Download module4.1

SECURITY
Security Threats, Policies, and
Mechanisms
• There are four types of security threats to
consider
• 1. Interception
• 2 Interruption
• 3. Modification
• 4. Fabrication
• Interception - an unauthorized party has
gained access to a service or data
• Interruption - attempts to make a service
inaccessible to other parties
• Modification - unauthorized changing of data
or tampering with a service
• Fabrication - additional data or activity are
generated that would normally not exist.
security mechanisms
•
•
•
•
•
Important security mechanisms are:
1. Encryption
2. Authentication
3. Authorization
4. Auditing
• Encryption - transforms data into something an
attacker cannot understand
• Authentication - used to verify the claimed
identity of a user, client, server, host, or other
entity
• Authorization - Permission may be granted to
read records, to modify certain fields in a record,
or to add or remove a record
• Auditing tools - used to trace which clients
accessed what, and which way.
SECURE CHANNELS
• Authentication
Authentication Based on a Shared
Secret Key
• Bob subsequently sends a challenge RB to
Alice, shown as message 2.
• Such a challenge could take the form of a
random number.
• Alice is required to encrypt the challenge with
the secret key KA,B that she shares with Bob,
and return the encrypted challenge to Bob.
• This response is shown as message 3 in
Fig. 9-12 containing KA,B(RB)·
The protocol
proceeds as
• First, Alice sends her identity to Bob (message 1), indicating that
she wants to set up a communication channel between the two.
• Bob subsequently sends a challenge RB to Alice, shown as message
2.
• Alice is required to encrypt the challenge with the secret key KA,B
that she shares with Bob, and return the encrypted challenge to
Bob.
• This response is shown as message 3 in Fig. 9-12 containing
KA,B(RB)
• When Bob receives the response KA,B(RB) to his challenge RB, he
can decrypt the message using the shared key again to see if it
contains RB·
• If so, he then knows that Alice is on the other side
Authentication Using a Key
Distribution Center
Message Integrity and Confidentiality
Digital Signatures
Secure Group Communication
• Confidential Group Communication
•
First, consider the problem of protecting communication
between a group of N users against eavesdropping.
• To ensure confidentiality, a simple scheme is to let all group members
share the same secret key, which is used to encrypt and decrypt all
messages transmitted between group members.
• Because the secret key in this scheme is shared by all members, it is
necessary that all members are trusted to indeed keep the key a secret.
• An alternative solution is to use a separate shared secret key between
each pair of group members.
• As soon as one member turns out to be leaking information, the others
can simply stop sending messages to that member, but still use the keys
they were using to communicate with each other .
• However, instead of having to maintain one key, it is now necessary to
maintain N(N - 1)/2 keys, which may be a difficult problem by itself.
• Using a public-key cryptosystem can improve
matters.
• In that case, each member has its own (public
key, private key) pair, in which the public key
can be used by all members for sending
confidential messages.
• In this case, a total of N key pairs are needed.
If one member ceases to be trustworthy, it is
simply removed
Secure Replicated Servers
Example: Kerberos
ACCESS CONTROL
General Issues in Access Control
Access Control Matrix
Protection Domains
Firewalls