TL;DR

There is no TL;DR, because it is so easy that any summary would be too long.

The steps are so simple:

Edit /etc/conf.d/dmcrypt

Checkout the samples and enable swap like this:

swap=crypt-swap
source='/dev/nvme0nXXX'

(obviously set the correct source device)

Start dmcrypt

Run

/etc/init.d/dmcrypt start

Enable swap

swapon /dev/mapper/crypt-swap

Make it persistent

# Enable automatic start of encryption & swap
rc-update add dmcrypt
rc-update add swap

# Enable automatic swapon
echo /dev/mapper/crypt-swap swap swap defaults 0 0 >> /etc/fstab

Background

Until today I did not know (or care) how to enable swap in Alpine Linux. As firefox (sic!) is killing my system more often these days, I decided to enable swap using a partition I left for it some years ago.

As I did not know how it works, I grep'ed for crypt through /etc/init.d, found the dmcrypt init script quickly which references /etc/conf.d/dmcrypt. I opened the config file and within minutes my encrypted swap was up.

Note that above instructions will reset/re-encrypt swap on every boot and thus no suspend to disk works with it. That's fine with me, as that is not a feature I am ever going to use.

The point I need to make is that at the moment there is no single other Linux distribution out there that I know off that is allowing me to make changes, even those which I don't know yet how to, that easily and I want to say many thanks for keeping Alpine Linux as simple as other Linux distributions used to be in the 90's.