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

March, 2007
However, it is often the case that different algorithms must be employed in order to provide all
the desired services.
Example:
Consider the example system where the secure exchange of information between pairs of
Internet entities is needed. Some of the exchanged information requires just integrity
protection, while other information requires both integrity and confidentiality protection. It is
also a requirement that each entity that participates in an information exchange knows the
identity of the other entity.
The designers of this example system decide that a Public Key Infrastructure (PKI) needs to
be established and that each entity wishing to communicate securely is required to physically
authenticate his or her identity at a Registration Authority (RA). This authentication requires
the presentation of proper credentials such as a driver’s license, passport, or birth certificate.
The authenticated individuals then generate a public static key pair in a smart card that is
used for key agreement. The public static key agreement key of each net member is
transferred from the smart card to the RA where it is incorporated with the user identifier and
other information into a digitally signed message for transmission to a Certificate Authority
(CA). The CA then composes the user’s public key certificate by signing the public key of
the user and the user’s identifier along with other information. This certificate is returned to
the public key owner so that it may be used in conjunction with the private key (under the
sole control of the owner) for entity authentication and key agreement purposes.
In this example, any two entities wishing to communicate may exchange public key
certificates containing public keys that are checked by verifying the CA signature on the
certificate (using the CA public key). The public static key agreement key of each entity and
each entity's own private static key agreement key is then used in a key agreement scheme to
produce a secret value shared between the two entities. The shared secret may then be used to
derive one or more shared symmetric keys. If the mode of the symmetric encryption
algorithm is designed to support all the desired services, then only one shared key is
necessary. Otherwise, multiple shared keys and algorithms are used. One of the shared keys
is used to encrypt for confidentiality, while another key is used for integrity and
authentication. The receiver of the data protected by the key(s) has assurance that the data
came from the other entity indicated by the public key certificate, that the data remains
confidential, and that the integrity of the data is preserved.
Alternatively, if confidentiality is not required, integrity protection, entity authentication, and
non-repudiation can be attained by establishing a signature key pair and corresponding
certificate for each entity. The private signature key of the sender is used to sign the data, and
the sender's public signature verification key is used by the receiver to verify the signature. In
this case a single algorithm provides all three services.
The above example provides a basic sketch of how cryptographic algorithms may be used to
support multiple security services. However, it can be easily seen that the security of such a
system depends on many factors including:
a. The strength of the entity’s credentials (e.g., driver’s license, passport, or birth certificate)
and authentication mechanism,
b. The strength of the cryptographic algorithms used,
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