Certificate Repositories

Once the requestor's identity has been proven, a certificate is registered with the public side of the key pair provided by the requestor. Public keys must be available to anybody who requires them to communicate within a PKI environment. These keys, and their corresponding certificates, are usually held in a publicly available repository. Repository is a general term that describes a centralized directory that can be accessed by a subset of individuals. The directories are usually Lightweight Directory Access Protocol (LDAP)-compliant, meaning that they can be accessed and searched via the LDAP.

When an individual initializes communication with another, the sender can send her certificate and public key to the receiver, which will allow the receiver to communicate with the sender using encryption or digital signatures (or both) without needing to track down the necessary public key in a certificate repository. This is equivalent to the sender saying, "If you would like to encrypt any future messages you send to me, or if you would like the ability to verify my digital signature, here are the necessary components."

But if a person wants to encrypt the first message sent to the receiver, the sender will need to find the receiver's public key in a certificate repository.

Trust and Certificate Verification

We need to use a PKI if we do not automatically trust individuals we do not know. Security is about being suspicious and being safe, so we need a third party that we do trust to vouch for the other individual before confidence can be instilled and sensitive communication can take place. But what does it mean that we trust a CA, and how can we use this to our advantage?

Validating a digital certificate


Encrypted with CA's private key

| Message digest В


When a user chooses to trust a CA, she will download that CA's digital certificate and public key, which will be stored on her local computer.

Most browsers have a list of CAs configured to be trusted by default, so when a user installs a new web browser, several of the most well-known and most trusted CAs will be trusted without any change of settings.

Message digest A

I -Joyce sends the message and digital certificate to Maynard.

2. Maynard extracts the certificate.

3. He puts the certificate through a hashing algorithm.

4. The algorithm calculates a value of A.

5. Maynard extracts the encrypted message digest from the certificate.

6. Maynard decrypts the value with the CA's public key.

7. Maynard checks to see if the certificate was revoked.

Maynard compares values A and B.

8. The values are the same, so Maynard reads the message.


In the Microsoft CAPI environment, the user can add and remove CAs from this list as needed. In production environments that require a higher degree of protection, this list will be pruned, and possibly the only CAs listed will be the company's internal CAs. This ensures that digitally signed software will be automatically installed only if it was signed by the company's CA. Other products, such as Entrust, use centrally controlled policies to determine which CAs are to be trusted instead of expecting the user to make these critical decisions.

Digital Certificates

A digital certificate binds an individual's identity to a public key, and it contains all the information a receiver needs to be assured of the identity of the public key owner. After an RA verifies an individual's identity, the CA generates the digital certificate, but how does the CA know what type of data to insert into the certificate? The certificates are created and formatted based on the X.509 standard, which outlines the necessary fields of a certificate and the possible values that can be inserted into the fields. As of this writing, X.509 version 3 is the most current version of the standard. X.509 is a standard of the International Telecommunication Union (www.itu.int). The

IETF's Public-Key Infrastructure (X.509), or PKIX, working group has adapted the X.509 standard to the more flexible organization of the Internet, as specified in RFC 3280, and is commonly referred to as PKIX for Public Key Infrastructure (X.509).

The following fields are included within a X.509 digital certificate:

  • Version number Identifies the version of the X.509 standard that was followed to create the certificate; indicates the format and fields that can be used.

  • Subject Specifies the owner of the certificate.

  • Public key Identifies the public key being bound to the certified subject; also identifies the algorithm used to create the private/public key pair.

  • Issuer Identifies the CA that generated and digitally signed the certificate.

  • Serial number Provides a unique number identifying this one specific certificate issued by a particular CA.

  • Validity Specifies the dates through which the certificate is valid for use.

  • Certificate usage Specifies the approved use of certificate, which dictates intended use of this public key.

  • Signature algorithm Specifies the hashing and digital signature algorithms used to digitally sign the certificate.

  • Extensions Allow additional data to be encoded into the certificate to expand the functionality of the certificate. Companies can customize the use of certificates within their environments by using these extensions. X.509 version 3 has extended the extension possibilities.

The subject of a certificate is commonly a person, but it does not have to be. The subject can be a network device (router, web server, firewall, and so on), an application, a department, a company, or a person. Each has its own identity that needs to be verified and proven to another entity before secure, trusted communication can be initiated.

If a network device is using a certificate for authentication, the certificate may contain the network address of that device. This means that if the certificate has a network address of, the receiver will compare this to the address from which it received the certificate to make sure a man-in-the-middle attack is not being attempted.

Certificate Attributes

Four main types of certificates are used:

  • End-entity certificates

  • CA certificates

  • Cross-certification certificates

  • Policy certificates

such as Joyce, the is the identity document

or root CA, or it can be

End-entity certificates are issued by a CA to a specific subject,

Accounting department, or a firewall. An end-entity certificate provided by PKI implementations.

A CA certificate can be self-signed, in the case of a standalone issued by a superior CA within a hierarchical model. The superior CA gives the authority and allows the subordinate CA to accept certificate requests and generate the individual certificates itself. This may be necessary when a company needs to have multiple internal CAs, and different departments within an organization need to have their own CAs servicing their specific end-entities in their sections. In these situations, a representative from each department requiring a CA registers with the higher trusted CA and requests a Certificate Authority certificate. Cross-certificates, or cross-certification certificates, are used when independent CAs establishes peer-to-peer trust relationships. Simply put, they are a mechanism through which one CA can issue a certificate allowing its users to trust another CA. Within sophisticated CAs used for high-security applications, a mechanism is required to provide centrally controlled policy information to PKI clients. This is often done by placing the policy information in a policy certificate.

Certificate Extensions

Certificate extensions allow for further information to be inserted within the certificate, which can be used to provide more functionality in a PKI implementation. Certificate extensions can be standard or private. Standard certificate extensions are implemented for every PKI implementation. Private certificate extensions are defined for specific organizations (or domains within one organization), and they allow companies to further define different, specific uses for digital certificates to best fit their business needs.

Several different extensions can be implemented, one being key usage extensions, which dictate how the public key that is held within the certificate can be used. Remember that public keys can be used for different functions: symmetric key encryption, data encryption, verifying digital signatures, and more. Following are some key examples of certificate extension:

  • DigitalSignature The key used to verify a digital signature

  • KeyEncipherment The key used to encrypt other keys used for secure key distribution

  • DataEncipherment The key used to encrypt data, which cannot be used to encrypt other keys

  • CRLSign The key used to verify a CA signature on a revocation list

  • KeyCertSign The key used to verify CA signatures on certificates

  • NonRepudiation The key used when a nonrepudiation service is being provided

Critical and Noncritical Extensions

Certificate extensions are considered either critical or noncritical, which is indicated by a specific flag within the certificate itself. When this flag is set to critical, it means that the extension must be understood and processed by the receiver. If the receiver is not configured to understand a particular extension marked as critical, and thus cannot process it properly, the certificate cannot be used for its proposed purpose. If the flag does not indicate that the extension is critical, the certificate can be used for the intended purpose, even if the receiver does not process the appended extension.

So how does this work? When an extension is marked as critical, it means that the CA is certifying the key for only that specific purpose. If Joe receives a certificate with a

DigitalSignature key usage extension and the critical flag is set, Joe can use the public key only within that certificate to validate digital signatures, and no more. If the extension was marked as noncritical, the key can be used for purposes outside of those listed in the extensions, so in this case it is up to Joe (and his applications) to decide how the key will be used.

Certificate Lifecycles

Keys and certificates should have lifetime settings that will force the user to register for a new certificate after a certain amount of time. Determining the proper length of these lifetimes is a trade-off: Shorter lifetimes limit the ability of attackers to crack them, but longer lifetimes lower system overhead. More sophisticated PKI implementations perform automated and often transparent key updates to avoid the time and expense of having users register for new certificates when old ones expire. This means that the certificate and key pair has a lifecycle that must be managed. Certificate management involves administrating and managing each of these phases, including registration, certificate and key generation, renewal, and revocation.

Registration and Generation

A key pair (public and private keys) can be generated locally by an application and stored in a local key store on the user's workstation. The key pair can also be created by a central key-generation server, which will require secure transmission of the keys to the user. The key pair that is created on the centralized server can be stored on the user's workstation or on the user's smart card, which will allow for more flexibility and mobility. In most modern PKI implementations, users have two key pairs. One key pair is often generated by a central server and used for encryption and key transfers. This allows the corporate PKI to retain a copy of the encryption key pair for recovery, if necessary. The second key pair, a digital signature key pair, is usually generated by the user to make sure that she is the only one with a copy of the private key. Nonrepudiation can be challenged if there is any doubt about someone else obtaining a copy of an individual's signature private key. If the key pair was created on a centralized server, that could weaken the case that the individual was the only one who had a copy of her private key.

If a copy of a user's signature private key is stored anywhere other than in her possession, or if there is a possibility of someone obtaining the user's key, then true nonrepudiation cannot be provided.


The certificate itself has its own lifetime, which can be different than the key pair's lifetime. The certificate's lifetime is specified by the validity dates inserted into the digital certificate. These are beginning and ending dates indicating the time period during which the certificate is valid. The certificate cannot be used before the start date, and once the end date is met, the certificate is expired and a new certificate will need to be issued.

A renewal process is different from the registration phase in that the RA assumes that the individual has already successfully completed one registration round. If the certificate has not actually been revoked, the original keys and certificate can be used to provide the necessary authentication information and proof of identity for the renewal phase.


A certificate can be revoked when its validity needs to be ended before its actual expiration date is met, and this can occur for many reasons: for example, a user may have lost a laptop or a smart card that stored a private key, an improper software implementation may have been uncovered that directly affected the security of a private key, a user may have fallen victim to a social engineering attack and inadvertently given up a private key, data held within the certificate may no longer apply to the specified individual, or perhaps an employee left a company and should not be identified as a member of an in-house PKI any longer. In the last instance, the certificate, which was bound to the user's key pair, identified the user as an employee of the company, and the administrator would want to ensure that the key pair could not be used in the future to validate this person's affiliation with the company. Revoking the certificate does this.

If any of these things happen, a user's private key has been compromised or should no longer be mapped to the owner's identity. A different individual may have access to that user's private key and could use it to impersonate and authenticate as the original user. If the impersonator used the key to digitally sign a message, the receiver would verify the authenticity of the sender by verifying the signature by using the original user's public key, and the verification would go through perfectly-the receiver would believe it came from the proper sender and not the impersonator. If receivers could look at a list of certificates that had been revoked before verifying the digital signature, however, they would know not to trust the digital signatures on the list. Because of issues associated with the private key being compromised, revocation is permanent and final-once revoked, a certificate cannot be reinstated. If these were allowed and a user revoked his certificate, the unauthorized holder of the private key could use it to restore the certificate validity.

CRL Distribution

CRL files can be requested by individuals who need to verify and validate a newly received certificate, or the files can be periodically pushed down (sent) to all users participating within a specific PKI. This means the CRL can be pulled (downloaded) by individual users when needed or pushed down to all users within the PKI on a timed interval.

The actual CRL file can grow substantially, and transmitting this file and requiring PKI client software on each workstation to save and maintain it can use a lot of resources, so the smaller the CRL is, the better. It is also possible to first push down the full CRL, and after that initial load, the following CRLs pushed down to the users are delta CRLs, meaning that they contain only the changes to the original or base CRL. This can greatly reduce the amount of bandwidth consumed when updating CRLs


Instead of being revoked, a certificate can be suspended, meaning it is temporarily put on hold. If, for example, Bob is taking an extended vacation and wants to ensure that his certificate will not be used during that time, he can make a suspension request to the CA. The CRL would list this certificate and its serial number, and in the field that describes why the certificate is revoked, it would instead indicate a hold state. Once Bob returns to work, he can make a request to the CA to remove his certificate from the list.

Key Destruction

Key pairs and certificates have set lifetimes, meaning that they will expire at some specified time. It is important that the certificates and keys are properly destroyed when that time comes, wherever the keys are stored (on users' workstations, centralized key servers, USB token devices, smart cards, and so on).

Centralized or Decentralized Infrastructures

Keys used for authentication and encryption within a PKI environment can be generated in a centralized or decentralized manner. In a decentralized approach, software on individual computers generates and stores cryptographic keys local to the systems themselves. In a centralized infrastructure, the keys are generated and stored on a central server, and the keys are transmitted to the individual systems as needed. You might choose one type over the other for several reasons.

If a company uses an asymmetric algorithm that is resource-intensive to generate the public/private key pair, and if large (and resource-intensive) key sizes are needed, then the individual computers may not have the necessary processing power to produce the keys in an acceptable fashion. In this situation, the company can choose a centralized approach in which a very high-end server with powerful processing abilities is used, probably along with a hardware-based random number generator.

Hardware Storage Devices

PKIs can be constructed in software without special cryptographic hardware, and this is perfectly suitable for many environments. But software can be vulnerable to viruses, hackers, and hacking. If a company requires a higher level of protection than a purely software-based solution can provide, several hardware-based solutions are available.

Private Key Protection

Although a PKI implementation can be complex, with many different components and options, a critical concept common to all PKIs must be understood and enforced: the private key needs to stay private. A digital signature is created solely for the purpose of proving who sent a particular message by using a private key. This rests on the assumption that only one person has access to this private key. If an imposter obtains a user's private key, authenticity and nonrepudiation can no longer be claimed or proven.

When a private key is generated for the first time, it must be stored somewhere for future use. This storage area is referred to as a key store, and it is usually created by the application registering for a certificate, such as a web browser, smart card software, or other application. In most implementations, the application will prompt the user for a password, which will be used to create an encryption key that protects the key store. So, for example, if Cheryl used her web browser to register for a certificate, her private key would be generated and stored in the key store. Cheryl would then be prompted for a password, which the software would use to create a key that will encrypt the key store.

When Cheryl needs to access this private key later that day, she will be prompted for the same password, which will decrypt the key store and allow her access to her private key.

Key Recovery

One individual could have one, two, or many key pairs that are tied to his or her identity. That is because users can have different needs and requirements for public/private key pairs. As mentioned earlier, certificates can have specific attributes and usage requirements dictating how their corresponding keys can and cannot be used. For example, David can have one key pair he uses to encrypt and transmit symmetric keys.

He can also have one key pair that allows him to encrypt data and another key pair to perform digital signatures. David can also have a digital signature key pair for his work related activities and another pair for personal activities, such as e-mailing his friends.

These key pairs need to be used only for their intended purposes, and this is enforced through certificate attributes and usage values.

Key Escrow

Key recovery and key escrow are terms that are often used interchangeably, but they actually describe two different things. You should not use them interchangeably after you have read this section. Key recovery is a process that allows for lost keys to be recovered. Key escrow is a process of giving keys to a third party so that they can decrypt and read sensitive information when this need arises. Key escrow almost always pertains to handing over encryption keys to the government, or to another higher authority, so that the keys can be used to collect evidence during investigations. A key pair used in a person's place of work may be required to be escrowed by the employer for obvious reasons. First, the keys are property of the enterprise, issued to the worker for use.

Second, the firm may have need for them after an employee leaves the firm.