bug#47222: Serious bug in Nettle's ecdsa_verify

bug#47222: Serious bug in Nettle's ecdsa_verify

[Mark H Weaver]

FYI...

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From: nisse@lysator.liu.se (Niels Möller)
To: nettle-bugs@lists.lysator.liu.se
Subject: ANNOUNCE: Serious bug in Nettle's ecdsa_verify
Date: Tue, 16 Mar 2021 09:07:56 +0100

I've been made aware of a bug in Nettle's code to verify ECDSA
signatures. Certain signatures result in the ecc point multiply function
being called with out-of-range scalars, which may give incorrect
results, or crash in an assertion failure. It's an old bug, probably
since Nettle's initial implementation of ECDSA.

I've just pushed fixes for ecdsa_verify, as well as a few other cases of
potentially out-of-range scalars, to the master-updates branch. I haven't
fully analysed the implications, but I'll describe my current
understanding.

I think an assertion failure, useful for a denial-of-service attack, is
easy on the curves where the bitsize of q, the group order, is not an
integral number of words. That's secp224r1, on 64-bit platforms, and
secp521r1.

Even when it's not possible to trigger an assertion failure, it's easy
to produce valid-looking input "signatures" that hit out-of range
intermediate scalar values where point multiplication may misbehave.
This applies to all the NIST secp* curves as well as the GOST curves.

To me, it looks very difficult to make it misbehave in such a way that
ecdsa_verify will think an invalid signature is valid, but it might be
possible; further analysis is needed. I will not be able to analyze it
properly now, if anyone else would like to look into it, I can provide a
bit more background.

ed25519 and ed448 may be affected too, but it appears a bit harder to
find inputs that hit out of range values. And since point operations are
inherently more robust on these curves, I think they will produce
correct results as long as they don't hit the assert.

Advise on how to deal best with this? My current plan is to prepare a
3.7.2 bugfix release (from a new bugfix-only branch, without the new
arm64 code). Maybe as soon as tomorrow (Wednesday, european time), or in
the weekend.

Regards,
/Niels

-- 
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Internet email is subject to wholesale government surveillance.

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bug#47222: [Niels Möller] ANNOUNCE: Nettle-3.7.2

[Mark H Weaver]

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-------------------- Start of forwarded message --------------------
From: nisse@lysator.liu.se (Niels Möller)
To: nettle-bugs@lists.lysator.liu.se, info-gnu@gnu.org
Subject: ANNOUNCE: Nettle-3.7.2
Date: Sun, 21 Mar 2021 10:24:11 +0100


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I've prepared a new bug-fix release of Nettle, a low-level
cryptographics library, to fix a serious bug in the function to verify
ECDSA signatures. Implications include an assertion failure, which could
be used for denial-of-service, when verifying signatures on the
secp_224r1 and secp521_r1 curves. More details in NEWS file below.

Upgrading is strongly recomended.

The Nettle home page can be found at
https://www.lysator.liu.se/~nisse/nettle/, and the manual at
https://www.lysator.liu.se/~nisse/nettle/nettle.html.

The release can be downloaded from

  https://ftp.gnu.org/gnu/nettle/nettle-3.7.2.tar.gz
  ftp://ftp.gnu.org/gnu/nettle/nettle-3.7.2.tar.gz
  https://www.lysator.liu.se/~nisse/archive/nettle-3.7.2.tar.gz

Regards,
/Niels

NEWS for the Nettle 3.7.2 release

	This is a bugfix release, fixing a bug in ECDSA signature
	verification that could lead to a denial of service attack
	(via an assertion failure) or possibly incorrect results. It
	also fixes a few related problems where scalars are required
	to be canonically reduced modulo the ECC group order, but in
	fact may be slightly larger.

	Upgrading to the new version is strongly recommended.

	Even when no assert is triggered in ecdsa_verify, ECC point
	multiplication may get invalid intermediate values as input,
	and produce incorrect results. It's trivial to construct
	alleged signatures that result in invalid intermediate values.
	It appears difficult to construct an alleged signature that
	makes the function misbehave in such a way that an invalid
	signature is accepted as valid, but such attacks can't be
	ruled out without further analysis.

	Thanks to Guido Vranken for setting up the fuzzer tests that
	uncovered this problem.

	The new version is intended to be fully source and binary
	compatible with Nettle-3.6. The shared library names are
	libnettle.so.8.3 and libhogweed.so.6.3, with sonames
	libnettle.so.8 and libhogweed.so.6.

	Bug fixes:

	* Fixed bug in ecdsa_verify, and added a corresponding test
          case.

	* Similar fixes to ecc_gostdsa_verify and gostdsa_vko.

	* Similar fixes to eddsa signatures. The problem is less severe
          for these curves, because (i) the potentially out or range
          value is derived from output of a hash function, making it
          harder for the attacker to to hit the narrow range of
          problematic values, and (ii) the ecc operations are
          inherently more robust, and my current understanding is that
          unless the corresponding assert is hit, the verify
          operation should complete with a correct result.

	* Fix to ecdsa_sign, which with a very low probability could
          return out of range signature values, which would be
          rejected immediately by a verifier.

-- 
Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677.
Internet email is subject to wholesale government surveillance.


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bug#47222: Serious bug in Nettle's ecdsa_verify

[Ludovic Courtès]

Hi Niels,

> I've prepared a new bug-fix release of Nettle, a low-level
> cryptographics library, to fix a serious bug in the function to verify
> ECDSA signatures. Implications include an assertion failure, which could
> be used for denial-of-service, when verifying signatures on the
> secp_224r1 and secp521_r1 curves. More details in NEWS file below.
>
> Upgrading is strongly recomended.

Are there plans to make a new 3.5 release including these fixes?
Alternatively, could you provide guidance as to which commits should be
cherry-picked in 3.5 for downstream distros?

I’m asking because in Guix, the easiest way for us to deploy the fixes
on the ‘master’ branch would be by “grafting” a new Nettle variant
ABI-compatible with 3.5.1, which is the one packages currently depend on.

Thanks in advance,
Ludo’.

bug#47222: Serious bug in Nettle's ecdsa_verify

[Niels Möller]

Ludovic Courtès <ludo@gnu.org> writes:

> Are there plans to make a new 3.5 release including these fixes?

No, I don't plan any 3.5.x release.

> Alternatively, could you provide guidance as to which commits should be
> cherry-picked in 3.5 for downstream distros?

Look at the branch release-3.7-fixes
(https://git.lysator.liu.se/nettle/nettle/-/commits/release-3.7-fixes/).
The commits since 3.7.1 are the ones you need.

Changes to gostdsa and ed448 will not apply, since those curves didn't
exist in nettle-3.5. Changes to ed25519 might not apply cleanly, due to
refactoring when adding ed448.

> I’m asking because in Guix, the easiest way for us to deploy the fixes
> on the ‘master’ branch would be by “grafting” a new Nettle variant
> ABI-compatible with 3.5.1, which is the one packages currently depend on.

I still recommend upgrading to the latest version. There were an abi
break in 3.6 (so you'd need to recompile lots of guix packages), but no
incompatible changes to the (source level) api.

Regards,
/Niels

-- 
Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677.
Internet email is subject to wholesale government surveillance.

bug#47222: Serious bug in Nettle's ecdsa_verify

[Leo Famulari]

On Thu, Mar 25, 2021 at 05:21:40PM +0100, Niels Möller wrote:
> Changes to gostdsa and ed448 will not apply, since those curves didn't
> exist in nettle-3.5. Changes to ed25519 might not apply cleanly, due to
> refactoring when adding ed448.

Okay.

> > I’m asking because in Guix, the easiest way for us to deploy the fixes
> > on the ‘master’ branch would be by “grafting” a new Nettle variant
> > ABI-compatible with 3.5.1, which is the one packages currently depend on.
> 
> I still recommend upgrading to the latest version. There were an abi
> break in 3.6 (so you'd need to recompile lots of guix packages), but no
> incompatible changes to the (source level) api.

Unfortunately, non-ABI compatible upgrades of nettle cannot be done
quickly in Guix. As you point out, we'd have to recompile over >10000
packages, and then we'd have to fix any breakage that might occur from
the upgrade.

We will have to try to cherry-pick the bug fix patches.

bug#47222: Serious bug in Nettle's ecdsa_verify

[Ludovic Courtès]

Hi!

(- Niels, - nettle-bugs)

nisse@lysator.liu.se (Niels Möller) skribis:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>> Are there plans to make a new 3.5 release including these fixes?
>
> No, I don't plan any 3.5.x release.
>
>> Alternatively, could you provide guidance as to which commits should be
>> cherry-picked in 3.5 for downstream distros?
>
> Look at the branch release-3.7-fixes
> (https://git.lysator.liu.se/nettle/nettle/-/commits/release-3.7-fixes/).
> The commits since 3.7.1 are the ones you need.
>
> Changes to gostdsa and ed448 will not apply, since those curves didn't
> exist in nettle-3.5. Changes to ed25519 might not apply cleanly, due to
> refactoring when adding ed448.

I confirm these patches don’t apply, and I’m not comfortable fiddling
with that.

Leo and I checked and found that Debian doesn’t have 3.5.  Do other
distros have backports of these patches to 3.5?

If not, our options are:

  1. to invest in the backport ourselves, with good peer review, ideally
     getting it stamped by Niels & co;

  2. to wait until a full rebuild has come.

It’s not an ideal situation.  Thoughts?

Ludo’.

bug#47222: Serious bug in Nettle's ecdsa_verify

[Léo Le Bouter via Bug reports for GNU Guix]

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I am no expert cryptographer, it is likely that if I try backporting
such patches I will get something wrong that introduces more flaws.

https://security-tracker.debian.org/tracker/CVE-2021-20305 - no patch
backported yet
https://packages.ubuntu.com/source/focal/nettle - no patch backported
either

It would be best if Nettle adopted a forever (or almost) backwards
compatible ABI from now on like curl (https://curl.se/libcurl/abi.html)
so that such things don't happen again.

Thank you,
Léo

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bug#47222:

[paren--- via Bug reports for GNU Guix]

We now have nettle 3.7.3, so this isn't an issue anymore. Closing.

    -- (