ANNOUNCE: Nettle-3.4.1

Niels Möller nisse at
Tue Dec 4 23:14:00 CET 2018

I'm happy to announce a new release of GNU Nettle, a low-level
cryptographics library. The main change in this release is that RSA
private key operations are now side-channel silent, thanks to
contributions by Simo Sorce, at Red Hat Inc. The release also includes a
few smaller bugfixes.

The Nettle home page can be found at, and the manual at

The release can be downloaded from

There are no code changes since the release candidate announced on the
Nettle mailing list November 30.

Release timing is prompted by the publication of
Nettle and GnuTLS authors (as well as developers of other TLS
implementations) were notified by the research team a few months ago.
Related CVE ids:

  CVE-2018-16868 gnutls: Bleichenbacher-like side channel leakage in
  PKCS#1 1.5 verification and padding oracle verification

  CVE-2018-16869 nettle: Leaky data conversion exposing a manager oracle

For Nettle, the RSA code, which was written some 15 years ago, have seen
an overhawl. Not only making the handling of PKCS#1 on decryption
side-channel silent (the vulnerabilities that could be exploited by the
methods of the above paper), but also ensuring that underlying bignum
arithmetic uses side-channel silent functions.

The attack directly affects RSA decryption, not signatures. And it
requires some resources to be pulled off. As far as I understand it, a
successful attack lets the attacker decrypt or sign a targeted message,
e.g., compromising the TLS "premaster secret" of a particular session,
corresponding session keys, and any transmitted passwords or login
cookies supposedly protected by those session keys, but it does not
expose the private key itself.

Upgrading the Nettle and GnuTLS libraries is recommended. If you operate
a TLS server, you should consider if you can completely disable key
exchange based on RSA decryption. If you need to keep it for backwards
compatibility, it is *strongly* encouraged to use a separate RSA key for
this purpose, *not* reused or authorized for any other purpose.


NEWS for the Nettle 3.4.1 release

	This release fixes a few bugs, and makes the RSA private key
	operations side channel silent. The RSA improvements are
	contributed by Simo Sorce and Red Hat, and include one new
	public function, rsa_sec_decrypt, see below.

	All functions using RSA private keys are now side-channel
	silent, meaning that they try hard to avoid any branches or
	memory accesses depending on secret data. This applies both to
	the bignum calculations, which now use GMP's mpn_sec_* family
	of functions, and the processing of PKCS#1 padding needed for
	RSA decryption.

	Nettle's ECC functions were already side-channel silent, while
	the DSA functions still aren't. There's also one caveat
	regarding the improved RSA functions: due to small table
	lookups in relevant mpn_sec_* functions in GMP-6.1.2, the
	lowest and highest few bits of the secret factors p and q may
	still leak. I'm not aware of any attacks on RSA where knowing
	a few bits of the factors makes a significant difference. This
	leak will likely be plugged in later GMP versions.

	Changes in behavior:

	* The functions rsa_decrypt and rsa_decrypt_tr may now clobber
	  all of the provided message buffer, independent of the
	  actual message length. They are side-channel silent, in that
	  branches and memory accesses don't depend on the validity or
	  length of the message. Side-channel leakage from the
	  caller's use of length and return value may still provide an
	  oracle useable for a Bleichenbacher-style chosen ciphertext
	  attack. Which is why the new function rsa_sec_decrypt is

	New features:

	* A new function rsa_sec_decrypt. It differs from
	  rsa_decrypt_tr in that the length of the decrypted message
	  is given a priori, and PKCS#1 padding indicating a different
	  length is treated as an error. For applications that may be
	  subject to chosen ciphertext attacks, it is recommended to
	  initialize the message area with random data, call this
	  function, and ignore the return value. This applies in
	  particular to RSA-based key exchange in the TLS protocol.

	Bug fixes:

	* Fix bug in pkcs1-conv, missing break statements in the
	  parsing of PEM input files.

	* Fix link error on the pss-mgf1-test test, affecting builds
	  without public key support.

	Performance regression:

	* All RSA private key operations employing RSA blinding, i.e.,
	  rsa_decrypt_tr, rsa_*_sign_tr, the new rsa_sec_decrypt, and
	  rsa_compute_root_tr, are significantly slower. This is
	  because (i) RSA blinding now use side-channel silent
	  operations, (ii) blinding includes a modular inversion, and
	  (iii) side-channel silent modular inversion, implemented as
	  mpn_sec_invert, is very expensive. A 60% slowdown for
	  2048-bit RSA keys have been measured.


	* Building the public key support of nettle now requires GMP
	  version 6.0 or later (unless --enable-mini-gmp is used).

	The shared library names are and, with sonames still and It is intended to be fully binary compatible
	with nettle-3.1.

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

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