[Freeipa-devel] Certificate Identity Mapping

Mon Dec 19 11:13:44 UTC 2016

On Mon, Dec 19, 2016 at 10:02:58AM +0100, Jan Cholasta wrote:
> I agree with *almost* everything Sumit said. See my inline comments below.
> 
> On 16.12.2016 11:53, Sumit Bose wrote:
> > On Tue, Dec 06, 2016 at 04:39:10PM +0100, Florence Blanc-Renaud wrote:
> > > Hi,
> > > 
> > > I have started a feature description for the Certificate Identity Mapping at
> > > the following location:
> > > http://www.freeipa.org/page/V4/Certificate_Identity_Mapping
> > > 
> > > This is a first step, focusing on the interface we would like to provide. It
> > > still contains open questions, some of which are linked to the corresponding
> > > design on SSSD side:
> > > https://fedorahosted.org/sssd/wiki/DesignDocs/MatchingAndMappingCertificates
> > > https://fedorahosted.org/sssd/wiki/DesignDocs/SmartcardsAndMultipleIdentities
> > > 
> > > Comments, concerns and suggestions are welcome. Thanks!
> > 
> > Hi Flo,
> > 
> > thank you very much for setting up the page.
> > 
> > My comments are mostly about the commands.
> > 
> > certmappingconfig-mod:
> > 
> > * --enable=Boolean: if this option is 'False' SSSD will basically show
> >   the current behavior and just look up the certificates directly. But I
> >   wonder if the option is needed at all because not adding any mapping
> >   rules would have the same effect.
> > 
> >   What is the scope here, only the IPA domain, or all trusted domains as
> >   well? If it is for trusted domains as well will the certmappingrule-*
> >   commands and user-{add/remove}-certmapping return an error?
> > 
> >   So, in general I see an overlap with the mapping rules and I think it
> >   would be clearer to drop this option and do the lookups according to
> >   the mapping rules.
> > 
> > * --prompt-username=Boolean: the description implies that this option is
> >   synonymous to 1:1 mapping, but it is not. On Linux authentication in
> >   most cases use a user name either by directly asking (e.g. /bin/login)
> >   or using the current user name (e.g. sudo). So, according to its name
> >   it would only control if gdm is allowed to ask for an (optional) user
> >   name.
> > 
> >   If the option is renamed to e.g. --force-1-to-1-mapping to really
> >   enforce a 1:1 mapping then it would make sense to derived to gdm
> >   behavior. I.e. if 1:1 mapping is enforce it makes no sense for gdm to
> >   ask for a user name and if it is not enforced then it makes sense to
> >   offer and optional user name input field.
> > 
> > * --enable-username-mismatch=Boolean: I think this option can be
> >   dropped. My test so far show that if a non-matching hint is given on a
> >   Windows client authentication fails.
> > 
> > * --alternate-attribute=STRING: I think this option isn't needed as
> >   well. For IPA server-side we should decide on an attribute name and
> >   add it to the schema for user objects. On the client side the
> >   attribute name can be taken from the mapping rule.A
> > 
> > 
> > certmappingrule.*:
> > 
> > * ISSUERDN: it looks like you want to use issuerName here. In
> >   certificateRecord it it used with LDAP ordering and I would prefer
> >   LDAP ordering at all points where we have a choice. Unfortunately in the
> >   issuer-subject mapping AD dictates X.500 ordering.
> 
> LDAP ordering should indeed be preferred, as it is used everywhere else in
> IPA. We can convert to/from X.500 ordering where necessary, when possible.
> 
> > 
> > * DOMAINDN: does this refer to the nsslapd-certmap-basedn attribute in
> >   the example? My intention in the SSSD design-page was to specify the
> >   domain (as in DNS domain/IPA domain/trusted domain) where the matching
> >   user should be searched. Different domains might certificates from
> >   different issuers and some domains might not even use certificates.
> >   With this information SSSD does not have to search any domain trusted
> >   by IPA from a given certificate, but look only at domains listed here
> >   (the attribute should be a multi-value one).
> > 
> >   There are objects in the LDAP tree for each trusted domain which are
> >   used by SSSD so using a DN syntax would be valid here.
> 
> We use domain names rather than DNs to refer to domains everywhere else in
> the framework. I don't think this place should be an exception.

I'm fine with domain names as well. In fact I didn't thought of using
DNs for this before I read DOMAINDN on the design page.

> 
> > 
> > * LDAPSEARCHFILTER: I think a separate option is not need. LDAP search
> >   filters should just be a special kind of mapping rules. I can image in
> >   syntax like: <LDAPFILTER:(&(cn=%A)(email=%B)(authType=pkinit))>. I
> >   think the difficult part with the LDAP filters will to define sensible
> >   templates.
> 
> I'm not sure I understand. Could you please elaborate a little bit?

A LDAP search filter which would cover the AD behavior would look like: 

(|(altSecurityIdentities=<I>%A<S>%B)(userPrincipalName=%C)(samAccountName=%D))

where

%A: must be replaced with the issuer of the certificate in X.500 order
%B: must be replaced with the subject of the certificate in X.500 order

it would be possible of course to use a specific template here which
would generate the complete search attribute value.

%C: must be replaced by the principal from AD's SAN
    szOID_NT_PRINCIPAL_NAME
%D: must be replaced with only then name component (the part before the
    realm) of the principal from szOID_NT_PRINCIPAL_NAME

As %C and %D imply this filter will only work for certificates which
have szOID_NT_PRINCIPAL_NAME but for those it must be used to be
compatible with AD. For certificates without

(altSecurityIdentities=<I>%A<S>%B)

is sufficient. It is possible to select the right filter with matching
rules.

So we have to find suitable names for the %A, %B, %C and %D templates
and also allow different representations (e.g. LDAP or X.500 order for
DNs).

> 
> >   But as long as we keep the general mapping rule syntax
> >   flexible the LDAP filter rules can be added in a later version.
> 
> IMHO it should be the other way round and LDAP filters should be implemented
> first, as they offer all the flexibility we need (all of the other fields
> can be easily implemented on top of LDAP filters) and are by default
> extensible without having to update servers and clients.

In general I agree, as long we can find a suitable scheme to handle the
templates to add content from the certificate in a specific format to
the search filters.

But from the user/admin perspective there should be special rules for
common use-cases which do not require to know too much details about
certificates and LDAP trees. E.g. for AD (either via direct or indirect
integration) there should be a <AD-LIKE> rule which just does all which
AD would do depending on the certificate type. For IPA something like
<ALT-SEC-ID-I-S> might be a good start for handling external
certificates which do not contain user specific data which can be mapped
to user object because the syntax is already known from AD.

> 
> > 
> > * enable/disable: I think this is a good idea and would be consistent
> >   with other rules like HBAC and sudo
> > 
> > * user-{add/mod} LOGIN --certmappingdata DATA: I think it might be
> >   better to not add this option and only implement the
> >   'user-{add/remove}-certmapping' commands
> > 
> > * user-{add/remove}-certmapping: you say '... almost any type of mapping,
> >   or a more user-friendly API ...'. I would not say 'or' but 'and' and
> >   implement both
> > 
> > * ipaCertMappingEnableMismatch and ipaCertMappingAlternateIdAttribute; I
> >   think both are note needed, see above
> > 
> > * altSecurityIdentities: I would prefer to use a different name and OID.
> >   Using the same definition as AD would imo imply that it can be used in
> >   the same way as in AD. But e.g. AD also supports other content like
> >   KERBEROS:alternative_user_principal at AD.DOMAIN which we will not
> >   support.
> > 
> > * issuerName vs ipaCAIssuerDN: I would prefer issuerName because it is
> >   general UTF-8 and not DN syntax (1.3.6.1.4.1.1466.115.121.1.12). Since
> >   the issuer DN in general will not be a DN from the local LDAP tree I
> >   think the UTF-8 version fits better.
> 
> I think it's worth mentioning that if the attribute used DN syntax and
> matching, we wouldn't have to worry about normalizing the issuer name before
> searching for it, as DS would do that for us.

Good point, but I think the main use case for this attribute is on the
client side to determine if a rule should be applied to a certificate or
not. So I guess LDAP searches with this attribute would be rare because
the client will load all rules in one run.

> 
> > 
> > * nsslapd-certmap-basedn, see DOMAINDN above
> > 
> > * altSecurityIdentities example: X.500 ordering is used by AD here and
> >   unfortunately I think we have to adopt it at least for this specific
> >   usage, here is an ldapsearch output from AD:
> > 
> > altSecurityIdentities:
> > X509:<I>DC=devel,DC=ad,CN=ad-AD-SERVER-CA<S>DC=devel,DC
> >  =ad,CN=Users,CN=t u,E=test.user at email.domain
> > altSecurityIdentities: X509:<I>O=Red Hat,OU=prod,CN=Certificate
> > Authority<S>DC
> >  =com,DC=redhat,OU=users,OID.0.9.2342.19200300.100.1.1=sbose,E=sbose at redhat.co
> >  m,CN=Sumit Bose Sumit Bose
> > 
> > * Certificate Mapping Administrators or re-use Certificate
> >   Administrators: I would prefer a new 'Certificate Mapping
> >   Administrators'
> > 
> > * Users can manage their own X.509 certificate mappings? I'm not sure
> >   here, at the first glance I would say no. How are OTP tokens handled?
> >   Maybe this would be a candidate for certmappingconfig-* option?
> 
> I think a better question is "How is userCertificate handled?"
> 
> Anyway, self-service permissions can be enabled/disabled, so there is really
> no need for a new certmappingconfig option.

Yes, this makes sense.

bye,
Sumit
> 
> > 
> > That's all :-)
> > 
> > bye,
> > Sumit
> > 
> 
> 
> -- 
> Jan Cholasta