Detect and report keys using old rsa

[dolmen]

From GitHub documentation:

Note: GitHub improved security by dropping older, insecure key types on March 15, 2022.
As of that date, DSA keys (ssh-dss) are no longer supported. You cannot add new DSA keys to your personal account on GitHub.com.
RSA keys (ssh-rsa) with a valid_after before November 2, 2021 may continue to use any signature algorithm. RSA keys generated after that date must use a SHA-2 signature algorithm. Some older clients may need to be upgraded in order to use SHA-2 signatures.

So we should report old RSA keys that don't use SHA-2.

[dolmen]

Any resources on how to inspect an RSA key would be welcome.

[dolmen]

This article contains information of the OpenSSH private key file format: https://coolaj86.com/articles/the-openssh-private-key-format/