[Freeipa-devel] DNSSEC design page: key wrapping

[Simo Sorce]

On Tue, 2014-03-04 at 13:51 +0100, Petr Spacek wrote:
> On 26.2.2014 16:00, Simo Sorce wrote:
> >>>> need to be protected as carefully as the private key.
> >>> > >
> >>> > >This is something I meant to discuss too, how do we protect them ?
> >>> > >Clearly we have ACIs but I am wondering if we want to encrypt them with
> >>> > >keys not immediately or easily available via LDAP ?
> >>> > >
> >>> > >It's kind of catastrofic if they get inadvertently exposed like if
> >>> > >someone does a ldapsearch as "Directory Manager", which is one of the
> >>> > >reasons why we encrypt kerberos key material before storing it into the
> >>> > >db.
> >> >
> >> >PKCS#8 allows encryption, I guess we can use that. There needs to be
> >> >some metadata on how to decrypt the blob though, so that the PKCS#11
> >> >module can actually decrypt it when necessary.
> > Yep, and we also need to decide what master key is used and where it is
> > placed, and who access it, and how:-)
> 
> Let's move the discussion forward, we need to implement the schema for 4.0.
> 
> Do I understand correctly that the whole purpose of
> krbPrincipalName=K/M at IPA.EXAMPLE,cn=IPA.EXAMPLE,cn=kerberos,dc=ipa,dc=example
> is just to encrypt keys with some other key which is located at some other 
> place? I.e. the goal is to lower the probability that a random ldapsearch will 
> return encrypted blob and master key at once, right?

Yes, it would also be nice if we could finally offload this key from
LDAP for added security, but we are not there yet.

> What algorithm/method should we use for key wrapping? As far as I remember 
> from my studies key wrapping is very sensitive thing and we definitely need to 
> use some existing standard&tool for doing it. Can we just call some NSS 
> function with default/system-wide parameters and let it do it's job?

I think that would be what we want to do in some form.

> That would mean that, after all, we just need to provide some blob as key 
> wrapping key :-) (Generated with care it deserves etc.)

The key must be self describing somehow (for algorithm agility).


> We have had discussion with Honza and the first idea is to add attribute like 
> 'wrappingKeyId' to each encrypted blob and use it for locating appropriate key 
> when necessary.
> - During decryption: Do a LDAP search with filter like (keyId=<wrappingKeyId>) 
> to find appropriate wrapping key.
> - The harder part is how to pick a wrapping key for encryption. It can be 
> tricky if the wrapped key is shared among more users (DNS servers) etc.
> - It is possible to easily use multiple wrapping keys at once so key rollover 
> is easy. You can re-encrypt keys one by one.

This makes things complicated fast.
One thing I was thinking was to use a keytab and krb functions to do the
wrapping but that would be pretty IPA specific.

> The other idea is to add 'wrappingKeyId' to PKCS#11 token. So all PKCS#11 
> objects inside the same token will be encrypted with the same key.
> - Decryption is easy - the same as in previous case.
> - Encryption is basically the same as encryption.
> - Key rollover is hard. You would have to re-encrypt all keys and change 
> wrappingKeyId associated with given token at once - but it is impossible 
> because we don't have LDAP transactions. As a result, clients will be confused 
> during rollover. (Consider problems with syncrepl when clients can see changes 
> immediately.)

Yeah this is a problem we need to address.

> The third approach is 'hybrid':
> A 'wrappingKeyId' associated with PKCS#11 token is 'the active one' and is 
> used for encrypting new objects stored into PKCS#11 token. Each key stored in 
> the token has own wrappingKeyId attribute and it is used for decryption.
> - Decryption is easy - the same as in previous case.
> - Encryption always use wrappingKeyId associated with given token.
> - Key roll over can start from wrappingKeyId associated with the token and 
> then re-encrypt keys in the token one by one. All keys can be decrypted in any 
> step (assuming that changes in one LDAP object are atomic).
> 
> 
> Where is a hole in this design? :-)

I do not like the idea of having to add a wrappingKeyId everywhere.

My initial though was to have a central place where wrapping keys are
stored, and during a rollover period you try all the keys until one
works or decryption fails. At the end of rollover you delete the old key
so you avoid the additional load.

> Where should we store wrapping keys for users/services and DNS servers? User 
> object or cn=dns doesn't sound like a good idea because it would defeat the 
> purpose.

Indeed. And this is the biggest problem. It would be nice to have a
mechanism to securely transfer the key to the DNS servers w/o having to
store it permanently in LDAP. If we had this mechanism we'd be able to
apply it to the Kerberos master key too. But it can't be confined to
installation time only, which is easy, it needs to be possible to update
it at a later time, which is where we have issues, as our replication
mechanism is LDAP.

If we solve the DNA plugin issue with the ability to use groups for
authentication I guess we could build a control/extend operation that
would allow masters to transfer keys to each other w/o exposing them as
LDAP searches, do we want to try to go in that direction ?

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York