Network Security: IPsec. Tuomas Aura T Network security Aalto University, Nov-Dec 2014
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1 Network Security: IPsec Tuomas Aura T Network security Aalto University, Nov-Dec 2014
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 applications; security policy defined and enforced on network level IP addresses used to as host identifiers Two steps: 1. IKE authenticated key exchange creates security associations 2. ESP session protocol protects data Specified by Internet Engineering Task Force (IETF) Original goal: encryption and authentication 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 associations (SA) created by IKE, used by IPsec ESP Security policy guides SA creation and selection 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 authenticated key-exchange protocols, typically with Diffie-Hellman Multiple authentication methods: certificates, 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 (identity-protection mode) and optimized aggressive mode Interoperability problems: too complex to implement and test all modes; specification incomplete IKEv2 [RFC 5996] Redesign of IKE: fewer modes and messages, simpler to implement Initial exchanges create the IKE SA and the first IPsec SA pair CREATE_CHILD_SA exchange can create further IPsec SAs EAP authentication for extensions Works over UDP port 500 5
6 Internet Key Exchange (IKEv2) Initial exchanges (using the notation from the IPsec RFC): 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 identity the protocol run and the created IKE SA Nx = nonces SAx1 = offered and chosen algorithms, DH group (p and g) KEx = Diffie-Hellman public key (g x or g y ) CERTREQ = accepted root CAs (or other trust roots) IDx, CERT = identity, certificate AUTHi = Sign I (Message 1, Nr, h(sk, IDi)) AUTHr = Sign R (Message 2, Ni, h(sk, IDr)) SK = h(ni, Nr, g xy ) actually, 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)
7 Internet Key Exchange (IKEv2) The same protocol using our Alice-and-Bob notation: 1. I R: SPI i, SPI r, SA i1, g x, N i 2. R I: SPI i, SPI r, SA r1, g y, N r, CERTREQ r 3. I R: SPI i, SPI r, E SK (ID i, CERT i, CERTREQ i, ID r, Sign i (Message 1, N r, h(sk, ID i )), SA i2, TS i, TS r, MAC SK ( )) 4. R I: SPI i, SPI r, E SK (ID r, CERT r, Sign R (Message 2, N i, h(sk, ID r )), SA r2, TS i, TS r, MAC SK ( )) SPI x = values that identity the protocol run and the created IKE SA SA x1 = offered and chosen algorithms, DH group (p and g) CERTREQ x = accepted root CAs (or other trust roots) ID x, CERT x = identity, certificate SK = h(ni, Nr, g xy ) actually, 6 different keys are derived from this SA x2, TS x = parameters for the first IPsec SA (algorithms, SPIs, traffic selectors)
8 Internet Key Exchange (IKEv2) Initial 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 identity the protocol run and the created IKE SA Nx = nonces SAx1 = offered and chosen algorithms, DH group Secret, (p and fresh g) session key? KEx = Diffie-Hellman public key (g x or g y ) Mutual authentication? CERTREQ = accepted root CAs (or other trust roots) Entity authentication and IDx, CERT = identity, certificate key confirmation? 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 ) actually, 6 different keys are Integrity derived check from this for initial negotiation? SK { } = E SK (, MAC SK ( )) MAC and encrypt Non-repudiation with session keyor plausible deniability? SAx2, TSx = parameters for the first IPsec SA (algorithms, Identity protection? SPIs, traffic selectors)
9 IKEv2 with a cookie exchange In the second message, responder may send a cookie (a nonce) Goal: prevent DOS attacks 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
10 Security Associations (SA) One IKE SA for each pair of nodes Stores the master key SK = h(ni, Nr, g xy ) for creating IPsec SAs At least one IPsec SA pair for each pair of nodes Stores the negotiated session protocol, encryption and authentication algorithms, keys and other session parameters Stores the encryption algorithm state IPsec SAs always come in pairs, one in each direction IPsec SAs are identified by a 32-bit security parameter index (SPI) [RFC4301] The destination node selects the SPI value Node stores IPsec SAs in a security association database (SAD) 10
11 Session protocol Encapsulated Security Payload (ESP) [RFC 4303] Encryption and/or MAC for each packet Optional replay prevention with sequence numbers Protects the IP payload (= transport-layer PDU) only ESP with encryption only is insecure! Old protocol: Authentication Header (AH) Do not use for new applications Authentication only, no encryption Protects payload and some IP header fields 11
12 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 Entire original IP packet encapsulated in a new IP header plus ESP header In practice, 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 semantics 12
13 Session protocol modes Transport mode Encryption and/or authentication from end host to end host Network Encrypted Tunnel mode Encryption and/or authentication between two gateways (VPN) Intranet IPsec gateway Internet IPsec gateway Intranet Encrypted 13
14 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 connection) Untrusted access network Internet IPsec gateway Intranet 14
15 Nested protection Nested tunnel and transport mode (less common but possible) IPsec gateway Internet IPsec gateway Intranet Intranet Internet IPsec gateway Untrusted access network Intranet 15
16 ESP packet format Original packet: IP header IP Payload ESP in transport mode: Original Original ESP header and trailer = SPI + Sequence number + Padding ESP authentication trailer = message authentication code (MAC) IP header ESP header IP Payload ESP trailer Auth trailer ESP in tunnel mode: Encrypted Authenticated Original! IP header ESP header IP header IP Payload ESP trailer Auth trailer Encrypted Authenticated 16
17 ESP packet format (2) ESP packets in a more abstract notation: 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 17
18 IPsec databases Security association database (SAD) Contains the IPsec SAs i.e.t the dynamic protection state Security policy database (SPD) Contains the static security policy Usually set by system administrators (e.g. Windows group policy), although some protocols and applications make dynamic changes Peer authorization database (PAD) Needed in IKE for mapping between authenticated names and IP addresses Conceptual; not implemented as an actual database Additionally, the IKE service/daemon stores IKE SAs: Master secret created with Diffie-Hellman key exchange Used for instantiating new IPsec Sas (Note: our description of SPD differs somewhat from RFC4301 and is probably closer to most implementations) 18
19 Gateway SPD/SAD example SPD of gateway A, interface 2 Protocol Local IP Port Remote IP Port Action Comment UDP BYPASS IKE * /24 * /24 * ESP tunnel to Protect VPN traffic * * * * * BYPASS All other peers Pointers to created associations SAD of gateway A SPI SPD selector values Protocol Algorithms, keys, algorithm state spi1 TCP, /24, /24 ESP tunnel from spi2 ESP tunnel to Intranet /24 Internet Intranet /24 interface1 interface2 interface1 interface IPsec gateway A IPsec gateway B 19
20 Security policy database (SPD) Specifies the static security policy Multi-homed nodes have a separate SPD for each network interface (in practice, they can be stored on one database) (multihomed = node with multiple network interfaces) Policy maps inbound and outbound packets to actions SPD = linearly ordered list of policies Policy = selectors + action 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 destination ports Actions: BYPASS (allow), DISCARD (block), or PROTECT PROTECT specifies also the session protocol and algorithms Packet is mapped to a suitable IPsec security association (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 20
21 Security association database (SAD) Contains the dynamic encryption and authentication 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 software, e.g. to create fixed VPN tunnels Each SAD entry remembers also the policy selector values that were used when creating it 21
22 Host SPD example SPD for host in intranet /24, connecting to server in network /24 (DMZ) and to the Internet Protocol Local IP Port Remote IP Port Action 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 encryption * * /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 protection for the same packets 22
23 IPsec policy implementation 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 implementations, the policy is specified in terms of source and destination addresses (like a typical firewall policy), instead of local and remote addresses mirror flag is shorthand notation to indicates that the policy applies also with the source and destination reversed Mirror Protocol Source IP Port Destination IP Port Action 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 protection, IPsec maps the packet to the right security association (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 waiting for the IPsec SA, at most one outbound packet (often 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 creating 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 action is BYPASS If the action is not BYPASS, the packet is dropped In Windows, it is possible to allow the first inbound packet (often 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 multiplex IPsec: impossible to modify SPI or port number because they are authenticated Host behind a NAT could not use IPsec NAT traversal (NAT-T): UDP-encapsulated ESP (port 4500) NAT detection: extension of IKEv1 and IKEv2 for sending the original source address in initial 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: Home address (HoA) in Mobile IP v6 is stable. But Mobile IPv6 itself depends on IPsec for the tunnel between HA and MN. chicken-and-egg situation Solutions: 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 solutions Proposed MOBIKE and HIP mobility protocols intergrate IPsec and mobility 28
29 IPsec and identifiers Application opens a connection to an IP address. IPsec uses the IP addresses as policy selector IKE usually authenticates the remote node by its DNS name Problem: No secure mapping between the two identifier spaces: DNS names and IP addresses 29
30 IPsec and name resolution 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 selection depends on secure name resolution Attacker spoofs DNS response to circumvent the IPsec policy 31
32 IPsec and Certificates server-b. example.org 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 certificate only contains the name How does IPsec decide whether the certificate 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 Certificates - Attack 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 certificate contains the correct name Secure DNS (forward lookup) does not help why? Result: group authentication of those certified by the same CA maybe ok for protecting an intranet where the goal is to keep outsiders out 34
35 Peer authorization database (PAD) IPsec spec [RFC4301] defines a database that maps authenticated names to the IP addresses which they are allowed to represent How implemented? Secure reverse DNS would be the best solution but it does not exist Other solutions: Secure DNS both secure forward and reverse lookup needed, which is unrealistic Give up IPsec transparency extend the socket API so that applications can query for the authenticated 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 situation: IPsec is only used for VPN where the gateway names and IP addresses are preconfigured 35
36 Related reading William Stallings. Network security essentials: applications 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 implementations? Does IPsec VPN suffer from any of the problems mentioned 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 connections. Try to configure the IPsec policy between two computers. What difficulties did you meet? Use ping and TCP to test connectivity. Use a network sniffer to observe the key exchange and to check that packets on the wire are encrypted. What administrative problems arise from the fact that IPsec security policies in two communicating 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 decorrelation. Why?) Each SAD entry stores (caches) policy selector values from the policy that was used when creating 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 decorrelation solve the problem? 37
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