Asyncssh

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* Added support for forwarding X11 connections. When requested, AsyncSSH
clients will allow remote X11 applications to tunnel data back to a local
X server and AsyncSSH servers can request an X11 DISPLAY value to export
to X11 applications they launch which will tunnel data back to an X
server associated with the client.

* Improved ssh-agent forwarding support on UNIX to allow AsyncSSH
servers to request an SSH_AUTH_SOCK value to export to applications
they launch in order to access the client's ssh-agent. Previously,
there was support for agent forwarding on server connections within
AsyncSSH itself, but they did not provide this forwarding to other
applications.

* Added support for PuTTY's Pageant agent on Windows systems, providing
functionality similar to the OpenSSH agent on UNIX. AsyncSSH client
connections from Windows can now access keys stored in the Pageant
agent when they perform public key authentication.

* Added support for the umac-64 and umac-128 MAC algorithms, compatible
with the implementation in OpenSSH. These algorithms are preferred
over the HMAC algorithms when both are available and the cipher chosen
doesn't already include a MAC.

* Added curve25519-sha256 as a supported key exchange algorithm. This
algorithm is identical to the previously supported algorithm named
'curve25519-sha256\libssh.org', matching what was done in OpenSSH 7.3.
Either name may now be used to request this type of key exchange.

* Changed the default order of key exchange algorithms to prefer the
curve25519-sha256 algorithm over the ecdh-sha2-nistp algorithms.

* Added support for a readuntil() function in SSHReader, modeled after
the readuntil() function in asyncio.StreamReader added in Python 3.5.2.
Thanks go to wwjiang for suggesting this and providing an example
implementation.

* Fixed issues where the explicitly provided event loop value was not
being passed through to all of the places which needed it. Thanks go
to Vladimir Rutsky for pointing out this problem and providing some
initial fixes.

* Improved error handling when port forwarding is requested for a port
number outside of the range 0-65535.

* Disabled use of IPv6 in unit tests when opening local loopback sockets
to avoid issues with incomplete IPv6 support in TravisCI.

* Changed the unit tests to always start with a known set of environment
variables rather than inheriting the environment from the shell
running the tests. This was leading to test breakage in some cases.

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* Updated unit tests to run properly in environments where OpenSSH
and OpenSSL are not installed.

* Updated a process unit test to not depend on the system's default
file encoding being UTF-8.

* Updated Mac TravisCI builds to use Xcode 8.1.

* Cleaned up some wording in the documentation.

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* Fixed an issue with preserving file access times in SFTP, and update
the unit tests to more accurate detect this kind of failure.

* Fixed some markup errors in the documentation.

* Fixed a small error in the change log for release 1.7.0 regarding
the newly added Diffie Hellman key exchange algorithms.

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* Fix an error that prevented the docs from building.

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* Added support for group 14, 16, and 18 Diffie Hellman key exchange
algorithms which use SHA-256 and SHA-512.

* Added support for using SHA-256 and SHA-512 based signature algorithms
for RSA keys and support for OpenSSH extension negotiation to advertise
these signature algorithms.

* Added new load_keypairs and load_public_keys API functions which
support explicitly loading keys using the same syntax that was
previously available for specifying client_keys, authorized_client_keys,
and server_host_keys arguments when creating SSH clients and servers.

* Enhanced the SSH agent client to support adding and removing keys
and certificates (including support for constraints) and locking and
unlocking the agent. Support has also been added for adding and
removing smart card keys in the agent.

* Added support for getting and setting a comment value when generating
keys and certificates, and decoding and encoding this comment when
importing and exporting keys that support it. Currently, this is
available for OpenSSH format private keys and OpenSSH and RFC 4716
format public keys. These comment values are also passed on to the
SSH agent when keys are added to it.

* Fixed a bug in the generation of ECDSA certificates that showed up
when trying to use the nistp384 or nistp521 curves.

* Updated unit tests to use the new key and certificate generation
functions, eliminating the dependency on the ssh-keygen program.

* Updated unit tests to use the new SSH agent support when adding keys
to the SSH agent, eliminating the dependency on the ssh-add program.

* Incorporated a fix from Vincent Bernat for an issue with launching
ssh-agent on some systems during unit testing.

* Fixed some typos in the documentation found by Jakub Wilk.

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* Added generate_user_certificate() and generate_host_certificate() methods
to SSHKey class to generate SSH certificates, and export_certificate()
and write_certificate() methods on SSHCertificate class to export
certificates for use in other tools.

* Improved editor unit tests to eliminate timing dependency.

* Cleaned up a few minor documentation issues.