Cryptography Reference
In-Depth Information
tem might also require nonrepudiation. As another example, if we want to secure
software updates for a cell phone, the chief objectives might be integrity and mes-
sage authentication because the manufacturer primarily wants to assure that only
original updates are loaded into the handheld device. We note that message authen-
tication always implies data integrity; the opposite is not true.
The four security services can be achieved in a more or less straightforward man-
ner with the schemes introduced in this topic: For confidentiality one uses primarily
symmetric ciphers and less frequently asymmetric encryption. Integrity and mes-
sage authentication are provided by digital signatures and message authentication
codes which, are introduced in Chap. 12. Nonrepudiation can be achieved with dig-
ital signatures as discussed above.
In addition to the four core security services there are several other ones:
5.
Identification/entity authentication:
Establish and verify the identity of an en-
tity, e.g., a person, a computer or a credit card.
6.
Access control:
Restrict access to the resources to privileged entities.
7.
Availability:
Assures that the electronic system is reliably available.
8.
Auditing:
Provide evidence about security-relevant activities, e.g., by keeping
logs about certain events.
9.
Physical security:
Provide protection against physical tampering and/or re-
sponses to physical tampering attempts.
10.
Anonymity:
Provide protection against discovery and misuse of identity.
Which security services are desired in a given system is heavily application-
specific. For instance, anonymity might make no sense for an e-mail system since
e-mails are supposed to have a clearly identifiable sender. On the other hand, car-
to-car communication systems for collision avoidance (one of the many exciting
new applications for cryptography we will see in the next ten years or so) have a
strong need to keep cars and drivers anonymous in order to avoid tracking. As a fur-
ther example, in order to secure an operating system, access control to certain parts
of a computer system is often of paramount importance. Most but not all of these
advanced services can be achieved with the crypto algorithms from this topic. How-
ever, in some cases noncryptographic approaches need to be taken. For instance,
availability is often achieved by using redundancy, e.g., running redundant comput-
ing or storage systems in parallel. Such solutions are only indirectly, if at all, related
to cryptography.
10.2 The RSA Signature Scheme
The RSA signature scheme is based on RSA encryption introduced in Chap. 7. Its
security relies on the difficulty of factoring a product of two large primes (the integer
factorization problem). Since its first description in 1978 in [143], the RSA signature
scheme has emerged as the most widely used digital signatures scheme in practice.
