Add ssh signataure to step ssh inspect

[maraino]

Description

This PR adds the signature and signature type to the step ssh inspect command. Fixes #699

Now a certificate will inspect as:

$ step ssh inspect mariano-cert.pub
mariano-cert.pub:
        Type: ecdsa-sha2-nistp256-cert-v01@openssh.com user certificate
        Public key: ECDSA-CERT SHA256:a8SHPBTzbkmcfAp+FWBoOj87k7wnGEG3qleXFPXz8FM
        Signing CA: ECDSA SHA256:ExpOxCvxwkeqnHuUtTR5dhsTUFgcN1QeuAYqOGE6200 (using ecdsa-sha2-nistp256)
        Key ID: "mariano@smallstep.com"
        Serial: 6770237405177342977
        Valid: from 2022-02-01T12:01:07 to 2022-02-02T04:02:07
        Principals:
                mariano@smallstep.com
        Critical Options: (none)
        Extensions:
                permit-port-forwarding
                permit-pty
                permit-user-rc
                permit-X11-forwarding
                permit-agent-forwarding
        Signature:
                00:00:00:20:62:59:41:e3:96:ab:49:95:e8:6a:1d:a5:
                9e:63:21:c5:b6:1d:66:f9:40:e8:7b:ec:c4:4c:41:c9:
                33:8e:2b:a3:00:00:00:21:00:a9:f0:ee:9c:69:5a:4f:
                a8:f3:dd:8c:88:36:92:3f:3a:70:31:fe:52:53:cf:b4:
                05:cd:a3:ba:1b:69:5b:3c:e0

The (using ecdsa-sha2-nistp256) has been added too to match the output of ssh-keygen -L -f mariano-cert.pub. Note that ssh-keygen does not display the signature.

[maraino]

The JSON format looks like this:

{
  "Type": "user",
  "KeyName": "ecdsa-sha2-nistp256-cert-v01@openssh.com",
  "KeyID": "mariano@smallstep.com",
  "KeyAlgo": "ECDSA",
  "KeyFingerprint": "SHA256:a8SHPBTzbkmcfAp+FWBoOj87k7wnGEG3qleXFPXz8FM",
  "SigningKeyAlgo": "ECDSA",
  "SigningKeyFingerprint": "SHA256:ExpOxCvxwkeqnHuUtTR5dhsTUFgcN1QeuAYqOGE6200",
  "Signature": {
    "Type": "ecdsa-sha2-nistp256",
    "Value": "AAAAIGJZQeOWq0mV6GodpZ5jIcW2HWb5QOh77MRMQckzjiujAAAAIQCp8O6caVpPqPPdjIg2kj86cDH+UlPPtAXNo7obaVs84A=="
  },
  "Serial": 6770237405177342977,
  "ValidAfter": "2022-02-01T12:01:07-08:00",
  "ValidBefore": "2022-02-02T04:02:07-08:00",
  "Principals": [
    "mariano@smallstep.com"
  ],
  "CriticalOptions": {},
  "Extensions": {
    "permit-X11-forwarding": "",
    "permit-agent-forwarding": "",
    "permit-port-forwarding": "",
    "permit-pty": "",
    "permit-user-rc": ""
  }
}

[Slamdunk]

Awesome, thank you 💪

Note that ssh-keygen does not display the signature.

Indeed this is to me a big missing from ssh-keygen 🤷

[dopey]

lgtm