7: Network Security1 21: Network Security Basics Last Modified: 6/28/2015 2:03:02 PM Some slides...
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Transcript of 7: Network Security1 21: Network Security Basics Last Modified: 6/28/2015 2:03:02 PM Some slides...
7: Network Security 1
21: Network Security Basics
Last Modified: 04/18/23 10:04 PM
Some slides based on notes from cs515 at UMass
7: Network Security 2
Importance of Network Security? Think about…
The most private, embarrassing or valuable piece of information you’ve ever stored on a computer
How much you rely on computer systems to be available when you need them
The degree to which you question whether a piece of email really came from the person listed in the From field
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 3
Importance of Network Security Society is becoming increasingly reliant
on the correct and secure functioning of computer systems Medical records, financial transactions, etc.
It is our jobs as professional computer scientists: To evaluate the systems we use to
understand their weaknesses To educate ourselves and others to be wise
network consumers To design networked systems that are
secure
7: Network Security 4
Acceptable Use
In this section of the course, we will discuss the weaknesses of the protocol stack we have just learned
In the homework, you will examine a trace of some security exploits
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.
This is the only acceptable environment in which to experiment with security exploits. Doing so on any production network is unacceptable.
7: Network Security 5
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 6
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 7
Fundamentals of Defense
Cryptography Restricted Access
Restrict physical access, close network ports, isolate from the Internet, firewalls, NAT gateways, switched networks
Monitoring Know what normal is and watch for
deviations Heterogeneity/Randomness
Variety of Implementations, Random sequence numbers, Random port numbers
7: Network Security 8
Fundamentals of Defense
Cryptography: the study of mathematical techniques related to information security that have the following objectives:IntegrityNon-repudiationConfidentialityAuthentication
7: Network Security 9
Objectives of Cryptography
Integrity : ensuring information has not been altered by unauthorized or unknown means Integrity makes it difficult for a third party to
substitute one message for another. It allows the recipient of a message to verify it
has not been modified in transit. Nonrepudiation : preventing the denial of
previous commitments or actions makes it difficult for the originator of a
message to falsely deny later that they were the party that sent the message.
E.g., your signature on a document.
7: Network Security 10
Objectives of Cryptography
Secrecy/Confidentiality : ensuring information is accessible only by authorized persons Traditionally, the primary objective of cryptography. E.g. encrypting a message
Authentication : corroboration of the identity of an entity 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 11
Security Services
Authorization Access Control Availability Anonymity Privacy Certification Revocation
7: Network Security 12
Security Services
Authorization: conveyance of official sanction to do or be something to another entity. 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.
Access Control: restricting access to resources to privileged entities. 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 13
Security Services
Availability: ensuring a system is available to authorized entities when needed ensures that a service or information is
available to an (authorized) user upon demand and without delay.
Denial-of-service attacks seek to interrupt a service or make some information unavailable to legitimate users.
7: Network Security 14
Security Services
Anonymity : concealing the identity of an entity involved in some process 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.
Privacy: concealing personal information, a form of confidentiality.
7: Network Security 15
Security Services
Certification: endorsement of information by a trusted entity.
Revocation: retraction of certification or authorization
Certification and Revocation 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 16
The most widely used tool for securing information and services is cryptography.
Cryptography relies on ciphers: mathematical functions used for encryption and decryption of a message. 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.
Cryptography
Encryption DecryptionPlaintext Ciphertext
OriginalPlaintext
7: Network Security 17
Ciphers
The security of a cipher may rest in the secrecy of its restricted algorithm . 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 :(
Modern cryptography relies on keys, a selected value from a large set (a keyspace), e.g., a 1024-bit number. 21024 values! 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 18
Friends and enemies: Alice, Bob, Trudy
well-known in network security world Bob, Alice want to communicate “securely” Trudy, the “intruder” may intercept, delete, add
messages
Figure 7.1 goes here
7: Network Security 19
The language of cryptography
Figure 7.3 goes here
plaintext plaintext
ciphertext
KA
KB
7: Network Security 20
What makes a good cipher?
substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another
plaintext: abcdefghijklmnopqrstuvwxyz
ciphertext: mnbvcxzasdfghjklpoiuytrewq
Plaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbc
E.g.:
Q: How hard to break this simple cipher?:•brute force (how hard?)•other?
7: Network Security 21
Symmetric vs Assymetric Key
The most common cryptographic tools are Symmetric key ciphers
• DES, 3DES, 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
Asymmetric key ciphers • RSA, El Gamal• two keys• Slow, but versatile (usually requires exponentiation)• Typical key lengths are 512, 1024, 2048
7: Network Security 22
Keys
Symmetric key (private key) algorithms have a separate key for each pair of entities sharing a key.
Public-Key algorithms use a public-key and private-key pair over a message. 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. Often, a symmetric session key is generated
by one of participants and encrypted with the other’s public key. Further communication occurs with the symmetric
key.
7: Network Security 23
Symmetric key crypto: DES
DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64 bit plaintext input
initial permutation 16 identical “rounds” of function application, each using
different 48 bits of key final permutation
How secure is DES? DES Challenge: 56-bit-key-encrypted phrase decrypted
(brute force) in 4 months no known “backdoor” decryption approach
making DES more secure use three keys sequentially (3-DES) on each datum use cipher-block chaining
7: Network Security 24
Public key cryptography
7: Network Security 25
Public key encryption algorithms
need a decryption function dB ( ) and an encrption function eB ( ) such that
d (e (m)) = m BB
. .
need public and private keysfor dB ( ) and eB ( ). .
Two inter-related requirements:
1
2
7: Network Security 26
RSA
Rivest, Shamir, Adelson Want a function eB that is easy to do,
but hard to undo without a special decryption key
Based on the difficulty of factoring large numbers (especially ones that have only large prime factors)
7: Network Security 27
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 28
RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern (message), m, compute
c = m mod n
e (i.e., remainder when m is divided by n)e
2. To decrypt received bit pattern, c, compute
m = c mod n
d (i.e., remainder when c is divided by n)d
m = (m mod n)
e mod n
dMagichappens!
7: Network Security 29
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.
letter m me c = m mod ne
l 12 1524832 17
c m = c mod nd
17 481968572106750915091411825223072000 12
cdletter
l
encrypt:
decrypt:
7: Network Security 30
RSA: Why: m = (m mod n)
e mod n
d
(m mod n)
e mod n = m mod n
d ed
Number theory result: If p,q prime, n = pq, then
x mod n = x mod ny y mod (p-1)(q-1)
= m mod n
ed mod (p-1)(q-1)
= m mod n1
= m
(using number theory result above)
(since we chose ed to be divisible by(p-1)(q-1) with remainder 1 )
7: Network Security 31
Using Cryptography
7: Network Security 32
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 sender encrypts msg receiver decrypts msg
7: Network Security 33
Digital Signatures
Cryptographic technique analogous to hand-written signatures.
Sender (Bob) digitally signs document, establishing he is document owner/creator.
Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document.
Simple digital signature for message m:
Bob encrypts m with his public key dB, creating signed message, dB(m).
Bob sends m and dB(m) to Alice.
7: Network Security 34
Digital Signatures (more)
Suppose Alice receives msg m, and digital signature dB(m)
Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m.
If eB(dB(m) ) = m, whoever signed m must have used Bob’s private key.
Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not
m’.Non-repudiation:
Alice can take m, and signature dB(m) to court and prove that Bob signed m.
7: Network Security 35
Message Digests
Computationally expensive to public-key-encrypt long messages
Goal: fixed-length,easy to compute digital signature, “fingerprint”
apply hash function H to m, get fixed size message digest, H(m).
Hash function properties: Many-to-1 Produces fixed-size msg
digest (fingerprint) Given message digest x,
computationally infeasible to find m such that x = H(m)
computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).
7: Network Security 36
Digital signature = Signed message digestBob sends digitally signed
message:Alice verifies signature and
integrity of digitally signed message:
7: Network Security 37
Hash Function Algorithms
Internet checksum would make a poor message digest. Too easy to find
two messages with same checksum.
MD5 hash function widely used. Computes 128-bit
message digest in 4-step process.
arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.
SHA-1 is also used. US standard 160-bit message digest
7: Network Security 38
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
Failure scenario??
7: Network Security 39
Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.
Failure scenario?
7: Network Security 40
Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.
Failure scenario?
I am Aliceencrypt(password)
7: Network Security 41
ap4.0: Authentication: yet another try
Goal: avoid playback attack
Failures, drawbacks?
Figure 7.11 goes here
Nonce: number (R) used onlyonce in a lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice
must return R, encrypted with shared secret key
7: Network Security 42
Trusted Intermediaries
Problem: How do two
entities establish shared secret key over network?
Solution: trusted key
distribution center (KDC) acting as intermediary between entities
Problem: When Alice obtains
Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s?
Solution: trusted certification
authority (CA)
7: Network Security 43
Key Distribution Center (KDC)
Alice,Bob need shared symmetric key.
KDC: server shares different secret key with each registered user.
Alice, Bob know own symmetric keys, KA-
KDC KB-KDC , for communicating with KDC.
Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1)
Alice sends Bob KB-KDC(A,R1), Bob extracts R1
Alice, Bob now share the symmetric key R1.
7: Network Security 44
Figure 7.12 goes here
Authentication: ap5.0
ap4.0 requires shared symmetric key problem: how do Bob, Alice agree on key can we authenticate using public key
techniques?
ap5.0: use nonce, public key cryptography
7: Network Security 45
Figure 7.14 goes here
ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)
Need “certified” public keys
7: Network Security 46
Certification Authorities
Certification authority (CA) binds public key to particular entity.
Entity (person, router, etc.) can register its public key with CA. Entity provides “proof
of identity” to CA. CA creates certificate
binding entity to public key.
Certificate digitally signed by CA.
Public key of CA can be universally known (on billboard, embedded in software)
When Alice wants Bob’s public key:
gets Bob’s certificate (Bob or elsewhere).
Apply CA’s public key to Bob’s certificate, get Bob’s public key
7: Network Security 47
Administrators 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.• 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 $16,000,000 artwork, you might consider a security guard. (can you trust the guard?)
2. 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 48
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 49
Physical Security
Are you sure someone can just walk into your building and 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?
Can anyone sit down outside your building and get on your DHCP-enable 802.11 network?
7: Network Security 50
Social Engineering
Using tricks and lies that take advantage of people’s trust to gain access to an otherwise guarded system. 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 FedEx or police uniform, etc.
Bribery/Threats
7: Network Security 51
The Security Process
Security is an on-going process between these three steps.
Moreover, most security research can be categorized within these three topics.
Prevention
Detection
Response
Prevention: firewalls and filtering, secure shell, anonymous protocols
Detection: intrusion detection, IP traceback Response: dynamic firewall rule sets, employee
education (post-its are bad)
7: Network Security 52
More 3-faceted views of Security Security of an organization consists of
Computer and Network Security• Everything that we will learn about in this class• Firewalls, IDS, virus protection, ssh, passwords, etc.
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 Physical security
• Protected by alarm systems, cameras, and mean dogs.• Are you sure someone can’t just steal the hard drive?