Sumit Bose wrote:

I've started to write a SSSD design page about enhancing the current
mapping of certificates to users and how to select/match a suitable
certificate if multiple certificates are on a Smartcard.

My currently thoughts and idea and be found at
and for your convenience below as well.

Comments and suggestions are welcome. Please let me know about concerns,
alternatives and missing use-cases/user-stories.


= Matching and Mapping Certificates =

Related ticket(s):

=== Problem statement ===
==== Mapping ====
Currently it is required that a certificate used for authentication is either 
stored in the LDAP user entry or in a matching override. This might not always 
be applicable and other ways are needed to relate a user with a certificate.

==== Matching ====
Even if SSSD will support multiple certificates on a Smartcard in the context 
of it might be necessary to restrict 
(or relax) the current certificate selection in certain environments.

=== Use cases ===
==== Mapping ====
In some environments it might not be possible or would cause unwanted effort to 
add certificates to the LDAP entry of the users to allow Smartcard based 
authentication. Reasons might be:
* Certificates/Smartcards are issued externally
* LDAP schema extension is not possible or not allowed

==== Matching ====
A user might have multiple certificate on a Smartcard which are suitable for 
authentication. But on some host in the environment only certificates from a 
specific CA (while all other CAs are trusted as well) or with some special 
extension should be valid for login.

=== Overview of the solution ===
To match a certificate a language/syntax has to be defined which allows to 
reference items from the certificate and compare the values with the expected 
data. To map the certificates to a user the language/syntax should allow to 
relate certificate items with LDAP attributes so that the value(s) from the 
certificate item can be used in a LDAP search filter.

=== Implementation details ===
==== Matching ====
The pkinit plugin of MIT Kerberos must find a suitable certificate from a 
Smartcard as well and has defined the following syntax (see the 
pkinit_cert_match section of the krb5.conf man page or for 
details). The main components are

* <SUBJECT>regular-expression
* <ISSUER>regular-expression
* <SAN>regular-expression
* <EKU>extended-key-usage-list
* <KU>key-usage-list

and can be grouped together with a prefixed '&&' (and) or '`||`' (or) operator 
('&&' is the default). If multiple rules are given they are iterated with the order in 
the config file as long as a rule matches exactly one certificate.

'''Question: MIT Kerberos use case-sensitive matching and POSIX Extended 
Regular Expression syntax, shall we do the same?'''

While <SUBJECT> and <ISSUER> are (imo) already quite flexible I can see some 
potential extensions for the other components.

<EKU> and <KU> in MIT Kerberos only accept certain string values related to 
some allowed values in those field as defined in . The 
selection is basically determined by what is supported on server side of the pkinit plugin 
of MIT Kerberos. Since we plan to extend pkinit and support local authentication without 
pkinit as well I would suggest to allow OID strings for those components as well (the 
comparison is done on the OID level nonetheless).

The <SAN> component in MIT Kerberos only checks the otherName SAN component for the 
id-pkinit-san OID as defined in or the 
szOID_NT_PRINCIPAL_NAME OID as mentioned in 
While this is sufficient for the default pkinit user case of MIT Kerberos I would suggest 
to extend this component by allowing to specific an OID with <SAN:O.I.D>

==== Mapping ====
Since different certificates, e.g. issued by different CAs, might have 
different mapping rule, a matching rule must be added if there are more than 1 
mapping rule. A single mapping rule without a matching rule might be used as 
default/catch-all rule in this case.

If multiple rules matches the derived LDAP filter components can be grouped with the 
or-operator "|".

A mapping rule can use a similar syntax like the matching rule where the LDAP 
attribute can be added with a ':', e.g.
* <SUBJECT:ldapAttributeName>
* <SAN:O.I.D.:ldapAttributeName>

Currently I see no usage for <ISSUER>, <KU> and <EKU> in mapping rules because they 
do not contain any user-specific data. If at some point we will have personal CAs we might consider to 
add <ISSUER> based mappings.

'''Question, do we need search-and-replace at all (or at this stage)? Most of the 
interesting values from the SAN should be directly map-able to LDAP attributes. And 
processing the string representation of <SUBJECT> might be tricky as discussed 
below. Nevertheless the following might be possible:

* <SUBJECT:ldapAttributeName>/regexp/replacement/
* <SAN:O.I.D.:ldapAttributeName>/regexp/replacement/

'''where "/regexp/replacement/" stands for optional sed-like substitution 
rules. E.g. a rule like
'''would take the subject string 'CN=Certuser,CN=Users,DC=example,DC=com' from 
the certificate and generate a LDAP search filter component 
'(samAccountName=Certuser)' which can be included in a LDAP search filter which 
includes additional components like e.g. an objectClass.

'''The search-and-replace does not has to be sed-like because afaik there is 
not library which offers this and I would like to avoid implementing it. GLib 
e.g. has 
 g_regex_replace]. Since we already have a GLib dependency in SSSD due to soem 
utf8 helper functions using might be acceptable as well. Nevertheless it would 
be nice to hear if there are alternative libraries available as well.

===== Some notes about DNs =====
The X.500 family of standards define names as "SEQUENCE OF RelativeDistinguishedName" where the 
sequence is "starting with the root and ending with the object being named" (see X.501 section 9.2 
for details). On the other hand RFC4514 section 2.1 says "Otherwise, the output consists of the string 
encoding of each RelativeDistinguishedName in the RDNSequence (according to Section 2.2), starting with the 
last element of the sequence and moving backwards toward the first." This means that the ASN.1 encoded 
issuer and subject DN from the X.509 certificate can be either displayed as string in the
* X.500 order: DC=com,DC=example,CN=users,CN=Certuser
or in the
* LDAP order: CN=Certuser,CN=Users,DC=example,DC=com

As a consequence different tools will use a different order when printing the 
issuer and subject DN. While NSS's certutil will use the LDAP order, 'openssl 
x509' and gnutls's certtool will use the X.500 order (the latter might change 
due to

This makes it important to specific the order which is used by SSSD for mapping and 
matching. I would prefer the LDAP order here. E.g. by default the AD CA uses the DN 
of the users entry in AD as subject in the issues certificate. So a matching rule 
like '<SUBJECT:dn>' could tell SSSD to directly search the user based on its DN 
(which btw is the original intention of the subject field in the certificate, only 
that the DN should be looked up in a more general DAP as defined by X.500 and not in 
the lightweight version called LDAP)

Another issue is the limited set of attribute names/types required by the RFCs 
(see section of RFC 3280 and section 3 of RFC 4514). If e.g. the 
deprecated OID [ 
1.2.840.113549.1.9.1] is used all tools are able to identify it as an email 
address but OpenSSL displays it as '', certtool as 
'' and certutil as ''. So matching 
rules should try to avoid attribute names or only the ones from 
[ RFC 4514]:
* CN      commonName (
* L       localityName (
* ST      stateOrProvinceName (
* O       organizationName (
* OU      organizationalUnitName (
* C       countryName (
* STREET  streetAddress (
* DC      domainComponent (0.9.2342.19200300.100.1.25)
* UID     userId (0.9.2342.19200300.100.1.1)

==== About restricting or enforcing the mapping an matching any further ====
The goal of the matching rules in MIT Kerberos is to select a single 
certificate from a Smartcard which will then be used for PKINIT. Since we 
already plan to enhance SSSD to support multiple certificates on a Smartcard 
and if needed prompt the user which one to use for login we should not enforce 
that the matching rules should return only a single certificate or nothing.

Similar we plan to enhance SSSD to use the same certificate to log in with 
different user identities, e.g. as a user with standard privileges or as a user 
with administrator privileges. So it can make sense that multiple mapping rules 
apply to the same certificate and the related LDAP search filter components are 
or-ed together.

In many cases the login program will first ask for a user name which will help 
to restrict the number of suitable certificates even further and the mapping 
rules are only needed to check if the certificate belongs to the user trying to 
log in.

But gdm has a feature where gdm will detect when a Smartcard is inserted and 
call PAM without a user name. In this case SSSD has to determine the user name 
based on the certificates found on the Smartcard. If in this case multiple 
valid certificates are on the card and the mapping rules will return multiple 
users for each certificate gdm has to display a quite long selection of 
certificate-user pairs the user has to choose from.

So it should be underlined in the documentation that the matching and mapping 
rules should be detailed and specific so that for the given environment they 
help to avoid cases where the user is prompted to select a certificate (or user 
name in the gdm case) when trying to log in.

==== Storing matching and mapping configuration ====
On the IPA server a new objectclass can be created to store an matching-mapping 
rule pair. Both attributes are optional because a missing mapping rule would 
mean that the user entry will be search with the whole certificate. A missing 
matching rule will indicate catch-all rule with a default mapping.

Specifying matching-mapping rules in sssd.conf is a bit more complicated 
because SSSD does not respect multiple entries with the same keyword, only the 
last one is used. So all rules have to be added to a single line. To give it a 
little bit of structure the rules can be enclosed by curly-braces '{}{}' and 
each rule pair is separated by a comma ','. A single rule in curly braces 
indicates a matching rule and the mapping will be done with the whole 
certificate. A default/catch-all mapping rule will start with an empty pair of 
curly braces followed by a pair containing the mapping rule.

===== Examples =====
* '''certificate_rules = {<EKU>msScLogin}''': only allow certificates with have 
the Microsoft OID for Smartcard logon set. use the whole 
certificate to look-up the user. The same result can be achieved with
* '''certificate_rules = {<EKU>}''': see above
* '''certificate_rules = 
 only allow certificates form the 'my-company' issuer which have an email address from the 
'' domain in the rfc882Name SAN attribute. Use the email address in a LDAP search 
filter '(mail=email-address)' to find the matching user.

=== Configuration changes ===
Does your feature involve changes to configuration, like new options or options 
changing values? Summarize them here. There's no need to go into too many 
details, that's what man pages are for.

=== How To Test ===
This section should explain to a person with admin-level of SSSD understanding 
how this change affects run time behaviour of SSSD and how can an SSSD user 
test this change. If the feature is internal-only, please list what areas of 
SSSD are affected so that testers know where to focus.

=== How To Debug ===
Explain how to debug this feature if something goes wrong. This section might 
include examples of additional commands the user might run (such as keytab or 
certificate sanity checks) or explain what message to look for.

=== Authors ===
Give credit to authors of the design in this section.

Wow, this is really great.

I think I'd pre-plan to support different configuration per issuer subject, with one named default. It shouldn't be a lot more work and will future-proof things for you, particularly in how the rules are stored in LDAP.

I worry a bit about matching without comparing the certificate for the case where you don't examine issuer.

You may want to have an option to require that the presented cert match the one stored in LDAP (off by default). I realize that you specifically mention this can be problematic, but it can also be quite useful. It can be used, for example, to disable a login by removing the certificate from the user's entry. It also ensures that some carefully crafted certificate doesn't allow a bad actor to map to a user account.


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