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public-key infrastructures

A Public-Key Infrastructure is the backbone for large-scale use of public-key cryptography. There are many initiatives by businesses and governments to set up PKIs with Certification Authorities (CAs), in particular to promote the use of digital signatures.

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Terminology

CA Certification Authority; organization that issues a public-key certificate and usually also provides other cryptographic services, such as certificate distribtution and revocation; a CA is a TTP
digital signature digital equivalent of hand-written signature, performed with public-key cryptography (sender signs a message (usually a hash) with her private key, recipient can check with sender's public key)
electronic signature electronic equivalent of hand-written signature, through public-key cryptography (digital signature) or biometrics (e.g., dynamic signature)
public-key certificate certificate issued by a CA which certifies that the public key belongs to the holder mentioned in the certificate
PKI Public-Key Infrastructure; infrastructure with CAs (either hierarchic or web-like)
TTP Trusted Third Party; organization which offers cryptographic services, such as key certification, revocation, and time-stamping; may also denote a Key Escrow Agent or Key Recovery Party




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Digital Signatures

Digital signatures are becoming increasingly important, particularly in electronic commerce applications. Many states are recognizing the need to set guidelines and preconditions for digital signatures, and to adapt the many laws that currently only allow hand-written signatures.

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Certification Authorities

A Certification Authority (CA) issues a public-key certificate to users of public-key cryptography. It may have created the key pair itself, or the user may have created it and given the public key to the CA to certify (the user should in that case provide proof that he holds the corresponding private key). The main issue is that the user convinces the CA of his identity. The certificate may have various formats, but the standard format is X.509.

Apart from certification, a CA will generally also offer a certificate distribution list containing the certificates it issued, as well as a certificate revocation list (CRL) containing the certificates it revoked. A CA may also provide other TTP services, such as time-stamping or key generation.

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Public-Key Infrastructures

A Public-Key Infrastructure (PKI) is basically a coherent structure of CAs. Most PKIs are hierarchic, with a central root-CA (or top-level CA) which certifies lower CAs, which in turn certifiy lower CAs or users; CAs may also occasionally cross-certify. Depending on the application and scale, a root-CA may be an international organization certifiying national top-level CAs, but it may also be the in-company department of a multinational certifying lower department and branch CAs and employees.

In the information society, there will be many PKIs. Interoperability and mutual recognition will be key issues. Cross-certification between root-CAs may ensure interoperability and mutual trust. Still, it is likely that government should set at least some basic general requirements for CAs and PKIs to smooth interoperability and mutual recognition. Also, if digital signatures are to have force of law, there must be clear rules on who can issue public-key certificates, and liability for abuse or errors in certificates must be addressed. If governments do not have a steering role to establish PKIs, at least they must ensure there is a viable legal climate for PKIs to function in.

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Essential reading

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Recommended reading and links

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© Bert-Jaap Koops, 1997. All rights reserved.
Last updated on 24 September 1997.

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