The keys used for communication over TLS in swarm mode are Elliptic Curve (ECDSA) keys created with a length of 256 bits. The strength of an Elliptic Curve key of 256 bits is roughly equivalent to that of a 3072-bit RSA key. You can check the strength of your swarm keys yourself by looking at the files located on a Swarm manager in
/var/lib/docker/swarm/certificates/* and viewing the details of your Swarm certificates with the
openssl command. For example:
sudo openssl x509 -text -in /var/lib/docker/swarm/certificates/swarm-node.crt
This will tell you about the certificates used for swarm mode, as well as the encryption type and key strength used for Swarm mode.
Additionally, you can take a look at a section of the Swarmkit source within the Docker project where this is explicitly set.