Security of Communication Protocols and Services

87-30-01 DATA SECURITY MANAGEMENT SECURITY OF COMMUNICATION PROTOCOLS AND SERVICES William Hugh Murray INSIDE Protocols; Security Protocols; Secure Socket Layer (SSL); Secure-HTTP; Secure File Transfer Protocol (S-FTP); Secure Electronic Transaction (SET); Point-to-Point Tunneling Protocol (PPTP); Layer 2 Forwarding (L2F); Layer 2 Tunneling Protocol (L2TP); Secure Internet Protocol (Secure-IP or IPsec); Internet Security Association Key Management Protocol (ISAKMP); Password Authentication Protocol (PAP); Challenge Handshake Authentication Protocol (CHAP); Services; Telnet; File Transfer; Secure Shell (SSH 2) INTRODUCTION For the last century, people have trusted the dial-switched voice-analog network. It was operated by one of the most trusted enterprises in the history of the world. It was connection-switched and point-to-point. While there was some eavesdropping, most of it was initiated by law en- forcement and was, for the most part, legitimate. While a few people carefully considered what they would say, most used the telephone au- tomatically, without worrying about being overheard. Similarly, people were able to recognize most of the people who called, they trusted the millions of copies of the printed directories, and they trusted the network to connect only to the number dialed. While it is not completely justified, much of that automatic trust has been transferred to the modern digi- PAYOFF IDEA tal network and even to the Internet. The information security manager is confronted All other things equal, the infor- with a wide variety of communication protocols mation security manager would like and services. This article describes the popular to be able to ignore how information protocols and services, discusses about their in- moves from one place to another. He tended uses and applications, and describes their security properties and characteristics. or she would like to be able to as- Therefore, this should make life easier for the in- sume that information can be put formation security person faced with the need to into a pipe at point A and have it understand which way to go to best protect criti- cal/sensitive information. 12/99 Auerbach Publications © 1999 CRC Press LLC come out reliably only at B. Of course, in the real world of the modern integrated network, this is not the case. In this world, the traffic is vulner- able to eavesdropping, misdirection, interference, contamination, alter- ation, and even total loss. On the other hand, relatively little of this happens; the vast majority of all information is delivered when and how it is intended and without any compromise. This happens, in part, despite the way that the information is moved and, in part, because of how it is moved. The various protocols and services have different security properties and qualities. Some provide error detection, corrective action such as retransmission, error correction, guaranteed delivery, and even information hiding. The different levels of service exist because they have different costs and performances. They exist because different traffic, applications, and environments have different requirements. For example, the transfer of a program file has a requirement for bit-for-bit integrity; in some cases, losing a bit is as bad as losing the entire file. On the other hand, a few seconds, or even tens of seconds, of delay in the transfer of the file may have little impact. However, if one is moving voice traffic, the loss of tens of bits may be perfectly acceptable, while a delay in seconds is intolera- ble. These costs must be balanced against the requirements of the application and the environment. While the balance between performance and cost is often struck without regard to security, the reality is that there are security differences. The balance between performance, cost, and security is the province of the information security manager. Therefore, it is necessary to understand the properties and characteristics of the protocols so that he or she can make the necessary trade-offs or evaluate those that have already been made. Finally, all protocols have limitations and many have fundamental vulnerabilities. Implementations of protocols can compensate for such vulnerabilities only in part. Implementers may be faced with difficult design choices and they may make errors resulting in implementation-induced vulnerabilities. The manager must understand these so that he or she will know when and how to compensate. PROTOCOLS A protocol is an agreed-upon set of rules or conventions for communi- cating between two or more parties. “Hello” and “goodbye” for begin- ning and ending voice phone calls is an example of a simple protocol. A slightly more sophisticated protocol might include lines that begin with tags like “This is (name) calling.” Protocols are to codes as sentences and paragraphs are to words. In a protocol, the parties may agree to addressing, codes, format, packet size, speed, message order, error detection and correction, acknowledgments, key exchange, and other things. © 1999 CRC Press LLC 12/99 This article deals with a number of common protocols. It will describe their intended use or application, characteristics, design choices, and limitations. Internet Protocol The Internet Protocol, IP (pronounced “eye pea”), is a primitive and application-independent protocol for addressing and routing packets of data within a network. It is the IP in TCP/IP, the protocol suite, that is used in and defines the Internet. It is intended for use in a relatively flat, mesh, broadcast, connectionless, packet-switched nets like the Internet. IP is analogous to a postcard in the 18th century. The sender wrote the message on one side of the card, the address and return address on the other, and then gave it to someone who was going in the general direc- tion of the intended recipient. The message was not confidential; every- one who handled it could read it and might even make an undetected change to it. IP is a “best-efforts” protocol; it does not guarantee message delivery, nor does it provide any evidence as to whether or not the message was delivered. It is unchecked; the receiver does not know whether or not he received the entire intended message or whether or not it is correct. The addresses are unreliable; the sender cannot be sure that the message will go only where he intends or even where he intends. The receiver cannot be sure that the message came from the address specified as the return address in the packet. The protocol does not provide any checking or hiding. If the application requires these, they must be implied or specified someplace else, usually in a higher (i.e., closer to the application) protocol layer. IP specifies the addresses of the sending or receiving hardware device,1 but if that device supports multiple applications, IP does not specify for which of those it is intended. The IP protocol uses 32 bit addresses. However, the use or meaning of the bits within the address depends on the size and use of the network. Addresses are divided into five classes. Each class represents a different design choice between the number of networks and the number of addressable devices within the class. Class A addresses are used for very large networks where the number of such networks is expected to be low but the number of addressable devices is expected to be very high. Class A addresses are used for nation states and other very large domains such as .mil, .gov, and .com. A zero in bit position 0 of an address specifies it as a class A address. Positions 1 through 7 are used to specify the network, and positions 8 through 31 are used to specify devices within the network. Class C is used for networks where the possi- ble number of networks is expected to be high but the number of addressable devices in each net is less than 128. Thus, in general, class B © 1999 CRC Press LLC 12/99 EXHIBIT 1 — IP Network Address Formats Device Network Class Description Address Class Network Address Address A National 0 in bit 0 1–7 8–31 B Enterprise 10 in bits 0–1 2–15 16–31 C LAN 110 in 0–2 3–23 24–31 D Multicast 1110 in 0–3 4–31 E Reserved 1111 in 0–3 is used for enterprises, states, provinces, or municipalities, and class C is used for LANs. Class D is used for multicasting, and class E is reserved for future uses (see Exhibit 1). One often sees IP addresses written as nnn.nnn.nnn.nnn. While security is certainly not IP’s long suit, it is responsible for much of the success of the Internet. It is fast and simple. In practice, the security limitations of IP simply do not matter much. Applications rely on higher-level protocols for security. Internet Protocol v6.0 (IPng) IPv6, or next-generation IP is a backwardly compatible new version of IP. It is intended to permit the Internet to grow, both in terms of the number of addressable devices, particularly class A addresses, and in quantity of traffic. It expands the address to 128 bits, simplifies the format header, improves the support for extensions and options, adds a quality-of-service capability, and adds address authentication and message confidenti- ality and integrity. IPv6 also formalizes the concepts of packet, node, router, host, link, and neighbors that were only loosely defined in IPv4. In other words, IPng addresses most of the limitations of IP, specifical- ly including the security limitations. It provides for the use of encryption to ensure that information goes only where it is intended to go.

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