Increase diego-ssh RSA host key size from 2048 to 4096 bits

[plamen-bardarov]

Description:

As part of ongoing security hardening, the RSA host key used by diego-sshd in LRPs should be increased from 2048 bits to 4096 bits.

Currently, SSHKey in lib/cloud_controller/diego/ssh_key.rb defaults to generating 2048-bit RSA keys. These keys are used when SSH is enabled on an app process to initialize the diego-sshd daemon.

Proposed solution:

I've updated(in the linked draft PR) the default in SSHKey#initialize from 2048 to 4096. Since AppRecipeBuilder instantiates SSHKey.new without an explicit bit size, this change propagates automatically to all LRPs with SSH enabled.
No other code changes are required.

Would this simple default change be acceptable, or would it be preferable to make the key size configurable via the Cloud Controller config (e.g. under diego.sshd.host_key_bits)? Happy to adjust the approach based on feedback.

Links:

• Increase diego-ssh RSA host key from 2048 to 4096 bits cloud_controller_ng#5032

[jochenehret]

The change of the RSA key size should be ok. There is however a minimal risk that the longer strings in the desired LRP definition hit some limits. But this should be detected in the capi validation pipeline.

[plamen-bardarov]

From what I can see the host key is stored in routes.diego-ssh.private_key and in diego-sshd's run action's args array. That was exactly my concern, if doubling the size will have any implications on the DesiredLRP.