nettle-3.4.1

[Bruce Dubbs]

New point version.

[Bruce Dubbs]

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 recommended.

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.

Miscellaneous:

• 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 libnettle.so.6.5 and libhogweed.so.4.5, with sonames still libnettle.so.6 and libhogweed.so.4. It is intended to be fully binary compatible with nettle-3.1.

[Bruce Dubbs]

Fixed at revision 20787.