1. Dimitris Palyvos Giannas Utilizing Kurose-Ross Slides, 7th Global Ed.
Network Security
Dimitris Palyvos Giannas
Utilizing Kurose-Ross Slides, 7th Global Ed.

2. Learning Objectives Explain the requirements of secure communication
Classify different encryption techniques
Compare and contrast message integrity strategies
Give a high level explanation of real-world security protocols
Explain the principles of operational security mechanisms

3. A tale of two lovers Alice Bob data, control messages channel secure
sender
secure
receiver
well-known paradigm in network security world
Bob, Alice (lovers!) want to communicate “securely”
Trudy (intruder) may intercept, delete, add messages
Trudy

4. Network Security Principles
Confidentiality
Integrity
Authentication
Access & Availability
confidentiality: only sender, intended receiver should “understand” message contents
authentication: sender, receiver want to confirm identity of each other
message integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection
access and availability: services must be accessible and available to users

5. Learning Objectives Explain the requirements for secure communication
Classify various encryption techniques
Compare and contrast message integrity strategies
Give a high level explanation of real-world security protocols
Explain the principles of operational security mechanisms

6. Symmetric Key Cryptography
plaintext
ciphertext K
encryption
algorithm
decryption
common key
Alice
Bob
Trudy
plaintext, ciphertext definitions
Sender & Receiver share the same key
How are the keys exchanged?

7. Block Ciphers Break up the message into bit blocks of constant size N
Use a substitution table for all values e.g. 100 ➜ 101, 001 ➜ 111, …
For N = 64, we need more than 2,305,843 terabytes
Instead, use a function that simulates random permuted tables
Small permutation tables
total size 256KB

8. (encrypts and decrypts)
Block Cipher ECB Mode
Block cipher
Ciphertext
block
Symmetric Key
(encrypts and decrypts)
Plaintext
101
111 IN
OUT
000
110
001
111
010
100
011
101
111
Can you think of possible problems with this approach?
011
001
001

9. Information Leakage Block Effect
Encrypting the same plaintext gives us the exact same ciphertext

10. Randomization to the rescue! CBC Mode
Plaintext
block
101
111
111 IN
OUT
000
110
001
111
010
100
011
101
010
010
001 f
IV – init. vector
for first block
110
101
111
Block cipher
Cipher Block Chaining
Identical blocks are now encrypted differently
The initial vector is sent as plaintext
Each ciphertext block is used to randomize the encryption of the next block
Ciphertext
block
001
010
011

11. Symmetric key ciphers DES (Data Encryption Standard)
Designed by IBM 1975, Adopted by NIST* 1977
Criticized for short key length (56 bits) and mysterious “S-boxes”
Not considered secure anymore!
3-DES (repeating DES three times with different keys)
3-DES probably secure today but too computational intensive
AES (Advanced Encryption Standard)
Replaces DES as of 2001
Result of an official competition
Longer Key lengths: 128, 192 or 256 bits
Brute force decryption: if DES takes 1 second, AES-128 takes 149 trillion years, AES-256 would take 1052 years
explain brute force

12. Symmetric Key Cryptography
Encryption algorithms are public
Sender & Receiver share a common secret key
Messages are split into blocks of constant length (e.g. 64, 128, 256, …)
Many block ciphers & modes of operation
Brute force attacks – trying all possible combinations
Key Length
# Combinations 1 2 10
1024 20 64
128
(3 billion billion billion billion …)
Key size matters!

13. Public Key Cryptography+
Bob’s public key K B -
Bob’s private key K B
encryption
algorithm
ciphertext
decryption
algorithm
m = K (K (m)) B + - m
plaintext
message
K (m) B +
plaintext
message

14. Public Key Cryptography
One key is public – the other kept secret
One key is used to encrypt, the other to decrypt
Based on mathematically hard problems
RSA Cipher (Rivest, Shamir, Adleman)
Factorization of very large prime numbers
Slow because of the large numbers involved
Key Length 1024 bits and up in RSA
21024 = which means > 300 digit numbers
In practice, used to exchange symmetric keys!
Can also be used for digital signatures!

15. Cryptographic Hash Function
Unique summary of a large message
Small change in message results in completely different hash
Deterministic
Fast to compute
Irreversible
Infeasible to find collisions
Does not encrypt data!
MD5 (compromised), SHA-1 (compromised*), SHA-2, SHA-3
Torrent example
* Since 2005 SHA-1 has not been considered secure against well-funded opponents

16. Cryptographic Hash Function
SHA-1

17. Performance Comparison
Hash functions
SHA-1
MD5
Symmetric
ciphers
200-1,000 Mbyte/s
AES
DES
3-DES
performance
100 Mbyte/s
Public-Key ciphers
RSA
0.1 Mbyte/s

18. bit.ly/2EKDpVi (case sensitive!)
Your turn
bit.ly/2EKDpVi (case sensitive!)

19. Learning Objectives Explain the requirements for secure communication
Classify different encryption techniques
Compare and contrast message integrity strategies
Give a high level explanation of popular security protocols
Explain the principles of operational security mechanisms

20. Woman in the middle attack!
Bob
Alice
Trudy
altered
ciphertext
ciphertext
“I love you”
“I hate you”
Was the message altered by Trudy?
Message Integrity
Was the message was sent by Alice?
Digital Signatures + Authentication
Contents can be changed even if it is encrypted!

21. MAC Message Authentication Code
Sender
Receiver
Message S
HASH
MAC
Compare
Sender and receiver need a shared secret
Authenticates sender + Verifies Message Integrity
No encryption!

22. Digital Signatures No need for shared secret
Only you have the private key
Image:

23. Digital Signatures
Cryptographic technique analogous to hand-written signatures
No shared secret required! Based on public key ciphers
Sender (Alice) digitally signs document
This way, she establishes she is document owner/creator
Verifiable, nonforgeable: recipient (Bob) can prove to someone that Alice, and no one else, must have signed document
Everybody can prove that Alice signed the message, as long as they have her public key!

24. Public Key Cryptography & Digital Signatures
Everybody can encrypt
Only one can decrypt
Digital Signatures
Only one can encrypt (sign)
Everybody can decrypt (verify)

25. Digital Certificates The attacker can alter the public key!
Another layer of authentication: Public Key Certificates
A Certification Authority (CA) sings the name + public key
Everybody can use the CA’s public key to verify other public keys
How do we verify the CA’s public key?
Show certificate in browser

26. End-point Authentication
Bob wants Alice to “prove” her identity to him
“I am Alice”
“I am Alice”

27. End-point authentication
Alice could send her IP address and a secret password to Bob…
Alice’s
IP addr
encrypted
password
“I’m Alice” OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
encrypted
password
Playback attack!

28. End-point authentication
Use a unique number (nonce) R and a shared secret!
“I am Alice” R
K (R)
A-B
Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!

29. Learning Objectives Explain the requirements for secure communication
Classify different encryption techniques
Compare and contrast message integrity strategies
Give a high level explanation of real-world security protocols
Explain the principles of operational security mechanisms

30. Real World
Source: xkcd

31. Transport Layer Security SSL & TLS
SSL/TLS provides secure communication over TCP
Guarantees Confidentiality, Integrity & Authentication
Widely deployed (billions $/year over SSL)
Provides an API to TCP applications
Application
TCP IP
normal application
Application
SSL
TCP IP
application with SSL
widely deployed security protocol
supported by almost all browsers, web servers
https
billions $/year over SSL
variation -TLS: transport layer security, RFC 2246
provides
confidentiality
integrity
authentication

32. SSL Handshake
How do we make sure that the server’s public key was not altered?
Mention certificate authorities, digital certificates.

33. SSL Handshake Client sends (supported algorithms, client nonce)
Server chooses algorithms from list; sends back: (algorithms choice, certificate, server nonce)
Client verifies certificate, extracts server’s public key, generates PMS, encrypts with server’s public key, sends to server (encrypted PMS)
Client and Server independently compute encryption and MAC keys from PMS and nonces
Client sends a MAC of all the handshake messages
Server sends a MAC of all the handshake messages

34. The Devil is in the Details
The two final steps protect handshake from tampering (e.g. weak algorithm suggestions)
Why two random nonces?
Suppose Trudy sniffs all messages between Alice & Bob
Next day, Trudy sets up TCP connection with Bob, sends exact same sequence of records (without being able to decrypt it)
Bob (Amazon) thinks Alice made two separate orders for the same thing
solution: Bob sends different random nonce for each connection. This causes encryption keys to be different on the two days
Trudy’s messages will fail Bob’s integrity check

35. bit.ly/2CG3IGk (case sensitive!)
Your turn
bit.ly/2CG3IGk (case sensitive!)

36. Learning Objectives Explain the requirements for secure communication
Classify different encryption techniques
Compare and contrast message integrity strategies
Give a high level explanation of real-world security protocols
Explain the principles of operational security mechanisms

37. Packet Filtering untrusted “bad guys” Firewall trusted “good guys”
administered
network
public
Internet
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others
trusted “good guys”
untrusted “bad guys”
Firewall

38. Stateless Packet Filters
Should arriving packet be allowed in?
Departing packet let out?
Internal network connected to Internet via router firewall
Router Firewall, filters packet-by-packet. Decision to forward/drop based on:
IP addresses (Src & Dest)
Port numbers (Src & Dest)
ICMP message type
TCP SYN and ACK bits
Security
8-41

39. Stateless Packet Filters
Policy
Firewall Setting
No outside Web access.
Drop all outgoing packets to any IP address, port 80
No incoming TCP connections, except those for institution’s public Web server only.
Drop all incoming TCP SYN packets to any IP except , port 80
Prevent Web-radios from eating up the available bandwidth.
Drop all incoming UDP packets - except DNS and router broadcasts.
Prevent your network from being used for a smurf DoS attack.
Drop all ICMP packets going to a “broadcast” address (e.g ).
Prevent your network from being tracerouted
Drop all outgoing ICMP TTL expired traffic

40. Stateful packet filters
stateless packet filter: heavy handed tool
admits packets that make no sense e.g., dest port = 80, ACK bit set, even if no TCP connection established
stateful packet filter: track status of every TCP connection
track connection setup (SYN), teardown (FIN): determine whether incoming, outgoing packets “make sense”
timeout inactive connections at firewall, no longer admit packets

41. Firewalls Operate on TCP/IP headers only
Prevent denial of service attacks
e.g. SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections
Prevent illegal modification/access of internal data
e.g., attacker replaces CIA’s homepage with something else
allow only authorized access to inside network
set of authenticated users/hosts
Stateful/stateless

42. Application Gateways
application
gateway
host-to-gateway
telnet session
Firewall
gateway-to-remote
host telnet session
Filter packets on application data as well as on header data
Example: allow select internal users to telnet outside
Require all telnet users to telnet through gateway
For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections
Router filter blocks all telnet connections not originating from gateway

43. Challenges for Packet Filters/Gateways
IP spoofing
Firewall can’t know if data really comes from claimed source
Filters often use all or nothing policy for UDP
Filtering is a tradeoff
Degree of communication ↔︎ Level of security
Each app requires its own gateway → resource requirements
Client software must know how to contact gateway
e.g., must set IP address of proxy in Web browser
Not a panacea!
Many highly protected sites still suffer from attacks

44. IDS Intrusion Detection Systems
Packet Filtering
Operates on TCP/IP headers only
No correlation check among sessions
IDS
deep packet inspection – look at packet contents (e.g., check strings in packet against database of known virus, attack strings)
Examine correlation among multiple packets
Port scanning
Network mapping
DoS attack
Signature/Anomaly based

45. IDS Intrusion Detection Systems
One or more IDS sensors
Multiple IDS sensors divide load
Second firewall can be an application gateway

46. Security Courses @ Chalmers
Computer Security (SP3)
Cryptography (SP2)
Language-based Security (SP4)
Network security (SP4)