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+------------------------------------------------------+ ▌ Oblivion Underground Magazine - Issue 1 - 15/03/2000 ▌ ▌ IP Security Architecture by Slider ▌ ▌ E-Mail : Slider_100@hotmail.com ▌ +------------------------------------------------------+ IP SECURITY ARCHITECTURE (IPSec) This text examines in detail the IPSec framework and its three main components, Authentication header (AH), Encapsulating Security Payload (ESP), and Internet Key Exchange (IKE). The header formats, the specific cryptograhic features and the different modes of application are modes of application are discussed. IPSec was designed for interoperability. When correctly implemented, it does not affect networks and hosts that do not support it. IPSec is independent of the current cryptogaphic algorithums; it can accomodate new ones as they become avaliable. It works both with IPv4 and IPv6. Actually IPSec is a mandatory component of IPv6. IPSec uses state-of-the-art cryptographic algorithums. The specific implementation of an algorithum for use by an IPSec protocol is often called a transform. For example, the DES algorithm used in ESP is called the ESP DES-CBC transform. The transforms, as the protocols, are published in RFCs and in internet drafts. 1.1 Concepts! Two major IPSec concepts should be clarified before entering the details: - The Security Associations and the tunneling. In fact they are not new; IPSec just makes use of them. - The concept of a Security Association (SA) is fundamental to IPSec. An SA is an unidirectional (Simplex) logical connection between two IPSec systems, uniquely identified by the following triple : <Security Parameter Index, IP Destination Adress, Security Protocol> The definition of the members is as follows: 1.1.1 Security Parameter Index (SPI) This is a 32-bit value used to identifiy different SAs with the same destination address and security protocol. The SPI is carried in the header of the security protocol. IP Destination Address This address can be unicast, broadcast of Multicast adress. However, currently SA management mechanisms are defined only for unicast adresses. Security Protocol This can be either AH or ESP. An SA can be in either of two modes: transport or tunnel, depending on the mode of the protocol in that SA. Because SAs are simplex, for bidirectional communication between two IPSec systems, there must be two SAs defined, one in each direction. An SA gives security services to the traffic carried by it either by using AH or ESP, but not both. In other words, for a connection that should be protected by both AH and ESP, two SAs must be defined for each direction. In this case, the set of SAs that define the connection is referred to as an SA bundle. The SAs in the bundle do not have to terminate at the same endpoint. For, example a mobile host could use an AH SA between itself and a firewall and a nested ESP SA that extends to a host behind the firewall. An IPSec implementation maintains two databases related to SAs: Security Policy Database (SPD) The security policy database specifies what security services are to be offered to the IP traffic, depending factors such as source, destination, whether it is inbound and/or outbound traffic. These entries might specifiy that some traffic must not go through IPSec processing, some must be discarded and the rest must be processed by the IPSec module. Entries in this database are similar to the firewall rules or packet filters, these were described in last months KRASHmag issue. Security Association Database (SAD) The Security Association Database contains parameter information about each SA, such as AH or ESP algorithms and keys, sequence numbers, protocol mode and SA lifetime. For outbound processing, an SPD entry points to an entry in the SAD. That is, the SPD determines which SA is to be used for a given packet. For inbound processing, the SAD is consulted to determine how the packet must be processed. 1.1.2 Tunneling Tunneling or encapsulation is a common technique in packet-switched networks. It can consists of wrapping a packet in a new one. That is, a new header is attached to the orginal packet. The entire original packet becomes the payload of the new one. In general tunneling is used to carry traffic of one protocol over a network that does not support that protocol directly. For example, NETBIOS or IPX can be encapsulated in IP to carry it over a TCP/IP WAN link! In the case of IPSec, IP is tunneled through IP for a slightly different purpose: to provide total protection , including the header of the encapsulated packet. If the encapsulated packet is encrypted, an intruder cannot figure out for example the destination address of that packet. The internal structure of a private network can be concealed in this way. Tunneling requires intermediate processing of the orginal packet on its route. The destination specified in the outer header, usually an IPSec firewall or router, retrieves the orginal packet and sends it to the ultimate destination. The processing overhead is compensated by the extra security. A notable advantage of IP tunneling is the possibility to exchange packets with private IP addresses between two intranets over the public internet, which requres globally unique addresses. Since, the encapsulated header is not processed by the internet routers, only the endpoints of the tunnel have to have globally assigned adresses; the hosts in the intranets behind them can be assigned private adresses. IPSec tunneling is modeled after RFC 2003 "IP Encapsulation within IP" I will be covering Virtual Private Networks (VPN's) in more detail in the next issue of KrashMag. 1.2 Authentication Header (AH) AH is used to provide integrity and authentication to IP datagrams. Optional replay protection is also possible. Although its usage is optional, the replay protection service must be implemented by any IPSec-compliant systems. The mentioned services are connectionless; that is they work on a per-packet basis. AH authenticates as much of the IP datagram as possible. Some fields in the IP header change en-route and their value cannot be predicted by the reciever. These fileds are called mutable and are not protected by AH. The mutable IPv4 fields are : - Type of Service (TOS) - Flags - Fragment Offset - Time To Live (TTL) - Header Checksum When protection of these fields is required, tunneling should be used. The payload of the IP packet is considered immutable and is always protected by AH. However, an IP packet with AH applied can be fragmented by intermediate routers. In this case the destination first reassembles the packet and then applies AH processing to it. If an IP packet that appears to be fragment is input to the AH processing, it is then discarded. AH processing is applied only to non-fragment IP packets. This prevents the so-called "overlapping fragment attack", which misuses the fragment reassembly algoritm in order to create forged packets and force them through a firewall. Packets that failed authentication are discarded and never delivered to upper layers. This mode of operation greatly reduces the chances of successful Denial of Service (DoS) attacks, which aim to block the communication of a host or gateway by flooding it with bogus packets. 1.2.1 Ways Of Using AH AH can be used in two ways : transport mode or tunnel mode. AH in Transport Mode : In this mode the orginal IP datagram is taken and the AH header is inserted right after the IP header. If the datagram already has IPSec header(s),then the AH header is inserted before any of those. The transport mode is used by hosts, not by gateways. Gateways are not even required to support transport mode. The advantage of the transport mode is less processing overhead. The desadvantage is that the mutable fields are not authenticated. AH in Tunnel Mode : With this mode the tunneling concept is applied a new IP datagram is constructed and the orginal IP datagram is made the payload of it. Then AH in transport mode is applied to the resulting datagram. The tunnel mode is used whenever either end of a security association is a gateway. Thus, between two firewalls the tunnel mode is always used. Although gateways are supposed to support tunnel mode only, often they can also work in transport mode. This mode is allowed when the gateway acts as a host, that is in cases when traffic is destined to itself. Examples are SNMP commands or ICMP echo requests. In tunnel mode the outer headers' IP addresses does not need to be the same as the inner headers' addresses. For example two security gateways can operate an AH tunnel which is used to authenticate all traffic between the networks they connect together. This is a very typical mode of operation. Hosts are not required to support tunnel mode, but often they do. The advantages of the tunnel mode are total protection of the encapsulated IP datagram and the possibility of using private addresses. However, there is an extra processing overhead associated with this mode. 1.2.2 Encapsulating Security Payload (ESP) ESP is used to provide integrity check, authentication and encryption to IP datagrams. Optional replay protection is also possible. These services are connectionless; they operate on a per-packet basis. The set of desired services are selectable upon SA establishment. However, some restrictions apply: - Integrity check and authentication go together - Replay protection is selectable only with integrity check and authentication - Replay protection can be selected only by the receiver Encryption is selectable independent of the other services. It is highly recommended that if encryption is enabled, then integrity check and authentication be turned on. If only encryption is used, intruders could forge packets in order to mount cryptanalytic attacks. This is infeasible when integrity check and authentication are in place. Although both authentication (with integrity check) and encryption are optional, at least one of them is always selected. Otherwise it really does not make sense to use ESP at all. ESP is identified by protocol number 50, assigned by the IANA. The protocol header (IPv4, IPv6, or Extension) immediately preceding the AH header will contain this value in its Protocol (IPv4) or Next Header (IPv6, Extension) field. ESP processing is applied only to non-fragmented IP packets. However an IP packet with ESP applied can be fragmented by intermediate routers. In this case the destination first reassembles the packet and then applies ESP processing to it. If an IP packet that appears to be a fragment (offset field is non-zero, or the More Fragments bit is set) is input to ESP processing, it is discarded. This prevents the overlapping fragment attack . 1.2.3 Ways of Using ESP Like AH, ESP can be used in two ways: transport mode and tunnel mode. ESP in Transport Mode: In this mode the original IP datagram is taken and the ESP header is inserted right after the IP header. If the datagram already has IPSec header(s), then the ESP header is inserted before any of those. The ESP trailer and the optional authentication data are appended to the payload. ESP in transport mode provides neither authentication nor encryption for the IP header. This is a disadvantage, since false packets might be delivered for ESP processing. The advantage of transport mode is the lower processing overhead. As in the case of AH, ESP in transport mode is used by hosts, not gateways. Gateways are not even required to support transport mode. ESP in Tunnel Mode: As expected, this mode applies the tunneling principle. A new IP packet is constructed with a new IP header and then ESP in transport mode is applied, as illustrated in Figure 183. Since the original datagram becomes the payload data for the new ESP packet, its protection is total if both encryption and authentication are selected. However, the new IP header is still not protected. The tunnel mode is used whenever either end of a security association is a gateway. Thus, between two firewalls the tunnel mode is always used. Although gateways are supposed to support tunnel mode only, often they can also work in transport mode. This mode is allowed when the gateway acts as a host, that is in cases when traffic is destined to itself. Examples are SNMP commands or ICMP echo requests. In tunnel mode the outer headers' IP addresses does not need to be the same as the inner headers' addresses. For example two security gateways may operate an ESP tunnel which is used to secure all traffic between the networks they connect together. Hosts are not required to support tunnel mode, but often they do. The advantages of the tunnel mode are total protection of the encapsulated IP datagram and the possibility of using private addresses. However, there is an extra processing overhead associated with this mode. 1.4 Why Two Authentication Protocols? Knowing about the security services of ESP, one might ask if there is really a requirement for AH. Why does ESP authentication not cover the IP header as well? There is no official answer to these questions, but here are some points that justify the existence of two different IPSec authentication protocols: - ESP requires strong cryptographic algorithms to be implemented, whether it will actually be used or not. Strong cryptography is an over-hyped and sensitive topic in some countries, with restrictive regulations in place. It might be troublesome to deploy ESP-based solutions in such areas. However, authentication is not regulated and AH can be used freely around the world. - Often only authentication is needed. While ESP could have been specified to cover the IP header as well, AH is more performant compared to ESP with authentication only, because of the simpler format and lower processing overhead. It makes sense to use AH in these cases. - Having two different protocols means finer-grade control over an IPSec network and more flexible security options. By nesting AH and ESP for example, one can implement IPSec tunnels that combine the strengths of both protocols. 1.5 Combining IPSec Protocols The AH and ESP protocols can be applied alone or in combination. Given the two modes of each protocol, there is quite a number of possible combinations. To make things even worse, the AH and ESP SAs do not need to have identical endpoints, so the picture becomes rather complicated. Luckily, out of the many possibilities only a few make sense in real-world scenarios. I mentioned earlier that the combinations of IPSec protocols are realized with SA bundles. There are two approaches for an SA bundle creation: - Transport adjacency: Both security protocols are applied in transport mode to the same IP datagram. This method is practical for only one level of combination. - Iterated (nested) tunneling: The security protocols are applied in tunnel mode in sequence. After each application a new IP datagram is created and the next protocol is applied to it. This method has no limit in the nesting levels. However, more than three levels are impractical. These approaches can be combined, for example an IP packet with transport adjacency IPSec headers can be sent through nested tunnels. When designing a VPN, one should limit the IPSec processing stages applied to a certain packet to a reasonable level. In our view three applications is that limit over which further processing has no benefits. Two stages are sufficient for almost all the cases. Note that in order to be able to create an SA bundle in which the SAs have different endpoints, at least one level of tunneling must be applied. Transport adjacency does not allow for multiple source/destination addresses, because only one IP header is present. The practical principle of the combined usage is that upon the receipt of a packet with both protocol headers, the IPSec processing sequence should be authentication followed by decryption. It is a common sense decision not to bother with the decryption of packets of uncertain origin. Following the above principle, the sender first applies ESP and then AH to the outbound traffic. In fact this sequence is an explicit requirement for transport mode IPSec processing. When using both ESP and AH, a new question arises: should ESP authentication be turned on? AH authenticates the packet anyway. The answer is simple. Turning ESP authentication on makes sense only when the ESP SA extends beyond the AH SA, as in the case of the supplier scenario. In this case, not only does it make sense to use ESP authentication, but it is highly recommended to do so, to avoid spoofing attacks in the intranet. As far as the modes are concerned, the usual way is that transport mode is used between the endpoints of a connection and tunnel mode is used between two machines when at least one of them is a gateway. VPN's will be covered in next month's issue. Slider. This file was developed by the help of Red Bull, Pepsi Max and Eninem. This file is dedicated to my Ex-girlfriend who decided to dump me half-way through the making of the file, thus incresing my sex life 10 fold :) thanks babe. This file was developed over a month of research and writing, so dont say i did not put effort into it. Any comments and querys please be directed to slider_100@hotmail.com