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The flexibility and scalability of your solution should be taken into consideration. If you have a high level of confidence that you will not need to change or adapt your PKI solution you can have a fairly simple design. However, if you need a solution that will need to support a variety of technologies, different levels of security, and a global presence, then your solution can get much more complicated.
When designing your PKI solution you will have to determine the hierarchy that you will use. There are generally three types of hierarchies, and they are denoted by the number of tiers. A single tier Hierarchy consists of one CA. Any applications, users, or computers that trust the Root CA trust any certificates issued by the CA hierarchy. For security reasons, these two roles are normally separated.
When using a single tier hierarchy they are combined. This may be sufficient for simple implementations where ease of manageability and lower cost outweigh the need for greater levels of security or flexibility. In some ways it is a compromise between the One and Three Tier hierarchies. But more importantly the Root CA is offline, and so the private key of the Root CA is better protected from compromise.
It also increases scalability and flexibility. Cost is increased marginally. I say marginally, because all you need is a hard drive and Windows OS license to implement an Offline Root.
Install the hard drive, install your OS, build your PKI hierarchy, and then remove the hard drive and store it in a safe. The hard drive can be attached to existing hardware when CRLs need to be re-signed. A virtual machine could be used as the Root CA, although you would still want to store it on a separate hard drive that can be stored in a safe.
The placement of this CA can be for a couple different reasons. 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. 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. One of the key aspects of designing a PKI solution is to make sure the proper controls are in place. Clients and application verify the signature so that they can be assured that a certificate was issued by a particular CA.
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. 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:. In addition to these roles that have direct interaction with the CA, you also will have ancillary roles that support the CA. These include:. 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. 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. 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. The Certificate Policy explains what methods are used to establish the identity of a subject before issuing a certificate. 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. 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. Digital certificates have a lifetime, a start date and an end date for which they are considered valid.
For end-entity certificates there are a number of factors taken into account:.
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