| Network Working Group | S. Hartman, Ed. |
| Internet-Draft | Painless Security |
| Intended status: Standards Track | J. Howlett |
| Expires: September 08, 2012 | JANET(UK) |
| March 09, 2012 |
A GSS-API Mechanism for the Extensible Authentication Protocol
draft-ietf-abfab-gss-eap-05.txt
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 EAP mechanism. Through the GS2 family of mechanisms, these protocols also define how Simple Authentication and Security Layer (SASL, RFC 4422) applications use the Extensible Authentication Protocol.
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The ABFAB architecture [I-D.lear-abfab-arch] describes an architecture for providing federated access management to applications using the Generic Security Services Application Programming Interface (GSS-API) [RFC2743] and Simple Authentication and Security Layers (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 server via RADIUS [RFC3579] or Diameter [RFC4072]. In this mode, the 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, 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 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 prospective 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:
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 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.
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 passthrough authenticator may decapsulate EAP messages from a lower layer and reencapsulate them into an AAA protocol. The term EAP authenticator referrs to whichever of the passthrough 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 passthrough 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 provides guaranteed delivery, 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, RadSec 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 recieved, 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 an 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.
After authentication succeeds, GSS-API provides a number of per-message security services that can be used:
These services perform a function similar to security association protocols in network access. Like security 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.
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].
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.
An EAP GSS-API mechanism needs to provide GSS-API naming semantics in order to work with existing GSS-API applications. EAP channel binding [I-D.ietf-emu-chbind] 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 below. 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.
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 name form. This section first discusses the mechanism name form and then discusses what name forms are supported.
name-char = %x00-39/%x41-46/%x48-FF
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 ]
The string representation of the GSS-EAP mechanism name has the following ABNF [RFC5234] representation:
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; specifications 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 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 either of 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 proceeding 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 1.3.6.1.5.5.15.2.1) [I-D.smith-abfab-oidregistry] 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)}.
For the most part, GSS-EAP names are transported in other protocols; those protocols define the internationalization semantics. For example, if an 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.
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 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 hostbased 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.
Currently, GSS-EAP uses a RADIUS vendor-specific attribute for carrying the acceptor name. The 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-specific | 130 | service-specifics portion of name |
| GSS-Acceptor-Realm-Name | 131 | Realm portion of name |
All these items are strings. See Section 3.1 for details of the values in a name.
If RADIUS is used as an AAA transport, the acceptor MUST send the acceptor name in the VSA. That is, the acceptor decomposes its name and sends any non-empty portion as a sub-attribute in this VSA.
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.
Proxies may play a role in verification of the acceptor identity. For example, an 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.
The specification currently describes a single GSS-API mechanism. The peer and authenticator exchange EAP messages. The GSS-API mechanism specifies no constraints about what EAP method types are used; text in the specification says that negotiation of which EAP method to use happens at the EAP layer.
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 SSH 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.
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:
GSS-API DEFINITIONS ::=
BEGIN
MechType ::= OBJECT IDENTIFIER
-- representing EAP mechanism
GSSAPI-Token ::=
-- option indication (delegation, etc.) indicated within
-- mechanism-specific token
[APPLICATION 0] IMPLICIT SEQUENCE {
thisMech MechType,
innerToken ANY DEFINED BY thisMech
-- contents mechanism-specific
-- ASN.1 structure not required
}
END
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:
| Position | Description |
|---|---|
| 0..1 | token ID |
| 2..5 | first subtoken type |
| 6..9 | length of first subtoken |
| 10..10+n | first subtoken body |
| 10+n+1..10+n+4 | second subtoken type |
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 does not include the length of the type field or the length field; the length only covers the value.
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 8.1.
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.
This mechanism family uses the security services of the Kerberos cryptographic framework [RFC3961]. 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-cts-hmac-sha1-96 GSS-API mechanism. XXX define an OID for that and use the right language to get that into the appropriate SASL registry.
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 reciever 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.
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 8.5 |
Initiators MUST ignore octets beyond the GSS EAP error code for future extensibility. As indicated, the error token is always marked critical.
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.
The vendor ID token 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.
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 TLS Server Name Indication [RFC6066] which 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 hosntame.
The acceptor name response subtoken indicates what acceptor name is used. This is useful for example if the initiator supplied no target name to 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 send 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 |
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:
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 | 0x8000005 |
| 4..7 | Length of EAP message |
| 8..8+length | EAP message |
This subtoken is required in authentication state messages from the initiator to the acceptor. It is always critical.
| Pos | Description |
|---|---|
| 0..3 | 0x8000004 |
| 4..7 | Length of EAP message |
| 8..8+length | EAP message |
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 MIC subtoken.
Both the initiator and acceptor MUST include and verify a MIC subtoken to protect the extensions exchange.
This token 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. 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.
This token is required and always critical. 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 contextestablishment 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 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.
This token MUST be the last subtoken in the tokens sent in Extensions state. This token 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:
+----+------+----+------+-----+----------------------------+ | 60 | 24 | 06 | 0a | 2b | 06 01 04 01 a9 4a 16 01 12 | +----+------+----+------+-----+----------------------------+ |App0|Token |OID |OID | 1 3 | 6 1 4 1 5322 22 1 18 | |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" | +-------------------------------------------+
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.
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.
Open issue: handling of lifetime parameters.
The context establishment process may be passed through to a 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 MSK). The GMSK is the result of the random-to-key [RFC3961] operation consuming the appropriate number of bits from the EAP master session key. 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 master session key are input to random-to-key to form the GMSK.
Tn = pseudo-random(GMSK, n || "rfc4121-gss-eap") CRK = truncate(L, T0 || T1 || .. || Tn) L = output RFC 3961 key size
The CRK is derived from the GMSK using the following procedur
Where n is a 32-bit integer in network byte order starting at 0 and incremented to each call to the pseudo_random operation.
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 providing by the initiator if null channel bindings are passed into gss_accept_sec_context. Protocols such as HTTP Negotiate depend on this behavior of some Kerberos implementations.
As discussed, the GSS channel bindings subtoken is sent in the extensions state.
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.
The pseudo random function defined in [RFC4402] is used.
Section 1.3 of RFC 3748 provides the applicability statement for EAP. Among other constraints, EAP is scoped for use in network access. This specification anticipates using EAP beyond its current scope. The assumption is that some other document will discuss the issues surrounding the use of EAP for application authentication and expand EAP's applicability. That document will likely enumerate considerations that a specific use of EAP for application authentication needs to handle. Examples of such considerations might include the multi-layer negotiation issue, deciding when EAP or some other mechanism should be used, and so forth. This section serves as a placeholder to discuss any such issues with regard to the use of EAP and GSS-API.
This specification creates a number of IANA registries.
A new top level registry titled "Kerberos V GSS-API Mechanism Parameters," should be created. This registry should be separate from the existing "Kerberos Parameters" registry. If it has already been created by [I-D.ietf-krb-wg-gss-cb-hash-agility] then that registry should be used.
In this registry a sub-registry called "Kerberos GSS-API Token Type Identifiers" is created; the overall reference for this subregistry is section 4.1 of RFC 4121. 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 initial registrations are as follows:
| ID | Description | Reference |
|---|---|---|
| 01 00 | KRB_AP_REQ | RFC 4121 sect 4.1 |
| 02 00 | KRB_AP_REP | RFC 4121 sect 4.1 |
| 03 00 | KRB_ERROR | RFC 4121 sect 4.1 |
| 04 04 | MIC tokens | RFC 4121 sect 4.2.6.1 |
| 05 04 | wrap tokens | RFC 4121 sect 4.2.6.2 |
| 06 01 | GSS-EAP initiator context token | Section 5 |
| 06 02 | GSS EAP acceptor context token | Section 5 |
This document creates a top level registry called "The Extensible Authentication Protocol Mechanism for the Generic Security Services 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 authenitication 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. 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 |
XXX register RADIUS attributes.
Subject: Registration of SASL mechanism
eap-aes128-cts-hmac-sha1-96
SASL mechanism name :eap-aes128-cts-hmac-sha1-96
Security considerations: See RFC 5801 and draft-ietf-abfab-gss-eap
Published specification (recommended): draft-ietf-abfab-gss-eap
Person & email address to contact for further information: Sam
Hartman <hartmans-ietf@mit.edu>
Intended usage: common
Owner/Change controller: iesg@ietf.org
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 1.3.6.1.5.5.15.1.1.17
A new subregistry is created in the GSS EAP parameters registry titled "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. Values 128 through 255 are assigned with the specification required assignment policy. 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 xxx |
| 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 |
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 involved 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.
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. 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.
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 IDP and RP is critical to per-message security and verification of GSS-API channel binding [RFC5056]. Mechanisms such as RADIUS over TLS [I-D.ietf-radext-radsec] provide significantly better protection of key material than the base RADIUS specification.
Jim Schaad, Alejandro Perez Mendez, Alexey Melnikov and Sujing Zhou provided valuable reviews of this document.