Identifying PKI Requirements

16 Aug

In Windows Server 2008 networks, a PKI relies on one or more CAs deployed through AD CS.
However, deploying a PKI is not as simple as adding the AD CS role in Server Manager. For
most medium-sized and large organizations, implementing a PKI requires significant planning.
Once the introduction of PKI-enabled applications triggers the need to implement a PKI,
you need to review your organization’s security policy. Then you need to assess other requirements
for the PKI, such as business requirements, external requirements, and Active Directory
requirements.
After you assess the needs of your organization in this way, you can design the PKI as a means
to enforce your organization’s security policies and to ensure that the new PKI remains aligned
with the company’s business and IT strategy.
After this lesson, you will be able to:
■ Understand the function of a PKI.
■ Identify applications that require a PKI.
■ Understand many of the factors that you need to consider when performing a
needs assessment for a PKI in a Windows Server 2008 network.
Estimated lesson time: 20 minutes
Reviewing PKI Concepts
A PKI refers to the set of technologies that enable an organization to use public key cryptography.
In public key cryptography, a mathematically related key pair consisting of a public key
and a private key is used in the encryption and decryption processes. If the public key is used
for encryption, the private key is used for decryption. If the private key is used for encryption,
the public key is used for decryption.
More specifically, a PKI is a system of digital certificates, CAs, and other registration authorities
(RAs) that provides cryptographic keys for, and authenticates the validity of, each party
involved in an electronic transaction.
MORE INFO Public key cryptography
For an introduction to public key cryptography, see “Understanding Public Key Cryptography,”
available at http://technet.microsoft.com/en-us/library/aa998077(EXCHG.65).aspx.
A PKI consists of the following basic components:
■ Digital certificates Electronic credentials that include a public key and that are used to
sign and encrypt data. Digital certificates are the foundation of a PKI.
394 Chapter 9 Planning and Designing a Public Key Infrastructure
■ One or more CAs Trusted entities or services that issue digital certificates. When multiple
CAs are used, they are typically arranged in a carefully prescribed order and perform
specialized tasks, such as issuing certificates to subordinate CAs or issuing certificates to
users.
■ Certificate policy and practice statements The two documents that outline how the CA
and its certificates are to be used, the degree of trust that can be placed in these certificates,
legal liabilities if the trust is broken, and so on.
■ Certificate repositories A directory service or other location where certificates are stored
and published. In a domain environment, Active Directory is the most likely publication
point for certificates issued by Windows-based CAs.
■ Certificate revocation lists (CRLs) Lists of certificates that have been revoked before
reaching the scheduled expiration date.
MORE INFO Public key infrastructure
For an introduction to PKI, see “Cryptography and Microsoft Public Key Infrastructure,” available at

http://www.microsoft.com/technet/security/guidance/cryptographyetc/cryptpki.mspx.

Identifying PKI-Enabled Applications
Typically, an organization decides to deploy a PKI only when that organization introduces one
or more applications that depend on a PKI. After the need for a PKI arises, you can begin to
define the PKI in a way that best supports these applications.
The following list describes the most common applications and technologies that can lead an
organization to consider deploying a PKI:
■ 802.1x port-based authentication 802.1x authentication allows only authenticated
users or computers to access either an 802.11 wireless network or a wired Ethernet network.
A PKI is required to support 802.1x when the Extensible Authentication Protocol-
Transport Layer Security (EAP-TLS), Extensible Authentication Protocol-Tunneled
Transport Layer Security (EAP-TTLS), or Protected Extensible Authentication Protocol
(PEAP) authentication protocol is used.
■ Digital signatures A PKI is used for digital signing. Digital signatures secure Internet
transactions by providing a method for verifying who sent the data and that content was
not modified in transit. Depending on how a certificate is issued, digital signatures also
provide nonrepudiation. In other words, data signers cannot deny that they are the data
senders because they are the only users with access to the certificate’s private key.
■ Encrypting File System (EFS) EFS provides a confidentiality service to NTFS. It employs
user key pairs to encrypt and decrypt files and recovery agent key pairs for file recovery
purposes. Certificates used for EFS are available from enterprise CAs. In an environment
with no Microsoft enterprise CAs, all EFS certificates are self-signed.
Lesson 1: Identifying PKI Requirements 395
■ Internet Protocol security Certificates can be used to authenticate the two endpoints in
an Internet Protocol Security (IPsec) association. After authentication, IPsec can be used
to encrypt and digitally sign all communications between the two endpoints. Certificates
do not play a part in the actual encryption and signing of IPsec-protected data—they are
used only to authenticate the two endpoints. Note also that in AD DS domains, Kerberos,
not certificates, is typically used for authentication.
■ Secure e-mail (S/MIME) Secure e-mail, the industry standard for which is Secure/Multipurpose
Internet Mail Extensions (S/MIME), provides confidential communication,
data integrity, and nonrepudiation for e-mail messages. S/MIME uses certificates to verify
a sender’s digital identity, the message’s point of origin, and message authenticity. It
also protects the confidentiality of messages by encrypting their content.
■ Smart card logon Smart cards are credit card–sized cards that contain a user certificate.
You can use smart cards to provide strong authentication for interactive logons.
■ Code signing Code signing protects computers from installation of unauthorized controls,
drivers, or applications. Applications that support code signing, such as Microsoft
Internet Explorer, can be configured to prevent execution of unsigned controls.
■ Virtual private networks (VPNs) VPNs allow remote users to connect to a private network
by using tunneling protocols, such as Point-to-Point Tunneling Protocol (PPTP),
Layer 2 Tunneling Protocol (L2TP), or Secure Socket Tunneling Protocol (SSTP). Not all
VPN types use certificates. However, certificates increase the strength of user authentication
and can provide authentication for IPsec if using L2TP with IPsec encryption.
■ Web authentication and encryption Distributing Secure Sockets Layer (SSL) certificates
to a Web server on either an intranet or the Internet allows a Web client to validate the
Web server’s identity and encrypt all data sent to and from the Web server. All Web servers
offering SSL connections require a server certificate, typically issued by a third-party
CA. Optionally, SSL connections can also use client certificates (although this is rarely
implemented).
Identifying Certificate Requirements
After you have determined which PKI-enabled applications your organization plans to deploy,
you must determine who must acquire the certificates and the types of certificates that are
required. Typically, certificates are deployed to the following subjects:
■ Users A digital certificate uniquely identifies a user to a PKI-enabled application. A user
can be assigned a single certificate that enables all applications or can receive applicationspecific
certificates, such as an EFS encryption certificate that can be used for one purpose
only. The certificates issued to the user are stored in the Current User certificate
store.
■ Computers A digital certificate uniquely identifies the computer when a user or computer
connects to the computer where the certificate is installed. The certificate becomes
396 Chapter 9 Planning and Designing a Public Key Infrastructure
the computer’s identifier and is stored in the Local Machine certificate store. If the Client
Authentication object identifier (OID) is included in the certificate in either the
Enhanced Key Usage (EKU) extension or the Application Policies extension, an application
can use the computer certificate to initiate connections. If the Server Authentication
OID is included in the certificate in the EKU or Application Policies extension, the certificate
can be used to authenticate the computer’s identity when a client application
connects.
■ Network devices Several devices on a network allow the installation of certificates for
client/server authentication. These devices include, but are not limited to, VPN appliances,
firewalls, and routers. The actual process used to install a certificate on a network
device is subject to the type of operating system and interfaces of the actual network
device.
Exam Tip Network device enrollment is a new feature offered by Windows Server 2008,
and, therefore, you are likely to see a general question about it on the 70-647 exam. Network
device enrollment relies on the Network Device Enrollment Service (NDES). This service
is the Microsoft implementation of the Simple Certificate Enrollment Protocol (SCEP), a communication
protocol that enables software running on network devices (such as routers and
switches, which cannot otherwise be authenticated on the network) to enroll for X.509 certificates
from a CA.
■ Services Some services require computer certificates for either authentication or
encryption. Certificates are not actually issued to a service. Instead, the service certificate
is stored either on the Local Machine store or in the user’s profile of the associated service
account. For example, if a certificate is installed for the World Wide Web (WWW)
service of a Web server, the certificate is stored in the Local Machine store. However, the
EFS recovery agent certificate for the EFS service is stored in the user profile of the designated
EFS recovery agent.
NOTE Where should you install a certificate for a service?
The easiest way to determine where to install a certificate for a service is to investigate what
credentials the service uses to authenticate. If the service uses Local System, then the certificate
must be stored in the Local Machine store. If the service uses a user account and password,
then the certificate must be stored in that specific user’s profile.
Identifying Certificate Security Requirements
Certificate requirements are driven by the PKI-enabled applications your organization plans to
use. Identifying these requirements will let you determine the properties of the certificates
needed. For each set of certificates, you should identify the following security requirements:
Lesson 1: Identifying PKI Requirements 397
■ Length of the private key In a typical deployment, the length of private keys are nested
so that each level in the PKI hierarchy has a key whose length is half that of the level
above it. For example, in a PKI, issued user certificates might have 1024-bit keys, issuing
CAs might have 2048-bit keys, and root CAs might have 4096-bit keys. Note that,
because longer keys are harder to mathematically attack, they support proportionately
longer lifetimes.
MORE INFO CA hierarchies
CA hierarchies, issuing CAs, and root CAs are discussed in more detail in Lesson 2, “Designing
the CA Hierarchy.”
In choosing a length for each CA in the CA hierarchy, the biggest restriction is the set of
applications that will use the CA hierarchy for certificates. Some applications are known
not to support keys larger than a certain value.
NOTE Which technologies limit private key length?
Technologies known to have issues with CA certificates with key lengths greater than 2048
bits include Cisco VPN 3000 series appliances, Nortel Contivity devices, and some older Java
applications.
■ Cryptographic algorithms that are used with certificates The standard settings for certificates
issued by a Windows Server 2008 CA can meet typical security needs. However,
you might want to specify stronger security settings for certificates that are used by certain
user groups. For example, you can specify longer private key lengths and shorter
certificate lifetimes for certificates used to provide security for very valuable information.
You can also specify the use of smart cards for private key storage to provide additional
security.
■ Lifetime of certificates and private keys and the renewal cycle A certificate has a predefined
validity period that comprises a start date and time and an end date and time.
You cannot change an issued certificate’s validity period after it has been issued. Certificate
lifetimes are determined by the type of certificate, your security requirements, standard
practices in your industry, and government regulations.
NOTE Certificate lifetimes
When determining certificate lifetimes for a PKI, a good rule of thumb is to make the validity
period of the certificate for a parent CA at least twice as long as the certificate for a subordinate
CA. In addition, the validity period of the certificate for an issuing CA should be at
least twice as long as the maximum validity period of any certificates issued by that same CA.
For example, you might give issued user certificates a lifetime of 1 year, the certificate for the
issuing CA a lifetime of 5 years, and the certificate for the root CA of the PKI a lifetime of 10
years.
398 Chapter 9 Planning and Designing a Public Key Infrastructure
■ Special private key storage and management requirements An organization’s security
policy can require specific security measures for a CA’s private key. For example, an organization
might have to implement Federal Information Processing Standards (FIPS) 140-2
protection of the CA’s private key to meet industry or organizational security requirements.
MORE INFO Where can you read FIPS 140-2?
FIPS 140-2, “Security Requirements for Cryptographic Modules,” can be found at http://
csrc.nist.gov/publications/fips/fips140-2/fips1402.pdf.
Measures you can take to protect the CA’s private key include using a cryptographic software
provider (CSP), which stores the CA’s private key material on the computer’s local
hard disk; a smart card CSP, which stores the CA’s private key material on a smart card
associated with a PIN; and a hardware security module (HSM), which provides the highest
security for private keys in dedicated hardware devices.
NOTE What is a CSP?
A CSP defines how a certificate’s private key is protected and accessed. The CSP will determine
where to generate the certificate’s key pair when the certificate is requested and will
implement mechanisms to protect access to the private key. For example, a CSP might
require the input of a PIN to access a smart card’s private key. The default CSP in AD CS
in Windows Server 2008 is the RSA#Microsoft Software Key Storage Provider. This CSP
supports traditional cryptographic algorithms as well as the Suite B algorithms enabled by
Cryptographic Next Generation, which is a new feature of Windows Server 2008.
Quick Check
■ If the lifetime of an issued user certificate is two years, what should normally be the
minimum lifetime of the certificate for the issuing CA?
Quick Check Answer
■ Four years.
Reviewing the Company Security Policy
After the need for a PKI is established and the required certificates are identified, you should
review the organization’s security policy. A security policy is a document, created by members
of an organization’s legal, human resources, and IT departments, that defines an organization’s
security standards. The policy usually includes the assets an organization considers
valuable, the potential threats to those assets, and, in general terms, the measures that must be
taken to protect these assets.
Lesson 1: Identifying PKI Requirements 399
The security policy should be updated to answer high-level PKI questions, such as:
■ What applications should be secured with certificates?
■ What kind of security services should be offered by using certificates?
In general, when planning and designing a PKI, it is essential to remember that a PKI should
enforce your organization’s security policy. A PKI, after all, is only as secure as the policies and
procedures that the organization implements.
MORE INFO What does a security policy include?
One of the most commonly used resources for defining a security policy is ISO 27002 (a renumbering
of ISO 17799/BS 7799), “Information Technology: Code of Practice for Information Security
Management,” which is available for purchase online (for example, at http://www.standardsdirect.org
/iso17799.htm). Another popular resource is RFC 2196, “The Site Security Handbook,” which is available
for free at http://www.ietf.org/rfc/rfc2196.txt.
Assessing Business Requirements
Business requirements define an organization’s goals. Business requirements affect the design
of the PKI by allowing the PKI to enhance business goals and processes. For example, the following
business requirements can affect a CA hierarchy design.
■ Minimizing PKI-associated costs When reviewing CA hierarchy designs, you might have
to choose a CA hierarchy that deploys the fewest CAs. For example, some organizations
combine the roles of policy CAs and issuing CAs into a single CA in the hierarchy,
deploying a two-tier hierarchy rather than a three-tier hierarchy.
■ High availability of certificate issuance An organization can require that a CA be consistently
available to ensure that no certificate requests fail due to a CA being down for any
reason. To ensure that a CA is always available, you should implement clustering on the
issuing CA that issues certificates based on the defined certificate template. If your uptime
requirements are not as stringent, you might consider publishing the certificate
template at more than one CA in the CA hierarchy, protecting against the failure of a single
CA.
■ Liability of PKI participants A CA hierarchy includes policy CAs that define the liability
of the CA. The liability should provide sufficient coverage for transactions that use CAissued
certificates. Your organization’s legal department must review this liability definition
to ensure that the definitions are legally correct and binding upon all participants in
the PKI.
400 Chapter 9 Planning and Designing a Public Key Infrastructure
Assessing External Requirements
In some cases, an organization might have to meet external requirements, such as those
defined by other organizations or by the governments of countries in which the organization
conducts business.
Examples of external requirements include the following:
■ Enabling external organizations to recognize employee-used certificates If you need other
organizations to recognize the certificates assigned to entities in your organization, you
can choose not to deploy an internal PKI and simply obtain certificates from commercial
CAs, such as VeriSign or RSA. Alternatively, you can use cross-certification or qualified
subordination to define which external certificates you trust.
■ Using your organization’s certificate at partner organizations Your employees might use
the certificates issued by your CA hierarchy for encryption or signing purposes at
another organization. In this case, you might have to create custom certificates to meet
the requirements of the other organization.
■ Industry or government legislation Several countries have legislation that affects the
design of a CA hierarchy. For example, Canada enforces the controllersInformation Protection
and Electronic Documents Act, which regulates the management of a customer’s
personal information when held by a private-sector company. The act requires that
someone be accountable for compliance and that this person be involved in the deployment
and design of the CA hierarchy to ensure that all requirements of the act are
enforced in the design.
■ Certificates for nonemployees If you issue certificates to nonemployees, you can use a
CA hierarchy to deploy a separate certificate policy that includes greater detail for external
clients.
Assessing Active Directory Requirements
You should make several preparations before you install a Windows Server 2008 enterprise
CA in a Windows 2000 or 2003 Active Directory environment. These preparations include the
following:
■ Determining the number of forests in the environment The number of forests will affect
the number of enterprise CAs that you require in your AD CS deployment. An enterprise
CA can issue certificates only to users and computers with accounts in the same forest.
If multiple forests must consume certificates from the PKI, you must deploy at least one
enterprise CA per forest.
■ Determining the number of domains in the forest If more than one domain is in the forest,
one of the major design decisions is which domain will host the CAs. The selection
of which domain will host the computer accounts of the CA computers will depend
largely on whether your organization uses centralized or decentralized management. In
Lesson 1: Identifying PKI Requirements 401
a centralized model, the CAs will typically be placed in the same domain. In a decentralized
environment, you might end up deploying CAs in multiple domains.
■ Determining the membership of the local Administrators groups for a member server I f
you use CSPs to protect a CA’s private key, all members of the CA’s local Administrators
group will be able to export the CA’s private key. You should start identifying which
domain or organizational unit in a domain will best limit the number of local administrators.
For example, an organization that has deployed an empty forest root might
choose to deploy all enterprise CAs as members of the forest root domain to limit the
number of local administrators on the CA.
■ Determining the schema version of the domain To implement Windows Server 2008
CAs and take advantage of all the new features introduced for AD CS, you must implement
the latest version of the Active Directory Domain Services schema. The Windows
Servers 2008 schema can be deployed in forests that contain Windows 2000 Server,
Windows Server 2003, and Windows Server 2008 domain controllers.
Assessing Certificate Template Requirements
Certificate templates provide a practical way to implement certificate enrollment in a managed
Active Directory environment. Because of the different versions of certificate templates
released with each version of Windows Server, compatibility issues must be identified as part
of your PKI planning.
Historically, static V1 certificate templates were introduced with Windows 2000. With Windows
Server 2003, customization was introduced with V2 certificate templates. With Windows
Server 2008, more certificate templates and certificate template properties compared with the
Windows Server 2003 templates became available (including properties related to CNG). The
new template types in Windows Server 2008 are called V3 templates.
Because of dependencies to the underlying operating system, Windows Server 2008 templates
can be assigned only to CAs that are running on a Windows Server 2008. Only Windows Vista
client computers and Windows Server 2008 computers can enroll for V3 certificate templates.
If you have installed only V2 certificates in your AD DS forest, you should upgrade the existing
templates and add the new V3 certificate templates. If you do not have any certificate templates,
all V1, V2, and V3 certificate templates are simply added to the configuration container
of your AD DS forest.
Lesson Summary
■ A PKI is a system of digital certificates, CAs, and other registration authorities (RAs) that
enables an organization to use public key cryptography.
402 Chapter 9 Planning and Designing a Public Key Infrastructure
■ The following technologies require a PKI or digital certificates: digital signatures, EFS,
SSL, S/MIME, smart cards, and code signing. In addition, the following technologies
sometimes require a PKI or digital certificates: 802.1x, IPsec, and VPNs.
■ After you have determined which PKI-enabled applications your organization plans to
deploy, you must determine who must acquire the certificates, the types of certificates
that are required, and the security requirements for those certificates.
■ As part of the process of planning a PKI for an organization, you should review the organization’s
existing security policy, along with its business requirements, Active Directory
requirements, certificate template requirements, and any other external requirements.
Lesson Review
You can use the following questions to test your knowledge of the information in Lesson 1,
“Identifying PKI Requirements.” The question is also available on the companion CD if you
prefer to review it in electronic form.
NOTE Answers
Answers to these questions and explanations of why each answer choice is correct or incorrect are
located in the “Answers” section at the end of the book.
1. Which of the following applications does NOT require the use of certificates?
A. Encrypting File System (EFS)
B. Secure/Multipurpose Internet Mail Extensions (S/MIME)
C. Internet Protocol Security (IPsec)
D. Secure Sockets Layer (SSL)

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