Diagram of a public key infrastructure
A public key infrastructure (PKI) 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 (like people and organizations).
The binding is established through a process of registration and
issuance of certificates at and by a certificate authority
(CA). 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 (RA). 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 (VA) 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
Public key cryptography 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 (PKI) 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:
- A certificate authority (CA) that stores, issues and signs the digital certificates;
- A registration authority (RA) which verifies the identity of entities requesting their digital certificates to be stored at the CA;
- A central directory—i.e., a secure location in which keys are stored and indexed;
- A certificate management system managing things like the access to stored certificates or the delivery of the certificates to be issued;
- A certificate policy stating the PKI's requirements concerning its procedures. Its purpose is to allow outsiders to analyze the PKI's trustworthiness.
Methods of certification
Broadly speaking, there have traditionally been three approaches to getting this trust: certificate authorities (CAs), web of trust (WoT), and simple public key infrastructure (SPKI).
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 (RA), 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 (TTP) may also be used for certificate authority (CA). Moreover, PKI is itself often used as a synonym for a CA implementation.
In this model of trust relationships, a CA is a trusted third party –
trusted both by the subject (owner) 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
(TLS) certificates, states that- "Although the global [TLS] ecosystem
is competitive, it is dominated by a handful of major CAs — three
certificate authorities (Symantec, Sectigo, GoDaddy)
account for three-quarters of all issued [TLS] certificates on
public-facing web servers. The top spot has been held by Symantec (or VeriSign
before it was purchased by Symantec) ever since [our] 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 (Pretty Good Privacy) and GnuPG (an implementation of OpenPGP, the standardized specification of PGP). 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 (such as an internal CA in a company) 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:
As time goes on, you will accumulate keys from other people that you may want to designate as trusted introducers. Everyone else will each choose their own trusted introducers. And everyone will gradually accumulate and distribute with their key a collection of certifying signatures from other people, with the expectation that anyone receiving it will trust at least one or two of the signatures. This will cause the emergence of a decentralized fault-tolerant web of confidence for all public keys.
Simple public key infrastructure
Another alternative, which does not deal with public authentication of public key information, is the simple public key infrastructure (SPKI) 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.
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 (SSH, GPG, RFC 2230, etc.) 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:
- CertCoin
- FlyClient
- BlockQuick
History
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 (the Internet
and its predecessors), 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 (e.g., e-commerce, online access to proprietary databases from web browsers) were sufficient. Taher Elgamal and others at Netscape developed the SSL protocol ('https' in Web URLs);
it included key establishment, server authentication (prior to v3,
one-way only), 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 (or new projects at existing 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 (see ABA digital signature guidelines), and shortly thereafter, several U.S. states (Utah
being the first in 1995) 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 (manual or automatic) were not easy to correctly
design (nor even if so designed, to execute perfectly, which the
engineering required). 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 (DISA) 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:
- Encryption and/or sender authentication of e-mail messages (e.g., using OpenPGP or S/MIME);
- Encryption and/or authentication of documents (e.g., the XML Signature or XML Encryption standards if documents are encoded as XML);
- Authentication of users to applications (e.g., smart card logon, client authentication with SSL). There's experimental usage for digitally signed HTTP authentication in the Enigform and mod_openpgp projects;
- Bootstrapping secure communication protocols, such as Internet key exchange (IKE) and SSL. In both of these, initial set-up of a secure channel (a "security association") uses asymmetric key—i.e., public key—methods, whereas actual communication uses faster symmetric key—i.e., secret key—methods;
- Mobile signatures are electronic signatures that are created using a mobile device and rely on signature or certification services in a location independent telecommunication environment;
- Internet of things requires secure communication between mutually trusted devices. A public key infrastructure enables devices to obtain and renew X509 certificates which are used to establish trust between devices and encrypt communications using TLS.
Open source implementations
- OpenSSL is the simplest form of CA and tool for PKI. It is a toolkit, developed in C, that is included in all major Linux distributions, and can be used both to build your own (simple) CA and to PKI-enable applications. (Apache licensed)
- EJBCA is a full featured, Enterprise grade, CA implementation developed in Java. It can be used to set up a CA both for internal use and as a service. (LGPL licensed)
- XiPKI, CA and OCSP responder. With SHA3 support, implemented in Java. (Apache licensed)
- OpenCA is a full featured CA implementation using a number of different tools. OpenCA uses OpenSSL for the underlying PKI operations.
- XCA is a graphical interface, and database. XCA uses OpenSSL for the underlying PKI operations.
- (Discontinued) TinyCA was a graphical interface for OpenSSL.
- IoT_pki is a simple PKI built using the python cryptography library
- DogTag is a full featured CA developed and maintained as part of the Fedora Project.
- CFSSL open source toolkit developed by CloudFlare for signing, verifying, and bundling TLS certificates. (BSD 2-clause licensed)
- Vault tool for securely managing secrets (TLS certificates included) developed by HashiCorp. (Mozilla Public License 2.0 licensed)
- Libhermetik is a self-contained public-key infrastructure system embedded in a C-language library. Hermetik utilizes LibSodium for all cryptographic operations, and SQLite for all data persistence operations. The software is open-source and released under the ISC license.
Criticism
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.
