XS

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XS (Xperimental Shell) is a golang implementation of a simple remote shell client and server, similar in role to ssh, offering encrypted interactive and non-interactive sessions (remote commands), remote file copying and tunnels with optional traffic obfuscation ('chaffing').

It is stable to the point that I use it for day-to-day remote access in place of, and in preference to, ssh.

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NOTE: Due to the experimental nature of the KEX/KEM algorithms used, and the novelty of the overall codebase, this package SHOULD BE CONSIDERED EXTREMELY EXPERIMENTAL and USED WITH CAUTION. It DEFINITELY SHOULD NOT be used for any sensitive applications. USE AT YOUR OWN RISK. NEITHER WARRANTY NOR CLAIM OF FITNESS FOR PURPOSE IS EXPRESSED OR IMPLIED.

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The client and server programs (xs and xsd) use a mostly drop-in replacement for golang's standard golang/pkg/net facilities (net.Dial(), net.Listen(), net.Accept() and the net.Conn type), which automatically negotiate keying material for secure sockets using one of a selectable set of experimental key exchange (KEX) or key encapsulation mechanisms (KEM).

Key Exchange

Currently supported exchanges are:

• The HerraduraKEx key exchange algorithm first released at Omar Elejandro Herrera Reyna's HerraduraKEx project;
• The KYBER IND-CCA-2 secure key encapsulation mechanism, pq-crystals Kyber :: Yawning/kyber golang implementation
• The NEWHOPE algorithm newhopecrypto.org :: Yawning/go-newhope golang implementation

Currently supported session algorithms:

[Encryption]

• AES-256
• Twofish-128
• Blowfish-64
• CryptMTv1 (64bit) (https://eprint.iacr.org/2005/165.pdf)
• ChaCha20 (https://github.com/aead/chacha20)

[HMAC]

• HMAC-SHA256
• HMAC-SHA512

Conn

Calls to xsnet.Dial() and xsnet.Listen()/Accept() are generally the same as calls to the equivalents within the net package; however upon connection a key exchange automatically occurs whereby client and server independently derive the same keying material, and all following traffic is secured by a symmetric encryption algorithm.

Session Negotiation

Above the xsnet.Conn layer, the server and client apps in this repository (xsd/ and xs/ respectively) negotiate session settings (cipher/hmac algorithms, interactive/non-interactive mode, tunnel specifiers, etc.) to be used for communication.

Padding and Chaffing

Packets are subject to padding (random size, randomly applied as prefix or postfix), and optionally the client and server channels can both send chaff packets at random defineable intervals to help thwart analysis of session activity (applicable to interactive and non-interactive command sessions, file copies and tunnels).

Mux/Demux of Chaffing and Tunnel Data

Chaffing and tunnels, if specified, are set up during initial client->server connection. Packets from the client local port(s) are sent through the main secured connection to the server's remote port(s), and vice versa, tagged with a chaff or tunnel specifier so that they can be discarded as chaff or de-multiplexed and delivered to the proper tunnel endpoints, respectively.

Accounts and Passwords

Within the xspasswd/ directory is a password-setting utility, xspasswd, used if one wishes xs access to use separate credentials from those of the default (likely ssh) login method. In this mode, xsd uses its own password file distinct from the system /etc/passwd to authenticate clients, using standard bcrypt+salt storage. Activate this mode by invoking xsd with -s false.

HERRADURA KEX

As of this time (Oct 2018) no verdict by acknowledged 'crypto experts' as to the level of security of the HerraduraKEx algorithm for purposes of session key exchange over an insecure channel has been rendered. It is hoped that experts in the field will analyze the algorithm and determine if it is indeed a suitable one for use in situations where Diffie-Hellman or other key exchange algorithms are currently utilized.

KYBER IND-CCA-2 KEM

As of this time (Oct 2018) Kyber is one of the candidate algorithms submitted to the NIST post-quantum cryptography project. The authors recommend using it in "... so-called hybrid mode in combination with established "pre-quantum" security; for example in combination with elliptic-curve Diffie-Hellman." THIS PROJECT DOES NOT DO THIS (in case you didn't notice yet, THIS PROJECT IS EXPERIMENTAL.)

Dependencies:

• Recent version of go (tested, at various times, with go-1.9 to go-1.12.4)
• github.com/mattn/go-isatty //terminal tty detection
• github.com/kr/pty //unix pty control (server pty connections)
• github.com/jameskeane/bcrypt //password storage/auth
• blitter.com/go/goutmp // wtmp/lastlog C bindings for user accounting
• https://git.schwanenlied.me/yawning/kyber // golang Kyber KEM
• https://git.schwanenlied.me/yawning/newhope // golang NEWHOPE,NEWHOPE-SIMPLE KEX
• blitter.com/go/mtwist // 64-bit Mersenne Twister PRNG
• blitter.com/go/cryptmt // CryptMTv1 stream cipher

Get source code

$ go get -u blitter.com/go/xs
$ cd $GOPATH/src/blitter.com/go/xs
$ go build ./... # install all dependent go pkgs

To build

$ cd $GOPATH/src/blitter.com/go/xs
$ make clean all

To install, uninstall, re-install

$ sudo make [install | uninstall | reinstall]

To manage service (assuming openrc init)

An example init script (xsd.initrc) is provided. Consult your Linux distribution documentation for proper service/daemon installation. For openrc,

$ sudo cp xsd.initrc /etc/init.d/xsd
$ sudo rc-config add xsd default

The make system assumes installation in /usr/local/sbin (xsd, xspasswd) and /usr/local/bin (xs/xc symlink).

$ sudo rc-config [start | restart | stop] xsd

To set accounts & passwords:

$ sudo touch /etc/xs.passwd
$ sudo xspasswd/xspasswd -u joebloggs
$ <enter a password, enter again to confirm>

Testing Client and Server from $GOPATH dev tree (w/o 'make install')

In separate shells A and B:

[A]$ cd xsd && sudo ./xsd &  # add -d for debugging

Interactive shell

[B]$ cd xs && ./xs joebloggs@host-or-ip # add -d for debugging

One-shot command

[B]$ cd xs && ./xs -x "ls /tmp" joebloggs@host-or-ip

WARNING WARNING WARNING: the -d debug flag will echo passwords to the log/console! Logging on Linux usually goes to /var/log/syslog and/or /var/log/debug, /var/log/daemon.log.

NOTE if running client (xs) with -d, one will likely need to run 'reset' afterwards to fix up the shell tty afterwards, as stty echo may not be restored if client crashes or is interrupted.

Setting up an 'authtoken' for scripted (password-free) logins

Use the -g option of xs to request a token from the remote server, which will return a hostname:token string. Place this string into $HOME/.xs_id to allow logins without entering a password (obviously, $HOME/.xs_id on both server and client for the user should not be world-readable.)

File Copying using xc

xc is a symlink to xs, and the binary checks its own filename to determine whether it is being invoked in 'shell' or 'copy' mode. Refer to the '-h' output for differences in accepted options.

General remote syntax is: user@server:[/]src-or-dest-path If no leading / is specified in src-or-dest-path, it is assumed to be relative to $HOME of the remote user. File operations are all performed as the remote user, so account permissions apply as expected.

Local (client) to remote (server) copy:

$ xc fileA /some/where/fileB /some/where/else/dirC joebloggs@host-or-ip:remoteDir

Remote (server) to local (client) copy:

$ xc joebloggs@host-or-ip:/remoteDirOrFile /some/where/local/Dir

xc uses a 'tarpipe' to send file data over the encrypted channel. Use the -d flag on client or server to see the generated tar commands if you're curious.

NOTE: Renaming while copying (eg., 'cp /foo/bar/fileA ./fileB') is NOT supported. Put another way, the destination (whether local or remote) must ALWAYS be a directory.

If the 'pv' pipeview utility is available (http://www.ivarch.com/programs/pv.shtml) file transfer progress and bandwidth control will be available (suppress the former with the -q option, set the latter with -L <bytes_per_second>).

Tunnels

Simple tunnels (client -> server, no reverse tunnels for now) are supported.

Syntax: xs -T=<tunspec>{,<tunspec>...} .. where <tunspec> is <localport:remoteport>

Example, tunnelling ssh through xs

• [server side] $ sudo /usr/sbin/sshd -p 7002
• [client side, term A] $ xs -T=6002:7002 user@server
• [client side, term B] $ ssh user@localhost -p 6002

Building for FreeBSD

The Makefile(s) to build require GNU make (gmake). Please install and invoke build via: $ gmake clean all