IPSEC Working Group Douglas Maughan, Mark Schertler INTERNET-DRAFT National Security Agency draft-ietf-ipsec-isakmp-03.txt, .ps November 21, 1995 Internet Security Association and Key Management Protocol (ISAKMP) Abstract This memo describes a protocol utilizing security concepts necessary for establishing Security Associations (SA) and crypto- graphic 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 mecha- nisms 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 communicat- ing 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 es- tablish and maintain secure communications (via IP Security Ser- vice or any other security protocol) in an Internet environment. Status of this memo This document is being submitted to the IETF Internet Protocol Security (IPSEC) Working Group for consideration as a method for the establish- ment and management of security associations and their appropriate secu- rity attributes. Additionally, this document proposes a method for key management to support IPSP and IPv6. Publication of this document does not imply acceptance of the concepts discussed by the IPSEC Working Group. Comments are solicited and should be addressed to the authors and/or the working group mailing list at ipsec@ans.net. This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. INTERNET-DRAFT ISAKMP November 21, 1995 Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id- abstracts.txt'' listing contained in the Internet- Drafts Shadow Di- rectories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this document is unlimited. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 2]
INTERNET-DRAFT ISAKMP November 21, 1995 Contents 1 Introduction 5 1.1 Authentication . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.1Certificate Authorities . . . . . . . . . . . . . . . . . . . 6 1.1.2Entity Naming . . . . . . . . . . . . . . . . . . . . . . . . 7 1.1.3ISAKMP Requirements . . . . . . . . . . . . . . . . . . . . . 7 1.2 Security Associations and Management . . . . . . . . . . . . . . 8 1.2.1Security Associations and Registration . . . . . . . . . . . . 8 1.2.2ISAKMP Requirements . . . . . . . . . . . . . . . . . . . . . 8 1.3 Public Key Cryptography . . . . . . . . . . . . . . . . . . . . . 9 1.3.1Key Exchange Properties . . . . . . . . . . . . . . . . . . . 9 1.3.2ISAKMP Requirements . . . . . . . . . . . . . . . . . . . . . 11 1.4 ISAKMP Protection . . . . . . . . . . . . . . . . . . . . . . . . 11 1.4.1Anti-Clogging (Denial of Service) . . . . . . . . . . . . . . 11 1.4.2Connection Hijacking . . . . . . . . . . . . . . . . . . . . . 11 1.4.3Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . . . 11 1.5 Multicast Communications . . . . . . . . . . . . . . . . . . . . 12 2 Description of the Protocol 12 2.1 ISAKMP Header Format . . . . . . . . . . . . . . . . . . . . . . 13 2.1.1General Message Processing . . . . . . . . . . . . . . . . . . 15 2.2 ISAKMP Packet Exchanges . . . . . . . . . . . . . . . . . . . . . 17 2.2.1Base Exchange . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.2Identity Protection Exchange . . . . . . . . . . . . . . . . . 17 2.2.3Authentication Only Exchange . . . . . . . . . . . . . . . . . 18 2.3 ISAKMP Details . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.3.1Security Association Attributes . . . . . . . . . . . . . . . 19 2.3.2Transport Protocol . . . . . . . . . . . . . . . . . . . . . . 21 2.3.3RESERVED Fields . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.4Anti-Clogging Token (``Cookie'') Creation . . . . . . . . . . 21 2.3.5SA Flags Field . . . . . . . . . . . . . . . . . . . . . . . . 22 3 Security Association Establishment 22 3.1 Security Association Initialization . . . . . . . . . . . . . . . 22 3.1.1SA Initialization Procedures . . . . . . . . . . . . . . . . . 24 3.2 Authentication and Key Exchange . . . . . . . . . . . . . . . . . 25 3.2.1Authentication Payload Format . . . . . . . . . . . . . . . . 26 3.2.2Key Exchange Payload Format . . . . . . . . . . . . . . . . . 28 3.2.3Authentication and Key Exchange Procedures . . . . . . . . . . 29 3.3 Security Association Negotiation . . . . . . . . . . . . . . . . 30 3.3.1SA Negotiation Procedures . . . . . . . . . . . . . . . . . . 31 3.4 SA Negotiation Conclusion . . . . . . . . . . . . . . . . . . . . 34 3.4.1SA Negotiation Conclusion Procedures . . . . . . . . . . . . . 34 4 Security Association Modification 36 4.1 Modification Procedures . . . . . . . . . . . . . . . . . . . . . 36 5 Security Association Deletion 36 5.1 Deletion Procedures . . . . . . . . . . . . . . . . . . . . . . . 37 6 Notification Message 39 6.1 Notification Procedures . . . . . . . . . . . . . . . . . . . . . 40 Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 3]
INTERNET-DRAFT ISAKMP November 21, 1995 7 Conclusions 41 A ISAKMP Scenarios 43 A.1 Initial ISAKMP Daemon Scenerio . . . . . . . . . . . . . . . . . 43 A.2 Virtual Private Network Scenario . . . . . . . . . . . . . . . . 44 B Security Association Attributes 47 C Security Association Examples 51 C.1 ISAKMP SA Definition . . . . . . . . . . . . . . . . . . . . . . 51 C.1.1ISAKMP SA Examples . . . . . . . . . . . . . . . . . . . . . . 52 C.2 ESP SA and AH SA Definitions . . . . . . . . . . . . . . . . . . 53 C.2.1ESP and AH SA Examples . . . . . . . . . . . . . . . . . . . . 54 C.2.2Fortezza SA Examples . . . . . . . . . . . . . . . . . . . . . 55 Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 4]
INTERNET-DRAFT ISAKMP November 21, 1995 1 Introduction This document describes an Internet Security Association and Key Manage- ment Protocol (ISAKMP). ISAKMP combines the security concepts of authen- tication, key management, and security associations to establish the re- quired security for government, commercial, and private communications on the Internet. ISAKMP extends the assertion in [DOW92] that authentica- tion and key exchanges must be combined for better security to include se- curity association exchanges. The security required for communications depends on the individual network configurations and environments. Orga- nizations are setting up Virtual Private Networks (VPN) that will require one set of security functions for communications within the VPN and possi- bly 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 associ- ations to communicate inter-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 it supports in an authen- ticated and protected manner for agreement upon a common interoperable se- curity association. Security associations must support different encryption algorithms, au- thentication mechanisms, and key establishment algorithms for other secu- rity 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 mecha- nism independence is required in applications such as e-mail, remote lo- gin, 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 gener- ation 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 secu- rity mechanisms and algorithms provides a forward migration path to better mechanisms and algorithms. When improved security mechanisms are devel- oped or new attacks against current encryption algorithms, authentica- tion mechanisms and key exchanges are discovered, ISAKMP will allow the updating of the algorithms and mechanisms without having to develop a com- pletely new KMP or patch the current one. ISAKMP has basic requirements for its authentication and key exchanges Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 5]
INTERNET-DRAFT ISAKMP November 21, 1995 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. 1.1 Authentication A very important step in establishing secure network communications is au- thentication 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. Passwords are an exam- ple of a mechanism that provides weak authentication. Reasons for this include the fact that most users pick easy to guess passwords and when used over an unprotected network are easily read by network sniffers. Digital signatures, such as the Digital Signature Standard (DSS) and the Rivest-Shamir-Adleman (RSA) signature, are public key based strong authen- tication mechanisms. When using digital signatures each entity requires a public and a private key. Certificates are an essential part of a digital signature authentication mechanism. Certificates bind a specific enti- ties 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 [Schn94]. 1.1.1 Certificate Authorities Certificates require an infrastructure for generation, verification, man- agement 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 Sys- tem (DNS) Security Extensions [EK94] which will provide signed entity keys in the DNS. The Public Key Infrastucture (PKIX) working group is speci- fying an Internet profile for X.509 certificates. There is also work go- ing 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 infrastruc- ture for the U.S. Government. Alternatively, if no infrastructure exists, the PGP Web of Trust certificates can be used to provide user authentica- tion and privacy in a community of users who know and trust each other. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 6]
INTERNET-DRAFT ISAKMP November 21, 1995 1.1.2 Entity Naming An entity's name is its identity and is bound to its public keys in cer- tificates. The CA MUST define the naming semantics for the certificates it issues. See the UNINETT PCA Policy Statements [Berg] 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 associatied with the keys are IP addresses and domain names which have meaning to entities ac- cessing 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 usaully the entities e-mail address which has meaning to those, and only those, who understand e-mail (Do MCI and AOL e-mail ad- dresses tell the casual e-mailer anything about identity?). Another web could use an entirely different naming scheme. 1.1.3 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, this makes access control questionable. Encryption (e.g. ESP) and integrity (e.g. AH) will pro- tect subsequent communications from passive eavesdroppers, but the SA and key may be established with an adversary who performed an active man-in- the-middle attack and is now stealing all your personnal data. A digital signature algorithm MUST be used within ISAKMP's authentication component. However, ISAKMP does not mandate a specific mechanism. ISAKMP allows an entity initiating communications to indicate which signature al- gorithms it supports. After selection of a common algorithm, the protocol provides the messages required to support the actual authentication ex- change. As an example, if the DSA is selected as the signature algorithm, then the protocol provides a facility for identification of different cer- tificate authorities, certificate types (e.g. X.509v1 certificates, PKCS #7), and the exchange of the certificates identified. ISAKMP utilizes digital signatures, based on public cryptography, for au- thentication. 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 Ker- beros server, which holds secret keys for all clients and servers within it's network domain. A clients proof it holds it's secret key provides its authenticaton to a server. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 7]
INTERNET-DRAFT ISAKMP November 21, 1995 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. 1.2 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 communi- cate 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 in- stantiation of the existing relationship. The existence of this relation- ship, represented by the information, is what provides the agreed upon se- curity information needed by entities to securely interoperate. All enti- ties 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). 1.2.1 Security Associations and Registration The SA attributes required and recommended for the IP Security (AH, ESP) are defined in [RFC-1825]. 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 SA attributes requirements. The separation of ISAKMP from a specific SA definition is important to ensure ISAKMP can establish SAs for all possible security protocols and applications. NOTE: See Appendix B for a discussion of SA attributes that should be con- sidered 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 at- tributes must be assigned identifiers and these identifiers must be reg- istered by a central authority. The Internet Assigned Numbers Authority (IANA) provides this function for the Internet. 1.2.2 ISAKMP Requirements Security Association (SA) establishment MUST be part of the key manage- ment protocol defined for IP based networks. The SA concept is required to support security protocols in a diverse and dynamic networking envi- Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 8]
INTERNET-DRAFT ISAKMP November 21, 1995 ronment. Just as authentication and key exchange must be linked to pro- vide 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 entities. First, an initial protocol exchange allows a basic set of security attributes to be agreed upon. This basic set provides protec- tion for subsequent ISAKMP exchanges. It also indicates the authentica- tion 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 on the communicating entities the initial ISAKMP exchange may be skipped and the key and authentication exchanges issued directly. After the basic set of security attributes has been agreed upon, initial identity authenti- cated, and required keys generated, another security attribute exchange takes place to establish the complete SA which will be used for subsequent communications by the entity that invoked ISAKMP. The basic set of SA at- tributes that MUST be implemented to provide ISAKMP interoperability are defined in Appendix C. *These atributes will be moved to a separate docu- ment to maintain separation of protocol and attributes.* 1.3 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 gen- eration algorithms, that have different properties, are available to users (see [DOW92] and [ANSI94]). Properties of key exchange protocols include the key establishment method, authentication, symmetry, perfect forward secrecy, and back traffic protection. 1.3.1 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 al- gorithm to encrypt a randomly generated session key (for encrypting subse- quent communications) with the recipient's public key. The encrypted ran- dom 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 ben- efit of the key transport method is that it has less computational over- head then the following method. The Diffie-Hellman (D-H) algorithm illus- trates key generation using public key cryptography. The D-H algorithm is begun by two users exchanging public information. Each user then mathe- matically combines the other's public information along with their own se- cret information to compute a shared secret value. This secret value can Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 9]
INTERNET-DRAFT ISAKMP November 21, 1995 be used as a session key or as a key encryption key for encrypting a ran- domly 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 infor- mation held by both users. Assuming checks for weak values neither party can force the session key to a predetermined value. Detailed descrip- tions of these algorithms can be found in [Schn94]. 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 provide 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 ex- change. 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 estab- lished 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 with- out effecting the key that is generated. This is desirable so that com- putation of the keys does not require either party to know who initiated the exchange. While key exchange symmetry is desirable, symmetry in the entire KMP may provide a vulnerablity to reflection attacks. The entire ISAKMP SA establishment is asymetrical. Back Traffic Protection / Perfect Forward Secrecy Perfect forward secrecy is provided by a key exchange protocol if disclosure of long-term cryp- tographic keying material (e.g. public signature keys) does not compro- mise previously generated keys. Back traffic protection is provided by the independent generation of each key such that subsequent keys are not dependent on any previous key. There is a subtle difference. Past ses- sion keys will NOT be obtainable is the long-term key is compromised in perfect forward secrecy; Past session keys will NOT be obtainable if the current session key is compromised in back traffic protecion. The difficulty of numerical factoring of large numbers has proven that cryptographic keys can protect information for a considerable length of time. However, this is based on the assumption that keys used for protec- tion of communications are destroyed after use and not kept for any rea- son. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 10]
INTERNET-DRAFT ISAKMP November 21, 1995 1.3.2 ISAKMP Requirements An authenticate key exchange MUST be supported by ISAKMP. Users SHOULD choose additional key establishment algorithms based on their require- ments. ISAKMP does not specify a specific key exchange. Requirements that should be evaluated when choosing a key establishment algorithm in- clude establishment method (generation vs. transport), perfect forward secrecy, back traffic protection, computational overhead, key escrow, and key strength. Based on user requirements, ISAKMP allows an entity initi- ating 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. 1.4 ISAKMP Protection 1.4.1 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. Phil Karn in his Internet-Draft [Karn95] has introduced a mechanism to provide a measure of denial of service protection through the use of a ``cookie'' exchange. This anti-clogging token (ACT) is aimed at protecting the com- puting resources from attack without spending excessive CPU resources to determine its authenticity. As described in [Karn95], 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). As noted by Karn, absolute protection against denial of service is impossible, but this anti-clogging token provides a technique for making it easier to han- dle. 1.4.2 Connection Hijacking ISAKMP prevents connection hijacking by linking the authentication, key exchange and security association exchanges. This linking prevent an at- tacker from allowing the authentication to complete and then jumping in and impersonating one entity to the other during the key and security as- sociation exchanges. 1.4.3 Man-in-the-Middle Attacks Man-in-the-Middle attacks include interception, insertion, deletion, and modification of messages, reflecting messages back at the sender, re- playing old messages and redirecting messages. ISAKMP features prevent Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 11]
INTERNET-DRAFT ISAKMP November 21, 1995 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 mes- sage from causing harm. The requirement for a new cookie with time vari- ant material for each new SA establishment prevents attacks that involve replaying old messages. The ISAKMP strong authentication requirement pre- vents an SA from being established with other then 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 specifica- tion defines where abnormal processing has occurred and recommends notify- ing the appropriate party of this abnormality. 1.5 Multicast Communications While future Internet communications will increasingly be of a multicast nature, this document is presenting a security association and key man- agement protocol from the unicast point of view. It is expected that mul- ticast communications will require the same security services as unicast communications and may introduce the need for additional security ser- vices. The issues of distributing SPIs for multicast traffic are pre- sented in [RFC-1825]. Upon agreement and implementation of a security association protocol for the Internet unicast environment, we fully intend to examine any additional security requirements for multicast communica- tions. For an introduction to the issues related to multicast security consult the Internet Drafts, [Spar94a] and [Spar94b], describing Sparta's research in this area. 2 Description of the Protocol The Internet Security Association and Key Management Protocol (ISAKMP) de- fines procedures and packet formats to establish, negotiate, modify and delete Security Associations (SA). SAs contain all the information re- quired for execution of IP security services, such as the IP Authentica- tion Header (AH), the IP Encapsulating Security Payload (ESP), and routing protocol authentication mechanisms. ISAKMP includes packet formats for exchanging key generation and authentication data. These formats provide a consistent method of transferring key and authentication data that is independent of the key generation technique, encryption algorithm or au- thentication mechanism. The following sections contain the details of ISAKMP. Sections 2.1 through 2.3 cover details that are pertinent to the entire protocol. Sections 3 through 6 define the establishment, modification, and deletion services, defined as exchanges, offered by the protocol. The appendices provide Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 12]
INTERNET-DRAFT ISAKMP November 21, 1995 examples of SAs and key exchanges. 2.1 ISAKMP Header Format ISAKMP has a fixed header format (shown in Figure 1) followed by a vari- able length payload, optional digital signature, and optional padding. A fixed header simplifies parsing, providing the benefit of protocol parsing software that is less complex and easier to implement. The fixed header contains the information required by the protocol to maintain state, pro- cess payloads and prevent attacks (e.g. denial of service and replay). Based on the message type, each header is followed by a payload specific to the message type. The payload for each message is defined in sections 3 through 6. Following the payload portion of the ISAKMP packet is a dig- ital signature. This field is dependent on the negotiation of Security Association attributes and may not be present. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Message Type ! Exch ! Vers ! Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Security Parameter Index (SPI) ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! ! ~ Initiator-Cookie ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! ! ~ Responder-Cookie ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! ! ~ Payload ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! ! ~ Digital Signature ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! ! ~ Padding ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: ISAKMP Header Format Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 13]
INTERNET-DRAFT ISAKMP November 21, 1995 o Message Type (1 octet) - Indicates the type of message. A suffix of REQ denotes a Request message type and an RESP suffix denotes a Response message type. The format and processing for each message is defined in sections 3 through 6. __ISAKMP_Message__Message_Type_ RESERVED 0 ISA_INIT_REQ 1 ISA_INIT_RESP 2 ISA_KE_REQ 3 ISA_KE_RESP 4 ISA_AUTH_REQ 5 ISA_AUTH_RESP 6 ISA_AUTH&KE_REQ 7 ISA_AUTH&KE_RESP 8 ISA_NEG_REQ 9 ISA_NEG_RESP 10 ISA_MODIFY_REQ 11 ISA_MODIFY_RESP 12 ISA_NOTIFY 13 ISA_DELETE 14 ISA_COMMIT 15 IANA Use 16-127 Future Use 128-255 o Exchange (4 bits) - indicates the type of exchange, see section 2.2 for a description of the Message Types exchanged in each of these Exchange Types. ___ISAKMP_Exchange___Exchange_Type__ RESERVED 0 Base 1 Identity Protection 2 Authentication Only 3 Future Use 4 - 15 o Version (4 bits) - indicates the version of the ISAKMP protocol in use. o Length (2 octets) - Length of total message (header + payload) in octets. o SPI (4 octets) - Security Parameter Index. The receiving entity's SPI is always in this field, except for the ISA_INIT packets. The ISA_INIT packets contain the SPI the initiator expects to receive in all subsequent packets. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 14]
INTERNET-DRAFT ISAKMP November 21, 1995 o Initiator Cookie (16 octets) - Cookie of entity that initiated SA establishment, SA modify or SA delete. o Responder Cookie (16 octets) - Cookie of entity that is responding to an SA establishment, SA modify or SA delete request. o Payload (variable) - The format of the payload is based on the message type. These are defined in sections 3 through 6. o Signature - The digital signature of the initiator of the ISAKMP message. This field will not be included on all packets and will be determined by the negotiated SA attributes. o Padding - This is an optional field that may be added depending on the type of encryption algorithm. If the encryption mechanism is based on block encryption, then this field may be necessary to ensure the packet is a specific size. 2.1.1 General Message Processing Every ISAKMP message has basic processing applied to insure protocol re- liability, and to minimize threats, such as denial of service and replay attacks. When transmitting an ISAKMP packet, the transmitting entity (initiator or responder) does the following: 1. Sets a timer and initializes a retry counter. 2. If the timer expires, the ISAKMP packet is resent and the retry counter is decremented. 3. If the retry counter reaches zero (0), the event, RETRY LIMIT REACHED, is logged in the appropriate system audit file. 4. The ISAKMP protocol machine clears all states and returns to IDLE. When an ISAKMP packet is received, the receiving entity (initiator or re- sponder) does the following: 1. Verifies the Initiator and Responder ``cookies''. If the cookie validation fails, the message is discarded and the following actions are taken: (a) The event, INVALID COOKIE, is logged in the appropriate system Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 15]
INTERNET-DRAFT ISAKMP November 21, 1995 audit file. (b) No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 2. Check the Message Type field to confirm it is valid. If the Message Type field validation fails, the message is discarded and the following actions are taken: (a) The event, INVALID MESSAGE TYPE, is logged in the appropriate system audit file. (b) No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 3. Check the Exchange field to confirm it is valid for the Message Type requested. If the Exchange field validation fails, the message is discarded and the following actions are taken: (a) The event, INVALID EXCHANGE TYPE, is logged in the appropriate system audit file. (b) No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 4. Check SPI to ensure it is valid for the Message Type and Exchange being performed. If the SPI validation fails, the message is discarded and the following actions are taken: (a) The event, INVALID SPI, is logged in the appropriate system audit file. (b) No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 5. The message payload is processed. Individual message processing is described in sections 3 through 6. Depending on the Message Type, a valid message results in a response being sent to the transmitting entity (message originator). The procedures for sending these responses are also outline in sections 3 through 6. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 16]
INTERNET-DRAFT ISAKMP November 21, 1995 2.2 ISAKMP Packet Exchanges The Exchange field in the ISAKMP header currently has three values de- fined: the base exchange, the identity protection exchange, and the au- thentication only exchange. These exchanges define the flow of the ISAKMP packets during SA establishment. The diagrams in 2.2.1, 2.2.2, and 2.2.3 show the packet exchange ordering for each exchange type and provide basic notes describing what has happened after each packet exchange. 2.2.1 Base Exchange Sections 3.1 through 3.3 describe the three basic phases: SA Initial- ization, Key Exchange and Authentication, and SA Negotiation, that com- prise the base exchange. The base exchange contains the minimum number of packet exchanges in order to reduce latency associated with SA establish- ment. Base Exchange ____Initiator____Direction_____Responder____ Note ISA_INIT_REQ => <= ISA_INIT_RESP Basic SA selected ISA_AUTH&KE_REQ => <= ISA_AUTH&KE_RESP Identity Verified Key Generated Encryption Begun ISA_NEG_REQ => <= ISA_NEG_RESP SA Completed (optional) ISA_COMMIT => 2.2.2 Identity Protection Exchange The identity protection exchange starts and ends the same as the base ex- change, but separates the key exchange payload and the authentication pay- loads into separate packets. In this exchange, the key exchange is trans- mitted first followed by the strong authentication exchange. The benefit of this exchange is the ability to communicate with a person without dis- closing either party's identity to passive attackers on the network. The ISA_KE_REQ and ISA_KE_RESP packets used for the key exchange portion of this exchange contain an ISAKMP header followed by the key exchange pay- load. The ISA_AUTH_REQ and ISA_AUTH_RESP packet used for the authentication portion of this exchange contain an ISAKMP header followed by the authen- Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 17]
INTERNET-DRAFT ISAKMP November 21, 1995 tication payload. Identity Protection Exchange __Initiator___Direction___Responder___ Note ISA_INIT_REQ => <= ISA_INIT_RESP Basic SA selected ISA_KE_REQ => <= ISA_KE_RESP Key Generated Encryption Begun ISA_AUTH_REQ => <= ISA_AUTH_RESP Identity Verified ISA_NEG_REQ => <= ISA_NEG_RESP SA Completed (optional) ISA_COMMIT => 2.2.3 Authentication Only Exchange The authentication only exchange starts and ends the same as the base ex- change. In this exchange, the authentication information is the only in- formation transmitted. The benefit of this exchange is the ability to perform only an authentication exchange without the computational expense of computing keys. Using this exchange, none of the transmitted informa- tion will be encrypted. The ISA_AUTH_REQ and ISA_AUTH_RESP packet used for the authentication only exchange contain an ISAKMP header followed by the authentication payload. Identity Protection Exchange __Initiator___Direction___Responder___ Note ISA_INIT_REQ => <= ISA_INIT_RESP Basic SA selected ISA_AUTH_REQ => <= ISA_AUTH_RESP Identity Verified ISA_NEG_REQ => <= ISA_NEG_RESP SA Completed (optional) ISA_COMMIT => Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 18]
INTERNET-DRAFT ISAKMP November 21, 1995 2.3 ISAKMP Details 2.3.1 Security Association Attributes A Security Association (SA) is a relationship between two entities that describes how they will utilize security services. This relationship is represented by a collection of security related information. The SA At- tributes are the individual elements that comprise all security relevant information necessary to form the SA. The following syntax defines the security attributes to be exchanged by ISAKMP. This syntax is used in the ISA_INIT_REQ, ISA_INIT_RESP, ISA_NEG_REQ, ISA_NEG_RESP, ISA_MOD_REQ, and ISA_MOD_RESP messages. The syntax groups se- curity attributes needed to perform a security function into either an SA set or SA list format. The set format MUST be supported by ISAKMP imple- mentations. The list format is an optional format. Security Associations Sets The set format groups all necessary attributes together. Each set has a unique identifier (Set Number), supported secu- rity service (Supports), such as IP AH, IP ESP, OSPF authentication, and a list of Attribute Classes and Attribute Types. The Attribute Class is the broad category of Attribute Type, such as encryption algorithms. At- tribute Type is a specific attribute identifier. DES is an example of an attribute type for the encryption algorithm attribute class. A set has only one instance of an Attribute Class and one type for that class. This syntax maintains flexibility by allowing many different (and some still undefined) types of SA attributes to be exchanged. Figure 2 depicts the syntax for exchanging security attributes using the set format. It shows a single set from which multiple sets would be grouped for a specific message type. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Set Number ! Supports ! Num of Attr ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Attribute Class ! Attribute Type ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ..... ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Attribute Class ! Attribute Type ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Generic Set Exchange Format Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 19]
INTERNET-DRAFT ISAKMP November 21, 1995 o Set number (1 octet) - Unique identifier for each proposed set o Supports (2 octets) - Security service proposed set supports. Examples are IP AH, IP ESP, and OSPF authentication o Number of Attributes (1 octet) - Number of attribute classes contained in the proposed set o Attribute Class (2 octets) - examples are Encryption Algorithms, Key Exchange Algorithms, Authentication Mechanisms o Attribute Type (2 octets) - examples of attribute types for the encryption algorithms attribute class are DES, Triple DES, and IDEA. The size of a set formatted exchange is 4 octets + (Number of Attribute Classes * 4 octets). Computing the size of a particular set allows the determination of the beginning of the next set without completely parsing the current set. This is necessary when it is determined that the current set is not an acceptable SA set. This will improve the performance of SA Attribute determination. Security Association Lists The SA list format presents several attribute types for an attribute class. Each type within the class is then ordered to indicate its precedence. Specifically, the first attribute type is the highest priority type, followed by other choices. Each subsequent choice is listed in descending priority order. An attribute type must be chosen for each attribute class to establish a complete SA. Figure 3 shows the syntax for the optional list exchange format. The num- ber of types is determined by the Count field. The number of Attribute Types within an Attribute Class will depend on what is supported by the local machine. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Attribute Class ! Count ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Attribute Type ! Attribute Type ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Attribute Type ! Attribute Type ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Generic List Exchange Format Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 20]
INTERNET-DRAFT ISAKMP November 21, 1995 o Attribute Class (2 octets) - Examples are Encryption Algorithms, Key Exchange Algorithms o Count - Number of proposed Attribute Types for the given Attribute Class o Attribute Type (2 octets) - Presented in descending priority order Appendix B presents an outline containing a comprehensive listing of SA attributes. This listing of attributes are initial definitions and are presented to stimulate thought and discussion on SAs. The final SA for a protocol SHOULD be defined in that protocol so additions or modifica- tions to the attributes do not require a modification to the Internet Key Management Protocol (IKMP) RFC and vice versa. For example, Appendix C describes the sample security associations for ISAKMP and IPSP ESP and AH. 2.3.2 Transport Protocol The User Datagram Protocol (UDP) is the transport protocol for ISAKMP. UDP checksumming discards UDP packets with an incorrect or zero (0) checksum. ISAKMP is unaware of the discard, but will resend the packet when its re- send timer expires. 2.3.3 RESERVED Fields The existence of RESERVED fields are strictly used to preserve byte alignement. 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 zero (0) and discard the packet if other values are found. 2.3.4 Anti-Clogging Token (``Cookie'') Creation Phil Karn's Internet Draft [Karn95] states that cookie generation is im- plementation dependent, but must satisfy some basic requirements: 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 Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 21]
INTERNET-DRAFT ISAKMP November 21, 1995 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, SA modify and SA delete to help prevent replay attacks, therefore the date and time MUST be added to the information hashed. 2.3.5 SA Flags Field The SA Flags field may be set by the entity that initiated the negotia- tion to indicate that the ISA_COMMIT packet will follow the completion of the protocol exchange. The SA Flags field exists only in the ISA_INIT and ISA_NEG packets. If the initiating entity sets the flag, the responding entity cannot reset it. If the initiating entity does not set the flag, the responding entity may set it, thereby, forcing the initiating entity to issue an ISA_COMMIT packet. If neither entity sets the flag, then the ISA_COMMIT packet will not be issued. To set the flag the Least Signifi- cant Bit (LSB) in the SA Flags field is set to one (1) . All other bits in the SA Flags field are zero (0). 3 Security Association Establishment Security Association (SA) Establishment is the process of agreeing upon and exchanging all the security information that is required in an SA. The following sections, 3.1 to 3.3, describe the three basic phases that com- prise SA Establishment: SA Initialization, Key and Authentication infor- mation exchange, and SA Negotiation. 3.1 Security Association Initialization The initialization exchange of SA establishment is composed of the ISA_INIT_REQ and ISA_INIT_RESP packets shown in figure 4. The ISA_INIT pack- ets exchange ``cookies'', and options for a key generation technique, an encryption algorithm and an authentication mechanism. The ``cookies'' Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 22]
INTERNET-DRAFT ISAKMP November 21, 1995 are used to prevent replay and denial of service attacks. Authentication and encryption for the ISAKMP exchanges are provided by the authentication mechanism and encryption algorithm selected. The key generation technique selected creates keys for use by the authentication mechanism and encryp- tion algorithm. The keys may also be used as any of the following: ac- tual session keys, to create the session keys, or to protect the exchange of the actual session keys for the SA. If the key, encryption algorithm, and authentication mechanism are only used to protect ISAKMP exchanges, then new options can be picked during the negotiation phase (described in Section 3.3) for use in protecting the actual data communications. If en- cryption is not required for the SA, the encryption algorithm options are not exchanged. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ISAKMP Header ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! SA Syntax Type! SA Flags ! # Sets/Lists ! RESERVED ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ SA Attribute Set/List #1 ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ SA Attribute Set/List #2 ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ... ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ SA Attribute Set/List #N ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: ISA_INIT_REQ and ISA_INIT_RESP Packet Format o ISAKMP Header - Described in Section 2.1 o SA Syntax Type (1 octet) - Presentation format of SAs _SA_Syntax__SA_Syntax_Type_ RESERVED 0 Set 1 List 2 o SA Flags (1 octet) - Flags specific to an SA service. See section 2.3.5 for details. o Number of Sets (1 octet) - Number of SA Attribute Sets being proposed Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 23]
INTERNET-DRAFT ISAKMP November 21, 1995 o SA Attributes (variable) - A list of SA Attributes. The SA Attribute specifications are discussed in Section 2.3.1. 3.1.1 SA Initialization Procedures When issuing an ISA_INIT_REQ message, the initiating entity does the fol- lowing: 1. Create initiator cookie. See Section 2.3.4 for details. 2. Generate a unique pseudo-random SPI. See Section 2.1 for details. 3. Construct an ISA_INIT_REQ packet. If the initiator will send an ISA_COMMIT message upon completion of the SA establishment, then the SA Flags field MUST be set (see section 2.3.5 and 3.4). 4. Transmit the packet to the destination host as described in Section 2.1.1. When an ISA_INIT_REQ message is received, the receiving entity does the following: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Unpack the ISA_INIT_REQ payload and determine the highest priority attribute set (or attribute list) supported. If the proposed attribute set (or list) is rejected, then the protocol machine must clear all state and return to IDLE. 3. Create responder cookie. See Section 2.3.4 for details. 4. Generate a unique pseudo-random SPI. See Section 2.1 for details. 5. Construct an ISA_INIT_RESP packet. If the responder wants to request that an ISA_COMMIT message be sent upon completion of the SA establishment, then the SA Flags field MUST be set (see section 2.3.5 and 3.4). 6. Transmit the packet to the initiating host as described in Section 2.1.1. When an ISA_INIT_RESP message is received, the receiving entity (original initiator) does the following: Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 24]
INTERNET-DRAFT ISAKMP November 21, 1995 1. Check the ISAKMP header as described in Section 2.1.1. 2. Unpack the ISA_INIT_RESP payload. 3. Determine if the attribute set (or list) selected by the responder is valid. If the attribute set (or list) is invalid or the responder rejected all proposed attribute sets (or lists), the receiving entity does the following: (a) The event, INVALID ATTRIBUTES, is logged in the appropriate system audit file. (b) Clear all state and return to IDLE. Any further communication must start the SA initialization procedures from the beginning. If the attribute set (or list) is valid, the receiving entity does the following: (a) Configure protocol machine based on attribute set selected. (b) Transition to Authentication and Key Exchange (see Section 3.2). 3.2 Authentication and Key Exchange During the authentication and key exchange phase, information required to confirm the identities of the parties wishing to establish the SA and es- tablish a session key for use during the SA establishment is exchanged. Depending on the key exchange algorithm, the original key may be used dur- ing data communications or a new one may be created and exchanged during the negotiation phase (described in section 3.3). This original or new key would be used in protecting the actual data communications. The packets that carry the authentication and key exchange payloads have the format shown in Figure 5. When the ISA_AUTH&KE_REQ and ISA_AUTH&KE_RESP packets are used, the Authentication Payload SHOULD be processed first to strongly authenticate the packet issuer, followed by the processing of the Key Exchange Payload. The authentication and key exchange payloads (shown in Figures 6 and 7) are general formats which support many types of authentication and key exchange mechanisms. The detailed specification of these fields will be specified in companion RFCs. These companion RFCs will define the standard authentication and key exchange mechanisms that need to be implemented to assure compliance with this specification. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 25]
INTERNET-DRAFT ISAKMP November 21, 1995 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ISAKMP Header ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ! Authentication Payload ! ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ! Key Exchange Payload ! ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: ISA_AUTH&KE_REQ and ISA_AUTH&KE_RESP Packet Format 3.2.1 Authentication Payload Format This section specifies the encoding of the authentication payload for the ISA_AUTH_REQ, ISA_AUTH_RESP, ISA_AUTH&KE_REQ, and ISA_AUTH&KE_RESP messages. As described in section 2.2.3, when the ISA_AUTH_REQ and ISA_AUTH_RESP pack- ets are transmitted alone, the key exchange payload is not present. The format of these messages is shown in Figure 6. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Authentication Authority ! Reserved ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Authentication Type ! Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ! Authentication Data ! ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Authentication Payload Format Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 26]
INTERNET-DRAFT ISAKMP November 21, 1995 o Authentication Authority (2 octets) - This field identifies the party that generated the certificates used for authentication. Authorities must be assigned an identifier by the Internet Assigned Numbers Authority (IANA). Before being assigned an identifier, an authority must publish an RFC defining the authority's domain. [RFC-1422] describes the Internet Policy Registration Authority (IPRA) and the procedures for achieving this registration. If PGP certificates, based on the ``web of trust'', are carried in the authentication payload the Authentication Authority value is one (1). Example certificate authorities that would have to register for an identifier are: -- RSA Commercial Certificate Authority (http://www_csc.rsa.com/netsite) -- Stable Large E-mail Database (SLED) (http://www.four11.com) -- U.S. Postal Service. o Authentication Type (2 octets) - This field indicates the authentication payload format. This field is used by authentication authorities that support more than one certificate type. The authentication types supported by an authentication authority must be defined in the RFC required for authentication authority registration. Examples are: -- RSA certificates -- PGP certificates -- DSS certificates -- DNS Signed Keys -- Kerberos Tokens -- X.509 certificates o Length (2 octets) - Length of the Authentication Data field in octets. o Authentication Data (variable) - Actual authentication data. The type of certificate is indicated by the Authentication Type field. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 27]
INTERNET-DRAFT ISAKMP November 21, 1995 3.2.2 Key Exchange Payload Format A variety of key exchanges can be supported in the key exchange phase. Some examples of key exchanges which may be used in this protocol are Diffie-Hellman, the enhanced Diffie-Hellman key exchange described in X9.42 [ANSI94], the key exchange on the FORTEZZA card, and the RSA-based key exchange used by PGP. This protocol will also support key exchanges that include key escrow or data recovery techniques, but does not mandate their use. The encoding of the key exchange payload is dependent on the specific key exchange and, therefore, is not specified in this Internet draft. Each key exchange must define the following information: (a) System parame- ters, (b) Key establishment algorithm, and (c) Key derivation procedure (dependent on key exchange type). There can be both public and private key generation techniques. Both types must register with IANA to obtain a Key Exchange Identifier (KEI). Before published public key exchanges can obtain a KEI, an RFC defining the key exchange payload format and key generation procedures MUST be sub- mitted. Private key exchanges SHOULD be documented in an RFC when regis- tering for a KEI. As described in section 2.2.2, when the ISA_KE_REQ and ISA_KE_RESP packets are transmitted alone, the authentication payload is not present. Once the key exchange is completed, then the authentication payload is sent separately using the format described in section 3.2.1 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! KEI ! Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ! Key Exchange Payload ! ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Key Exchange Payload Format o KEI (2 octets) - Key Exchange Identifier o Length (2 octets) - Length of payload in octets o Key Exchange Payload (variable) - Data (i.e. public values) required to create session key. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 28]
INTERNET-DRAFT ISAKMP November 21, 1995 3.2.3 Authentication and Key Exchange Procedures When issuing an ISA_AUTH&KE_REQ packet, the initiating entity will do the following: 1. Create the ISAKMP Header. 2. Create the authentication payload. 3. Create the key exchange payload based on KEI. 4. Construct an ISA_AUTH&KE_REQ packet. 5. Generate an authentication signature using the authentication attributes and options selected in the initialization phase. 6. Transmit the packet to the responding host as described in Section 2.1.1. When an ISA_AUTH&KE_REQ packet is received, the receiving entity will do the following: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Verify the initiator's signature. The ISA_AUTH&KE_REQ packet is processed and the calculated signature is compared to the signature contained in the ISA_AUTH&KE_REQ packet. If these signatures are not identical, the message is discarded and the following actions are taken: (a) The event, INVALID SIGNATURE, is logged in the appropriate system audit file. (b) No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 3. Unpack the ISA_AUTH&KE_REQ packet. 4. Create the ISAKMP Header. 5. Create the authentication payload. 6. Create the key exchange payload based on KEI. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 29]
INTERNET-DRAFT ISAKMP November 21, 1995 7. Construct an ISA_AUTH&KE_RESP packet. 8. Generate an authentication signature, to authenticate responder to initiator, using the authentication attributes and options selected. 9. Transmit the packet to the initiating host as described in Section 2.1.1. 10. Begin key calculation in the background, if necessary. When an ISA_AUTH&KE_RESP message is received, the receiving entity (origi- nal initiator) will do the following: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Verify the initiator's signature. The ISA_AUTH&KE_RESP packet is processed and the calculated signature is compared to the signature contained in the ISA_AUTH&KE_RESP packet. If these signatures are not identical, the message is discarded and the following actions are taken: (a) The event, INVALID SIGNATURE, is logged in the appropriate system audit file. (b) No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 3. Calculate key, if necessary. 4. Transition to Security Association Negotiation. 3.3 Security Association Negotiation The SA Negotiation phase allows the initiating entity to present SA at- tributes that it wishes to use for secure communications to a respond- ing entity. These SA attributes may include additional options for the attributes agreed upon during the initialization phase, as well as ad- ditional attributes required for an SA. As an example, the SA parame- ters for the IP AH and IP ESP security mechanisms are cited in the Secu- rity Architecture for the Internet Protocol [RFC-1825]. The format for the ISA_NEG_REQ and ISA_NEG_RESP packets is the same as the ISA_INIT_REQ and ISA_INIT_RESP shown in Figure 4. All fields shown in Figure 4 exist for the ISA_NEG_REQ and ISA_NEG_RESP packets. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 30]
INTERNET-DRAFT ISAKMP November 21, 1995 3.3.1 SA Negotiation Procedures When issuing an ISA_NEG_REQ packet, the initiating entity does the follow- ing: 1. Determine SA attributes to be negotiated. This may include changing some attributes from the original SA initialization. 2. Construct an ISA_NEG_REQ packet. If the initiator will send an ISA_COMMIT message upon completion of the SA establishment, then the SA Flags field MUST be set (see section 2.3.5 and 3.4). 3. Depending on the SA Attributes established in the SA initialization phase, apply the agreed upon security services. (a) If the SA requires authentication, the ISA_NEG_REQ packet is pro- cessed (or signed) and the signature placed as noted in Figure 1. (b) If the SA requires encryption and the encryption algorithm is a block encryption algorithm, then padding up to the block size MUST be placed as noted in Figure 1. (c) If the SA requires encryption, the ISA_NEG_REQ payload and Signature are encrypted. 4. Transmit the packet to the responding host as described in Section 2.1.1. When an ISA_NEG_REQ packet is received, the receiving entity does the fol- lowing: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Depending on the SA Attributes, apply the agreed upon security services. (a) If the SA requires encryption, decrypt the ISA_NEG_REQ payload and Signature. If the decryption fails, the message is discarded and the following actions are taken: i. The event, DECRYPTION FAILED, is logged in the appropriate system audit file. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 31]
INTERNET-DRAFT ISAKMP November 21, 1995 ii. No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. (b) If the SA requires authentication, the ISA_NEG_REQ packet is processed and the calculated signature is compared to the signature contained in the ISA_NEG_REQ packet. If these signatures are not identical, the message is discarded and the following actions are taken: i. The event, INVALID SIGNATURE, is logged in the appropriate system audit file. ii. No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 3. Unpack the ISA_NEG_REQ packet payload and determine the highest priority SA attributes supported. If none of the SA attribute options are supported, the ISA_NEG_RESP packet will have the value zero (0) in the Number of Sets field and an SA will not be established. 4. If the SA negotiation is requesting a key change or new authentication mechanism, then generate the appropriate information and include it as an attribute in the ISA_NEG_RESP payload. 5. Construct an ISA_NEG_RESP packet. If the responder wants to request that an ISA_COMMIT message be sent upon completion of the SA establishment, then the SA Flags field MUST be set (see section 2.3.5 and 3.4). 6. Depending on the SA Attributes, apply the agreed upon security services. (a) If the SA requires authentication, the ISA_NEG_RESP packet is processed and the signature placed as noted in Figure 1. (b) If the SA requires encryption and the encryption algorithm is a block encryption algorithm, then padding up to the block size MUST be placed as noted in Figure 1. (c) If the SA requires encryption, the ISA_NEG_RESP payload and Signature are encrypted. 7. Transmit the packet to the initiating host as described in Section 2.1.1. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 32]
INTERNET-DRAFT ISAKMP November 21, 1995 8. If required, begin calculation of the new session key in the background. 9. Transition to SA Negotation Conclusion (see Section 3.4). When an ISA_NEG_RESP message is received, the receiving entity (original initiator) does the following: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Depending on the SA Attributes, apply the agreed upon security services. (a) If the SA requires encryption, decrypt the ISA_NEG_RESP payload and Signature. If the decryption fails, the message is discarded and the following actions are taken: i. The event, DECRYPTION FAILED, is logged in the appropriate system audit file. ii. No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. (b) If the SA requires authentication, the ISA_NEG_RESP packet is processed and the calculated signature is compared to the signature contained in the ISA_NEG_RESP packet. If these signatures are not identical, the message is discarded and the following actions are taken: i. The event, INVALID SIGNATURE, is logged in the appropriate system audit file. ii. No response is sent to the initiating entity. This will cause the transmission timer of the initiating entity to expire and force retransmission of the message. 3. Unpack the ISA_NEG_RESP payload and verify the SA attributes selected by responder are valid. If the attribute sets (or lists) are invalid or the responder rejected all proposed attribute sets (or lists), the receiving entity does the following: Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 33]
INTERNET-DRAFT ISAKMP November 21, 1995 (a) The event, INVALID ATTRIBUTES, is logged in the appropriate system audit file. (b) Clear all state and return to IDLE. If the attribute set (or list) is valid, the receiving entity does the following: (a) Configure the protocol machine based on the attribute set (or list) selected. 4. If required, begin calculation of the new session key in the background. 5. Transition to SA Negotiation Conclusion (see Section 3.4). 3.4 SA Negotiation Conclusion The SA negotiation concludes with the transmittal of the optional SA_COMMIT packet. This is determined by the setting of the SA Flags field. The SA_COMMIT message allows the initiating entity to inform the responding party that it has completed the processing required to set-up the SA and therefore, secure communications may begin. If the entity ini- tiating the SA establishment does not have the ability to queue incoming data it may receive prior to its completion of SA establishment process- ing, then it requires the responding entity to wait for an SA_COMMIT mes- sage before sending data. The transmittal of the ISA_COMMIT packet is op- tional and determined by the policy of the parties establishing the SA. All implementations MUST be able to generate, transmit, and receive this message. The ISA_COMMIT packet is the ISAKMP header, described in section 2.1, with no payload. 3.4.1 SA Negotiation Conclusion Procedures When issuing an ISA_COMMIT packet, the initiating entity does the follow- ing: 1. Construct an ISA_COMMIT packet (ISAKMP Header). 2. Depending on the SA Attributes established in the SA initialization phase, apply the agreed upon security services. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 34]
INTERNET-DRAFT ISAKMP November 21, 1995 (a) If the SA requires authentication, the ISA_COMMIT packet is pro- cessed (or signed) and the signature placed as noted in Figure 1. (b) If the SA requires encryption and the encryption algorithm is a block encryption algorithm, then padding up to the block size MUST be placed as noted in Figure 1. (c) If the SA requires encryption, the ISA_COMMIT Signature is encrypted. 3. Transmit the packet to the responding host as described in Section 2.1.1. 4. Clear all state and return to IDLE. When an ISA_COMMIT packet is received, the receiving entity does the fol- lowing: 1. Check the ISAKMP header as described in section 2.1.1. 2. Depending on the SA Attributes, apply the agreed upon security services. (a) If the SA requires encryption, decrypt the ISA_COMMIT Signature. If the decryption fails, the message is discarded and the following actions are taken: i. The event, DECRYPTION FAILED, is logged in the appropriate system audit file. ii. Because the ISA_COMMIT packet is a unidirectional message a retransmission will not be performed. Because the SA is established, we recommend that communications can proceed, however, the local security policy will dictate the procedures for continuing. We recommend that an ISA_NOTIFY packet with an Error Message Type (see Section 6) be sent to the originator of the ISA_COMMIT packet. (b) If the SA requires authentication, the ISA_COMMIT packet is processed and the calculated signature is compared to the signature contained in the ISA_COMMIT packet. If these signatures are not identical, the message is discarded and the following actions are taken: Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 35]
INTERNET-DRAFT ISAKMP November 21, 1995 i. The event, INVALID SIGNATURE, is logged in the appropriate system audit file. ii. Because the ISA_COMMIT packet is a unidirectional message a retransmission will not be performed. Because the SA is established, we recommend that communications can proceed, however, the local security policy will dictate the procedures for continuing. We recommend that an ISA_NOTIFY packet with an Error Message Type (see Section 6) be sent to the originator of the ISA_COMMIT packet. 3. Clear all state and return to IDLE. 4 Security Association Modification Security Association modification provides the ability to update security association attributes and parameters within an existing SA without having to establish a new SA. The use of this exchange can provide performance benefits without sacrificing the security of the existing communication. The most common use of this exchange will be to re-key an existing SA. The format for the ISA_MODIFY packet is the same as the ISA_INIT_REQ and ISA_INIT_RESP shown in Figure 4. All fields shown in Figure 4 exist for the ISA_MODIFY packets. 4.1 Modification Procedures The procedure for exchanging information to modify an SA are similiar to the SA negotiation exchange. The details of SA modification will be de- scribed in this section as they are solidified during prototype develop- ment. 5 Security Association Deletion During communications it is possible that hosts may be compromised or that information may be intercepted during transmission. Determining whether this has occurred is not an easy task and is outside the scope of this Internet-Draft. However, if it is discovered that transmissions are being compromised, then it is necessary to delete the current SA and establish a new SA. The ISA_DELETE packet (shown in Figure 8) provides a controlled method of informing a peer entity that the initiating entity has deleted an SA(s). The ISA_DELETE packet allows for the deletion of any number of SAs with Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 36]
INTERNET-DRAFT ISAKMP November 21, 1995 a single message. The receiving entity SHOULD clean up its local SA database. The receiving entity may be using the SA for secure communi- cations with more than one party and would not want to actually delete the SA from its database in this case. However, upon receipt of an ISA_DELETE packet the SAs listed in the SPIs field of the packet cannot be used with the initiating entity. The SA Establishment procedure must be invoked to re-establish secure communications. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ISAKMP Header ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! SPI Count ! Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ! SPIs ! ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: SA Delete Payload Format o SPI Count - Number of security associations to be deleted o Length - length of payload in octets o SPIs - Initiator's Security Parameter Index(s) to be deleted 5.1 Deletion Procedures When issuing an ISA_DELETE packet, the issuing entity (initiator or re- sponder) does the following: 1. Create initiator cookie. See Section 2.3.4 for details. 2. Determine SPI of receiving entity. 3. Construct the ISA_DELETE packet. 4. Depending on the SA Attributes, apply the agreed upon security services. (a) If the SA requires authentication, the ISA_DELETE packet is processed and the signature placed as noted in Figure 1. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 37]
INTERNET-DRAFT ISAKMP November 21, 1995 (b) If the SA requires encryption, the ISA_DELETE payload and Signature are encrypted. 5. Transmit the packet to the destination host as described in Section 2.1.1. 6. Update the local SA database to reflect the SPI deletions. Upon receipt of an ISA_DELETE packet, the receiving entity (initiator or responder) does the following: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Depending on the SA Attributes, apply the agreed upon security services in the following order. (a) If the SA requires encryption, decrypt the ISA_DELETE payload and Signature. If the decryption fails, the message is discarded and the following actions are taken: i. The event is logged in the appropriate system audit file. ii. Because the ISA_DELETE packet is a unidirectional message a retransmission will not be performed. The local security policy will dictate the procedures for continuing. However, we recommend that the SPIs in the ISA_DELETE packet be checked to see if the originator was the communicating party. If so, then these SAs can be deleted from the local SA database. We also recommend that an ISA_NOTIFY packet with an Error Message Type (see Section 6) be sent to the originator of the ISA_DELETE packet. If the SPIs do not match those of the originator, then no further action should be taken. (b) If the SA requires authentication, the ISA_DELETE packet is processed and the calculated signature is compared to the signature contained in the ISA_DELETE packet. If these signatures are not identical, the message is discarded and the following actions are taken: i. The event is logged in the appropriate system audit file. ii. Because the ISA_DELETE packet is a unidirectional message a retransmission will not be performed. The local security policy will dictate the procedures for continuing. However, Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 38]
INTERNET-DRAFT ISAKMP November 21, 1995 we recommend that the SPIs in the ISA_DELETE packet be checked to see if the originator was the communicating party. If so, then these SAs can be deleted from the local SA database. We also recommend that an ISA_NOTIFY packet with an Error Message Type (see Section 6) be sent to the originator of the ISA_DELETE packet. If the SPIs do not match those of the originator, then no further action should be taken. 3. Unpack the ISA_DELETE payload. 4. Update the local SA database to reflect the SPI deletions. 6 Notification Message The ISAKMP ISA_NOTIFY packet contains information one party wants to send to another. Notification information can be error messages specifying why a SA could not be established. It can also be status data that a process managing an SA database wishes to communicate with a peer pro- cess. For example, a secure front end or security gateway may use the ISA_NOTIFY message to synchronize SA communication (see Appendix A.2). The ISA_NOTIFY packet is unidirectional. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ISAKMP Header ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! Notify Message Type ! Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ! Notify Payload ! ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: ISA NOTIFY Payload Format o Notify Message Type (2 octets) _Notification__Notify_Message_Type_ Error 1 Status 2 Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 39]
INTERNET-DRAFT ISAKMP November 21, 1995 o Length (2 octets) - length of payload in octets o Notify Payload (variable) - Value dependent on the Notify Message Type 6.1 Notification Procedures When issuing an ISA_NOTIFY message, the issuing entity (initiator or re- sponder) does the following: 1. Create initiator cookie. See Section 2.3.4 for details. 2. Determine SPI of receiving entity. 3. Construct ISA_NOTIFY packet. 4. Depending on the SA Attributes, apply the agreed upon security services. (a) If the SA requires authentication, the ISA_NOTIFY packet is processed and the signature placed as noted in Figure 1. (b) If the SA requires encryption, the ISA_NOTIFY payload and Signature are encrypted. 5. Transmit the packet to the destination host as described in Section 2.1.1. Upon receipt of an ISA_NOTIFY message, the receiving entity (initiator or responder) does the following: 1. Check the ISAKMP header as described in Section 2.1.1. 2. Depending on the SA Attributes, apply the agreed upon security services in the following order. (a) If the SA requires encryption, decrypt the ISA_NOTIFY payload and Signature. If the decryption fails, the message is discarded and the following actions are taken: i. The event is logged in the appropriate system audit file. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 40]
INTERNET-DRAFT ISAKMP November 21, 1995 ii. Because the ISA_NOTIFY packet is a unidirectional message a retransmission will not be performed. The local security policy will dictate the procedures for continuing. (b) If the SA requires authentication, the ISA_NOTIFY packet is processed and the calculated signature is compared to the signature contained in the ISA_NOTIFY packet. If these signatures are not identical, the message is discarded and the following actions are taken: i. The event is logged in the appropriate system audit file. ii. Because the ISA_NOTIFY packet is a unidirectional message a retransmission will not be performed. The local security policy will dictate the procedures for continuing. 3. Unpack the ISA_NOTIFY payload. 4. Depending on the Notify Message Type, additional processing may be necessary. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 41]
INTERNET-DRAFT ISAKMP November 21, 1995 7 Conclusions The Internet Security Association and Key Management Protocol (ISAKMP) is a well designed protocol aimed at the Internet of the future. The massive growth of the Internet will lead to great diversity in network utiliza- tion, communications, and security requirements. ISAKMP contains all the features that will be needed for this dynamic and expanding communications environment. ISAKMP's Security Association (SA) feature coupled with authentication and key establishment provides the security and flexibility that will be needed for future growth and diversity. This security diversity of multi- ple key exchange techniques, encryption algorithms, authentication mecha- nisms, security services, and security attributes will allow users to se- lect the appropriate security for their network, communications, and secu- rity needs. The SA feature allows users to specify and negotiate security requirements with other users. An additional benefit of supporting multi- ple techniques in a single protocol is that as new techniques are devel- oped they can easily be added to the protocol. This provides a path for the growth of Internet security services. ISAKMP supports both publicly or privately defined SAs, making it ideal for government, commercial, and private communications. ISAKMP provides the ability to establish SAs for multiple security proto- cols and applications. These protocols and applications may be session- oriented or sessionless. Having one SA establishment protocol that sup- ports multiple security protocols eliminates the need for multiple, nearly identical authentication, key exchange and SA establishment protocols when more than one security protocol is in use or desired. Just as IP has pro- vided the common networking layer for the Internet, a common security es- tablishment protocol is needed if security is to become a reality on the Internet. ISAKMP provides the common base that allows all other security protocols to interoperate. ISAKMP follows good security design principles. It is not coupled to other insecure transport protocols, therefore it is not vulnerable or weakened by attacks on other protocols. Also, when more secure transport protocols are developed, ISAKMP can be easily migrated to them. ISAKMP also provides protection against protocol related attacks. This protec- tion provides the assurance that the SAs and keys established are with the desired party and not with an attacker. ISAKMP also follows good protocol design principles. Protocol specific information only is in the protocol header, following the design prin- ciples of IPv6. The data transported by the protocol is separated into functional payloads. As the Internet grows and evolves, new payloads to support new security functionality can be added without modifying the en- tire protocol. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 42]
INTERNET-DRAFT ISAKMP November 21, 1995 A ISAKMP Scenarios Examples scenerios are are presented to help illustrate the ISAKMP's abil- ity to support multiple authentication methods and key exchanges. A.1 Initial ISAKMP Daemon Scenerio This example steps through two ISAKMP daemons establishing an SA between themselves. This SA uses DNS Security Extentions [EK94] for authentica- tion and a Photuris [Karn95] compliant key exchange. Following the SA es- tablishment between the daemons, SAs are established for ESP and AH commu- nications between user processes. 1. The initiating daemon sends an ISA_INIT_REQ messages with ISAKMP SA #3, #2, and #1 (in priority order). These SAs are defined in C.1.1. 2. The responding daemon sends an ISA_INIT_RESP message indicating that ISAKMP SA #2 was selected, which requires DNS Signature and Key Records and a Photuris compliant key exchange [DOW92]. 3. The initiating daemon sends an ISA_KE_REQ packet with an index into well-known table of generator / prime pairs and it's public value. 4. Upon receipt of ISA_KE_REQ packet the responding daemon computes the shared secret and session key. 5. The responding daemon sends an ISA_KE_RESP packet with an its public value and both the initiator and responders public values signed using its Private (Signature) Key and encrypted in the session key created. 6. Upon receipt of ISA_KE_REQ packet the initiating daemon computes the shared secret and session key. 7. The initiating daemon sends an ISA_AUTH_REQ packet with both the initiator and responders public values signed using its Private (Signature) Key and it's DNS name and Public (Verification) Key signed by it nameserver. All encrypted in the session key created. 8. The responding daemon sends an ISA_AUTH_RESP packet with it's DNS name and Public (Verification) signed by it Secure DNS nameserver and encrypted in the session key created. 9. The initiating daemon sends an ISA_NEG_REQ packet with ESP SA #2, ESP SA #1, AH SA #1, and AH SA #2. These SAs are defined in C.2.1. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 43]
INTERNET-DRAFT ISAKMP November 21, 1995 10. The responding daemon sends an ISA_NEG_RESP packet indicating that ESP SA #2, and AH SA #1 was selected. A.2 Virtual Private Network Scenario This scenario show how ISAKMP can be used in a Virtual Public Network (VPN). The ability to establish SAs for more than just ESP and AH and one of the uses of the ISA_NOTIFY message are also illustrated. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 44]
INTERNET-DRAFT ISAKMP November 21, 1995 ___________________________Virtual_Public_Network_Scenario_______________________ End System#1 SFE#1 INTERNET SFE#2 End System #2 _______ _______ Establish ES#1 To | | | | SFE#1 Connection | | | | SYN | | | | ===> | | | | | |Establish Connection Between SFEs | | | | | | | | SYN | | | | ===> | | | | SYN, ACK | | | | <======= | | | | ACK | | | | ===> | | | | | | | | Establish SA Between SFEs | | | | | | | | ISA_INIT_REQ | | | | ============> | | | | ISA_INIT_RESP | | | | <============ | | | | ISA_KE&AUTH_REQ | | | | ==============> | | | | ISA_KE&AUTH_RESP | | | | <=============== | | | | Secure Connection | |Establish SFE#2 | | Between SFEs | |to ES#2 Connection | | | | | | | |SYN | | | |===> | | | |SYN, ACK | | | |<======= | | | |ACK | | | |===> | | ISA_NOTIFY(Status == Connected) | | SYN, ACK | | <==================== | | <======= | | | | ACK | | | | ===> | | | | | | | | | | Protected Traffic | | | | ES#1 to ES#2 | | |_______| <==============> |_______| The diagram shows an End System (ES) using a connection oriented proto- col (we use TCP as an example) establishing a connection with another ES. Both ES are behind Secure Front Ends (SFE) (e.g. firewalls). The connec- tion establishment from End System #1 (ES#1) is intercepted by its Secure Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 45]
INTERNET-DRAFT ISAKMP November 21, 1995 Front End (SFE #1). SFE#1 establishes a connection and then a Security Association (SA), using normal ISAKMP SA establishment procedures, with SFE #2. Next SFE #2 establishes a connection with ES #2. Upon successful completion SFE #2 sends an SA_NOTIFY with Status equal Connected. SFE #1 completes it's connection with ES #1 and normal end to end communications takes place secured between SFE #1 and SFE #2. If SFE #2 had been unable to establish a connection with ES #2 it would have returned an SA_NOTIFY with Status equal Not Connected with an optional reason code. Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 46]
INTERNET-DRAFT ISAKMP November 21, 1995 B Security Association Attributes This appendix contains a list of security attributes that should be con- sidered when defining a Security Association (SA) for a security proto- col or application. As an example, the security attributes culled from this list and required for an IP Security (AH, ESP) SA are defined in [RFC-1825]. The separation of ISAKMP from a specific SA definition is im- portant to ensure ISAKMP can establish SAs for all possible security func- tionality. Each security function will be required to maintain a database of current SAs. This list is based upon an e-mail message [Kent94] to the IPSEC mail list from Steve Kent. The authors welcome input on what are meaningful security attributes for an SA. 1. SAID.INBOUND 2. SAID.OUTBOUND 3. ENCAPSULATION 4. INBOUND-CRITERIA (a) IP-DESTINATION-ADDRESS (b) IP-SOURCE-ADDRESS (c) NEXT-PROTOCOL (d) IP-SECURITY-LABEL (e) TRANSPORT-DESTINATION-PORT (f) TRANSPORT-SOURCE-PORT 5. PEER-ADDRESS 6. AUTHENTICATION (a) ENABLED (b) MECHANISM o DIGITAL SIGNATURE Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 47]
INTERNET-DRAFT ISAKMP November 21, 1995 i. KEY.INBOUND (Peer's Public Key) ii. KEY.OUTBOUND (Initator's Private Key) 7. ENCRYPTION (a) ENABLED (b) ALGORTIHM (c) KEY.INBOUND (d) KEY.OUTBOUND (e) IV.INBOUND (f) IV.OUTBOUND 8. INTEGRITY (a) ENABLED (b) PLAINTEXT (c) DIRECTION.ENABLED (d) DIRECTION.VALUE (e) ALGORITHM (f) KEY.OUTBOUND (g) KEY.INBOUND 9. COMPRESSION (a) ENABLED (b) ALGORITHM 10. REPLAY (a) ENABLED Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 48]
INTERNET-DRAFT ISAKMP November 21, 1995 (b) SIZE (c) NUMBER.OUTBOUND (d) NUMBER.INBOUND (e) WINDOW.SIZE (f) WINDOW 11. FRAGMENTATION (a) INBOUND (b) OUTBOUND 12. KEY-MANAGEMENT (a) NEGOTIATED (b) TECHNIQUE (c) PARAMETERS (d) REKEY o GRACE o NEXT-SA o TIME-BASED i. ENABLE ii. TRIGGER o TRAFFIC-BASED i. ENABLE ii. PACKET-COUNT.INBOUND iii. PACKET-COUNT.OUTBOUND Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 49]
INTERNET-DRAFT ISAKMP November 21, 1995 iv. TRIGGER.INBOUND v. TRIGGER.OUTBOUND Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 50]
INTERNET-DRAFT ISAKMP November 21, 1995 C Security Association Examples C.1 ISAKMP SA Definition The ISAKMP SA contains the SA attributes that are exchanged in the ISA_INIT messages. ISAKMP Security Association _______________________SA_Attributes_______________________Requirement__ Peer ISAKMP Daemon Address REQUIRED Security Association Lifetime REQUIRED Certificate Authority REQUIRED Digital Signature Algorithm REQUIRED Signature Key(s) REQUIRED Security Association Lifetime REQUIRED Key Establishment Algorithm REQUIRED Cookie Generation Algorithm REQUIRED Sensitivity Level (e.g. Secret, Unclassified) RECOMMENDED Encryption Algorithm RECOMMENDED Encryption Mode RECOMMENDED Encryption Transform RECOMMENDED Encryption Key(s) RECOMMENDED Key Lifetime or Key Rollover RECOMMENDED Presence / Absence of cryptographic synchronization or IV RECOMMENDED Size of cryptographic synchronization or IV RECOMMENDED Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 51]
INTERNET-DRAFT ISAKMP November 21, 1995 C.1.1 ISAKMP SA Examples ISAKMP SA #1 _________________________SA_Class_________________________________SA_Type_________ Peer ISAKMP Daemon Address N/A Security Association Lifetime 86400 seconds (1day) Certificate Authority DMS Root CAW Certificate Type X.509v1m Digital Signature Algorithm DSA Signature Key(s) N/A Security Association Lifetime 86400 seconds (1day) Key Establishment Algorithm Fortezza KEA Cookie Generation Algorithm SHA_1 Sensitivity Level (e.g. Secret, Unclassified) Unclassified Encryption Algorithm Skipjack Encryption Mode CDC Encryption Transform NULL Encryption Key(s) N/A Key Lifetime or Key Rollover 3600 seconds (1 hour) Presence / Absence of cryptographic synchronization or IV Present Size of cryptographic synchronization or IV 64 bits ISAKMP SA #2 _________________________SA_Class___________________________________SA_Type__________ Peer ISAKMP Daemon Address N/A Security Association Lifetime 86400 seconds (1day) Certificate Authority DNSSEC janeway.ncsc.mil Certificate Type RR Digital Signature Algorithm RSA Signature Key(s) N/A Security Association Lifetime 86400 seconds (1day) Key Establishment Algorithm X9.42_STS Cookie Generation Algorithm MD5 Sensitivity Level (e.g. Secret, Unclassified) N/A Encryption Algorithm DES Encryption Mode CDC Encryption Transform RFC-1829 Encryption Key(s) N/A Key Lifetime or Key Rollover 600 seconds (10 minutes) Presence / Absence of cryptographic synchronization or IV Present Size of cryptographic synchronization or IV 64 bits Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 52]
INTERNET-DRAFT ISAKMP November 21, 1995 ISAKMP SA #3 _________________________SA_Class___________________________________SA_Type__________ Peer ISAKMP Daemon Address N/A Security Association Lifetime 86400 seconds (1day) Certificate Authority IPRA PCA UNINETT Certificate Type X.509v1 Digital Signature Algorithm RSA Signature Key(s) N/A Security Association Lifetime 86400 seconds (1day) Key Establishment Algorithm STS Cookie Generation Algorithm MD5 Sensitivity Level (e.g. Secret, Unclassified) N/A Encryption Algorithm DES Encryption Mode CDC Encryption Transform RFC-1829 Encryption Key(s) N/A Key Lifetime or Key Rollover 600 seconds (10 minutes) Presence / Absence of cryptographic synchronization or IV Present Size of cryptographic synchronization or IV 64 bits C.2 ESP SA and AH SA Definitions The following SAs are defined in [RFC-1825] and are presented here for comparative and completeness purposes. AH Security Association __________________SA_Attributes__________________Requirement_ Authentication Algorithm REQUIRED Authentication Mode REQUIRED Authentication Key(s) REQUIRED Key Lifetime or Key Rollover RECOMMENDED Security Association Lifetime RECOMMENDED Sensitivity Level (e.g. Secret, Unclassified) RECOMMENDED Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 53]
INTERNET-DRAFT ISAKMP November 21, 1995 ESP Security Association _______________________SA_Attributes_______________________Requirement__ Encryption Algorithm REQUIRED Encryption Mode REQUIRED Encryption Transform REQUIRED Encryption Key(s) REQUIRED Presence / Absence of cryptographic synchronization or IV REQUIRED Size of cryptographic synchronization or IV REQUIRED Authentication Algorithm RECOMMENDED Authentication Mode RECOMMENDED Authentication Key(s) RECOMMENDED Key Lifetime or Key Rollover RECOMMENDED Security Association Lifetime RECOMMENDED Sensitivity Level (e.g. Secret, Unclassified) RECOMMENDED C.2.1 ESP and AH SA Examples AH SA #1 ____________________SA_Class_____________________________SA_Type__________ Authentication Algorithm MD5 Authentication Mode Keyed Authentication Key(s) Photuris Key Lifetime or Key Rollover 600 seconds (10 minutes) Security Association Lifetime 3600 seconds (1 hour) Sensitivity Level (e.g. Secret, Unclassified) N/A AH SA #2 ____________________SA_Class_____________________________SA_Type__________ Authentication Algorithm SHA Authentication Mode NULL Authentication Key(s) NULL Key Lifetime or Key Rollover 600 seconds (10 minutes) Security Association Lifetime 3600 seconds (1 hour) Sensitivity Level (e.g. Secret, Unclassified) N/A Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 54]
INTERNET-DRAFT ISAKMP November 21, 1995 ESP SA #1 _________________________SA_Class___________________________________SA_Type__________ Encryption Algorithm DES Encryption Mode CBC Encryption Transform RFC-1829 Encryption Key(s) Phutoris Generated Presence / Absence of cryptographic synchronization or IV Present Size of cryptographic synchronization or IV 64 bits Authentication Algorithm NULL Authentication Mode NULL Authentication Key(s) NULL Key Lifetime or Key Rollover 600 seconds (10 minutes) Security Association Lifetime 3600 seconds (1 hour) Sensitivity Level (e.g. Secret, Unclassified) N/A ESP SA #2 _________________________SA_Class___________________________________SA_Type__________ Encryption Algorithm DES Encryption Mode CBC Encryption Transform RFC-1829 Encryption Key(s) X9.42_DH Generated Presence / Absence of cryptographic synchronization or IV Present Size of cryptographic synchronization or IV 64 bits Authentication Algorithm NULL Authentication Mode NULL Authentication Key(s) NULL Key Lifetime or Key Rollover 600 seconds (10 minutes) Security Association Lifetime 3600 seconds (1 hour) Sensitivity Level (e.g. Secret, Unclassified) N/A C.2.2 Fortezza SA Examples Fortezza AH SA ____________________SA_Class___________________________SA_Type________ Authentication Algorithm SHA Authentication Mode NULL Authentication Key(s) DMS Root CAW Key Lifetime or Key Rollover 86400 seconds (1day) Security Association Lifetime 86400 seconds (1day) Sensitivity Level (e.g. Secret, Unclassified) N/A Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 55]
INTERNET-DRAFT ISAKMP November 21, 1995 Fortezza ESP SA _________________________SA_Class__________________________________SA_Type_________ Encryption Algorithm Skipjack Encryption Mode CBC Encryption Transform NULL Encryption Key(s) Fortezza KEA Generated Presence / Absence of cryptographic synchronization or IV Present Size of cryptographic synchronization or IV 64 bits Authentication Algorithm DSA Authentication Mode NULL Authentication Key(s) DMS Root CAW Key Lifetime or Key Rollover 3600 seconds (1 hour) Security Association Lifetime 86400 seconds (1day) Sensitivity Level (e.g. Secret, Unclassified) Unclassified Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 56]
INTERNET-DRAFT ISAKMP November 21, 1995 Security Considerations Cryptographic analysis techniques are improving at a steady pace. The continuing improvement in processing power makes once computational pro- hibitive cryptographic attacks more realistic. New cryptographic algo- rithms and public key generation techniques are also being developed at a steady pace. New security services and mechanisms are being developed at an accelerated pace. A consistent method of choosing from a variety of security services and mechanisms and to exchange attributes required by the mechanisms is important to security in the complex structure of the Internet. However a system that locks itself into a single cryptographic algorithm, key exchange technique, or security mechanism will become in- creasingly vulnerable as time passes. UDP is an unreliable datagram protocol and therefore its use in ISAKMP in- troduces a number of security considerations. Since UDP is unreliable, but a key management protocol must be reliable, the reliability is built into ISAKMP. While ISAKMP utilizes UDP as its transport mechanism, it doesn't soley rely on any UDP information (e.g. checksum, length) for its processing. Another issue that must be considered in the development of IKMP is the effect of firewalls on the protocol. Many firewalls filter out all UDP packets, making reliance on UDP questionable in certian environments. A number of very important security considerations are presented in [RFC-1825]. One bares repeating. Once a private session key is created it must be safely stored. Failure to properly protect the private key from access both internal and external to the system completely nullifies any protect provided by the IP Security services. Acknowledgements Marsha Gross, Bill Kutz, Mike Oehler, Mark Schneider, and Pete Sell pro- vided significant input and review to this document. Thanks to Carl Muckenhirn of SPARTA, Inc. for his assistance with LaTeX. References [ANSI94] ANSI, X9.42: Public Key Cryptography for the Financial Services Industry -- Establishment of Symmetric Algorithm Keys Using Diffie-Hellman, Working Draft, October 26, 1995. [DOW92] W. Diffie, M.Wiener, P. Van Oorschot, Authtication and Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 57]
INTERNET-DRAFT ISAKMP November 21, 1995 Authenticated Key Exchanges, Designs, Codes, and Cryptography, 2, 107-125, Kluwer Academic Publishers, 1992. [Berg] Berge, N.H., UNINETT PCA Policy Statements, Internet-Draft, work in progress, November, 1995. [EK94] Eastlake III, D. and C. Kaufman, Domain Name System Protocol Security Extensions, Internet-Draft, work in progress, Oct, 1995. [Karn95] Karn P. and B. Simpson, The Photuris Key Management Protocol, Internet-Draft, work in progress, November, 1995. [Kent94] Steve Kent, IPSEC SMIB, e-mail to ipsec@ans.net, August 10, 1994. [RFC-1155] Rose M. and K. McCloghrie, Structure and Identification of Management Information for TCP/IP-based Internets, RFC-1155, May, 1990. [RFC-1212] McCloghrie K. and M. Rose, Concise MIB Definitions, RFC-1212, March 26, 1991. [RFC-1213] McCloghrie K. and M. Rose, Management Information Base for Network Management of TCP/IP-based Internets: MIB-II, RFC-1213, March 26, 1991. [RFC-1422] Steve Kent, Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management, RFC-1422, February 1993. [RFC-1825] Randell Atkinson, Security Architecture for the Internet Protocol, RFC-1825, August, 1995. [Secu] SECUREWARE INC., Peer Authentication and Key Management Protocol Specification, Version 2.2, October 27, 1995. [Schn94] Bruce Schneier, Applied Cryptography - Protocols, Algorithms, and Source Code in C, John Wiley & Sons, Inc., 1994. [Spar94a] Harney H., C. Muckenhirn, and T. Rivers, Group Key Management (GKMP) Architecture, SPARTA, Inc., Internet-Draft, September, 1994. [Spar94b] Harney H., C. Muckenhirn, and T. Rivers, Group Key Management (GKMP) Specification, SPARTA, Inc., Internet-Draft, September, 1994. Addresses of Authors The two authors are with: National Security Agency Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 58]
INTERNET-DRAFT ISAKMP November 21, 1995 ATTN: R23 9800 Savage Road Ft. Meade, MD. 20755-6000 Douglas Maughan Phone: 301-688-0847 E-mail:wdmaugh@tycho.ncsc.mil Mark Schertler Phone: 301-688-0849 E-mail:mjs@tycho.ncsc.mil Maughan/Schertler draft-ietf-ipsec-isakmp-03.txt, .ps [Page 59]
