[Bug 1310781] Re: bad bignum encoding for curve25519-sha256 at libssh.org

[Omega Weapon]

I've just come across a knock on effect of this bug when configuring the
Ubuntu SSH server for curve25519-sha256 at libssh.org key exchange only,
and using the latest Debian package openssh-client to connect.

Because Ubuntu's SSH server version doesnt match 6.6.1, the Debian SSH
client disables curve25519-sha256 at libssh.org completely, getting rid of
the (presumably) most secure algorithm available:

==========================================================================

debug1: Local version string SSH-2.0-OpenSSH_6.6.1p1 Debian-4
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6p1 Ubuntu-2ubuntu1
debug1: match: OpenSSH_6.6p1 Ubuntu-2ubuntu1 pat OpenSSH_6.5*,OpenSSH_6.6* compat 0x14000000
...
debug2: compat_kex_proposal: original KEX proposal: curve25519-sha256 at libssh.org
debug2: Compat: skipping algorithm "curve25519-sha256 at libssh.org"
debug2: compat_kex_proposal: compat KEX proposal: 
No supported key exchange algorithms found

==========================================================================

The compat value being hit is in compat.c:100.

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https://bugs.launchpad.net/bugs/1310781

Title:
  bad bignum encoding for curve25519-sha256 at libssh.org

Status in “openssh” package in Ubuntu:
  Fix Released
Status in “openssh” source package in Trusty:
  In Progress

Bug description:
  [Impact] Occasional key exchange failure with ED25519.
  [Test Case] I don't have a clear one, but perhaps attempting lots of connections to a fixed server would do it.
  [Regression Potential] We should test with an unpatched server to make sure that it properly falls back to skipping that key exchange method.

  There's an occasional (one in 512 or so) key exchange failure in the
  curve25519-sha256 key exchange method, which affects OpenSSH 6.5 and
  6.6.  Upstream gives more details here and has recommended that
  distributors apply this patch:

    https://lists.mindrot.org/pipermail/openssh-unix-
  dev/2014-April/032494.html

  We should issue this as an update for trusty.

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