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<concept id="GUID-A3B58436-07E4-565B-800B-86435D205461" xml:lang="en"><title>Certificate
Validation in PKIX</title><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Certificates of different types are trusted by different applications.
Therefore certificates need to be signed by certification authority (CA) and
the signature must be verified and validated. This section describes about
certificate chain and certificate validation processes. </p>
<section><title>Certificate Chain</title> <p>Certificate validation is a recursive
process. It begins with the need to verify the signature on some data presented
by an End Entity (EE). This involves checking that the key pair is actually
owned by that EE. To do this the public signing key of that EE is acquired
by getting its certificate. That certificate would have been signed by the
EE's certification authority (CA), so the signature on the certificate can
be verified by getting the CA's public signing key. In turn the CA's certificate
might require verifying in which case the process is repeated until the process
bottoms out when an entity which is already trusted is reached; that entity
is usually self signed. This process is illustrated in the figure below: </p> <fig id="GUID-A0956B2A-B12F-58E1-9172-C1EDBBE593CE">
<title>              Certificate chain            </title>
<image href="GUID-EA5E9A07-587C-5E64-A157-1077AD9E56ED_d0e390209_href.png" placement="inline"/>
</fig> <p>The set of certificates from an EE up to a trusted root CA certificate
is called a <keyword>certificate chain</keyword>. Once a certificate chain
has been constructed ,the EE's key pair at the start can be validated. </p> </section>
<section><title> Input to Certificate Validation</title> <p id="GUID-21CE5C18-856E-57B6-A5B6-3C1104EB8151"><b> End
Entity &amp; Intermediate Certificates</b> </p> <p>A set of certificates,
from the entity requesting authentication up to, but not including, one already
trusted by the relying party. </p> <p>Where these certificates come from is
potentially a difficult problem if certificate management is expected to search
for intermediate certificates in remote repositories; however for TLS at least
servers are required to supply a complete, ordered set of certificates in
the form of DER-encoded ASN.1; so client code can just pass this into the
certificate chain object. </p> <p id="GUID-2533ACD8-29FC-5297-A462-7D93BF029A59"><b> Trusted
Root Certificates</b> </p> <p>Authentication cannot be done entirely by software:
there must be a point at which the user confirms that they trust a particular
entity. The validation algorithm can only ascertain that if the user trusts
certificate X then they may also trust certificate Y. Certificates which the
user trusts directly are called root certificates because they are the root
of the validation chain. They are usually self-signed. </p> <p>It is likely
that different applications will have different requirements about which certificates
may be considered trust roots and for the level of protection they require
for trust roots. To this end, applications will pass in a unique ID (<codeph>TUid</codeph>)
which Certificate and Key Management will use to identify the application,
and so work out which certificates can be considered trusted for that application. </p> <p>For
additional flexibility, an overload is provided enabling clients to pass a
set of root certificates directly into the validation function. </p> <p>For
more detailed discussion of the storage and management of root certificates
see <xref href="GUID-2800C486-2FB4-5C5C-990F-CC1A290F7E0C.dita">Root Certificate
Management, Storage, and Client registration</xref>. </p> <p id="GUID-0E4B09EF-74B8-5272-98F4-60C423614F50"><b> Validation
Time</b> </p> <p>The time for which validation will be performed. </p> <p id="GUID-288B55BA-E0B0-5F67-BD62-A08832A09889"><b> Acceptable Policies</b> </p> <p>A
set of OIDs, each of which represents a certification policy acceptable to
the application. This enables client code to restrict the certificates that
may occur in a valid certificate chain. Client code does not have to specify
any preferred policies here. An empty set is interpreted as any policy. </p> </section>
<section id="GUID-AE46492C-5F13-5B73-B1F5-81DD6C15AFAF"><title>Configuring
for Certificate Validation</title> <p>The following settings can be configured
for the validation process: </p> <table id="GUID-D88105DD-347A-5E33-8B79-1A75CCC28ED0">
<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>
<tbody>
<row>
<entry><p>Supported critical extensions </p> </entry>
<entry><p>You can list supported X.509 v3 critical extensions. Once you have
set a list, you have full control over the processing of X.509 v3 critical
extensions. If a critical extension is found whose OID matches an element
in this list, certificate validation treats this as a warning instead of an
error. </p> <p>You can set, add, remove and reset the list of supported critical
extensions. </p> <p>If critical extensions are not configured, the validation
process uses a default set. This includes standard X.509 critical extensions
and Symbian specific critical extensions. </p> </entry>
</row>
<row>
<entry><p>Date validity checks </p> </entry>
<entry><p>You can specify whether a failed check on the certificate validity
date is treated as an error or a warning. You can use this to verify the certificate
validity period post-installation. By default the certificate validity period
only needs to be checked at installation. </p> </entry>
</row>
</tbody>
</tgroup>
</table> </section>
<section><title> Chain Validation</title> <p><b> Chain Construction</b> </p> <p>The
chain object parses the encoded set of certificates. The first certificate
is considered to be the EE certificate, and subsequent certificates must each
certify the preceding one. The chain object then searches the set of trusted
root certificates for a certificate trusted by the client and whose subject
name matches the issuer name in the last certificate in the decoded set. If
two or more match, it will attempt to resolve this be comparing the authority
key ID in the last certificate with the subject key ID in each candidate root.
If this extension is not present, it will attempt to find a single root by
signature verification. If no root is found validation will fail immediately
with an error. </p> <p><b> Initialisation of Chain Validation Algorithm</b> </p> <p>The
certificate chain initialises the following state variables: </p> <ul>
<li id="GUID-00BFA3CA-ECE2-5D69-B90F-B1B01D40006D"><p>Validation Time: time
for which validation is calculated. Initialised to the time supplied by client
code. </p> </li>
<li id="GUID-266D360B-2793-5963-B2B2-D79E9745AF4D"><p>Initial Policies: set
of policy OIDs, initialised to the policies supplied by client code </p> </li>
<li id="GUID-4DEBA756-99FA-54CB-9E93-85F8F54301B7"><p>Acceptable Policies:
set of X.509 policy information objects, initially any policy </p> </li>
<li id="GUID-B7FD8FF3-61E1-5CAB-8B6E-25092186727E"><p>Mapped Policies: set
of policy OIDs, initially empty </p> </li>
<li id="GUID-280A775A-4F34-5A1B-9A9A-BAFFE7A4E77B"><p>Excluded Subtrees: set
of X.500 General Name objects, initially empty </p> </li>
<li id="GUID-C4059DDE-16AC-5B3F-8001-09DC1FF789A8"><p>Permitted Subtrees:
set of X.500 General Name objects, initially any subtree </p> </li>
<li id="GUID-A27F8096-0F6B-5942-9019-3D7685AE3E10"><p>Max Path Length: integer
representing the maximum path length. Initially the actual path length </p> </li>
<li id="GUID-89B057B9-E26E-594C-B23F-E73FA97E07C8"><p>Inhibit Policy Mapping:
integer whose value is the number of certificates that may appear after the
current one before policy mapping is no longer permitted. Initially the chain
length </p> </li>
<li id="GUID-3D92E10D-2BF5-5DD3-B061-9CB332631880"><p>Require Explicit Policy:
integer whose value is the number of certificates that may appear after the
current one before an acceptable policy OID must appear in the certificate.
Initially the chain length </p> </li>
<li id="GUID-1CEA8529-2141-5083-9279-F464B217BDE1"><p>Current Cert: integer
whose value is the position of the current certificate in the chain. Initially
the chain length -1 </p> </li>
</ul> <p><b> Validation Algorithm</b> </p> <p>Validation of a certificate
chain starts at the root and ends at the End Entity. </p> <p><i>Validation
warnings </i></p> <p>Validation returns warnings. Warnings enable client code
to evaluate whether irregularities are errors. </p> <p>From Version 9.3, validation
returns a warning object for every certificate in the chain. Each warning
object has the following characteristics: </p> <ul>
<li id="GUID-4C016BA3-99EF-5D7C-9EDC-AC958C9AA937"><p>It includes the object's
index. Clients use the index to get the certificate from the certificate chain
object (<xref href="GUID-A919BE84-8257-3911-9AD1-B1DB736346CE.dita#GUID-A919BE84-8257-3911-9AD1-B1DB736346CE/GUID-9A843DCB-54C7-3BF4-BF30-2515969079A9"><apiname>CX509CertChain::Cert()</apiname></xref>). </p> </li>
<li id="GUID-81C9B8B1-F8B4-50E8-B7DC-53C131B646B9"><p>It encapsulates the
warnings and the critical extensions' OID values for that certificate. </p> <p> </p> </li>
</ul> <p>Clients can query the object for two sets of data: </p> <ul>
<li id="GUID-A7E6D671-1966-5DA7-821C-C7AD3880D7F1"><p>the critical extensions
for the certificate </p> </li>
<li id="GUID-5FCA36E5-CEDB-567C-8E95-759C13A64362"><p>a set of warning values,
each consisting of a reason for the warning and an integer identifying the
certificate that the warning is associated with. </p> </li>
</ul> <p>After querying the warning object for the critical extension, the
client must process any custom critical extensions it supplied that are not
in the supported list. This meets the X.509 certificate specification. </p><note> Prior
to version 9.3, the set of warning values returned included any warnings about
critical extensions. Warning values no longer include this information. But
backward compatibility has been maintained for warning objects prior to version
9.3. </note> <p><i>Certificate validation step</i>s </p> <p>Certificate
validation takes place through the following steps: </p> <ul>
<li id="GUID-70351BB4-CE56-559E-81A2-1EE0DBA19AFD"><p>Signature Verification
&amp; Name Chaining </p> <p>Each certificate must be signed by, and its subject
name should match the issuer name in, the certificate above it in the chain.
The only exception is the root certificate, which, if it claims to be self
signed (i.e. its subject and issuer names match) must really be self signed;
otherwise its signature is not verified, but a warning is generated. </p> </li>
<li id="GUID-9597F61C-7ECE-5754-85BC-12D50035D7D1"><p>Validity Period Checking </p> <p>For
each certificate, the Validation Time must lie within the validity period
in the certificate. The check is always carried out. The check treats a failure
as an error or a warning depending on how the validation process is configured
(See Date validity checks in <xref href="GUID-A3B58436-07E4-565B-800B-86435D205461.dita#GUID-A3B58436-07E4-565B-800B-86435D205461/GUID-AE46492C-5F13-5B73-B1F5-81DD6C15AFAF">Configuring
for Certificate Validation</xref>). By default the check treats the failure
as an error. </p> </li>
<li id="GUID-1EB6F83B-C10D-5809-B5A8-4AF45FFA9A92"><p>Extension Processing </p> <p>Extensions
can be marked critical. All critical extensions must be in the supported list
to be recognized and processed. When a critical extension that is not in the
supported list is encountered, an error results and validation fails. The
client must process critical extensions that are not in the supported list. </p> </li>
<li id="GUID-463F6C6E-7C3B-514E-A7E8-91D07CFFD102"><p>Revocation Checking </p> </li>
</ul> <p>Additionally, for each certificate: </p> <ul>
<li id="GUID-91579F6C-2C98-50DF-9637-E08EBDEC4CDE"><p>the <i>Max Path Length</i>, <i>Inhibit
Policy Mapping</i> and <i>Require Explicit Policy</i> variables are decremented, </p> </li>
<li id="GUID-05687909-8111-53D0-9760-88FF557C26AA"><p>and Current Cert must
be less than or equal to <i>Max Path Length</i>. </p> </li>
</ul> <p>When validation is complete the <i>Acceptable Policies</i> variable
will be copied into the set of policies in the result object. </p> </section>
<section><title>See also</title> <ul>
<li id="GUID-A6095947-6CFF-50CC-BBAC-181B91CD9132"><p><xref href="GUID-E326C00B-6E07-5902-AB19-F00D1761795C.dita">PKIXCert</xref>  </p> </li>
</ul> </section>
</conbody></concept>