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.