Network Security: IPsec. Tuomas Aura T Network security Aalto University, Nov- Dec 2012

Transcription

1 Network Security: IPsec Tuomas Aura T Network security Aalto University, Nov- Dec 2012

2 2 IPsec: Architecture and protocols

3 Internet protocol security (IPsec)! Network- layer security protocol! Protects IP packets between two hosts or gateways! Transparent to transport layer and applicafons; security policy defined and enforced on network level! IP addresses used to as host idenffiers! Two steps: 1. IKE authenfcated key exchange creates security associafons 2. ESP session protocol protects data! Specified by Internet Engineering Task Force (IETF)! Original goal: encrypfon and authenfcafon layer that will replace all others! Security (IPsec) was a sales point for IPv6, but it now works just as well for IPv4 3

4 IPsec architecture [RFC4301] Node A Untrusted network Node B PAD IKE(v2) Session Key 1. Key exchange Session IKE(v2) Key PAD SPD IKE SA SPD Security Policy Database IPSec IPsec SA Pair IPSec Security Policy Database SAD Security Association Database 2. ESP protects data Security Association Database SAD! Security associafons (SA) created by IKE, used by IPsec ESP! Security policy guides SA creafon and selecfon for use! IPsec is part of the IP layer in the OS kernel; IKE is a user- space service (daemon) 4

5 Internet Key Exchange (IKE)! IKE(v1) [RFC 2407, 2408, 2409]! Framework for authenfcated key- exchange protocols, typically with Diffie- Hellman! MulFple authenfcafon methods: cerfficates, pre- shared key, Kerberos! Two phases: Main Mode (MM) creates an ISAKMP SA (i.e. IKE SA) and Quick Mode (QM) creates IPsec SAs! Main mode (idenfty- protecfon mode) and opfmized aggressive mode! Interoperability problems: too complex to implement and test all modes; specificafon incomplete! IKEv2 [RFC 5996]! Redesign of IKE: fewer modes and messages, simpler to implement! IniFal exchanges create the IKE SA and the first IPsec SA pair! CREATE_CHILD_SA exchange can create further IPsec SAs! EAP authenfcafon for extensions! Works over UDP port 500 5

6 Internet Key Exchange (IKEv2) IniFal exchanges: 1. I R: HDR(A,0), SAi1, KEi, Ni 2. R I: HDR(A,B), SAr1, KEr, Nr, [CERTREQ] 3. I R: HDR(A,B), SK { IDi, [CERT,] [CERTREQ,] [IDr,] AUTHi, SAi2, TSi, TSr } 4. R I: HDR(A,B), SK { IDr, [CERT,] AUTHr, SAr2, TSi, TSr } A, B = SPI values that idenfty the protocol run and the created IKE SA Nx = nonces SAx1 = offered and chosen algorithms, DH group KEx = Diffie- Hellman public key (g x or g y ) IDx, CERT = idenfty, cerfficate AUTHi = Sign I (Message 1, Nr, h(sk, IDi)) AUTHr = Sign R (Message 2, Ni, h(sk, IDr)) SK = h(ni, Nr, g xy ) a bit simplified, 6 different keys are derived from this SK { } = E SK (, MAC SK ( )) MAC and encrypt with session key SAx2, TSx = parameters for the first IPsec SA (algorithms, SPIs, traffic selectors) CERTREQ = accepted root CAs (or other trust roots)

7 Internet Key Exchange (IKEv2) IniFal exchanges: 1. I R: HDR(A,0), SAi1, KEi, Ni 2. R I: HDR(A,B), SAr1, KEr, Nr, [CERTREQ] 3. I R: HDR(A,B), SK { IDi, [CERT,] [CERTREQ,] [IDr,] AUTHi, SAi2, TSi, TSr } 4. R I: HDR(A,B), SK { IDr, [CERT,] AUTHr, SAr2, TSi, TSr } A, B = SPI values that idenfty the protocol run and the created IKE SA Nx = nonces SAx1 = offered and chosen algorithms, DH Secret, group fresh session key? KEx = Diffie- Hellman public key (g x or g y ) Mutual authenfcafon? EnFty authenfcafon and IDx, CERT = idenfty, cerfficate key confirmafon? AUTHi = Sign I (Message 1, Nr, h(sk, IDi)) Contributory? AUTHr = Sign R (Message 2, Ni, h(sk, IDr)) Perfect forward secrecy? SK = h(ni, Nr, g xy ) a bit simplified, 6 keys Integrity are derived check from for inifal this negofafon? SK { } = E SK (, MAC SK ( )) MAC and encrypt Non- repudiafon or plausible deniability? SAx2, TSx = parameters for the first IPsec SA IdenFty (algorithms, protecfon? SPIs, traffic selectors) CERTREQ = accepted root CAs (or other trust roots)

8 IKEv2 with a cookie exchange! In the second message, responder may send a cookie (a nonce)! Goal: prevent DOS aoacks from a spoofed IP address 1. I R: HDR(A,0), SAi1, KEi, Ni 2. R I: HDR(A,0), N(COOKIE) // R stores no state 3. I R: HDR(A,0), N(COOKIE), SAi1, KEi, Ni 4. R I: HDR(A,B), SAr1, KEr, Nr, [CERTREQ] // R creates a state 5. I R: HDR(A,B), SK{ IDi, [CERT,] [CERTREQ,] [IDr,] AUTH, SAi2, TSi, TSr } 6. R I: HDR(A,B), E SK (IDr, [CERT,] AUTH, SAr2, TSi, TSr) How to bake a good cookie? For example: COOKIE = h(n R- periodic, IP addr of I, IP addr of R) where N R- periodic is a periodically changing secret random value know only by the responder R

9 Security AssociaJons (SA)! One IKE SA for each pair of nodes! Stores the master key SK = h(ni, Nr, g xy ) for creafng IPsec SAs! At least one IPsec SA pair for each pair of nodes! Stores the negofated session protocol, encrypfon and authenfcafon algorithms, keys and other session parameters! Stores the encrypfon algorithm state! IPsec SAs always come in pairs, one in each direcfon! SAs are idenffied by a 32- bit security parameter index (SPI) [RFC4301]! The desfnafon node selects the SPI value! Node stores SAs in a security associafon database (SAD) 9

10 Session protocol Encapsulated Security Payload (ESP) [RFC 4303] EncrypFon and/or MAC for each packet OpFonal replay prevenfon with sequence numbers Protects the IP payload (= transport- layer PDU) only ESP with encrypfon only is insecure! Old protocol: AuthenFcaFon Header (AH) Do not use for new applicafons AuthenFcaFon only, no encrypfon Protects payload and some IP header fields 10

11 Session protocol modes! Transport mode:! Host- to- host security! ESP header added between the original IP header and payload! Tunnel mode:! Typically used for tunnels between security gateways to create a VPN! EnFre original IP packet encapsulated in a new IP header plus ESP header! In pracfce, IPsec is mainly used in tunnel mode! Proposed BEET mode:! Like tunnel mode but inner IP header not sent explicitly! Transport- mode headers but tunnel mode semanfcs 11

12 Session protocol modes Transport mode EncrypFon and/or authenfcafon from end host to end host Network Encrypted Tunnel mode EncrypFon and/or authenfcafon between two gateways (VPN) Intranet IPsec gateway Internet IPsec gateway Intranet Encrypted 12

13 Using tunnel mode with hosts Tunnel mode - between end hosts (security equivalent to transport mode) Network Tunnel mode - between a host and a gateway (typical VPN connecfon) Untrusted access network Internet IPsec gateway Intranet 13

14 Nested protecjon Nested tunnel and transport mode (less common but possible) IPsec gateway Internet IPsec gateway Intranet Intranet Internet IPsec gateway Untrusted access network Intranet 14

15 ESP packet format Original packet: IP header IP Payload ESP in transport mode: Original Original ESP header and trailer = SPI + Sequence number + Padding ESP authenfcafon trailer = message authenfcafon code (MAC) IP header ESP header IP Payload ESP trailer Auth trailer ESP in tunnel mode: Encrypted AuthenFcated Original! IP header ESP header IP header IP Payload ESP trailer Auth trailer Encrypted AuthenFcated 15

16 ESP packet format (2)! ESP packets in a more abstract notafon:! Transport mode headers: IP(src host, dst host) ESP payload! Tunnel mode headers: IP(src gw, dst gw) ESP IP(src host,dst host) payload 16

17 IPsec databases! Security associafon database (SAD)! Contains the IPsec SAs i.e.t the dynamic protecfon state! Security policy database (SPD)! Contains the stafc security policy! Usually set by system administrators (e.g. Windows group policy), although some protocols and applicafons make dynamic changes! Peer authorizafon database (PAD)! Needed in IKE for mapping between authenfcated names and IP addresses! Conceptual; not implemented as an actual database! AddiFonally, the IKE service/daemon stores IKE SAs:! Master secret created with Diffie- Hellman key exchange! Used for instanfafng new IPsec Sas (Note: our descripfon of SPD differs somewhat from RFC4301 and is probably closer to most implementafons) 17

18 Gateway SPD/SAD example SPD of gateway A, interface 2 Protocol Local IP Port Remote IP Port AcFon Comment UDP BYPASS IKE * /24 * /24 * ESP tunnel to Protect VPN traffic * * * * * BYPASS All other peers SAD of gateway A SPI SPD selector values Protocol Algorithms, keys, algorithm state spi1 UDP, /24, /24 ESP tunnel from spi2 ESP tunnel to Intranet /24 Internet Intranet /24 interface1 interface2 interface1 interface IPsec gateway A IPsec gateway B 18

19 Security policy database (SPD)! Specifies the stafc security policy! MulF- homed nodes have a separate SPD for each network interface (in pracfce, they can be stored on one database) (mulfhomed = node with mulfple network interfaces)! Policy maps inbound and outbound packets to acfons! SPD = linearly ordered list of policies! Policy = selectors + acfon! The first policy with matching selectors applies to each packet! Policy selector is a 5- tuple:! Local and remote IP address! Transport protocol (TCP, UDP, ICMP)! Source and desfnafon ports! AcFons: BYPASS (allow), DISCARD (block), or PROTECT! PROTECT specifies also the session protocol and algorithms! Packet is mapped to a suitable IPsec security associafon (SA)! If the SA does not exist, IKE is triggered to create them! SPD stores pointers to previously created SAs! Policies at peer nodes must match if they are to communicate 19

20 Security associajon database (SAD)! Contains the dynamic encrypfon and authenfcafon state! IPsec SAs always come in pairs: inbound and outbound! SAD is keyed by SPI (for unicast packets)! SAs are typically created by IKE but they may also be configured manually or by other soxware, e.g. to create fixed VPN tunnels! Each SAD entry remembers also the policy selector values that were used when creafng it 20

21 Gateway SPD/SAD example SPD of gateway A, interface 2 Protocol Local IP Port Remote IP Port AcFon Comment UDP BYPASS IKE * /24 * /24 * ESP tunnel to Protect VPN traffic * * * * * BYPASS All other peers SAD of gateway A SPI SPD selector values Protocol Algorithms, keys, algorithm state spi1 UDP, /24, /24 ESP tunnel from spi2 ESP tunnel to Intranet /24 Internet Intranet /24 interface1 interface2 interface1 interface IPsec gateway A IPsec gateway B 21

22 Host SPD example! SPD for host in intranet /24, connecfng to server in network /24 (DMZ) and to the Internet Protocol Local IP Port Remote IP Port AcFon Comment UDP * 500 BYPASS IKE ICMP * * * BYPASS Error messages * * /24 * PROTECT: ESP in transport- mode TCP * PROTECT: ESP in transport- mode Encrypt intranet traffic Encrypt to server TCP BYPASS Allow TLS, no double encrypfon * * /24 * DISCARD Others in DMZ * * * * BYPASS Internet! What is the danger of bypassing TLS traffic (line 5)?! What is the danger of bypassing outbound ICMP (line 2)?! Note that the other endpoint (other intranet hosts and ) must have an IPsec policy that specifies the same protecfon for the same packets 22

23 IPsec policy implementajon differences! Historically, IPsec and firewalls have different models of the network:! Firewall is a packet filter: which packets to drop and which to allow?! IPsec sits between the secure and insecure areas (host and network at IPsec hosts, intranet and Internet at IPsec gateways) and encrypts packets that leave the secure side Firewall and IPsec policies can, however, be unified! In some IPsec implementafons, the policy is specified in terms of source and desfnafon addresses (like a typical firewall policy), instead of local and remote addresses mirror flag is shorthand notafon to indicates that the policy applies also with the source and desfnafon reversed Mirror Protocol Source IP Port DesFnaFon IP Port AcFon Comment yes UDP BYPASS IKE yes * /24 * /24 * ESP tunnel to Protect VPN traffic yes * * * * * BYPASS All other peers 23

24 Outbound packet processing! Processing outbound packets in IPsec: 1. For each outbound packet, IPsec finds the first matching policy in the security policy database (SDP) 2. If the policy requires protecfon, IPsec maps the packet to the right security associafon (SA) in the SA database (SAD) 3. If no SA exists, IPsec invokes the IKE service (running in user space) to create a new SA pair 4. While waifng for the IPsec SA, at most one outbound packet (oxen TCP SYN) is buffered in the kernel 5. When the SA exists, the packet is encrypted and a MAC added 24

25 Inbound packet processing! Processing inbound IPsec packets: 1. IPsec looks up the inbound SA in SAD based on the SPI 2. IPsec processes the packet with the SA, i.e. verifies the MAC and decrypts 3. IPsec compares the packet with the selector values that were used when creafng this SAD entry. For tunnel- mode packets, the comparison is done with the inner IP header! Processing of inbound non- IPsec packets:! IPsec finds the first matching policy in the SPD and checks that the acfon is BYPASS! If the acfon is not BYPASS, the packet is dropped! In Windows, it is possible to allow the first inbound packet (oxen TCP SYN) to bypass IPsec. The outbound response will trigger IKE! Helps in gradual deployment of host- to- host IPsec 25

26 Some problems with IPsec

27 ! Problems: IPsec and NAT! NAT cannot mulfplex IPsec: impossible to modify SPI or port number because they are authenfcated Host behind a NAT could not use IPsec! NAT traversal (NAT- T):! UDP- encapsulated ESP (port 4500)! NAT detecfon: extension of IKEv1 and IKEv2 for sending the original source address in inifal packets Enables host behind a NAT to use IPsec 27

28 IPsec and mobility! Problem: IPsec policies and SAs are bound to IP addresses. Mobile nodes change their IP address! Mobile IPv6 helps: home address (HoA) is stable. But Mobile IPv6 itself depends on IPsec for the tunnel between HA and MN. Chicken- and- egg problem! SoluFons:! IPsec was changed to look up inbound SAs by SPI only! IPsec- based VPNs from mobile hosts do not use the IP address as selector. Instead, proprietary solufons! MOBIKE mobility protocol 28

29 IPsec and IdenJfiers! ApplicaFon opens a connecfon to an IP address. IPsec uses the IP addresses as policy selector! IKE usually authenfcates the remote node by its DNS name! Problem: No secure mapping between the two idenffier spaces: DNS names and IP addresses 29

30 IPsec and name resolujon PC-A Application IP Network 1. Name resolution OS IPsec Policy: 1.2.*.* ESP * BYPASS 3. IPsec Protection Application Data 2. Key Exchange (IKE) Server-B Response: Query: server-b Name service! Typical TCP socket API use: resolve name into an IP address; then connect to the address! TCP SYN to the address triggers IKE (if the ESP SA does not exist yet) 30

31 Classic IPsec/DNS Vulnerability Honest host 1. Name resolution Application OS IPsec Policy 1.2.*.* ESP * BYPASS 3. IPsec Protection Application Data 2. Key Exchange (IKE) Attacker pc-c.example.org Spoofed Response: Query: server-b.example.org! IPsec policy selecfon depends on secure name resolufon 31

32 server-b. example.org IPsec and CerJficates Honest host Application OS Connect( ) 2. Key Exchange (IKE) 1. Name resolution = server-b? Certificate: { server-b.example.org, PublicKey C }CA Query: server-b. example.org Response: Name service! Let s assume DNS is secure! Another problem: IKE knows the peer IP address, not the peer name; the cerfficate only contains the name à How does IPsec decide whether the cerfficate is ok? 32

33 PC-A Application 1. Name resolution OS IPsec Policy: * ESP 3. IPsec Protection Application Data 2. Key Exchange (IKE) Certificate: { server-b, PublicKey B }CA Server-B Response: Query: server-b Name service! What can go wrong? 33

34 IPsec and CerJficates - AYack PC-A 1. Name resolution Application OS IPsec Policy: * ESP 3. IPsec Protection Application Data 2. Key Exchange (IKE) Certificate: { pc-c, PublicKey C }CA Attacker PC-C (using ) Response: Query: server-b Name service! IPsec cannot detect whether the cerfficate contains the correct name! Secure DNS (forward lookup) does not help why?! Result: group authenfcafon of those cerffied by the same CA maybe ok for protecfng an intranet where the goal is to keep outsiders out 34

35 Peer authorizajon database (PAD)! IPsec spec [RFC4301] defines a database that maps authenfcated names to the IP addresses which they are allowed to represent! How implemented? Secure reverse DNS would be the best solufon but it does not exist! Other solufons:! Secure DNS both secure forward and reverse lookup needed, which is unrealisfc! Give up IPsec transparency extend the socket API so that applicafons can query for the authenfcated name and other security state! Connect by name change the socket API so that the connect() call specifies the host name, not the IP address! Currents situafon: IPsec is only used for VPN where the gateway names and IP addresses are preconfigured 35

36 Related reading! William Stallings. Network security essenfals: applicafons and standards, 3rd ed. chapter 6; 4th ed. chapters 8, 11! William Stallings. Cryptography and Network Security, 4th ed.: chapter 16! Kaufmann, Perlman, Speciner. Network security, 2nd ed.: chapter 17 (ignore AH)! Note: chapter 18 on the older IKEv1 is historical! Dieter Gollmann. Computer Security, 2nd ed. chapter 13.3; 3rd ed. chapters ,

37 Exercises! Why is IPsec used mainly for VPN implementafons? Does IPsec VPN suffer from any of the problems menfoned in the lecture?! For the IPsec policy examples of this lecture, define the IPsec policy for the peer nodes i.e. the other ends of the connecfons.! Try to configure the IPsec policy between two computers. What difficulfes did you meet? Use ping and TCP to test connecfvity. Use a network sniffer to observe the key exchange and to check that packets on the wire are encrypted.! What administrafve problems arise from the fact that IPsec security policies in two communicafng nodes must match? How is this solved in Windows?! RFC 4301 requires that the SPD is decorrelated, i.e. that the selectors of policy entries not to overlap, i.e. that any IP packet will match at most one rule (excluding the default rule which matches all packet). Yet, the policies created by system administrators almost always have overlapping entries. Device an algorithm for transforming any IPsec policy to an equivalent decorrelated policy. (Real protocol stacks do not implement decorrelafon. Why?)! Each SAD entry stores (caches) policy selector values from the policy that was used when creafng it. Inbound packets are compared against these selectors to check that the packet arrives on the correct SA.! What security problem would arise without this check?! What security weakness does the caching have (compared to a lookup through the SPD)?! Does policy decorrelafon solve the problem? 37