[Freeipa-devel] Certificate Identity Mapping

Thu Jan 5 12:15:57 UTC 2017

On Mon, Jan 02, 2017 at 08:06:04AM +0100, Jan Cholasta wrote:
> On 19.12.2016 12:13, Sumit Bose wrote:
> > 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.
> 
> Right.
> 
> > 
> > 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).
> 
> I would personally prefer if we used Python-style formatting strings for the
> templates, I find them much more pleasant to work with than C-style

sure, I used %A etc as arbitrary templates, I didn't wanted to imply
that C-style syntax should be used.

> formatting strings. The filter which covers AD behavior could be written as:
> 
> 
> (|(altSecurityIdentities=<I>{issuer_dn!x500}<S>{subject_dn!x500})(userPrincipalName={subject_nt_principal})(samAccountName={subject_nt_principal.name}))

I think this is quite readable and understandable to an admin with basic
knowledge of LDAP search filters and certificate components. Are the
names used here (issuer_dn, subject_dn, subject_nt_principal) already
used by e.g. some Python modules or do we have to define the list of
names?

If there a reason for using '!x500' to indicate X500 ordering? Wouldn't
e.g. issuer_dn.x500 work as well similar to Python's str.lower()? Having
only one symbol to indicate a special formatting of the value from the
certificate would make the syntax easier to learn.

> 
> > 
> > > 
> > > >   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.
> 
> This could be handled in the IPA plugin by converting from the user-friendly
> representation to LDAP filter template internally when a mapping rule is
> added or modified.

Yes, but it can also be expanded by the component/library which will
replace the templates with the actual values from the certificate. This
would have the befit that the user-friendly representation is visible
even with low-level LDAP commands?

bye,
Sumit

> 
> > 
> > > 
> > > > 
> > > > * 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
> 
> 
> -- 
> Jan Cholasta