Category Archives: Encryption

Trustworthy email: authentication using exim4, SPF, DKIM and DMARC

The email authentication technologies we’re about to implement are, according to the authentication authorities, all you need to be regarded as being from your domain when you send email, and someone else not being from your domain.  Effect: your emails can be considered trustworthy by email receivers who use these technologies. If they don’t use these technologies, they can’t tell.

At the very least, Google will be less likely to think your email is spam.

PTR record

A PTR record can be obtained from your host’s nameserver – it’s a reverse DNS record for your IP address. If the PTR record points at rather than (your domain), and you’re claiming to be sending mail from, what’s the email recipient meant to think?


will tell you what the host for that IP is. Lodge a ticket with your hosting provider and get that PTR record changed to This might take about a day.

SPF record

Create a Sender Policy Framework record on your nameserver:

TXT @ "v=spf1 a mx -all"

This says “for my domain, I will only send email from IP addresses listed on the nameserver”.  Nameserver changes take time to propagate.

After your nameserver changes have propagated, you can go to to check out if you got it right.


DomainKeys Identified Mail is where things get more involved.  We’re doing this on a Debian Linux, like Ubuntu for exim4. We’re making our signing key 2048 bits, which is long enough to make life slightly unpleasant for us. Fortunately for you I’ve written a bash script that outputs the TXT record we need to create on the nameserver – because some nameservers (I’m looking at you, Gandi) can’t hold “long” strings – it’s broken into “small” strings:

sudo apt install openssl
cd /etc/exim4
sudo openssl genrsa -out dkim.private 2048
sudo openssl rsa -in dkim.private -out dkim.public -pubout -outform PEM
echo $(echo $(date -u +%Y%m%d && echo '') | sed -e 's/[ ]//g' && echo $(echo ' TXT "v=DKIM1; p="' && echo $(grep 'PUBLIC KEY' -v dkim.public) | sed -e 's/[ ]//g' | fold -w200 | sed -e 's/\(.*\)/"\1"/g'))

which gives something like
20170419._domainkey TXT “v=DKIM1; p=” “MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvCNqU0Njd4YQ4e89T3FNc+uyOS2JwUqynGk7uwcSYHjIE2MGRuTxi56s4JgPKSnCVlBkJlUnXQHXFp2UGnLm8SADtjRMfWwpNxz6TmzXBpMnNZV1zvuoBBdcxh0Qg1TtCSACtWM6ehml0BmOHVA8Ippqj9iRlP2HMjuVMxZXewN9eJl”

That all goes into one nameserver TXT record, spaces and all.  The world will join up the ” ” and get one big string. Note the bold number up there? That’s the selector. That a number needs to get larger with each new key.  Periodically you’re going to have to reissue your key because security.  You know what gets larger as time goes by?  The date.  Use the date.  If you screw up, use tomorrow’s date, etc.

Once you’ve got our public key out to the public via our public nameserver, we need to get exim to sign the payloads:

sudo nano conf.d/main/01_exim4-config_listmacrosdefs

After the line CONFDIR = /etc/exim4, add:

#DKIM loading
DKIM_CANON = relaxed
DKIM_DOMAIN = ${sender_address_domain}
DKIM_SELECTOR = 20170419

and reload the mail server

sudo service exim4 restart

After an appropriate delay for nameserver propagation, use to check your work.
If that works out, from to ensure everything checks out:

echo -e "Test my DKIM plz\nMsg Body\n.\n\n" | mail -v


Domain-based Message Authentication, Reporting and Conformance is where the wheels can come off if you screwed anything up.  We’re going to set things up so that when you screw it up, computers scold you rather than putting your emails in the bin.

You will need to create two dmarc reporting accounts.  Servers will email you a (surprisingly detailed) report card on how you’re doing with your implementation. It’s best if these accounts are on the same domain, because technically you need to be or it’ll be ignored (Google will happily mail reports off-domain even if the other domain hasn’t said that’s okay).  Yours are and, according to the following nameserver entry: 1800 IN TXT "v=DMARC1;p=none;pct=100;;"

none is the consequence for screwing up. none is where we’ll start at, and see what the reporting records say to us.  After a while, you’ll be comfortable that everything is ticking along nicely, and you’ll up the consequent to quarantine (shove it in spam) or reject (burn it).

After your nameserver changes have propagated, you can go to to check out if you got it right.

As a human, to read the records sent to you, upload the files to

How to to install the Crypt::Eksblowfish::Bcrypt module, and Crypt::Random

Have you gotten the error
Can't locate Crypt/Eksblowfish/ in @INC (you may need to install the Crypt::Eksblowfish::Bcrypt module)
and wondered what to do? Wonder no more!

sudo apt install libcrypt-eksblowfish-perl

and perhaps

sudo apt install libdigest-bcrypt-perl

What about
Can't locate Crypt/ in @INC (you may need to install the Crypt::Random module)

sudo apt install unzip make gcc
cd Math-Pari-2.01080900/
perl Makefile.PL
sed -i 's/CLK_TCK/CLOCKS_PER_SEC/g' pari-2.1.7/src/language/init.c
make test
sudo make install
cd ..
tar zxvf Crypt-Random-1.25.tar.gz
cd Crypt-Rando1.25.tar
perl Makefile.PL

Easy! Only takes a few hours if you don’t know what you’re doing.

Disney: evil, but defeatably evil

Disney DVD’s slogan is Moves, Magic and More.  They got the more part right for sure.

There I am trying to back up my copy of Wall_e_lic2_d1 so that once the kids have scratched the living bejesus out of the playing version, a new one can be generated from the master. And also as to avoid the annoying ads, language selection and other remote-control-based activity at the start – just shove it in the DVD player and walk away. Thankfully Australian copyright law lets me do this.

The studio have been dicking around with the disk’s table of contents, giving it over seventy files it claims are five gig in size – which, giving the DVD specification, is not possible. What you need to do in circumstances like this is play it in some player that will tell you what the magic track that actually contains the movie, not some hacked version of it. Then back that one up.

In the case of this particular disk it’s track 53, 1:33:26 long weighting in at 5425.95MB in size.

Thing is, DVDShrink barfs on it. Like it does Cars, but for different reasons. Thankfully I’ve recently discovered that Linux has an equivalent to DVDShrink, but this one is still being maintained. K9copy is it’s name; Cars was processed with no problems, and it was only the tomfoolery on Wall-E that caused a pause in activity.

So there’s one less application that I need a copy of Windows to run.

Distributed computing, capitalist style

Here’s an idea: rather than sending advertisements to user’s computers, why not send scripting code to calculate [the valuable thing, like, I dunno, pi or hacking the encryption on HDDVD or something] and send the results back to your central computer?

Come on, you know you want to. And it’s free! Primer: GPG encryption in five minutes or your money back

A while back I was given the task of setting up encryption for sending files around. Ooh. Sounds tricky, I thought. I’ve seen PGP signatures on privacy freaks’ e-mail for years now, but it all seemed a bit like black magic. I had no idea how it worked.

I went looking, and it turns out it’s not really particularly difficult to figure out or get working. But I had to wade through a few hefty (in web terms) manuals to find all the info I needed. I never really found a web page which detailed the basics in an easy to digest format. This could be that page.


PGP is Pretty Good Privacy, invented by Phil Zimmerman and now run by the PGP Corporation. It’s the defacto standard for this kind of stuff. It’s fairly secure, and has the added benefit of compressing text quite well. PGP sell a number of solutions, but if you’re wondering about freebies, then…

GPG is Gnu Privacy Guard, which is the free implementation of (most of) PGP. It lives here:

How to use it

Encryption of this type is all about keys. If you haven’t grasped the key concept before, here it is in brief: a recipient has a public and a private key. The public key is given to anybody. Senders encrypt stuff using the public key. Only the recipient has the private key, and uses this to decrypt stuff. Obviously if communication is two-way, you need multiple public and private keys. Okay? Easy.
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