Part 1: General - Computer Security Resource Center - National ...

8.1.5.1.1 Distribution of Static Public Keys
March, 2007
Static public keys are relatively long lived and are typically used for a number of executions of
an algorithm. The distribution of the public key should provide assurance to the receiver of that
key that the true owner of the key is known (i.e., the owner of the key pair); this requirement
may be disregarded if anonymity is acceptable. However, the strength of the overall architecture
and trust in the validity of the protected data depends, in large part, on the assurance of the public
key owner’s identity.
In addition, the distribution of the public key shall provide assurance to the receiver that:
1. the purpose/usage of the key is known (e.g., RSA digital signatures or elliptic curve key
agreement),
2. any parameters associated with the public key are known (e.g., domain parameters),
3. the public key is valid (e.g., the public key satisfies the known arithmetical properties),
and
4. the owner actually possesses the corresponding private key.
8.1.5.1.1.1 Distribution of a Trust Anchor's Public Key in a PKI
The public key of a certificate authority is the foundation for all PKI-based security services. The
trust anchor is not a secret, but the authenticity of the trust anchor is the crucial assumption for
PKI. Trust anchors may be obtained through many different mechanisms, providing different
levels of assurance. The types of mechanisms that are provided may depend on the role of the
user in the infrastructure. A user that is only a “relying party” – that is, a user that does not have
keys registered with the infrastructure – may use different mechanisms than a user that possesses
keys registered by the infrastructure. Trust anchors are frequently distributed as "self-signed"
X.509 certificates, that is, certificates that are signed by the subject public key of the certificate.
Trust anchors are often embedded within an application and distributed with the application. For
example, the installation of a new web browser typically includes the installation or replacement
of the user’s trust anchor list. Operating systems often are shipped with "code signing" trust
anchor public keys. The user relies upon the authenticity of the software distribution mechanism
to ensure that only valid trust anchors are installed during installation or replacement. However,
in some cases other applications may install trust anchor keys in web browsers.
Trust anchors in web browsers are used for several purposes, including the validation of
S/MIME e-mail certificates and web server certificates for "secure websites" that use the
SSL/TLS protocol to authenticate the web server and provide confidentiality. Users who visit
"secure" websites that have a certificate not issued by a trust anchor CA may be given an
opportunity to accept that certificate, either for a single session, or permanently. Relying users
should be cautious about accepting certificates from unknown certification authorities so that
they do not, in effect, inadvertently add new permanent trust anchors.
Roaming users should be aware that they must trust all software on the host systems that they
use. They may have particular concerns about trust anchors used by web browsers when they use
systems in kiosks, libraries, Internet cafes, or hotels and systems provided by conference
organizers to access "secure websites." The user has no control over the trust anchors installed in
the host system, and therefore must rely upon the host systems to make sound choices of trust
94