[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

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