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A tutorial and survey covering both cryptography and network security protocols and technology. Each of the basic topics of cryptography, including conventional and public-key cryptography, authentication, and digital signatures, are covered. Thorough mathematical background is provided for such algorithms as AES and RSA. The book covers important network security tools and applications, including S/MIME, IP Security, Kerberos, SSL/TLS, and X509v3. In addition, methods for countering hackers and viruses are explored. Second edition received the TAA award for the best Computer Science and Engineering Textbook of 1999.
Computer data often travels from one computer to another, leaving the safety of its
protected physical surroundings. Once the data is out of hand, people with bad intention could
modify or forge your data, either for amusement or for their own benefit.
Cryptography can reformat and transform our data, making it safer on its trip between
computers. The technology is based on the essentials of secret codes, augmented by modern
mathematics that protects our data in powerful ways.
• Computer Security - generic name for the collection of tools designed to protect data and to
thwart hackers
• Network Security - measures to protect data during their transmission
• Internet Security - measures to protect data during their transmission over a collection of
interconnected networks
Security Attacks, Services and Mechanisms
To assess the security needs of an organization effectively, the manager responsible for
security needs some systematic way of defining the requirements for security and characterization
of approaches to satisfy those requirements. One approach is to consider three aspects of
information security:
Security attack – Any action that compromises the security of information owned by an
organization.
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 the data processing systems and the
information transfers of an organization. The services are intended to counter security attacks and
they make use of one or more security mechanisms to provide the service.
Basic Concepts
Cryptography The art or science encompassing the principles and methods of transforming an
intelligible message into one that is unintelligible, and then retransforming that message back to its
original form
Plaintext The original intelligible message
Cipher text The transformed message
Cipher An algorithm for transforming an intelligible message into one that is unintelligible by
transposition and/or substitution methods
Key Some critical information used by the cipher, known only to the sender& receiver
Encipher (encode) The process of converting plaintext to cipher text using a cipher and a key
Decipher (decode) the process of converting cipher text back into plaintext using a cipher and a
key
Cryptanalysis The study of principles and methods of transforming an unintelligible message
back into an intelligible message without knowledge of the key. Also called code breaking
Cryptology Both cryptography and cryptanalysis
Code An algorithm for transforming an intelligible message into an unintelligible one using a
code-book
Cryptography
Cryptographic systems are generally classified along 3 independent dimensions:
Type of operations used for transforming plain text to cipher text
All the encryption algorithms are based on two general principles: substitution, in which each
element in the plaintext is mapped into another element, and transposition, in which
elements in the plaintext are rearranged.
The number of keys used
If the sender and receiver uses same key then it is said to be symmetric key (or)
single key (or) conventional encryption.
If the sender and receiver use different keys then it is said to be public key encryption.
The way in which the plain text is processed
A block cipher processes the input and block of elements at a time, producing output block for
each input block.
A stream cipher processes the input elements continuously, producing output element one at a
time, as it goes along.
Cryptanalysis
The process of attempting to discover X or K or both is known as cryptanalysis. The
strategy used by the cryptanalysis depends on the nature of the encryption scheme and the
information available to the cryptanalyst.
There are various types of cryptanalytic attacks based on the amount of
information known to the cryptanalyst.
Cipher text only – A copy of cipher text alone is known to the cryptanalyst.
Known plaintext – The cryptanalyst has a copy of the cipher text and the corresponding
plaintext.
Chosen plaintext – The cryptanalysts gains temporary access to the encryption machine. They
cannot open it to find the key, however; they can encrypt a large number of suitably chosen
plaintexts and try to use the resulting cipher texts to deduce the key.
Chosen cipher text – The cryptanalyst obtains temporary access to the decryption
machine, uses it to decrypt several string of symbols, and tries to use the results to deduce the
key.