way, by compile-time flags. So I've
broken up each of these modules into lots of sub-pieces: a file called
(e.g.) 'foo-common.c' containing common definitions across all
implementations (such as round constants), one called 'foo-select.c'
containing the top-level vtable(s), and a separate file for each
implementation exporting just the vtable(s) for that implementation.
One advantage of this is that it depends a lot less on compiler-
specific bodgery. My particular least favourite part of the previous
setup was the part where I had to _manually_ define some Arm ACLE
feature macros before including <arm_neon.h>, so that it would define
the intrinsics I wanted. Now I'm enabling interesting architecture
features in the normal way, on the compiler command line, there's no
need for that kind of trick: the right feature macros are already
defined and <arm_neon.h> does the right thing.
Another change in this reorganisation is that I've stopped assuming
there's just one hardware implementation per platform. Previously, the
accelerated vtables were called things like sha256_hw, and varied
between FOO-NI and NEON depending on platform; and the selection code
would simply ask 'is hw available? if so, use hw, else sw'. Now, each
HW acceleration strategy names its vtable its own way, and the
selection vtable has a whole list of possibilities to iterate over
looking for a supported one. So if someone feels like writing a second
accelerated implementation of something for a given platform - for
example, I've heard you can use plain NEON to speed up AES somewhat
even without the crypto extension - then it will now have somewhere to
drop in alongside the existing ones.
Similarly to 'utils', I've moved all the stuff in the crypto
build-time library into a source directory of its own, and while I'm
at it, split up the monolithic sshauxcrypt.c into its various
unrelated parts.
This is also an opportunity to remove the annoying 'ssh' prefix from
the front of the file names, and give several of them less cryptic
names.
