1. CSS432 Network Security Textbook Ch8
Professor: Munehiro Fukuda
CSS432: Network Security

2. Overview Cryptography functions
Secret key (e.g., DES): sharing a common single secret key
Public key (e.g., RSA): owning a private key, while publishing the corresponding public key
Message digest (e.g., MD5): generating a cryptographic checksum
Security services
Privacy: preventing unauthorized release of information
Authentication: verifying identity of the remote participant
Integrity: making sure message has not been altered
Security
Cryptography
algorithms
Public
key
(e.g., RSA)
Secret
(e.g., DES)
Message
digest
(e.g., MD5)
services
Authentication
Privacy
integrity
CSS432: Network Security

3. Secret Key (DES) Plaintext Encrypt with secret key Ciphertext
Decrypt with
CSS432: Network Security

4. 64-bit key (56-bits + 8-bit parity) 16 rounds
Initial permutation
Round 1
Round 2
Round 16
56-bit
key
Final permutation …
Each Round
Left half
Right half + F L i – 1 R K
Combiner function
Ex-OR
CSS432: Network Security

5. Repeat for larger messages
64bits
64bits
64bits
64bits
Block 1 IV
DES
Cipher 2 3 4 +
Initialization vector
(Random number)
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6. Public Key (RSA)
Plaintext
Encrypt with
public key
Ciphertext
Decrypt with
private key
Encryption: c = memod n where <e, n> is a public key
Decryption: m = cdmod n where <d, n> is a private key
The main goal is to find a pair of e and d
CSS432: Network Security

7. RSA (cont) Algorithm Example
Choose two large prime numbers p and q (each 256 bits)
n = p x q
Choose the encryption key e, such that e and (p - 1) x (q - 1) are relatively prime.
Two numbers are relatively prime if they have no common factor greater than one
Compute decryption key d such that
d = e-1mod ((p - 1) x (q - 1))
Construct public key as (e, n)
Construct private key as (d, n)
Discard (do not disclose) original primes p and q
Encryption: c = memod n
Decryption: m = cdmod n
p = 3, q = 5
n = 3 x 5 = 15
(3-1) x (5 – 1) = 8
e = 7
7d = 1 mod 8
d = 15
7 x 15 = 105
= 1 mod 8
m = 2
c = 27 mod 15 = 8
m = 815 mod 15 = 2
CSS432: Network Security

8. Message Digest Sender message message message checksum checksum
Adversary knows
the checksum computation
Receiver
Errors
altered
message
message
message
Wrong
altered
checksum
checksum
checksum
checksum
checksum
checksum
No errors
Errors detected
No alternation detected
CSS432: Network Security

9. Message Digest message message message MD MD MD Errors altered message
Adversary doesn’t know
the key used in MD
Errors
altered
message
message
message
Wrong
altered MD MD MD MD MD MD
No errors nor alternations
Errors detected
Alternation detected
CSS432: Network Security

10. Message Digest Message (padded) 128-bit Initial digest (constant)
512 bits
512 bits
512 bits
Bitwise transform
Bitwise transform
Bitwise transform
Message digest
CSS432: Network Security

11. Authentication Protocols Three-Way Handshake
E(msg, key): msg was encrypted with key
CHK: client handshake key
SHK: server handshake key
Both client and server must know CHK and SHK.
Client sends x encrypted with CHK.
Server decrypts x with CHK.
Server sends x + 1 and y encrypted with SHK.
Client decrypts x + 1 and y with SHK and knows Server has CHK.
Client sends y + 1 encrypted with CHK.
Server decrypts y + 1 with CHK and knows Client has SHK.
What if client and server know nothing about each other?
CSS432: Network Security

12. Authentication Protocols Trusted Third Party (Kerberos)
The authentication server S assumes that A and B each share a secret key with S.
T: timestamp
L: Lifetime
K: Session key
KA: Secret key shared between A and S
KB: Secret key shared between B and S B A , E (( T , L , K ,
A knows T, L, and K
A checks if B has received K E B (( T ), , K L , ), K , A A ), K ) B E (( A , E T (( ), T , K L ), , K , A ), K )
B knows T, L, and K
B responds T+1 to A B K )
+ 1, E ( T
A session has been established
CSS432: Network Security

13. Public key authentication
A authenticated B
CSS432: Network Security

14. Message Integrity Protocols
Digital signature using RSA
Original cryptography:
Encryption with a public key : c = memod n
Decryption with a private key: m = cdmod n
Compute signature with private key and verify with public key
Encryption with a private key : m = cdmod n
Decryption with a public key: c = memod n
Large computational complexity
Keyed MD5
sender: m + MD5(m + k)
receiver
Sender and receiver share the key k
Computes MD5(m + k) and compare it with the received MD5(m + k)
MD5 with RSA signature
sender: m + E(MD5(m), private)
decrypts MD5(m) with sender’s public key
compares result with MD5 checksum sent with message
CSS432: Network Security

15. Key Distribution Certificate
Special type of digitally signed document:
“I certify that the public key in this document belongs to the entity named in this document, signed X.”
The name of the entity being certified
The public key of the entity
The name of the certified authority
A digital signature
Certified Authority (CA)
Administrative entity that issues certificates
Useful only to someone that already holds the CA’s public key.
PGP Example:
CA imports a public key from a person who wants it to certify his/her public key.
CA retrieves the finger print from this public key.
CA verify it with the key’s owner over a phone or through any other means.
CSS432: Network Security

16. Key Distribution (cont)
Chain of Trust
X certifies that a certain public key belongs to Y:
X.cert( Y, publicKeyY )
Y certifies that another public key belongs to Z:
Y.cert( Z, publicKeyZ )
Someone, say A wants to verify Z’s public key has to know X’s public key:
A.knows( X, publicKeyX )
Certificate Revocation List
A digitally signed list of revoked certificates
Periodically updated and made available in public
A revoked certificate will be listed for a certain time period
A certificate has an expiration date.
PGP Example:
A person who wants to revoke his/her public key generates a revocation certificate. (This should be done before his/her public key becomes useless.)
He/she prints out the revocation certificate and sends it to CA.
CSS432: Network Security

17. Example Systems Pretty Good Privacy (PGP) Secure Shell (SSH)
Encryption and authentication for
Secure Shell (SSH)
Secured version of Unix R commands (rlogin, rsh, and rcp)
Transport Layer Security (TLS, SSL, HTTPS)
Additional handshake to encrypt and authenticate TCP communication, in particular HTTP connections
IP Security (IPSEC)
Framework to make IP packets secured
CSS432: Network Security

18. PGP
A popular approach to providing encryption ad authentication capabilities for .
A user decides the level of establishing trust
You know A well, and thus trust A’s public key.
You have received B’s certificate from A.
You trust it if B is A’s colleague but not a politician.
You have received B’s certificate from C and D and then trust B’s certificate.
A user collects a set of certificates with varying trust level in a key ring (~/.gnupg/trustdb.gpg)
Systems available at:
MIT distribution site for PGP:
The International PGP Home Page:
The Gnu Privacy Guard:
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19. PGP (Message Encryption)
Generating a key: gpg --gen-key
Required to type your real name, comments, and address(
Exporting a public key: gpg --output mfukuda.pub --armor --export mfukuda
Importing a public key: gpg --import --armor mfukuda.pub
Validating a public key: gpg --edit-key
Command > fpr // check his finger print
Command > sign // sign this public key at a favorite trust level
Encrypting a document: gpg –output doc.enc –encrypt –recipient doc
Decrypting a document: gpg –output doc –descript doc.enc
Create a random secret key k
Encrypt message using
DES with secret key k
Decrypt message using
DES with secret key k
Encrypt k using RSA with
recipient’s public key
Decrypt E(k) using RSA with
my private key -> k
Encode message + E(k)
in ASCII for transmission
Convert ASCII message
CSS432: Network Security

20. PGP (Message Integrity and Authentication)
Making a signature: gpg --output doc.asc --sign doc
Making a signature in format: gpg --clearsign doc // doc.asc is automatically created
Verifying a signature: gpg --output doc --decrypt doc.asc
Sender identity and message
integrity confirmed
if checksums match
Calculate MD5 checksum on
received message and compare
against received value
Calculate MD5 checksum
over message contents
Sign checksum using RSA
with sender’s private key
Decrypt signed checksum
with sender’s public key
CSS432: Network Security

21. SSH Secured version of Unix R commands (rlogin, rsh, and rcp)
SSH-TRANS:
a transport layer protocol on top a TCP connection
SSH-AUTH:
An authentication protocl using RSA
SSH-CONN:
a connection protocol enabling SSH tunnelling
CSS432: Network Security

22. SSH (Protocol Exchange)
(1) Contact to port 22
/etc/ssh/ssh_host_rsa_key
host1
/etc/ssh/ssh_known_hosts
~/.ssh/known_hosts
host1
host2
host3
(2) SSH version
host1
/etc/ssh/ssh_host_rsa_key.pub
host1
(3)
server
(4) Server Authentication
session
E( , )
server
host1
session
server
(5) Secure connection established
server
~/.ssh/id_rsa
(~/.ssh/identity)
(6) Client Authentication
~/.ssh/authorized_keys
E( random 256-bit string, )
MD5(random 256-bit string, )
session
(7) All authentications completed
local1
server1
CSS432: Network Security

23. SSH (Key Generation Example)
Munehiro ~$ ssh-keygent –t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/Munehiro Fukuda/.ssh/id_rsa):
Enter passphrase (empty for no passphrase): ********
Enter same passphrase again: ********
Your identification has been saved in /home/Munehiro Fukuda/.ssh/id_rsa.
Your public key has been saved in /home/Munehiro Fukuda/.ssh/id_rsa.pub.
The key finger print is:
3b:4c:e7:db:68:25:13:7e:77:a2:c8:b8:43:af:44:82 Munehiro
Munehiro ~$ cd .ssh
Munehiro ~$ ls
Id_rsa id_rsa.pub known_hosts known_hosts~
Munehiro ~$ sftp perseus.bothell.washington.edu
Connecting to perseus.bothell.washington.edu…
password: ********
sftp> put id_rsa.pub
sftp> quit
Munehiro ~$ ssh –l mfukuda perseus.bothell.washington.edu
passowrd: ********
mfukuda]$ mv id_rsa.pub .ssh/id_rsa.fukuda-thinkpad.pub
mfukuda]$ cd .ssh
mfukuda]$ cat id_rsa.fukuda-thinkpad.pub >> authorized_keys
mfukuda]$ chmod 644 authorized_keys
mfukuda]$ exit
Munehiro ~$ ssh –l mfukuda perseus bothell.washington.edu
Enter passphrase for key ‘/honme/Munehiro Fukuda/.ssh/id_rsa’: ********
mfukuda]$
CSS432: Network Security

24. SSH (SSH Agent) $ ssh-add ssh-agent SSH protocol exchange server local
~/.ssh/id_rsa
(~/.ssh/identity)
server
local
Munehiro ~$ eval `ssh-agent`
Agent pid 228
Munehiro ~$ ssh-add
Enter passphrase for /home/Munehiro Fukuda/.ssh/id_rsa: ********
Identity added: /home/Munehiro Fukuda/.ssh/id_rsa (home/Munehiro Fukuda/..ssh/id_rsa)
Munehiro ~$ ssh –l mfukuda perseus.bothell.washington.edu
mfukuda]$ exit
Munehiro ~$ eval `ssh-agent –k`
Agent pid 228 killd
Munehiro ~$ ssh –l mfukuda perseus bothell.washington.edu
Enter passphrase for key ‘/honme/Munehiro Fukuda/.ssh/id_rsa’: ********
mfukuda]$
CSS432: Network Security

25. SSH (Agent Forwarding)
SSH protocol
exchange
sshd
proxy
agent
SSH protocol
exchange
$ ssh-add
ssh-agent
sshd
~/.ssh/id_rsa
authentication
authentication
forwarded
authentication
scp
scp
scp
local
Server X
Server X
#Enable agent forwarding
ssh –A ….
sftp
CSS432: Network Security

26. SSH (Port Forwarding) Local forwarding Remote forwarding
ssh -L x:appserver:y
Ex: ssh –L 80:localhost:8080
Localhost = perseus in this example
Remote forwarding
ssh -R x:appclient:y
Ex: ssh –R 80:localhost:
Localhost = my local computer (fukuda-thinkpad)
=localhost x y 22
appclient
sshclient
sshdserver
appserver
=localhost 22 y x
appclient
sshdserver
appserver
sshclient
CSS432: Network Security

27. Firewalls Filter-Based Solution Example
Rest of the Internet
Local site
Firewall
Filter-Based Solution
Example
( , 1234, , 80 )
filter all packets from port 1234 on host addressed to port 80 on host
(*,*, , 80 )
filter all packets address to port 80 on
Default: forward or not forward?
All ports initialized to forward packets
All ports initialized to filter packets
How dynamic?
Filter initialized upon a boot
Filter reinitialized at run time
CSS432: Network Security

28. CISCO Example X 172.16.2.0 Internet Local Network hostname router1
hostname router1
! Link to the Internet
interface ethernet 0
ip address
ip access-group 1 in
! Link to the local network
interface ethernet 1
ip address !
Access-list 1 deny
Access-list 1 permit
CSS432: Network Security

29. Proxy-Based Firewalls
Problem: complex policy
Example: web server
Solution: proxy
Question:
What if we need to run different applications (HTTP, FTP, IMAP, etc)?
Company net
Firewall W eb
server
Random
external
user
Remote
company
Internet
Filter port 80 or not
Some web pages should be visible,
While the other should not.
External
client
External HTTP/TCP connection
Proxy
Firewall
Internal HTTP/TCP connection
Local
server
CSS432: Network Security

30. DMZ and Bastion Servers
1. Forward packets from /4
Filter the others
Internet
Local Network
Border
Router
Firewall
System
2. What if someone spoofs
X ?
/24
De-Militarized Zone (DMZ)
HTTP
FTP
IMAP
Bastion Hosts
3. Solution:
hostname router1
! Link to the Internet
interface ethernet 0
ip address
ip access-group 1 in
! Link to the local network
interface ethernet 1
ip address !
Access-list 1 deny
Access-list 1 permit
4. Limitations:
Attacks from within local network
CSS432: Network Security

31. Reviews Cryptography algorithms: DES, RSA, and MD5
Authentication: three way handshake and Kerberos
Message integrity: RSA, keyed MD5, and MD5 with RSA signature
Examples: PGP and SSH
Firewall: filter-based, proxy-based, and DMZ
CSS432: Network Security