Security Attacks, Mechanisms, and Services
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Security Attacks, Mechanisms,
and Services
Attacks, Services and
Mechanisms
◦ Security Attack: Any action thatcompromises the security of information.
◦ Security Mechanism: A mechanism that isdesigned to detect, prevent, or recover from asecurity attack.
◦ Security Service: A service that enhancesthe security of data processing systems andinformation transfers. A security servicemakes use of one or more securitymechanisms.
Passive Attacks
◦ These are in the nature of eavesdropping on, ormonitoring of, transmission
◦ Attempts to learn or make use of informationfrom system
◦ Does not affect system resources
◦ Difficult to detect because they do not involveany alteration of data
◦ Traffic normally sent and received in a normalfashion
◦ Attempt is made to prevent the success of theseattacks, usually by means of encryption
◦ Thus emphasis is on prevention rather thandetection
Passive Attacks(cont….)
Release of message contents is easily
understood
◦ Telephonic conversation, e-mail message, or a
transferred file may contain sensitive
information
◦ Emphasis is to prevent opponent from
learning
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
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
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
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”
Active Attacks(cont….)
Denial of Service
◦ Prevents normal use of communication
facilities
◦ Disabling or overloading of entire network
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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
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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
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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
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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
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