Certificate-specific ssh_key method suite.

Certificate keys don't work the same as normal keys, so the rest of
the code is going to have to pay attention to whether a key is a
certificate, and if so, treat it differently and do cert-specific
stuff to it. So here's a collection of methods for that purpose.

With one exception, these methods of ssh_key are not expected to be
implemented at all in non-certificate key types: they should only ever
be called once you already know you're dealing with a certificate. So
most of the new method pointers can be left out of the ssh_keyalg
initialisers.

The exception is the base_key method, which retrieves the base key of
a certificate - the underlying one with the certificate stripped off.
It's convenient for non-certificate keys to implement this too, and
just return a pointer to themselves. So I've added an implementation
in nullkey.c doing that. (The returned pointer doesn't transfer
ownership; you have to use the new ssh_key_clone() if you want to keep
the base key after freeing the certificate key.)

The methods _only_ implemented in certificates:

Query methods to return the public key of the CA (for looking up in a
list of trusted ones), and to return the key id string (which exists
to be written into log files).

Obviously, we need a check_cert() method which will verify the CA's
actual signature, not to mention checking all the other details like
the principal and the validity period.

And there's another fiddly method for dealing with the RSA upgrade
system, called 'related_alg'. This is quite like alternate_ssh_id, in
that its job is to upgrade one key algorithm to a related one with
more modern RSA signing flags (or any other similar thing that might
later reuse the same mechanism). But where alternate_ssh_id took the
actual signing flags as an argument, this takes a pointer to the
upgraded base algorithm. So it answers the question "What is to this
key algorithm as you are to its base?" - if you call it on
opensshcert_ssh_rsa and give it ssh_rsa_sha512, it'll give you back
opensshcert_ssh_rsa_sha512.

(It's awkward to have to have another of these fiddly methods, and in
the longer term I'd like to try to clean up their proliferation a bit.
But I even more dislike the alternative of just going through
all_keyalgs looking for a cert algorithm with, say, ssh_rsa_sha512 as
the base: that approach would work fine now but it would be a lurking
time bomb for when all the -cert-v02@ methods appear one day. This
way, each certificate type can upgrade itself to the appropriately
related version. And at least related_alg is only needed if you _are_
a certificate key type - it's not adding yet another piece of
null-method boilerplate to the rest.)

1 files changed, 6 insertions, 0 deletions

diff --git a/utils/nullkey.c b/utils/nullkey.c
index d4f2a691..1b01c891 100644
--- a/utils/nullkey.c
+++ b/utils/nullkey.c
@@ -11,3 +11,9 @@ const char *nullkey_alternate_ssh_id(const ssh_keyalg *self, unsigned flags)
     /* There are no alternate ids */
     return self->ssh_id;
 }
+
+ssh_key *nullkey_base_key(ssh_key *key)
+{
+    /* When a key is not certified, it is its own base */
+    return key;
+}