1. Network Security
8: Network Security

2. What is network security?
Confidentiality: only sender, intended receiver should “understand” message contents
sender encrypts message
receiver decrypts message
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
8: Network Security

3. Friends and enemies: Alice, Bob, Trudy
well-known in network security world
Bob, Alice (lovers!) want to communicate “securely”
Trudy (intruder) may intercept, delete, add messages
Alice
Bob
channel
data, control messages
secure
sender
secure
receiver
data
data
Trudy
8: Network Security

4. Who might Bob, Alice be? … well, real-life Bobs and Alices!
Web browser/server for electronic transactions (e.g., on-line purchases)
on-line banking client/server
DNS servers
routers exchanging routing table updates
other examples?
8: Network Security

5. There are bad guys (and girls) out there!
Q: What can a “bad guy” do?
A: a lot!
eavesdrop: intercept messages
actively insert messages into connection
impersonation: can fake (spoof) source address in packet (or any field in packet)
hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place
denial of service: prevent service from being used by others (e.g., by overloading resources)
8: Network Security

6. The language of cryptography
Alice’s
encryption
key
Bob’s
decryption
key K A K B
encryption
algorithm
plaintext
ciphertext
decryption
algorithm
plaintext
symmetric key crypto: sender, receiver keys identical
public-key crypto: encryption key public, decryption key secret (private)
8: Network Security

7. Symmetric key cryptography
substitution cipher: substituting one thing for another
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
8: Network Security

8. Symmetric key cryptography
A-B K
A-B
encryption
algorithm
plaintext
message, m
ciphertext
decryption
algorithm
plaintext
K (m)
K (m)
A-B
m = K ( )
A-B
symmetric key crypto: Bob and Alice share know same (symmetric) key: K
e.g., key is knowing substitution pattern in mono alphabetic substitution cipher
Q: how do Bob and Alice agree on key value?
A-B
8: Network Security

9. Public Key Cryptography
symmetric key crypto
requires sender, receiver know shared secret key
Q: how to agree on key in first place (particularly if never “met”)?
public key cryptography
radically different approach [Diffie-Hellman76, RSA78]
sender, receiver do not share secret key
public encryption key known to all
private decryption key known only to receiver
8: Network Security

10. Public key cryptography+
Bob’s public
key K B -
Bob’s private
key K B
plaintext
message, m
encryption
algorithm
ciphertext
decryption
algorithm
plaintext
message
K (m) B +
m = K (K (m)) B + -
8: Network Security

11. Public key encryption algorithms
Requirements: . . + - 1
need K ( ) and K ( ) such that B B
K (K (m)) = m B - + + 2
given public key K , it should be impossible to compute private key K B - B
RSA: Rivest, Shamir, Adelson algorithm
8: Network Security

12. RSA: another important property
The following property will be very useful later:
K (K (m)) = m B - +
K (K (m)) =
use public key first, followed by private key
use private key first, followed by public key
Result is the same!
8: Network Security

13. Protocol ap1.0: Alice says “I am Alice”
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
“I am Alice”
Failure scenario??
8: Network Security

14. Authentication Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
in a network,
Bob can not “see” Alice, so Trudy simply declares
herself to be Alice
“I am Alice”
8: Network Security

15. Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
“I am Alice”
Alice’s
IP address
Failure scenario??
8: Network Security

16. Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
Trudy can create
a packet “spoofing”
Alice’s address
“I am Alice”
Alice’s
IP address
8: Network Security

17. Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
“I’m Alice”
Alice’s
IP addr
password
Failure scenario?? OK
Alice’s
IP addr
8: Network Security

18. Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Alice’s
IP addr
Alice’s
password
“I’m Alice”
playback attack: Trudy records Alice’s packet
and later
plays it back to Bob OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
password
8: Network Security

19. Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
“I’m Alice”
Alice’s
IP addr
encrypted
password
Failure scenario?? OK
Alice’s
IP addr
8: Network Security

20. Authentication: another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
Alice’s
IP addr
encrypted
password
“I’m Alice”
record
and
playback
still works! OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
encrypted
password
8: Network Security

21. Authentication: yet another try
Goal: avoid playback attack
Nonce: number (R) used only once –in-a-lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice
must return R, encrypted with shared secret key
“I am Alice” R
K (R)
A-B
Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!
Failures, drawbacks?
8: Network Security

22. “send me your public key”
Authentication: ap5.0
ap4.0 requires shared symmetric key
can we authenticate using public key techniques?
ap5.0: use nonce, public key cryptography
“I am Alice”
Bob computes R
(K (R)) = R A - K +
K (R) A -
and knows only Alice could have the private key, that encrypted R such that
“send me your public key” K A +
(K (R)) = R A - K +
8: Network Security

23. sends m to Alice encrypted with Alice’s public key
ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)
I am Alice
I am Alice R T
K (R) - R A
K (R) -
Send me your public key T K +
Send me your public key A K + T
K (m) +
Trudy gets T
m = K (K (m)) + - A
K (m) +
sends m to Alice encrypted with Alice’s public key A
m = K (K (m)) + -
8: Network Security

24. ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)
Difficult to detect:
Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation)
problem is that Trudy receives all messages as well!
8: Network Security

25. 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 prove to someone that Bob, and no one else (including Alice), must have signed document
8: Network Security

26. Bob’s message, m, signed (encrypted) with his private key
Digital Signatures
Simple digital signature for message m:
Bob signs m by encrypting with his private key KB, creating “signed” message, KB(m) - - K B -
Bob’s message, m
Bob’s private
key K B -
(m)
Dear Alice
Oh, how I have missed you. I think of you all the time! …(blah blah blah)
Bob
Bob’s message, m, signed (encrypted) with his private key
Public key
encryption
algorithm
8: Network Security

27. Digital Signatures (more)-
Suppose Alice receives msg m, digital signature KB(m)
Alice verifies m signed by Bob by applying Bob’s public key KB to KB(m) then checks KB(KB(m) ) = m.
If KB(KB(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 KB(m) to court and prove that Bob signed m. -
8: Network Security

28. Message Digests
large
message m
H: Hash
Function
Computationally expensive to public-key-encrypt long messages
Goal: fixed-length, easy- to-compute digital “fingerprint”
apply hash function H to m, get fixed size message digest, H(m).
H(m)
Hash function properties:
produces fixed-size msg digest (fingerprint)
given message digest x, computationally infeasible to find m such that x = H(m)
8: Network Security

29. Digital signature = signed message digest
Alice verifies signature and integrity of digitally signed message:
Bob sends digitally signed message:
large
message m
H: Hash
function
KB(H(m)) -
encrypted
msg digest
H(m)
digital
signature
(encrypt)
Bob’s
private
key
large
message m K B -
Bob’s
public
key
digital
signature
(decrypt) K B +
KB(H(m)) -
encrypted
msg digest
H: Hash
function +
H(m)
H(m)
equal ?
8: Network Security

30. Hash Function Algorithms
MD5 hash function widely used (RFC 1321)
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 [NIST, FIPS PUB 180-1]
160-bit message digest
8: Network Security

31. Trusted Intermediaries
Symmetric key problem:
How do two entities establish shared secret key over network?
Solution:
trusted key distribution center (KDC) acting as intermediary between entities
Public key problem:
When Alice obtains Bob’s public key (from web site, , diskette), how does she know it is Bob’s public key, not Trudy’s?
Solution:
trusted certification authority (CA)
8: Network Security

32. Key Distribution Center (KDC)
Alice, Bob need shared symmetric key.
KDC: server shares different secret key with each registered user (many users)
Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC.
KDC
KB-KDC
KP-KDC
KA-KDC
KX-KDC
KP-KDC
KY-KDC
KZ-KDC
KA-KDC
KB-KDC
8: Network Security

33. Certification Authorities
Certification authority (CA): binds public key to particular entity, E.
E (person, router) registers its public key with CA.
E provides “proof of identity” to CA.
CA creates certificate binding E to its public key.
certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key”
digital
signature
(encrypt) K B +
Bob’s
public
key K B + CA
private
key
certificate for Bob’s public key, signed by CA -
Bob’s
identifying information K CA
8: Network Security

34. Certification Authorities
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 K B +
digital
signature
(decrypt)
Bob’s
public
key K B + CA
public
key + K CA
8: Network Security

35. A certificate contains:
Serial number (unique to issuer)
info about certificate owner, including algorithm and key value itself (not shown)
info about certificate issuer
valid dates
digital signature by issuer
8: Network Security

36. Secure sockets layer (SSL)
server authentication:
SSL-enabled browser includes public keys for trusted CAs.
Browser requests server certificate, issued by trusted CA.
Browser uses CA’s public key to extract server’s public key from certificate.
check your browser’s security menu to see its trusted CAs.
transport layer security to any TCP-based app using SSL services.
used between Web browsers, servers for e-commerce (shttp).
security services:
server authentication
data encryption
client authentication (optional)
8: Network Security

37. Certificate (CA signed Pub Key)
SSL (continued)
Client
Server
Open secure socket
Certificate (CA signed Pub Key)
Verify CA trusted
Extract Server Pub Key
Generate symmetric Session Key
Encrypt Session Key with Server Pub Key
Encrypted Session Key
Extract Session Key (using Private Key)
In Java all of this happens behind the scenes!
SSLSocket s = (SSLSocket)sslFact.createSocket(host, port);
8: Network Security

38. SSL Observations Previous example does not
Show how public/private key pairs are generated
Manually
Enable the Server to authenticate the client
Client can use trusted certificate, or another scheme such as passwords
Show how the Server receives a signed certificate
CA!
8: Network Security

39. Project 3 Project 3 Overview Offline, manual tasks
Secure your time/date client and server
Mutual authentication between client and server
Implement a trusted CA that can generate certificates for the client and server (by signing their public keys)
Offline, manual tasks
CA, Client, and Server generate public/private keys
CA generates self-signed certificate
Client and Server import CA certificate
8: Network Security

40. Project 3
CA behavior
Accepts connections from clients – does not require SSL-based client authentication
Note: The T/D Server can act as a client when connecting to the CA
Authenticates clients using passwords – can be sent in plaintext
Receives Public Key from client
Generates certificate by signing client Public Key with CA Private Key
Returns certificate to client
8: Network Security

41. Project 3 Server behavior Connect to trusted CA
Send password for CA authentication of Server
Send Public Key to CA
Receive certificate from CA
Wait for secure client connection
Require client authentication – client must have CA-signed certificate
Allow client to request time or date
Send response
8: Network Security

42. Project 3 Client behavior Connect to trusted CA
Send password for CA authentication of Server
Send Public Key to CA
Receive certificate from CA
Securely connect to server
Request time or date
Receive/display response
8: Network Security

43. Java and SSL
keytool – command line tool used to generate public/private keys, generate self-signed certificates, and import other trusted certificates
javax.net.ssl java.security java.security.[spec,cert] org.bouncycastle.[x509,jce,asn1]
KeyManager – object that stores my keys/certificates
used during handshake with server
TrustManager – object that stores trusted certificates
can be initialized from same file as KeyStore
8: Network Security

44. Java and SSL
SSLContext – context that specifies keys, certificates, and trusted certificates
initialize context with appropriate KeyManagers and TrustManagers
SSLSocketFactory/SSLServerSocketFactory – will create socket based on given context
default context will not contain appropriate certificates
SSLSocket – performs regular socket-like communication only with encryption
8: Network Security

45. Java and SSL
CertificateFactory – can create a certificate from an InputStream
used to create certificate sent from CA
Certificate, PublicKey, PrivateKey – what you’d expect
8: Network Security

46. Java and SSL X509Certificate – certificate based on X.509 standard
defines structure of certificate, etc
X509Name – object to represent distinguished name of subject/issuer of certificate
X509EncodedKeySpec/KeyFactory – create public key from byte stream
X509V3CertificateGenerator – use to generate certificate
use specified private key to generate certificate for holder of specified public key
8: Network Security

47. Firewalls
firewall
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.
administered
network
public
Internet
firewall
8: Network Security

48. Firewalls: Why prevent denial of service attacks:
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)
two types of firewalls:
application-level
packet-filtering
8: Network Security

49. Should arriving packet be allowed in? Departing packet let out?
Packet Filtering
Should arriving packet be allowed in? Departing packet let out?
internal network connected to Internet via router firewall
router filters packet-by-packet, decision to forward/drop packet based on:
source IP address, destination IP address
TCP/UDP source and destination port numbers
ICMP message type
TCP SYN and ACK bits
8: Network Security

50. Packet Filtering
Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23.
All incoming and outgoing UDP flows and telnet connections are blocked.
Example 2: Block inbound TCP segments with ACK=0.
Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.
8: Network Security

51. Application gateways
gateway-to-remote
host telnet session
host-to-gateway
telnet session
Filters packets on application data as well as on IP/TCP/UDP fields.
Example: allow select internal users to telnet outside.
application
gateway
router and filter
1. Require all telnet users to telnet through gateway.
2. For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections
3. Router filter blocks all telnet connections not originating from gateway.
8: Network Security

52. Internet security threats
Mapping:
before attacking: “case the joint” – find out what services are implemented on network
Use ping to determine what hosts have addresses on network
Port-scanning: try to establish TCP connection to each port in sequence (see what happens)
nmap ( mapper: “network exploration and security auditing”
Countermeasures?
8: Network Security

53. Internet security threats
Mapping: countermeasures
record traffic entering network
look for suspicious activity (IP addresses, pots being scanned sequentially)
8: Network Security

54. Internet security threats
Packet sniffing:
broadcast media
promiscuous NIC reads all packets passing by
can read all unencrypted data (e.g. passwords)
e.g.: C sniffs B’s packets A C
src:B dest:A payload B
Countermeasures?
8: Network Security

55. Internet security threats
Packet sniffing: countermeasures
all hosts in organization run software that checks periodically if host interface in promiscuous mode.
one host per segment of broadcast media (switched Ethernet at hub) A C
src:B dest:A payload B
8: Network Security

56. Internet security threats
IP Spoofing:
can generate “raw” IP packets directly from application, putting any value into IP source address field
receiver can’t tell if source is spoofed
e.g.: C pretends to be B A C
src:B dest:A payload B
Countermeasures?
8: Network Security

57. Internet security threats
IP Spoofing: ingress filtering
routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network)
great, but ingress filtering can not be mandated for all networks A C
src:B dest:A payload B
8: Network Security

58. Internet security threats
Denial of service (DOS):
flood of maliciously generated packets “swamp” receiver
Distributed DOS (DDOS): multiple coordinated sources swamp receiver
e.g., C and remote host SYN-attack A A C
SYN
SYN
SYN
SYN
SYN B
SYN
Countermeasures?
SYN
8: Network Security

59. Internet security threats
Denial of service (DOS): countermeasures
filter out flooded packets (e.g., SYN) before reaching host: throw out good with bad
traceback to source of floods (most likely an innocent, compromised machine) A C
SYN
SYN
SYN
SYN
SYN B
SYN
SYN
8: Network Security