EEC 688/788 Secure and Dependable Computing Lecture 3 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University

2 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Outline Threats in computer networks –Threats –Method of defense Three security principles Introduction to cryptography –Terminology –Basic encryption methods

3 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Threats in Networks Networks are specialized collections of hardware, software, and data –Each network node is itself a computing system –It experiences all normal security problems A network must also confront communication problems that involve the interaction of system components and outside resources

4 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Threats in Networks The challenges to achieve network security are rooted in –A network's lack of physical proximity –Use of insecure, shared media, and –The inability of a network to identify remote users positively

5 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao What Makes a Network Vulnerable Anonymity. An attacker can mount an attack from thousands of miles away and never come into direct contact with the system, its administrators, or users Many points of attack—both targets and origins. An attack can come from any host to any host, so that a large network offers many points of vulnerability

6 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao What Makes a Network Vulnerable Sharing. Because networks enable resource and workload sharing, more users have the potential to access networked systems than on single computers Complexity of system. A network combines two or more possibly dissimilar operating systems Unknown network boundary. A network's expandability also implies uncertainty about the network boundary

7 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao What Makes a Network Vulnerable Unknown network boundary

8 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao What Makes a Network Vulnerable Unknown path in message routing. There may be many paths from one host to another. Some intermediate node might not be trustworthy

9 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Methods of Defense Harm occurs when a threat is realized against a vulnerability To protect against harm, we can neutralize the threat, close the vulnerability, or both The possibility for harm to occur is called risk

10 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Methods of Defense We can deal with harm in several ways. We can seek to –Prevent it, by blocking the attack or closing the vulnerability –Deter it, by making the attack harder, but not impossible –Deflect it, by making another target more attractive (or this one less so) –Detect it, either as it happens or some time after the fact –Recover from its effects

11 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Methods of Defense – Multiple Controls

12 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Countermeasures / Controls Encryption –Scrambling process Software controls Hardware controls –hardware or smart card implementations of encryption Policies and Procedures –Example: change password periodically Physical Controls –Example: Locks on doors, guards at entry points

13 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Software Controls Internal program controls: parts of the program that enforce security restrictions, such as access limitations Operating system and network system controls: limitations enforced by the operating system or network to protect each user from all other users Independent control programs: application programs, such as password checkers, intrusion detection utilities, or virus scanners, that protect against certain types of vulnerabilities Development controls: quality standards under which a program is designed, coded, tested, and maintained, to prevent software faults from becoming exploitable vulnerabilities

14 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Principle of Easiest Penetration An intruder may use any available means of penetration –The penetration may not necessarily be by the most obvious means –Nor is it the one against which the most solid defense has been installed

15 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Principle of Easiest Penetration This principle implies that –Computer security specialists must consider all possible means of penetration –The penetration analysis must be done repeatedly, and especially whenever the system and its security changes –Strengthening one aspect of a system may simply make another means of penetration more appealing to intruders

16 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Principle of Adequate Protection Principle of Adequate Protection: –Computer items must be protected only until they lose their value –They must be protected to a degree consistent with their value

17 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Principle of Effectiveness Principle of Effectiveness: Controls must be used — and used properly — to be effective. They must be efficient, easy to use, and appropriate

18 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptography Terminology Encryption is the process of encoding a message so that its meaning is not obvious –Equivalent terms: encode, encipher Decryption is the reverse process, transforming an encrypted message back into its normal, original form –Equivalent terms: decode, decipher Plaintext: message to be encrypted Ciphertext: encrypted message

19 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptography Terminology The cryptosystem involves a set of rules for how to encrypt the plaintext and how to decrypt the ciphertext Why encryption? –It addresses the need for confidentiality of data, also helps to ensure integrity –It forms the basis of protocols that enable us to provide security while accomplishing system or network tasks

20 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptography Terminology The encryption and decryption rules are called encryption and decryption algorithms Encryption/decryptions algorithms often use a device called a key, denoted by K, so that the resulting ciphertext depends on the original plaintext message, the algorithm, and the key value An encryption scheme that does not require the use of a key is called a keyless cipher

21 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Symmetric Encryption The encryption and decryption keys are the same, so P = D(K, E(K,P)) D and E are closely related. They are mirror- image processes The symmetric systems provide a two-way channel to their users The symmetry of this situation is a major advantage of this type of encryption, but it also leads to a problem: key distribution

22 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Symmetric Encryption D K (E K (P)) = P

23 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Asymmetric Encryption Encryption and decryption keys come in pairs. The decryption key, K D, inverts the encryption of key K E, so that P = D(K D, E(K E,P)) Asymmetric encryption systems excel at key management

24 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptology Cryptology is the research into and study of encryption and decryption; it includes both cryptography and cryptanalysis Cryptography – art of devising ciphers –Comes from Greek words for “secret writing”. It refers to the practice of using encryption to conceal text Cryptanalysis – art of breaking ciphers –Study of encryption and encrypted messages, hoping to find the hidden meanings

25 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptanalysis Attempt to break a single message Attempt to recognize patterns in encrypted messages, to be able to break subsequent ones Attempt to deduce the key, in order to break subsequent messages easily Attempt to find weaknesses in the implementation or environment of use of encryption Attempt to find general weaknesses in an encryption algorithm

26 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptanalysis Traffic analysis: attempt to infer some meaning without even breaking the encryption, e.g., –Noticing an unusual frequency of communication –Determining something by whether the communication was short or long

27 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Basic Encryption Methods Substitution ciphers: one letter is exchanged for another Transposition ciphers: order of letters is rearranged

28 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Substitution Ciphers Idea: each letter or group of letters is replaced by another letter or group of letters Caesar cipher – circularly shift by 3 letters –a -> D, b -> E, … z -> C –More generally, shift by k letters, k is the key Monoalphabetic cipher – map each letter to some other letter –A b c d e f … w x y z –Q W E R T Y … V B N M <= the key

29 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Cryptanalysis of Substitution Ciphers Brute force cryptanalysis would have to try 26! permutations of a particular ciphertext message Smarter way: use frequencies of letters, pairs of letter etc., or by guessing a probable word or phrase. Most frequently occurred –Letters: e, t, o, a, n, … –Digrams: th, in, er, re, an, … –Trigrams: the, ing, and, ion, ent –Words: the, of, and, to, a, in, that, … When messages are long enough, the frequency distribution analysis quickly betrays many of the letters of the plaintext

30 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Transposition Ciphers Substitution cipher – preserves order of plaintext symbols but disguises them Transposition cipher – reorders (rearrange) symbols but does not disguise them. It is also called permutation With transposition, the cryptography aims for –Widely spreading the information from the message or the key across the ciphertext –Transpositions try to break established patterns

31 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Columnar Transposition Plaintext written in rows, number of columns = key length Key is used to number the columns Ciphertext read out by columns, starting with column whose key letter is lowest

32 6/24/2015EEC688: Secure & Dependable ComputingWenbing Zhao Columnar Transposition A transposition cipher example