Public Key Infrastructure (PKI) provides the means to establish trust by binding public keys and identities, thus giving reasonable assurance that we’re communicating securely with who we think we are. PKI is important to using public key cryptography effectively, and is essential to understanding and using the SSL protocol.

Using public key cryptography, we can be sure that only the encrypted data can be decrypted with the corresponding private key. If we combine this with the use of a message digest algorithm to compute a signature, we can be sure that the encrypted data has not been tampered with. What’s missing is some means of ensuring that the party we’re communicating with is actually who they say they are. In other words, trust has not been established. This is where PKI fits in.



  • At the heart of PKI is something called a certificate .
  • In simple terms, a certificate binds a public key with a distinguished name .
  • A distinguished name is simply the name of the person or entity that owns the public key to which it’s bound.
  • A certificate is signed with the issuer’s private key, and it contains almost all of the information necessary to verify its validity. It contains information about the subject, the issuer, and the period for which it is valid. The key component that is missing is the issuer’s certificate. The issuer’s certificate is the key component for verifying the validity of a certificate because it contains the issuer’s public key, which is necessary for verifying the signature on the subject’s certificate.
  • A certificate is signed with the issuer’s private key, and it contains almost all of the information necessary to verify its validity
    Certificates are also created with a serial number embedded in them. The serial number is unique only to the issuer of the certificate. No two certificates issued by the same issuer should ever be assigned the same serial number. The certificate’s serial number is often used to identify a certificate quickly


Certification Authorities

A Certification Authority (CA) is an organization or company that issues certificates
The CA must ensure beyond all reasonable doubt that every certificate it issues contains a public key that was issued by the party that claims to have issued it.

There are two basic types of CAs.

  • A private CA has the responsibility of issuing certificates only for members of its own organization, and is likewise trusted only by members of its own organization.
  • A public CA, such as VeriSign or Thawte, has the responsibility of issuing certificates for any member of the public, and must be trusted by the public


Certificate Hierarchies

A certificate that is issued by a CA can be used to issue and sign another certificate, if the issued certificate is created with the appropriate permissions to do so. In this way, certificates can be chained.
At the root of the chain is the root CA’s certificate. Because it is at the root of the chain and there is no other authority to sign its certificate, the root CA signs its own certificate. Such a certificate is known as a self-signed certificate.


Certificate Extensions

The most widely accepted format for certificates is the X.509 format. There are three versions of the format, known as X.509v1, X.509v2, and X.509v3. The most recent revision of the standard was introduced in 1996, and most, if not all, modern software now supports it. A large number of changes were made between X.509v1 and X.509v3, but perhaps one of the most significant features introduced in the X.509v3 standard is its support of extensions.


Certificate Revocation Lists

Once a certificate has been issued, it is generally put into production, where it will be distributed to many clients. If an attacker compromises the associated private key, he now has the ability to use the certificate even though it doesn’t belong to him. Assuming the proper owner is aware of the compromise, a new certificate with a new key pair should be obtained and put into use. In this situation there are two certificates for the same entity—both are technically valid, but one should not be trusted. The compromised certificate will eventually expire, but in the meantime, how will the world at large know not to trust it?

The answer lies in something called a certificate revocation list (CRL). A CRL contains a list of all of the revoked certificates a CA has issued that have yet to expire. When a certificate is revoked, the CA declares that the certificate should no longer be trusted.


Online Certificate Status Protocol

The Online Certificate Status Protocol (OCSP), formally specified in RFC 2560, is a relatively new addition to PKI. Its primary aim is to address some of the distribution problems that have traditionally plagued CRLs.

Using OCSP, an application makes a connection to an OCSP responder and requests the status of a certificate by passing the certificate’s serial number. The responder replies “good,” “revoked,” or “unknown.” A “good” response indicates that the certificate is valid, so far as the responder knows. This does not necessarily mean that the certificate was ever issued, just that is hasn’t been revoked. A “revoked” response indicates that the certificate has been issued and that it has indeed been revoked. An “unknown” response indicates that the responder doesn’t know anything about the certificate. A typical reason for this response could be that the certificate was issued by a CA that is unknown to the responder.


Network Security with OpenSSL

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