Computer and Information Security (PDF)

COMPUTER AND INFORMATION SECURITY SECOND YEAR  1. Information flow control: controls the flow of information between network segments.  2. Access Control Matrix (ACM): The access control matrix exists both in operating systems and in database and it describes allowed access...

COMPUTER AND INFORMATION SECURITY SECOND YEAR  1. Information flow control: controls the flow of information between network segments.  2. Access Control Matrix (ACM): The access control matrix exists both in operating systems and in database and it describes allowed accesses.  3. Firewalls: control traffic between the Internet and the local system or intranet  4. Cryptography: is the process of converting between readable text, called plaintext, and an unreadable form, called ciphertext. The sender converts the plaintext message to ciphertext, and the receiver converts the ciphertext message back to its plaintext form.  5. Digital signature: ensures the authenticity of documents, programs, and messages.  6. Intrusion detection: detects attempted or successful attacks on a system  7. Auditing: bookkeeping on all security- related events in a system 1. Identify: The first step in cyber security strategy is to understand your resources and risks. 2. Protect: Deploy security protection mechanisms. 3. Detect: If an attack occurs, you’ll want mechanisms in place that will alert you as quickly as possible. 4. Respond: If a cyber security breach happens, you’ll want to contain and reduce any damage. 5. Recover: After a cyber security breach, you’ll need mechanisms in place to help resume normal operations.  All the techniques for providing security have two components:  A security-related transformation on the information to be sent making it unreadable by the opponent,.  Some secret information shared by the two principals and unknown to the opponent. A Model for Network Security  This general model shows that there are four basic tasks in designing a particular security service:  1. Design an algorithm for performing the security- related transformation. The algorithm should be such that an opponent cannot defeat its purpose.  2. Generate the secret information to be used with the algorithm.  3. Develop methods for the distribution and sharing of the secret information.  4. Specify a protocol to be used by the two principals that makes use of the security algorithm and the secret information to achieve a particular security service.  Chapter 2 Classical Encryption Techniques  Symmetric encryption, also referred to as conventional encryption or single-key Encryption  A symmetric encryption scheme has five ingredients:  Plaintext: This is the original message that is fed into the algorithm as input.  Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext.  Secret key: The secret key is an input to the encryption algorithm.  The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time.  Ciphertext: This is the scrambled message produced as output. It depends on  the plaintext and the secret key.  Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext.  CRYPTOGRAPHY - study of encryption principles/methods  CRYPTANALYSIS (CODEBREAKING) - the study of principles/ methods of deciphering ciphertext without knowing key  CRYPTOLOGY - the field of both cryptography and cryptanalysis.  There are two requirements for secure use of conventional encryption:  1- a strong encryption algorithm.  2- Sender and receiver must have obtained copies of the secret key in a secure fashion   Cryptographic systems are generally classified along 3 independent dimensions:  Type of operations used for transforming plain text to cipher text  substitution, in which each element in the plaintext is mapped into another element. 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.  the objective of attacking an encryption system is to recover the key in use rather than simply to recover the plaintext of a single ciphertext.  There are two general approaches to attacking a conventional encryption scheme:  Cryptanalysis: Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the plaintext or even some sample plaintext–ciphertext pairs.  This type of attack exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used.  Brute-force attack: The attacker tries every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained.  There are various types of cryptanalytic attacks :  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. 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.  A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers or symbols. If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with cipher text bit patterns.  CAESAR CIPHER The Caesar cipher involves replacing each letter of the alphabet with the letter standing 3 places further down the alphabet. Example 1  plain text: pay more money  Cipher text: SDB PRUH PRQHB  Example 2 plain: meet me after the toga party cipher: PHHW PH DIWHU WKH WRJD SDUWB  Note that the alphabet is wrapped around, so that letter following „z‟ is „a‟.  We can define the transformation by listing all possibilities, as follows:  Let us assign a numerical equivalent to each letter:  For each plaintext letter p, substitute the cipher text letter c such that C = E(p) = (p+3) mod 26  A shift may be any amount, so that general Caesar algorithm is C = E (p) = (p+k) mod 26 Where k takes on a value in the range 1 to 25.  The decryption algorithm is simply P = D(C) = (C-k) mod 26  If it is known that a given ciphertext is a Caesar cipher, then a brute-force cryptanalysis is easily performed: simply try all the 25 possible keys.  Three important characteristics enabled us to use a brute-force cryptanalysis:  1. The encryption and decryption algorithms are known.  2. There are only 25 keys to try.  3. The language of the plaintext is known and easily recognizable.  permutation of a finite set of elements S is an ordered sequence of all the elements of S, with each element appearing exactly once.  For example, if S = {a, b, c}, there are six permutations of S: abc, acb, bac, bca, cab, cba  In general, there are n! permutations of a set of n elements, because the first element can be chosen in one of n ways, the second in n - 1 ways, the third in n – 2 ways, and so on.  Recall the assignment for the Caesar cipher:  If, instead, the “cipher” line can be any permutation of the 26 alphabetic characters, then there are 26! or greater than 4 * 1026 possible keys.  Such an approach is referred to as monoalphabetic substitution cipher, because a single cipher alphabet (mapping from plain alphabet to cipher alphabet) is used per message.  If the cryptanalyst knows the nature of the plaintext (e.g., non compressed English text), then the analyst can exploit the regularities of the language.  The ciphertext to be solved is UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ  As a first step, the relative frequency of the letters can be determined and compared to a standard frequency distribution for English If the message were long enough, this technique alone might be sufficient, but because this is a relatively short message, we cannot expect an exact match. In any case, the relative frequencies of the letters in the ciphertext (in percentages) are as follows:  it seems likely that cipher letters P and Z are the equivalents of plain letters e and t, but it is not certain which is which.  The letters S, U, O, M, and H are all of relatively high frequency and probably correspond to plain letters from the set {a, h, i, n, o, r, s}.  The letters with the lowest frequencies (namely, A, B, G, Y, I, J) are likely included in the set {b, j, k, q, v, x, z}.  The complete plaintext, with spaces added between words, follows:  Monoalphabetic ciphers are easy to break because they reflect the frequency data of the original alphabet.  The best-known multiple-letter encryption cipher is the Playfair, which treats digrams in the plaintext as single units and translates these units into ciphertextDigrams.  The Playfair algorithm is based on the use of a 5x 5 matrix of letters constructed using a keyword.  In this case, the keyword is monarchy.  The matrix is constructed by filling in the letters of the keyword (minus duplicates) from left to right and from top to bottom, and then filling in the remainder of the matrix with the remaining letters in alphabetic order.  The letters I and J count as one letter. Plaintext is encrypted two letters at a time, according to the following rules:  1. Repeating plaintext letters that are in the same pair are separated with a filler letter, such as x, so that balloon would be treated as ba lx lo on.  2. Two plaintext letters that fall in the same row of the matrix are each replaced by the letter to the right, with the first element of the row circularly following the last. For example, ar is encrypted as RM.  3. Two plaintext letters that fall in the same column are each replaced by the letter beneath, with the top element of the column circularly following the last.  For example, mu is encrypted as CM.  4. Otherwise, each plaintext letter in a pair is replaced by the letter that lies in its own row and the column occupied by the other plaintext letter. Thus, hs becomes BP and ea becomes IM (or JM, as the encipherer wishes).  The Playfair cipher is a great advance over simple monoalphabetic ciphers.  For one thing, whereas there are only 26 letters, there are 26 * 26 = 676 digrams, so that identification of individual digrams is more difficult.  Furthermore, the relative frequencies of individual letters exhibit a much greater range than that of digrams, making frequency analysis much more difficult. For these reasons, the Playfair cipher was for a long time considered unbreakable.