From Wikipedia, the free encyclopedia
In
cryptography, a certificate authority
or certification authority (CA) is
an entity which issues digital
certificates for use by other parties. It is
an example of a
trusted third party. CAs are characteristic
of many
public key infrastructure (PKI) schemes.
There are many
commercial CAs that charge for their services.
Institutions and governments may have their own
CAs, and there are free CAs.
Issuing a certificate
A CA will
issue a
public key certificate which states that the
CA attests that the
public key contained in the certificate
belongs to the person, organization, server, or
other entity noted in the certificate. A CA's
obligation in such schemes is to verify an
applicant's credentials, so that users (relying
parties) can trust the information in the
CA's certificates. The usual idea is that if the
user trusts the CA and can verify the CA's
signature, then they can also verify that a
certain public key does indeed belong to
whomever is identified in the certificate.
If the CA can
be subverted, then the security of the entire
system is lost. Suppose an attacker, Mallory (to
use the
Alice and Bob convention), manages to get a
certificate authority to issue a false
certificate tying Alice to the wrong public key,
which corresponding private key is known to
Mallory. If Bob subsequently obtains and uses
Alice's public key in this (bogus) certificate,
the security of his communications to her could
be compromised by Mallory — for example, his
messages could be decrypted, or he could be
tricked into accepting forged signatures.
Security
The problem of
assuring correctness of match between data and
entity when the data are presented to the CA
(perhaps over an electronic network), and when
the credentials of the person/company/program
asking for a certificate is likewise presented,
is difficult, which is why commercial CAs often
use a combination of authentication techniques
including leveraging government bureaus, the
payment infrastructure, third parties' databases
and services, and custom heuristics. In some
enterprise systems, local forms of
authentication such as
Kerberos can be used to obtain a certificate
which can in turn be used by external relying
parties. Notaries are required in some cases to
personally know the party whose signature is
being notarized; this is a higher standard than
can be reached for many CAs. According to the
American Bar Association outline on
Online Transaction Management the primary
points of federal and state statutes that have
been enacted regarding
digital signatures in the
United States has been to "prevent
conflicting and overly burdensome local
regulation and to establish that electronic
writings satisfy the traditional requirements
associated with paper documents." Further the
E-Sign and UETA code help ensure that:
(1)
a signature, contract or other record
relating to such transaction may not be
denied legal effect, validity, or
enforceability solely because it is in
electronic form; and
(2) a contract relating to such transaction
may not be denied legal effect, validity or
enforceability solely because an electronic
signature or electronic record was used in
its formation.
In large-scale
deployments, Alice may not be familiar with
Bob's certificate authority (perhaps they each
have a different CA), so Bob's certificate may
also include his CA's public key signed by a
different CA2, which is presumably
recognizable by Alice. This process typically
leads to a hierarchy or mesh of CAs and CA
certificates.
Largest providers
A 2005
Netcraft survey determined that
VeriSign and its acquisitions such as
Thawte have a 53% share of the certificate
authority market, followed by GeoTrust (25%),
Comodo (12%),
Go Daddy (4%) and
Entrust (2%).[1]
(GeoTrust has since been acquired by VeriSign.
