Windows Server 2008 - Working with Certificate Services

Certificate Services in Windows Server 2008 is an easier venture than ever before. As we look at what is entailed in the components involved in establishing and supporting a PKI in Windows Server 2008 we need to quickly discuss what Certificate Services do for us. In Active Directory and Windows Server 2008, Certificate Services allow administrators to establish and manage the PKI environment. More generally, they allow for a trust model to be established within a given organization. The trust model is the framework that will hold all the pieces and components of the PKI in place. Typically, there are two options for a trust model within PKI: a single CA model and a hierarchical model. The certificate services within Windows Server 2008 provide the interfaces and underlying technology to setup and manage both of these type of deployments.

Configuring a Certificate Authority
By definition, a certificate authority is an entity (computer or system) that issues digital certificates of authenticity for use by other parties. With the ever increasing demand for effective and efficient methods to verify and secure communications, our technology market has seen the rise of many trusted third parties into the market. If you have been in the technology field for any length of time, you are likely familiar with many such vendors by name: VeriSign, Entrust, Thawte, GeoTrust, DigiCert and GoDaddy are just a few.

While these companies provide an excellent and useful resource for both the IT administrator and the consumer, companies and organizations desired a way to establish their own certificate authorities. In a third-party, or external PKI, it is up to the thirdparty CA to positively verify the identity of anyone requesting a certificate from it. Beginning with Windows 2000, Microsoft has allowed the creation of a trusted internal CA—possibly eliminating the need for an external third party. With a Windows Server 2008 CA, the CA verifies the identity of the user requesting a certificate by checking that user’s authentication credentials (using Kerberos or NTLM). If the credentials of the requesting user check out, a certificate is issued to the user. When the user needs to transmit his or her public key to another user or application, the certificate is then used to prove to the receiver that the public key inside can be used safely.

Certificate Authorities
Certificates are a way to transfer keys securely across an insecure network. If any arbitrary user were allowed to issue certificates, it would be no different than that user simply signing the data. In order for a certificate to be of any use, it must be issued by a trusted entity—an entity that both the sender and receiver trust. Such a trusted entity is known as a Certification Authority (CA). Third-party CAs such as VeriSign or Entrust can be trusted because they are highly visible, and their public keys are well known to the IT community. When you are confident that you hold a true public key for a CA, and that public key properly decrypts a certificate, you are then certain that the certificate was digitally signed by the CA and no one else. Only then can you be positive that the public key contained inside the certificate is valid and safe.

In the analogy we used earlier, the state driver’s licensing agency is trusted because it is known that the agency requires proof of identity before issuing a driver’s license. In the same way, users can trust the certification authority because they know it verifies the authentication credentials before issuing a certificate. Within an organization leveraging Windows Server 2008, several options exist for building this trust relationship. Each of these begins with the decisions made around selecting and implementing certificate authorities. With regard to the Microsoft implementation of PKI, there are at least four major roles or types of certificate authorities to be aware of:

• Enterprise CA
• Standard CA
• Root CA
• Subordinate CA

Believe it or not, beyond this list at least two variations exist: intermediate CAs and leaf CAs, each of which is a type of subordinate CA implementation.

Standard vs. Enterprise
An enterprise CA is tied into Active Directory and is required to use it. In fact, a copy of its own CA certificate is stored in Active Directory. Perhaps the biggest difference between an enterprise CA and a stand-alone CA is that enterprise CAs use Kerberos or NTLM authentication to validate users and computers before certificates are issued. This provides additional security to the PKI because the validation process relies on the strength of the Kerberos protocol, and not a human administrator. Enterprise CAs also use templates.

There are also several downsides to an enterprise CA. In comparison to a stand-alone CA, enterprise CAs are more difficult to maintain and require a much more in-depth knowledge about Active Directory and authentication. Also, because an enterprise CA requires Active Directory, it is nearly impossible to remove it from the network. If you were to do so, the Directory itself would quickly become outdated—making it difficult to resynchronize with the rest of the network when brought back online. Such a situation would force an enterprise CA to remain attached to the network, leaving it vulnerable to attackers.

Root vs. Subordinate Certificate Authorities
As discussed earlier, there are two ways to view PKI trust models: single CA and hierarchical. In a single CA model PKIs are very simplistic; only one CA is used within the infrastructure. Anyone who needs to trust parties vouched for by the CA is given the public key for the CA. That single CA is responsible for the interactions that ensue when parties request and seek to verify the information for a given certificate.

In a hierarchical model, a root CA functions as a top-level authority over one or more levels of CAs beneath it. The CAs below the root CA are called subordinate CAs. Root CAs serve as a trust anchor to all the CA’s beneath it and to the users who trust the root CA. A trust anchor is an entity known to be trusted without requiring that it be trusted by going to another party, and therefore can be used as a base for trusting other parties. Since there is nothing above the root CA, no one can vouch for its identity; it must create a self-signed certificate to vouch for itself. With a self-signed certificate, both the certificate issuer and the certificate subject are exactly the same. Being the trust anchor, the root CA must make its own certificate available to all of the users (including subordinate CAs) that will ultimately be using that particular root CA.

Hierarchical models work well in larger hierarchical environments, such as large government organizations or corporate environments. Often, a large organization also deploys a Registration Authority, Directory Services and optionally Timestamping Services in an organization leveraging a hierarchical approach to PKI. In situations where different organization are trying to develop a hierarchical model together (such as post acquisition or merger companies or those that are partnered for collaboration), a hierarchical model can be very difficult to establish as both parties must ultimately agree upon a single trust anchor.

When you first set up an internal PKI, no CA exists. The first CA created is known as the root CA, and it can be used to issue certificates to users or to other CAs. As mentioned above, in a large organization there usually is a hierarchy where the root CA is not the only certification authority. In this case, the sole purpose of the root CA is to issue certificates to other CAs in order to establish their authority.

Any certification authority that is established after the root CA is a subordinate
CA. Subordinate CAs gain their authority by requesting a certificate from either the root CA or a higher level subordinate CA. Once the subordinate CA receives the certificate, it can control CA policies and/or issue certificates itself, depending on your PKI structure and policies. Sometimes, subordinate CAs also issue certificates to other CAs below them on the tree. These CAs are called intermediate CAs. Is most hierarchies, there is more than one intermediate CA. Subordinate CAs that issue certificates to end users, server, and other entities but do not issue certificates to other CAs are called leaf CAs.

Certificate Requests
In order to receive a certificate from a valid issuing CA, a client—computer or user—must request a certificate from a CA.

There are three ways that this request can be made:
• Autoenrollment
• Use of the Certificates snap-in
• Via a web browser

It is very likely that the most common method for requesting a certificate is autoenrollment. A client can also request a certificate by use of the Certificates snap in. The snap-in, shown can be launched by clicking Start | Run, and then typing in certmgr.msc and pressing Enter. Note that the Certificates snap-in does not appear in the Administrative Tools folder as the Certification Authority snap-in does after installing certificate services. Once you open the Certificate Snap-in, expand the Personal container, and then right-clicking the Certificates container beneath it. You can start the Certificate Request Wizard by choosing All Tasks | Request New Certificate …,

Certificate Practice Statement
As the use of X.509-based certificates continues to grow it becomes increasingly important that the management an organization of certificates be as diligent as possible. We know what a digital certificate is and what its critical components are, but a CA can issue a certificate for a number of different reasons. The certificate, then, must indicate exactly what the certificate will be used for. The set of rules that indicates exactly how a certificate may be used (what purpose it can e trusted for, or perhaps the community for which it can be trusted) is called a certificate policy. The X.509 standard defines certificate policies as “a named set of rules that indicates the applicability of a certificate to a particular community and/or class of application with common security requirements.”

Different entities have different security requirements. For example, users want a digital certificate for securing e-mail (either encrypting the incoming messages signing outgoing mail), Syngress (as other Web vendors do) wants a digital certificate for their online store, etc. Every user will want to secure their information, and a certificate owner will use the policy information to determine if they want to accept a certificate.

It is important to have a policy in place to state what the appropriate protocol is for use of certificates—how they are requested, how and when they may be used, etc.—but it is equally as important to explain exactly how to implement those policies. This is where the Certificate Practice Statement (CPS) comes in. A CPS describes how the CA plans to manage the certificates it issues.

Source of Information : Syngress The Best Damn Windows Server 2008 Book Period 2nd Edition

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