CSE 127: Lecture 2
The topics covered in this lecture are
next time: distributed systems: finish up on confidentiality
by covering briefly DES/AES as standard symmetric block ciphers and
covering in more detail RSA as a standard asymmetric (public key) scheme;
cover integrity and non-repudiation.
Availability is the notion that resources should remain
usable by the authorized users. This is often a difficult security
property to provide, especially in distributed systems -- distributed
denial of service (DDOS) attacks are difficult to prevent.
Confidentiality refers to the notion that application/user
data that is confidential can be protected from unauthorized access.
In some systems, there is an explicit Mandatory Access Control (MAC)
policy where data is marked with a sensitivity label, and users and
their processes are marked with an access clearance. In other systems
such as Unix or Windows, the access control is called Discretionary
Access Control (DAC) -- it is up to the owner of the file to control
access, via either the RWX access bits or an Access Control List
(ACL). This can be thought of as ``read'' access to a file.
In distributed systems, confidentiality of messages is typically
provided by encrypting the communications.
Integrity refers to the notion that data cannot be modified
without appropriate authorization. The same MAC/DAC ideas apply. The
type of access correspond to ``write'' access. Note that the ability to
read the data is not required to (over)write it.
In distributed systems, integrity of messages is typically provided by
sending the message with a cryptographic checksum.
Traceability and non-repudiation refer to the idea
that users can be held accountable for their actions. This means that
``significant'' operations are logged by the system, and that users
must authenticate themselves to use the system, so their identities
are known. This means that there has to be a login process, which
attaches a user-ID to every process that run as a result of the login.
What gets logged depends on what ``signficant'' means, and the
system programmers must take care to identify all security relevent
events and include code to generate log entries.
In distributed systems, digital signatures are associated with request
messages; the digital signatures cannot be falsified (unless the user
releases his/her private key), so by logging the digital signatures an
audit trail can be created. Note that great care must be taken to
ensure that the signed message contain enough context information or a
``nonce'' value which is recorded to prevent re-use: otherwise an
attacker might be able to take a digitally signed request from an
earlier exchange and reuse it inappropriately to implicate a
legitimate user. This sort of attack is known as a replay
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them is optional unless otherwise stated.
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