Three Tier Hierarchy - Part I design and Planning

Three Tier Hierarchy
Specifically the difference between a Two Tier Hierarchy is that second tier is placed between the Root CA and the issuing CA. The placement of this CA can be for a couple different reasons. The first reason would be to use the second tier CA as a Policy CA. In other words the Policy CA is configured to issue certificates to the Issuing CA that is restricted in what type of certificates it issues. The Policy CA can also just be used as an administrative boundary. In other words, you only issue certain certificates from subordinates of the Policy CA, and perform a certain level of verification before issuing certificates, but the policy is only enforced from an administrative not technical perspective.
The other reason to have the second tier added is so that if you need to revoke a number of CAs due to a key compromise, you can perform it at the Second Tier level, leaving other “branches from the root” available. It should be noted that Second Tier CAs in this hierarchy can, like the Root, be kept offline.
Following the paradigm, security increases with the addition of a Tier, and flexibility and scalability increase due to the increased design options. On the other hand, manageability increases as there are a larger number of CAs in the hierarchy to manage. And, of course, cost goes up.
Security
Private Key Protection
One of the key aspects of designing a PKI solution is to make sure the proper controls are in place. Security for a PKI solution mostly revolves around protecting the private key pair of the CA. Each CA has a private/public key pair. The private key is used to sign CRL’s as well as certificates that are issued by the CA. Clients and application verify the signature so that they can be assured that a certificate was issued by a particular CA. If you install a Microsoft CA, the private key is protected by software, or more specifically the Data Protection API (DPAPI). Although this method does provide protection it does not prevent a user that is a member of the Administrators group on the CA from accessing the private key. This can be a cause for concern, because you may have administrators whose job is just to patch the system, and yet they have access to the private key which violates the concept of least privilege.
There are generally two methods for protecting the private key of a CA. The first method is to keep the CA offline and the hard drive stored in a safe. By controlling the conditions the hard drive can be used, the opportunities for key compromise are reduced. The second method is to use a hardware device to protect the private key. For example, a smartcard can be used to store the private key of the CA. This is not the best solution since the smart card must remain in the reader attached to the CA in order to be used. Also, a smart card may not be as resilient, or provide the level of security that is required. It is however a low cost solution. A more standard solution is to use a Hardware Security Module (HSM). HSM’s are fairly expensive, but are normally certified for FIPS compliance -- a standardized measure of relative security. HSM’s are accepted as the most secure way to protect the private key for a CA.
Role Separation
Aside from private key protection you will most likely want to have some control as to the level of administrative access to a CA. In some cases you may have administrators that are responsible for performing every function on the CA. But in larger or higher security environments you will want to have some granular control over what access different role holders have. Below is a list of common roles on a CA:
CA or PKI Administrator whose role is to manage the CA itself.
Certificate Manager who issues and revokes certificates.
Enrollment Agent is typically a role used in conjunction with smart cards; an Enrollment Agent enrolls for a certificate on behalf of another user.
Key Recovery Manager if using key archival. If you are using key archival, the Key Recovery Manager is responsible for recovering private keys. Also, if you are using EFS an EFS Recovery Agent role may be created to recover data encrypted using EFS.
In addition to these roles that have direct interaction with the CA, you also will have ancillary roles that support the CA. These include:
Backup Operator who is responsible for backing up the CA and restoring data in case of failure.
Auditor who is responsible for reviewing audit logs and ensuring policy is not being violated.
Physical Security
Certificates issued by CAs are used in many cases for very sensitive operations such as establishment of identity, authentication and encryption. As such, it is important to not only protect the private key but to protect physical access. Law #3 of the Ten Immutable Laws of Security, states: “If a bad guy has unrestricted physical access to your computer, it's not your computer anymore.” For this reason you will want to protect physical access to the CAs. This will depend on the resources you have available, but typically in larger organizations the CAs are stored in a locked cage in a data center. Only individuals that need physical access to the CA to perform their duties should be given access.
Policy
Generally the security requirements, such as those mentioned above, are dictated by a corporate security policy. A security policy usually takes into consideration regulatory and industry requirements as well as unique requirements for the individual company. The policy may also specify technical aspects of the PKI such as the encryption algorithms that must be used as well as operation of the Certificate Authorities.
In addition to security policies there may be CA-specific policies that need to be developed before implementing the PKI. These include Certificate Policy and Certification Practice Statement. The Certificate Policy explains what methods are used to establish the identity of a subject before issuing a certificate. A Certification Practice Statement outlines the operation of the PKI from a security perspective. Many companies, especially third parties companies that issue certificates, have their Certificate Policies and Certification Practice Statements available publicly. It may be helpful to view one of these public documents when writing your own policy documents.
Additional Security Considerations
In addition to the topics discussed it is important to apply any relevant security patches to your online CAs and to install them in a timely manner. In addition to patches, you should have an anti-malware solution installed on your CA.
So far we have covered reasons to deploy a Public Key Infrastructure. We also have covered the various costs involved in a PKI infrastructure, as well as the impact of various design considerations. Now we will dive a little deeper into specific configuration decisions and technical aspects of the Certificate Authorities.
CA Configuration
Certificate Validity Period
Digital certificates have a lifetime, a start date and an end date for which they are considered valid. You should determine what values for this lifetime are appropriate for each CA certificate and end-entity certificate issued by your CA’s. For CA’s, this lifetime is set when the CA is installed and when the private key is renewed. For end-entity certificates there are a number of factors taken into account:
These include the validity period for the issuing CA. The CA will not issue certificates that are valid past the CA’s lifetime.
The validity period specified in the Certificate Template.
The value of this registry key, specified in this KB article: http://support.microsoft.com/kb/254632
The certificate issued will be configured with the validity period that is the shortest of these items.
Key Length
The length of a key definitely affects security of information protected with that key. Thus, you will need to determine the key lengths you will use with each key pair. First you will need to determine the key lengths that will be used for each of the CA key pairs. Additionally, you will need to determine the key lengths for any certificates issued by the issuing CA. The key lengths for the CA certificates are determined by the key size requested when the CA is installed and when the key pair is renewed. The key length at installation is set during the CA Setup process. The key length for renewal is determined by a value set in the CAPolicy.inf configuration file installed on the CA.
For certificates issued by the issuing CA the maximum key size is limited by the CSP that is being used. The specific key size that is required can be specified in the certificate request or in the Certificate Template if using an Enterprise CA. As a general guideline, the longer the lifetime of the certificate the longer the key length should be. For applications that will be using certificates you will need to determine the maximum key length they support. Some applications have limitation on the key size not only in the actual certificate it is using, but also for any certificates in the CA hierarchy. From a security standpoint it is recommend that 4096 bit key is used for Certification Authorities key pair. However, if you wanted to insure maximum compatibility with network devices and applications a 2048 bit key would be the better choice.
AIA Locations
When a client or application is validating a certificate it needs to not only validate the certificate that is being used but also the entire chain of the certificate. In other words, the application or client needs a certificate from each CA in the chain beginning with the issuing CA and ending with the Root CA. If the application or client does not have access to the certificates in the chain locally the application or client needs a place from which to obtain the certificates. This location is called the Authority Information Access or AIA. The AIA location is the repository where the CA certificate is stored so that it can be downloaded by clients or applications validating a certificate. The AIA location is included in the AIA extension of a certificate. Before implementing your PKI it is important to think about what types of clients will be validating the certificates and where they reside. If you are using Windows clients that are internal to your network and are domain members then LDAP locations in Active Directory are a good place for clients to access the AIA repository. If you have non-Windows clients or Windows clients that are not domain members that are internal then an internally hosted web site would be the ideal location for the AIA repository. However, if clients may need to validate a certificate when outside the network, then you will need an AIA repository that is available externally, perhaps on the public network.
CDP Locations
A CRL Distribution Point (CDP) is where clients or applications that are validating a certificate download the certificate revocation list (CRL) to obtain revocation status. CA’s periodically publish CRLs to allow clients and applications to determine if a certificate has been revoked. CRLs contain the serial number of the certificate that has been revoked, a timestamp indicating when the certificate was revoked, as well as the reason for revocation. Similar to AIA Locations, you need to keep in mind what types of clients you are supporting and where they are located.
CRL Validity and Overlap Periods
Like certificates, CRLs have a start date and an end date denoting a period for which they are valid. As such, you will need to consider what the CRL lifetime should be for each CA. In general, the CRL lifetime is proportional to the number of certificates the CA is expected to issue. Offline CA’s that issue relatively few certificates, and those only to other CAs, would tend to have CRLs with a more extended lifetime, for example, six months to a year. This reflects the fact that, in a properly managed PKI, an offline CA would rarely revoke a certificate. Issuing CAs, on the other hand, can be expected to issue large numbers of certificates to end-entities. It is quite common to revoke an end-entity certificate for any number of reasons, so the lifetime of an issuing CA’s CRL can be quite short; a few days or even hours.
Another thing to consider is the overlap period. The overlap period is a short time interval beyond the expiration date of the CRL, and reflects the period between when a new CRL is published, and when the old CRL actually expires. During this time both CRLs are valid. This allows time for the new CRL to replicate to all of the repositories before the old CRL expires.
Delta CRLs
Delta CRLs are CRLs that contain revocation information for certificates that have been revoked since the base CRL was last published. For example, you have a Certificate A and it is revoked. The CA then publishes a new Base CRL that includes the revocation information for Certificate A. Shortly thereafter, Certificate B is revoked. At the designated interval a Delta CRL is published which contains the serial number and reason for revocation for Certificate B. When a client needs to determine revocation status Certificates A or B it downloads both the base CRL and the Delta CRL. The client determines that Certificate A is revoked from the base CRL, and then determines that Certificate B is revoked from the Delta CRL.
The reason for Delta CRLs is due to limitations with base CRLs. Base CRLs can grow rather large over time as they contain the serial number and revocation reason for every valid certificate that has been revoked from a CA. Instead of publishing a large CRL over and over again, revocation status can be updated with the smaller Delta CRL. In this way clients that have a time valid CRL will just need to download the Delta CRL. Like base CRLs you will need to determine how often Delta CRLs are published. It should be noted that the use of Delta CRLs is completely optional and is not normally used with offline CAs for obvious reasons.
OCSP URIs
Windows Vista, Windows 7, Windows 2008 and Windows 2008 R2 can obtain revocation information from an Online Responder via the Online Certificate Status Protocol. If you are using an Online Responder to provide revocation status, you should include the URI that points to the Online Responder.
Microsoft PKI
Other than the benefits of the Windows PKI, most of the things I have mentioned so far apply to any Public Key Infrastructure. I am now going to focus on a Microsoft-specific implementation.
Operating System Version
Currently both Windows 2003 and Windows 2008 are supported so you need to determine which OS you’re going to use for your CAs. In order to make that decision you will need to know what additional features Windows Server 2008 has over Windows Server 2003. Here are a few of the many new features in Windows Server 2008:
Supports Suite B Cryptographic Algorithms
http://blogs.technet.com/pki/archive/2009/04/15/suite-b-pki-in-windows-server-2008-and-windows-server-2008r2.aspx
New Crypto API called Cryptography Next Generation (CNG)
http://technet.microsoft.com/en-us/library/cc730763.aspx
PKIView is installed with the Certificate Services Role (Really nice when troubleshooting) http://technet.microsoft.com/en-us/library/cc732261.aspx
Certificate Authorities can be clustered (Active/Passive)
http://technet.microsoft.com/en-us/library/cc742517.aspx
Version 3 Certificate Templates
http://technet.microsoft.com/en-us/library/cc730826.aspx
New Certificate Templates
http://technet.microsoft.com/en-us/library/cc730826.aspx
Network Service permissions can be configured on Version 3 Templates
http://technet.microsoft.com/en-us/library/cc725621.aspx
Here is an article that goes into more detail on new features in Windows 2008 PKI
http://technet.microsoft.com/en-us/magazine/cc137986.aspx
Windows Server 2008 R2
Windows Server 2008 R2 adds a number of new features to Certificate Services. These features include:
Cross-Forest enrollment- Windows 2008 R2 Supports Cross-Forest enrollment which will allow a CA or multiple CAs in one forest to support clients in multiple Forests.
Certificate Enrollment Web Service and Policy Service- Allows clients to enroll for certificates over web interfaces. This new capability allows clients to retrieve certificates even if they are not located on the same physical network as Active Directory and the CA. Clients query the Enrollment Policy Service, to determine which Certificates they should enroll for, the Enrollment Policy Service contacts Active Directory and responds to the client with CA and Certificate Template information. The client then queries the Enrollment Web Service, to enroll for certificates. The Enrollment Web Service than contacts the CA on behalf of the client, and returns the enrolled certificates back to the client.
Non-persistent certificates (not stored in the CA database) - Certificate Templates can be configured to not store certificates in the CA database. The is useful for CAs that issue certificates for network authentication, in which certificates have a lifetime of hours or days and the storage of the certificates in the database would impact CA performance unnecessarily.
Operating System Edition
There are three editions of the OS on which you can install the Certificate Authority role. Those editions are Standard, Enterprise, and Datacenter. Standard or Enterprise Editions are normally used.
Below are the key features that Enterprise and Datacenter Edition supports and Standard Edition does not. It is important to note that Datacenter Edition does not offer any additional functionality in terms of Certificate Services over Enterprise Edition.
Version 2 Templates
The ability to duplicate and modify Certificate Templates
Certificate Autoenrollment (requires version 2 templates)
Common Criteria Role Separation enforcement
Key Archival and Retrieval
Certificate Authority Type
Next, you need to consider what type of Windows CA is required. A Standalone CA does not require Active Directory and can be installed on a non-domain member server. Requests for certificate enrollment can be sent through Web Enrollment if installed, or by sending a request file to the CA. The request files are usually generated through the Certreq.exe tool. Also, in Windows Server 2008 and Vista you can use the Certificate Management Console to build custom requests.
An Enterprise CA is integrated with Active Directory and requires AD in order to function. The Enterprise CA supports the same enrollment methods as the Standalone CA. In addition, however, an Enterprise can receive requests submitted through the Certificates MMC console. An Enterprise CA also allows for computer or user Autoenrollment which allows certificate request and issuance to be automated through a Group Policy setting. Enterprise CAs also use certificate templates which allow define what types of certificates a user or computer request from the CA.
Additionally, PKI related configuration information is stored in Active Directory. This makes it easy for applications and clients to locate an issuing CA, associated Certificate Templates, CRL’s and AIA information.
Certificate Templates
Certificate Templates allow you to create a template from which the values in the certificate request are generated. When a user requests a certificate based on a Certificate Template, the request generated by the client is built based upon the configuration of the template. Certificate templates are key to Autoenrollment, since Autoenrollment requires certificate templates. The importance of certificate templates is that it reduces management cost, and makes the CA easier to use. This is due to the fact that the user does not have to use complicated methods such as using certreq.exe and the associated configuration file to generate requests. Certificate Templates are stored in Active Directory and replicated to all domain controllers in the forest. This makes them highly available in order to support all clients in the forest.
For more information about Certificate Templates, please visit the following URLs:
Certificate Templates Overview: http://technet.microsoft.com/en-us/library/cc730826.aspx
Implementing and Administering Certificate Templates in Windows Server 2003: http://technet.microsoft.com/en-us/library/cc755996.aspx
Certificate Templates: http://technet.microsoft.com/en-us/library/cc758496.aspx
Enrollment
Another important aspect to consider is how clients will enroll for certificates. There are several methods to enroll for certificates and the actual method you choose may vary. But for the majority of customers most enrollments will be done through Autoenrollment. For one off situations and testing enrollment, manual enrollment or Web Enrollment will typically be used. Beginning with Windows 2008 there is Network Device Enrollment Service that can be used for network devices to enroll.
Autoenrollment
Autoenrollment adds a high level of automation to certificate issuance. In order to deploy Autoenrollment, you must first configure a certificate template from which clients will generate their requests. You then need to enable a Group Policy setting that enables Autoenrollment. Once you enable autoenrollment the computer or user, depending on what template is configured will automatically request a certificate based on the template after Group Policy refreshes.
For more information about Autoenrollment, please visit the following URLs:
How Autoenrollment Works: http://technet.microsoft.com/en-us/library/cc787781.aspx
Configure Autoenrollment in Group Policy: http://technet.microsoft.com/en-us/library/cc771025.aspx
Troubleshooting: http://technet.microsoft.com/en-us/library/cc755801.aspx
Manual Enrollment
There are several different methods for manual enrollment. The first method is to use the Certificates Management Console to enroll for certificates directly. The second method is to use the Certificate Management Console to generate a request file that can later be submitted to the Certification Authority. Third, a tool called certreq.exe can be used to create requests that can later be submitted to the CA. Certreq.exe can also be used to submit the request to the CA and also to download, and install the resulting certificate.
Web Enrollment
Web Enrollment is a web page that can be used to submit requests and download issued certificates from a CA. Web enrollment has typically been used to generate custom requests. However, with the ability to create custom requests in Windows Vista and Windows 2008 there is less of a need to use Web Enrollment
Application Specific Enrollment
Many Microsoft applications, such as Internet Information Services and Office Communication Server have built in wizards that assist with enrolling for certificates that are used by those applications.
Network Device Enrollment Service (NDES)
NDES is a role that uses the SCEP protocol to allow network devices to enroll for certificates.
Additional Considerations
Key Archival and Retrieval
Key Archival is a feature that allows the CA to archive the private key associated with a certificate in its database. A Key Recovery Manager can than recover the private key for a certificate if required. Although separate from Key Archival, an EFS Recovery Agent can be configured to recover EFS encrypted files.
For more information about Key Archival and Retrieval, please visit the following URLs:
Key Archival and Management in Windows Server 2003: http://technet.microsoft.com/en-us/library/cc755395.aspx
Implementing Key Archival Walkthrough: http://technet.microsoft.com/en-us/library/cc738977.aspx
Troubleshooting: http://technet.microsoft.com/en-us/library/cc787039.aspx
Active Directory and Group Policy
Aside from Autoenrollment, Active Directory and Group Policy allow the configuration of PKI related settings for clients. This includes EFS-related configuration, automatically publishing Root CA certificates to the Trusted Root Certification Store on clients, revocation checking configuration, and more…
For more information about PKI-related Group Policy settings, please visit the following URL:
Configure Public Key Group Policy: http://technet.microsoft.com/en-us/library/cc962057.aspx
Conclusion
I attempted to cover many aspects of the PKI implementation you will need to consider before deploying a Public Key Infrastructure. There is a great deal of information out there. As I mentioned before I would strongly recommend reading the Brian Komar books. I would also invite you to review PKI related posts on the Ask Directory Services Team Blog:
http://blogs.technet.com/askds/archive/tags/PKI/default.aspx
and the Windows PKI Blog:
http://blogs.technet.com/pki/default.aspx
As well as the Best Practices for Implementing a Microsoft Windows Server 2003 Public Key Infrastructure, which is located at:
http://www.microsoft.com/downloads/details.aspx?familyid=0BC67F4E-4FCF-4717-89E8-D0EE5E23A242&displaylang=en.
And the Windows Server 2003 PKI Operations Guide that can be downloaded from:
http://www.microsoft.com/downloads/details.aspx?FamilyID=8e25369f-bc5a-4083-a42d-436bdb363e7e&DisplayLang=en .
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