OpenSSL/SSH

openssl rsa -text -noout -in id_rsa

Print out key attributes (o.a. key length)

openssl s_client -showcerts -connect <system>:<port>

Get the certificates available and test connectivity

openssl x509 -outform der -in <pemfile> -out <filename>.der -text

Convert to DER format.

ssh-keygen -t ed25519

Create a keypair (accept all defaults, put a password if you want)

ssh-keygen -y -f <privatkeyfile>

Print the public key for the private key in <privatkeyfile>

ssh user@target

SSH into target system as user, if user is omitted the current username is used

ssh -vvv user@target

Be very verbose with debug information

ssh -i keyfile user@target

Use the key in keyfile to connect