Public key infrastructure


A public key infrastructure is a set of roles, policies, hardware, software and procedures needed to create, manage, distribute, use, store and revoke digital certificates and manage public-key encryption. The purpose of a PKI is to facilitate the secure electronic transfer of information for a range of network activities such as e-commerce, internet banking and confidential email. It is required for activities where simple passwords are an inadequate authentication method and more rigorous proof is required to confirm the identity of the parties involved in the communication and to validate the information being transferred.
In cryptography, a PKI is an arrangement that binds public keys with respective identities of entities. The binding is established through a process of registration and issuance of certificates at and by a certificate authority. Depending on the assurance level of the binding, this may be carried out by an automated process or under human supervision.
The PKI role that assures valid and correct registration is called a registration authority. An RA is responsible for accepting requests for digital certificates and authenticating the entity making the request. In a Microsoft PKI, a registration authority is usually called a subordinate CA.
An entity must be uniquely identifiable within each CA domain on the basis of information about that entity. A third-party validation authority can provide this entity information on behalf of the CA.
The X.509 standard defines the most commonly used format for public key certificates.

Design

is a cryptographic technique that enables entities to securely communicate on an insecure public network, and reliably verify the identity of an entity via digital signatures.
A public key infrastructure is a system for the creation, storage, and distribution of digital certificates which are used to verify that a particular public key belongs to a certain entity. The PKI creates digital certificates which map public keys to entities, securely stores these certificates in a central repository and revokes them if needed.
A PKI consists of:
Broadly speaking, there have traditionally been three approaches to getting this trust: certificate authorities, web of trust, and simple public key infrastructure.

Certificate authorities

The primary role of the CA is to digitally sign and publish the public key bound to a given user. This is done using the CA's own private key, so that trust in the user key relies on one's trust in the validity of the CA's key. When the CA is a third party separate from the user and the system, then it is called the Registration Authority, which may or may not be separate from the CA. The key-to-user binding is established, depending on the level of assurance the binding has, by software or under human supervision.
The term trusted third party may also be used for certificate authority. Moreover, PKI is itself often used as a synonym for a CA implementation.

Issuer market share

In this model of trust relationships, a CA is a trusted third party – trusted both by the subject of the certificate and by the party relying upon the certificate.
According to NetCraft report from 2015, the industry standard for monitoring Active Transport Layer Security certificates, states that- "Although the global ecosystem is competitive, it is dominated by a handful of major CAs — three certificate authorities account for three-quarters of all issued certificates on public-facing web servers. The top spot has been held by Symantec ever since survey began, with it currently accounting for just under a third of all certificates. To illustrate the effect of differing methodologies, amongst the million busiest sites Symantec issued 44% of the valid, trusted certificates in use — significantly more than its overall market share."
Following to major issues in how certificate issuing were managed, all major players gradually distrusted Symantec issued certificates starting from 2017.

Temporary certificates and single sign-on

This approach involves a server that acts as an offline certificate authority within a single sign-on system. A single sign-on server will issue digital certificates into the client system, but never stores them. Users can execute programs, etc. with the temporary certificate. It is common to find this solution variety with X.509-based certificates.

Web of trust

An alternative approach to the problem of public authentication of public key information is the web-of-trust scheme, which uses self-signed certificates and third party attestations of those certificates. The singular term "web of trust" does not imply the existence of a single web of trust, or common point of trust, but rather one of any number of potentially disjoint "webs of trust". Examples of implementations of this approach are PGP and GnuPG. Because PGP and implementations allow the use of e-mail digital signatures for self-publication of public key information, it is relatively easy to implement one's own web of trust.
One of the benefits of the web of trust, such as in PGP, is that it can inter-operate with a PKI CA fully trusted by all parties in a domain that is willing to guarantee certificates, as a trusted introducer. If the "web of trust" is completely trusted then, because of the nature of a web of trust, trusting one certificate is granting trust to all the certificates in that web. A PKI is only as valuable as the standards and practices that control the issuance of certificates and including PGP or a personally instituted web of trust could significantly degrade the trustworthiness of that enterprise's or domain's implementation of PKI.
The web of trust concept was first put forth by PGP creator Phil Zimmermann in 1992 in the manual for PGP version 2.0:

Simple public key infrastructure

Another alternative, which does not deal with public authentication of public key information, is the simple public key infrastructure that grew out of three independent efforts to overcome the complexities of X.509 and PGP's web of trust. SPKI does not associate users with persons, since the key is what is trusted, rather than the person. SPKI does not use any notion of trust, as the verifier is also the issuer. This is called an "authorization loop" in SPKI terminology, where authorization is integral to its design. This type of PKI is specially useful for making integrations of PKI that do not rely on third parties for certificate authorization, certificate information, etc.; A good example of this is an Air-gapped network in an office.

Decentralized PKI

eliminates dependence on centralized registries for identifiers as well as centralized certificate authorities for key management, which is the standard in hierarchical PKI. In cases where the DID registry is a distributed ledger, each entity can serve as its own root authority. This architecture is referred to as decentralized PKI.

Blockchain-based PKI

An emerging approach for PKI is to use the blockchain technology commonly associated with modern cryptocurrency. Since blockchain technology aims to provide a distributed and unalterable ledger of information, it has qualities considered highly suitable for the storage and management of public keys. Some cryptocurrencies support the storage of different public key types and provides open source software that directly supports PKI for OpenSSH servers. While blockchain technology can approximate the proof of work often underpinning the confidence in trust that relying parties have in a PKI, issues remain such as administrative conformance to policy, operational security and software implementation quality. A Certificate Authority paradigm has these issues regardless of the underlying cryptographic methods and algorithms employed, and PKI that seeks to endow certificates with trustworthy properties must also address these issues.
Here is a list of known Blockchain-based PKI:
Developments in PKI occurred in the early 1970s at the British intelligence agency GCHQ, where James Ellis, Clifford Cocks and others made important discoveries related to encryption algorithms and key distribution. Because developments at GCHQ are highly classified, the results of this work were kept secret and not publicly acknowledged until the mid-1990s.
The public disclosure of both secure key exchange and asymmetric key algorithms in 1976 by Diffie, Hellman, Rivest, Shamir, and Adleman changed secure communications entirely. With the further development of high-speed digital electronic communications, a need became evident for ways in which users could securely communicate with each other, and as a further consequence of that, for ways in which users could be sure with whom they were actually interacting.
Assorted cryptographic protocols were invented and analyzed within which the new cryptographic primitives could be effectively used. With the invention of the World Wide Web and its rapid spread, the need for authentication and secure communication became still more acute. Commercial reasons alone were sufficient. Taher Elgamal and others at Netscape developed the SSL protocol ; it included key establishment, server authentication, and so on. A PKI structure was thus created for Web users/sites wishing secure communications.
Vendors and entrepreneurs saw the possibility of a large market, started companies, and began to agitate for legal recognition and protection from liability. An American Bar Association technology project published an extensive analysis of some of the foreseeable legal aspects of PKI operations, and shortly thereafter, several U.S. states and other jurisdictions throughout the world began to enact laws and adopt regulations. Consumer groups raised questions about privacy, access, and liability considerations, which were more taken into consideration in some jurisdictions than in others.
The enacted laws and regulations differed, there were technical and operational problems in converting PKI schemes into successful commercial operation, and progress has been much slower than pioneers had imagined it would be.
By the first few years of the 21st century, the underlying cryptographic engineering was clearly not easy to deploy correctly. Operating procedures were not easy to correctly design. The standards that existed were insufficient.
PKI vendors have found a market, but it is not quite the market envisioned in the mid-1990s, and it has grown both more slowly and in somewhat different ways than were anticipated. PKIs have not solved some of the problems they were expected to, and several major vendors have gone out of business or been acquired by others. PKI has had the most success in government implementations; the largest PKI implementation to date is the Defense Information Systems Agency PKI infrastructure for the Common Access Cards program.

Uses

PKIs of one type or another, and from any of several vendors, have many uses, including providing public keys and bindings to user identities which are used for:
Some argue that purchasing certificates for securing websites by SSL and securing software by code signing is a costly venture for small businesses. However, the emergence of free alternatives such as Let's Encrypt, has changed this. HTTP/2, the latest version of HTTP protocol allows unsecured connections in theory, in practice major browser companies have made it clear that they would support this protocol only over a PKI secured TLS connection. Web browser implementation of HTTP/2 including Edge from Microsoft, Chrome from Google, Firefox from Mozilla, and Opera supports HTTP/2 only over TLS by using ALPN extension of TLS protocol. This would mean that to get the speed benefits of HTTP/2, website owners would be forced to purchase SSL certificates controlled by corporations.
Current web browsers carry pre-installed intermediary certificates issued and signed by a Certificate Authority. This means browsers need to carry a large number of different certificate providers, increasing the risk of a key compromise.
When a key is known to be compromised it could be fixed by revoking the certificate, but such a compromise is not easily detectable and can be a huge security breach. Browsers have to issue a security patch to revoke intermediary certificates issued by a compromised root certificate authority. Some practical security vulnerabilities of X.509 certificates and known cases where keys were stolen from a major Certificate Authority are listed below.