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https://github.com/stealth/psc

E2E encryption for multi-hop tty sessions or portshells + TCP/UDP port forward
https://github.com/stealth/psc

anti-censorship censorship-resistance console dialup e2e encryption modem pentesting portshell proxy pty screen socks-proxy ssh tcp tmux uart udp

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E2E encryption for multi-hop tty sessions or portshells + TCP/UDP port forward

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README 

        
PortShellCrypter -- PSC

=======================



This project - as well as its sister project [crash](https://github.com/stealth/crash) - belongs

to my anti-censorship tool-set that allows to setup fully working encrypted shells and TCP/UDP

forwarding in hostile censoring environments. It is also useful for forensics to dump data from

devices via UART or adb when no other means are available.



[![asciicast](https://asciinema.org/a/383043.svg)](https://asciinema.org/a/383043)

*DNS lookup and SSH session forwarded across an UART connection to a Pi*



PSC allows to e2e encrypt shell sessions, single- or multip-hop, being

agnostic of the underlying transport, as long as it is reliable and can send/receive

Base64 encoded data without modding/filtering. Along with the e2e pty that

you receive (for example inside a port-shell), you can forward TCP and UDP

connections, similar to OpenSSH's `-L` parameter. This works transparently

and without the need of an IP address assigned locally at the starting

point. This allows forensicans and pen-testers to create network connections

for example via:



* UART sessions to a device

* `adb shell` sessions, if the OEM `adbd` doesn't support TCP forwarding

* telnet sessions

* modem dial-ups without ppp

* other kinds of console logins

* mixed SSH/telnet/modem sessions

* ...



Just imagine you would have an invisible ppp session inside your shell session,

without the remote peer actually supporting ppp.



It runs on *Linux, Android, OSX, Windows, FreeBSD, NetBSD* and (possibly) *OpenBSD*.



PSC also includes *SOCKS4* and *SOCKS5* proxy support in order to have actual

web browsing sessions via port-shells or modem dial-ups remotely.



Build

-----



Edit the `Makefile` to reflect your pre shared keys, as defined at the top of the `Makefile`.



Then just type `make` on *Linux* and *OSX*.



On *BSD* you need to install *GNU make* and invoke `gmake` instead.



On *Windows* you need to install [cygwin](https://cygwin.com/install.html) and select

the appropriate `gcc, gcc-g++, make` and `git` packages.



On *Linux*, PSC will use *Unix98* pseudo terminals, on other systems it will use *POSIX*

pty's but that should be transparent to you. I once added *4.4BSD* pty and *SunOS*

support back in the stone age for a particular reason, so it may or may not

build even with *Solaris*.



*proudly sponsored by:*












Usage

-----



Plain and simple. On your local box, execute `pscl`, and pass any

TCP or UDP ports you want to forward *from* the remote site to a particular

address. For example:



```

linux:~ > ./pscl -T 1234:[192.168.0.254]:22 -U 1234:[8.8.8.8]:53



PortShellCrypter [pscl] v0.60 (C) 2006-2020 stealth -- github.com/stealth/psc



pscl: set up local TCP port 1234 to proxy to 192.168.0.254:22 @ remote.

pscl: set up local UDP port 1234 to proxy to 8.8.8.8:53 @ remote.



pscl: Waiting for [pscr] session to appear ...

linux:~ >



[ UART / SSH / ... login to remote side ... ]

```



On the remote site (the last hop) with the shell session, no matter if its in

a port-shell, SSH, console login etc, you execute `pscr`:



```

linux:~ > ./pscr



PortShellCrypter [pscr] v0.60 (C) 2006-2020 stealth -- github.com/stealth/psc



pscl: Seen STARTTLS sequence, enabling crypto.

linux:~ >

```



Once you execute `pscr`, both ends establish a crypto handshake and lay an additional

protocol over your existing session that is transparent for you. You can then

connect to `127.0.0.1:1234` on your local box to reach `192.168.0.254:22` via

TCP or the `8.8.8.8` resolver via UDP. This also works with [IPv6] addresses,

if the remote site has IPv6 connectivity. Actually, you can even use it to translate

IPv4 software to IPv6, since you always connect to `127.0.0.1` on the local side.



You can pass multiple `-T` and `-U` parameters. If you lost track if your session

is already e2e encrypted, you can send a `SIGUSR1` to the local `pscl` process, and it

will tell you.



PSC is also useful if you want to use tor from a remote SSH shell, where you

can forward the socks5 and the DNS port to the remote hosts `127.0.0.1` address.

Since SSH does not forward UDP packets, you would normally use two `socat` connectors

or similar to resolve via the tor node. PSC has the advantage of keeping the UDP

datagram boundaries, while `socat` over `SSH -L` may break datagram boundaries

and create malformed DNS requests.



The session will be encrypted with `aes_256_ctr` of a PSK that you choose in the

`Makefile`. This crypto scheme is malleable, but adding AAD or OAD data blows up

the packet size, where every byte counts since on interactive sessions and due to

Base64 encoding, each typed character already causes much more data to be sent.



UART sessions may be used via `screen` but for example not via `minicom` since

minicom will create invisible windows with status lines and acts like a filter

that destroys PSC's protocol. PSC tries to detect filtering and can live with

certain amount of data mangling, but in some situations it is not possible to recover.

Similar thing with `tmux`. You should avoid stacking pty handlers with PSC that

mess/handle their incoming data too much.



The `SHELL` environment variable needs to be set for both `pscl` and `pscr` in order

for PSC to know which shell to execute on the pty. `SHELL` is set in most environments

by default, but in case it isn't, PSC needs to be executed like `SHELL=/bin/bash pscl`

etc.



SOCKS4 and SOCKS5 support

-------------------------



`pscl` also supports forwarding of TCP connections via *SOCKS4* (`-4 port`) and *SOCKS5*

(`-5 port`). This sets up *port* as SOCKS port for TCP connections, so for instance you

can browse remote networks from a port-shell session without the need to open any other

connection during a pen-test. If you pass `-N` to `pscl`, it enables DNS name resolution

on the remote side, so you can also use chrome with it. But be warned: There is a privacy

problem with browsers that try to resolve a sequence of DNS names upon startup that

is not under your control. Also, if your remote side has a broken DNS setup, your typing

shell may block for several seconds if DNS reply packets are missing. There are no good

async resolver functions which are embeddable and portable so I had to rely on

`getaddrinfo()` in the single thread at the price of possible blockings for several seconds

if DNS problems exist. Thats why name resolving has to be enabled explicitly. `pscr`

tries to minimize this potential problem with DNS lookup caches though, so in most

situation it should just work painlessly.

If you pass `-X IP-address` (must be the first argument), you can bind your local proxy

to an address different from `127.0.0.1`, so you can share the proxy in your local network.



Bounce commands

---------------



*psc* features allow TCP-connections or binary data blobs being forwarded from/to remote

devices across multiple hops even if it is not possible to install the `pscr` binary at

the remote site. This is very useful for forensic purposes if you do not have any means

to otherwise download artefacts from the device (which can be an UART connected phone for example)

or need to forward connections without touching the FS to not destroy evidence on the system

or when the root-FS is ro mounted and you can't upload your tool-set.



This is a really cool feature, as you can see your TCP connection hop through your local tty

to a remote box without the need to install anything remotely.



This solely works by local pty punkrock and handing over a bounce-command to `pscl` that it will

drop on the remote shell (without `pscr` running) and some state engine magic that filters out

and handles the data at the local side. Usually this requires to set the remote pty to raw mode

at first before issuing the actual command and some other details that are passed to `-B`. The

argument is split into the following parts:



* The local port to trigger the command upon connect, followed by `:`, e.g. `1234:`.

* The cmd that sets the remote tty to raw mode, usually `stty -echo raw` or

  `python -c "import tty;tty.setraw(0)"` (take care to get the quotes right, as `-B` also needs

  to be quoted) or anything similar.

* A "GO" marker issued by remote that tells `pscl` to start sending data to avoid a race between

  `stty` actually happen and the start of the cmd, e.g. a `echo GO` is perfect.

* The trigger command itself, e.g. `nc 127.0.0.1 22` to bounce local port 1234 to remote's SSH

  server

* optionally a FIN marker issued by remote so you notice that trigger command has been finished

  i.e. you can kill your local connection to port 1234, which allows `pscl` to reset its tty state.

  `echo FIN` will do it. Recommended, as otherwise you can have trouble recognizing the end of

  your command.

* All four previous commands are separated by `;` and enclosed in brackets.



Examples:



If you want to forward a TCP connection, this example requires `stty` and `nc` installed on the

device, but it could theoretically be anything else that does equivalent.



Start a local session:



`./pscl -B '1234:[stty -echo raw;echo GO;nc example.com 22;echo FIN]'`



This will issue the command `stty -echo raw;echo GO;nc example.com 22;echo FIN` to the remote

device if you connect locally to port 1234 and then just forwards any data it sees back and forth

and rate-limiting the traffic so it will not exceed the devices' tty speed (115200 is the default).



When the pscl session is started, connect to the remote device by UART, `ssh -e none ...` or

whatever it is and once you have the remote shell, also type locally:



`ssh [email protected] -p 1234` to bounce the SSH connection from your local box across the remote

device to the `example.com` destination. Of course the `pscr` variant is preferred as `-B` can only

bounce a single connection at a time (although you can pass multiple `-B` commands for various

forwards) and theres a chance to hang the shell after the TCP session since the pty is in `raw -echo`

mode and depending on whether the final remote peer also closes the connection, it might be

that the shell just hangs after that. If you happen to find a pscl notification that the connection

has finished and see a prompt, you should `reset` it, so that a new connection can be started.

While data is being forwarded, you will see 7bit ASCII `<` and `>` notifications in `pscl` which

are just local for easier debugging and progress detection.



Note that the connection to the remote site has to be 8bit clean, i.e. the ssh, telnet, UART or

whatever channel *must not handle escape sequences* (unlike when using `pscr`). For ssh connections

this means you have to use `ssh -e none` in the `pscl` session.



Next, following some examples to handle binary file xfer where *rfile* denotes the remote file and

*lfile* the local file.



To start a session to drop remote files, locally:



`./pscl -B '1234:[stty -echo raw;echo GO;dd of=rfile.bin bs=1 count=7350;echo FIN]'`



Where you need to specify the amount of data that the remote side is expecting. It would also

work without (e.g. `cat>...`) but then the session will hang after transmission has finished as

`cat` is endlessly expecting input. By using `dd count=...`, you will get a clean exit and be notified

about it by the FIN marker.



Then, ssh or whatever is necessary to get a shell on the remote device from within the just

started `pscl` session. On a second terminal locally:



`dd if=lfile.bin|nc 127.0.0.1 1234`



which will connect to `pscl`'s local port 1234 and trigger the dump command on the remote side,

forwarding the binary data of the local `lfile.bin` to remotes `rfile.bin`. Due to rate-limiting

this can take a while and you *only trust your psc progress screen* whether the transfer is finished.

The local `dd ...|nc ...` command will only show you the local status which can eat entire files

in msecs due to local TCP buffers while the file is still being transfered through the pty.

So make sure you only press `Ctrl-C` when the *pscl* screen tells you it is finished or you see

the `FIN` end marker being echoed back to you on the `dd ...|nc ...` session.



Likewise, similar commands could be used to transfer binary data from a remote device to the

local box for forensic purposes. Again, start of the session locally:



`./pscl -B '1234:[stty -echo raw;echo GO;dd if=rfile.bin]'` or



`./pscl -B '1234:[stty -echo raw;echo GO;cat rfile.bin]'`



Then, ssh to remote device to get the shell, then again locally:



`nc 127.0.0.1 1234|dd of=lfile.bin bs=1 count=7350`



To obtain `rfile.bin` of size 7350 copied to local file `lfile.bin`



If `stty -echo raw` is not available on the device, something like

`python -c "import tty;tty.setraw(0)"` also works. Note that on the remote device you need to have

a tty (not just a port-shell) when using bounce commands, since the `stty` command to set raw mode

requires a real tty.



UART / modems / Flow Control

----------------------------



If *psc* runs across a serial connection, lost bits can kill all your fun. If you run

without HW FC you will eventually experience bitloss and hung connections, in particular

as there is no throttling when the device is sending data in your direction when using

*bounce commands*. Dumping data to the device works better as this data goes through

the `pscl` rate limits.



However, here are some tips that worked for me under circumstances when it is not

possible to use `pscr` on the device and HW FC. This only applies when using UARTs, as this

is a potentially unreliable transport channel.



* do not enable soft FC, as this would tamper the 8-bit channel

* when possible use HW FC - or if not - you have to disable FC alltogether

* Use `pscr` on the device so you can set rate limiting for data being sent into your direction.

  As the direction towards the device is always rate limited, you can use bounce commands to

  dump a cross-compiled `pscr` binary to the device and start a two-way rate limited session with it.

* use high quality cables with proper shielding and UART chipsets with large buffers

* apply the tio-limit patch from the contrib folder, as tio is buffering input bytes which could

  lead to writing-peeks that exceed the set rate

* use `tio -o 1` or `-o 2` to add delays between sent output-bytes

* use a conservative rate limitig (i.e. prefer `38400` although serial line has set `115200`)

* compile `psc` with `-DRESPECT_UART_BUFSIZE=4096`, however this will make the session very slow



Inside the `contrib` folder you will also find a `tio-noprefix` patch to disable escape-character

processing but this patch is only necessary for older versions, as upstream already accepted and

integrated this patch. I really recommend using `tio` when using UARTs.



When using bounce commands across *tio*, you have to add to your `~/.tioconfig` file:



```

[default]



prefix-ctrl-key = none

```



which disables ESC-handling and gives you an 8-bit clean channel.



SIGUSR1 / SIGUSR2

-----------------



You can send `SIGUSR1` to `pscl` so it tells you whether the session is encrypted. If the remote

`pscr` dies or exits without possibility to signal that to the local part, `pscl` will stay

in encryption mode and therefore hang. In this case you can force a reset to plaintext mode

by sending `SIGUSR2`, so a new session can be started.



Scripting

---------



As of version 0.64, *psc* supports scripting-sockets so you no longer need `screen` to

get/put files or dump paste buffers to the remote console. Instead, you start your local

session like so:



```

~ > ./pscl -S ~/psc.script_sock

```



You can then go ahead and use it as before. If you need to 'paste' something you do like:



```

~ > ./pscsh -S ~/psc.script_sock -f script_/helloworld

```



This will 'type' the content of `script_/helloworld` to the console. While scripting,

the stdin of `pscl` is blocked so that the injected input does not mix up with any

typing. If `-S` is omitted in `pscsh`, `~/psc.script_sock` is used automatically.

For safety reasons, scripts must start with the `script_` prefix.



As a bonus, `pscr` now contains the ability to base64 en/decode files, even with CR

embedded characters for convenience. It is compatible to `uuencode -m`.