RFC 2078
Generic Security Service Application Program Interface, Version 2
Network Working Group J. Linn Request for Comments: 2078 OpenVision Technologies Category: Standards Track January 1997 Obsoletes: 1508 Generic Security Service Application Program Interface, Version 2 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract The Generic Security Service Application Program Interface (GSS-API), as defined in RFC-1508, provides security services to callers in a generic fashion, supportable with a range of underlying mechanisms and technologies and hence allowing source-level portability of applications to different environments. This specification defines GSS-API services and primitives at a level independent of underlying mechanism and programming language environment, and is to be complemented by other, related specifications: documents defining specific parameter bindings for particular language environments documents defining token formats, protocols, and procedures to be implemented in order to realize GSS-API services atop particular security mechanisms This memo revises RFC-1508, making specific, incremental changes in response to implementation experience and liaison requests. It is intended, therefore, that this memo or a successor version thereto will become the basis for subsequent progression of the GSS-API specification on the standards track. Table of Contents 1: GSS-API Characteristics and Concepts.......................... 3 1.1: GSS-API Constructs.......................................... 6 1.1.1: Credentials.............................................. 6 1.1.1.1: Credential Constructs and Concepts...................... 6 1.1.1.2: Credential Management................................... 7 1.1.1.3: Default Credential Resolution........................... 8
1.1.2: Tokens.................................................... 9 1.1.3: Security Contexts........................................ 10 1.1.4: Mechanism Types.......................................... 11 1.1.5: Naming................................................... 12 1.1.6: Channel Bindings......................................... 14 1.2: GSS-API Features and Issues................................ 15 1.2.1: Status Reporting......................................... 15 1.2.2: Per-Message Security Service Availability................. 17 1.2.3: Per-Message Replay Detection and Sequencing............... 18 1.2.4: Quality of Protection.................................... 20 1.2.5: Anonymity Support......................................... 21 1.2.6: Initialization............................................ 22 1.2.7: Per-Message Protection During Context Establishment....... 22 1.2.8: Implementation Robustness................................. 23 2: Interface Descriptions....................................... 23 2.1: Credential management calls................................ 25 2.1.1: GSS_Acquire_cred call.................................... 26 2.1.2: GSS_Release_cred call.................................... 28 2.1.3: GSS_Inquire_cred call.................................... 29 2.1.4: GSS_Add_cred call........................................ 31 2.1.5: GSS_Inquire_cred_by_mech call............................ 33 2.2: Context-level calls........................................ 34 2.2.1: GSS_Init_sec_context call................................ 34 2.2.2: GSS_Accept_sec_context call.............................. 40 2.2.3: GSS_Delete_sec_context call.............................. 44 2.2.4: GSS_Process_context_token call........................... 46 2.2.5: GSS_Context_time call.................................... 47 2.2.6: GSS_Inquire_context call................................. 47 2.2.7: GSS_Wrap_size_limit call................................. 49 2.2.8: GSS_Export_sec_context call.............................. 50 2.2.9: GSS_Import_sec_context call.............................. 52 2.3: Per-message calls.......................................... 53 2.3.1: GSS_GetMIC call.......................................... 54 2.3.2: GSS_VerifyMIC call....................................... 55 2.3.3: GSS_Wrap call............................................ 56 2.3.4: GSS_Unwrap call.......................................... 58 2.4: Support calls.............................................. 59 2.4.1: GSS_Display_status call.................................. 60 2.4.2: GSS_Indicate_mechs call.................................. 60 2.4.3: GSS_Compare_name call.................................... 61 2.4.4: GSS_Display_name call.................................... 62 2.4.5: GSS_Import_name call..................................... 63 2.4.6: GSS_Release_name call.................................... 64 2.4.7: GSS_Release_buffer call.................................. 65 2.4.8: GSS_Release_OID_set call................................. 65 2.4.9: GSS_Create_empty_OID_set call............................ 66 2.4.10: GSS_Add_OID_set_member call.............................. 67 2.4.11: GSS_Test_OID_set_member call............................. 67
2.4.12: GSS_Release_OID call..................................... 68 2.4.13: GSS_OID_to_str call...................................... 68 2.4.14: GSS_Str_to_OID call...................................... 69 2.4.15: GSS_Inquire_names_for_mech call.......................... 69 2.4.16: GSS_Inquire_mechs_for_name call.......................... 70 2.4.17: GSS_Canonicalize_name call............................... 71 2.4.18: GSS_Export_name call..................................... 72 2.4.19: GSS_Duplicate_name call.................................. 73 3: Data Structure Definitions for GSS-V2 Usage................... 73 3.1: Mechanism-Independent Token Format.......................... 74 3.2: Mechanism-Independent Exported Name Object Format........... 77 4: Name Type Definitions......................................... 77 4.1: Host-Based Service Name Form................................ 77 4.2: User Name Form.............................................. 78 4.3: Machine UID Form............................................ 78 4.4: String UID Form............................................. 79 5: Mechanism-Specific Example Scenarios......................... 79 5.1: Kerberos V5, single-TGT..................................... 79 5.2: Kerberos V5, double-TGT..................................... 80 5.3: X.509 Authentication Framework............................. 81 6: Security Considerations...................................... 82 7: Related Activities........................................... 82 Appendix A: Mechanism Design Constraints......................... 83 Appendix B: Compatibility with GSS-V1............................ 83 1: GSS-API Characteristics and Concepts GSS-API operates in the following paradigm. A typical GSS-API caller is itself a communications protocol, calling on GSS-API in order to protect its communications with authentication, integrity, and/or confidentiality security services. A GSS-API caller accepts tokens provided to it by its local GSS-API implementation and transfers the tokens to a peer on a remote system; that peer passes the received tokens to its local GSS-API implementation for processing. The security services available through GSS-API in this fashion are implementable (and have been implemented) over a range of underlying mechanisms based on secret-key and public-key cryptographic technologies. The GSS-API separates the operations of initializing a security context between peers, achieving peer entity authentication (This security service definition, and other definitions used in this document, corresponds to that provided in International Standard ISO 7498-2-1988(E), Security Architecture.) (GSS_Init_sec_context() and GSS_Accept_sec_context() calls), from the operations of providing per-message data origin authentication and data integrity protection (GSS_GetMIC() and GSS_VerifyMIC() calls) for messages subsequently transferred in conjunction with that context. When establishing a
security context, the GSS-API enables a context initiator to optionally permit its credentials to be delegated, meaning that the context acceptor may initiate further security contexts on behalf of the initiating caller. Per-message GSS_Wrap() and GSS_Unwrap() calls provide the data origin authentication and data integrity services which GSS_GetMIC() and GSS_VerifyMIC() offer, and also support selection of confidentiality services as a caller option. Additional calls provide supportive functions to the GSS-API's users. The following paragraphs provide an example illustrating the dataflows involved in use of the GSS-API by a client and server in a mechanism-independent fashion, establishing a security context and transferring a protected message. The example assumes that credential acquisition has already been completed. The example assumes that the underlying authentication technology is capable of authenticating a client to a server using elements carried within a single token, and of authenticating the server to the client (mutual authentication) with a single returned token; this assumption holds for presently- documented CAT mechanisms but is not necessarily true for other cryptographic technologies and associated protocols. The client calls GSS_Init_sec_context() to establish a security context to the server identified by targ_name, and elects to set the mutual_req_flag so that mutual authentication is performed in the course of context establishment. GSS_Init_sec_context() returns an output_token to be passed to the server, and indicates GSS_S_CONTINUE_NEEDED status pending completion of the mutual authentication sequence. Had mutual_req_flag not been set, the initial call to GSS_Init_sec_context() would have returned GSS_S_COMPLETE status. The client sends the output_token to the server. The server passes the received token as the input_token parameter to GSS_Accept_sec_context(). GSS_Accept_sec_context indicates GSS_S_COMPLETE status, provides the client's authenticated identity in the src_name result, and provides an output_token to be passed to the client. The server sends the output_token to the client. The client passes the received token as the input_token parameter to a successor call to GSS_Init_sec_context(), which processes data included in the token in order to achieve mutual authentication from the client's viewpoint. This call to GSS_Init_sec_context() returns GSS_S_COMPLETE status, indicating successful mutual authentication and the completion of context establishment for this example. The client generates a data message and passes it to GSS_Wrap(). GSS_Wrap() performs data origin authentication, data integrity, and (optionally) confidentiality processing on the message and
encapsulates the result into output_message, indicating
GSS_S_COMPLETE status. The client sends the output_message to the
server.
The server passes the received message to GSS_Unwrap(). GSS_Unwrap()
inverts the encapsulation performed by GSS_Wrap(), deciphers the
message if the optional confidentiality feature was applied, and
validates the data origin authentication and data integrity checking
quantities. GSS_Unwrap() indicates successful validation by
returning GSS_S_COMPLETE status along with the resultant
output_message.
For purposes of this example, we assume that the server knows by
out-of-band means that this context will have no further use after
one protected message is transferred from client to server. Given
this premise, the server now calls GSS_Delete_sec_context() to flush
context-level information. Optionally, the server-side application
may provide a token buffer to GSS_Delete_sec_context(), to receive a
context_token to be transferred to the client in order to request
that client-side context-level information be deleted.
If a context_token is transferred, the client passes the
context_token to GSS_Process_context_token(), which returns
GSS_S_COMPLETE status after deleting context-level information at the
client system.
The GSS-API design assumes and addresses several basic goals,
including:
Mechanism independence: The GSS-API defines an interface to
cryptographically implemented strong authentication and other
security services at a generic level which is independent of
particular underlying mechanisms. For example, GSS-API-provided
services can be implemented by secret-key technologies (e.g.,
Kerberos) or public-key approaches (e.g., X.509).
Protocol environment independence: The GSS-API is independent of
the communications protocol suites with which it is employed,
permitting use in a broad range of protocol environments. In
appropriate environments, an intermediate implementation "veneer"
which is oriented to a particular communication protocol (e.g.,
Remote Procedure Call (RPC)) may be interposed between
applications which call that protocol and the GSS-API, thereby
invoking GSS-API facilities in conjunction with that protocol's
communications invocations.
Protocol association independence: The GSS-API's security context
construct is independent of communications protocol association
constructs. This characteristic allows a single GSS-API
implementation to be utilized by a variety of invoking protocol
modules on behalf of those modules' calling applications. GSS-API
services can also be invoked directly by applications, wholly
independent of protocol associations.
Suitability to a range of implementation placements: GSS-API
clients are not constrained to reside within any Trusted Computing
Base (TCB) perimeter defined on a system where the GSS-API is
implemented; security services are specified in a manner suitable
to both intra-TCB and extra-TCB callers.
1.1: GSS-API Constructs
This section describes the basic elements comprising the GSS-API.
1.1.1: Credentials
1.1.1.1: Credential Constructs and Concepts
Credentials provide the prerequisites which permit GSS-API peers to
establish security contexts with each other. A caller may designate
that the credential elements which are to be applied for context
initiation or acceptance be selected by default. Alternately, those
GSS-API callers which need to make explicit selection of particular
credentials structures may make references to those credentials
through GSS-API-provided credential handles ("cred_handles"). In all
cases, callers' credential references are indirect, mediated by GSS-
API implementations and not requiring callers to access the selected
credential elements.
A single credential structure may be used to initiate outbound
contexts and to accept inbound contexts. Callers needing to operate
in only one of these modes may designate this fact when credentials
are acquired for use, allowing underlying mechanisms to optimize
their processing and storage requirements. The credential elements
defined by a particular mechanism may contain multiple cryptographic
keys, e.g., to enable authentication and message encryption to be
performed with different algorithms.
A GSS-API credential structure may contain multiple credential
elements, each containing mechanism-specific information for a
particular underlying mechanism (mech_type), but the set of elements
within a given credential structure represent a common entity. A
credential structure's contents will vary depending on the set of
mech_types supported by a particular GSS-API implementation. Each
credential element identifies the data needed by its mechanism in
order to establish contexts on behalf of a particular principal, and
may contain separate credential references for use in context initiation and context acceptance. Multiple credential elements within a given credential having overlapping combinations of mechanism, usage mode, and validity period are not permitted. Commonly, a single mech_type will be used for all security contexts established by a particular initiator to a particular target. A major motivation for supporting credential sets representing multiple mech_types is to allow initiators on systems which are equipped to handle multiple types to initiate contexts to targets on other systems which can accommodate only a subset of the set supported at the initiator's system. 1.1.1.2: Credential Management It is the responsibility of underlying system-specific mechanisms and OS functions below the GSS-API to ensure that the ability to acquire and use credentials associated with a given identity is constrained to appropriate processes within a system. This responsibility should be taken seriously by implementors, as the ability for an entity to utilize a principal's credentials is equivalent to the entity's ability to successfully assert that principal's identity. Once a set of GSS-API credentials is established, the transferability of that credentials set to other processes or analogous constructs within a system is a local matter, not defined by the GSS-API. An example local policy would be one in which any credentials received as a result of login to a given user account, or of delegation of rights to that account, are accessible by, or transferable to, processes running under that account. The credential establishment process (particularly when performed on behalf of users rather than server processes) is likely to require access to passwords or other quantities which should be protected locally and exposed for the shortest time possible. As a result, it will often be appropriate for preliminary credential establishment to be performed through local means at user login time, with the result(s) cached for subsequent reference. These preliminary credentials would be set aside (in a system-specific fashion) for subsequent use, either: to be accessed by an invocation of the GSS-API GSS_Acquire_cred() call, returning an explicit handle to reference that credential to comprise default credential elements to be installed, and to be used when default credential behavior is requested on behalf of a process
1.1.1.3: Default Credential Resolution The gss_init_sec_context and gss_accept_sec_context routines allow the value GSS_C_NO_CREDENTIAL to be specified as their credential handle parameter. This special credential-handle indicates a desire by the application to act as a default principal. While individual GSS-API implementations are free to determine such default behavior as appropriate to the mechanism, the following default behavior by these routines is recommended for portability: GSS_Init_sec_context: (i) If there is only a single principal capable of initiating security contexts that the application is authorized to act on behalf of, then that principal shall be used, otherwise (ii) If the platform maintains a concept of a default network- identity, and if the application is authorized to act on behalf of that identity for the purpose of initiating security contexts, then the principal corresponding to that identity shall be used, otherwise (iii) If the platform maintains a concept of a default local identity, and provides a means to map local identities into network-identities, and if the application is authorized to act on behalf of the network-identity image of the default local identity for the purpose of initiating security contexts, then the principal corresponding to that identity shall be used, otherwise (iv) A user-configurable default identity should be used. GSS_Accept_sec_context: (i) If there is only a single authorized principal identity capable of accepting security contexts, then that principal shall be used, otherwise (ii) If the mechanism can determine the identity of the target principal by examining the context-establishment token, and if the accepting application is authorized to act as that principal for the purpose of accepting security contexts, then that principal identity shall be used, otherwise (iii) If the mechanism supports context acceptance by any principal, and mutual authentication was not requested, any principal that the application is authorized to accept security contexts under may be used, otherwise
(iv) A user-configurable default identity shall be used.
The purpose of the above rules is to allow security contexts to be
established by both initiator and acceptor using the default behavior
wherever possible. Applications requesting default behavior are
likely to be more portable across mechanisms and platforms than ones
that use GSS_Acquire_cred to request a specific identity.
1.1.2: Tokens
Tokens are data elements transferred between GSS-API callers, and are
divided into two classes. Context-level tokens are exchanged in order
to establish and manage a security context between peers. Per-message
tokens relate to an established context and are exchanged to provide
protective security services (i.e., data origin authentication,
integrity, and optional confidentiality) for corresponding data
messages.
The first context-level token obtained from GSS_Init_sec_context() is
required to indicate at its very beginning a globally-interpretable
mechanism identifier, i.e., an Object Identifier (OID) of the
security mechanism. The remaining part of this token as well as the
whole content of all other tokens are specific to the particular
underlying mechanism used to support the GSS-API. Section 3 of this
document provides, for designers of GSS-API support mechanisms, the
description of the header of the first context-level token which is
then followed by mechanism-specific information.
Tokens' contents are opaque from the viewpoint of GSS-API callers.
They are generated within the GSS-API implementation at an end
system, provided to a GSS-API caller to be transferred to the peer
GSS-API caller at a remote end system, and processed by the GSS-API
implementation at that remote end system. Tokens may be output by
GSS-API calls (and should be transferred to GSS-API peers) whether or
not the calls' status indicators indicate successful completion.
Token transfer may take place in an in-band manner, integrated into
the same protocol stream used by the GSS-API callers for other data
transfers, or in an out-of-band manner across a logically separate
channel.
Different GSS-API tokens are used for different purposes (e.g.,
context initiation, context acceptance, protected message data on an
established context), and it is the responsibility of a GSS-API
caller receiving tokens to distinguish their types, associate them
with corresponding security contexts, and pass them to appropriate
GSS-API processing routines. Depending on the caller protocol
environment, this distinction may be accomplished in several ways.
The following examples illustrate means through which tokens' types
may be distinguished:
- implicit tagging based on state information (e.g., all tokens on
a new association are considered to be context establishment
tokens until context establishment is completed, at which point
all tokens are considered to be wrapped data objects for that
context),
- explicit tagging at the caller protocol level,
- a hybrid of these approaches.
Commonly, the encapsulated data within a token includes internal
mechanism-specific tagging information, enabling mechanism-level
processing modules to distinguish tokens used within the mechanism
for different purposes. Such internal mechanism-level tagging is
recommended to mechanism designers, and enables mechanisms to
determine whether a caller has passed a particular token for
processing by an inappropriate GSS-API routine.
Development of GSS-API support primitives based on a particular
underlying cryptographic technique and protocol (i.e., conformant to
a specific GSS-API mechanism definition) does not necessarily imply
that GSS-API callers using that GSS-API mechanism will be able to
interoperate with peers invoking the same technique and protocol
outside the GSS-API paradigm, or with peers implementing a different
GSS-API mechanism based on the same underlying technology. The
format of GSS-API tokens defined in conjunction with a particular
mechanism, and the techniques used to integrate those tokens into
callers' protocols, may not be interoperable with the tokens used by
non-GSS-API callers of the same underlying technique.
1.1.3: Security Contexts
Security contexts are established between peers, using credentials
established locally in conjunction with each peer or received by
peers via delegation. Multiple contexts may exist simultaneously
between a pair of peers, using the same or different sets of
credentials. Coexistence of multiple contexts using different
credentials allows graceful rollover when credentials expire.
Distinction among multiple contexts based on the same credentials
serves applications by distinguishing different message streams in a
security sense.
The GSS-API is independent of underlying protocols and addressing
structure, and depends on its callers to transport GSS-API-provided
data elements. As a result of these factors, it is a caller
responsibility to parse communicated messages, separating GSS-API- related data elements from caller-provided data. The GSS-API is independent of connection vs. connectionless orientation of the underlying communications service. No correlation between security context and communications protocol association is dictated. (The optional channel binding facility, discussed in Section 1.1.6 of this document, represents an intentional exception to this rule, supporting additional protection features within GSS-API supporting mechanisms.) This separation allows the GSS-API to be used in a wide range of communications environments, and also simplifies the calling sequences of the individual calls. In many cases (depending on underlying security protocol, associated mechanism, and availability of cached information), the state information required for context setup can be sent concurrently with initial signed user data, without interposing additional message exchanges. 1.1.4: Mechanism Types In order to successfully establish a security context with a target peer, it is necessary to identify an appropriate underlying mechanism type (mech_type) which both initiator and target peers support. The definition of a mechanism embodies not only the use of a particular cryptographic technology (or a hybrid or choice among alternative cryptographic technologies), but also definition of the syntax and semantics of data element exchanges which that mechanism will employ in order to support security services. It is recommended that callers initiating contexts specify the "default" mech_type value, allowing system-specific functions within or invoked by the GSS-API implementation to select the appropriate mech_type, but callers may direct that a particular mech_type be employed when necessary. The means for identifying a shared mech_type to establish a security context with a peer will vary in different environments and circumstances; examples include (but are not limited to): use of a fixed mech_type, defined by configuration, within an environment syntactic convention on a target-specific basis, through examination of a target's name lookup of a target's name in a naming service or other database in order to identify mech_types supported by that target
explicit negotiation between GSS-API callers in advance of
security context setup
When transferred between GSS-API peers, mech_type specifiers (per
Section 3, represented as Object Identifiers (OIDs)) serve to qualify
the interpretation of associated tokens. (The structure and encoding
of Object Identifiers is defined in ISO/IEC 8824, "Specification of
Abstract Syntax Notation One (ASN.1)" and in ISO/IEC 8825,
"Specification of Basic Encoding Rules for Abstract Syntax Notation
One (ASN.1)".) Use of hierarchically structured OIDs serves to
preclude ambiguous interpretation of mech_type specifiers. The OID
representing the DASS MechType, for example, is 1.3.12.2.1011.7.5,
and that of the Kerberos V5 mechanism, once advanced to the level of
Proposed Standard, will be 1.2.840.113554.1.2.2.
1.1.5: Naming
The GSS-API avoids prescribing naming structures, treating the names
which are transferred across the interface in order to initiate and
accept security contexts as opaque objects. This approach supports
the GSS-API's goal of implementability atop a range of underlying
security mechanisms, recognizing the fact that different mechanisms
process and authenticate names which are presented in different
forms. Generalized services offering translation functions among
arbitrary sets of naming environments are outside the scope of the
GSS-API; availability and use of local conversion functions to
translate among the naming formats supported within a given end
system is anticipated.
Different classes of name representations are used in conjunction
with different GSS-API parameters:
- Internal form (denoted in this document by INTERNAL NAME),
opaque to callers and defined by individual GSS-API
implementations. GSS-API implementations supporting multiple
namespace types must maintain internal tags to disambiguate the
interpretation of particular names. A Mechanism Name (MN) is a
special case of INTERNAL NAME, guaranteed to contain elements
corresponding to one and only one mechanism; calls which are
guaranteed to emit MNs or which require MNs as input are so
identified within this specification.
- Contiguous string ("flat") form (denoted in this document by
OCTET STRING); accompanied by OID tags identifying the namespace
to which they correspond. Depending on tag value, flat names may
or may not be printable strings for direct acceptance from and
presentation to users. Tagging of flat names allows GSS-API
callers and underlying GSS-API mechanisms to disambiguate name
types and to determine whether an associated name's type is one
which they are capable of processing, avoiding aliasing problems
which could result from misinterpreting a name of one type as a
name of another type.
- The GSS-API Exported Name Object, a special case of flat name
designated by a reserved OID value, carries a canonicalized form
of a name suitable for binary comparisons.
In addition to providing means for names to be tagged with types,
this specification defines primitives to support a level of naming
environment independence for certain calling applications. To provide
basic services oriented towards the requirements of callers which
need not themselves interpret the internal syntax and semantics of
names, GSS-API calls for name comparison (GSS_Compare_name()),
human-readable display (GSS_Display_name()), input conversion
(GSS_Import_name()), internal name deallocation (GSS_Release_name()),
and internal name duplication (GSS_Duplicate_name()) functions are
defined. (It is anticipated that these proposed GSS-API calls will be
implemented in many end systems based on system-specific name
manipulation primitives already extant within those end systems;
inclusion within the GSS-API is intended to offer GSS-API callers a
portable means to perform specific operations, supportive of
authorization and audit requirements, on authenticated names.)
GSS_Import_name() implementations can, where appropriate, support
more than one printable syntax corresponding to a given namespace
(e.g., alternative printable representations for X.500 Distinguished
Names), allowing flexibility for their callers to select among
alternative representations. GSS_Display_name() implementations
output a printable syntax selected as appropriate to their
operational environments; this selection is a local matter. Callers
desiring portability across alternative printable syntaxes should
refrain from implementing comparisons based on printable name forms
and should instead use the GSS_Compare_name() call to determine
whether or not one internal-format name matches another.
The GSS_Canonicalize_name() and GSS_Export_name() calls enable
callers to acquire and process Exported Name Objects, canonicalized
and translated in accordance with the procedures of a particular
GSS-API mechanism. Exported Name Objects can, in turn, be input to
GSS_Import_name(), yielding equivalent MNs. These facilities are
designed specifically to enable efficient storage and comparison of
names (e.g., for use in access control lists).
The following diagram illustrates the intended dataflow among name-
related GSS-API processing routines.
GSS-API library defaults
|
|
V text, for
text --------------> internal_name (IN) -----------> display only
import_name() / display_name()
/
/
/
accept_sec_context() /
| /
| /
| / canonicalize_name()
| /
| /
| /
| /
| /
| |
V V <---------------------
single mechanism import_name() exported name: flat
internal_name (MN) binary "blob" usable
----------------------> for access control
export_name()
1.1.6: Channel Bindings
The GSS-API accommodates the concept of caller-provided channel
binding ("chan_binding") information. Channel bindings are used to
strengthen the quality with which peer entity authentication is
provided during context establishment, by limiting the scope within
which an intercepted context establishment token can be reused by an
attacker. Specifically, they enable GSS-API callers to bind the
establishment of a security context to relevant characteristics
(e.g., addresses, transformed representations of encryption keys) of
the underlying communications channel, of protection mechanisms
applied to that communications channel, and to application-specific
data.
The caller initiating a security context must determine the
appropriate channel binding values to provide as input to the
GSS_Init_sec_context() call, and consistent values must be provided
to GSS_Accept_sec_context() by the context's target, in order for
both peers' GSS-API mechanisms to validate that received tokens
possess correct channel-related characteristics. Use or non-use of
the GSS-API channel binding facility is a caller option. GSS-API mechanisms can operate in an environment where NULL channel bindings are presented; mechanism implementors are encouraged, but not required, to make use of caller-provided channel binding data within their mechanisms. Callers should not assume that underlying mechanisms provide confidentiality protection for channel binding information. When non-NULL channel bindings are provided by callers, certain mechanisms can offer enhanced security value by interpreting the bindings' content (rather than simply representing those bindings, or integrity check values computed on them, within tokens) and will therefore depend on presentation of specific data in a defined format. To this end, agreements among mechanism implementors are defining conventional interpretations for the contents of channel binding arguments, including address specifiers (with content dependent on communications protocol environment) for context initiators and acceptors. (These conventions are being incorporated in GSS-API mechanism specifications and into the GSS-API C language bindings specification.) In order for GSS-API callers to be portable across multiple mechanisms and achieve the full security functionality which each mechanism can provide, it is strongly recommended that GSS-API callers provide channel bindings consistent with these conventions and those of the networking environment in which they operate. 1.2: GSS-API Features and Issues This section describes aspects of GSS-API operations, of the security services which the GSS-API provides, and provides commentary on design issues. 1.2.1: Status Reporting Each GSS-API call provides two status return values. Major_status values provide a mechanism-independent indication of call status (e.g., GSS_S_COMPLETE, GSS_S_FAILURE, GSS_S_CONTINUE_NEEDED), sufficient to drive normal control flow within the caller in a generic fashion. Table 1 summarizes the defined major_status return codes in tabular fashion.
Table 1: GSS-API Major Status Codes
FATAL ERROR CODES
GSS_S_BAD_BINDINGS channel binding mismatch
GSS_S_BAD_MECH unsupported mechanism requested
GSS_S_BAD_NAME invalid name provided
GSS_S_BAD_NAMETYPE name of unsupported type provided
GSS_S_BAD_STATUS invalid input status selector
GSS_S_BAD_SIG token had invalid integrity check
GSS_S_CONTEXT_EXPIRED specified security context expired
GSS_S_CREDENTIALS_EXPIRED expired credentials detected
GSS_S_DEFECTIVE_CREDENTIAL defective credential detected
GSS_S_DEFECTIVE_TOKEN defective token detected
GSS_S_FAILURE failure, unspecified at GSS-API
level
GSS_S_NO_CONTEXT no valid security context specified
GSS_S_NO_CRED no valid credentials provided
GSS_S_BAD_QOP unsupported QOP value
GSS_S_UNAUTHORIZED operation unauthorized
GSS_S_UNAVAILABLE operation unavailable
GSS_S_DUPLICATE_ELEMENT duplicate credential element requested
GSS_S_NAME_NOT_MN name contains multi-mechanism elements
INFORMATORY STATUS CODES
GSS_S_COMPLETE normal completion
GSS_S_CONTINUE_NEEDED continuation call to routine
required
GSS_S_DUPLICATE_TOKEN duplicate per-message token
detected
GSS_S_OLD_TOKEN timed-out per-message token
detected
GSS_S_UNSEQ_TOKEN reordered (early) per-message token
detected
GSS_S_GAP_TOKEN skipped predecessor token(s)
detected
Minor_status provides more detailed status information which may
include status codes specific to the underlying security mechanism.
Minor_status values are not specified in this document.
GSS_S_CONTINUE_NEEDED major_status returns, and optional message
outputs, are provided in GSS_Init_sec_context() and
GSS_Accept_sec_context() calls so that different mechanisms'
employment of different numbers of messages within their
authentication sequences need not be reflected in separate code paths
within calling applications. Instead, such cases are accommodated
with sequences of continuation calls to GSS_Init_sec_context() and GSS_Accept_sec_context(). The same mechanism is used to encapsulate mutual authentication within the GSS-API's context initiation calls. For mech_types which require interactions with third-party servers in order to establish a security context, GSS-API context establishment calls may block pending completion of such third-party interactions. On the other hand, no GSS-API calls pend on serialized interactions with GSS-API peer entities. As a result, local GSS-API status returns cannot reflect unpredictable or asynchronous exceptions occurring at remote peers, and reflection of such status information is a caller responsibility outside the GSS-API. 1.2.2: Per-Message Security Service Availability When a context is established, two flags are returned to indicate the set of per-message protection security services which will be available on the context: the integ_avail flag indicates whether per-message integrity and data origin authentication services are available the conf_avail flag indicates whether per-message confidentiality services are available, and will never be returned TRUE unless the integ_avail flag is also returned TRUE GSS-API callers desiring per-message security services should check the values of these flags at context establishment time, and must be aware that a returned FALSE value for integ_avail means that invocation of GSS_GetMIC() or GSS_Wrap() primitives on the associated context will apply no cryptographic protection to user data messages. The GSS-API per-message integrity and data origin authentication services provide assurance to a receiving caller that protection was applied to a message by the caller's peer on the security context, corresponding to the entity named at context initiation. The GSS-API per-message confidentiality service provides assurance to a sending caller that the message's content is protected from access by entities other than the context's named peer.
The GSS-API per-message protection service primitives, as the category name implies, are oriented to operation at the granularity of protocol data units. They perform cryptographic operations on the data units, transfer cryptographic control information in tokens, and, in the case of GSS_Wrap(), encapsulate the protected data unit. As such, these primitives are not oriented to efficient data protection for stream-paradigm protocols (e.g., Telnet) if cryptography must be applied on an octet-by-octet basis. 1.2.3: Per-Message Replay Detection and Sequencing Certain underlying mech_types offer support for replay detection and/or sequencing of messages transferred on the contexts they support. These optionally-selectable protection features are distinct from replay detection and sequencing features applied to the context establishment operation itself; the presence or absence of context- level replay or sequencing features is wholly a function of the underlying mech_type's capabilities, and is not selected or omitted as a caller option. The caller initiating a context provides flags (replay_det_req_flag and sequence_req_flag) to specify whether the use of per-message replay detection and sequencing features is desired on the context being established. The GSS-API implementation at the initiator system can determine whether these features are supported (and whether they are optionally selectable) as a function of mech_type, without need for bilateral negotiation with the target. When enabled, these features provide recipients with indicators as a result of GSS-API processing of incoming messages, identifying whether those messages were detected as duplicates or out-of-sequence. Detection of such events does not prevent a suspect message from being provided to a recipient; the appropriate course of action on a suspect message is a matter of caller policy. The semantics of the replay detection and sequencing services applied to received messages, as visible across the interface which the GSS- API provides to its clients, are as follows: When replay_det_state is TRUE, the possible major_status returns for well-formed and correctly signed messages are as follows: 1. GSS_S_COMPLETE indicates that the message was within the window (of time or sequence space) allowing replay events to be detected, and that the message was not a replay of a previously-processed message within that window.
2. GSS_S_DUPLICATE_TOKEN indicates that the cryptographic
checkvalue on the received message was correct, but that the
message was recognized as a duplicate of a previously-processed
message.
3. GSS_S_OLD_TOKEN indicates that the cryptographic checkvalue on
the received message was correct, but that the message is too old
to be checked for duplication.
When sequence_state is TRUE, the possible major_status returns for
well-formed and correctly signed messages are as follows:
1. GSS_S_COMPLETE indicates that the message was within the window
(of time or sequence space) allowing replay events to be detected,
that the message was not a replay of a previously-processed
message within that window, and that no predecessor sequenced
messages are missing relative to the last received message (if
any) processed on the context with a correct cryptographic
checkvalue.
2. GSS_S_DUPLICATE_TOKEN indicates that the integrity check value
on the received message was correct, but that the message was
recognized as a duplicate of a previously-processed message.
3. GSS_S_OLD_TOKEN indicates that the integrity check value on the
received message was correct, but that the token is too old to be
checked for duplication.
4. GSS_S_UNSEQ_TOKEN indicates that the cryptographic checkvalue
on the received message was correct, but that it is earlier in a
sequenced stream than a message already processed on the context.
[Note: Mechanisms can be architected to provide a stricter form of
sequencing service, delivering particular messages to recipients
only after all predecessor messages in an ordered stream have been
delivered. This type of support is incompatible with the GSS-API
paradigm in which recipients receive all messages, whether in
order or not, and provide them (one at a time, without intra-GSS-
API message buffering) to GSS-API routines for validation. GSS-
API facilities provide supportive functions, aiding clients to
achieve strict message stream integrity in an efficient manner in
conjunction with sequencing provisions in communications
protocols, but the GSS-API does not offer this level of message
stream integrity service by itself.]
5. GSS_S_GAP_TOKEN indicates that the cryptographic checkvalue on
the received message was correct, but that one or more predecessor
sequenced messages have not been successfully processed relative
to the last received message (if any) processed on the context
with a correct cryptographic checkvalue.
As the message stream integrity features (especially sequencing) may
interfere with certain applications' intended communications
paradigms, and since support for such features is likely to be
resource intensive, it is highly recommended that mech_types
supporting these features allow them to be activated selectively on
initiator request when a context is established. A context initiator
and target are provided with corresponding indicators
(replay_det_state and sequence_state), signifying whether these
features are active on a given context.
An example mech_type supporting per-message replay detection could
(when replay_det_state is TRUE) implement the feature as follows: The
underlying mechanism would insert timestamps in data elements output
by GSS_GetMIC() and GSS_Wrap(), and would maintain (within a time-
limited window) a cache (qualified by originator-recipient pair)
identifying received data elements processed by GSS_VerifyMIC() and
GSS_Unwrap(). When this feature is active, exception status returns
(GSS_S_DUPLICATE_TOKEN, GSS_S_OLD_TOKEN) will be provided when
GSS_VerifyMIC() or GSS_Unwrap() is presented with a message which is
either a detected duplicate of a prior message or which is too old to
validate against a cache of recently received messages.
1.2.4: Quality of Protection
Some mech_types provide their users with fine granularity control
over the means used to provide per-message protection, allowing
callers to trade off security processing overhead dynamically against
the protection requirements of particular messages. A per-message
quality-of-protection parameter (analogous to quality-of-service, or
QOS) selects among different QOP options supported by that mechanism.
On context establishment for a multi-QOP mech_type, context-level
data provides the prerequisite data for a range of protection
qualities.
It is expected that the majority of callers will not wish to exert
explicit mechanism-specific QOP control and will therefore request
selection of a default QOP. Definitions of, and choices among, non-
default QOP values are mechanism-specific, and no ordered sequences
of QOP values can be assumed equivalent across different mechanisms.
Meaningful use of non-default QOP values demands that callers be
familiar with the QOP definitions of an underlying mechanism or
mechanisms, and is therefore a non-portable construct. The
GSS_S_BAD_QOP major_status value is defined in order to indicate that a provided QOP value is unsupported for a security context, most likely because that value is unrecognized by the underlying mechanism. 1.2.5: Anonymity Support In certain situations or environments, an application may wish to authenticate a peer and/or protect communications using GSS-API per- message services without revealing its own identity. For example, consider an application which provides read access to a research database, and which permits queries by arbitrary requestors. A client of such a service might wish to authenticate the service, to establish trust in the information received from it, but might not wish to disclose its identity to the service for privacy reasons. In ordinary GSS-API usage, a context initiator's identity is made available to the context acceptor as part of the context establishment process. To provide for anonymity support, a facility (input anon_req_flag to GSS_Init_sec_context()) is provided through which context initiators may request that their identity not be provided to the context acceptor. Mechanisms are not required to honor this request, but a caller will be informed (via returned anon_state indicator from GSS_Init_sec_context()) whether or not the request is honored. Note that authentication as the anonymous principal does not necessarily imply that credentials are not required in order to establish a context. The following Object Identifier value is provided as a means to identify anonymous names, and can be compared against in order to determine, in a mechanism-independent fashion, whether a name refers to an anonymous principal: {1(iso), 3(org), 6(dod), 1(internet), 5(security), 6(nametypes), 3(gss-anonymous-name)} The recommended symbolic name corresponding to this definition is GSS_C_NT_ANONYMOUS. Four possible combinations of anon_state and mutual_state are possible, with the following results: anon_state == FALSE, mutual_state == FALSE: initiator authenticated to target. anon_state == FALSE, mutual_state == TRUE: initiator authenticated to target, target authenticated to initiator.
anon_state == TRUE, mutual_state == FALSE: initiator authenticated
as anonymous principal to target.
anon_state == TRUE, mutual_state == TRUE: initiator authenticated
as anonymous principal to target, target authenticated to
initiator.
1.2.6: Initialization
No initialization calls (i.e., calls which must be invoked prior to
invocation of other facilities in the interface) are defined in GSS-
API. As an implication of this fact, GSS-API implementations must
themselves be self-initializing.
1.2.7: Per-Message Protection During Context Establishment
A facility is defined in GSS-V2 to enable protection and buffering of
data messages for later transfer while a security context's
establishment is in GSS_S_CONTINUE_NEEDED status, to be used in cases
where the caller side already possesses the necessary session key to
enable this processing. Specifically, a new state Boolean, called
prot_ready_state, is added to the set of information returned by
GSS_Init_sec_context(), GSS_Accept_sec_context(), and
GSS_Inquire_context().
For context establishment calls, this state Boolean is valid and
interpretable when the associated major_status is either
GSS_S_CONTINUE_NEEDED, or GSS_S_COMPLETE. Callers of GSS-API (both
initiators and acceptors) can assume that per-message protection (via
GSS_Wrap(), GSS_Unwrap(), GSS_GetMIC() and GSS_VerifyMIC()) is
available and ready for use if either: prot_ready_state == TRUE, or
major_status == GSS_S_COMPLETE, though mutual authentication (if
requested) cannot be guaranteed until GSS_S_COMPLETE is returned.
This achieves full, transparent backward compatibility for GSS-API V1
callers, who need not even know of the existence of prot_ready_state,
and who will get the expected behavior from GSS_S_COMPLETE, but who
will not be able to use per-message protection before GSS_S_COMPLETE
is returned.
It is not a requirement that GSS-V2 mechanisms ever return TRUE
prot_ready_state before completion of context establishment (indeed,
some mechanisms will not evolve usable message protection keys,
especially at the context acceptor, before context establishment is
complete). It is expected but not required that GSS-V2 mechanisms
will return TRUE prot_ready_state upon completion of context
establishment if they support per-message protection at all (however
GSS-V2 applications should not assume that TRUE prot_ready_state will
always be returned together with the GSS_S_COMPLETE major_status, since GSS-V2 implementations may continue to support GSS-V1 mechanism code, which will never return TRUE prot_ready_state). When prot_ready_state is returned TRUE, mechanisms shall also set those context service indicator flags (deleg_state, mutual_state, replay_det_state, sequence_state, anon_state, trans_state, conf_avail, integ_avail) which represent facilities confirmed, at that time, to be available on the context being established. In situations where prot_ready_state is returned before GSS_S_COMPLETE, it is possible that additional facilities may be confirmed and subsequently indicated when GSS_S_COMPLETE is returned. 1.2.8: Implementation Robustness This section recommends aspects of GSS-API implementation behavior in the interests of overall robustness. If a token is presented for processing on a GSS-API security context and that token is determined to be invalid for that context, the context's state should not be disrupted for purposes of processing subsequent valid tokens. Certain local conditions at a GSS-API implementation (e.g., unavailability of memory) may preclude, temporarily or permanently, the successful processing of tokens on a GSS-API security context, typically generating GSS_S_FAILURE major_status returns along with locally-significant minor_status. For robust operation under such conditions, the following recommendations are made: Failing calls should free any memory they allocate, so that callers may retry without causing further loss of resources. Failure of an individual call on an established context should not preclude subsequent calls from succeeding on the same context. Whenever possible, it should be possible for GSS_Delete_sec_context() calls to be successfully processed even if other calls cannot succeed, thereby enabling context-related resources to be released.
(page 23 continued on part 2)
