RFC 2408
Internet Security Association and Key Management Protocol (ISAKMP)
Network Working Group D. Maughan Request for Comments: 2408 National Security Agency Category: Standards Track M. Schertler Securify, Inc. M. Schneider National Security Agency J. Turner RABA Technologies, Inc. November 1998 Internet Security Association and Key Management Protocol (ISAKMP) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. Abstract This memo describes a protocol utilizing security concepts necessary for establishing Security Associations (SA) and cryptographic keys in an Internet environment. A Security Association protocol that negotiates, establishes, modifies and deletes Security Associations and their attributes is required for an evolving Internet, where there will be numerous security mechanisms and several options for each security mechanism. The key management protocol must be robust in order to handle public key generation for the Internet community at large and private key requirements for those private networks with that requirement. The Internet Security Association and Key Management Protocol (ISAKMP) defines the procedures for authenticating a communicating peer, creation and management of Security Associations, key generation techniques, and threat mitigation (e.g. denial of service and replay attacks). All of these are necessary to establish and maintain secure communications (via IP Security Service or any other security protocol) in an Internet environment.
Table of Contents 1 Introduction 4 1.1 Requirements Terminology . . . . . . . . . . . . . . . . . 5 1.2 The Need for Negotiation . . . . . . . . . . . . . . . . . 5 1.3 What can be Negotiated? . . . . . . . . . . . . . . . . . 6 1.4 Security Associations and Management . . . . . . . . . . . 7 1.4.1 Security Associations and Registration . . . . . . . . 7 1.4.2 ISAKMP Requirements . . . . . . . . . . . . . . . . . 8 1.5 Authentication . . . . . . . . . . . . . . . . . . . . . . 8 1.5.1 Certificate Authorities . . . . . . . . . . . . . . . 9 1.5.2 Entity Naming . . . . . . . . . . . . . . . . . . . . 9 1.5.3 ISAKMP Requirements . . . . . . . . . . . . . . . . . 10 1.6 Public Key Cryptography . . . . . . . . . . . . . . . . . . 10 1.6.1 Key Exchange Properties . . . . . . . . . . . . . . . 11 1.6.2 ISAKMP Requirements . . . . . . . . . . . . . . . . . 12 1.7 ISAKMP Protection . . . . . . . . . . . . . . . . . . . . . 12 1.7.1 Anti-Clogging (Denial of Service) . . . . . . . . . . 12 1.7.2 Connection Hijacking . . . . . . . . . . . . . . . . . 13 1.7.3 Man-in-the-Middle Attacks . . . . . . . . . . . . . . 13 1.8 Multicast Communications . . . . . . . . . . . . . . . . . 13 2 Terminology and Concepts 14 2.1 ISAKMP Terminology . . . . . . . . . . . . . . . . . . . . 14 2.2 ISAKMP Placement . . . . . . . . . . . . . . . . . . . . . 16 2.3 Negotiation Phases . . . . . . . . . . . . . . . . . . . . 16 2.4 Identifying Security Associations . . . . . . . . . . . . . 17 2.5 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . 20 2.5.1 Transport Protocol . . . . . . . . . . . . . . . . . . 20 2.5.2 RESERVED Fields . . . . . . . . . . . . . . . . . . . 20 2.5.3 Anti-Clogging Token ("Cookie") Creation . . . . . . . 20 3 ISAKMP Payloads 21 3.1 ISAKMP Header Format . . . . . . . . . . . . . . . . . . . 21 3.2 Generic Payload Header . . . . . . . . . . . . . . . . . . 25 3.3 Data Attributes . . . . . . . . . . . . . . . . . . . . . . 25 3.4 Security Association Payload . . . . . . . . . . . . . . . 27 3.5 Proposal Payload . . . . . . . . . . . . . . . . . . . . . 28 3.6 Transform Payload . . . . . . . . . . . . . . . . . . . . . 29 3.7 Key Exchange Payload . . . . . . . . . . . . . . . . . . . 31 3.8 Identification Payload . . . . . . . . . . . . . . . . . . 32 3.9 Certificate Payload . . . . . . . . . . . . . . . . . . . . 33 3.10 Certificate Request Payload . . . . . . . . . . . . . . . 34 3.11 Hash Payload . . . . . . . . . . . . . . . . . . . . . . 36 3.12 Signature Payload . . . . . . . . . . . . . . . . . . . . 37 3.13 Nonce Payload . . . . . . . . . . . . . . . . . . . . . . 37 3.14 Notification Payload . . . . . . . . . . . . . . . . . . 38 3.14.1 Notify Message Types . . . . . . . . . . . . . . . . 40 3.15 Delete Payload . . . . . . . . . . . . . . . . . . . . . 41 3.16 Vendor ID Payload . . . . . . . . . . . . . . . . . . . . 43
4 ISAKMP Exchanges 44 4.1 ISAKMP Exchange Types . . . . . . . . . . . . . . . . . . . 45 4.1.1 Notation . . . . . . . . . . . . . . . . . . . . . . . 46 4.2 Security Association Establishment . . . . . . . . . . . . 46 4.2.1 Security Association Establishment Examples . . . . . 48 4.3 Security Association Modification . . . . . . . . . . . . . 50 4.4 Base Exchange . . . . . . . . . . . . . . . . . . . . . . . 51 4.5 Identity Protection Exchange . . . . . . . . . . . . . . . 52 4.6 Authentication Only Exchange . . . . . . . . . . . . . . . 54 4.7 Aggressive Exchange . . . . . . . . . . . . . . . . . . . . 55 4.8 Informational Exchange . . . . . . . . . . . . . . . . . . 57 5 ISAKMP Payload Processing 58 5.1 General Message Processing . . . . . . . . . . . . . . . . 58 5.2 ISAKMP Header Processing . . . . . . . . . . . . . . . . . 59 5.3 Generic Payload Header Processing . . . . . . . . . . . . . 61 5.4 Security Association Payload Processing . . . . . . . . . . 62 5.5 Proposal Payload Processing . . . . . . . . . . . . . . . . 63 5.6 Transform Payload Processing . . . . . . . . . . . . . . . 64 5.7 Key Exchange Payload Processing . . . . . . . . . . . . . . 65 5.8 Identification Payload Processing . . . . . . . . . . . . . 66 5.9 Certificate Payload Processing . . . . . . . . . . . . . . 66 5.10 Certificate Request Payload Processing . . . . . . . . . 67 5.11 Hash Payload Processing . . . . . . . . . . . . . . . . . 69 5.12 Signature Payload Processing . . . . . . . . . . . . . . 69 5.13 Nonce Payload Processing . . . . . . . . . . . . . . . . 70 5.14 Notification Payload Processing . . . . . . . . . . . . . 71 5.15 Delete Payload Processing . . . . . . . . . . . . . . . . 73 6 Conclusions 75 A. ISAKMP Security Association Attributes 77 A.1 Background/Rationale . . . . . . . . . . . . . . . . . . . 77 A.2 Internet IP Security DOI Assigned Value . . . . . . . . . . 77 A.3 Supported Security Protocols . . . . . . . . . . . . . . . 77 A.4 ISAKMP Identification Type Values . . . . . . . . . . . . . 78 A.4.1 ID_IPV4_ADDR . . . . . . . . . . . . . . . . . . . . . 78 A.4.2 ID_IPV4_ADDR_SUBNET . . . . . . . . . . . . . . . . . . 78 A.4.3 ID_IPV6_ADDR . . . . . . . . . . . . . . . . . . . . . 78 A.4.4 ID_IPV6_ADDR_SUBNET . . . . . . . . . . . . . . . . . 78 B. Defining a new Domain of Interpretation 79 B.1 Situation . . . . . . . . . . . . . . . . . . . . . . . . . 79 B.2 Security Policies . . . . . . . . . . . . . . . . . . . . . 80 B.3 Naming Schemes . . . . . . . . . . . . . . . . . . . . . . 80 B.4 Syntax for Specifying Security Services . . . . . . . . . . 80 B.5 Payload Specification . . . . . . . . . . . . . . . . . . . 80 B.6 Defining new Exchange Types . . . . . . . . . . . . . . . . 80 Security Considerations 81 IANA Considerations 81 Domain of Interpretation 81 Supported Security Protocols 82
Acknowledgements 82 References 82 Authors' Addresses 85 Full Copyright Statement 86 List of Figures 1 ISAKMP Relationships . . . . . . . . . . . . . . . . . . . 16 2 ISAKMP Header Format . . . . . . . . . . . . . . . . . . . 22 3 Generic Payload Header . . . . . . . . . . . . . . . . . . 25 4 Data Attributes . . . . . . . . . . . . . . . . . . . . . . 26 5 Security Association Payload . . . . . . . . . . . . . . . 27 6 Proposal Payload Format . . . . . . . . . . . . . . . . . . 28 7 Transform Payload Format . . . . . . . . . . . . . . . . . 30 8 Key Exchange Payload Format . . . . . . . . . . . . . . . . 31 9 Identification Payload Format . . . . . . . . . . . . . . . 32 10 Certificate Payload Format . . . . . . . . . . . . . . . . 33 11 Certificate Request Payload Format . . . . . . . . . . . . 34 12 Hash Payload Format . . . . . . . . . . . . . . . . . . . . 36 13 Signature Payload Format . . . . . . . . . . . . . . . . . 37 14 Nonce Payload Format . . . . . . . . . . . . . . . . . . . 38 15 Notification Payload Format . . . . . . . . . . . . . . . . 39 16 Delete Payload Format . . . . . . . . . . . . . . . . . . . 42 17 Vendor ID Payload Format . . . . . . . . . . . . . . . . . 44 1 Introduction This document describes an Internet Security Association and Key Management Protocol (ISAKMP). ISAKMP combines the security concepts of authentication, key management, and security associations to establish the required security for government, commercial, and private communications on the Internet. The Internet Security Association and Key Management Protocol (ISAKMP) defines procedures and packet formats to establish, negotiate, modify and delete Security Associations (SA). SAs contain all the information required for execution of various network security services, such as the IP layer services (such as header authentication and payload encapsulation), transport or application layer services, or self-protection of negotiation traffic. ISAKMP defines payloads for exchanging key generation and authentication data. These formats provide a consistent framework for transferring key and authentication data which is independent of the key generation technique, encryption algorithm and authentication mechanism.
ISAKMP is distinct from key exchange protocols in order to cleanly separate the details of security association management (and key management) from the details of key exchange. There may be many different key exchange protocols, each with different security properties. However, a common framework is required for agreeing to the format of SA attributes, and for negotiating, modifying, and deleting SAs. ISAKMP serves as this common framework. Separating the functionality into three parts adds complexity to the security analysis of a complete ISAKMP implementation. However, the separation is critical for interoperability between systems with differing security requirements, and should also simplify the analysis of further evolution of a ISAKMP server. ISAKMP is intended to support the negotiation of SAs for security protocols at all layers of the network stack (e.g., IPSEC, TLS, TLSP, OSPF, etc.). By centralizing the management of the security associations, ISAKMP reduces the amount of duplicated functionality within each security protocol. ISAKMP can also reduce connection setup time, by negotiating a whole stack of services at once. The remainder of section 1 establishes the motivation for security negotiation and outlines the major components of ISAKMP, i.e. Security Associations and Management, Authentication, Public Key Cryptography, and Miscellaneous items. Section 2 presents the terminology and concepts associated with ISAKMP. Section 3 describes the different ISAKMP payload formats. Section 4 describes how the payloads of ISAKMP are composed together as exchange types to establish security associations and perform key exchanges in an authenticated manner. Additionally, security association modification, deletion, and error notification are discussed. Section 5 describes the processing of each payload within the context of ISAKMP exchanges, including error handling and associated actions. The appendices provide the attribute values necessary for ISAKMP and requirement for defining a new Domain of Interpretation (DOI) within ISAKMP. 1.1 Requirements Terminology The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC-2119]. 1.2 The Need for Negotiation ISAKMP extends the assertion in [DOW92] that authentication and key exchanges must be combined for better security to include security association exchanges. The security services required for
communications depends on the individual network configurations and environments. Organizations are setting up Virtual Private Networks (VPN), also known as Intranets, that will require one set of security functions for communications within the VPN and possibly many different security functions for communications outside the VPN to support geographically separate organizational components, customers, suppliers, sub-contractors (with their own VPNs), government, and others. Departments within large organizations may require a number of security associations to separate and protect data (e.g. personnel data, company proprietary data, medical) on internal networks and other security associations to communicate within the same department. Nomadic users wanting to "phone home" represent another set of security requirements. These requirements must be tempered with bandwidth challenges. Smaller groups of people may meet their security requirements by setting up "Webs of Trust". ISAKMP exchanges provide these assorted networking communities the ability to present peers with the security functionality that the user supports in an authenticated and protected manner for agreement upon a common set of security attributes, i.e. an interoperable security association. 1.3 What can be Negotiated? Security associations must support different encryption algorithms, authentication mechanisms, and key establishment algorithms for other security protocols, as well as IP Security. Security associations must also support host-oriented certificates for lower layer protocols and user- oriented certificates for higher level protocols. Algorithm and mechanism independence is required in applications such as e-mail, remote login, and file transfer, as well as in session oriented protocols, routing protocols, and link layer protocols. ISAKMP provides a common security association and key establishment protocol for this wide range of security protocols, applications, security requirements, and network environments. ISAKMP is not bound to any specific cryptographic algorithm, key generation technique, or security mechanism. This flexibility is beneficial for a number of reasons. First, it supports the dynamic communications environment described above. Second, the independence from specific security mechanisms and algorithms provides a forward migration path to better mechanisms and algorithms. When improved security mechanisms are developed or new attacks against current encryption algorithms, authentication mechanisms and key exchanges are discovered, ISAKMP will allow the updating of the algorithms and mechanisms without having to develop a completely new KMP or patch the current one.
ISAKMP has basic requirements for its authentication and key exchange components. These requirements guard against denial of service, replay / reflection, man-in-the-middle, and connection hijacking attacks. This is important because these are the types of attacks that are targeted against protocols. Complete Security Association (SA) support, which provides mechanism and algorithm independence, and protection from protocol threats are the strengths of ISAKMP. 1.4 Security Associations and Management A Security Association (SA) is a relationship between two or more entities that describes how the entities will utilize security services to communicate securely. This relationship is represented by a set of information that can be considered a contract between the entities. The information must be agreed upon and shared between all the entities. Sometimes the information alone is referred to as an SA, but this is just a physical instantiation of the existing relationship. The existence of this relationship, represented by the information, is what provides the agreed upon security information needed by entities to securely interoperate. All entities must adhere to the SA for secure communications to be possible. When accessing SA attributes, entities use a pointer or identifier refered to as the Security Parameter Index (SPI). [SEC-ARCH] provides details on IP Security Associations (SA) and Security Parameter Index (SPI) definitions. 1.4.1 Security Associations and Registration The SA attributes required and recommended for the IP Security (AH, ESP) are defined in [SEC-ARCH]. The attributes specified for an IP Security SA include, but are not limited to, authentication mechanism, cryptographic algorithm, algorithm mode, key length, and Initialization Vector (IV). Other protocols that provide algorithm and mechanism independent security MUST define their requirements for SA attributes. The separation of ISAKMP from a specific SA definition is important to ensure ISAKMP can es tablish SAs for all possible security protocols and applications. NOTE: See [IPDOI] for a discussion of SA attributes that should be considered when defining a security protocol or application. In order to facilitate easy identification of specific attributes (e.g. a specific encryption algorithm) among different network entites the attributes must be assigned identifiers and these identifiers must be registered by a central authority. The Internet Assigned Numbers Authority (IANA) provides this function for the Internet.
1.4.2 ISAKMP Requirements Security Association (SA) establishment MUST be part of the key management protocol defined for IP based networks. The SA concept is required to support security protocols in a diverse and dynamic networking environment. Just as authentication and key exchange must be linked to provide assurance that the key is established with the authenticated party [DOW92], SA establishment must be linked with the authentication and the key exchange protocol. ISAKMP provides the protocol exchanges to establish a security association between negotiating entities followed by the establishment of a security association by these negotiating entities in behalf of some protocol (e.g. ESP/AH). First, an initial protocol exchange allows a basic set of security attributes to be agreed upon. This basic set provides protection for subsequent ISAKMP exchanges. It also indicates the authentication method and key exchange that will be performed as part of the ISAKMP protocol. If a basic set of security attributes is already in place between the negotiating server entities, the initial ISAKMP exchange may be skipped and the establishment of a security association can be done directly. After the basic set of security attributes has been agreed upon, initial identity authenticated, and required keys generated, the established SA can be used for subsequent communications by the entity that invoked ISAKMP. The basic set of SA attributes that MUST be implemented to provide ISAKMP interoperability are defined in Appendix A. 1.5 Authentication A very important step in establishing secure network communications is authentication of the entity at the other end of the communication. Many authentication mechanisms are available. Authentication mechanisms fall into two catagories of strength - weak and strong. Sending cleartext keys or other unprotected authenticating information over a network is weak, due to the threat of reading them with a network sniffer. Additionally, sending one- way hashed poorly-chosen keys with low entropy is also weak, due to the threat of brute-force guessing attacks on the sniffed messages. While passwords can be used for establishing identity, they are not considered in this context because of recent statements from the Internet Architecture Board [IAB]. Digital signatures, such as the Digital Signature Standard (DSS) and the Rivest-Shamir-Adleman (RSA) signature, are public key based strong authentication mechanisms. When using public key digital signatures each entity requires a public key and a private key. Certificates are an essential part of a digital signature authentication mechanism. Certificates bind a specific entity's identity (be it host, network, user, or
application) to its public keys and possibly other security-related information such as privileges, clearances, and compartments. Authentication based on digital signatures requires a trusted third party or certificate authority to create, sign and properly distribute certificates. For more detailed information on digital signatures, such as DSS and RSA, and certificates see [Schneier]. 1.5.1 Certificate Authorities Certificates require an infrastructure for generation, verification, revocation, management and distribution. The Internet Policy Registration Authority (IPRA) [RFC-1422] has been established to direct this infrastructure for the IETF. The IPRA certifies Policy Certification Authorities (PCA). PCAs control Certificate Authorities (CA) which certify users and subordinate entities. Current certificate related work includes the Domain Name System (DNS) Security Extensions [DNSSEC] which will provide signed entity keys in the DNS. The Public Key Infrastucture (PKIX) working group is specifying an Internet profile for X.509 certificates. There is also work going on in industry to develop X.500 Directory Services which would provide X.509 certificates to users. The U.S. Post Office is developing a (CA) hierarchy. The NIST Public Key Infrastructure Working Group has also been doing work in this area. The DOD Multi Level Information System Security Initiative (MISSI) program has begun deploying a certificate infrastructure for the U.S. Government. Alternatively, if no infrastructure exists, the PGP Web of Trust certificates can be used to provide user authentication and privacy in a community of users who know and trust each other. 1.5.2 Entity Naming An entity's name is its identity and is bound to its public keys in certificates. The CA MUST define the naming semantics for the certificates it issues. See the UNINETT PCA Policy Statements [Berge] for an example of how a CA defines its naming policy. When the certificate is verified, the name is verified and that name will have meaning within the realm of that CA. An example is the DNS security extensions which make DNS servers CAs for the zones and nodes they serve. Resource records are provided for public keys and signatures on those keys. The names associated with the keys are IP addresses and domain names which have meaning to entities accessing the DNS for this information. A Web of Trust is another example. When webs of trust are set up, names are bound with the public keys. In PGP the name is usually the entity's e-mail address which has meaning to those, and only those, who understand e-mail. Another web of trust could use an entirely different naming scheme.
1.5.3 ISAKMP Requirements Strong authentication MUST be provided on ISAKMP exchanges. Without being able to authenticate the entity at the other end, the Security Association (SA) and session key established are suspect. Without authentication you are unable to trust an entity's identification, which makes access control questionable. While encryption (e.g. ESP) and integrity (e.g. AH) will protect subsequent communications from passive eavesdroppers, without authentication it is possible that the SA and key may have been established with an adversary who performed an active man-in-the-middle attack and is now stealing all your personal data. A digital signature algorithm MUST be used within ISAKMP's authentication component. However, ISAKMP does not mandate a specific signature algorithm or certificate authority (CA). ISAKMP allows an entity initiating communications to indicate which CAs it supports. After selection of a CA, the protocol provides the messages required to support the actual authentication exchange. The protocol provides a facility for identification of different certificate authorities, certificate types (e.g. X.509, PKCS #7, PGP, DNS SIG and KEY records), and the exchange of the certificates identified. ISAKMP utilizes digital signatures, based on public key cryptography, for authentication. There are other strong authentication systems available, which could be specified as additional optional authentication mechanisms for ISAKMP. Some of these authentication systems rely on a trusted third party called a key distribution center (KDC) to distribute secret session keys. An example is Kerberos, where the trusted third party is the Kerberos server, which holds secret keys for all clients and servers within its network domain. A client's proof that it holds its secret key provides authenticaton to a server. The ISAKMP specification does not specify the protocol for communicating with the trusted third parties (TTP) or certificate directory services. These protocols are defined by the TTP and directory service themselves and are outside the scope of this specification. The use of these additional services and protocols will be described in a Key Exchange specific document. 1.6 Public Key Cryptography Public key cryptography is the most flexible, scalable, and efficient way for users to obtain the shared secrets and session keys needed to support the large number of ways Internet users will interoperate. Many key generation algorithms, that have different properties, are
available to users (see [DOW92], [ANSI], and [Oakley]). Properties of key exchange protocols include the key establishment method, authentication, symmetry, perfect forward secrecy, and back traffic protection. NOTE: Cryptographic keys can protect information for a considerable length of time. However, this is based on the assumption that keys used for protection of communications are destroyed after use and not kept for any reason. 1.6.1 Key Exchange Properties Key Establishment (Key Generation / Key Transport): The two common methods of using public key cryptography for key establishment are key transport and key generation. An example of key transport is the use of the RSA algorithm to encrypt a randomly generated session key (for encrypting subsequent communications) with the recipient's public key. The encrypted random key is then sent to the recipient, who decrypts it using his private key. At this point both sides have the same session key, however it was created based on input from only one side of the communications. The benefit of the key transport method is that it has less computational overhead than the following method. The Diffie-Hellman (D-H) algorithm illustrates key generation using public key cryptography. The D-H algorithm is begun by two users exchanging public information. Each user then mathematically combines the other's public information along with their own secret information to compute a shared secret value. This secret value can be used as a session key or as a key encryption key for encrypting a randomly generated session key. This method generates a session key based on public and secret information held by both users. The benefit of the D-H algorithm is that the key used for encrypting messages is based on information held by both users and the independence of keys from one key exchange to another provides perfect forward secrecy. Detailed descriptions of these algorithms can be found in [Schneier]. There are a number of variations on these two key generation schemes and these variations do not necessarily interoperate. Key Exchange Authentication: Key exchanges may be authenticated during the protocol or after protocol completion. Authentication of the key exchange during the protocol is provided when each party provides proof it has the secret session key before the end of the protocol. Proof can be provided by encrypting known data in the secret session key during the protocol echange. Authentication after the protocol must occur in subsequent commu nications. Authentication during the protocol is preferred so subsequent communications are not initiated if the secret session key is not established with the desired party.
Key Exchange Symmetry: A key exchange provides symmetry if either party can initiate the exchange and exchanged messages can cross in transit without affecting the key that is generated. This is desirable so that computation of the keys does not require either party to know who initated the exchange. While key exchange symmetry is desirable, symmetry in the entire key management protocol may provide a vulnerablity to reflection attacks. Perfect Forward Secrecy: As described in [DOW92], an authenticated key exchange protocol provides perfect forward secrecy if disclosure of longterm secret keying material does not compromise the secrecy of the exchanged keys from previous communications. The property of perfect forward secrecy does not apply to key exchange without authentication. 1.6.2 ISAKMP Requirements An authenticated key exchange MUST be supported by ISAKMP. Users SHOULD choose additional key establishment algorithms based on their requirements. ISAKMP does not specify a specific key exchange. However, [IKE] describes a proposal for using the Oakley key exchange [Oakley] in conjunction with ISAKMP. Requirements that should be evaluated when choosing a key establishment algorithm include establishment method (generation vs. transport), perfect forward secrecy, computational overhead, key escrow, and key strength. Based on user requirements, ISAKMP allows an entity initiating communications to indicate which key exchanges it supports. After selection of a key exchange, the protocol provides the messages required to support the actual key establishment. 1.7 ISAKMP Protection 1.7.1 Anti-Clogging (Denial of Service) Of the numerous security services available, protection against denial of service always seems to be one of the most difficult to address. A "cookie" or anti-clogging token (ACT) is aimed at protecting the computing resources from attack without spending excessive CPU resources to determine its authenticity. An exchange prior to CPU-intensive public key operations can thwart some denial of service attempts (e.g. simple flooding with bogus IP source addresses). Absolute protection against denial of service is impossible, but this anti-clogging token provides a technique for making it easier to handle. The use of an anti-clogging token was introduced by Karn and Simpson in [Karn].
It should be noted that in the exchanges shown in section 4, the anticlogging mechanism should be used in conjuction with a garbage- state collection mechanism; an attacker can still flood a server using packets with bogus IP addresses and cause state to be created. Such aggressive memory management techniques SHOULD be employed by protocols using ISAKMP that do not go through an initial, anti- clogging only phase, as was done in [Karn]. 1.7.2 Connection Hijacking ISAKMP prevents connection hijacking by linking the authentication, key exchange and security association exchanges. This linking prevents an attacker from allowing the authentication to complete and then jumping in and impersonating one entity to the other during the key and security association exchanges. 1.7.3 Man-in-the-Middle Attacks Man-in-the-Middle attacks include interception, insertion, deletion, and modification of messages, reflecting messages back at the sender, replaying old messages and redirecting messages. ISAKMP features prevent these types of attacks from being successful. The linking of the ISAKMP exchanges prevents the insertion of messages in the protocol exchange. The ISAKMP protocol state machine is defined so deleted messages will not cause a partial SA to be created, the state machine will clear all state and return to idle. The state machine also prevents reflection of a message from causing harm. The requirement for a new cookie with time variant material for each new SA establishment prevents attacks that involve replaying old messages. The ISAKMP strong authentication requirement prevents an SA from being established with anyone other than the intended party. Messages may be redirected to a different destination or modified but this will be detected and an SA will not be established. The ISAKMP specification defines where abnormal processing has occurred and recommends notifying the appropriate party of this abnormality. 1.8 Multicast Communications It is expected that multicast communications will require the same security services as unicast communications and may introduce the need for additional security services. The issues of distributing SPIs for multicast traffic are presented in [SEC-ARCH]. Multicast security issues are also discussed in [RFC-1949] and [BC]. A future extension to ISAKMP will support multicast key distribution. For an introduction to the issues related to multicast security, consult the Internet Drafts, [RFC-2094] and [RFC-2093], describing Sparta's research in this area.
2 Terminology and Concepts 2.1 ISAKMP Terminology Security Protocol: A Security Protocol consists of an entity at a single point in the network stack, performing a security service for network communication. For example, IPSEC ESP and IPSEC AH are two different security protocols. TLS is another example. Security Protocols may perform more than one service, for example providing integrity and confidentiality in one module. Protection Suite: A protection suite is a list of the security services that must be applied by various security protocols. For example, a protection suite may consist of DES encryption in IP ESP, and keyed MD5 in IP AH. All of the protections in a suite must be treated as a single unit. This is necessary because security services in different security protocols can have subtle interactions, and the effects of a suite must be analyzed and verified as a whole. Security Association (SA): A Security Association is a security- protocol- specific set of parameters that completely defines the services and mechanisms necessary to protect traffic at that security protocol location. These parameters can include algorithm identifiers, modes, cryptographic keys, etc. The SA is referred to by its associated security protocol (for example, "ISAKMP SA", "ESP SA", "TLS SA"). ISAKMP SA: An SA used by the ISAKMP servers to protect their own traffic. Sections 2.3 and 2.4 provide more details about ISAKMP SAs. Security Parameter Index (SPI): An identifier for a Security Assocation, relative to some security protocol. Each security protocol has its own "SPI-space". A (security protocol, SPI) pair may uniquely identify an SA. The uniqueness of the SPI is implementation dependent, but could be based per system, per protocol, or other options. Depending on the DOI, additional information (e.g. host address) may be necessary to identify an SA. The DOI will also determine which SPIs (i.e. initiator's or responder's) are sent during communication. Domain of Interpretation: A Domain of Interpretation (DOI) defines payload formats, exchange types, and conventions for naming security-relevant information such as security policies or cryptographic algorithms and modes. A Domain of Interpretation (DOI) identifier is used to interpret the payloads of ISAKMP payloads. A system SHOULD support multiple Domains of Interpretation simultaneously. The concept of a DOI is based on previous work by
the TSIG CIPSO Working Group, but extends beyond security label
interpretation to include naming and interpretation of security
services. A DOI defines:
o A "situation": the set of information that will be used to
determine the required security services.
o The set of security policies that must, and may, be supported.
o A syntax for the specification of proposed security services.
o A scheme for naming security-relevant information, including
encryption algorithms, key exchange algorithms, security policy
attributes, and certificate authorities.
o The specific formats of the various payload contents.
o Additional exchange types, if required.
The rules for the IETF IP Security DOI are presented in [IPDOI].
Specifications of the rules for customized DOIs will be presented in
separate documents.
Situation: A situation contains all of the security-relevant
information that a system considers necessary to decide the security
services required to protect the session being negotiated. The
situation may include addresses, security classifications, modes of
operation (normal vs. emergency), etc.
Proposal: A proposal is a list, in decreasing order of preference, of
the protection suites that a system considers acceptable to protect
traffic under a given situation.
Payload: ISAKMP defines several types of payloads, which are used to
transfer information such as security association data, or key
exchange data, in DOI-defined formats. A payload consists of a
generic payload header and a string of octects that is opaque to
ISAKMP. ISAKMP uses DOI- specific functionality to synthesize and
interpret these payloads. Multiple payloads can be sent in a single
ISAKMP message. See section 3 for more details on the payload types,
and [IPDOI] for the formats of the IETF IP Security DOI payloads.
Exchange Type: An exchange type is a specification of the number of
messages in an ISAKMP exchange, and the payload types that are
contained in each of those messages. Each exchange type is designed
to provide a particular set of security services, such as anonymity
of the participants, perfect forward secrecy of the keying material,
authentication of the participants, etc. Section 4.1 defines the
default set of ISAKMP exchange types. Other exchange types can be added to support additional key exchanges, if required. 2.2 ISAKMP Placement Figure 1 is a high level view of the placement of ISAKMP within a system context in a network architecture. An important part of negotiating security services is to consider the entire "stack" of individual SAs as a unit. This is referred to as a "protection suite". +------------+ +--------+ +--------------+ ! DOI ! ! ! ! Application ! ! Definition ! <----> ! ISAKMP ! ! Process ! +------------+ --> ! ! !--------------! +--------------+ ! +--------+ ! Appl Protocol! ! Key Exchange ! ! ^ ^ +--------------+ ! Definition !<-- ! ! ^ +--------------+ ! ! ! ! ! ! !----------------! ! ! v ! ! +-------+ v v ! API ! +---------------------------------------------+ +-------+ ! Socket Layer ! ! !---------------------------------------------! v ! Transport Protocol (TCP / UDP) ! +----------+ !---------------------------------------------! ! Security ! <----> ! IP ! ! Protocol ! !---------------------------------------------! +----------+ ! Link Layer Protocol ! +---------------------------------------------+ Figure 1: ISAKMP Relationships 2.3 Negotiation Phases ISAKMP offers two "phases" of negotiation. In the first phase, two entities (e.g. ISAKMP servers) agree on how to protect further negotiation traffic between themselves, establishing an ISAKMP SA. This ISAKMP SA is then used to protect the negotiations for the Protocol SA being requested. Two entities (e.g. ISAKMP servers) can negotiate (and have active) multiple ISAKMP SAs.
The second phase of negotiation is used to establish security associations for other security protocols. This second phase can be used to establish many security associations. The security associations established by ISAKMP during this phase can be used by a security protocol to protect many message/data exchanges. While the two-phased approach has a higher start-up cost for most simple scenarios, there are several reasons that it is beneficial for most cases. First, entities (e.g. ISAKMP servers) can amortize the cost of the first phase across several second phase negotiations. This allows multiple SAs to be established between peers over time without having to start over for each communication. Second, security services negotiated during the first phase provide security properties for the second phase. For example, after the first phase of negotiation, the encryption provided by the ISAKMP SA can provide identity protection, potentially allowing the use of simpler second-phase exchanges. On the other hand, if the channel established during the first phase is not adequate to protect identities, then the second phase must negotiate adequate security mechanisms. Third, having an ISAKMP SA in place considerably reduces the cost of ISAKMP management activity - without the "trusted path" that an ISAKMP SA gives you, the entities (e.g. ISAKMP servers) would have to go through a complete re-authentication for each error notification or deletion of an SA. Negotiation during each phase is accomplished using ISAKMP-defined exchanges (see section 4) or exchanges defined for a key exchange within a DOI. Note that security services may be applied differently in each negotiation phase. For example, different parties are being authenticated during each of the phases of negotiation. During the first phase, the parties being authenticated may be the ISAKMP servers/hosts, while during the second phase, users or application level programs are being authenticated. 2.4 Identifying Security Associations While bootstrapping secure channels between systems, ISAKMP cannot assume the existence of security services, and must provide some protections for itself. Therefore, ISAKMP considers an ISAKMP Security Association to be different than other types, and manages ISAKMP SAs itself, in their own name space. ISAKMP uses the two
cookie fields in the ISAKMP header to identify ISAKMP SAs. The Message ID in the ISAKMP Header and the SPI field in the Proposal payload are used during SA establishment to identify the SA for other security protocols. The interpretation of these four fields is dependent on the operation taking place. The following table shows the presence or absence of several fields during SA establishment. The following fields are necessary for various operations associated with SA establishment: cookies in the ISAKMP header, the ISAKMP Header Message ID field, and the SPI field in the Proposal payload. An 'X' in the column means the value MUST be present. An 'NA' in the column means a value in the column is Not Applicable to the operation. # Operation I-Cookie R-Cookie Message ID SPI (1) Start ISAKMP SA negotiation X 0 0 0 (2) Respond ISAKMP SA negotiation X X 0 0 (3) Init other SA negotiation X X X X (4) Respond other SA negotiation X X X X (5) Other (KE, ID, etc.) X X X/0 NA (6) Security Protocol (ESP, AH) NA NA NA X In the first line (1) of the table, the initiator includes the Initiator Cookie field in the ISAKMP Header, using the procedures outlined in sections 2.5.3 and 3.1. In the second line (2) of the table, the responder includes the Initiator and Responder Cookie fields in the ISAKMP Header, using the procedures outlined in sections 2.5.3 and 3.1. Additional messages may be exchanged between ISAKMP peers, depending on the ISAKMP exchange type used during the phase 1 negotiation. Once the phase 1 exchange is completed, the Initiator and Responder cookies are included in the ISAKMP Header of all subsequent communications between the ISAKMP peers. During phase 1 negotiations, the initiator and responder cookies determine the ISAKMP SA. Therefore, the SPI field in the Proposal payload is redundant and MAY be set to 0 or it MAY contain the transmitting entity's cookie. In the third line (3) of the table, the initiator associates a Message ID with the Protocols contained in the SA Proposal. This Message ID and the initiator's SPI(s) to be associated with each protocol in the Proposal are sent to the responder. The SPI(s) will be used by the security protocols once the phase 2 negotiation is completed.
In the fourth line (4) of the table, the responder includes the same Message ID and the responder's SPI(s) to be associated with each protocol in the accepted Proposal. This information is returned to the initiator. In the fifth line (5) of the table, the initiator and responder use the Message ID field in the ISAKMP Header to keep track of the in- progress protocol negotiation. This is only applicable for a phase 2 exchange and the value MUST be 0 for a phase 1 exchange because the combined cookies identify the ISAKMP SA. The SPI field in the Proposal payload is not applicable because the Proposal payload is only used during the SA negotiation message exchange (steps 3 and 4). In the sixth line (6) of the table, the phase 2 negotiation is complete. The security protocols use the SPI(s) to determine which security services and mechanisms to apply to the communication between them. The SPI value shown in the sixth line (6) is not the SPI field in the Proposal payload, but the SPI field contained within the security protocol header. During the SA establishment, a SPI MUST be generated. ISAKMP is designed to handle variable sized SPIs. This is accomplished by using the SPI Size field within the Proposal payload during SA establishment. Handling of SPIs will be outlined by the DOI specification (e.g. [IPDOI]). When a security association (SA) is initially established, one side assumes the role of initiator and the other the role of responder. Once the SA is established, both the original initiator and responder can initiate a phase 2 negotiation with the peer entity. Thus, ISAKMP SAs are bidirectional in nature. Additionally, ISAKMP allows both initiator and responder to have some control during the negotiation process. While ISAKMP is designed to allow an SA negotiation that includes multiple proposals, the initiator can maintain some control by only making one proposal in accordance with the initiator's local security policy. Once the initiator sends a proposal containing more than one proposal (which are sent in decreasing preference order), the initiator relinquishes control to the responder. Once the responder is controlling the SA establishment, the responder can make its policy take precedence over the initiator within the context of the multiple options offered by the initiator. This is accomplished by selecting the proposal best suited for the responder's local security policy and returning this selection to the initiator.
2.5 Miscellaneous 2.5.1 Transport Protocol ISAKMP can be implemented over any transport protocol or over IP itself. Implementations MUST include send and receive capability for ISAKMP using the User Datagram Protocol (UDP) on port 500. UDP Port 500 has been assigned to ISAKMP by the Internet Assigned Numbers Authority (IANA). Implementations MAY additionally support ISAKMP over other transport protocols or over IP itself. 2.5.2 RESERVED Fields The existence of RESERVED fields within ISAKMP payloads are used strictly to preserve byte alignment. All RESERVED fields in the ISAKMP protocol MUST be set to zero (0) when a packet is issued. The receiver SHOULD check the RESERVED fields for a zero (0) value and discard the packet if other values are found. 2.5.3 Anti-Clogging Token ("Cookie") Creation The details of cookie generation are implementation dependent, but MUST satisfy these basic requirements (originally stated by Phil Karn in [Karn]): 1. The cookie must depend on the specific parties. This prevents an attacker from obtaining a cookie using a real IP address and UDP port, and then using it to swamp the victim with Diffie-Hellman requests from randomly chosen IP addresses or ports. 2. It must not be possible for anyone other than the issuing entity to generate cookies that will be accepted by that entity. This implies that the issuing entity must use local secret information in the generation and subsequent verification of a cookie. It must not be possible to deduce this secret information from any particular cookie. 3. The cookie generation function must be fast to thwart attacks intended to sabotage CPU resources. Karn's suggested method for creating the cookie is to perform a fast hash (e.g. MD5) over the IP Source and Destination Address, the UDP Source and Destination Ports and a locally generated secret random value. ISAKMP requires that the cookie be unique for each SA establishment to help prevent replay attacks, therefore, the date and time MUST be added to the information hashed. The generated cookies are placed in the ISAKMP Header (described in section 3.1) Initiator
and Responder cookie fields. These fields are 8 octets in length, thus, requiring a generated cookie to be 8 octets. Notify and Delete messages (see sections 3.14, 3.15, and 4.8) are uni-directional transmissions and are done under the protection of an existing ISAKMP SA, thus, not requiring the generation of a new cookie. One exception to this is the transmission of a Notify message during a Phase 1 exchange, prior to completing the establishment of an SA. Sections 3.14 and 4.8 provide additional details.
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