== Using Attestation

=== Introduction
This feature is only available in YubiKey 4.3 and newer.

A high level description of the thinking and how this can be used can be found
at https://developers.yubico.com/PIV/Introduction/PIV_attestation.html

=== Usage
Attestation works through a special key slot called “f9” this comes
pre-loaded from factory with a key and cert signed by Yubico, but can be
overwritten.
After a key has been generated in a normal slot it can be attested by this
special key, this can be realised by using the yubico-piv-tool action attest:

  $ yubico-piv-tool --action=generate --slot=9a
  ...
  $ yubico-piv-tool --action=attest --slot=9a

The output of this is a PEM encoded certificate, signed by the key in slot f9.

=== Verifying
To verify an attestation step 1 is to build the certificate chain. Put the
attestation root certificate in a file (or if you trust several put all
of them in said file). The Yubico root certificate can be found at
https://developers.yubico.com/PIV/Introduction/piv-attestation-ca.pem

Then add the keys attestation certificate to that file:

  $ yubico-piv-tool --action=read-certificate --slot=f9 >> certs.pem

Now we're ready to verify the attestation:

  $ yubico-piv-tool --action=attest --slot=9a > attestation.pem
  $ openssl verify -CAfile certs.pem attestation.pem
  attestation.pem: OK