Computer Networks. Network Security 1. Professor Richard Harris School of Engineering and Advanced Technology

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1 Network Security 1 Professor Richard Harris School of Engineering and Advanced Technology

2 Presentation Outline Overview of Identification and Authentication The importance of identification and Authentication in secure transactions Cryptography introduction Cryptography Protocols Cryptography as a basis for Identification and Authentication The Digital Signature A Secure and Authenticated Communication over an Open Network Computer Networks - 1/2

3 Additional References [1] Kaufman, Perlman and Speciner, Network Security, 2 nd Edition, Prentice Hall, [2] Stallings, Networking Standards: A Guide to OSI, ISDN LAN, and MAN Standards, (Addison-Wesley), 1993 [3] Stallings, Networking and InterNetwork Security, (Prentice Hall), 1995, ISBN [4] FIPS 186, Digital Signature Standard (DSS). [5] FIPS 180, Secure Hash Standard (SHS). [6] ANSI X , American National Standard for Financial Institution Key Management Computer Networks - 1/3

4 Need for Security Computer Networks - 1/4

5 Problems for network security Secrecy: keeping information out of the hands of unauthorized users. Authentication: determining whom you are talking to before revealing sensitive information or entering a business deal. Non-repudiation: dealing with signature, how do you prove that your customer really placed an electronic order. Integrity control: how can you be sure that a message you received was really the one sent and not something that a malicious adversary modified in transit or concocted? Computer Networks - 1/5

6 Where in the protocol network does security belong? Every layer has something to contribute Physical layer, wiretapping can be foiled by enclosing transmission lines in sealed tubes containing gas at high pressure. Any attempt to drill into a tube will release some gas, reducing the pressure and triggering an alarm. Data link layer, packets can be encrypted as they leave one machine and decrypted as they enter another, vulnerable to attacks from within the router. However, link Encryption can be added to any network easily and often is useful. In the network layer, firewalls can be installed to keep good packets and bad packets out. In the transport layer, entire connections can be encrypted, end to end, process to process. Issues such as user authentication and non-repudiation can be only handled in the application layer. Computer Networks - 1/6

7 OSI Security Mechanisms (Controls) Encipherment The use of algorithms to transform data into a form that is not readily intelligible. The transformation and subsequent recovery of the data depend on an algorithm and one or more encryption keys Authentication exchange A mechanism intended to ensure the identity of an entity by means of information exchange Digital Signature Data appended to, or a cryptographic transformation of a data unit that allows the recipient to prove the source and integrity of the data unit and protects against forgery (e.g. by the recipient) Access control A variety of mechanisms that enforce access rights to resources Computer Networks - 1/7

8 Identification and Authentication (Overview) One of the first steps towards securing the resources of a system is the development of the ability to verify the identity of its users. Since all users communicate via messages this comes down to verifying that messages come from the alleged source and have not been altered The process of verifying a user s identity is typically referred to as user identification and authentication Identification and Authentication are distinct steps Computer Networks - 1/8

9 Identification and Authentication (Overview) Identification concerns the manner in which a user provides his/her unique identity to a system The identity: May be (for example), a name or a number (account number) Must be unique so that the system can distinguish between different users, or between different classes of users (remember the Control Selection Criteria of need to know ) May describe one individual, more than one individual.some or all of the time Example System Security Officer is a class identity Computer Networks - 1/9

10 Identification and Authentication (Overview) Authentication is the process of associating an individual with his/her unique identity or that of associating a message with a sending entity An important distinction between Identification and Authentication: Identities can be public (but aren t always) Authentication information (but not necessarily the methodology) is kept secret and becomes the means by which a person proves that they are who they say they are There are three basic means by which an individual may authenticate his/her identity Computer Networks - 1/10 10

11 Identification and Authentication (The Three Basic Approaches) Something the person knows password combination history other. Something the person possesses a token or a card a key to a lock other. Something the person is (Biometrics) Fingerprints retinal pattern voice pattern other. Computer Networks - 1/11 11

12 An Introduction to Cryptography Computer Networks - 1/12 12

13 Cryptography (Introduction) Encryption Is a process designed to conceal meaning by changing intelligible messages to unintelligible messages.. Covers both encypherment and encoding Encypherment The translation of individual letters (or tokens) to other letters of tokens Encoding The translation of words or phrases (or groups of tokens) to other words or phrases Cryptography relies on two basic components An algorithm (also called a cryptographic methodology) A Key (one or more) Computer Networks - 1/13 13

14 Cryptography (Example) Example In a simple system where letters are substituted for other letters The Key? The chart of paired letters The Algorithm? Substitution Computer Networks - 1/14 14

15 Cryptography (The Two Basic Types) There are two basic types of Cryptographic Systems Secret Key (also called symmetric systems) The same key is used to encrypt and decrypt data Two or more parties share the key The key must remain secret Public Key (also called asymmetric systems) Computer Networks - 1/15 15

16 Cryptography (Secret Key and Public Key) Secret Key Encryption - Key must remain secret Key Shared by the parties involved - Reliance on all parties PlainText Encryption Algorithm CypherText Decryption Algorithm PlainText - Data Encryption Standard (DES) Federal Information Processing Standards (FIPS) DES has been widely adopted by the commercial sector in the U.S. - Chips available, so low cost encryption/decryption is available, but access is restricted - Produces output that is dependent on the key - Powerful enough to defy decryption from examination of the cyphertext and/or knowledge of the algorithm - Security is dependent on the secrecy of the key - How do you distribute the key..?? Computer Networks - 1/16 16

17 Cryptography (Secret Key and Public Key) One of the major difficulties with Secret Key systems is the secure distribution of the Key Public Key Systems don t require Key distribution although you still need keys to encrypt and decrypt The Public Key algorithms are asymmetric. That is, you cannot decrypt the message with the same key that you used to encrypt it. This system uses key pairs, one to encrypt and one to decrypt. If you want to receive secure messages then you can make one key public (otherwise known as the Public Key). and so long as the other key is known only to you, then you will be the only person that can read it Needless to say, it is a requirement of this type of system that you cannot derive one key from the other Computer Networks - 1/17 17

18 Cryptography (Public Key example) Public Key Encryption Directory of Public Keys - X places his/her Public Key (Xp) in an accessible place. and keeps the Private Key (Xs) hidden Xp Xp Xs A PlainText Encryption Algorithm CypherText Decryption Algorithm PlainText X - A obtains X s public Key, encrypts a message and sends it to X - X uses his/her Private Key to decrypt the message RSA, named after its three creators, Ronald Rivest, Adi Shamir and Len Adlemen The Digital Signature Standard (DSS) Computer Networks - 1/18 18

19 Cryptography Some uses To ensure confidentiality and integrity of information Public Key is particularly useful when key secrecy is a problem Public Key can be used to distribute secret keys To support controls such as authentication (how do I know you are who you say you are) Other.. Having set a foundation for cryptographic systems we shall take another look at the important (and related) issue of identification and authentication Computer Networks - 1/19 19

20 Internet Communications and Cryptography The rush towards Internet Related Electronic Business Activities Funds transfer associated with sales Authorisations Would you send your Credit Card number over the Internet?? The Problems of - Identification and Authentication The Problems of - Security of Information once the Sender / Receiver have been authenticated Interruption Interception Modification Fabrication Computer Networks - 1/20

21 Cryptography as a Basis for Identification & Authentication Drawbacks of Secret Key Systems Relies on one or more parties sharing the Secret Key In practice this means that communication can only occur between people with some prior relationship. (because they must be entrusted with the Secret Key) The same key that allows for communication allows any of the parties to create forgeries in the name of others Public Key Systems Provide a basis for Authentication In RSA each key of a key pair can undo what the other does If a user can unscramble a message using say, Jack s Public Key, then it must have been created in the first place with Jack s Private Key. This is the basis for Digital Signatures Computer Networks - 1/21

22 Transposition Ciphers Transposition ciphers reorder the letters but don t disguise them. The cipher is keyed by a word or phrase not containing any repeated letters. In this example, MEGABUCK is the key. The purpose in the example is to number the columns, column 1 is under the key letter closest to the start of the alphabet. To break a transposition cipher 1. Be aware it is a transposition cipher by looking at the frequency of E, T, A, etc. 2. Guess the number of columns by first guessing a word or phrase. Say: Million dollars 3. The remaining step is to order the columns. Computer Networks - 1/22

23 P-Box Transposition can be implemented with simple electrical circuits. If the 8 bits are designated from top to bottom as , then the output of this particular P-box is By appropriate internal wiring, a P-box can be made to perform any transposition and do it at practically the speed of light, since no computation is involved; just signal propagation This design follows Kerckhoff s principle: the attacker knows that the general method is permuting the bits. What he doesn t know is which bit goes where, which is the key. Output Input Computer Networks - 1/23

24 Substitution ciphers In substitution cipher each letter or group of letters is replaced by another letter or group of letters to disguise it. For instance a b c d e f g h i j k l m n o p q r s t u v w x y z Q W E R T Y U I O P A S D F G H J K L Z X C V B N M Substitution ciphers preserve the order of the plaintext symbols but disguise them. The substitution ciphers can be broken by starting out with counting the relative frequencies of all letters in the ciphertext. Then one might tentatively assign the most common one to letter e, etc. The general system of symbol-to-symbol substitution is called mono-alphabetic substitution Computer Networks - 1/24

25 S-Box Substitutions are performed by S-box In the example, the 3-bit input selects one of the eight lines existing from the first stage and sets it to 1; all the other lines are 0. The second stage is a P-box. The third stage encodes the selected input line in binary again. With the wiring shown, if the eight octal numbers were input one after another, the output sequence would be In other words, 0 has been replaced by 2 and 1 has been replaced by 4. By appropriate wiring of the P-box inside the S-box, any substitution can be accomplished. Computer Networks - 1/25

26 One-time pads 1. Choose a random bit string as the key. 2. Then convert the plaintext into a bit string, for example by using its ASCII representation. 3. Finally compute the XOR of these two strings, bit by bit. The resulting ciphertext cannot be broken, because in a sufficiently large sample of ciphertext, each letter will occur equally often. The biggest disadvantage is that both sender and receiver must carry the pads which greatly reduces their practical utility. Computer Networks - 1/26

27 One-time pads Message 1, I love you. Message 2, Elvis lives Computer Networks - 1/27

28 Prime Numbers Prime numbers only have divisors of 1 and self they cannot be written as a product of other numbers note: 1 is prime, but is generally not of interest eg. 2,3,5,7 are prime, 4,6,8,9,10 are not Prime numbers are central to number theory List of prime number less than 200 is: Computer Networks - 1/28

29 Prime Factorisation To factor a number n involves writing it as a product of other numbers: n = a b c Note that factoring a number is relatively hard compared to multiplying the factors together to generate the number! The prime factorisation of a number n is when its written as a product of primes eg. 91=7 13 Computer Networks - 1/29

30 Relatively Prime Numbers & GCD Two numbers a, b are relatively prime if they have no common divisors apart from 1 eg. 8 & 15 are relatively prime since factors of 8 are 1,2,4,8 and of 15 are 1,3,5,15 and 1 is the only common factor Conversely can determine the Greatest Common Divisor by comparing their prime factorizations and using least powers eg. 300 = = hence GCD(18,300) = = 6 Computer Networks - 1/30

31 Fermat's Theorem ap-1 mod p = 1 where p is prime and gcd(a,p) = 1 Also known as Fermat s Little Theorem Useful in public key Computer Networks - 1/31

32 Euler Totient Function ø(n) (1) When doing arithmetic modulo n, complete set of residues is: 0 n-1 Reduced set of residues is those numbers (residues) which are relatively prime to n eg for n = 10, complete set of residues is {0,1,2,3,4,5,6,7,8,9} reduced set of residues is {1,3,7,9} Number of elements in a reduced set of residues is called the Euler Totient Function ø(n) Computer Networks - 1/32

33 Euler Totient Function ø(n) (2) To compute ø(n) need to count number of elements to be excluded In general need prime factorization, but eg. for p (p is a prime) ø(p) = p -1 for p.q (p and q are primes) ø(37) = 36 ø(21) = (3 1) (7 1) = 2 6 = 12 ø(p.q) = (p-1)(q-1) Computer Networks - 1/33

34 Generalization of Euler s theorem For numbers n = pq where p and q are primes, akø(n) + 1 = a mod n, for all a < n, as long as k is a non-negative integer. Computer Networks - 1/34

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