Michael Rash, Security Researcher

Software Release: fwknop-2.5 with HMAC Support

fwknop-2.5 with HMAC support After a long development cycle started over a year ago that has focused on how fwknop uses cryptography, the 2.5 release of fwknop is available for download. This release now includes support for HMAC authenticated encryption, with SHA-256 being the default digest algorithm though others such as SHA-512 are supported as well. The HMAC mode can be applied to SPA packets that have been encrypted with either Rijndael or GnuPG, and the order of operation is always encrypt-then-authenticate which is considered to be the most secure option among all possible orders. Not only does using the new HMAC mode provide a cryptographically strong authentication step for SPA communications, it also affords a significant security benefit because maliciously constructed SPA packets can be discarded before they are even sent through decryption routines. I.e. HMAC verification is a much more simplistic operation than decryption, and therefore generally less prone to programming bugs and potential security vulnerabilities.

There are many other enhancements in fwknop-2.5 as well such as usage of the Coverity static analyzer, a new ~/.fwknoprc stanza saving feature for fwknop client usage simplification, support for automatic Rijndael+HMAC key generation with the --key-gen option, many test suite improvements, an updated tutorial, and more. There is a robust roadmap for fwknop, and new releases will come faster now that a solid foundation is made upon HMAC authenticated encryption for SPA packets.

I wish to thank all who contributed to this effort - particularly Damien Stuart, Franck Joncourt, Blair Zajac, Michael T. Dean, and Ryman. Additional contributors are listed in the git history.

NOTE: If you are upgrading from a previous version of fwknop you will want to read the following information on backwards compatibility. In short, fwknop-2.5 is compatible with prior versions, but it requires one configuration tweak to either the client (add "-M legacy" to the command line) or server config (add "ENCRYPTION_MODE legacy" to each stanza in the access.conf file) if you wish to run a mixed environment of older clients and/or older servers. The reason for the incompatibility is that prior to 2.5, fwknop was not properly using PBKDF1 for Rijndael key derivation - this has been fixed.

Here is the complete ChangeLog for fwknop-2.5:

  • ***** IMPORTANT *****: If you are upgrading from an older version of fwknop, you will want to read the "Backwards Compatibility" section of the fwknop tutorial available here:

    In summary, it is possible to have a mixed environment of fwknop-2.5 clients and/or servers with older client and/or servers, but this requires some configuration in order to work properly. On the server side, the directive "ENCRYPTION_MODE legacy" will need to be added to every access.conf stanza that uses Rijndael and that needs to support SPA packets from pre-2.5 clients. On the client side when generating Rijndael-encrypted SPA packets from a pre-2.5 server, the command line argument "-M legacy" will need to be given. GnuPG operations are not affected however and don't require the above steps whenever the new HMAC authenticated encryption feature (offered in fwknop-2.5) is not used.
  • Major release of new functionality - HMAC authenticated encryption support in the encrypt-then-authenticate model for SPA communications. Supported HMAC digests include MD5, SHA1, SHA256, SHA384, and SHA512. The default is HMAC-SHA256 when an HMAC is used. The HMAC mode is supported for both Rijndael and GPG encrypted SPA packet data, and provides a significant security benefit for the fwknopd server since the HMAC verification is more simplistic than decryption operations. This is particularly true for GPG. Beyond this, HMAC authenticated encryption in the encrypt-then-authenticate mode does not suffer from things like CBC-mode padding oracle attacks (see the Vaudenay attack and the more recent "Lucky 13" attack against SSL). HMAC verifications are performed with a constant time comparison function.
  • [libfko] Significant bug fix to honor the full encryption key length for user-supplied Rijndael keys > 16 bytes long. Previous to this fix, only the first 16 bytes of a key were actually used in the encryption/ decryption process even if the supplied key was longer. The result was a weakening of expected security for users that had keys > 16 bytes. Note that "passphrase" is perhaps technically a better word for "user-supplied key" in this context since the actual key is generated with the PBKDF1 key derivation algorithm. This issue was reported by Michael T. Dean. Closes issue #18 on github.
  • [libfko] Added the ability to maintain backwards compatibility with the now deprecated "zero padding" key derivation strategy in AES mode that was a hold over from the old perl fwknop implementation. This is NOT compliant with PBKDF1 and is only brought forward into fwknop-2.5 for backwards compatibility. Future versions of fwknop will remove this code altogether since PBKDF1 is now implemented.
  • [libfko+server] Ensure that all HMAC, digest, and other comparisons are done via a dedicated constant_runtime_cmp() function so that a potential attacker cannot gain any information about fail/success just by mounting a timing attack. This function always compares two buffers from beginning to end regardless of whether a difference is detected early on in the comparison, and this strategy mirrors changes in SSL libraries such as yaSSL to protect against potential timing attacks. This change fixes #85 on github which was reported by Ryman.
  • [test suite] Added --enable-openssl-checks to send all SPA packets encrypted via libfko through the OpenSSL library to ensure that the libfko usage of AES is always compatible with OpenSSL. This ensures that the fwknop usage of AES is properly implemented as verified by the OpenSSL library, which is a frequently audited high profile crypto engine. If a vulnerability is discovered in OpenSSL and a change is made, then the --enable-openssl-checks mode will allow the test suite to discover this in a automated fashion for fwknop.
  • The fwknop project is using Coverity for source code validation (in addition to other tools such as the CLANG static analyzer). Many bugs have been fixed in this release that were discovered by Coverity. These bugs spanned multiple classes of problems from memory leaks, improper use of sizeof(), potential double-free conditions, and more. Full details on these fixes are available in the git history. Any open source project that is written in a language supported by Coverity would benefit highly from participating. As of the 2.5 release, fwknop has a Coverity defect score of zero.
  • [test suite] Changed how the test suite interacts with the fwknop client and server by looking for indications that SPA packets are actually received. This is done by first waiting for 'main event loop' in fwknopd log output to ensure that fwknopd is ready to receive packets, sending the SPA packet(s), and then watching for for 'SPA Packet from IP' in fwknopd output. This is an improvement over the previous strategy that was only based on timeout values since it works identically regardless of whether fwknop is being run under valgrind or when the test suite is run on an embedded system with very limited resources. Another check is run for fwknopd receiving the SIGTERM signal to shutdown via 'fwknopd -K', and that failing, the test suite manually kills the process (though this should be rarely needed). This change was implemented based on discussions with George Herlin.
  • (Franck Joncourt) Added support for resolving hostnames in various NAT modes (fixes issue #43 in github).
  • (Franck Joncourt) Bug fix in the client for resolving hostnames in '-P icmp' mode (fixes issue #64).
  • (Franck Joncourt) Added support for saving fwknop client command line arguments via a new options --save-rc-stanza.
  • (Franck Joncourt) Added log module support for the client.
  • [client] Bug fix for --nat-rand-port mode to ensure that the port to be NAT'd is properly defined so that the fwknopd server will NAT connections to this port instead of applying the NAT operation to the port that is to be accessed via -A. This change also prints the randomly assigned port to stdout regardless of whether --verbose mode is used (since if not then the user will have no idea which port is actually going to be NAT'd on the fwknopd side).
  • (Vlad Glagolev) Submitted an OpenBSD port for fwknop-2.0.4, and this has been checked in under the extras/openbsd/fwknop-2.0.4 directory.
  • (Shawn Wilson) Added better SPA source IP logging for various fwknopd logging messages. This helps to make it more clear why certain SPA packets are rejected from some systems.
  • [client] Added --get-hmac-key to allow HMAC keys to be acquired from the specified file similarly to the --get-key option. This is a convenience only, and the fwknop rc file feature should be used instead since it is far more powerful.