CLASSICAL ENCRYPTION
TECHNIQUES
INFORMATION SECURITY
BS_CS 6TH 2
MULTI-LEVEL SECURITY (MLS)
There are security classifications or security levels
Users/principals/subjects have security clearances
Objects have security classifications
Example of security levels
Top Secret
Secret
Confidential
Unclassified
In this case Top Secret > Secret > Confidential > Unclassified
Security goal (confidentiality): ensures that information do not flow to those
not cleared for that level
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MULTI-LEVEL SECURITY (MLS)
The capability of a computer system to carry information with different
sensitivities (i.e. classified information at different security levels), permit
simultaneous access by users with different security clearances and needs-to-
know, and prevent users from obtaining access to information for which they
lack authorization.
Discretionary access control fails to achieve MLS
Typically use Mandatory Access Control
Primary Security Goal: Confidentiality
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MANDATORY ACCESS CONTROL(MAC)
Mandatory access controls (MAC) restrict the access of subjects to
objects based on a system-wide policy
Denying users full control over the access to resources that they create. The
system security policy (as set by the administrator) entirely determines the
access rights granted
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WHAT IS A SECURITY MODEL?
A model describes the system
e.g., a high level specification or an abstract machine description of what the system does
A security policy
defines the security requirements for a given system
Verification techniques that can be used to show that a policy is satisfied by a system
System Model + Security Policy = Security Model
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BELL-LAPADULA MODEL(BLP): A MAC MODEL FOR ACHIEVING
MULTI-LEVEL SECURITY
Introduce in 1973
Air-Force was concerned with security in time-sharing systems
Many OS bugs
Accidental misuse
Main Objective:
Enable one to formally show that a computer system can securely process classified
information
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THE BLP SECURITY MODEL
A computer system is modeled as a state-transition system
There is a set of subjects; some are designated as trusted.
Each state has objects, an access matrix, and the current access
information.
There are state transition rules describing how a system can go from
one state to another
Each subject s has a maximal sec level Lm(s), and a current sec level Lc(s)
Each object has a classification level
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ELEMENTS OF THE BLP MODEL
Lm: Max Lc: Current L: Class.
Sec. Level Sec. Level Level
Subjects Objects
Current
Accesses
Trusted
Subjects
Access Matrix 9
Security levels, e.g.: {TS, S, C, U}
CLASSICAL ENCRYPTION TECHNIQUES
As opposed to modern cryptography
Goals:
to introduce basic concepts & terminology of encryption
to prepare us for studying modern cryptography
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BASIC TERMINOLOGY
Plaintext: original message to be encrypted
Ciphertext: the encrypted message
Enciphering or encryption: the process of converting plaintext into ciphertext
Encryption algorithm: performs encryption
Two inputs: a plaintext and a secret key
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Deciphering or decryption: recovering plaintext from ciphertext
Decryption algorithm: performs decryption
Two inputs: ciphertext and secret key
Secret key: same key used for encryption and decryption
Also referred to as a symmetric key
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Cipher or cryptographic system : a scheme for encryption and decryption
Cryptography: science of studying ciphers
Cryptanalysis: science of studying attacks against cryptographic systems
Cryptology: cryptography + cryptanalysis
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Cryptography
Basics
Cryptography is the science of secret, or hidden writing
It has two main Components:
1. Encryption
– Practice of hiding messages so that they can not be read by anyone other than the intended
recipient
2. Authentication & Integrity
– Ensuring that users of data/resources are the persons they claim to be and that a message has
not been surreptitiously altered
CIPHERS
Symmetric cipher: same key used for encryption and
decryption
Block cipher: encrypts a block of plaintext at a time
(typically 64 or 128 bits)
Stream cipher: encrypts data one bit or one byte at a time
Asymmetric cipher: different keys used for
encryption and decryption
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Encryption
Symmetric Algorithms
Algorithms in which the key for encryption and decryption are the same are
Symmetric
Example: Caesar Cipher
Types:
1. Block Ciphers
– Encrypt data one block at a time (typically 64 bits, or 128 bits)
– Used for a single message
2. Stream Ciphers
– Encrypt data one bit or one byte at a time
– Used if data is a constant stream of information
SYMMETRIC ENCRYPTION
or conventional / secret-key / single-key
sender and recipient share a common key
all classical encryption algorithms are symmetric
The only type of ciphers prior to the invention of asymmetric-key ciphers in
1970’s
by far most widely used
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Encryption
Cipher
Plain Text Encryption Cipher Text Decryption Plain Text
Algorithm Algorithm
Cipher is a method for encrypting messages
Key A Key B
Encryption algorithms are standardized & published
The key which is an input to the algorithm is secret
Key is a string of numbers or characters
If same key is used for encryption & decryption the algorithm is called symmetric
If different keys are used for encryption & decryption the algorithm is called asymmetric
Symmetric Encryption
Key Strength
Strength of algorithm is determined by the size of the key
The longer the key the more difficult it is to crack
Key length is expressed in bits
Typical key sizes vary between 48 bits and 448 bits
Set of possible keys for a cipher is called key space
For 40-bit key there are 240 possible keys
For 128-bit key there are 2128 possible keys
Each additional bit added to the key length doubles the security
To crack the key the hacker has to use brute-force
(i.e. try all the possible keys till a key that works is found)
Super Computer can crack a 56-bit key in 24 hours
It will take 272 times longer to crack a 128-bit key
(Longer than the age of the universe)
SYMMETRIC CIPHER MODEL
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SYMMETRIC ENCRYPTION
Mathematically:
Y = EK(X) or Y = E(K, X)
X = DK(Y) or X = D(K, Y)
X = plaintext
Y = ciphertext
K = secret key
E = encryption algorithm
D = decryption algorithm
Both E and D are known to public
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