21 Network Security Basics Last Modified 3 19 2018 11

21: Network Security Basics Last Modified: 3/19/2018 11: 40: 55 AM Some slides based on notes from cs 515 at UMass 7: Network Security 1

Importance of Network Security? r Think about… m The most private, embarrassing or valuable piece of information you’ve ever stored on a computer m How much you rely on computer systems to be available when you need them m The degree to which you question whether a piece of email really came from the person listed in the From field m How convenient it is to be able to access private information online (e. g. buy without entering all data, look up your transcript without requesting a copy, …) 7: Network Security 2

Importance of Network Security r Society is becoming increasingly reliant on the correct and secure functioning of computer systems m Medical records, financial transactions, etc. r It is our jobs as professional computer scientists: m To evaluate the systems we use to understand their weaknesses m To educate ourselves and others to be wise network consumers m To design networked systems that are secure 7: Network Security 3

Acceptable Use r In this section of the course, we will discuss the weaknesses of the protocol stack we have just learned r In the homework, you will examine a trace of some security exploits r This trace was taken in network that was completely disconnected from the Internet. We had root privileges on all machines. The experiments were conducted with the full knowledge and consent of all participants. r This is the only acceptable environment in which to experiment with security exploits. Doing so on any production network is unacceptable. 7: Network Security 4

Taxonomy of Attacks (1) § Process based model to classify methods of attack § Passive: § Interception: attacks confidentiality. a. k. a. , eavesdropping, “man-in-the-middle” attacks. § Traffic Analysis: attacks confidentiality, or anonymity. Can include traceback on a network, CRT radiation. § Active: § Interruption: attacks availability. (a. k. a. , denial-of-service attacks § Modification: attacks integrity. § Fabrication: attacks authenticity. 7: Network Security 5

Taxonomy of Attacks (2) § ‘Result of the attack’ taxonomy § Increased Access the quest for root § Disclosure of Information credit card numbers § Corruption of Information changing grades, etc § Denial of Service self explanatory § Theft of Resources stealing accounts, bandwidth 7: Network Security 6

Fundamentals of Defense r Cryptography r Restricted Access m Restrict physical access, close network ports, isolate from the Internet, firewalls, NAT gateways, switched networks r Monitoring m Know what normal is and watch for deviations r Heterogeneity/Randomness m Variety of Implementations, Random sequence numbers, Random port numbers 7: Network Security 7

Fundamentals of Defense r Cryptography: the study of mathematical techniques related to information security that have the following objectives: m Integrity m Non-repudiation m Confidentiality m Authentication 7: Network Security 8

Objectives of Cryptography r Integrity : ensuring information has not been altered by unauthorized or unknown means m Integrity makes it difficult for a third party to substitute one message for another. m It allows the recipient of a message to verify it has not been modified in transit. r Nonrepudiation : preventing the denial of previous commitments or actions m makes it difficult for the originator of a message to falsely deny later that they were the party that sent the message. m E. g. , your signature on a document. 7: Network Security 9

Objectives of Cryptography r Secrecy/Confidentiality : ensuring information is accessible only by authorized persons m m Traditionally, the primary objective of cryptography. E. g. encrypting a message r Authentication : corroboration of the identity of an entity m m m allows receivers of a message to identify its origin makes it difficult for third parties to masquerade as someone else e. g. , your driver’s license and photo authenticates your image to a name, address, and birth date. 7: Network Security 10

Security Services r Authorization r Access Control r Availability r Anonymity r Privacy r Certification r Revocation 7: Network Security 11

Security Services r Authorization: conveyance of official sanction to do or be something to another entity. m m Allows only entities that have been authenticated and who appear on an access list to utilize a service. E. g. , your date of birth on your driver’s license authorizes you to drink as someone who is over 21. r Access Control: restricting access to resources to privileged entities. m m ensures that specific entities may perform specific operations on a secure object. E. g. Unix access control for files (read, write, execute for owner, group, world) 7: Network Security 12

Security Services r Availability: ensuring a system is available to authorized entities when needed m ensures that a service or information is available to an (authorized) user upon demand without delay. m Denial-of-service attacks seek to interrupt a service or make some information unavailable to legitimate users. 7: Network Security 13

Security Services r Anonymity : concealing the identity of an entity involved in some process m Concealing the originator of a message within a set of possible entities. • The degree of anonymity of an entity is the sum chance that everyone else in the set is the originator of the message. • Anonymity is a technical means to privacy. r Privacy: concealing personal information, a form of confidentiality. 7: Network Security 14

Security Services r Certification: endorsement of information by a trusted entity. r Revocation: retraction of certification or authorization r Certification and Revocation m Just as important as certifying an entity, we need to be able to take those rights away, in case the system is compromised, we change policy, or the safety that comes from a “refresh”. 7: Network Security 15

Cryptography r The most widely used tool for securing information and services is cryptography. r Cryptography relies on ciphers: mathematical functions used for encryption and decryption of a message. m m m Encryption: the process of disguising a message in such a way as to hide its substance. Ciphertext: an encrypted message Decryption: the process of returning an encrypted message back into plaintext. Plaintext Encryption Ciphertext Decryption Original Plaintext 7: Network Security 16

Ciphers r The security of a cipher may rest in the secrecy of its restricted algorithm. m m m Whenever a user leaves a group, the algorithm must change. Can’t be scrutinized by people smarter than you. But, secrecy is a popular approach : ( r Modern cryptography relies on keys, a selected value from a large set (a keyspace), e. g. , a 1024 -bit number. 21024 values! m m Security is based on secrecy of the key, not the details of the algorithm. Change of authorized participants requires only a change in key. 7: Network Security 17

Friends and enemies: Alice, Bob, Trudy Figure 7. 1 goes here r well-known in network security world r Bob, Alice want to communicate “securely” r Trudy, the “intruder” may intercept, delete, add messages 7: Network Security 18

The language of cryptography plaintext K K B A plaintext ciphertext Figure 7. 3 goes here 7: Network Security 19

What makes a good cipher? substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq E. g. : Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard to break this simple cipher? : • brute force (how hard? ) • other? 7: Network Security 20

Symmetric vs Assymetric Key r The most common cryptographic tools are m Symmetric key ciphers • • DES, 3 DES, AES, Blowfish, Twofish, IDEA Fast and simple (based on addition, masks, and shifts) One key shared and kept secret Typical key lengths are 40, 128, 256, 512 m Asymmetric • • key ciphers RSA, El Gamal two keys Slow, but versatile (usually requires exponentiation) Typical key lengths are 512, 1024, 2048 7: Network Security 21

Keys r Symmetric key (private key) algorithms have a separate key for each pair of entities sharing a key. r Public-Key algorithms use a public-key and privatekey pair over a message. m m Only the public-key can decrypt a message encrypted with the private key. Similarly, only the private key can decrypt a message decrypted with the public key. r Often, a symmetric session key is generated by one of participants and encrypted with the other’s public key. m Further communication occurs with the symmetric key. 7: Network Security 22

Symmetric key crypto: DES: Data Encryption Standard r US encryption standard [NIST 1993] r 56 -bit symmetric key, 64 bit plaintext input m m m initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation r How secure is DES? m DES Challenge: 56 -bit-key-encrypted phrase decrypted (brute force) in 4 months m no known “backdoor” decryption approach r making DES more secure m use three keys sequentially (3 -DES) on each datum m use cipher-block chaining 7: Network Security 23

Public key cryptography 7: Network Security 24

Public key encryption algorithms Two inter-related requirements: 1 . . need a decryption function d. B ( ) and an encrption function e. B ( ) such that d (e (m)) = m 2 B B need public and private keys for d. B ( ) and e. B ( ) . . 7: Network Security 25

RSA r Rivest, Shamir, Adelson r Want a function e. B that is easy to do, but hard to undo without a special decryption key r Based on the difficulty of factoring large numbers (especially ones that have only large prime factors) 7: Network Security 26

RSA: Choosing keys 1. Choose two large prime numbers p, q. (e. g. , 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”). 4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5. Public key is (n, e). Private key is (n, d). Why? (Will hint at) How? (Won’t discuss) 7: Network Security 27

RSA: Encryption, decryption 0. Given (n, e) and (n, d) as computed above 1. To encrypt bit pattern (message), m, compute e mod n (i. e. , remainder when m e is divided by n) c=m 2. To decrypt received bit pattern, c, compute d m = c d mod n (i. e. , remainder when c is divided by n) Magic d m = (m e mod n) mod n happens! 7: Network Security 28

RSA example: Bob chooses p=5, q=7. Then n=35, z=24. e=5 (so e, z relatively prime). d=29 (so ed-1 exactly divisible by z. encrypt: decrypt: letter m me l 12 1524832 c 17 d c 48196857210675091411825223072000 c = me mod n 17 m = cd mod n letter 12 l 7: Network Security 29

RSA: Why: m = (m e mod n) d mod n Number theory result: If p, q prime, n = pq, then y y mod (p-1)(q-1) x mod n = x mod n e (m mod n) d mod n = med mod n = m ed mod (p-1)(q-1) mod n (using number theory result above) 1 = m mod n (since we chose ed to be divisible by (p-1)(q-1) with remainder 1 ) = m 7: Network Security 30

Using Cryptography 7: Network Security 31

Using Cryptography for: Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection Authentication: sender, receiver want to confirm identity of each other Secrecy: only sender, intended receiver should “understand” msg contents m sender encrypts msg m receiver decrypts msg 7: Network Security 32

Digital Signatures Cryptographic technique analogous to handwritten signatures. Simple digital signature for message m: r Sender (Bob) digitally signs public key d. B, creating signed message, d. B(m). r Bob sends m and d. B(m) to Alice. document, establishing he is document owner/creator. r Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document. r Bob encrypts m with his 7: Network Security 33

Digital Signatures (more) r Suppose Alice receives Alice thus verifies that: msg m, and digital m Bob signed m. signature d. B(m) m No one else signed m. r Alice verifies m signed m Bob signed m and not m’. by Bob by applying Non-repudiation: Bob’s public key e. B to m Alice can take m, and d. B(m) then checks signature d. B(m) to court e. B(d. B(m) ) = m. and prove that Bob r If e. B(d. B(m) ) = m, signed m. whoever signed m must have used Bob’s private key. 7: Network Security 34

Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easy to compute digital signature, “fingerprint” r apply hash function H to m, get fixed size message digest, H(m). Hash function properties: r Many-to-1 r Produces fixed-size msg digest (fingerprint) r Given message digest x, computationally infeasible to find m such that x = H(m) r computationally infeasible to find any two messages m and m’ such that H(m) = H(m’). 7: Network Security 35

Digital signature = Signed message digest Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: 7: Network Security 36

Hash Function Algorithms r Internet checksum would make a poor message digest. m Too easy to find two messages with same checksum. r MD 5 hash function widely used. m Computes 128 -bit message digest in 4 -step process. m arbitrary 128 -bit string x, appears difficult to construct msg m whose MD 5 hash is equal to x. r SHA-1 is also used. m US standard m 160 -bit message digest 7: Network Security 37

Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap 1. 0: Alice says “I am Alice” Failure scenario? ? 7: Network Security 38

Authentication: another try Protocol ap 3. 0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario? 7: Network Security 39

Authentication: yet another try Protocol ap 3. 1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. I am Alice encrypt(password) Failure scenario? 7: Network Security 40

ap 4. 0: Authentication: yet another try Goal: avoid playback attack Nonce: number (R) used onlyonce in a lifetime ap 4. 0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key Figure 7. 11 goes here Failures, drawbacks? 7: Network Security 41

Trusted Intermediaries Problem: m How do two entities m When Alice obtains establish shared Bob’s public key secret key over (from web site, enetwork? mail, diskette), how does she know it is Solution: Bob’s public key, not m trusted key Trudy’s? distribution center Solution: (KDC) acting as intermediary m trusted certification between entities authority (CA) 7: Network Security 42

Key Distribution Center (KDC) r Alice, Bob need shared symmetric key. r KDC: server shares different secret key with each registered user. r Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC. r Alice communicates with KDC, gets session key R 1, and KBKDC(A, R 1) r Alice sends Bob KB-KDC(A, R 1), Bob extracts R 1 r Alice, Bob now share the symmetric key R 1. 7: Network Security 43

Authentication: ap 5. 0 ap 4. 0 requires shared symmetric key m problem: how do Bob, Alice agree on key m can we authenticate using public key techniques? ap 5. 0: use nonce, public key cryptography Figure 7. 12 goes here 7: Network Security 44

ap 5. 0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Figure 7. 14 goes here Need “certified” public keys 7: Network Security 45

Certification Authorities r Certification authority (CA) binds public key to particular entity. r Entity (person, router, etc. ) can register its public key with CA. m Entity provides “proof of identity” to CA. m CA creates certificate binding entity to public key. m Certificate digitally signed by CA. m Public key of CA can be universally known (on billboard, embedded in software) r When Alice wants Bob’s public key: r gets Bob’s certificate (Bob or elsewhere). r Apply CA’s public key to Bob’s certificate, get Bob’s public key 7: Network Security 46

Administrators r Persons managing the security of a valued resource consider five steps: 1. Risk assessment: the value of a resource should determine how much effort (or money) is spent protecting it. • • 2. E. g. , If you have nothing in your house of value do you need to lock your doors other than to protect the house itself? If you have an $16, 000 artwork, you might consider a security guard. (can you trust the guard? ) Policy: define the responsibilities of the organization, the employees and management. It should also fix responsibility for implementation, enforcement, audit and review. 7: Network Security 47

Administrators 3. Prevention: taking measures that prevent damage. • E. g. , firewalls or one-time passwords (e. g. , s/key) 4. Detection: measures that allow detection of when an asset has been damaged, altered, or copied. • E. g. , intrusion detection, trip wire, network forensics 5. Recovery/Response: restoring systems that were compromised; patch holes. 7: Network Security 48

Physical Security r Are you sure someone can just walk into your building and m m m Steal floppies or CD-ROMs that are lying around? Bring in a laptop and plug into your dhcp-enable ethernet jacks? Reboot your computer into single user mode? (using a bios password? ) Reboot your computer with a live CD-ROM and mount the drives? Sit down at an unlocked screen? r Can anyone sit down outside your building and get on your DHCP-enable 802. 11 network? 7: Network Security 49

Social Engineering r Using tricks and lies that take advantage of people’s trust to gain access to an otherwise guarded system. m m m Social Engineering by Phone: “Hi this is your visa credit card company. We have a charge for $3500 that we would like to verify. But, to be sure it’s you, please tell me your social security number, pin, mother’s maiden name, etc” Dumpster Diving: collecting company info by searching through trash. Online: “hi this is Alice from my other email account on yahoo. I believe someone broke into my account, can you please change the password to “Sucker”? Persuasion: Showing up in a Fed. Ex or police uniform, etc. Bribery/Threats 7: Network Security 50

The Security Process Detection r Security is an on-going process between these three steps. r Moreover, most security research can be categorized within these three topics. Prevention Response r Prevention: firewalls and filtering, secure shell, anonymous protocols r Detection: intrusion detection, IP traceback r Response: dynamic firewall rule sets, employee education (post-its are 7: Network Security bad) 51

More 3 -faceted views of Security r Security of an organization consists of m Computer and Network Security • Everything that we will learn about in this class • Firewalls, IDS, virus protection, ssh, passwords, etc. m Process security • Protected by good policy! • No one should be able to get an account by phone: a form should be filled out, an email/phone call sent to a manager, and then the password picked up in person. Don’t send notifications after accounts are set up! • http: //www. nstissc. gov/html/library. html m Physical security • Protected by alarm systems, cameras, and mean dogs. • Are you sure someone can’t just steal the hard drive? 7: Network Security 52