1. Virtual Private Networks (VPN)
Generic Routing Encapsulation (GRE)
TLS (SSL-VPN)
CN8814: Network Security

2. 1. Generic Routing Encapsulation (GRE)
Tunneling
Encapsulation with delivery header
The addresses in the delivery header are the addresses of the head-end and the tail-end of the tunnel
Delivery header /
GRE / /
tunnel
Private
network site
Private
network site
/16
/16
Public Network
CN8816: Network Security
Virtual Private Networks (VPN)

3. 1. Generic Routing Encapsulation (GRE)
Structure of a GRE encapsulated packet
CN8816: Network Security
Virtual Private Networks (VPN)

4. 1. Generic Routing Encapsulation (GRE)
IP access of the tunnel through the tunnel interface
/ GRE /
Gateway
Gateway
Internet
serial 0/0
serial 0/0
tunnel 0
tunnel 0 / e0 e0 / /
/16
/16
Customer Sites
CN8816: Network Security
Virtual Private Networks (VPN)

5. 1. Generic Routing Encapsulation (GRE)
Tunneling mechanism at IP
Outbound traffic
Routing table of R1 /
/16 e0
/30 s0
/16 tunnel0
/0 s0
(6)
(3) IP
GRE
(7)
(4)
(5)
(2)
(8) … s0 e0
Tunnel0
(9)
(1)
/ [ / ] /
CN8816: Network Security
Virtual Private Networks (VPN)

6. 1. Generic Routing Encapsulation (GRE)
Inbound traffic
Routing table of R2
/16 e0
/30 s0
/16 tunnel0
/0 s0
(6)
(3) IP
GRE
(5)
(2)
(4)
(7) / … s0 e0
Tunnel
(8)
(1)
/ [ / ] /
CN8816: Network Security
Virtual Private Networks (VPN)

7. 1. Generic Routing Encapsulation (GRE)
Example
interface tunnel0
ip unnumbered s0
tunnel source s0
tunnel destination !
ip route tunnel0
interface tunnel0
ip unnumbered s0
tunnel source s0
tunnel destination !
ip route tunnel0
Routing table of R1
/16 e0
/30 s0
/16 tunnel0
/0 s0
CN8816: Network Security
Virtual Private Networks (VPN)

8. 1. Generic Routing Encapsulation (GRE)
GRE tunneling with routing
Routing updates
(subnets of /8)
CN8816: Network Security
Virtual Private Networks (VPN)

9. Virtual Private Networks (VPN)
3. IP Security (IPsec)
IPsec and Dynamic Routing
IPsec-protected traffic must be pre-defined
IPsec only supports static routing
The IPsec tunnel is setup for the traffic between /16 and /16
IPsec tunnel
/16
/16
/16
The tunnel will not support the traffic to the new site
New site
CN8816: Network Security
Virtual Private Networks (VPN)

10. Virtual Private Networks (VPN)
3. IP Security (IPsec)
IPsec/GRE
GRE defines a tunnel interface
IPsec transport mode provides the security
Routing table
/ inside
/8 outside
/ tunnel0
(4) IP
GRE
(5)
/16 tunnel0
IPsec
(transport)
(2)
(1)
(3)
(6)
outside
inside
Tunnel0 / /
Ipsec_protected /
CN8816: Network Security
Virtual Private Networks (VPN)

11. Virtual Private Networks (VPN)
3. IP Security (IPsec)
Virtual Tunnel Interface
Provide secure tunnel by associating the virtual interface with IPsec
Routing table /
Protected packet
/ inside
/8 outside
/ tunnel0
(5) IP
IPsec
/16 tunnel0
(2)
(3)
(6)
(4)
inside
outside
Tunnel0 / /
CN8816: Network Security
Virtual Private Networks (VPN)

12. 2. Transport-Layer Security (TLS)
TLS architecture
Provide security connection between two application entities
Error and alert reporting
-Authentication
-Cipher suit
negotiation
-Keys
generation
Encryption,
MIC &
Compression
CN8816: Network Security
Virtual Private Networks (VPN)

13. 2. Transport-Layer Security
TLS Record protocol
Four connection states: current read and write states, and the pending read and write states
Security parameters:
Connection end (client/server)
Bulk encryption algorithm: type, key_size, …
MAC algorithm: hash_size
Compression algorithm
Master Secret
Client random, Nc
Server random, Ns
Keys
Client_MAC_Write, Server_MAC_Write, Client_Write_Key, Server_Write_Key, IVs
CN8816: Network Security
Virtual Private Networks (VPN)

14. 2. Transport-Layer Security
Messages are processed using the current read or write states
Type
Version
Length
Write/read
MAC secret
Fragment
(Compressed)
Write/read
Key
ICV
Padding (for CBC block cipher)
Pad length
CN 8816: Network Security
Virtual Private Networks (VPN)

15. 2. Transport-Layer Security
Key Generation
Master_secret = PRF(gxy mod N, “master secret”, Nc+Ns)
x and y are the DE parameters
Nc and Ns are the nonces generated by the client and the server, respectively
Master_secret is always 48 bytes
PRF(secret, label, seed) = P_hash(secret, label+seed)
P_hash(secret, seed) = HMAC(secret, A(1)+seed)
+ HMAC(secret, A(2)+seed)
+ …
+ HMAC(secret, A(n)+seed)
A(0) = seed
A(i) = HMAC(secret, A(i-1)
CN 8816: Network Security
Virtual Private Networks (VPN)

16. 2. Transport-Layer Security
By default, the hash used in HMAC is SHA-256
Need two HMAC iterations to generate the Master secret
Key generation
Key_block = PRF(Master_secret, “key expansion”, Ns+Nc)
Truncated
(16 bytes)
Master secret (48 bytes)
MAC client
Write (Kmc)
MAC server
Write (Kms)
Client Write
Key (Kwc)
Server Write
Key (Kws)
IVs (options) …
CN 8816: Network Security
Virtual Private Networks (VPN)

17. 2. Transport-Layer Security
Handshake protocol
Client hello and server response phase
Client
Server
Client Hello: session ID, Nc, cipher suit list, compression alg. list
Server Hello: Ns, cipher suit, compression alg.
Server Certificate O+
Key Exchange: Signature O+
Certificate request O-
Server Hello Done
O+ => Often; O- => Less often
CN 8816: Network Security
Virtual Private Networks (VPN)

18. 2. Transport-Layer Security
Client reponse and change-cipher specification phase
Client
Server
Client Certificate O-
Key exchange O-
Certificate verify: signature
Change cipher specification
Finished: session authentication
Change cipher specification
Finished: session authentication
Application Data
CN 8816: Network Security
Virtual Private Networks (VPN)

19. 2. Transport-Layer Security
The handshake protocol allows quick setup of new TLS connection using the old session master secret
The master secret is allowed to be kept for 24 hours
Client
Server
Client Hello: session ID, Nc
Generate keys
Server Hello: Ns
Generate keys
Change cipher spec
Finished
Change cipher spec
Finished
Data
CN 8816: Network Security
Virtual Private Networks (VPN)

20. 2. Transport-Layer Security
Change Cipher Spec Protocol
invoke the transition from the pending states to current states
Client
Server
Pending
states
Kmc, Kwc
Kmc, Kwc
Pending
states
Kms, Kws
Kms, Kws
(1) ChangeCipherSpecs
ChangeCipher
Specs Protocol
(1)
(4)
ChangeCipher
Specs Protocol
(5)
(2)
(4) ChangeCipherSpecs
(3)
(3) finished
(3)
Handshake
Handshake
(6)
(6) finished
(6)
Current
states
Current
states
CN8816: Network Security
Virtual Private Networks (VPN)

21. 2. Transport-Layer Security
Alert Protocol
Alert messages convey the severity of the message and a description of the alert
Alert levels: warning or fatal
Alert messages with a level of fatal result in the immediate termination of the connection
Alert types:
Close notification
Error alerts
CN8816: Network Security
Virtual Private Networks (VPN)

22. 2. Transport-Layer Security
SSL (TLS) VPN
Tunnel interface is used to support secure tunnel connection
SSL-VPN
client
Upload of SSL-VPN client
SSL-VPN web server
Appl.
Web browser
(1)
(9)
(3)
(17)
(5)
(2) TLS handshake protocol
TLS
TLS
(18)
(10)
(16)
TCP/UDP
TCP/UDP
(15)
(6)
(11)
(8) IP IP
(19)
(7)
(12)
(14)
(20)
(13)TLS protected datagram
tun0
eth0
Outside
Inside
tun0
CN8816: Network Security
Virtual Private Networks (VPN)