Public key infrastructure (PKI) consists of programs, data formats, procedures, communication protocols, security policies, and public key cryptographic mechanisms working in a comprehensive manner to enable a wide range of dispersed people to communicate in a secure and predictable fashion. In other words, a PKI establishes a level of trust within an environment. PKI is an ISO authentication framework that uses public key cryptography and the X.509 standard. The framework was set up to enable authentication to happen across different networks and the Internet. Particular protocols and algorithms are not specified, which is why PKI is called a framework and not a specific technology.

PKI provides authentication, confidentiality, non-repudiation, and integrity of the messages exchanged. PKI is a hybrid system of symmetric and asymmetric key algorithms and methods.

There is a difference between public key cryptography and PKI. Public key cryptography is another name for asymmetric algorithms, while PKI is what its name states – it is an infrastructure. The infrastructure assumes that the receiver’s identity can be positively ensured through certificates and that an asymmetric algorithm will automatically carry out the process of key exchange. The infrastructure therefore contains the pieces that will identify users, create and distribute certificates, maintain and revoke certificates, distribute and maintain encryption keys, and enable all technologies to communicate and work together for the purpose of encrypted communication and authentication.

Public key cryptography is one piece in PKI, but many other pieces make up this infrastructure. An analogy can be drawn with the email protocol Simple Mail Transfer Protocol (SMTP). SMTP is the technology used to get email messages from here to there, but many other things must be in place before this protocol can be productive.

We need email clients, email servers, and email messages, which together build a type of infrastructure – an email infrastructure. PKI is made up of many different parts: certificate authorities, registration authorities, certificates, keys, and users. The following sections explain these parts and how they all work together.

Each person who wants to participate in a PKI requires a digital certificate, which is a credential that contains the public key for that individual along with other identifying information. The certificate is created and signed (digital signature) by a trusted third party, which is a certificate authority (CA). When the CA signs the certificate, it binds the individual’s identity to the public key, and the CA takes liability for the authenticity of that individual. It is this trusted third party (the CA) that allows people who have never met to authenticate to each other and communicate in a secure method. If Shahbaz has never met X, but would like to communicate securely with her, and they both trust the same CA, then Shahbaz could retrieve X’s digital certificate and start the process.

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