Bump, Sumit, have you seen my comments? I haven't heard back from you.

On 17.10.2016 09:50, Jan Cholasta wrote:

On 13.10.2016 18:52, Sumit Bose wrote:
On Tue, Oct 11, 2016 at 01:37:09PM +0200, Sumit Bose wrote:
On Thu, Oct 06, 2016 at 12:49:30PM +0200, 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
alternatives and missing use-cases/user-stories.



Rob, Fraser, Alexander, thank you for your comments. I think both the
issuer specific matching and the OID in the SUBJECT matching are good
ideas. I updated the design page accordingly. The changes can be shown

The updated version can be found below as well. Of course more
comments and
suggestions are still very welcome.

I did another update. A "Compatibility with Active Director" section is
added which made me realize that there are use-cases for using the
issuer in the mapping as well and the sub-strings in LDAP search filters
might be useful as well.

The changes can be seen with

Please let me know your comments and suggestions.


= 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 https://fedorahosted.org/sssd/ticket/3050 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.

Note that in some cases it might be possible to map a certificate to a
user without having to do an extra LDAP search, for example when the
certificate contains the principal name of the user. Does the design
allow this? Or is there no extra LDAP search?

=== 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.

I don't think regular expressions are a particularly good choice for DN
matching. It is difficult to express assertions which are quite natural
for DNs (matching multi-attribute RDNs, matching the same attribute type
by different identifiers, respecting the defined matching rules of
attribute types) and at the same time it is easy to express assertions
which do not make much sense for DNs (matching substrings in attribute
names, matching accross multiple syntactical elements, etc.).

That said, does the design have to be based on the MIT pkinit matching?
To me it looks like something quickly hacked together rather than
thoughtfully designed. I would personally base the design on the
concepts of CertificateMatch, which is the standard way of matching
certificates, defined in X.509, rather than reinvent the wheel.

<EKU> and <KU> in MIT Kerberos only accept certain string values
related to some allowed values in those field as defined in
https://www.ietf.org/rfc/rfc3280.txt . 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
https://www.ietf.org/rfc/rfc4556.txt or the szOID_NT_PRINCIPAL_NAME
OID as mentioned in https://support.microsoft.com/en-us/kb/287547.
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>

===== Issuer specific matching =====
Although the MIT Kerberos rules allow to select the issuer of a
certificate there are use cases where a more specific selection is
needed. E.g. if there are some default matching rules for all issuers
and some other issuer specific rules where the default rules should
not apply. To make this possible with the above scheme the default
rules must have an <ISSUER> clause which matches all but the issuer
with the specific rules. Writing regular-expressions to not match a
specific string or a list of strings is at least error-prone if not

To make it easier to define issuer specific rules and default rules at
the same time and optional issuer string can be added to the rule to
indicate that for the given issuer only those rules should be
considered. Given the use-case I think it is acceptable to require
that the full issuer must be specified here in LDAP order (see below)
and case-sensitive matching is used.

This could also be solved by adding priority to rules - if two rules
match, the one with higher priority (the issuer specific rule) is
preferred over the one with lower priority (the default rule). IMO this
is better than an optional issuer string as it offers greater flexibility.

How the issuer string is linked to the matching rules depends on the
storage (LDAP or sssd.conf, see below for details).
==== 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.
* <ISSUER:O.I.D.:ldapAttributeName:*>
* <SUBJECT:O.I.D.:ldapAttributeName:*>
* <SAN:O.I.D.:ldapAttributeName:*>

where O.I.D. is either the OID or name of a RDN type or the OID or
some well-known-name of the SAN component respectively. Since the
SUBJECT might contain multiple RDNs of the same type always the "most
specific" is selected because in general this will be the most suited
one to map the certificate to a specific user. "most specific" means
the last in X.500 order and the first in LDAP order (see discussion
below for details).

If the O.I.D. is missing the full SUBJECT/ISSUER is used for mapping.
If 'DN' is used as ldapAttributeName SUBJECT is expected to be the DN
of the user. If the O.I.D. is missing in the SAN case the same default
as with matching (id-pkinit-san and szOID_NT_PRINCIPAL_NAME OID) is
used. If both SAN values can be found in the certificate and are
different the LDAP search filter will combine both with the or-operator.

The optional '*' in the end indicates that a sub-string search
(ldapAttributeName=*value*) should be used and not an exact match
(ldapAttributeName=value). Please note that it depends on the
server-side definition of the LDAP attribute if case-sensitive or
case-insensitve matching is used.

This seems like a rather quirky way to write down an LDAP filter. IMHO a
better way would be to use a single attribute containing a filter
template, e.g.:


Currently I see no usage for <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.

===== Future consideration =====
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 it might
be possible to add to following in a future release if more complex
operations on the values are needed:

* <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.

Maybe even search-and-replace are not sufficient for all cases and
something like embedded lua scripts are needed. But since certificate
mapping is about access control and authorization it should be always
considered if adding a new attribute to the users LDAP entry which
makes mapping easy and straight-forward wouldn't be the better solution.

===== 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
'emailAddress=u...@example.com', certtool as 'EMAIL=u...@example.com'
and certutil as 'E=u...@example.com'. So matching rules should try to
avoid attribute names or only the ones from
[https://www.ietf.org/rfc/rfc4514.txt 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

==== Storing matching and mapping configuration ====
On the IPA server a new objectclass can be created to store an
matching-mapping rule pair together with a specific issuer. All
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. If
only a specific issuer is given certificates from this issuer must be
stored in the LDAP entry of the user to make authentication possible.

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. Issuer specific
rules will have three pairs of curly braces where the first pair must
contain an issuer string.

===== Future considerations =====
If it turns out that this option is used quite often and it gets
complicated to manage a larger set of rules with it and storing the
rules in LDAP/IPA/AD is not an option we might add support to read the
rules from a separate file (certificate_rules =
FILE:///etc/sssd/cert_rules) with a more suitable format, e.g. ini
where a list can be defined by given the same option multiple times.

===== 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 'my-company.com' domain in the rfc882Name SAN
attribute. Use the email address in a LDAP search filter
'(mail=email-address)' to find the matching user.

==== Compatibility with Active Directory ====
Active Directory uses a per-user LDAP attribute
altSecurityIdentities] to allow arbitrary user-certificate mappings is
there is no suitable user-principal-name entry in the SAN of the

Unfortunately it is more or less undocumented how AD use the values of
this attribute. The best overview I found is in

It looks like the most important variant is the issuer-subject pair.
This one is e.g. created when a certificate is added via the 'Name
Mappings' context menu entry in AD's 'Users and Computers' utility
('Advanced Features' must be activated in the 'View' menu). The
attribute value might look like
altSecurityIdentities: X509:<I>O=Red Hat,OU=prod,CN=Certificate

 m,CN=Sumit Bose Sumit Bose
First it can be seen that X.500 ordering is used. Second, if RDN types
not explicitly mentioned in the RFCs are used, you are on your own. As
can be seen AD can translate the deprecated OID
1.2.840.113549.1.9.1] and uses 'E' as NSS. But the OID
0.9.2342.19200300.100.1.1] which is explicitly mentioned in RFC4514 is
not translated as UID but the plain OID syntax is used (my guess it
that Microsoft tries to be compatible with "older" versions because
the UID was added in RFC2253 from 1997 but was not present in the
RFC1779 from 1995 and RFC1485 from 1993).

Nevertheless with the mapping rules described above a rule like
would product a LDAP search filter like
(&(altSecurityIdentities=*Red Hat*)(altSecurityIdentities=*Sumit Bose
Sumit Bose*))
which should quite reliable find the right LDAP entry.

As an alternative it would be possible to add special mapping rules
like <ALT-SEC-ID-I-S:ldapAttributeName> which would try in a best
effort to produce the exact attribute value AD is using. This should
work reliable with standard RDN types (see above). I think an optional
'ldapAttributeName' is useful here so that the same mapping rule can
be used with different LDAP servers (e.g. IPA) where user-specific
mapping attributes are used with the same content but a different
attribute name.

According to the blob post describing altSecurityIdentities some other
additional mapping rules might be useful too. This will give us
* <ALT-SEC-ID-I-S:ldapAttributeName>
* <ALT-SEC-ID-S:ldapAttributeName>
* <ALT-SEC-ID-SKI:ldapAttributeName>
* <ALT-SEC-ID-I-SR:ldapAttributeName>
* <ALT-SEC-ID-SHA1-PUBKEY:ldapAttributeName>
* <ALT-SEC-ID-RFC822:ldapAttributeName>

So far I didn't found a AD tool which creates to other mappings, if
you know one, please let me know.
=== 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.


Jan Cholasta

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