Tag: Certificates

Certificates

chdelay

1              Certificates

1.1                 Background

This blog posting covers certificates. The purpose of this post is for those who would like to better understand certificates, their structure, and their contents. This blog post also covers basic management of certificates.

1.2                 What is a certificate?

A certificate is a digital document that binds an identity to a public key.  The identity contained in a certificate can be associated with any number of things including computers, users, and services such as a web site.  Certificates also contain additional information that makes the certificate more useful in many respects. 

1.3                 Keys

Public Key cryptography makes use of two related keys.  There is a public key and a private key.  Any data that is encrypted with the public key can be decrypted the private key.  Also, any data that encrypted with the private key can be decrypted with the public key. The public key is distributed via certificates and the private key is protected on a computer or on a cryptographic device such as a smart card.

1.4                 Validity Periods

Certificates also have a period of time for which they are valid.  In other words, a certificate is valid for a certain period of time. This built in expiration mechanism helps increase the security of certificates.  This is due to the fact that if a certificate is compromised and that compromise is not detected it will eventually expire and become invalid. 

1.5                 Digital Signature

A certificate is digitally signed by the CA that issued the certificate.  This signature can be validated to verify that the certificate was in fact

1.6                                                                                                                                                             Viewing Certificates in the Windows GUI

There is some basic information you can get from a certificate by viewing it in the Windows UI.  However, for most troubleshooting you will want to look at the certificate fields which will be discussed in the next section.

1.6.1   General Tab

In the General Tab you can look at a description of the purposes of the certificate.  You can also get the identity of the subject and the issuer.  You can view the validity period of the certificate.  If there is a private key associated with the certificate available in the certificate store you will see that as well.

1.6.2   Details Tab

In the Details Tab you will see all of the fields listed in the certificate as well as some additional information supplied by the Operating System.  We will discuss these fields in the next section.  You can also use the Copy to File… button to export a certificate.

1.6.3   Certification Path Tab

The Certification Path Tab shows the chain of the certificate, starting at the current certificate and going up as far as the Root CA.  There is also a Certificate status section that can be used for identifying issues with the certificate in terms of trust or validation.

1.7                 Certificate Fields

To better understand certificates including how they are used and validated we need to understand the various fields that are contained in a certificate.

Certificates contain two types of fields Version 1 Fields and Extensions.

1.7.1   Version 1 Fields

Version 1 fields are those that were specified in the original specification for X.509 certificates defined in RFC 1422. 

1.7.1.1          Version

This field indicates the X.509 version that is being used.  The current version of the X.509 standard is Version 3.

1.7.1.2          Serial Number

Serial Number is a unique identifier for each certificate.  The Serial Number should be unique at least on a per CA basis.  In other words, a Certification Authority should not issue more than one certificate with the same Serial Number.

1.7.1.3          Signature Algorithm

This field contains the name of the Public Key Algorithm and Hashing Algorithm that were used to sign the certificate.

1.7.1.4          Issuer

This field indicates the name of the CA that issued the certificate

1.7.1.5          Valid From (Validity Period)

This field indicates at what date and time the certificate becomes valid.

1.7.1.6          Valid To (Validity Period)

This field indicates at what date and time the certificate expires

1.7.1.7          Subject

This field contains the identity that is bound to the public key of the certificate.  The identity is bound to that public key duet to the fact that the certificate contains both the public key as well as the identity are contained within a certificate that is signed by a trusted certification authority.

1.7.1.8          Public key

This field contains the public key that is bound to the certificate.

1.7.2   Extensions

After the publication of RFC 1422 the idea of extensions in certificates were introduced in a later RFC. Extensions are optional fields that can be included in a certificate.  However, generally the extensions included in RFC 5280 should be understood by PKI enabled applications.

1.7.2.1          Subject Key Identifier

This field contains a hash of the public key in the certificate, this information is one way to identify the certificate.

1.7.2.2          Authority Key Identifier

This field contains a hash of the public key that is contained in the CA certificate, this is one way to identify the certificate of the CA that issued the current certificate.

1.7.2.3          CRL Distribution Points

This field indicates where Certificate Revocation Lists can be downloaded from to determine if the certificate has been revoked.

1.7.2.4          Authority Information Access

This field indicates where the certificate of the CA that issued the certificate can be downloaded. This field will also indicate where the OSCP service can be accessed, if one is available in the environment.

1.7.2.5          Key Usage

This field indicates what purposes the keys in the certificate can be used for and these purposes include Digital Signature, Key Encryption, Key Agreement, Certificate Signing, CRL Signing, Key Agreement Enciphering, Key Agreement Deciphering.

1.7.2.6          Certificate Template Information

This is a Microsoft specific extension which means it is note one included in the RFC 5280, as such it would only be expected to be understood by Microsoft applications. This field contains the name and OID of the Certificate Template.

1.7.2.7          Certificate Template Name

This is a Microsoft specific extension which means it is note one included in the RFC 5280, as such it would only be expected to be understood by Microsoft applications. This field contains the name of the Certificate Template.

1.7.2.8          Enhanced Key Usage

This extension specifies for what purposes the public key may be used.

1.7.2.9          Application Policies

This extension specifies for what applications the certificate can be used.

1.7.2.10      Subject Alternative Name

The Subject Alternative Name or SAN is an optional field that contains the identity or identities that are bound to the certificate.  The SAN extension is typically used when the certificate will contain more than one identity. To elaborate further identity information can be stored in either the subject field, the Subject Alternative Name extension or both.  Some services such as SSL with ignore the subject field if the Subject Alternative Name extension is populated. 

1.7.2.11      Basic Constraints

Typically, this extension is only included in CA certificate and identifies the certificate as a CA certificate.

1.8                 View the Certificate via Command Line

You can also view the raw certificate via the command line if the certificate is saved as a flat file on the file system. 

To view the certificate in the console run: Certutil <Certificate File Name>

It is often more desirable to output the text to a file. 

Below is an example of the output of the certificate file dumped to a text document.

1.9                 Certificate Storage

In Windows certificates can be stored in a number of places, so let’s discuss the various locations where certificates are stored and the benefits of storing certificates in these locations.

1.9.1   Active Directory Certificate Services Database

Certification Authorities by default store a copy of every issued certificate in its database. Storing certificates in the CA Database enables the following capabilities:

  • Retrieval- Having a copy of each certificate provides a central repository from where certificates can be retrieved by enrollees or a certificate manager.
  • Recovery- Certificates can be recovered for a user or computer since a copy of the certificate is stored in the CA Database.  Additionally, if Key Archival is enabled the private key can be recovered for encryption certificates.
  • Forensics and Analysis: The database can be searched to determine what certificates have been issued and provide useful information such as when a certificate expires.
  • Revocation: If a certificate has been compromised or is no longer trusted that certificate can easily be located in the CA database and revoked.

1.9.2   Active Directory

Certificates can be stored on the userCertificate attribute of a user or computer that enrolls for a certificate. Publishing certificates to a computer or user object is typically not necessary for most applications.  However, some applications such as EFS or Email Encryption can query Active Directory for the certificate of the user that the document or email for which it is being encrypted.

As previously mentioned the certificate is published to the userCertificate during enrollment.  The certificate is published by the Certification Authority.  The Cert Publishers security group is the group that has permission to write to the userCertificate attribute.  In order for the CA to be able to publish a certificate to the userCertificate attribute it must be a member of the Cert Publishers group in the corresponding Active Directory domain.

Additionally, the certificate template must be configured to Publish certificate in Active Directory as seen in the screenshot below:

1.9.2.1          Viewing the userCertificate Attribute

There are several ways to view certificates that are stored in the userCertificate attribute.

1.9.2.1.1                Certmgr.msc

If a user opens Certmgr.msc they can view what certificates are stored on their user object by navigating to the Active Directory User Object node.

1.9.2.1.2                Active Directory Users and Computers

To view the userCertificate attribute in Active Directory Users and Computers navigate to the user or computer whose userCertificate attribute you wish to view.  Open the properties for the user or computer account and navigate to the Published Certificates tab.  If the Published Certificates tab is not available, you need to enable Advanced Features in the Active Directory Users and Computers View menu.

1.9.2.1.3                ADSIEDIT

To view the userCertificate attribute connect to the Domain Partition (Default Naming Context).  Go to properties on the corresponding object and navigate to the userCertificate attribute.

1.9.3   Local Storage of Certificates and Private Keys

For certificates that are stored on the local machine for the computer or user these certificates are stored on the local file system or registry.  The only exception to this is if you are storing certificates and/or keys on a cryptographic device such as a TPM or Smart Card. 

The following tables illustrate where certificates are stored on the local computer.

The following table shows where Computer certificates are stored on the computer. 

StoreStorage Location
RootHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\Root\Certificates
MyHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\My\Certificates
RequestHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\Request\Certificates
TrustedPublisherHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\TrustedPublisher\Certificates
TrustedPeopleHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\TrustedPeople\Certificates
DisallowedHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\Disallowed\Certificates
CAHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\CA\Certificates
ACRAHKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\ACRS\CTLs

http://msdn.microsoft.com/en-us/library/windows/desktop/bb204781(v=vs.85).aspx

The following table shows where User certificates are stored on the computer.

StoreStorage Location
RootHKEY_CURRENT_USER\SOFTWARE\Microsoft\SystemCertificates\Root\Certificates
Myfile:\\%APPDATA%\Microsoft\SystemCertificates\My\Certificates
Requestfile:\\%APPDATA%\Microsoft\SystemCertificates\Request\Certificates
TrustedPublisherHKEY_CURRENT_USER\SOFTWARE\Microsoft\SystemCertificates\TrustedPublisher\Certificates
TrustedPeopleHKEY_CURRENT_USER\SOFTWARE\Microsoft\SystemCertificates\TrustedPeople\Certificates
DisallowedHKEY_CURRENT_USER\SOFTWARE\Microsoft\SystemCertificates\Disallowed\Certificates
CAHKEY_CURRENT_USER\SOFTWARE\Microsoft\SystemCertificates\CA\Certificates

http://msdn.microsoft.com/en-us/library/windows/desktop/bb204781(v=vs.85).aspx

The following table shows where CNG Private Keys are stored.

Key typeDirectory
User Private%APPDATA%\Microsoft\Crypto\Keys
Local system private%ALLUSERSPROFILE%\Application Data\Microsoft\Crypto\SystemKeys
Local service private%WINDIR%\ServiceProfiles\LocalService
Network service private%WINDIR%\ServiceProfiles\NetworkService
Shared private%ALLUSERSPROFILE%\Application Data\Microsoft\Crypto\Keys

http://msdn.microsoft.com/en-us/library/bb204778%28VS.85%29.aspx

1.9.3.1          Protection of Private Keys that are stored locally

Private keys are protected by the Data Protection Application Programming Interface (DPAPI).  DPAPI protects the private keys with encryption and that encryption is accessed with a key that is a derivation of the user or computer’s password. 

1.9.4   Smart Card

Certificates can also be stored on a cryptographic device called a Smart Card.  Smart cards are typically credit card size and require a smart card reader.  However, there are also USB smart cards that have an integrated smart card reader.  Regardless, these devices have an integrated circuit and cryptographic functions are performed on that hardware.  Certificates and private keys are stored on the smart card. To use the keys on that smart card a user must have knowledge of the PIN associated with that smart card and must enter the PIN when using the smart card. The key advantage is that the private keys are protected by hardware.  A secondary advantage is the smart card provides a mechanism to roam the user’s certificates and keys.  This is due to the fact that the certificates and keys can be used on any system where the user inserts their smart card.

1.9.5   Virtual Smart Card / Trusted Platform Module

Starting in Windows 8.0 the capability to store certificates on the Trusted Platform Module chip was introduced. A Virtual Smart Card is created on the TPM and the certificate is stored on the Virtual Smart Card which is protected by the TPM.  The Virtual Smart Card offers some of the advantages of physical smart card in the sense that the private keys are protected by hardware and cryptographic functions that require use of the private key occur on the TPM. However, since the TPM is physically part of the computer certificates do not roam with the user as they do in the case of physical smart cards.  However, in some cases that is a practical advantage since physical smart cards can be lost or forgotten by the user. 

1.9.6   Hardware Security Modules

A Hardware Security Module (HSM) is a dedicated cryptographic security device for protecting keys.  In PKI we use an HSM to protect the private keys that are linked to a certificate and that are part of a public/private key pair. HSMs are commonly used to protect the Private Keys of   Private Keys are isolated to prevent tampering or exportation of the Private Keys.  Cryptographic functions that require use of the private key occur in the HSM. HSMs typically use K of N security to prevent a single individual from having control of the HSM.  K of N security means that a certain number of credentials have to presented in order to make changes to the HSM.  For example, if 5 credentials are provisioned for administrative actions on HSM, you can configure the HSM so that at least 3 of those 5 credentials must be presented to the HSM for administrative actions to be performed. There are several types of HSMs currently used.  There are USB HSMs that are typically used for an Offline CA or for Code Signing.  There are PCI Express HSMs that can be used with a Physical CA.  However, the most commonly type used of HSM that I have seen in use are Network HSMs. Network HSMs are connected to via Ethernet. 

1.10             Certificate Administration

There are a number of tools that are used for certificate administration.  The most common tool that you may end up using for certificate administration is the Certificates MMC.  The Certificates MMC itself can be targeted at the certificate store of a user, a computer, or a service account. However, you will most commonly use the Certificates MMC for administration of user or computer certificates, and hence we will focus on administration of user and computer certificates.

1.10.1                  Certmgr.msc

Certmgr.msc is the certificates MMC for managing user certificates.  Using this MMC you can perform the following actions for the user:

  • View Certificates
  • Import Certificates
  • Export Certificates
  • Enroll for Certificates
  • View Requests
  • Generate Certificate Request
  • Process Approved Certificate Requests

Below is a screenshot of the Certmgr.msc:

The table below describes the purpose of each container in Certmgr.msc.

Folder NameContents
PersonalCertificates associated with private keys to which you have access. These are the certificates that have been issued to you, or to the computer or service for which you are managing certificates.
Trusted Root Certification AuthoritiesImplicitly trusted certification authorities. Includes all of the certificates in the Third-Party Root Certification Authorities store plus root certificates from your organization and Microsoft.
Enterprise TrustA container for certificate trust lists. A certificate trust list provides a mechanism for trusting self-signed root certificates from other organizations and limiting the purposes for which these certificates are trusted.
Intermediate Certification AuthoritiesCertificates issued to subordinate certification authorities.
Active Directory User ObjectCertificates associated with your user object and published in Active Directory.
Trusted PublishersCertificates from certification authorities that are trusted by Software Restriction policies.
Untrusted CertificatesAn untrusted certificate is a certificate that a certification authority has revoked, or a certificate that for other reasons has been placed in the Untrusted Certificates folder on your computer.
Third-Party Root Certification AuthoritiesTrusted root certificates from certification authorities other than Microsoft and your organization.
Trusted PeopleCertificates issued to people or end entities that are explicitly trusted. Most often these are self-signed certificates or certificates explicitly trusted in an application such as Microsoft Outlook.
Client Authentication IssuersA list of issuers whose certificates are trusted for TLS/SSL Client Authentication.
Certificate Enrollment RequestsPending or rejected certificate requests.
Smart Card Trusted RootsOn smart card insertion, the Certificate Propagation service propagates any root certificates on the card to the Smart Card Trusted Roots certificate stores on the local computer. This process establishes a trust relationship with the organization.

1.10.2                  Certlm.msc

Certlm.msc is the Certificate MMC for managing certificates for the local computer.  Prior to Windows 8.1 the shortcut (certlm.msc) was not available for launching the Certificate MMC targeted to the local computer. So, for pre-Win8 or pre-Windows Server 2012 machines the following steps can be used to launch the Certificates MMC targeted for the local computer.

Steps for Opening Certificates MMC Targeted for Local Computer

Step 1: Click the Start key and then the R key

Step 2: Type mmc.exe and click OK

Step 3: In the new console select the File menu and from the File menu select Add/Remove Snap-in…

Step 4: Select Certificates and then click the Add button

Step 5: Select Computer account and then click Next

Step 6: Select Local computer:  (the computer this console is running on), and the click Finish

Step 7: Click OK

The Certificates MMC targeted for the Local Computer will now be Open

1.10.2.1      Certlm.msc Uses

The following tasks for the local computer can be performed using certlm.msc:

  • View Certificates
  • Import Certificates
  • Export Certificates
  • Enroll for Certificates
  • Generate Certificate Request

Below is a screenshot of the Certmgr.msc:

The table below describes the purpose of each container in Certlm.msc.

Folder NameContents
PersonalCertificates associated with private keys to which you have access. These are the certificates that have been issued to you, or to the computer or service for which you are managing certificates.
Trusted Root Certification AuthoritiesImplicitly trusted certification authorities. Includes all of the certificates in the Third-Party Root Certification Authorities store plus root certificates from your organization and Microsoft.
Enterprise TrustA container for certificate trust lists. A certificate trust list provides a mechanism for trusting self-signed root certificates from other organizations and limiting the purposes for which these certificates are trusted.
Intermediate Certification AuthoritiesCertificates issued to subordinate certification authorities.
Trusted PublishersCertificates from certification authorities that are trusted by Software Restriction policies.
Untrusted CertificatesAn untrusted certificate is a certificate that a certification authority has revoked, or a certificate that for other reasons has been placed in the Untrusted Certificates folder on your computer.
Third-Party Root Certification AuthoritiesTrusted root certificates from certification authorities other than Microsoft and your organization.
Trusted PeopleCertificates issued to people or end entities that are explicitly trusted. Most often these are self-signed certificates or certificates explicitly trusted in an application such as Microsoft Outlook.
Client Authentication IssuersA list of issuers whose certificates are trusted for TLS/SSL Client Authentication.
Remote DesktopCertificates that are used to provide SSL security for RDP connections.
Smart Card Trusted RootsOn smart card insertion, the Certificate Propagation service propagates any root certificates on the card to the Smart Card Trusted Roots certificate stores on the local computer. This process establishes a trust relationship with the organization.
Web HostingHighly scalable certificate store for SSL certificates that can support thousands of certificates to support increased secure site density.

The following video provides additional information on certificates:

-Chris

Certificate Validation

chdelay

1              Certificate Validation and Revocation

1.1                 Background

One important concept to understand when troubleshooting certificates. This blog covers the basics of understanding certificate validation. Then towards the end, I cover troubleshooting steps.

1.2                 Identity Validation

Certificates bind an identity to a public key.  And one of the reasons that we use certificates is to ensure that we are accessing the resource that we intend to access.  Therefore, we expect the identity in the certificate to match the identity we are trying to access.  For example, if you enter the following URL in your web browser: https://www.microsoft.com, you would expect that the identity www.microsoft.com would be included in the certificate.  There are two possible fields in the certificate where this identity information can be located: Subject and Subject Alternative Name.  The Subject is a Version 1 field that will typically only contain one identity.  The Subject Alternative Name field is an extension and is typically used when there is more than one identity in the certificate.  For some applications such as SSL, the subject will be ignored if the SAN is populated.  Below is an example of an error a user might receive if there is a mismatch between the identity that they are trying to contact and the identity in the certificate.

1.3                 Chain Validation

In order to determine if a certificate is trusted, you need to build a chain to the root certificate.  Once the chain is built to the root certificate you can determine whether or not the root certificate is trusted.  To validate the chain, you must have possession all of the certificates in the chain and then you can build the chain from the end-entity certificate all the way up to the root certificate, which is a self-signed certificate where the chain terminates.  I will discuss more about Chain Validation later on in this document.

1.4                 Validity Period Validation

Certificates also have a period of time for which they are valid.  In other words, a certificate is valid for a certain period of time. This built in expiration mechanism helps increase the security of certificates.  This is due to the fact that if a certificate is compromised and that compromise is not detected it will eventually expire and become invalid. 

The Valid From field is when the certificate begins its validity period and the Valid To field indicates when a certificate ends its validity period.  In order for a certificate to be valid from a time perspective, the current date should fall in between the Valid From and Valid To dates specified in the certificate.

1.5                 Certificate Revocation Validation

When a Windows client checks the revocation status of a certificate it checks revocation for that certificate and every certificate in the chain excluding the Root CA certificate.

  1. The client will first check to see if it has any time valid OCSP Responses or CRLs cached locally.
  2. If an OSCP URI has been deployed via Group Policy the client will send and OCSP response to the OCSP Responder.  The OCSP responder then should send a response back to the client informing the client whether or not the certificate has been revoked. 
  3. If an OCSP URI is included in the AIA extension of the certificate the client will send and OCSP response to the OCSP Responder.  The OCSP responder then should send a response back to the client informing the client whether or not the certificate has been revoked. 
  4. If there is no OCSP URI Specified the client will download the Base CRL.  If the Base CRL has a Freshest CRL extension the client will then download the Delta CRL.  The client will then parse the CRL(s) to determine if the certificate has been revoked.

1.6                 Certificate Chaining

As mentioned earlier a certificate needs to be chained to a root certificate in order to determine if the certificate chains to a trusted root.  There is information included in certificates to assist in this chain building process.  The most common type of information I see used to help the system identify the chain or the Subject Key Identifier (SKI) field and the Authority Key Identifier (AKI) field.  The Authority Key Identifier is a hash of the certificate issuers public key. The Subject Key Identifier is a hash of the public key in the current certificate. The Operating System can chain a certificate by matching the Authority Key Identifier field in a certificate to the Subject Key Identifier in the issuer’s certificate. Matching the Authority Key Identifier field in the certificate to the Subject Key Identifier of the issuer is commonly called a key match.

Keep in mind not only do the fields have to match but the signature needs to verify as well.  A certificate is digitally signed by the issuer.  This means the certificate data was passed through a hashing algorithm and then the resulting hash is encrypted with the private key of the issuer.  The signature can be verified by hashing the same certificate data, decrypting the digital signature with the public key of the issuer, and comparing the hashes.  If the resulting hashes match and the digital signature can be decrypted with the issuer’s public key then the signature is verified.

Below is an example of a key match:

1.6.1   Retrieving CA Certificates / AIA

Remember that the verifier of the certificate must have all certificates in the chain.  The verifier will have access to the certificate it is verifying usually during a handshake during a protocol setup.  Intermediate certificates may also be distributed the same way.  However, if the intermediate certificates are not distributed during the protocol handshake they can be downloaded from the AIA repository.  There is an extension named Authority Information Access that is commonly included with certificates.  The AIA extension usually has a url which indicates from where the issuer’s certificate can be retrieved from. Windows supports two protocols for retrieval of CA certificates via the AIA Repository: LDAP and HTTP.  Also, keep in mind that you have multiple AIA locations specified in the AIA extension of the certificate.  Those AIA locations are processed in the order in which they are present the AIA extension. 

Below is an example of the AIA extension in a certificate:

1.6.1.1          LDAP Repository

When we talk about using an LDAP repository to host CA certificates we typically mean using the LDAP functionality of Active Directory.  Although, other ldap providers can be used for hosting CA certificates it is less common and does not provide the same level of integration as Active Directory.

There are some significant advantages and disadvantages to using LDAP as an AIA repository:

1.6.1.1.1                Advantages

High Availability: When using Active Directory as the LDAP provider, there is built-in fault tolerance.  This is due to the fact that the AIA location is stored in the Configuration container that is replicated to all Doman Controllers in the forest.

Site Awareness: Active Directory has built-in site awareness.  Clients use DCLocator to find the nearest DC, so clients will be able to pull certificates from a local Domain Controller.

1.6.1.1.2                Disadvantages

3rd Party Support: PKI can be leveraged by 3rd party Operating Systems and applications.  Many of these are not integrated into Active Directory and hence cannot access the Active Directory LDAP provider.

Insecure Networks: Providing access to Active Directory LDAP over insecure networks such as a DMZ or Public Network would be a security nightmare so it would not be a good practice to use LDAP if clients on an insecure network need to retrieve the CA Certificates from AIA.

1.6.1.2          HTTP Repository

Using an HTTP or Web Repository is a more common practice.  This is especially true for Public or 3rd Party CAs.  If using an HTTP repository, it is important that the repository can be located by a DNS name and that the DNS name is not tied to a specific machine.  In other words, you would likely use a CNAME or an A record that is not the computer name of the Web Server.

1.6.1.2.1                Advantages

Insecure Networks: Just about every organization is familiar on how to make the http protocol accessible in DMZ’s or over the Internet. 

3rd Party Support: The virtual directory where the CA certificates are made accessible is normally configured to allow anyone to read and hence download the CA certificates. 

1.6.1.2.2                Disadvantages

High Availability: A web server by itself is not highly available.  Additional work and resources must be brought to bear to make a website highly available including having multiple web servers typically behind a load balancer. 

Site Awareness: If you wish to make the HTTP AIA repository available locally to every site it requires additional web servers and a load balancer that will re-direct clients to the closest web server.

1.6.1.3          AIA Design Considerations

When you are in the process of building a PKI you have to decide where the AIA locations are going to be located and how the names used to access those locations are going to be formed.  A common practice for 3rd Party Certification Authorities is just to use a single http URL inside of the AIA extension.  Then that web location is made available from a highly available provider such as a Content Delivery Network (CDN).  For your own internal PKI you have a number of possible ways to make the HTTP AIA location highly available.  You could use for instance Azure CDN to provide a highly available solution.  Alternatively, you could stand up several web servers behind a load balancer to provide this functionality.  Regardless, when you design the name for the AIA location you will want to choose a name that is not tied to a specific machine name.  Even if you do decide to go against best practices and host the HTTP AIA location on the CA itself, you would still want to use some sort of alias instead of the machine name.  This configuration would allow you flexibility in the future if you ever decide to move that location to another machine.  This is due to the fact that aside from reconfiguring where the certificate was published you would simply just need to update where that DNS alias is pointed instead of having to re-configure the AIA location which would require renewing the CA certificate to update the extension in that certificate. Additionally, if you plan on using the PKI to secure internet facing resources you would want to ensure that the DNS name used for the AIA location is not just resolvable internally, but also externally. Aside from the HTTP location some organizations decide to leverage LDAP as a secondary AIA location that would be available if the HTTP location is inaccessible.

1.7                 Root Certification Authority Certificates

The Root CA’s primary function is to be the trust anchor for the PKI.  When a system trusts a Root CA, then that trust extends to subordinate CAs that chain to that same root and the certificates that they issue.

In order for a Root CA to be trusted on a Windows system the Root CA certificate must be present in the Trusted Root Certification Authorities store.  Keep in mind there Trusted Certification Authorities container in both the Computer certificate store and the User certificate store.  Trusted Root CA Certificates present in the Computer store are inherited by the user. There are a number of ways that a Root CA certificate can be populated in that store. 

1.7.1   Microsoft Root Certificate Program

Microsoft has a program where 3rd Party CAs can submit their certificate to be installed in the Trusted Root Certification Authorities store.  These are the Root CA certificates that ship with Windows.  The advantage to this approach is that a user or organization does not need to go out download and install the Trusted Roots for popular 3rd Party CAs. 

1.7.2   Group Policy

Root CA Certificates can be deployed via Group Policy.  This is a common way organizations deploy Root CA Certificates for the PKI of a trusted organization.  Root CA certificates may also be deployed this way during a Merger or Acquisition.

1.7.3   Autoenrollment

Autoenrollment performs a variety of functions.  One of those functions is to download Enterprise Certificates from Active Directory.  Enterprise Certificates includes the Root CA certificates that are published to the Certification Authorities container as well as downloading Subordinate CA certificates from the Authority Information Access container in Active Directory.  All of these containers are located in the Configuration Partition of Active Directory.

1.7.4   Manual

Unless otherwise prevented by Group Policy users can add Root CA certificates in the Trusted Root Certification Authority container in the user store.  A user that has administrator privileges can also add Root CA certificates to the Trusted Root Certification Authorities container in the Computer certificate store.

1.8                 Chain Building Summary

In order to determine whether or not a certificate should be trusted, the certificate chain must be built.  Starting with the certificate that is being validated, the validator locates the next certificate in the chain.  If that certificate is an intermediate CA that cert can be retrieved locally from the Intermediate Certification Authorities container, passed to the validator during the protocol handshake for whatever protocol is using the certificate, or retrieved from the AIA extension in the certificate.  This process continues until it reaches a self-signed certificate or in other words a Root CA certificate.  If that Root CA certificate exists in the Trusted Root Certification Authority store that the certificate being validated is considered trusted.

1.9                 Revocation

Certificates do have a limited time for which they are valid.  This built-in expiration mechanism enhances the security of certificates.  However, there are times where you may no longer trust a certificate before it expires. There are several possible reasons why you may no longer trust a certificate.  Some of those reasons include: the certificate was compromised, the computer was compromised, or the user left the company.  When you no longer trust a certificate you can revoke.  Once the certificate is revoked, applications that perform revocation checking will no longer trust the certificate.  More specifically the CA Manager and Certificate Manager can revoke the certificate.  When an administrator revokes a certificate they must specify a reason for the revocation.  These reason include: Unspecified, Key Compromise, CA Compromise, Change of Affiliation, Superseded, Cease of Operation, and Certificate Hold. If the Certificate Hold reason is selected, the certificate can later be unrevoked.

1.9.1   Revoking a Certificate

The following steps can be taken to revoke a certificate

Step 1: To revoke a certificate the Certificate Manager open the Certification Authority MMC (certsrv.msc)

Step 2: Click on Issued Certificates

Step 3: Right-click on the certificate that needs to be revoked

Step 4: From the context menu click All Tasks and the Revoke Certificate

Step 5: Select a reason for the revocation

Step 6: Click OK

The Revoked Certificate will show up under Revoked Certificates.  The certificate will not be known to be revoked until the CRL is published to the CDP and subsequently downloaded and parsed by clients checking revocation.

1.9.2   Certificate Revocation List (CRL)

A Certificate Revocation List contains a list of certificates that have been revoked.  Specifically, CRLs contain the Serial Number of each certificate that has been revoked, along with the reason for revocation, and the date and time that the certificate was revoked. There are two types of CRLs: Base CRLs and Delta CRLs. Base CRLs contain a list of all non-expired certificates that have been revoked. Delta CRLs contain a list of non-expired certificates that have been revoked since the last Base CRL was published.  The lists from the latest Base CRL and Delta CRL can be combined to get a list of all non-expired certificates that have been revoked. Delta CRLs were implemented so that clients would not have repeatedly download a base CRL which would be larger in size.  So, instead of publishing a Base CRL more frequently, delta CRL can be published instead.  Delta CRLs are optional you can just use a Base CRL if that better fits the requirements for your organization.

1.9.3   CRL Fields

In this section I will discuss the various fields that are included in a CRL.

1.9.3.1          Version

This is the Version of the CRL Profile that is being used.  The current version of the CRL Profiles is Version2.

1.9.3.2          Issuer

The Issuer field indicates which CA issued the CRL.

1.9.3.3          Effective date

This is the date and time at which a CRL becomes effective.

1.9.3.4          Next update

Next update indicates the date and time at which the CRL will be expired.

1.9.3.5          Signature Algorithm

The Signature Algorithm field indicates what Asymmetric Cryptographic Algorithm and Hashing Algorithm were used to sign the CRL.

1.9.3.6          Signature Hash Algorithm

The Signature Hash Algorithm field indicates the Hashing Algorithm used when signing the CRL

1.9.3.7          Authority Key Identifier

The Authority Key Identifier is another way of identifying the issuer of the CRL.  Specifically, the Authority Key Identifier field contains a hash of the issuer’s public key

1.9.3.8          CA Version

The CA Version Field list the version number for the CA Certificate that was used to sign the CRL.

1.9.3.9          CRL Number

The CRL Number indicates the version of the CRL.  The CRL Number increments each time a new CRL is published.

1.9.3.10      Next CRL Publish

The Next CRL Publish field indicates the date and time at which the next CRL will be published.

1.9.3.11      Published CRL Location

Published CRL Location is the location at which the CRL will be published in Active Directory.  This is typically

1.9.3.12      Freshest CRL

If a CA publishes Delta CRLs then the Freshest CRL extension will show the location(s) where Delta CRLs can be retrieved.

1.9.3.13      Revocation List

The Revocation List will list every non-expired certificate that has been revoked.  This list will include the Serial Number and revocation date.  Depending on the reason code selected it may also list the reason for the revocation.

1.9.4   Delta CRL

A Delta CRL contains a list of all non-expired certificates that have been revoked after the publication of the Base CRL.  To determine the location of the Delta CRL the Freshest CRL extension of the Base CRL must be read or known.  To determine revocation, the current Base CRL and latest Delta CRL must be available.

1.9.4.1          Delta CRL Indicator

A Delta CRL has the same extension and formatting as a base CRL.  A Delta CRL also has a field called Delta CRL Indicator.  This field indicates which base CRL the client must have to have complete knowledge of revocations.

1.9.5   CRL Distribution Points (CDP)

The CRL Distribution Points extension is an extension in a certificate that indicates where the CRL can be retrieved from for the purposes of checking revocation. That location where the CRLs can be retrieved is often referred to as a CDP repository. Some of the same concepts that apply to the Authority Information Access extension apply here, but I have listed them below for completeness. 

1.9.5.1          LDAP Repository

When we talk about using an LDAP repository to host CA certificates we typically mean using the LDAP functionality of Active Directory.  Although, other ldap providers can be used for hosting CA certificates it is less common and does not provide the same level of integration as Active Directory.

There are some significant advantages and disadvantages to using LDAP as an AIA repository:

1.9.5.1.1                Advantages

High Availability: When using Active Directory as the LDAP provider, there is built-in fault tolerance.  This is due to the fact that the AIA location is stored in the Configuration container that is replicated to all Doman Controllers in the forest.

Site Awareness: Active Directory has built-in site awareness.  Clients use DCLocator to find the nearest DC, so clients will be able to pull certificates from a local Domain Controller.

1.9.5.1.2                Disadvantages

3rd Party Support: PKI can be leveraged by 3rd party Operating Systems and applications.  Many of these are not integrated into Active Directory and hence cannot access the Active Directory LDAP provider.

Insecure Networks: Providing access to Active Directory LDAP over insecure networks such as a DMZ or Public Network would be a security nightmare so it would not be a good practice to use LDAP if clients on an insecure network need to retrieve the CA Certificates from AIA.

1.9.5.2          HTTP Repository

Using an HTTP or Web Repository is a more common practice.  This is especially true for Public or 3rd Party CAs.  If using an HTTP repository, it is important that the repository can be located by a DNS name and that the DNS name is not tied to a specific machine.  In other words, you would likely use a CNAME or an A record that is not the computer name of the Web Server.

1.9.5.2.1                Advantages

Insecure Networks: Just about every organization is familiar on how to make the http protocol accessible in DMZ’s or over the Internet. 

3rd Party Support: The virtual directory where the CA certificates are made accessible is normally configured to allow anyone to read and hence download the CA certificates. 

1.9.5.2.2                Disadvantages

High Availability: A web server by itself is not highly available.  Additional work and resources must be brought to bear to make a website highly available including having multiple web servers typically behind a load balancer. 

Site Awareness: If you wish to make the HTTP AIA repository available locally to every site it requires additional web servers and a load balancer that will re-direct clients to the closest web server.

1.9.5.3          CDP Design Considerations

When you are in the process of building a PKI you have to decide where the CDP locations are going to be located and how the names used to access those locations are going to be formed.  A common practice for 3rd Party Certification Authorities is just to use a single http URL inside of the CDP extension.  Then that web location is made available from a highly available provider such as a Content Delivery Network (CDN).  For your own internal PKI you have a number of possible ways to make the HTTP CDP location highly available.  You could use for instance Azure CDN to provide a highly available solution.  Alternatively, you could stand up several web servers behind a load balancer to provide this functionality.  Regardless, when you design the name for the CDP location you will want to choose a name that is not tied to a specific machine name.  Even if you do decide to go against best practices and host the CDP AIA location on the CA itself, you would still want to use some sort of alias instead of the machine name.  This configuration would allow you flexibility in the future if you ever decide to move that location to another machine.  This is due to the fact that aside from reconfiguring where the CRL was published you would simply just need to update where that DNS alias is pointed instead of having to re-configure the CDP location which would require renewing the CA certificate to update the extension in that certificate. Additionally, if you plan on using the PKI to secure internet facing resources you would want to ensure that the DNS name used for the CDP location is not just resolvable internally, but also externally. Aside from the HTTP location some organizations decide to leverage LDAP as a secondary CDP location that would be available if the HTTP location is inaccessible.

1.9.5.4          CRL Publishing

So, how do we maximize the amount of time we have for recovery, and at the same time increase the “freshness” of the CRL?  We can do this with the use of Overlap Periods.

So, let’s say we want to implement this solution that maximizes recovery time and increases CRL freshness.  The first consideration is Delta CRLs.  Since Delta CRLs in my view are not necessary in most instances, we will first get rid of Delta CRLs.  So, now that gives us just Base CRLs to work with.  Let us take our Base CRL and make it valid for a period of 3 days.  So, at first you would think great, now I have 3 days to recover the CA if the CA fails since the CRL is valid for 3 days.  Well this is not true.  If the validity period of the CRL and the publishing interval coincide, then you could have as little time as 1 second depending on when the CA failed.  The illustration below exemplifies this scenario.  The problem is without changing the default configuration the Next Update and Next Publish fields will be very close together, providing very little time for recovery.

1.9.5.5          Introducing CRL Overlap Period

As mentioned there is some additional configuration that can be performed that can optimize your CRL publishing intervals so that you have adequate time to perform Emergency CRL Signing or to recover your CA.  What will need to be configured is the CRL Overlap Period.  In order to configure the CRL Overlap Period both the CRLOverlapUnits and CRLOverlapPeriod registry settings need to be configured. 
So, in my previous example my CRL had a validity period of 3 days.  What I can do now is add a CRL Overlap Period of 3 days.  With this configuration, my CRL will be valid for a period of 6 Days.  However, at 3 days a new CRL will be published as well.  This is illustrated in the graphic below:


 
In the example illustrated in the graphic above, CRL 1 will be valid for a period of 6 days.  CRL 2 will be published at Day 3.  So, if my CA fails between Day 1 and Day 3, I would still have 3 Days (Day 3 through Day 6) to perform Emergency CRL signing or to recover my CA in event of failure.  If my CA fails between Day 3 and Day 6, there is a new CRL (CRL 2) that is valid through Day 9.  So, in short if my CA fails between Day 3 and Day 6, I still have at least 3 days to perform Emergency CRL Signing or to recovery my CA, before revocation checking starts to fail.  And the reason that I have the 3 days is the CRL Overlap Period extended out my CRL for 3 days and staggered the Next Publish and Next Update times by 3 days. 

1.10             Troubleshooting Certificate Validation

There are several tools that you can use to troubleshooting Chain Building and Revocation.

1.10.1                  Windows UI

You can use the errors visible in the Windows UI to troubleshoot Certificate Validation.  Below are a sample of some errors you may encounter in the Windows UI.

This CA Root certificate is not trusted.  To enable trust, install this certificate in the Trusted Root Certification Authorities store.

This error simply indicates that the Root CA Certificate is not trusted.

Windows does not have enough information to verify this certificate.

This error indicates that the client verify the certificate does not have access to all of the certificates in the chain.

The issuer of this certificate could not be found.

This error indicates that the client verify the certificate does not have access to all of the certificates in the chain.

This certificate has expired or is not yet valid.

This error typically indicates that the certificate is expired.

1.10.2                  Certutil -verify -urlfetch

Certutil -verify -urlfetch is perhaps the best tool to check the validation of a certificate.  You will want to run this tool on a machine which appears to have validation issues.  For example, if a client is visiting a website that is secured with TLS you would want to copy the certificate to the client and run this tool from the client. This command does all of the validation checks including checking of the AIA, CDP, and OCSP locations are accessible and contain valid configuration.  The normal syntax for running the command is: certutil -verify -urlfetch [File Name of Certificate] > [Name of Output Text File].

Below are screenshots or a sample output of certutil – verify -urlfetch to help you better understand how to read this file.

Here are some of the information or error messages you can receive at the end of the output from the command:

Leaf certificate revocation check passed

Indicates that the certificate passed the verification check.

A required certificate is not within its validity period when verifying against the current system clock or the timestamp in the signed file. 0x800b0101 (-2146762495 CERT_E_EXPIRED)

Typically means the certificate is expired.

ERROR: Verifying leaf certificate revocation status returned The revocation function was unable to check revocation because the revocation server was offline. 0x80092013 (-2146885613 CRYPT_E_REVOCATION_OFFLINE)

CertUtil: The revocation function was unable to check revocation because the revocation server was offline.

This means that the verifier was unable download a CRL.

The certificate is revoked. 0x80092010 (-2146885616 CRYPT_E_REVOKED)

Indicates that the certificate has been revoked.

Conclusion

That covers certificate validation. For additional information see the following video.

-Chris