A simple alternative to ssh (<5% total SLOCC) written from scratch in Go. A testbed for candidate PQC (post-quantum cryptography) KEMs (Key-Encapsulation Mechanisms) and symmetric session encryption algorithms.

xs also features integrated traffic chaffing to obscure interactive session and file copy activity, and is intended as a playground for other attack-resistance techniques.

Runs on Linux, FreeBSD, Windows (client only, MSYS) and Android (within Termux).

Russtopia d1076c9bd9 Update 'README.md' with xc progress info 2 years ago
bacillus e320725e07 Added unit tests for auth.go 2 years ago
logger c2c43c1c50 Fix for issue #23: null panic if logger obj is null due no active syslog sink 2 years ago
spinsult 94e06abbcb Updates to main Makefile (and added spinsult Makefile) 3 years ago
vendor e9aa0072a5 Initial aead/chacha20 support (ChaCha20_12) 2 years ago
xs 2e16c9a692 Remote->Client cp with pv status TODO: Remote total bytesize estimate 2 years ago
xsd f3bf6d9041 Added login timeout (30s) 2 years ago
xsnet e9aa0072a5 Initial aead/chacha20 support (ChaCha20_12) 2 years ago
xspasswd 89b6e8bce7 Version tags now reflect mod/vendor build setup. 2 years ago
.golangci.yml d3ebcab1d4 Added golangci-lint config 2 years ago
CODE_OF_MERIT.md 8920184ae4 Updated to stable archived link 3 years ago
LICENSE.mit b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
Makefile 6ac6f02b3b Bumped version 2 years ago
README.md d1076c9bd9 Update 'README.md' with xc progress info 2 years ago
TODO.txt faf4d5c50a Add (as default) option to use system shadow passwds 2 years ago
auth.go e84f432033 auth.go: added new unit tests 2 years ago
auth_test.go e84f432033 auth.go: added new unit tests 2 years ago
consts.go b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
cp.cmd 5859131678 Continuing groundwork for cp mode - refactor main client code into shell/copy subroutines; -r option 3 years ago
cptest.sha1sum 306ea64299 Sorted cptest.sha1sum 3 years ago
fixup-gv.sh fc05a9a4a3 Added fixup-gv.sh and Makefile rules to generate annotated goroutines in graphviz output. 3 years ago
go.mod e9aa0072a5 Initial aead/chacha20 support (ChaCha20_12) 2 years ago
go.sum e9aa0072a5 Initial aead/chacha20 support (ChaCha20_12) 2 years ago
grml.yaml b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
session.go b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
session_test.go 8de76520e4 Added session_test.go 2 years ago
termmode_unix.go b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
termmode_windows.go b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
viz_xs_dot.png b19687c80b The Great Renaming: hkexsh -> xs (Xperimental Shell) 2 years ago
xsd.initrc 15a1a39d81 Fixed xsd.initrc $ARGS passing 2 years ago





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, file copying and tunnels with traffic obfuscation ('chaffing').

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.

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:

Currently supported session algorithms:



  • HMAC-SHA256
  • HMAC-SHA512


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. XS uses its own password file distinct from the system /etc/passwd to authenticate clients, using standard bcrypt+salt storage. This is currently done to allow alternate login credentials via xs vs. console/ssh login, due to the experimental nature of the program. At some point in the future an option to use the system's /etc/passwd and /etc/shadow may be implemented, making the use of the auxilliary xspasswd utility optional or obsolete.


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.


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.)


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>).


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