1. Security Attacks, Mechanisms, and Services

2. Attacks, Services and Mechanisms
Security Attack: Any action that compromises the security of information.
Security Mechanism: A mechanism that is designed to detect, prevent, or recover from a security attack.
Security Service: A service that enhances the security of data processing systems and information transfers. A security service makes use of one or more security mechanisms.

4. Passive Attacks
These are in the nature of eavesdropping on, or monitoring of, transmission
Attempts to learn or make use of information from system
Does not affect system resources
Difficult to detect because they do not involve any alteration of data
Traffic normally sent and received in a normal fashion
Attempt is made to prevent the success of these attacks, usually by means of encryption
Thus emphasis is on prevention rather than detection

5. Passive Attacks(cont….)
Release of message contents is easily understood
Telephonic conversation, message, or a transferred file may contain sensitive information
Emphasis is to prevent opponent from learning

6. Passive Attacks(cont….)
Traffic Analysis
Observe patterns of messages
Could determine identity and location of communicating hosts
Could observe the frequency and length of messages being exchanged

7. Active Attacks
Involves modification of data, or false creation of data
Attempt to alter system resources or affect their operations
Difficult to prevent active attacks
Emphasis is to detect active attacks and recover from the damage

8. Active Attacks(cont….)
Masquarade
One entity pretendes to be a different entity
E.g. Authentication sequences can be captured
Replay
Involves passive capture of data unit and its subsequent retransmission to produce unauthorized effect

9. Active Attacks(cont….)
Modification of Message
Means some portion of a legitimate message is altered, or messages are reordered or delayed, to produce unauthorized affect
E.g. ” Allow John Smith to read confidential file accounts” is modified to ” Allow Fred Brown to read confidential file accounts”

10. Active Attacks(cont….)
Denial of Service
Prevents normal use of communication facilities
Disabling or overloading of entire network

11. Conventional Encryption Principles
An encryption scheme has five ingredients:
Plaintext
Encryption algorithm
Secret Key
Ciphertext
Decryption algorithm
Security depends on the secrecy of the key, not the secrecy of the algorithm

12. Conventional Encryption Principles

13. Cryptography Classified along three independent dimensions:
The type of operations used for transforming plaintext to ciphertext
The number of keys used
symmetric (single key)
asymmetric (two-keys, or public-key encryption)
The way in which the plaintext is processed

14. Cryptography can be characterized by:
type of encryption operations used
substitution / transposition / product
number of keys used
single-key or private / two-key or public
way in which plaintext is processed
block / stream

15. Average time required for exhaustive key search
Key Size (bits)
Number of Alternative Keys
Time required at 106 Decryption/µs 32
232 = 4.3 x 109
2.15 milliseconds 56
256 = 7.2 x 1016
10 hours
128
2128 = 3.4 x 1038
5.4 x 1018 years
168
2168 = 3.7 x 1050
5.9 x 1030 years