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pssht

pssht is a PHP library that provides an SSH server that can be embedded in other applications.

What we're aiming for:

  • Clean code (eg. PSR-2 & PSR-4 compliance, tests, ...)
  • Extensibility
  • Interoperability with as much SSH clients as possible, but mainly with the OpenSSH client

What we're not specifically aiming for, but still interested in:

  • Completeness (support for TCP port forwarding, TUN/TAP tunneling, the scp/sftp subsystems, ...)
  • Strong security (peer reviews, security audits, ...)

Disclaimer

This should be obvious from the get-go, but DO NOT USE pssht in production. This project merely exists for two reasons:

  • First, I wanted to provide a """somewhat secure""" cross-platform way to expose Erebot internals for introspection purposes and I did not want to install an external SSH daemon.
  • Secondly, I wanted to learn more about the SSH protocol itself.

The implementation did not pass any specific security audit. In addition, no attempt has been made to avoid some common classes of vulnerabilities, eg. timing attacks. Not to mention that the PHP interpreter itself is known to be frequently subject to vulnerabilities of its own.

If you are looking for an SSH daemon with thorough testing and code audits to integrate with your PHP code, we recommend that you look into the OpenSSH project.

If you still aren't convinced that you shouldn't use this code in production, see this reddit page which relates part of the story of a project similar to pssht by MtGox's CEO.

In no event shall the authors of pssht be liable for anything that happens while using this library. Please read the license for the full disclaimer.

Installation

The requirements for pssht are quite basic:

  • PHP 5.3.3 or later with the following PHP extensions enabled:
    • OpenSSL
    • mcrypt
    • gmp
    • pcre
    • Sockets
    • SPL
  • Some external packages (they will automatically be installed when installing pssht):
    • erebot/plop for logging
    • symfony/config for configuration handling
    • symfony/dependency-injection for dependency injection
    • symfony/filesystem (dependency for symfony/config)

Moreover, you may be interested in enable the following PHP extensions to get additional features:

  • HTTP: adds support for zlib-compression
  • hash: adds support for more encryption and message authentication code algorithms

First things first, download the composer.phar executable or use the installer:

$ curl -sS https://getcomposer.org/installer | php

Now, you can either install pssht:

  • As a basic SSH server for evaluation purposes (standalone).
  • As a library/framework in your own project (embedded) to create a custom SSH server.

Standalone installation

To install pssht as a standalone SSH server, clone this repository and then run Composer on it:

$ git clone https://github.com/fpoirotte/pssht.git
$ cd pssht
$ php /path/to/composer.phar update --no-dev

Embedded installation

To install pssht as an embedded library in your application, create or update a composer.json file in your project's root directory with a requirement on pssht.

For example, for a new empty project, your composer.json file would look somewhat like this:

{
    "require": {
        "fpoirotte/pssht": "*"
    }
}

Run Composer:

$ php /path/to/composer.phar install --no-dev

Finally, copy pssht.xml to your project's root directory:

$ cp -a vendor/fpoirotte/pssht/pssht.xml ./

Basic usage

Start the server:

$ php bin/pssht         # for standalone installations
$                       # ...or...
$ php vendor/bin/pssht  # for embedded installations

Note

When run like that, pssht will just act as a basic echo server, responding with the exact same data that was sent to it.

pssht will display various debugging messages while initializing. When ready, you will see something like this in the console:

[Fri, 08 May 2015 20:23:21 +0200] INFO: Listening for new connections on 0.0.0.0:22222

You can now connect to the server with the same user that was used to start pssht by using your regular SSH client (eg. OpenSSH/PuTTy). For example, using the OpenSSH client and assuming pssht was run by clicky:

$ ssh -T -p 22222 clicky@localhost
Hello world!
clicky@localhost's password: pssht

The default pssht.xml configuration file automatically loads the public keys stored in ~/.ssh/authorized_keys. You can thus connect with the matching private key. It will also accept password-based authentication using "pssht" as the password.

Note

The -T option is used to disable pseudo-tty allocation as it is not yet supported (see #21). Without it, OpenSSH displays a warning in the console (PTY allocation request failed on channel 0).

Configuration

pssht uses the Dependency Injection component from the Symfony2 framework for its configuration.

Have a look at the default pssht.xml configuration file for ways to customize pssht. The file contains numerous comments and the options should thus be very straightforward.

Compatibility

pssht supports the mechanisms and algorithms defined in the following documents for compatibility with other Secure Shell implementations:

The rest of this section describes precisely which algorithms and features are supported.

TL;DR here's a feature chart for comparison with OpenSSH 6.7p1:

  • ☑ Services (2 in pssht; 2 in OpenSSH)
  • ☐ Authentication methods (4 in pssht; ? in OpenSSH)
  • ☐ Key exchange methods (6 in pssht; 8 in OpenSSH)
  • ☑ Encryption algorithms (34 in pssht; 16 in OpenSSH) [1]
  • ☑ MAC algorithms (20 in pssht; 19 in OpenSSH) [1]
  • ☐ Public key algorithms (6 in pssht; 14 in OpenSSH)
  • ☑ Compression algorithms (2 in pssht; 2 in OpenSSH) [1]
[1](1, 2, 3) The "none" algorithm has been excluded from those counts.

Services

The following services are supported:

  • ssh-userauth
  • ssh-connection

Authentication methods

The following authentication methods are supported:

  • publickey
  • password
  • hostbased
  • none

Key exchange methods

The following key exchange methods are supported:

  • [email protected]
  • diffie-hellman-group1-sha1
  • diffie-hellman-group14-sha1
  • ecdh-sha2-nistp256
  • ecdh-sha2-nistp384
  • ecdh-sha2-nistp521

The PHP hash extension must be installed for [email protected] and the ecdsa-sha2-* family of algorithms to work properly. Also, elliptic curve points encoded using point compression are not accepted or generated.

Encryption algorithms

The following encryption algorithms are supported:

  • 3des-cbc
  • 3des-ctr
  • aes128-cbc
  • aes128-ctr
  • [email protected]
  • aes192-cbc
  • aes192-ctr
  • aes256-cbc
  • aes256-ctr
  • [email protected]
  • arcfour
  • arcfour128
  • arcfour256
  • blowfish-cbc
  • blowfish-ctr
  • cast128-cbc
  • cast128-ctr
  • [email protected]
  • idea-cbc
  • idea-ctr
  • none
  • [email protected] (as an alias for aes256-cbc)
  • serpent128-cbc
  • serpent192-cbc
  • serpent256-cbc
  • serpent128-ctr
  • serpent192-ctr
  • serpent256-ctr
  • twofish-cbc
  • twofish128-cbc
  • twofish192-cbc
  • twofish256-cbc
  • twofish128-ctr
  • twofish192-ctr
  • twofish256-ctr

MAC algorithms

The following MAC algorithms are supported:

All these algorithms except for the umac-* family require the PHP hash extension in order to work properly.

Public key algorithms

The following public key algorithms are supported:

  • ecdsa-sha2-nistp256
  • ecdsa-sha2-nistp384
  • ecdsa-sha2-nistp521
  • ssh-dss
  • ssh-ed25519
  • ssh-rsa

The PHP hash extension must be installed for the ssh-ed25519 and ecdsa-sha2-* family of algorithms to work properly. Also, elliptic curve points encoded using point compression are not accepted or generated.

Compression algorithms

The following compression algorithms are supported:

The PHP http extension must be installed for the zlib and [email protected] algorithms to work properly.

Integration

pssht is mainly intended to be used as an embedded SSH server for PHP applications. By default, only the bare structure for an SSH server is provided. The application using pssht is responsible for adding it's own logic on top of this structure.

Contributions

Want to contribute back to the project?

  • Fork the code to your own account.
  • Create a new branch.
  • Hack around.
  • Create a pull request with your changes.

License

The MIT License (MIT)

Copyright (c) 2014 François Poirotte

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Changelog

v0.1.1

  • [#28] Temporarily fix Diffie–Hellman key exchange by disabling public key validation for Elliptic Curve Diffie–Hellman. This code will be revisited later on as it currently represents a possible security threat when ECDH is used.
  • Improve this README (installation instruction, changelog).
  • Change the default pssht.xml so that it accepts connections from the same user as the one starting the server (prior to this change, it used an hardcoded username).

v0.1.0

  • Initial release with lots of features already.

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SSH server written in PHP

pssht.readthedocs.io

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May 8, 2015
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