Specification for the Derivation of Root Keys from an Extended Master Session Key (EMSK)
Author(s): M. Nakhjiri, J. Salowey, V. Narayanan, L. Dondeti
The Extensible Authentication Protocol (EAP) defined the Extended Master Session Key (EMSK) generation, but reserved it for unspecified future uses. This memo reserves the EMSK for the sole purpose of deriving root keys. Root keys are master keys that...
Network Working Group J. Salowey Request for Comments: 5295 Cisco Systems Updates: 5247 L. Dondeti Category: Standards Track V. Narayanan Qualcomm, Inc M. Nakhjiri Motorola August 2008 Specification for the Derivation of Root Keys from an Extended Master Session Key (EMSK) Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract The Extensible Authentication Protocol (EAP) defined the Extended Master Session Key (EMSK) generation, but reserved it for unspecified future uses. This memo reserves the EMSK for the sole purpose of deriving root keys. Root keys are master keys that can be used for multiple purposes, identified by usage definitions. This document also specifies a mechanism for avoiding conflicts between root keys by deriving them in a manner that guarantees cryptographic separation. Finally, this document also defines one such root key usage: Domain-Specific Root Keys are root keys made available to and used within specific key management domains. Salowey, et al. Standards Track [Page 1] RFC 5295 EMSK Root Key Derivation August 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Applicable Usages of Keys Derived from the EMSK . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2. Cryptographic Separation and Coordinated Key Derivation . . . 6 3. EMSK Key Root Derivation Framework . . . . . . . . . . . . . . 7 3.1. USRK Derivation . . . . . . . . . . . . . . . . . . . . . 8 3.1.1. On the KDFs . . . . . . . . . . . . . . . . . . . . . 9 3.1.2. Default KDF . . . . . . . . . . . . . . . . . . . . . 9 3.2. EMSK and USRK Name Derivation . . . . . . . . . . . . . . 10 4. Domain-Specific Root Key Derivation . . . . . . . . . . . . . 11 4.1. Applicability of Multi-Domain Usages . . . . . . . . . . . 12 5. Requirements for Usage Definitions . . . . . . . . . . . . . . 13 5.1. Root Key Management Guidelines . . . . . . . . . . . . . . 13 6. Requirements for EAP System . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 7.1. Key Strength . . . . . . . . . . . . . . . . . . . . . . . 15 7.2. Cryptographic Separation of Keys . . . . . . . . . . . . . 15 7.3. Implementation . . . . . . . . . . . . . . . . . . . . . . 15 7.4. Key Distribution . . . . . . . . . . . . . . . . . . . . . 16 7.5. Key Lifetime . . . . . . . . . . . . . . . . . . . . . . . 16 7.6. Entropy Consideration . . . . . . . . . . . . . . . . . . 16 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8.1. Key Labels . . . . . . . . . . . . . . . . . . . . . . . . 17 8.2. PRF Numbers . . . . . . . . . . . . . . . . . . . . . . . 18 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . . 19 10.2. Informative References . . . . . . . . . . . . . . . . . . 19 Salowey, et al. Standards Track [Page 2] RFC 5295 EMSK Root Key Derivation August 2008 1. Introduction This document deals with keys generated by authenticated key exchange mechanisms defined within the EAP framework [RFC3748]. EAP defines two types of keying material: a Master Session Key (MSK) and an Extended Master Session Key (EMSK). The EAP specification implicitly assumes that the MSK produced by EAP will be used for a single purpose at a single device; however, it does reserve the EMSK for future use. This document defines the EMSK to be used solely for deriving root keys using the key derivation specified. The root keys are meant for specific purposes called usages; a special usage class is the Domain-Specific Root Keys made available to and used within specific key management domains. This document also provides guidelines for creating usage definitions for the various uses of EAP key material and for the management of the root keys. In this document, the terms application and usage (or "usage definition") refer to a specific use case of the EAP keying material. Different uses for keys derived from the EMSK have been proposed. Some examples include hand-off across access points in various mobile technologies, mobile IP authentication, and higher-layer application authentication. In order for a particular usage of EAP key material to make use of this specification, it must specify a so-called usage definition. This document does not define how the derived Usage- Specific Root Keys (USRK) are used; see the following section for discussion of applicable usages. It does define a framework for the derivation of USRKs for different purposes such that different usages can be developed independently from one another. The goal is to have security properties of one usage have minimal or no effect on the security properties of other usages. This document does define a special class of USRK, called a Domain- Specific Root Key (DSRK) for use in deriving keys specific to a key management domain. Each DSRK is a root key used to derive Domain- Specific Usage-Specific Root Keys (DSUSRK). The DSUSRKs are USRKs specific to a particular key management domain. In order to keep root keys for specific purposes separate from one another, two requirements are defined in the following sections. One is coordinated key derivation and another is cryptographic separation. 1.1. Applicable Usages of Keys Derived from the EMSK The EMSK is typically established as part of network access authentication and authorization. It is expected that keys derived from EMSK will be used in protocols related to network access, such Salowey, et al. Standards Track [Page 3] RFC 5295 EMSK Root Key Derivation August 2008 as handover optimizations, and the scope of these protocols is usually restricted to the endpoints of the lower layers over which EAP packets are sent. In particular, it is inappropriate for the security of higher-layer applications to solely rely on keys derived from network access authentication. Even when used together with another, independent security mechanism, the use of these keys needs to be carefully evaluated with regards to the benefits of the optimization and the need to support multiple solutions. Performance optimizations may not warrant the close tie-in that may be required between the layers in order to use EAP-based keys. Such optimizations may be offset by the complexities of managing the validity and usage of key materials. Keys generated from subsequent EAP authentications may be beyond the knowledge and control of applications. From an architectural point of view, applications should not make assumptions about the lower-layer technology (such as network access authentication) used on any particular hop along the path between the application endpoints. From a practical point of view, making such assumptions would complicate using those applications over lower layers that do not use EAP, and make it more difficult for applications and network access technologies to evolve independently of each other. Parties using keys derived from EMSK also need trust relationships with the EAP endpoints, and mechanisms for securely communicating the keys. For most applications, it is not appropriate to assume that all current and future access networks are trusted to secure the application function. Instead, applications should implement the required security mechanisms in an access-independent manner. Implementation considerations may also complicate communication of keys to an application from the lower layer. For instance, in many configurations, application protocol endpoints may be different from the EAP endpoints. Given all this, it is NOT RECOMMENDED to use keys derived from the EMSK as an exclusive security mechanism, when their usage is not inherently, and by permanent nature, tied to the lower layer where network access authentication was performed. Salowey, et al. Standards Track [Page 4] RFC 5295 EMSK Root Key Derivation August 2008 Keys derived from EAP are pair-wise by nature and are not directly suitable for multicast or other group usages such as those involved in some routing protocols. It is possible to use keys derived from EAP in protocols that distribute group keys to group participants. The definition of these group key distribution protocols is beyond the scope of this document and would require additional specification. 1.2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. The following terms are taken from [RFC3748]: EAP Server, peer, authenticator, Master Session Key (MSK), Extended Master Session Key (EMSK), Cryptographic Separation. Usage Definition An application of cryptographic key material to provide one or more security functions such as authentication, authorization, encryption, or integrity protection for related applications or services. This document provides guidelines and recommendations for what should be included in usage definitions. This document does not place any constraints on the types of use cases or services that create usage definitions. Usage-Specific Root Key (USRK) Keying material derived from the EMSK for a particular usage definition. It is used to derive child keys in a way defined by its usage definition. Key Management Domain A key management domain is specified by the scope of a given root key. The scope is the collection of systems authorized to access key material derived from that key. Systems within a key management domain may be authorized to (1) derive key materials, (2) use key materials, or (3) distribute key materials to other systems in the same domain. A derived key's scope is constrained to a subset of the scope of the key from which it is derived. In this document, the term "domain" refers to a key management domain unless otherwise qualified. Domain Specific Root Key (DSRK) Keying material derived from the EMSK that is restricted to use in a specific key management domain. It is used to derive child keys for a particular usage definition. The child keys derived from a Salowey, et al. Standards Track [Page 5] RFC 5295 EMSK Root Key Derivation August 2008 DSRK are referred to as Domain-Specific Usage-Specific Root Keys (DSUSRKs). A DSUSRK is similar to the USRK, except in the fact that its scope is restricted to the same domain as the parent DSRK from which it is derived. 2. Cryptographic Separation and Coordinated Key Derivation The EMSK is used to derive keys for multiple use cases, and thus it is required that the derived keys are cryptographically separate. Cryptographic separation means that when multiple keys are derived from an EMSK, given any derived key, it is computationally infeasible to derive any of the other derived keys. Note that deriving the EMSK from any combinations of the derived keys must also be computationally infeasible. In practice, this means that derivation of an EMSK from a derived key or derivation of one child key from another must require an amount of computation equivalent to that required to, say, reversing a cryptographic hash function. Cryptographic separation of keys derived from the same key can be achieved in many ways. Two obvious methods are as follows: o Use a Pseudo-Random Function (PRF) on the EMSK and generate a key stream. Keys of various lengths may be provided as required from the key stream for various uses. o Derive keys from EMSK by providing different inputs to the PRF. However, it is desirable that derivation of one child key from the EMSK is independent of derivation of another child key. Independent derivation of keys from the EMSK allows child keys to be derived in any order, independent of other keys. Thus, it is desirable to use option 2 from above. Using the second option implies the additional input to the PRF must be different for each child key derivation. This additional input to the PRF must be coordinated properly to meet the requirement of cryptographic separation and to prevent reuse of key material between usages. If cryptographic separation is not maintained, then the security of one usage depends upon the security of all other usages that use keys derived from the EMSK. If a system does not have this property, then a usage's security depends upon all other usages deriving keys from the same EMSK, which is undesirable. In order to prevent security problems in one usage from interfering with another usage, the following cryptographic separation is required: o It MUST be computationally infeasible to compute the EMSK from any root key derived from it. Salowey, et al. Standards Track [Page 6] RFC 5295 EMSK Root Key Derivation August 2008 o Any root key MUST be cryptographically separate from any other root key derived from the same EMSK or DSRK. o Derivation of USRKs MUST be coordinated so that two separate cryptographic usages do not derive the same key. o Derivation of DSRKs MUST be coordinated so that two separate key management domains do not derive the same key. o Derivation of DSRKs and USRKs MUST be specified such that no domain can obtain a USRK by providing a domain name identical to a Usage Key Label. This document provides guidelines for a key derivation mechanism that can be used with existing and new EAP methods to provide cryptographic separation between usages of EMSK. This allows for the development of new usages without cumbersome coordination between different usage definitions. 3. EMSK Key Root Derivation Framework The EMSK key derivation framework provides a coordinated means for generating multiple root keys from an EMSK. Further keys may then be derived from the root key for various purposes, including encryption, integrity protection, entity authentication by way of proof of possession, and subsequent key derivation. A root key is derived from the EMSK for a specific set of uses set forth in a usage definition described in Section 5. The basic EMSK root key hierarchy looks as follows: EMSK / \ USRK1 USRK2 This document defines how to derive Usage-Specific Root Keys (USRKs) from the EMSK and also defines a specific USRK called a Domain- Specific Root Key (DSRK). DSRKs are root keys restricted to use in a particular key management domain. From the DSRK, Usage-Specific Root Keys for a particular application may be derived (DSUSRKs). The DSUSRKs are equivalent to USRKs that are restricted to use in a particular domain. The details of lower levels of key hierarchy are outside scope of this document. The key hierarchy looks as follows: Salowey, et al. Standards Track [Page 7] RFC 5295 EMSK Root Key Derivation August 2008 EMSK / \ USRK DSRK / \ DSUSRK1 DSUSRK2 3.1. USRK Derivation The EMSK Root Key Derivation Function (KDF) derives a USRK from the EMSK, a key label, optional data, and output length. The KDF is expected to give the same output for the same input. The basic key derivation function is given below. USRK = KDF(EMSK, key label | "\0" | optional data | length) where: | denotes concatenation "\0" is a NULL octet (0x00 in hex) length is a 2-octet unsigned integer in network byte order The key labels are printable ASCII strings unique for each usage definition and are a maximum of 255 octets. In general, they are of the form label-string@specorg, where specorg is the organization that controls the specification of the usage definition of the Root Key. The key label is intended to provide global uniqueness. Rules for the allocation of these labels are given in Section 8. The NULL octet after the key label is used to avoid collisions if one key label is a prefix of another label (e.g., "foobar" and "foobarExtendedV2"). This is considered a simpler solution than requiring a key label assignment policy that prevents prefixes from occurring. For the optional data, the KDF MUST be capable of processing at least 2048 opaque octets. The optional data must be constant during the execution of the KDF. Usage definitions MAY use the EAP Session-ID [RFC5247] in the specification of the optional data parameter that goes into the KDF function. In this case, the advantage is that data provided into the key derivation is unique to the session that generated the keys. The KDF must be able to process input keys of up to 256 bytes. It may do this by providing a mechanism for "hashing" long keys down to a suitable size that can be consumed by the underlying derivation algorithm. Salowey, et al. Standards Track [Page 8] RFC 5295 EMSK Root Key Derivation August 2008 The length is a 2-octet unsigned integer in network byte order of the output key length in octets. An implementation of the KDF MUST be capable of producing at least 2048 octets of output; however, it is RECOMMENDED that Root Keys be at least 64 octets long. A usage definition requiring derivation of a Root Key must specify all the inputs (other than EMSK) to the key derivation function. USRKs MUST be at least 64 octets in length. 3.1.1. On the KDFs This specification allows for the use of different KDFs. However, in order to have a coordinated key derivation function, the same KDF function MUST be used for all key derivations for a given EMSK. If no KDF is specified, then the default KDF specified in Section 3.1.2 MUST be used. A system may provide the capability to negotiate additional KDFs. KDFs are assigned numbers through IANA following the policy set in Section 8. The rules for negotiating a KDF are as follows: o If no other KDF is specified, the KDF specified in this document MUST be used. This is the "default" KDF. o The initial authenticated key exchange MAY specify a favored KDF. For example, an EAP method may define a preferred KDF to use in its specification. If the initial authenticated key exchange specifies a KDF, then this MUST override the default KDF. Note that usage definitions MUST NOT concern themselves with the details of the KDF construction or the KDF selection, they only need to worry about the inputs specified in Section 3. 3.1.2. Default KDF The default KDF for deriving root keys from an EMSK is taken from the PRF+ key expansion specified in [RFC4306] based on HMAC-SHA-256 [SHA256]. The PRF+ construction was chosen because of its simplicity and efficiency over other mechanisms such as those used in [RFC 4346]. The motivation for the design of PRF+ is described in [SIGMA]. The definition of PRF+ from [RFC4306] is given below: Salowey, et al. Standards Track [Page 9] RFC 5295 EMSK Root Key Derivation August 2008 PRF+ (K,S) = T1 | T2 | T3 | T4 | ... where: T1 = PRF (K, S | 0x01) T2 = PRF (K, T1 | S | 0x02) T3 = PRF (K, T2 | S | 0x03) T4 = PRF (K, T3 | S | 0x04) continuing as needed to compute the required length of key material. The key, K, is the EMSK and S is the concatenation of the key label, the NULL octet, optional data, and length defined in Section 3.1. For this specification, the PRF is taken as HMAC-SHA-256 [SHA256]. Since PRF+ is only defined for 255 iterations, it may produce up to 8160 octets of key material. 3.2. EMSK and USRK Name Derivation The EAP keying framework [RFC5247] specifies that the EMSK MUST be named using the EAP Session-ID and a binary or textual indication. Following that requirement, the EMSK name SHALL be derived as follows: EMSKname = KDF ( EAP Session-ID, "EMSK" | "\0" | length ) where: | denotes concatenation "EMSK" consists of the 4 ASCII values for the letters "\0" = is a NULL octet (0x00 in hex) length is the 2-octet unsigned integer 8 in network byte order It is RECOMMENDED that all keys derived from the EMSK are referred to by the EMSKname and the context of the descendant key usage. This is the default behavior. Any exceptions SHALL be signaled by individual usages. USRKs MAY be named explicitly with a name derivation specified as follows: USRKName = KDF(EAP Session-ID, key label|"\0"|optional data|length) where: key label and optional data MUST be the same as those used in the corresponding USRK derivation length is the 2-octet unsigned integer 8 in network byte order Salowey, et al. Standards Track [Page 10] RFC 5295 EMSK Root Key Derivation August 2008 USRKName derivation and usage are applicable when there is ambiguity in referencing the keys using the EMSKname and the associated context of the USRK usage. The usage SHALL signal such an exception in key naming, so both parties know the key name used. 4. Domain-Specific Root Key Derivation A specific USRK called a Domain-Specific Root Key (DSRK) is derived from the EMSK for a specific set of usages in a particular key management domain. Usages derive specific keys for specific services from this DSRK. The DSRK may be distributed to a key management domain for a specific set of usages so that keys can be derived within the key management domain for those usages. DSRK-based usages will follow a key hierarchy similar to the following: EMSK / \ / \ / \ / \ DSRK1 DSRK2 / \ / \ / \ / \ DSUSRK11 DSUSRK12 DSUSRK21 DSUSRK22 The DSRK is a USRK with a key label of "email@example.com" and the optional data containing a domain label. The optional data MUST contain an ASCII string representing the key management domain for which the root key is being derived. The DSRK MUST be at least 64 octets long. Domain-Specific Usage-Specific Root Keys (DSUSRKs) are derived from the DSRK. The KDF is expected to give the same output for the same input. The basic key derivation function is given below. DSUSRK = KDF(DSRK, key label | "\0" | optional data | length) The key labels are printable ASCII strings unique for each usage definition within a DSRK usage and are a maximum of 255 octets. In general, they are of the form label-string@specorg where specorg is the organization that controls the specification of the usage definition of the DSRK. The key label is intended to provide global uniqueness. Rules for the allocation of these labels are given in Section 8. For the optional data, the KDF MUST be capable of processing at least 2048 opaque octets. The optional data must be constant during the execution of the KDF. The length is a 2-octet unsigned integer in network byte order of the output key length in octets. An implementation of the KDF MUST be capable of producing at Salowey, et al. Standards Track [Page 11] RFC 5295 EMSK Root Key Derivation August 2008 least 2048 octets of output; however, it is RECOMMENDED that DSUSRKs be at least 64 octets long. Usages that make use of the DSRK must define how the peer learns the domain label to use in a particular derivation. A multi-domain usage must define how both DSRKs and specific DSUSRKs are transported to different key management domains. Note that usages may define alternate ways to constrain specific keys to particular key management domains. To facilitate the use of EMSKname to refer to keys derived from DSRKs, EMSKname SHOULD be sent along with the DSRK. The exception is when a DSRKname is expected to be used. The usage SHALL signal such an exception in key naming, so both parties know the key name used. DSUSRKs MAY be named explicitly with a name derivation specified as follows: DSUSRKName = KDF(EMSKName,key label | "\0" | optional data | length) where length is the 2-octet unsigned integer 8 in network byte order. 4.1. Applicability of Multi-Domain Usages The DSUSRKs generated by a domain can be used to authorize entities in a domain to perform specific functions. In cases where it is appropriate for only a specific domain to be authorized to perform a function, the usage SHOULD NOT be defined as multi-domain. In some cases, only certain domains are authorized for a particular multi-domain usage. In this case, domains that do not have full authorization should not receive the DSRK and should only receive DSUSRKs for the usages for which they are authorized. If it is possible for a peer to know which domains are authorized for a particular usage without relying on restricting access to the DSRK to specific domains, then this recommendation may be relaxed. Salowey, et al. Standards Track [Page 12] RFC 5295 EMSK Root Key Derivation August 2008 5. Requirements for Usage Definitions In order for a usage definition to meet the guidelines for USRK usage, it must meet the following recommendations: o The usage must define if it is a domain-enabled usage. o The usage definition MUST NOT use the EMSK in any other way except to derive Root Keys using the key derivation specified in Section 3 of this document. They MUST NOT use the EMSK directly. o The usage definition SHOULD NOT require caching of the EMSK. It is RECOMMENDED that the Root Key derived specifically for the usage definition (rather than the EMSK) should be used to derive child keys for specific cryptographic operations. o Usage definitions MUST define distinct key labels and optional data used in the key derivation described in Section 3. Usage definitions are encouraged to use the key name described in Section 3.2 and include additional data in the optional data to provide additional entropy. o Usage definitions MUST define the length of their Root Keys. It is RECOMMENDED that the Root Keys be at least as long as the EMSK (at least 64 octets). o Usage definitions MUST define how they use their Root Keys. This includes aspects of key management covered in the next section on Root Key management guidelines. 5.1. Root Key Management Guidelines This section makes recommendations for various aspects of key management of the Root Key including lifetime, child key derivation, caching, and transport. It is RECOMMENDED that the Root Key is only used for deriving child keys. A usage definition must specify how and when the derivation of child keys should be done. It is RECOMMENDED that usages following similar considerations for key derivation are as outlined in this document for the Root Key derivation with respect to cryptographic separation and key reuse. In addition, usages should take into consideration the number of keys that will be derived from the Root Key and ensure that enough entropy is introduced in the derivation to support this usage. It is desirable that the entropy is provided by the two parties that derive the child key. Salowey, et al. Standards Track [Page 13] RFC 5295 EMSK Root Key Derivation August 2008 Root Keys' lifetimes should not be more than that of the EMSK. Thus, when the EMSK expires, the Root Keys derived from it should be removed from use. If a new EMSK is derived from a subsequent EAP transaction, then a usage implementation should begin to use the new Root Keys derived from the new EMSK as soon as possible. Whether or not child keys associated with a Root Key are replaced depends on the requirements of the usage definition. It is conceivable that some usage definition forces the child key to be replaced and others allow child keys to be used based on the policy of the entities that use the child key. Recall that the EMSK never leaves the EAP peer and server. That also holds true for some Root Keys; however, some Root Keys may be provided to other entities for child key derivation and delivery. Each usage definition specification will specify delivery caching and/or delivery procedures. Note that the purpose of the key derivation in Section 3 is to ensure that Root Keys are cryptographically separate from each other and the EMSK. In other words, given a Root Key, it is computationally infeasible to derive the EMSK, any other Root Keys, or child keys associated with other Root Keys. In addition to the Root Key, several other parameters may need to be sent. Root Key names may be derived using the EAP Session-ID, and thus the key name may need to be sent along with the key. When Root Keys are delivered to another entity, the EMSKname and the lifetime associated with the specific root keys MUST also be transported to that entity. Recommendations for transporting keys are discussed in the Security Considerations (Section 7.4). Usage definitions may also define how keys are bound to particular entities. This can be done through the inclusion of usage parameters and identities in the child key derivation. Some of this data is described as "channel bindings" in [RFC3748]. 6. Requirements for EAP System The system that wishes to make use of EAP root keys derived from the EMSK must take certain things into consideration. The following is a list of these considerations: o The EMSK MUST NOT be used for any other purpose than the key derivation described in this document. o The EMSK MUST be secret and not known to someone observing the authentication mechanism protocol exchange. Salowey, et al. Standards Track [Page 14] RFC 5295 EMSK Root Key Derivation August 2008 o The EMSK MUST be maintained within a protected location inside the entity where it is generated. Only root keys derived according to this specification may be exported from this boundary. o The EMSK MUST be unique for each EAP session o The EAP method MUST provide an identifier for the EAP transaction that generated the key. o The system MUST define which usage definitions are used and how they are invoked. o The system may define ways to select an alternate PRF for key derivation as defined in Section 3.1. The system MAY use the MSK transmitted to the Network Access Server (NAS) in any way it chooses in accordance with [RFC3748], [RFC5247], and other relevant specifications. This is required for backward compatibility. New usage definitions following this specification MUST NOT use the MSK. If more than one usage uses the MSK, then the cryptographic separation is not achieved. Implementations MUST prevent such combinations. 7. Security Considerations 7.1. Key Strength The effective key strength of the derived keys will never be greater than the strength of the EMSK (or a master key internal to an EAP mechanism). 7.2. Cryptographic Separation of Keys The intent of the KDF is to derive keys that are cryptographically separate: the compromise of one of the Usage-Specific Root Keys (USRKs) should not compromise the security of other USRKs or the EMSK. It is believed that the KDF chosen provides the desired separation. 7.3. Implementation An implementation of an EAP framework should keep the EMSK internally as close to where it is derived as possible and only provide an interface for obtaining Root Keys. It may also choose to restrict which callers have access to which keys. A usage definition MUST NOT assume that any entity outside the EAP server or EAP peer has access to the EMSK. In particular, it MUST NOT assume that a lower layer has access to the EMSK. Salowey, et al. Standards Track [Page 15] RFC 5295 EMSK Root Key Derivation August 2008 7.4. Key Distribution In some cases, it will be necessary or convenient to distribute USRKs from where they are generated. Since these are secret keys, they MUST be transported with their integrity and confidentiality maintained. They MUST be transmitted between authenticated and authorized parties. It is also important that the context of the key usage be transmitted along with the key. This includes information to identify the key and constraints on its usage such as lifetime. This document does not define a mechanism for key transport. It is up to usage definitions and the systems that use them to define how keys are distributed. Usage definition designers may enforce constraints on key usage by various parties by deriving a key hierarchy and by providing entities only with the keys in the hierarchy that they need. 7.5. Key Lifetime The key lifetime is dependent upon how the key is generated and how the key is used. Since the Root Key is the responsibility of the usage definition, it must determine how long the key is valid for. If key lifetime or key strength information is available from the authenticated key exchange, then this information SHOULD be used in determining the lifetime of the key. If possible, it is recommended that key lifetimes be coordinated throughout the system. Setting a key lifetime shorter that a system lifetime may result in keys becoming invalid with no convenient way to refresh them. Setting a key lifetime to longer may result in decreased security since the key may be used beyond its recommended lifetime. 7.6. Entropy Consideration The number of root keys derived from the EMSK is expected to be low. Note that there is no randomness required to be introduced into the EMSK-to-Root-Key derivation beyond the root key labels. Thus, if many keys are going to be derived from a Root Key, it is important that Root-Key-to-child-key derivation introduce fresh random numbers in deriving each key. 8. IANA Considerations The keywords "Private Use", "Specification Required", and "IETF Consensus" that appear in this document when used to describe namespace allocation are to be interpreted as described in [RFC 5226]. Salowey, et al. Standards Track [Page 16] RFC 5295 EMSK Root Key Derivation August 2008 8.1. Key Labels This specification introduces a new name space for "USRK Key Labels". Key labels MUST be printable US-ASCII strings, and MUST NOT contain the characters at-sign ("@") except as noted below, comma (","), whitespace, control characters (ASCII codes 32 or less), or the ASCII code 127 (DEL). Labels are case-sensitive and MUST NOT be longer than 64 characters. Labels can be assigned based on Specification Required policy [RFC 5226]. In addition, the labels "experimental1" and "experimental2" are reserved for experimental use. The following considerations apply to their use: Production networks do not necessarily support the use of experimental code points. The network scope of support for experimental values should carefully be evaluated before deploying any experiment across extended network domains, such as the public Internet. The potential to disrupt the stable operation of EAP devices is a consideration when planning an experiment using such code points. The network administrators should ensure that each code point is used consistently to avoid interference between experiments. Particular attention should be given to security vulnerabilities and the freedom of different domains to employ their own experiments. Cross-domain usage is NOT RECOMMENDED. Similarly, labels "private1" and "private2" have been reserved for Private Use within an organization. Again, cross-domain usage of these labels is NOT RECOMMENDED. Labels starting with a string and followed by the "@" and a valid, fully qualified Internet domain name [RFC 1034] can be requested by the person or organization that is in control of the domain name. Such labels can be allocated based on Expert Review with Specification Required. Besides the review needed for Specification Required (see Section 4.1 of [RFC 5226]), the expert needs to review the proposed usage for conformance to this specification, including the suitability of the usage according to the applicability statement outlined in Section 1.1. It is RECOMMENDED that the specification contain the following information: o A description of the usage o The key label to be used o Length of the Root Key Salowey, et al. Standards Track [Page 17] RFC 5295 EMSK Root Key Derivation August 2008 o If optional data is used, what it is and how it is maintained o How child keys will be derived from the Root Key and how they will be used o How lifetime of the Root Key and its child keys will be managed o Where the Root Keys or child keys will be used and how they are communicated if necessary The following labels are reserved by this document: "EMSK", "firstname.lastname@example.org". 8.2. PRF Numbers This specification introduces a new number space for "EMSK PRF numbers". The numbers are in the range 0 to 255. Numbers from 0 to 220 are assigned through the policy IETF Consensus, and numbers in the range 221 to 255 are left for Private Use. The initial registry contains the following values: 0 RESERVED 1 HMAC-SHA-256 PRF+ (Default) 9. Acknowledgements This document expands upon previous collaboration with Pasi Eronen. This document reflects conversations with Bernard Aboba, Jari Arkko, Avi Lior, David McGrew, Henry Haverinen, Hao Zhou, Russ Housley, Glen Zorn, Charles Clancy, Dan Harkins, Alan DeKok, Yoshi Ohba, and members of the EAP and HOKEY working groups. Thanks to Dan Harkins for the idea of using a single root key name to refer to all keys. Salowey, et al. Standards Track [Page 18] RFC 5295 EMSK Root Key Derivation August 2008 10. References 10.1. Normative References [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC3748, June 2004. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC4306, December 2005. [RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible Authentication Protocol (EAP) Key Management Framework", RFC5247, August 2008. [SHA256] National Institute of Standards and Technology, "Secure Hash Standard", FIPS 180-2, With Change Notice 1 dated February 2004, August 2002. 10.2. Informative References [RFC 1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC 4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [SIGMA] Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' Approach to Authenticated Diffie-Hellman and its Use in the IKE Protocols", LNCS 2729, Springer, 2003, <http://www.informatik.uni-trier.de/~ley/db/conf/ crypto/crypto2003.html>. Salowey, et al. Standards Track [Page 19] RFC 5295 EMSK Root Key Derivation August 2008 Authors' Addresses Joseph Salowey Cisco Systems EMail: email@example.com Lakshminath Dondeti Qualcomm, Inc EMail: firstname.lastname@example.org Vidya Narayanan Qualcomm, Inc EMail: email@example.com Madjid Nakhjiri Motorola EMail: firstname.lastname@example.org Salowey, et al. Standards Track [Page 20] RFC 5295 EMSK Root Key Derivation August 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at email@example.com. Salowey, et al. Standards Track [Page 21]