[Bug 1409798] Re: enable hmac-sha2-256, hmac-sha2-512 MAC algorithms

[Launchpad Bug Tracker]

** Branch linked: lp:~a7x/ubuntu/trusty/dropbear/bug1409798

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Title:
  enable hmac-sha2-256, hmac-sha2-512 MAC algorithms

Status in dropbear package in Ubuntu:
  Fix Released
Status in dropbear source package in Trusty:
  New
Status in dropbear source package in Utopic:
  New
Status in dropbear package in Debian:
  New

Bug description:
  [Impact]

  Since version 2013.56, dropbear has supported the hmac-sha2-256 and
  hmac-sha2-512 MAC algorithms.  However, until version 2015.67
  (released 2015-01-28), they have not been enabled by default.
  According to the dropbear changelog, enabling them is a matter of
  uncommenting the following two lines in options.h:

      /*#define DROPBEAR_SHA2_256_HMAC*/
      /*#define DROPBEAR_SHA2_512_HMAC*/

  Due to recent NSA revelations, some people are recommending users
  disable certain algorithms.  If the recommendations at
  <https://stribika.github.io/2015/01/04/secure-secure-shell.html> are
  followed, there are no MAC algorithms left that dropbear supports
  unless hmac-sha2-256 and hmac-sha2-512 are enabled.

  [Test Case]

   Setup steps:
    1. install dropbear and openssh-client:
         sudo apt-get install dropbear openssh-client
    2. if you have openssh-server installed, stop it:
         sudo service ssh stop
    3. make sure that the dropbear service is enabled by editing
       /etc/default/dropbear and setting the NO_START variable to 0 like
       this:
         NO_START=0
    4. start the dropbear service:
         sudo service dropbear start

   Test steps:
    5. try the hmac-sha2-256 MAC:
         ssh -o UserKnownHostsFile=/dev/null \
             -o MACs=hmac-sha2-256 localhost
    6. try the hmac-sha2-512 MAC:
         ssh -o UserKnownHostsFile=/dev/null \
             -o MACs=hmac-sha2-512 localhost

   Cleanup steps:
    7. stop the dropbear service:
         sudo service dropbear stop
    8. if you edited /etc/default/dropbear to enable the dropbear
       service, disable it again by editing /etc/default/dropbear and
       setting the NO_START variable to 1 like this:
         NO_START=1
    9. if you stopped the OpenSSH server, restart it:
         sudo service ssh start

  [Regression Potential]

    * Upstream recently enabled these algorithms by default (version
      2015.67 released 2015-01-28), so it is believed they are ready for
      production use.  The code implementing these hash algorithms has
      not changed since 2013.56, so enabling these algorithms for
      Trusty (based on 2013.60) should be as safe as enabling them for
      Utopic (based on 2014.65).  However, because the code has not been
      enabled by default for long, it has probably not had as much field
      testing and may be more likely to contain compatibility or
      security bugs.
    * SSH clients are likely to prefer the new MAC algorithms over the
      old MAC algorithms.  If either the client or server has an
      implementation bug, the ability to connect may be impaired unless
      the user configures the client to disallow the new algorithms or
      prefer the old algorithms (e.g., via the ssh_config "MACs"
      directive in OpenSSH).
    * The new MAC algorithms may negatively impact performance.

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