BT0088 - Cryptography and Network Security
Question 1 - Define attack and explain the types of Threats. The Int Intern ernet et con contin tinue ues s to grow grow expon exponent ential ially ly.. Person Personal al,, gov govern ernmen ment, t, and bus busine iness ss applicat appl ications ions continue continue to mult multiply iply on the Internet, Internet, with imme immediat diate e ben benefits efits to end users. Howeve However, r, the these se netwo network rkbas based ed app applilicat cation ions s and servic services es can pos pose e securi security ty risks risks to individuals and to the information resources of companies and governments. Information is an asset asset tha thatt mus mustt be protec protected ted.. !itho !ithout ut ade"u ade"uate ate#$ #$ HH$ HH$ #H$$## #H$$##$H% $H%$ $ net netwo work rk security security,, many individuals individuals,, business businesses, es, and gove governme rnments nts risk losing that asset asset is called called attack. Types of Threats Interception& This type of threat occurs when an unauthori'ed party (outsider) has gained access. The outside party can be a person, a program, or a computing system. *xamples of this type of failure are illicit copying of program or data files, or wiretapping to obtain data in a network. +lt*H$ +lt*H$ #H*hough a loss may be discovered fairly "uickly, "uickly, a silent interceptor may leave no traces bH*#$y which the interception can be readily detected. detected. !hen an unauthori'ed unauthori'ed party modifies or corrupts corrupts the asset, the threat is a modification. or example, someone might change the values in a database, alter a program so that it performs an addi*$H$Htional computation. It is even possible to modify hardware. -nly some cases are detected easily using simple measures, but others are almost impossible to detect.
Interruption& This occurs when an asset of the system becomes lost, unavailable, or unusable. +n +n example is the malicious destruction of a hardware device, erasure of a program or data file, or malfunction of an operating system file manager so that it cannot find a particular disk file. The useful means of classifying security attacks is in terms of passive attacks and active attacks. + passive attack attempts to learn or make use of information from the system but does not affect the system resources. +n active attack attempts attempts to alter system resources resources or affect affect their operation. operation.
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Question 2 - !hat is security attack *xplain with examples. !hen you test any computer system, one of your /obs is to imagine how the system could malfunction. Then, you improve the system0s design so that the system can withstand any of the problems you have identified. In the same way, we analy'e a system from a security perspective, thinking about the ways in which the system0s security can malfunction and diminish the value of its assets. +ny action that compromises the security of information owned by an organi'ation is called security attack. Those who execute such actions, or cause them to be executed, are called attackers or opponents. 1omputerbased system has three interrelated and valuable components namely, hardware, software, and data. *ach of these assets offers value to different members of the community affected by the system. To analy'e security, we can brainstorm about the ways in which the system or its information can experience some kind of loss or harm. or example, we can identify data whose format or contents should be protected in some way. !e want our security system to make sure that no data is disclosed to an unauthori'ed parties. 2either do we want the data being modified in illegitimate ways nor do we want the illegitimate users to access the data. 3y this we identify weaknesses of a system. i.e. + process whereby a person compromises your computer by installing harmful malicious software in your computer without your knowledge. These malicious software includes viruses, spywares, adwares, and tro/an horses. These software often deletes certain vital files on your computer, making your computer to function abnormally, spying on your online surfing habits, and cause advertisements to pop up on your screen when you are online.
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Question 3 - *xplain different characteristics that identify a good encryption techni"ue. #everal characteristics that identify a good *ncryption techni"ue.
The implementation of the process should be as simple as possible. Principle % was formulated with hand implementation in mind& + complicated algorithm is prone to error or likely to be forgotten. !ith the development and popularity of digital computers, algorithms far too complex for hand implementation became feasible. #till, the issue of complexity is important. People will avoid an encryption algorithm whose implementation process severely hinders message transmission, thereby undermining security. +nd a complex algorithm is more likely to be programmed incorrectly.
The enciphering algorithm and set of keys used should be less complex. This principle implies that we should restrict neither the choice of keys nor the types of plaintext on which the algorithm can work. or instance, an algorithm that works only on plaintext having an e"ual number of +s and *s is useless. #imilarly, it would be difficult to select keys such that the sum of the values of the letters of the key is a prime number. $estrictions such as these make the use of the encipherment prohibitively complex. If the process is too complex, it will not be used. urthermore, the key must be transmitted, stored, and remembered, so it must be short.
The amount of secrecy needed should determine the amount of labor appropriate for the encryption and decryption. Principle 4 is a reiteration of the principle of timeliness and of the earlier observation that even a simple cipher may be strong enough to deter the casual interceptor or to hold off any interceptor for a short time.
*rrors in ciphering should not propagate and cause corruption of further information in the message. Principle 5 acknowledges that humans make errors in their use of enciphering algorithms. -ne error early in the process should not throw off the entire remaining ciphertext.
The si'e of the original message and that of enciphered text should be at most same. The idea behind principle 6 is that a ciphertext that expands dramatically si'e cannot possibly carry more information than the plaintext, yet it gives the cryptanalyst more data from which to infer a pattern. urthermore, a longer ciphertext implies more space for storage and more time to communicate.
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Question 4 - 1ompare #ymmetric and +symmetric *ncryption #ystems. 3ased on 7ey !e have two types of encryptions based on keys they are symmetric (also called 8secret key8) and asymmetric (also called 8public key8). #ymmetric algorithms use one key, which works for both encryption and decryption. 9sually, the decryption algorithm is closely related to the encryption one. (or example, the 1aesar cipher with a shift of % uses the encryption algorithm 8substitute the character three letters later in the alphabet8 with the decryption 8substitute the character three letters earlier in the alphabet.8) The symmetric system means both encryption and the decryption are performed using the same key. They provide a two way channel to their users& + and 3 share a secret key, and they can both encrypt information to send to the other as well as decrypt information from the other. +s long as the key remains secret, the system also provides authentication, proof that a message received was not fabricated by someone other than the declared sender. +uthenticity is ensured because only the legitimate sender can produce a message that will decrypt properly with the shared key. The symmetry of this situation is a ma/or advantage of this type of encryption, but it also leads to a problem& key distribution. How do + and 3 obtain their shared secret key +nd only + and 3 can use that key for their encrypted communications. If + wants to share encrypted communication with another user 1, + and 1 need a different shared key. 7ey distribution is the ma/or difficulty in using symmetric encryption. In general, n users who want to communicate in pairs need n : (n ; 4)<= keys. In other words, the number of keys needed increases at a rate proportional to the s"uare of the number of users> #o a property of symmetric encryption systems is that they re"uire a means of key distribution. 3ased on 3lock 3lock based encryption system is classified as stream and block encryption system. #tream encryption algorithm convert one symbol of plaintext immediately into a symbol of ciphertext. (The exception is the columnar transposition cipher.) The transformation depends only on the symbol, the key, and the control information of the encipherment algorithm. #ome kinds of errors, such as skipping a character in the key during encryption, affect the encryption of all future characters. However, such errors can sometimes be recogni'ed during decryption because the plaintext will be properly recovered up to a point, and then all following characters will be wrong. If that is the case, the receiver may be able to recover from the error by dropping a character of the key on the receiving end. -nce the receiver has successfully recalibrated the key with the ciphertext, there will be no further effects from this error. In the columnar transposition, the entire message is translated as one block. The block si'e need not have any particular relationship to the si'e of a character. 3lock ciphers work on blocks of plaintext and produce blocks of ciphertext, as shown in figure %.=. In this figure, the central box represents an encryption machine& The previous plaintext pair is converted to po, the current one being converted is IH, and the machine is soon to convert *#.
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Question - ?ive the -verview of D*# +lgorithm. The Data *ncryption algorithm is a combination of both substitution as well as transposition techni"ue. The strength of D*# techni"ue is improved when it uses both the techni"ues together. It uses both the techni"ue repeatedly i.e., one on the top of other for a total of 4@ cycles. The sheer complexity of tracing a single bit through 4@ iterations of substitutions and transpositions has so far stopped researchers in the public from identifying more than a handful of general properties of the algorithm. The algorithm begins by encrypting the plaintext as blocks of @5 bits. The key is @5 bits long, but in fact it can be any 6@bit number. (The extra A bits are often used as check digits and do not affect encryption in normal implementations.) The user can change the key at will any time there is uncertainty about the security of the old key.
D*# uses only standard arithmetic and logical operations on numbers up to @5 bits long, so it is suitable for implementation in software on most current computers. +lthough complex, the algorithm is repetitive, making it suitable for implementation on a singlepurpose chip.
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Question ! - *xplain $#+ techni"ue with an example. $#+ is an exponentiation cipher. Bou have to follow the following two steps. 4. 1hoose two large prime numbers p and ", and let n C p". The totient (n) of n is the number of numbers less than n with no factors in common with n. *xample& Eet n C 4F. The numbers that are less than 4F and are relatively prime to (have no factors in common with) n are 4, %, G, and . Hence, (4F) C 5. #imilarly, if n C =4, the numbers that are relatively prime to n are 4, =, 5, 6, A, 4F, 44, 4%, 4@, 4G, 4, and =F. #o (=4) C 4=. =. 1hoose an integer e n that is relatively prime to (n). ind a second integer d such that ed mod (n) C 4. The public key is (e, n), and the private key is d. Eet m be a message. Then& c C me mod n and m C cd mod n. *xample& Eet p C G and " C 44. Then n C GG and (n) C @F. +lice chooses e C 4G, so her private key is d C 6%. In this cryptosystem, each plaintext character is represented by a number between FF (+) and =6 (J)K =@ represents a blank. 3ob wants to send +lice the message 8H*EE- !-$ED.8 9sing the representation above, the plaintext is FG F5 44 44 45 =@ == 45 4G 44 F%. 9sing +lice0s public key, the ciphertext is FG4G mod GG C =A F54G mod GG C 4@ 444G mod GG C 55 ... F%4G mod GG C G6 or =A 4@ 55 55 5= %A == 5= 4 55 G6. In addition to confidentiality, $#+ can provide data and origin authentication. If +lice enciphers her message using her private key, anyone can read it, but if anyone alters it, the (altered) ciphertext cannot be deciphered correctly. *xample& #uppose +lice wishes to send 3ob the message 8H*EE- !-$ED8 in such a way that 3ob will be sure that +lice sent it. #he enciphers the message with her private key and sends it to 3ob. +s indicated above, the plaintext is represented as FG F5 44 44 45 =@ == 45 4G 44 F%. 9sing +lice0s private key, the ciphertext is FG6% mod GG C %6 F56% mod GG C F 446% mod GG C 55 ... F%6% mod GG C F6 or %6 F 55 55 % 4= =5 5 F5 F6.
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