blog/content/2019/04/using-signal-on-server.html

---
title: Using Signal on a server
description: >
  Using Signal on a server
created: !!timestamp '2019-04-08'
time: 1:54 PM
tags:
  - signal
---

For a long while, I'd been using an email to SMS gateway to push
important notifications from my server, such as SMART error messages,
to my phone.  After all the
[NSA warrantless surveillance](https://en.wikipedia.org/wiki/NSA_warrantless_surveillance_(2001%E2%80%932007)),
I made a commitment to encrypt as much of my communications as possible.
When Signal came out, I adopted it because of it's strong encryption and
privacy.  Ever since I've been wanting to use it for notifications from
my server.  I finally got around to trying out the CLI version, and got
it to work.

The installation of the command line utility for Signal was more straight
forward than I was expecting.  I decided to use
[signal-cli](https://github.com/AsamK/signal-cli) and I was a bit worried,
as it uses Java.  Java has historically been difficult to run on FreeBSD
due to lack of support and draconian licensing terms.  I was surprised
that the packages for OpenJDK 8 were both present and just worked on my
server.  A simple `pkg install openjdk8` got Java up and running.

One thing to note is that the package said that fdesc and proc needed to
be mounted for Java to work, but I did not, and things still worked.
There are likely other parts of Java that may not work w/o those mounted,
but not for Signal.

As I have been using OSS for a long time, I like to build things from
source, so I followed the instructions at
[Building signal-cli](https://github.com/AsamK/signal-cli#building) and
got the command built with out any trouble.

Once the command was built, the
[Usage guide](https://github.com/AsamK/signal-cli#usage) provided the
basics, but didn't include instructions on how to verify the safety
numbers to ensure that the initial exchange was not MitM'd.  There is a
[man page](https://github.com/AsamK/signal-cli/blob/master/man/signal-cli.1.adoc),
but it requires a2x and separate steps to build, but a little bit of
digging got me the necessary steps (also, it turns out that the adoc
format is a simple text format).

With a bit of searching, I found the `listIdentities` and `verify`
commands.  There may have been another way, but because I had sent a
test message, my phone was listed:
```
$ signal-cli -u +XXXXXXXXXXX listIdentities
+YYYYYYYYYYY: TRUSTED_UNVERIFIED Added: Sat Apr 06 18:43:15 PDT 2019 Fingerprint: ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ ZZ  Safety Number: WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW
```

And then I needed to use the `trust` subcommand:
```
$ signal-cli -u +XXXXXXXXXXX trust -v 'WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW WWWWW' +YYYYYYYYYYY
```

The hardest part of this was figuring out how to invoke the command upon
reception of an email.  I used an alias listed in `/etc/aliases` to forward
the email to both the SMS gateway and myself.  The issue with trying to
invoke the command from here was that the command was run as the `mailnull`
user, which of course didn't have access to my user's home directory to
read the private key.  After a bit of debating, I remembered I use
`procmail`, and realized this was the best way to send the message.

I created a symlink for the command into my user's bin directory, created
a short script called `sendcell`:
```
$ cat ~/bin/sendcell
#!/bin/sh -

~user/bin/signal-cli -u +XXXXXXXXXXX send +YYYYYYYYYYY
```

and then added a filter to my `.procmailrc` file.  The filter at first
looked like this:
```
:0Wf
* ^TO_celluser@([^@\.]*\.)*example.com
| sendcell
```

But after the first test, it included all the headers, including all the
`Received` headers, so I updated it to use `formail` to remove all but the
`From`, `Subject` and `Date` (in case the message gets significantly delayed,
I can see by how much) headers:
```
:0c
* ^TO_celluser@([^@\.]*\.)*example.com
{
:0Wf
| formail -k -X From: -X Subject: -X Date:

:0
| sendcell
}
```

and now I get the messages delivered to my phone securely!

It is tempting to use this to be able to invoke commands on my server
remotely, but there isn't much I need to do when I don't have my laptop
with me.