Internet Engineering Task Force (IETF)
Request for Comments: 7055
Category: Standards Track
ISSN: 2070-1721
S. Hartman, Ed.
Painless Security
J. Howlett
December 2013

A GSS-API Mechanism for the Extensible Authentication Protocol


This document defines protocols, procedures, and conventions to be employed by peers implementing the Generic Security Service Application Program Interface (GSS-API) when using the Extensible Authentication Protocol mechanism. Through the GS2 family of mechanisms defined in RFC 5801, these protocols also define how Simple Authentication and Security Layer (SASL) applications use the Extensible Authentication Protocol.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Discovery ..................................................4
      1.2. Authentication .............................................4
      1.3. Secure Association Protocol ................................6
   2. Requirements Notation ...........................................6
   3. EAP Channel Binding and Naming ..................................6
      3.1. Mechanism Name Format ......................................7
      3.2. Internationalization of Names .............................10
      3.3. Exported Mechanism Names ..................................10
      3.4. Acceptor Name RADIUS AVP ..................................11
      3.5. Proxy Verification of Acceptor Name .......................11
   4. Selection of EAP Method ........................................12
   5. Context Tokens .................................................13
      5.1. Mechanisms and Encryption Types ...........................14
      5.2. Processing Received Tokens ................................15
      5.3. Error Subtokens ...........................................16
      5.4. Initial State .............................................16
           5.4.1. Vendor Subtoken ....................................17
           5.4.2. Acceptor Name Request ..............................17
           5.4.3. Acceptor Name Response .............................18
      5.5. Authenticate State ........................................18
           5.5.1. EAP Request Subtoken ...............................19
           5.5.2. EAP Response Subtoken ..............................19
      5.6. Extensions State ..........................................20
           5.6.1. Flags Subtoken .....................................20
           5.6.2. GSS Channel Bindings Subtoken ......................20
           5.6.3. MIC Subtoken .......................................21
      5.7. Example Token .............................................22
      5.8. Context Options ...........................................23
   6. Acceptor Services ..............................................23
      6.1. GSS-API Channel Binding ...................................24
      6.2. Per-Message Security ......................................24
      6.3. Pseudorandom Function .....................................24
   7. IANA Considerations ............................................25
      7.1. OID Registry ..............................................25
      7.2. RFC 4121 Token Identifiers ................................26
      7.3. GSS-EAP Subtoken Types ....................................26
      7.4. RADIUS Attribute Assignments ..............................27
      7.5. Registration of the EAP-AES128 SASL Mechanisms ............28
      7.6. GSS-EAP Errors ............................................28
      7.7. GSS-EAP Context Flags .....................................30
   8. Security Considerations ........................................30
   9. Acknowledgements ...............................................32
   10. References ....................................................32
   Appendix A. Pre-publication RADIUS VSA ............................33

1. Introduction

The Application Bridging for Federated Access Beyond Web (ABFAB) document [ABFAB-ARCH] describes an architecture for providing federated access management to applications using the Generic Security Service Application Programming Interface (GSS-API) [RFC2743] and Simple Authentication and Security Layer (SASL) [RFC4422]. This specification provides the core mechanism for bringing federated authentication to these applications.

The Extensible Authentication Protocol (EAP) [RFC3748] defines a framework for authenticating a network access client and server in order to gain access to a network. A variety of different EAP methods are in wide use; one of EAP's strengths is that for most types of credentials in common use, there is an EAP method that permits the credential to be used.

EAP is often used in conjunction with a backend Authentication, Authorization and Accounting (AAA) server via RADIUS [RFC3579] or Diameter [RFC4072]. In this mode, the Network Access Server (NAS) simply tunnels EAP packets over the backend authentication protocol to a home EAP/AAA server for the client. After EAP succeeds, the backend authentication protocol is used to communicate key material to the NAS. In this mode, the NAS need not be aware of or have any specific support for the EAP method used between the client and the home EAP server. The client and EAP server share a credential that depends on the EAP method; the NAS and AAA server share a credential based on the backend authentication protocol in use. The backend authentication server acts as a trusted third party, enabling network access even though the client and NAS may not actually share any common authentication methods. As described in the architecture document [ABFAB-ARCH], using AAA proxies, this mode can be extended beyond one organization to provide federated authentication for network access.

The GSS-API provides a generic framework for applications to use security services including authentication and per-message data security. Between protocols that support GSS-API directly or protocols that support SASL [RFC4422], many application protocols can use GSS-API for security services. However, with the exception of Kerberos [RFC4121], few GSS-API mechanisms are in wide use on the Internet. While GSS-API permits an application to be written independent of the specific GSS-API mechanism in use, there is no facility to separate the server from the implementation of the mechanism as there is with EAP and backend authentication servers.

The goal of this specification is to combine GSS-API's support for application protocols with EAP/AAA's support for common credential types and for authenticating to a server without requiring that server to specifically support the authentication method in use. In addition, this specification supports the architectural goal of transporting attributes about subjects to relying parties. Together this combination will provide federated authentication and authorization for GSS-API applications. This specification meets the applicability requirements for EAP to application authentication [RFC7057].

This mechanism is a GSS-API mechanism that encapsulates an EAP conversation. From the perspective of RFC 3748, this specification defines a new lower-layer protocol for EAP. From the perspective of the application, this specification defines a new GSS-API mechanism.

Section 1.3 of [RFC5247] outlines the typical conversation between EAP peers where an EAP key is derived:

   Phase 0: Discovery
   Phase 1: Authentication
            1a: EAP authentication
            1b: AAA Key Transport (optional)
   Phase 2: Secure Association Protocol
            2a: Unicast Secure Association
            2b: Multicast Secure Association (optional)

1.1. Discovery

GSS-API peers discover each other and discover support for GSS-API in an application-dependent mechanism. SASL [RFC4422] describes how discovery of a particular SASL mechanism such as a GSS-API EAP mechanism is conducted. The Simple and Protected Negotiation mechanism (SPNEGO) [RFC4178] provides another approach for discovering what GSS-API mechanisms are available. The specific approach used for discovery is out of scope for this mechanism.

1.2. Authentication

GSS-API authenticates a party called the "GSS-API initiator" to the GSS-API acceptor, optionally providing authentication of the acceptor to the initiator. Authentication starts with a mechanism-specific message called a "context token" sent from the initiator to the acceptor. The acceptor responds, followed by the initiator, and so on until authentication succeeds or fails. GSS-API context tokens are reliably delivered by the application using GSS-API. The application is responsible for in-order delivery and retransmission.

EAP authenticates a party called a "peer" to a party called the "EAP server". A third party called an "EAP pass-through authenticator" may decapsulate EAP messages from a lower layer and re-encapsulate them into a AAA protocol. The term EAP authenticator refers to whichever of the pass-through authenticator or EAP server receives the lower-layer EAP packets. The first EAP message travels from the authenticator to the peer; a GSS-API message is sent from the initiator to acceptor to prompt the authenticator to send the first EAP message. The EAP peer maps onto the GSS-API initiator. The role of the GSS-API acceptor is split between the EAP authenticator and the EAP server. When these two entities are combined, the division resembles GSS-API acceptors in other mechanisms. When a more typical deployment is used and there is a pass-through authenticator, most context establishment takes place on the EAP server and per-message operations take place on the authenticator. EAP messages from the peer to the authenticator are called responses; messages from the authenticator to the peer are called requests.

Because GSS-API applications provide guaranteed delivery of context tokens, the EAP retransmission timeout MUST be infinite and the EAP layer MUST NOT retransmit a message.

This specification permits a GSS-API acceptor to hand off the processing of the EAP packets to a remote EAP server by using AAA protocols such as RADIUS, Transport Layer Security (TLS) Encryption thereof [RFC6929], or Diameter. In this case, the GSS-API acceptor acts as an EAP pass-through authenticator. The pass-through authenticator is responsible for retransmitting AAA messages if a response is not received from the AAA server. If a response cannot be received, then the authenticator generates an error at the GSS-API level. If EAP authentication is successful, and where the chosen EAP method supports key derivation, EAP keying material may also be derived. If a AAA protocol is used, this can also be used to replicate the EAP Key from the EAP server to the EAP authenticator.

See Section 5 for details of the authentication exchange.

1.3. Secure Association Protocol

After authentication succeeds, GSS-API provides a number of per- message security services that can be used:

GSS_Wrap() provides integrity and optional confidentiality for a message.

GSS_GetMIC() provides integrity protection for data sent independently of the GSS-API

GSS_Pseudo_random [RFC4401] provides key derivation functionality.

These services perform a function similar to secure association protocols in network access. Like secure association protocols, these services need to be performed near the authenticator/acceptor even when a AAA protocol is used to separate the authenticator from the EAP server. The key used for these per-message services is derived from the EAP key; the EAP peer and authenticator derive this key as a result of a successful EAP authentication. In the case that the EAP authenticator is acting as a pass-through, it obtains it via the AAA protocol. See Section 6 for details.

2. Requirements Notation

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 [RFC2119].

3. EAP Channel Binding and Naming

EAP authenticates a user to a realm. The peer knows that it has exchanged authentication with an EAP server in a given realm. Today, the peer does not typically know which NAS it is talking to securely. That is often fine for network access. However, privileges to delegate to a chat server seem very different than privileges for a file server or trading site. Also, an EAP peer knows the identity of the home realm, but perhaps not even the visited realm.

In contrast, GSS-API takes a name for both the initiator and acceptor as inputs to the authentication process. When mutual authentication is used, both parties are authenticated. The granularity of these names is somewhat mechanism dependent. In the case of the Kerberos mechanism, the acceptor name typically identifies both the protocol in use (such as IMAP) and the specific instance of the service being connected to. The acceptor name almost always identifies the administrative domain providing service.

A GSS-API EAP mechanism needs to provide GSS-API naming semantics in order to work with existing GSS-API applications. EAP channel binding [RFC6677] is used to provide GSS-API naming semantics. Channel binding sends a set of attributes from the peer to the EAP server either as part of the EAP conversation or as part of a secure association protocol. In addition, attributes are sent in the backend authentication protocol from the authenticator to the EAP server. The EAP server confirms the consistency of these attributes. Confirming attribute consistency also involves checking consistency against a local policy database as discussed in Section 3.5. In particular, the peer sends the name of the acceptor it is authenticating to as part of channel binding. The acceptor sends its full name as part of the backend authentication protocol. The EAP server confirms consistency of the names.

EAP channel binding is easily confused with a facility in GSS-API also called "channel binding". GSS-API channel binding provides protection against man-in-the-middle attacks when GSS-API is used as authentication inside some tunnel; it is similar to a facility called "cryptographic binding" in EAP. See [RFC5056] for a discussion of the differences between these two facilities and Section 6.1 for how GSS-API channel binding is handled in this mechanism.

3.1. Mechanism Name Format

Before discussing how the initiator and acceptor names are validated in the AAA infrastructure, it is necessary to discuss what composes a name for an EAP GSS-API mechanism. GSS-API permits several types of generic names to be imported using GSS_Import_name(). Once a mechanism is chosen, these names are converted into a mechanism- specific name called a "Mechanism Name". Note that a Mechanism Name is the name of an initiator or acceptor, not of a GSS-API mechanism. This section first discusses the mechanism name form and then discusses what name forms are supported.

The string representation of the GSS-EAP mechanism name has the following ABNF [RFC5234] representation:

        char-normal = %x00-2E/%x30-3F/%x41-5B/%x5D-FF
        char-escaped = "\" %x2F / "\" %x40 / "\" %x5C
        name-char = char-normal / char-escaped
        name-string = 1*name-char
        user-or-service = name-string
        host = [name-string]
        realm = name-string
        service-specific = name-string
        service-specifics = service-specific 0*("/" service-specifics)
        name = user-or-service ["/" host [ "/" service-specifics]] [ "@"
                realm ]

Special characters appearing in a name can be backslash escaped to avoid their special meanings. For example, "\\" represents a literal backslash. This escaping mechanism is a property of the string representation; if the components of a name are transported in some mechanism that will keep them separate without backslash escaping, then backslash SHOULD have no special meaning.

The user-or-service component is similar to the portion of a network access identifier (NAI) before the '@' symbol for initiator names and the service name from the registry of GSS-API host-based services in the case of acceptor names [GSS-IANA]. The NAI specification provides rules for encoding and string preparation in order to support internationalization of NAIs; implementations of this mechanism MUST NOT prepare the user-or-service according to these rules; see Section 3.2 for internationalization of this mechanism. The host portion is empty for initiators and typically contains the domain name of the system on which an acceptor service is running. Some services MAY require additional parameters to distinguish the entity being authenticated against. Such parameters are encoded in the service-specifics portion of the name. The EAP server MUST reject authentication of any acceptor name that has a non-empty service-specifics component unless the EAP server understands the service-specifics and authenticates them. The interpretation of the service-specifics is scoped by the user-or-service portion. The realm is similar to the realm portion of a NAI for initiator names; again the NAI specification's internationalization rules MUST NOT be applied to the realm. The realm is the administrative realm of a service for an acceptor name.

The string representation of this name form is designed to be generally compatible with the string representation of Kerberos names defined in [RFC1964].

The GSS_C_NT_USER_NAME form represents the name of an individual user. From the standpoint of this mechanism, it may take the form of either an undecorated user name or a name semantically similar to a network access identifier (NAI) [RFC4282]. The name is split at the first at-sign ('@') into the part preceding the realm, which is the user-or-service portion of the mechanism name, and the realm portion, which is the realm portion of the mechanism name.

The GSS_C_NT_HOSTBASED_SERVICE name form represents a service running on a host; it is textually represented as "service@host". This name form is required by most SASL profiles and is used by many existing applications that use the Kerberos GSS-API mechanism. While support for this name form is critical, it presents an interesting challenge in terms of EAP channel binding. Consider a case where the server communicates with a "server proxy," or a AAA server near the server. That server proxy communicates with the EAP server. The EAP server and server proxy are in different administrative realms. The server proxy is in a position to verify that the request comes from the indicated host. However, the EAP server cannot make this determination directly. So, the EAP server needs to determine whether to trust the server proxy to verify the host portion of the acceptor name. This trust decision depends both on the host name and the realm of the server proxy. In effect, the EAP server decides whether to trust that the realm of the server proxy is the right realm for the given hostname and then makes a trust decision about the server proxy itself. The same problem appears in Kerberos: there, clients decide what Kerberos realm to trust for a given hostname. The service portion of this name is imported into the user-or-service portion of the mechanism name; the host portion is imported into the host portion of the mechanism name. The realm portion is empty. However, authentication will typically fail unless some AAA component indicates the realm to the EAP server. If the application server knows its realm, then it should be indicated in the outgoing AAA request. Otherwise, a proxy SHOULD add the realm. An alternate form of this name type MAY be used on acceptors; in this case, the name form is "service" with no host component. This is imported with the service as user-or-service and an empty host and realm portion. This form is useful when a service is unsure which name an initiator knows it by.

If the null name type or the GSS_EAP_NT_EAP_NAME (OID (see Section 7.1 ) is imported, then the string representation above should be directly imported. Mechanisms MAY support the GSS_KRB5_NT_KRB5_PRINCIPAL_NAME name form with the OID {iso(1) member-body(2) United States(840) mit(113554) infosys(1) gssapi(2) krb5(2) krb5_name(1)}. In many circumstances, Kerberos GSS-API mechanism names will behave as expected when used with the GSS-API EAP mechanism, but there are some differences that may cause some confusion. If an implementation does support importing Kerberos names it SHOULD fail the import if the Kerberos name is not syntactically a valid GSS-API EAP mechanism name as defined in this section.

3.2. Internationalization of Names

For the most part, GSS-EAP names are transported in other protocols; those protocols define the internationalization semantics. For example, if a AAA server wishes to communicate the user-or-service portion of the initiator name to an acceptor, it does so using existing mechanisms in the AAA protocol. Existing internationalization rules are applied. Similarly, within an application, existing specifications such as [RFC5178] define the encoding of names that are imported and displayed with the GSS-API.

This mechanism does introduce a few cases where name components are sent. In these cases, the encoding of the string is UTF-8. Senders SHOULD NOT normalize or map strings before sending. These strings include RADIUS attributes introduced in Section 3.4.

When comparing the host portion of a GSS-EAP acceptor name supplied in EAP channel binding by a peer to that supplied by an acceptor, EAP servers SHOULD prepare the host portion according to [RFC5891] prior to comparison. Applications MAY prepare domain names prior to importing them into this mechanism.

3.3. Exported Mechanism Names

GSS-API provides the GSS_Export_name call. This call can be used to export the binary representation of a name. This name form can be stored on access control lists for binary comparison.

The exported name token MUST use the format described in Section 3.2 of RFC 2743. The mechanism specific portion of this name token is the string format of the mechanism name described in Section 3.1.

RFC 2744 [RFC2744] places the requirement that the result of importing a name, canonicalizing it to a Mechanism Name and then exporting it needs to be the same as importing that name, obtaining credentials for that principal, initiating a context with those credentials and exporting the name on the acceptor. In practice, GSS mechanisms often, but not always, meet this requirement. For names expected to be used as initiator names, this requirement is met. However, permitting empty host and realm components when importing host-based services may make it possible for an imported name to differ from the exported name actually used. Other mechanisms such as Kerberos have similar situations where imported and exported names may differ.

3.4. Acceptor Name RADIUS AVP

See Section 7.4 for registrations of RADIUS attribute types to carry the acceptor service name. All the attribute types registered in that section are strings. See Section 3.1 for details of the values in a name.

If RADIUS is used as a AAA transport, the acceptor MUST send the acceptor name in these attribute types. That is, the acceptor decomposes its name and sends any non-empty portion as a RADIUS attribute. With the exception of the service-specifics portion of the name, the backslash escaping mechanism is not used in RADIUS attributes; backslash has no special meaning. In the service- specifics portion, a literal "/" separates components. In this one attribute, "\/" indicates a slash character that does not separate components and "\\" indicates a literal backslash character.

The initiator MUST require that the EAP method in use support channel binding and MUST send the acceptor name as part of the channel binding data. The client MUST NOT indicate mutual authentication in the result of GSS_Init_sec_context unless all name elements that the client supplied are in a successful channel binding response. For example, if the client supplied a hostname in channel binding data, the hostname MUST be in a successful channel binding response.

If an empty target name is supplied to GSS_Init_sec_context, the initiator MUST fail context establishment unless the acceptor supplies the acceptor name response (Section 5.4.3). If a null target name is supplied, the initiator MUST use this response to populate EAP channel bindings.

3.5. Proxy Verification of Acceptor Name

Proxies may play a role in verification of the acceptor identity. For example, a AAA proxy near the acceptor may be in a position to verify the acceptor hostname, while the EAP server is likely to be too distant to reliably verify this on its own.

The EAP server or some proxy trusted by the EAP server is likely to be in a position to verify the acceptor realm. In effect, this proxy is confirming that the right AAA credential is used for the claimed realm and thus that the acceptor is in the organization it claims to be part of. This proxy is also typically trusted by the EAP server to make sure that the hostname claimed by the acceptor is a reasonable hostname for the realm of the acceptor.

A proxy close to the EAP server is unlikely to be in a position to confirm that the acceptor is claiming the correct hostname. Instead, this is typically delegated to a proxy near the acceptor. That proxy is typically expected to verify the acceptor hostname and to verify the appropriate AAA credential for that host is used. Such a proxy may insert the acceptor realm if it is absent, permitting realm configuration to be at the proxy boundary rather than on acceptors.

Ultimately, specific proxy behavior is a matter for deployment. The EAP server MUST assure that the appropriate validation has been done before including acceptor name attributes in a successful channel binding response. If the acceptor service is included, the EAP server asserts that the service is plausible for the acceptor. If the acceptor hostname is included, the EAP server asserts that the acceptor hostname is verified. If the realm is included the EAP server asserts that the realm has been verified, and if the hostname was also included, that the realm and hostname are consistent. Part of this verification MAY be delegated to proxies, but the EAP server configuration MUST guarantee that the combination of proxies meets these requirements. Typically, such delegation will involve business or operational measures such as cross-organizational agreements as well as technical measures.

It is likely that future technical work will be needed to communicate what verification has been done by proxies along the path. Such technical measures will not release the EAP server from its responsibility to decide whether proxies on the path should be trusted to perform checks delegated to them. However, technical measures could prevent misconfigurations and help to support diverse environments.

4. Selection of EAP Method

EAP does not provide a facility for an EAP server to advertise what methods are available to a peer. Instead, a server starts with its preferred method selection. If the peer does not accept that method, the peer sends a NAK response containing the list of methods supported by the client.

Providing multiple facilities to negotiate which security mechanism to use is undesirable. Section 7.3 of [RFC4462]describes the problem referencing the Secure Shell (SSH) Protocol key exchange negotiation and the SPNEGO GSS-API mechanism. If a client preferred an EAP method A, a non-EAP authentication mechanism B, and then an EAP method C, then the client would have to commit to using EAP before learning whether A is actually supported. Such a client might end up using C when B is available.

The standard solution to this problem is to perform all the negotiation at one layer. In this case, rather than defining a single GSS-API mechanism, a family of mechanisms should be defined. Each mechanism corresponds to an EAP method. The EAP method type should be part of the GSS-API OID. Then, a GSS-API rather than EAP facility can be used for negotiation.

Unfortunately, using a family of mechanisms has a number of problems. First, GSS-API assumes that both the initiator and acceptor know the entire set of mechanisms that are available. Some negotiation mechanisms are driven by the client; others are driven by the server. With EAP GSS-API, the acceptor does not know what methods the EAP server implements. The EAP server that is used depends on the identity of the client. The best solution so far is to accept the disadvantages of multi-layer negotiation and commit to using EAP GSS- API before a specific EAP method. This has two main disadvantages. First, authentication may fail when other methods might allow authentication to succeed. Second, a non-optimal security mechanism may be chosen.

5. Context Tokens

All context establishment tokens emitted by the EAP mechanism SHALL have the framing described in Section 3.1 of [RFC2743], as illustrated by the following pseudo-ASN.1 structures:


            MechType ::= OBJECT IDENTIFIER
            -- representing EAP mechanism
            GSSAPI-Token ::=
            -- option indication (delegation, etc.) indicated within
            -- mechanism-specific token
                    thisMech MechType,
                    innerToken ANY DEFINED BY thisMech
                       -- contents mechanism-specific
                       -- ASN.1 structure not required

The innerToken field starts with a 16-bit network byte order token type identifier. The remainder of the innerToken field is a set of type-length-value subtokens. The following figure describes the structure of the inner token:

              | Octet Position | Description               |
              | 0..1           | token ID                  |
              |                |                           |
              | 2..5           | first subtoken type       |
              |                |                           |
              | 6..9           | length  of first subtoken |
              |                |                           |
              | 10..10+n-1     | first subtoken body       |
              |                |                           |
              | 10+n..10+n+3   | second subtoken type      |

Structure of Inner Token

The inner token continues with length, second subtoken body, and so forth. If a subtoken type is present, its length and body MUST be present.

The length is a four-octet length of the subtoken body in network byte order. The length does not include the length of the type field or the length field; the length only covers the body.

Tokens from the initiator to acceptor use an inner token type with ID 06 01; tokens from acceptor to initiator use an inner token type with ID 06 02. These token types are registered in the registry of RFC 4121 token types; see Section 7.2.

See Section 5.7 for the encoding of a complete token. The following sections discuss how mechanism OIDs are chosen and the state machine that defines what subtokens are permitted at each point in the context establishment process.

5.1. Mechanisms and Encryption Types

This mechanism family uses the security services of the Kerberos cryptographic framework [RFC3961]. The root of the OID ARC for mechanisms described in this document is; a Kerberos encryption type number [RFC3961] is appended to that root OID to form a mechanism OID. As such, a particular encryption type needs to be chosen. By convention, there is a single object identifier arc for the EAP family of GSS-API mechanisms. A specific mechanism is chosen by adding the numeric Kerberos encryption type number to the root of this arc. However, in order to register the SASL name, the specific usage with a given encryption type needs to be registered. This document defines the EAP-AES128 GSS-API mechanism.

5.2. Processing Received Tokens

Whenever a context token is received, the receiver performs the following checks. First, the receiver confirms the object identifier is that of the mechanism being used. The receiver confirms that the token type corresponds to the role of the peer: acceptors will only process initiator tokens and initiators will only process acceptor tokens.

Implementations of this mechanism maintain a state machine for the context establishment process. Both the initiator and acceptor start out in the initial state; see Section 5.4 for a description of this state. Associated with each state are a set of subtoken types that are processed in that state and rules for processing these subtoken types. The receiver examines the subtokens in order, processing any that are appropriate for the current state. Unknown subtokens or subtokens that are not expected in the current state are ignored if their critical bit (see below) is clear.

A state may have a set of required subtoken types. If a subtoken type is required by the current state but no subtoken of that type is present, then the context establishment MUST fail.

The most significant bit (0x80000000) in a subtoken type is the critical bit. If a subtoken with this bit set in the type is received, the receiver MUST fail context establishment unless the subtoken is understood and processed for the current state.

The subtoken type MUST be unique within a given token.

5.3. Error Subtokens

The acceptor may always end the exchange by generating an error subtoken. The error subtoken has the following format:

   | Pos    | Description                                              |
   | 0..3   | 0x80 00 00 01                                            |
   |        |                                                          |
   | 4..7   | length of error token                                    |
   |        |                                                          |
   | 8..11  | major status from RFC 2744 as 32-bit network byte order  |
   |        |                                                          |
   | 12..15 | GSS-EAP error code as 32-bit network byte order; see     |
   |        | Section 7.6                                              |

Initiators MUST ignore octets beyond the GSS-EAP error code for future extensibility. As indicated, the error token is always marked critical.

5.4. Initial State

Both the acceptor and initiator start the context establishment process in the initial state.

The initiator sends a token to the acceptor. It MAY be empty; no subtokens are required in this state. Alternatively, the initiator MAY include a vendor ID subtoken or an acceptor name request subtoken.

The acceptor responds to this message. It MAY include an acceptor name response subtoken. It MUST include a first EAP request; this is an EAP request/identity message (see Section 5.5.1 for the format of this subtoken).

The initiator and acceptor then transition to authenticate state.

5.4.1. Vendor Subtoken

The vendor ID subtoken has type 0x0000000B and the following structure:

                 | Pos         | Description            |
                 | 0..3        | 0x0000000B             |
                 |             |                        |
                 | 4..7        | length of vendor token |
                 |             |                        |
                 | 8..8+length | Vendor ID string       |

The vendor ID string is an UTF-8 string describing the vendor of this implementation. This string is unstructured and for debugging purposes only.

5.4.2. Acceptor Name Request

The acceptor name request token is sent from the initiator to the acceptor indicating that the initiator wishes a particular acceptor name. This is similar to Transport Layer Security (TLS) Server Name Indication [RFC6066] that permits a client to indicate which one of a number of virtual services to contact. The structure is as follows:

                  | Pos  | Description                  |
                  | 0..3 | 0x00000002                   |
                  |      |                              |
                  | 4..7 | length of subtoken           |
                  |      |                              |
                  | 8..n | string form of acceptor name |

It is likely that channel binding and thus authentication will fail if the acceptor does not choose a name that is a superset of this name. That is, if a hostname is sent, the acceptor needs to be willing to accept this hostname.

5.4.3. Acceptor Name Response

The acceptor name response subtoken indicates what acceptor name is used. This is useful, for example, if the initiator supplied no target name to the context initialization. This allows the initiator to learn the acceptor name. EAP channel bindings will provide confirmation that the acceptor is accurately naming itself.

This token is sent from the acceptor to initiator. In the Initial state, this token would typically be sent if the acceptor name request is absent, because if the initiator already sent an acceptor name, then the initiator knows what acceptor it wishes to contact. This subtoken is also sent in Extensions state Section 5.6, so the initiator can protect against a man-in-the-middle modifying the acceptor name request subtoken.

                  | Pos  | Description                  |
                  | 0..3 | 0x00000003                   |
                  |      |                              |
                  | 4..7 | length of subtoken           |
                  |      |                              |
                  | 8..n | string form of acceptor name |

5.5. Authenticate State

In this state, the acceptor sends EAP requests to the initiator and the initiator generates EAP responses. The goal of the state is to perform a successful EAP authentication. Since the acceptor sends an identity request at the end of the initial state, the first half- round-trip in this state is a response to that request from the initiator.

The EAP conversation can end in a number of ways:

  • If the EAP state machine generates an EAP Success message, then the EAP authenticator believes the authentication is successful. The acceptor MUST confirm that a key has been derived (Section 7.10 of [RFC3748]). The acceptor MUST confirm that this success indication is consistent with any protected result indication for combined authenticators and with AAA indication of success for pass-through authenticators. If any of these checks fail, the acceptor MUST send an error subtoken and fail the context establishment. If these checks succeed, the acceptor sends the Success message using the EAP Request subtoken type and transitions to Extensions state. If the initiator receives an EAP

Success message, it confirms that a key has been derived and that the EAP Success is consistent with any protected result indication. If so, it transitions to Extensions state. Otherwise, it returns an error to the caller of GSS_Init_sec_context without producing an output token.

  • If the acceptor receives an EAP failure, then the acceptor sends this in the EAP Request subtoken type. If the initiator receives an EAP Failure, it returns GSS failure.
  • If there is some other error, the acceptor MAY return an error subtoken.

5.5.1. EAP Request Subtoken

The EAP Request subtoken is sent from the acceptor to the initiator. This subtoken is always critical and is REQUIRED in the authentication state.

                  | Pos         | Description           |
                  | 0..3        | 0x80000005            |
                  |             |                       |
                  | 4..7        | length of EAP message |
                  |             |                       |
                  | 8..8+length | EAP message           |

5.5.2. EAP Response Subtoken

This subtoken is REQUIRED in authentication state messages from the initiator to the acceptor. It is always critical.

                  | Pos         | Description           |
                  | 0..3        | 0x80000004            |
                  |             |                       |
                  | 4..7        | length of EAP message |
                  |             |                       |
                  | 8..8+length | EAP message           |

5.6. Extensions State

After EAP Success, the initiator sends a token to the acceptor including additional subtokens that negotiate optional features or provide GSS-API channel binding (see Section 6.1). The acceptor then responds with a token to the initiator. When the acceptor produces its final token, it returns GSS_S_COMPLETE; when the initiator consumes this token, it returns GSS_S_COMPLETE if no errors are detected.

The acceptor SHOULD send an acceptor name response (Section 5.4.3) so that the initiator can get a copy of the acceptor name protected by the Message Integrity Check (MIC) subtoken.

Both the initiator and acceptor MUST include and verify a MIC subtoken to protect the extensions exchange.

5.6.1. Flags Subtoken

This subtoken is sent to convey initiator flags to the acceptor. The flags are sent as a 32-bit integer in network byte order. The only flag defined so far is GSS_C_MUTUAL_FLAG, indicating that the initiator successfully performed mutual authentication of the acceptor. This flag is communicated to the acceptor because some protocols [RFC4462] require the acceptor to know whether the initiator has confirmed its identity. This flag has the value 0x2 to be consistent with RFC 2744.

                     | Pos   | Description           |
                     | 0..3  | 0x0000000C            |
                     |       |                       |
                     | 4..7  | length of flags token |
                     |       |                       |
                     | 8..11 | flags                 |

Initiators MUST send 4 octets of flags. Acceptors MUST ignore flag octets beyond the first 4 and MUST ignore flag bits other than GSS_C_MUTUAL_FLAG. Initiators MUST send undefined flag bits as zero.

5.6.2. GSS Channel Bindings Subtoken

This subtoken is always critical when sent. It is sent from the initiator to the acceptor. The contents of this token are an RFC 3961 get_mic token of the application data from the GSS channel bindings structure passed into the context establishment call.

      | Pos         | Description                                   |
      | 0..3        | 0x80000006                                    |
      |             |                                               |
      | 4..7        | length of token                               |
      |             |                                               |
      | 8..8+length | get_mic  of  channel binding application data |

Again, only the application data is sent in the channel binding. Any initiator and acceptor addresses passed by an application into context establishment calls are ignored and not sent over the wire. The checksum type of the get_mic token SHOULD be the mandatory-to- implement checksum type of the Context Root Key (CRK). The key to use is the CRK and the key usage is 60 (KEY_USAGE_GSSEAP_CHBIND_MIC). An acceptor MAY accept any MIC in the channel bindings subtoken if the channel bindings input to GSS_Accept_sec_context is not provided. If the channel binding input to GSS_Accept_sec_context is provided, the acceptor MUST return failure if the channel binding MIC in a received channel binding subtoken fails to verify.

The initiator MUST send this token if channel bindings including application data are passed into GSS_Init_sec_context and MUST NOT send this token otherwise.

5.6.3. MIC Subtoken

This subtoken MUST be the last subtoken in the tokens sent in Extensions state. This subtoken is sent both by the initiator and acceptor.

    | Pos         | Description                                      |
    | 0..3        | 0x8000000D for initiator 0x8000000E for acceptor |
    |             |                                                  |
    | 4..7        | length of RFC 3961 MIC token                     |
    |             |                                                  |
    | 8..8+length | RFC 3961 result of get_mic                       |

As with any call to get_mic, a token is produced as described in RFC 3961 using the CRK (Section 6) as the key and the mandatory checksum type for the encryption type of the CRK as the checksum type. The key usage is 61 (KEY_USAGE_GSSEAP_ACCTOKEN_MIC) for the subtoken from the acceptor to the initiator and 62 (KEY_USAGE_GSSEAP_INITTOKEN_MIC) for the subtoken from the initiator to the acceptor. The input is as follows:

  1. The DER-encoded object identifier of the mechanism in use; this value starts with 0x06 (the tag for object identifier). When encoded in an RFC 2743 context token, the object identifier is preceded by the tag and length for [Application 0] SEQUENCE. This tag and the length of the overall token is not included; only the tag, length, and value of the object identifier itself.
  1. A 16-bit token type in network byte order of the RFC 4121 token identifier (0x0601 for initiator, 0x0602 for acceptor).
  1. For each subtoken, other than the MIC subtoken itself, the order the subtokens appear in the token is as follows:
  1. A four-octet subtoken type in network byte order
  1. A four-byte length in network byte order
  1. Length octets of value from that subtoken

5.7. Example Token

   | 60 |  23  | 06 |  09  | 2b  | 06 01 05 05 0f 01 01 11 |
   |App0|Token |OID |OID   | 1 3 |  6  1  5  5 15  1  1 17 |
   |Tag |length|Tag |length|      Mechanism object ID      |
   |  06 01   | 00 00 00 02 | 00 00 00 0e |
   |Initiator | Acceptor    | Length      |
   |context   | name        | (14 octets) |
   |token ID  | request     |             |
   | 68 6f 73 74 2f 6c 6f 63 61 6c 68 6f 73 74 |
   | String form of acceptor name              |
   | "host/localhost"                          |

Example Initiator Token

5.8. Context Options

GSS-API provides a number of optional per-context services requested by flags on the call to GSS_Init_sec_context and indicated as outputs from both GSS_Init_sec_context and GSS_Accept_sec_context. This section describes how these services are handled. Which services the client selects in the call to GSS_Init_sec_context controls what EAP methods MAY be used by the client. Section 7.2 of RFC 3748 describes a set of security claims for EAP. As described below, the selected GSS options place requirements on security claims that MUST be met.

This GSS mechanism MUST only be used with EAP methods that provide dictionary-attack resistance. Typically, dictionary-attack resistance is obtained by using an EAP tunnel method to tunnel an inner method in TLS.

The EAP method MUST support key derivation. Integrity, confidentiality, sequencing, and replay detection MUST be indicated in the output of GSS_Init_sec_context and GSS_Accept_sec_context regardless of which services are requested.

The PROT_READY service defined in Section 1.2.7 of [RFC2743] is never available with this mechanism. Implementations MUST NOT offer this flag or permit per-message security services to be used before context establishment.

The EAP method MUST support mutual authentication and channel binding. See Section 3.4 for details on what is required for successful mutual authentication. Regardless of whether mutual authentication is requested, the implementation MUST include channel bindings in the EAP authentication. If mutual authentication is requested and successful mutual authentication takes place as defined in Section 3.4, the initiator MUST send a flags subtoken Section 5.6.1 in Extensions state.

6. Acceptor Services

The context establishment process may be passed through to an EAP server via a backend authentication protocol. However, after the EAP authentication succeeds, security services are provided directly by the acceptor.

This mechanism uses an RFC 3961 cryptographic key called the Context Root Key (CRK). The CRK is derived from the GMSK (GSS-API Master Session Key). The GMSK is the result of the random-to-key [RFC3961] operation of the encryption type of this mechanism consuming the appropriate number of bits from the EAP MSK. For example, for aes128-cts-hmac-sha1-96, the random-to-key operation consumes 16 octets of key material; thus, the first 16 bytes of the MSK are input to random-to-key to form the GMSK. If the MSK is too short, authentication MUST fail.

In the following, pseudorandom is the RFC 3961 pseudorandom operation for the encryption type of the GMSK and random-to-key is the RFC 3961 random-to-key operation for the enctype of the mechanism. The truncate function takes the initial l bits of its input. The goal in constructing a CRK is to call the pseudorandom function enough times to produce the right number of bits of output and discard any excess bits of output.

The CRK is derived from the GMSK using the following procedure:

   Tn = pseudorandom(GMSK, n || "rfc4121-gss-eap")
   CRK = random-to-key(truncate(L, T0 || T1 || .. || Tn))
   L = random-to-key input size

Where n is a 32-bit integer in network byte order starting at 0 and incremented to each call to the pseudo_random operation.

6.1. GSS-API Channel Binding

GSS-API channel binding [RFC5554] is a protected facility for exchanging a cryptographic name for an enclosing channel between the initiator and acceptor. The initiator sends channel binding data and the acceptor confirms that channel binding data has been checked.

The acceptor SHOULD accept any channel binding provided by the initiator if null channel bindings are passed into gss_accept_sec_context. Protocols such as HTTP Negotiate [RFC4559] depend on this behavior of some Kerberos implementations.

As discussed, the GSS channel bindings subtoken is sent in the Extensions state.

6.2. Per-Message Security

The per-message tokens of Section 4 of RFC 4121 are used. The CRK SHALL be treated as the initiator sub-session key, the acceptor sub- session key and the ticket session key.

6.3. Pseudorandom Function

The pseudorandom function defined in [RFC4402] is used to provide GSS_Pseudo_Random functionality to applications.

7. IANA Considerations

This specification creates a number of IANA registries.

7.1. OID Registry

IANA has created a registry of ABFAB object identifiers titled "Object Identifiers for Application Bridging for Federated Access". The initial contents of the registry are specified below. The registration policy is IETF Review or IESG Approval [RFC5226]. Early allocation is permitted. IANA has updated the reference for the root of this OID delegation to point to the newly created registry.

   Decimal   Name        Description                         References
   -------   ----        ----------------------------------  ----------
         0   Reserved    Reserved                            RFC 7055
         1   mechanisms  A sub-arc containing ABFAB          RFC 7055
         2   nametypes   A sub-arc containing ABFAB          RFC 7055
                         GSS-API Name Types

NOTE: the following mechanisms registry is the root of the OID for the mechanism in question. As discussed in Section 5.1, a Kerberos encryption type number [RFC3961] is appended to the mechanism version OID below to form the OID of a specific mechanism.

   Decimal   Name          Description                      References
   -------   ----          -------------------------------  ----------
         0   Reserved      Reserved                         RFC 7055
         1   gss-eap-v1    The GSS-EAP mechanism            RFC 7055
   Decimal   Name          Description            References
   -------   ----          ---------------------  ----------
         0   Reserved      Reserved               RFC 7055
         1   GSS_EAP_NT_EAP_NAME                  RFC 7055, Section 3.1

7.2. RFC 4121 Token Identifiers

In the top-level registry titled "Kerberos V GSS-API Mechanism Parameters", a subregistry called "Kerberos GSS-API Token Type Identifiers" was created; the references for this subregistry are RFC 4121 and this document. The allocation procedure is Expert Review [RFC5226]. The Expert's primary job is to make sure that token type identifiers are requested by an appropriate requester for the RFC 4121 mechanism in which they will be used and that multiple values are not allocated for the same purpose. For RFC 4121 and this mechanism, the Expert is currently expected to make allocations for token identifiers from documents in the IETF stream; effectively, for these mechanisms, the Expert currently confirms the allocation meets the requirements of the IETF Review process.

The ID field is a hexadecimal token identifier specified in network byte order.

The initial registrations are as follows:

   | ID    | Description                   | Reference                 |
   | 01 00 | KRB_AP_REQ                    | RFC 4121, Section 4.1     |
   |       |                               |                           |
   | 02 00 | KRB_AP_REP                    | RFC 4121, Section 4.1     |
   |       |                               |                           |
   | 03 00 | KRB_ERROR                     | RFC 4121, Section 4.1     |
   |       |                               |                           |
   | 04 04 | MIC tokens                    | RFC 4121, Section |
   |       |                               |                           |
   | 05 04 | wrap tokens                   | RFC 4121, Section |
   |       |                               |                           |
   | 06 01 | GSS-EAP initiator context     | RFC 7055, Section 5       |
   |       | token                         |                           |
   |       |                               |                           |
   | 06 02 | GSS EAP acceptor context      | RFC 7055, Section 5       |
   |       | token                         |                           |

7.3. GSS-EAP Subtoken Types

This document creates a top-level registry called "The Extensible Authentication Protocol Mechanism for the Generic Security Service Application Programming Interface (GSS-EAP) Parameters". In any short form of that name, including any URI for this registry, it is important that the string GSS come before the string EAP; this will help to distinguish registries if EAP methods for performing GSS-API authentication are ever defined.

In this registry is a subregistry of subtoken types. Identifiers are 32-bit integers; the upper bit (0x80000000) is reserved as a critical flag and should not be indicated in the registration. Assignments of GSS-EAP subtoken types are made by Expert Review [RFC5226]. The Expert is expected to require a public specification of the subtoken similar in detail to registrations given in this document. The security of GSS-EAP depends on making sure that subtoken information has adequate protection and that the overall mechanism continues to be secure. Examining the security and architectural consistency of the proposed registration is the primary responsibility of the Expert.

         | Type       | Description              | Reference     |
         | 0x00000001 | Error                    | Section 5.3   |
         |            |                          |               |
         | 0x0000000B | Vendor                   | Section 5.4.1 |
         |            |                          |               |
         | 0x00000002 | Acceptor name request    | Section 5.4.2 |
         |            |                          |               |
         | 0x00000003 | Acceptor name response   | Section 5.4.3 |
         |            |                          |               |
         | 0x00000005 | EAP request              | Section 5.5.1 |
         |            |                          |               |
         | 0x00000004 | EAP response             | Section 5.5.2 |
         |            |                          |               |
         | 0x0000000C | Flags                    | Section 5.6.1 |
         |            |                          |               |
         | 0x00000006 | GSS-API channel bindings | Section 5.6.2 |
         |            |                          |               |
         | 0x0000000D | Initiator MIC            | Section 5.6.3 |
         |            |                          |               |
         | 0x0000000E | Acceptor MIC             | Section 5.6.3 |

7.4. RADIUS Attribute Assignments

The following RADIUS attribute type values [RFC3575] are assigned. The allocation instructions in Section 10.3 of [RFC6929] have been followed.

   | Description                    | Value | More Information         |
   | GSS-Acceptor-Service-Name      | 164   | user-or-service portion  |
   |                                |       | of name                  |
   |                                |       |                          |
   | GSS-Acceptor-Host-Name         | 165   | host portion of name     |
   |                                |       |                          |
   | GSS-Acceptor-Service-Specifics | 166   | service-specifics        |
   |                                |       | portion of name          |
   |                                |       |                          |
   | GSS-Acceptor-Realm-Name        | 167   | Realm portion of name    |

7.5. Registration of the EAP-AES128 SASL Mechanisms

   Subject:  Registration of SASL mechanisms EAP-AES128 and
   SASL mechanism names:  EAP-AES128 and EAP-AES128-PLUS
   Security considerations:  See RFC 5801 and RFC 7055
   Published specification (recommended):  RFC 7055

Person & email address to contact for further information:

Abfab Working Group,

   Intended usage:  common
   Owner/Change controller:
   Note:  This mechanism describes the GSS-EAP mechanism used with the
      aes128-cts-hmac-sha1-96 enctype.  The GSS-API OID for this
      mechanism is

As described in RFC 5801, a PLUS variant of this mechanism is also required.

7.6. GSS-EAP Errors

A new subregistry is created in the GSS-EAP parameters registry titled "GSS-EAP Error Codes". The error codes in this registry are unsigned 32-bit numbers. Values less than or equal to 127 are assigned by Standards Action [RFC5226]. Values 128 through 255 are assigned with the Specification Required assignment policy [RFC5226].

Values greater than 255 are reserved; updates to registration policy may make these values available for assignment and implementations MUST be prepared to receive them.

This table provides the initial contents of the registry.

        | Value | Description                                    |
        | 0     | Reserved                                       |
        |       |                                                |
        | 1     | Buffer is incorrect size                       |
        |       |                                                |
        | 2     | Incorrect mechanism OID                        |
        |       |                                                |
        | 3     | Token is corrupted                             |
        |       |                                                |
        | 4     | Token is truncated                             |
        |       |                                                |
        | 5     | Packet received by direction that sent it      |
        |       |                                                |
        | 6     | Incorrect token type identifier                |
        |       |                                                |
        | 7     | Unhandled critical subtoken received           |
        |       |                                                |
        | 8     | Missing required subtoken                      |
        |       |                                                |
        | 9     | Duplicate subtoken type                        |
        |       |                                                |
        | 10    | Received unexpected subtoken for current state |
        |       |                                                |
        | 11    | EAP did not produce a key                      |
        |       |                                                |
        | 12    | EAP key too short                              |
        |       |                                                |
        | 13    | Authentication rejected                        |
        |       |                                                |
        | 14    | AAA returned an unexpected message type        |
        |       |                                                |
        | 15    | AAA response did not include EAP request       |
        |       |                                                |
        | 16    | Generic AAA failure                            |

7.7. GSS-EAP Context Flags

A new subregistry is created in the GSS-EAP parameters registry. This registry holds registrations of flag bits sent in the flags subtoken (Section 5.6.1). There are 32 flag bits available for registration represented as hexadecimal numbers from the most significant bit 0x80000000 to the least significant bit 0x1. The registration policy for this registry is IETF Review or, in exceptional cases, IESG Approval. The following table indicates initial registrations; all other values are available for assignment.

               | Flag | Name              | Reference     |
               | 0x2  | GSS_C_MUTUAL_FLAG | Section 5.6.1 |

8. Security Considerations

RFC 3748 discusses security issues surrounding EAP. RFC 5247 discusses the security and requirements surrounding key management that leverages the AAA infrastructure. These documents are critical to the security analysis of this mechanism.

RFC 2743 discusses generic security considerations for the GSS-API. RFC 4121 discusses security issues surrounding the specific per- message services used in this mechanism.

As discussed in Section 4, this mechanism may introduce multiple layers of security negotiation into application protocols. Multiple layer negotiations are vulnerable to a bid-down attack when a mechanism negotiated at the outer layer is preferred to some but not all mechanisms negotiated at the inner layer; see Section 7.3 of [RFC4462] for an example. One possible approach to mitigate this attack is to construct security policy such that the preference for all mechanisms negotiated in the inner layer falls between preferences for two outer-layer mechanisms or falls at one end of the overall ranked preferences including both the inner and outer layer. Another approach is to only use this mechanism when it has specifically been selected for a given service. The second approach is likely to be common in practice because one common deployment will involve an EAP supplicant interacting with a user to select a given identity. Only when an identity is successfully chosen by the user will this mechanism be attempted.

EAP channel binding is used to give the GSS-API initiator confidence in the identity of the GSS-API acceptor. Thus, the security of this mechanism depends on the use and verification of EAP channel binding.

Today, EAP channel binding is in very limited deployment. If EAP channel binding is not used, then the system may be vulnerable to phishing attacks where a user is diverted from one service to another. If the EAP method in question supports mutual authentication then users can only be diverted between servers that are part of the same AAA infrastructure. For deployments where membership in the AAA infrastructure is limited, this may serve as a significant limitation on the value of phishing as an attack. For other deployments, use of EAP channel binding is critical to avoid phishing. These attacks are possible with EAP today although not typically with common GSS-API mechanisms. For this reason, implementations are required to implement and use EAP channel binding; see Section 3 for details.

The security considerations of EAP channel binding [RFC6677] describe the security properties of channel binding. Two attacks are worth calling out here. First, when a tunneled EAP method is used, it is critical that the channel binding be performed with an EAP server trusted by the peer. With existing EAP methods, this typically requires validating the certificate of the server tunnel endpoint back to a trust anchor and confirming the name of the entity who is a subject of that certificate. EAP methods may suffer from bid-down attacks where an attacker can cause a peer to think that a particular EAP server does not support channel binding. This does not directly cause a problem because mutual authentication is only offered at the GSS-API level when channel binding to the server's identity is successful. However, when an EAP method is not vulnerable to these bid-down attacks, additional protection is available. This mechanism will benefit significantly from new strong EAP methods such as [TEAP].

Every proxy in the AAA chain from the authenticator to the EAP server needs to be trusted to help verify channel bindings and to protect the integrity of key material. GSS-API applications may be built to assume a trust model where the acceptor is directly responsible for authentication. However, GSS-API is definitely used with trusted- third-party mechanisms such as Kerberos.

RADIUS does provide a weak form of hop-by-hop confidentiality of key material based on using MD5 as a stream cipher. Diameter can use TLS or IPsec but has no mandatory-to-implement confidentiality mechanism. Operationally, protecting key material as it is transported between the Identity Provider (IdP) and Relying Party (RP) is critical to per-message security and verification of GSS-API channel binding [RFC5056]. Mechanisms such as RADIUS over TLS [RFC6614] provide significantly better protection of key material than the base RADIUS specification.

9. Acknowledgements

Luke Howard, Jim Schaad, Alejandro Perez Mendez, Alexey Melnikov, and Sujing Zhou provided valuable reviews of this document.

Rhys Smith provided the text for the OID registry section. Sam Hartman's work on this document has been funded by JANET.

10. References

10.1. Normative References

[GSS-IANA] IANA, "GSS-API Service Name Registry",


   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.
   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, January 2000.
   [RFC3575]  Aboba, B., "IANA Considerations for RADIUS (Remote
              Authentication Dial In User Service)", RFC 3575, July
   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
              3748, June 2004.
   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.
   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121, July
   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.
   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401, February 2006.
   [RFC4402]  Williams, N., "A Pseudo-Random Function (PRF) for the
              Kerberos V Generic Security Service Application Program
              Interface (GSS-API) Mechanism", RFC 4402, February 2006.
   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.
   [RFC5554]  Williams, N., "Clarifications and Extensions to the
              Generic Security Service Application Program Interface
              (GSS-API) for the Use of Channel Bindings", RFC 5554, May
   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.
   [RFC6677]  Hartman, S., Clancy, T., and K. Hoeper, "Channel-Binding
              Support for Extensible Authentication Protocol (EAP)
              Methods", RFC 6677, July 2012.
   [RFC7057]  Winter, S. and J. Salowey, "Update to the Extensible
              Authentication Protocol (EAP) Applicability Statement for
              Application Bridging for Federated Access Beyond Web
              (ABFAB)", RFC 7057, December 2013.

10.2. Informative References


              Howlett, J., Hartman, S., Tschofenig, H., Lear, E., and J.
              Schaad, "Application Bridging for Federated Access Beyond
              Web (ABFAB) Architecture", Work in Progress, July 2013.
   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
              1964, June 1996.
   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.
   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005.
   [RFC4178]  Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
              Simple and Protected Generic Security Service Application
              Program Interface (GSS-API) Negotiation Mechanism", RFC
              4178, October 2005.
   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.
   [RFC4462]  Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
              "Generic Security Service Application Program Interface
              (GSS-API) Authentication and Key Exchange for the Secure
              Shell (SSH) Protocol", RFC 4462, May 2006.
   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, June 2006.
   [RFC5178]  Williams, N. and A. Melnikov, "Generic Security Service
              Application Program Interface (GSS-API)
              Internationalization and Domain-Based Service Names and
              Name Type", RFC 5178, May 2008.
   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.
   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.
   [RFC6614]  Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
              "Transport Layer Security (TLS) Encryption for RADIUS",
              RFC 6614, May 2012.
   [RFC6929]  DeKok, A. and A. Lior, "Remote Authentication Dial In User
              Service (RADIUS) Protocol Extensions", RFC 6929, April
   [TEAP]     Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
              "Tunnel EAP Method (TEAP) Version 1", Work in Progress,
              September 2013.

Appendix A. Pre-publication RADIUS VSA

As described in Section 3.4, RADIUS attributes are used to carry the acceptor name when this family of mechanisms is used with RADIUS. Prior to the publication of this specification, a vendor-specific RADIUS attribute was used. This non-normative appendix documents that attribute as it may be seen from older implementations.

Prior to IANA assignment, GSS-EAP used a RADIUS vendor-specific attribute for carrying the acceptor name. The Vendor-Specific Attribute (VSA) with enterprise ID 25622 is formatted as a VSA according to the recommendation in the RADIUS specification. The following sub-attributes are defined:

   | Name                           | Attribute | Description          |
   | GSS-Acceptor-Service-Name      | 128       | user-or-service      |
   |                                |           | portion of name      |
   |                                |           |                      |
   | GSS-Acceptor-Host-Name         | 129       | host portion of name |
   |                                |           |                      |
   | GSS-Acceptor-Service-Specifics | 130       | service-specifics    |
   |                                |           | portion of name      |
   |                                |           |                      |
   | GSS-Acceptor-Realm-Name        | 131       | Realm portion of     |
   |                                |           | name                 |

Authors' Addresses

Sam Hartman (editor)
Painless Security


Josh Howlett