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Lecture 7: Network Level
Security – IPSec
CS 336/536: Computer Network Security
Fall 2013
Nitesh Saxena
Adopted from previous lecture by Keith Ross, and Tony Barnard
Course Admin
• Mid-Term Exam
– Hope it went well
– Will provide solution set soon
– We are grading
• HW2
– Will provide solution set soon
– We are grading
10/29/2013 Lecture 7 - SSL/TLS 2
10/29/2013
2
Course Admin
• HW3 Posted
– Covers SSL/TLS (lecture 7)
– Due 11am on Nov 11 (Monday)
– Lab exercise involves capturing SSL/TLS packets
using Wireshark
– Labs active this Friday, and next Friday
10/29/2013 Lecture 7 - SSL/TLS 3
Outline
IPSec
– Modes and Protocols
– IKE Protocol Basics
10/29/2013 Lecture 7 - SSL/TLS 4
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Relative Location of Security Facilities in the TCP/IP Protocol Stack
IKE
UDP
Which Layer to Add Security to?
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Security at different layersr Application layer: PGPr Transport layer: SSLr Network layer: IPsec r Link layer: WEP / 802.11iIPsec approach:
IPsec
TCP/UDP/ICMP
HTTP/SMTP/IM
IPsec can provide security between any pair of network-layerentities (eg, between two hosts, two routers, or a host and a router).
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IP Security
r IP datagrams have no inherent securitym IP source address can be spoofedm Content of IP datagrams can be sniffedm Content of IP datagrams can be modifiedm IP datagrams can be replayed
r IPSec is a method for protecting IP datagramsm Standardized by IETF: dozens of RFCs.m Only sender and receiver have to be IPsec compliant• Rest of network can be regular IP
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What is security at the network-layer?
Between two network entities:
r Sending entity encrypts/authenticates the payloads of datagrams. Payload could be:m TCP segment, UDP segment, ICMP message, OSPF (routing) message, and so on.
r All data sent from one entity to the other would be hidden:m Web pages, e-mail, P2P file transfers, TCP SYN packets, and so on.
r That is, “blanket coverage”.
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Virtual Private Networks (VPNs)
r Institutions often want private networks for security. m Costly! Separate routers, links, DNS infrastructure.
r With a VPN, institution’s inter-office traffic is sent over public Internet instead. m But inter-office traffic is encrypted and/or authenticated before entering public Internet
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IPheader
IPsecheader
Securepayload
headquartersbranch office
salespersonin hotel
PublicInternet
laptop w/ IPsec
Router w/IPv4 and IPsec
Router w/IPv4 and IPsec
Virtual Private Network (VPN)
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IPsec services
r Integrity
r Authentication
r Replay attack prevention
r Confidentiality
r Two modes: m Transport and Tunnel
r Two protocols providing different service models:m AH – Authentication Header
m ESP – Encrypted Security Payload
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IPSec Modes: Transport and Tunnel Modes r Transport Mode provides protection primarily for
upper-layer protocols – that is, for the IP datagram payload.
m Transport mode is typically used in end-to-end communication between two hosts.
r Tunnel Mode extends protection to the entire datagram, by encapsulating it in a new “outer” datagram.
m Tunnel mode is typically used in communication between two routers (must be used if a router is involved).
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IPsec Transport Mode
r IPsec datagram emitted and received by end-system.
r Protects upper level protocols
IPsec IPsec
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IPsec – tunneling mode (1)
r End routers are IPsec aware. Hosts need not be.
IPsec IPsec
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IPsec – tunneling mode (2)
r Also tunneling mode.
IPsecIPsec
IPSec protocols
r Authentication Header (AH) protocolm provides source authentication & integrity but not confidentiality
r Encapsulation Security Protocol (ESP)m provides source authentication, integrity, and confidentiality
m more widely used than AH
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Four combinations are possible!
Host mode with AH
Host mode with ESP
Tunnel modewith AH
Tunnel modewith ESP
Most common andmost important
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Security associations (SAs)
r Before sending data, a virtual connection is established from sending entity to receiving entity.
r Called “security association (SA)”m SAs are simplex: for only one direction
r Both sending and receiving entites maintain state information about the SA
m Recall that TCP endpoints also maintain state information.
m IP is connectionless; IPsec is connection-oriented!
r How many SAs in VPN w/ headquarters, branch office, and n traveling salesperson?
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Tunnel Mode and Transport Mode Functionality
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ESP in transport mode
End-to-end encryption
ESP in transport mode conceals what Alice is saying to Bob,
but not that Alice and Bob are communicating.
Alice Bob
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ESP in tunnel mode
VPN
ESP in tunnel mode over the VPN also
conceals the fact that Alice is talking to Bob
Alice
Bob
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Original datagram
ESP in
transport mode
ESP in
tunnel mode
Protocol = 6
Protocol = 50
Protocol = 50
Next = 6
Next = 4
Scope of ESP encryption
and authentication
ESP authentication
does not extend to
the IP header
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193.68.2.23200.168.1.100
172.16.1/24172.16.2/24
SA
InternetHeadquartersBranch Office
R1R2
Example SA from R1 to R2
R1 stores for SAr 32-bit identifier for SA: Security Parameter Index (SPI)r the origin interface of the SA (200.168.1.100)r destination interface of the SA (193.68.2.23)r type of encryption to be used (for example, 3DES with CBC)r encryption keyr type of integrity check (for example, HMAC with with MD5)r authentication key
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Security Association Database (SAD)
r Endpoint holds state of its SAs in a SAD, where it can locate them during processing.
r With n salespersons, 2 + 2n SAs in R1’s SAD
r When sending IPsec datagram, R1 accesses SAD to determine how to process datagram.
r When IPsec datagram arrives to R2, R2 examines SPI in IPsec datagram, indexes SAD with SPI, and processes datagram accordingly.
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IPsec datagram
Focus for now on tunnel mode with ESP
new IPheader
ESPhdr
originalIP hdr
Original IPdatagram payload
ESPtrl
ESPauth
encrypted
“enchilada” authenticated
paddingpad
lengthnext
headerSPI
Seq#
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What happens?
193.68.2.23200.168.1.100
172.16.1/24172.16.2/24
SA
InternetHeadquartersBranch Office
R1R2
new IPheader
ESPhdr
originalIP hdr
Original IPdatagram payload
ESPtrl
ESPauth
encrypted
“enchilada” authenticated
paddingpad
lengthnext
headerSPI
Seq#
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R1 converts original datagraminto IPsec datagram
r Appends to back of original datagram (which includes original header fields!) an “ESP trailer” field.
r Encrypts result using algorithm & key specified by SA.
r Appends to front of this encrypted quantity the “ESP header, creating “enchilada”.
r Creates authentication MAC over the whole enchilada, using algorithm and key specified in SA;
r Appends MAC to back of enchilada, forming payload;
r Creates brand new IP header, with all the classic IPv4 header fields, which it appends before payload.
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Inside the enchilada:
r ESP trailer: Padding for block ciphersr ESP header:
m SPI, so receiving entity knows what to dom Sequence number, to thwart replay attacks
r MAC in ESP auth field is created with shared secret key
new IPheader
ESPhdr
originalIP hdr
Original IPdatagram payload
ESPtrl
ESPauth
encrypted
“enchilada” authenticated
paddingpad
lengthnext
headerSPI
Seq#
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IPsec sequence numbers
r For new SA, sender initializes seq. # to 0
r Each time datagram is sent on SA:m Sender increments seq # counter
m Places value in seq # field
r Goal:m Prevent attacker from sniffing and replaying a packet
• Receipt of duplicate, authenticated IP packets may disrupt service
r Method: m Destination checks for duplicates
m But doesn’t keep track of ALL received packets; instead uses a window
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Algorithm at receiver
1. If rcvd packet falls in window, packet is new, and MAC is valid ➜ slot in window marked
2. If rcvd packet is to right of window, MAC is valid ➜window advanced & right-most slot marked
3. If rcvd packet is left of window, or already marked, or MAC not valid ➜ packet is discarded
N is highestsequence #rcvd.
Default W=64
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Security Policy Database (SPD)
r Policy: For a given datagram, sending entity needs to know if it should use IPsec.
r Needs also to know which SA to usem May use: source and destination IP address; protocol number.
r Info in SPD indicates “what” to do with arriving datagram;
r Info in the SAD indicates “how” to do it.
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Focus on an outbound IP datagram crossing the boundary
between an intranet and the Internet.
How is it decided what security processes are applied to this
datagram?
This is a policy decision by administration.
The decision for each category of traffic is entered into
a Security Policy Database.
The menu of available processes is collected into a
Security Association Database.
Adopt 2-step process - entries in the Security Policy Database point
to entries in the Security Association Database.
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Linux example: ESP in tunnel mode
• In each host, create config file:• /etc/setkey.conf
• Execute setkey command in both hosts, which reads the setkey.conf file• setkey –f /etc/setkey.conf• Creates SAD and SPD databases
193.68.2.23200.168.1.100
172.16.1/24172.16.2/24
SA
InternetHeadquartersBranch Office
R1R2
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# Flush the SAD and SPD
flush;
spdflush;
# SAs encrypt w/ 192 bit keys & auth w/ 128 bit keys
Add 200.168.1.100 193.68.2.23 esp 0x201 -m tunnel -E 3des-cbc
0x7aeaca3f87d060a12f4a4487d5a5c3355920fae69a96c831 -A
hmac-md5 0xc0291ff014dccdd03874d9e8e4cdf3e6;
Add 193.68.2.23 200.168.1.100 esp 0x301 -m tunnel -E 3des-cbc
0xf6ddb555acfd9d77b03ea3843f2653255afe8eb5573965df -A
hmac-md5 0x96358c90783bbfa3d7b196ceabe0536b;
# Security policies
spdadd 172.16.1.0/24 172.16.2.0/24 any -P out ipsec
esp/tunnel/ 200.168.1.100 - 193.68.2.23 /require;
spdadd 172.16.2.0/24 172.16.1.0/24 any -P in ipsec
esp/tunnel/ 193.68.2.23 - 200.168.1.100 /require;
setkey.conf for R1
2 SAsaddedto SAD
SPI
ESP protocol
2 policiesaddedto SPD
apply to all packets
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# Flush the SAD and SPD flush; spdflush;
# AH SAs using 128 bit long keysAdd 200.168.1.100 193.68.2.23 ah 0x200 -A hmac-md5 0xc0291ff014dccdd03874d9e8e4cdf3e6; Add 193.68.2.23 200.168.1.100 ah 0x300 -A hmac-md5 0x96358c90783bbfa3d7b196ceabe0536b;
# Security policies Spdadd 200.168.1.100 193.68.2.23 any -P out ipsec
ah/transport//require; Spdadd 193.68.2.23 200.168.1.100 any -P in ipsec
ah/transport//require;
Another Example: AH in Transport Mode between R1 and R2
2 SAsaddedto SAD
2 policiesaddedto SPD
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Possible encryption algorithms
r DES
r 3DES
r AES
r RC5
r IDEA
r 3-IDEA
r CAST
r Blowfish
r ….
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IPsec Security
r Suppose Trudy sits somewhere between R1 and R2. She doesn’t know the keys. m Will Trudy be able to see contents of original datagram? How about source, dest IP address, transport protocol, application port?
m Flip bits without detection?
m Masquerade as R1 using R1’s IP address?
m Replay a datagram?
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Internet Key Exchange
r In previous examples, we manually established IPsec SAs in IPsec endpoints:
Example SASPI: 12345Source IP: 200.168.1.100Dest IP: 193.68.2.23 Protocol: ESPEncryption algorithm: 3DES-cbcHMAC algorithm: MD5Encryption key: 0x7aeaca…
HMAC key:0xc0291f…
r Such manually keying is impractical for large VPN with, say, hundreds of sales people.
r Instead use IPsec IKE (Internet Key Exchange)
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IKE: PSK and PKI
r Authentication (proof who you are) with eitherm pre-shared secret (PSK) or m with PKI (pubic/private keys and certificates).
r With PSK, both sides start with secret:m then run IKE to authenticate each other and to generate IPsec SAs (one in each direction), including encryption and authentication keys
r With PKI, both sides start with public/private key pair and certificate.m run IKE to authenticate each other and obtain IPsec SAs (one in each direction).
m Similar with handshake in SSL.
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Summary of relationship between SPD, IKE, and SAD:
Note that SA must first be established between the two IKEs
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Note that BYPASS IPsec is a possible policy for non-
sensitive traffic that requires no security processing.
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PROTOCOL = 50 or 51
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Before a pair of security gateways can exchange user data protected
by IPsec, they must complete a preliminary handshake, the Internet
Key Exchange.
During the handshake they establish algorithms,
keys that will be used, and authenticate each other.
IKE itself has two phases:
► phase 1: a secure channel, the IKE Security Association
pair is set up between the two security gateways
► phase 2: the two gateways use this channel to negotiate
safely one or more IPsec SA pairs that will be
used to protect transfer of user data between
the two intranets
Both phases must be complete before user data can flow
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First we must establish a secure channel between the two security gateways – the IKE SA
Then the SPD tells IKE what SAs are needed for user traffic.
IKE negotiates the user SAs and enters them in the SAD.
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IKE Phases
r IKE has two phasesm Phase 1: Establish bi-directional IKE SA
• Note: IKE SA different from IPsec SA• Also called ISAKMP security association
m Phase 2: ISAKMP is used to securely negotiate the IPsec pair of SAs
r Phase 1 has two modes: aggressive mode and main modem Aggressive mode uses fewer messagesm Main mode provides identity protection and is more flexible
Diffie-Hellman (DH) Key Exchange
1. A � B: Ka = ga mod p
2. B � A: Kb = gb mod p
3. A outputs Kab = Kba
4. B outputs Kba = Kab
r Note Kab = Kba = gab mod p
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Given (g, ga) hard to
compute a – Discrete
Logarithm Assumption
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Security of DH key exchange
r No authentication of either party
r Secure only against a passive adversarym Under the computational Diffie-Hellman assumption • Given (g, ga,gb), hard to compute gab
r Not secure against an active attackerm Man-in-the-middle attack…
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Authenticated DH Key Exchange
1. A � B: Ka = ga mod p
2. B � A: Certb, Kb = gb mod p SigSKb(Kb, Ka )
3. A � B: Certa, SigSKa(Ka,Kb)
4. A outputs Kab = Kba
5. B outputs Kba = Kab
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IKE phase 1:
The Identity Protection Exchange consists of 6 messages:
Messages (1) and (2): Peers negotiate algorithms to be used for
establishment of the secure channel between security gateways
Messages (3) and (4) generate the keys to be used
Messages (5) and (6) authenticate the peers.
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IKE Phase 2
We could go through the six-message exchange again, selecting
algorithms, producing totally new keys and checking authentication.
Instead, use Quick Mode, which takes just three messages.
Quick Mode accepts the choice of algorithms made in phase1.
It also accepts the DH key generated in phase 1, but hashes it
to make “new” keys (“new keys from old”)
End of Handshake.
IKE and IPSec Security Associations established.
We can proceed to transmit user data.
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Summary of IPsec
r IKE message exchange for algorithms, secret keys, SPI numbers
r Either the AH or the ESP protocol (or both)
r The AH protocol provides integrity and source authentication
r The ESP protocol (with AH) additionally provides encryption
r IPsec peers can be two end systems, two routers/firewalls, or a router/firewall and an end system
Further Reading
r Stallings Chapter 9
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