RFC2853 - Generic Security Service API Version 2 : Java Bindings

Network Working Group J. Kabat
Request for Comments: 2853 ValiCert, Inc.
Category: Standards Track M. Upadhyay
Sun Microsystems, Inc.
June 2000
Generic Security Service API Version 2 : Java Bindings
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.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
The Generic Security Services Application Program Interface (GSS-API)
offers application programmers uniform Access to security services
atop a variety of underlying cryptographic mechanisms. This document
specifies the Java bindings for GSS-API which is described at a
language independent conceptual level in RFC2743 [GSSAPIv2-UPDATE].
The GSS-API allows a caller application to authenticate a principal
identity, to delegate rights to a peer, and to apply security
services sUCh as confidentiality and integrity on a per-message
basis. Examples of security mechanisms defined for GSS-API are The
Simple Public-Key GSS-API Mechanism [SPKM] and The Kerberos Version 5
GSS-API Mechanism [KERBV5].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5
2. GSS-API Operational Paradigm . . . . . . . . . . . . . . . 6
3. Additional Controls . . . . . . . . . . . . . . . . . . . 8
3.1. Delegation . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Mutual Authentication . . . . . . . . . . . . . . . . . 10
3.3. Replay and Out-of-Sequence Detection . . . . . . . . . . 10
3.4. Anonymous Authentication . . . . . . . . . . . . . . . . 11
3.5. Confidentiality . . . . . . . . . . . . . . . . . . . . 12
3.6. Inter-process Context Transfer . . . . . . . . . . . . . 12
3.7. The Use of Incomplete Contexts . . . . . . . . . . . . . 13
4. Calling Conventions . . . . . . . . . . . . . . . . . . . 13
4.1. Package Name . . . . . . . . . . . . . . . . . . . . . . 13
4.2. Provider Framework . . . . . . . . . . . . . . . . . . . 13
4.3. Integer types . . . . . . . . . . . . . . . . . . . . . 14
4.4. Opaque Data types . . . . . . . . . . . . . . . . . . . 14
4.5. Strings . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6. Object Identifiers . . . . . . . . . . . . . . . . . . . 15
4.7. Object Identifier Sets . . . . . . . . . . . . . . . . . 15
4.8. Credentials . . . . . . . . . . . . . . . . . . . . . . 16
4.9. Contexts . . . . . . . . . . . . . . . . . . . . . . . . 18
4.10. Authentication tokens . . . . . . . . . . . . . . . . . 18
4.11. Interprocess tokens . . . . . . . . . . . . . . . . . . 18
4.12. Error Reporting . . . . . . . . . . . . . . . . . . . . 19
4.12.1. GSS status codes . . . . . . . . . . . . . . . . . . 19
4.12.2. Mechanism-specific status codes . . . . . . . . . . . 21
4.12.3. Supplementary status codes . . . . . . . . . . . . . 21
4.13. Names . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.14. Channel Bindings . . . . . . . . . . . . . . . . . . . 25
4.15. Stream Objects . . . . . . . . . . . . . . . . . . . . 26
4.16. Optional Parameters . . . . . . . . . . . . . . . . . . 26
5. Introduction to GSS-API Classes and Interfaces . . . . . . 26
5.1. GSSManager class . . . . . . . . . . . . . . . . . . . . 26
5.2. GSSName interface . . . . . . . . . . . . . . . . . . . 27
5.3. GSSCredential interface . . . . . . . . . . . . . . . . 28
5.4. GSSContext interface . . . . . . . . . . . . . . . . . . 28
5.5. MessageProp class . . . . . . . . . . . . . . . . . . . 30
5.6. GSSException class . . . . . . . . . . . . . . . . . . . 30
5.7. Oid class . . . . . . . . . . . . . . . . . . . . . . . 30
5.8. ChannelBinding class . . . . . . . . . . . . . . . . . . 31
6. Detailed GSS-API Class Description . . . . . . . . . . . . 31
6.1. public abstract class GSSManager . . . . . . . . . . . . 31
6.1.1. Example Code . . . . . . . . . . . . . . . . . . . . . 32
6.1.2. getInstance . . . . . . . . . . . . . . . . . . . . . 33
6.1.3. getMechs . . . . . . . . . . . . . . . . . . . . . . . 33
6.1.4. getNamesForMech . . . . . . . . . . . . . . . . . . . 33
6.1.5. getMechsForName . . . . . . . . . . . . . . . . . . . 33
6.1.6. createName . . . . . . . . . . . . . . . . . . . . . . 33
6.1.7. createName . . . . . . . . . . . . . . . . . . . . . . 34
6.1.8. createName . . . . . . . . . . . . . . . . . . . . . . 35
6.1.9. createName . . . . . . . . . . . . . . . . . . . . . . 35
6.1.10. createCredential . . . . . . . . . . . . . . . . . . 36
6.1.11. createCredential . . . . . . . . . . . . . . . . . . 36
6.1.12. createCredential . . . . . . . . . . . . . . . . . . 37
6.1.13. createContext . . . . . . . . . . . . . . . . . . . . 37
6.1.14. createContext . . . . . . . . . . . . . . . . . . . . 38
6.1.15. createContext . . . . . . . . . . . . . . . . . . . . 38
6.1.16. addProviderAtFront . . . . . . . . . . . . . . . . . 38
6.1.16.1. Example Code . . . . . . . . . . . . . . . . . . . 39
6.1.17. addProviderAtEnd . . . . . . . . . . . . . . . . . . 40
6.1.17.1. Example Code . . . . . . . . . . . . . . . . . . . 41
6.2. public interface GSSName . . . . . . . . . . . . . . . . 42
6.2.1. Example Code . . . . . . . . . . . . . . . . . . . . . 42
6.2.2. Static Constants . . . . . . . . . . . . . . . . . . . 43
6.2.3. equals . . . . . . . . . . . . . . . . . . . . . . . . 44
6.2.4. equals . . . . . . . . . . . . . . . . . . . . . . . . 44
6.2.5. canonicalize . . . . . . . . . . . . . . . . . . . . . 44
6.2.6. eXPort . . . . . . . . . . . . . . . . . . . . . . . . 45
6.2.7. toString . . . . . . . . . . . . . . . . . . . . . . . 45
6.2.8. getStringNameType . . . . . . . . . . . . . . . . . . 45
6.2.9. isAnonymous . . . . . . . . . . . . . . . . . . . . . 45
6.2.10. isMN . . . . . . . . . . . . . . . . . . . . . . . . 45
6.3. public interface GSSCredential implements Cloneable . . 45
6.3.1. Example Code . . . . . . . . . . . . . . . . . . . . . 46
6.3.2. Static Constants . . . . . . . . . . . . . . . . . . . 47
6.3.3. dispose . . . . . . . . . . . . . . . . . . . . . . . 48
6.3.4. getName . . . . . . . . . . . . . . . . . . . . . . . 48
6.3.5. getName . . . . . . . . . . . . . . . . . . . . . . . 48
6.3.6. getRemainingLifetime . . . . . . . . . . . . . . . . . 48
6.3.7. getRemainingInitLifetime . . . . . . . . . . . . . . . 49
6.3.8. getRemainingAcceptLifetime . . . . . . . . . . . . . . 49
6.3.9. getUsage . . . . . . . . . . . . . . . . . . . . . . . 49
6.3.10. getUsage . . . . . . . . . . . . . . . . . . . . . . 49
6.3.11. getMechs . . . . . . . . . . . . . . . . . . . . . . 50
6.3.12. add . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.3.13. equals . . . . . . . . . . . . . . . . . . . . . . . 51
6.4. public interface GSSContext . . . . . . . . . . . . . . 51
6.4.1. Example Code . . . . . . . . . . . . . . . . . . . . . 52
6.4.2. Static Constants . . . . . . . . . . . . . . . . . . . 54
6.4.3. initSecContext . . . . . . . . . . . . . . . . . . . . 54
6.4.3.1. Example Code . . . . . . . . . . . . . . . . . . . . 55
6.4.4. initSecContext . . . . . . . . . . . . . . . . . . . . 56
6.4.4.1. Example Code . . . . . . . . . . . . . . . . . . . . 56
6.4.5. acceptSecContext . . . . . . . . . . . . . . . . . . . 57
6.4.5.1. Example Code . . . . . . . . . . . . . . . . . . . . 58
6.4.6. acceptSecContext . . . . . . . . . . . . . . . . . . . 59
6.4.6.1. Example Code . . . . . . . . . . . . . . . . . . . . 59
6.4.7. isEstablished . . . . . . . . . . . . . . . . . . . . 60
6.4.8. dispose . . . . . . . . . . . . . . . . . . . . . . . 60
6.4.9. getWrapSizeLimit . . . . . . . . . . . . . . . . . . . 61
6.4.10. wrap . . . . . . . . . . . . . . . . . . . . . . . . 61
6.4.11. wrap . . . . . . . . . . . . . . . . . . . . . . . . 62
6.4.12. unwrap . . . . . . . . . . . . . . . . . . . . . . . 63
6.4.13. unwrap . . . . . . . . . . . . . . . . . . . . . . . 64
6.4.14. getMIC . . . . . . . . . . . . . . . . . . . . . . . 65
6.4.15. getMIC . . . . . . . . . . . . . . . . . . . . . . . 65
6.4.16. verifyMIC . . . . . . . . . . . . . . . . . . . . . . 66
6.4.17. verifyMIC . . . . . . . . . . . . . . . . . . . . . . 67
6.4.18. export . . . . . . . . . . . . . . . . . . . . . . . 68
6.4.19. requestMutualAuth . . . . . . . . . . . . . . . . . . 68
6.4.20. requestReplayDet . . . . . . . . . . . . . . . . . . 69
6.4.21. requestSequenceDet . . . . . . . . . . . . . . . . . 69
6.4.22. requestCredDeleg . . . . . . . . . . . . . . . . . . 69
6.4.23. requestAnonymity . . . . . . . . . . . . . . . . . . 69
6.4.24. requestConf . . . . . . . . . . . . . . . . . . . . . 70
6.4.25. requestInteg . . . . . . . . . . . . . . . . . . . . 70
6.4.26. requestLifetime . . . . . . . . . . . . . . . . . . . 70
6.4.27. setChannelBinding . . . . . . . . . . . . . . . . . . 71
6.4.28. getCredDelegState . . . . . . . . . . . . . . . . . . 71
6.4.29. getMutualAuthState . . . . . . . . . . . . . . . . . 71
6.4.30. getReplayDetState . . . . . . . . . . . . . . . . . . 71
6.4.31. getSequenceDetState . . . . . . . . . . . . . . . . . 71
6.4.32. getAnonymityState . . . . . . . . . . . . . . . . . . 72
6.4.33. isTransferable . . . . . . . . . . . . . . . . . . . 72
6.4.34. isProtReady . . . . . . . . . . . . . . . . . . . . . 72
6.4.35. getConfState . . . . . . . . . . . . . . . . . . . . 72
6.4.36. getIntegState . . . . . . . . . . . . . . . . . . . . 72
6.4.37. getLifetime . . . . . . . . . . . . . . . . . . . . . 73
6.4.38. getSrcName . . . . . . . . . . . . . . . . . . . . . 73
6.4.39. getTargName . . . . . . . . . . . . . . . . . . . . . 73
6.4.40. getMech . . . . . . . . . . . . . . . . . . . . . . . 73
6.4.41. getDelegCred . . . . . . . . . . . . . . . . . . . . 73
6.4.42. isInitiator . . . . . . . . . . . . . . . . . . . . . 73
6.5. public class MessageProp . . . . . . . . . . . . . . . . 74
6.5.1. Constructors . . . . . . . . . . . . . . . . . . . . . 74
6.5.2. getQOP . . . . . . . . . . . . . . . . . . . . . . . . 75
6.5.3. getPrivacy . . . . . . . . . . . . . . . . . . . . . . 75
6.5.4. getMinorStatus . . . . . . . . . . . . . . . . . . . . 75
6.5.5. getMinorString . . . . . . . . . . . . . . . . . . . . 75
6.5.6. setQOP . . . . . . . . . . . . . . . . . . . . . . . . 75
6.5.7. setPrivacy . . . . . . . . . . . . . . . . . . . . . . 75
6.5.8. isDuplicateToken . . . . . . . . . . . . . . . . . . . 76
6.5.9. isOldToken . . . . . . . . . . . . . . . . . . . . . . 76
6.5.10. isUnseqToken . . . . . . . . . . . . . . . . . . . . 76
6.5.11. isGapToken . . . . . . . . . . . . . . . . . . . . . 76
6.5.12. setSupplementaryStates . . . . . . . . . . . . . . . 76
6.6. public class ChannelBinding . . . . . . . . . . . . . . 77
6.6.1. Constructors . . . . . . . . . . . . . . . . . . . . . 77
6.6.2. getInitiatorAddress . . . . . . . . . . . . . . . . . 78
6.6.3. getAcceptorAddress . . . . . . . . . . . . . . . . . . 78
6.6.4. getApplicationData . . . . . . . . . . . . . . . . . . 78
6.6.5. equals . . . . . . . . . . . . . . . . . . . . . . . . 78
6.7. public class Oid . . . . . . . . . . . . . . . . . . . . 79
6.7.1. Constructors . . . . . . . . . . . . . . . . . . . . . 79
6.7.2. toString . . . . . . . . . . . . . . . . . . . . . . . 80
6.7.3. equals . . . . . . . . . . . . . . . . . . . . . . . . 80
6.7.4. getDER . . . . . . . . . . . . . . . . . . . . . . . . 80
6.7.5. containedIn . . . . . . . . . . . . . . . . . . . . . 80
6.8. public class GSSException extends Exception . . . . . . 80
6.8.1. Static Constants . . . . . . . . . . . . . . . . . . . 81
6.8.2. Constructors . . . . . . . . . . . . . . . . . . . . . 83
6.8.3. getMajor . . . . . . . . . . . . . . . . . . . . . . . 84
6.8.4. getMinor . . . . . . . . . . . . . . . . . . . . . . . 84
6.8.5. getMajorString . . . . . . . . . . . . . . . . . . . . 84
6.8.6. getMinorString . . . . . . . . . . . . . . . . . . . . 84
6.8.7. setMinor . . . . . . . . . . . . . . . . . . . . . . . 84
6.8.8. toString . . . . . . . . . . . . . . . . . . . . . . . 85
6.8.9. getMessage . . . . . . . . . . . . . . . . . . . . . . 85
7. Sample Applications . . . . . . . . . . . . . . . . . . . 85
7.1. Simple GSS Context Initiator . . . . . . . . . . . . . . 85
7.2. Simple GSS Context Acceptor . . . . . . . . . . . . . . 89
8. Security Considerations . . . . . . . . . . . . . . . . . 93
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 94
10. Bibliography . . . . . . . . . . . . . . . . . . . . . . 94
11. Authors" Addresses . . . . . . . . . . . . . . . . . . . 95
12. Full Copyright Statement. . . . . . . . . . . . . . . . . 96
1. Introduction
This document specifies Java language bindings for the Generic
Security Services Application Programming Interface Version 2 (GSS-
API). GSS-API Version 2 is described in a language independent
format in RFC2743 [GSSAPIv2-UPDATE]. The GSS-API allows a caller
application to authenticate a principal identity, to delegate rights
to a peer, and to apply security services such as confidentiality and
integrity on a per-message basis.
This document leverages the work performed by the WG in the area of
RFC2743 [GSSAPIv2-UPDATE] and the C-bindings RFC2744 [GSSAPI-C].
Whenever appropriate, text has been used from the C-bindings RFC2744
to explain generic concepts and provide direction to the
implementors.
The design goals of this API have been to satisfy all the
functionality defined in RFC2743 and to provide these services in an
object oriented method. The specification also aims to satisfy the
needs of both types of Java application developers, those who would
like access to a "system-wide" GSS-API implementation, as well as
those who would want to provide their own "custom" implementation.
A "system-wide" implementation is one that is available to all
applications in the form of a library package. It may be a standard
package in the Java runtime environment (JRE) being used or it may be
additionally installed and accessible to any application via the
CLASSPATH.
A "custom" implementation of the GSS-API, on the other hand, is one
that would, in most cases, be bundled with the application during
distribution. It is expected that such an implementation would be
meant to provide for some particular need of the application, such as
support for some specific mechanism.
The design of this API also aims to provide a flexible framework to
add and manage GSS-API mechanisms. GSS-API leverages the Java
Cryptography Architecture (JCA) provider model to support the
plugability of mechanisms. Mechanisms can be added on a "system-
wide" basis, where all users of the framework will have them
available. The specification also allows for the addition of
mechanisms per-instance of the GSS-API.
Lastly, this specification presents an API that will naturally fit
within the operation environment of the Java platform. Readers are
assumed to be familiar with both the GSS-API and the Java platform.
2. GSS-API Operational Paradigm
The Generic Security Service Application Programming Interface
Version 2 [GSSAPIv2-UPDATE] defines a generic security API to calling
applications. It allows a communicating application to authenticate
the user associated with another application, to delegate rights to
another application, and to apply security services such as
confidentiality and integrity on a per-message basis.
There are four stages to using GSS-API:
1) The application acquires a set of credentials with which it may
prove its identity to other processes. The application"s
credentials vouch for its global identity, which may or may not
be related to any local username under which it may be running.
2) A pair of communicating applications establish a joint security
context using their credentials. The security context
encapsulates shared state information, which is required in
order that per-message security services may be provided.
Examples of state information that might be shared between
applications as part of a security context are cryptographic
keys, and message sequence numbers. As part of the
establishment of a security context, the context initiator is
authenticated to the responder, and may require that the
responder is authenticated back to the initiator. The
initiator may optionally give the responder the right to
initiate further security contexts, acting as an agent or
delegate of the initiator. This transfer of rights is termed
"delegation", and is achieved by creating a set of credentials,
similar to those used by the initiating application, but which
may be used by the responder.
A GSSContext object is used to establish and maintain the
shared information that makes up the security context. Certain
GSSContext methods will generate a token, which applications
treat as cryptographically protected, opaque data. The caller
of such GSSContext method is responsible for transferring the
token to the peer application, encapsulated if necessary in an
application-to-application protocol. On receipt of such a
token, the peer application should pass it to a corresponding
GSSContext method which will decode the token and extract the
information, updating the security context state information
accordingly.
3) Per-message services are invoked on a GSSContext object to
apply either:
integrity and data origin authentication, or
confidentiality, integrity and data origin authentication
to application data, which are treated by GSS-API as arbitrary
octet-strings. An application transmitting a message that it
wishes to protect will call the appropriate GSSContext method
(getMIC or wrap) to apply protection, and send the resulting
token to the receiving application. The receiver will pass the
received token (and, in the case of data protected by getMIC,
the accompanying message-data) to the corresponding decoding
method of the GSSContext interface (verifyMIC or unwrap) to
remove the protection and validate the data.
4) At the completion of a communications session (which may extend
across several transport connections), each application uses a
GSSContext method to invalidate the security context and
release any system or cryptographic resources held. Multiple
contexts may also be used (either successively or
simultaneously) within a single communications association, at
the discretion of the applications.
3. Additional Controls
This section discusses the optional services that a context initiator
may request of the GSS-API before the context establishment. Each of
these services is requested by calling the appropriate mutator method
in the GSSContext object before the first call to init is performed.
Only the context initiator can request context flags.
The optional services defined are:
Delegation
The (usually temporary) transfer of rights from initiator to
acceptor, enabling the acceptor to authenticate itself as an
agent of the initiator.
Mutual Authentication
In addition to the initiator authenticating its identity to the
context acceptor, the context acceptor should also authenticate
itself to the initiator.
Replay Detection
In addition to providing message integrity services, GSSContext
per-message operations of getMIC and wrap should include
message numbering information to enable verifyMIC and unwrap
to detect if a message has been duplicated.
Out-of-Sequence Detection
In addition to providing message integrity services, GSSContext
per-message operations (getMIC and wrap) should include
message sequencing information to enable verifyMIC and unwrap
to detect if a message has been received out of sequence.
Anonymous Authentication
The establishment of the security context should not reveal the
initiator"s identity to the context acceptor.
Some mechanisms may not support all optional services, and some
mechanisms may only support some services in conjunction with others.
The GSSContext interface offers query methods to allow the
verification by the calling application of which services will be
available from the context when the establishment phase is complete.
In general, if the security mechanism is capable of providing a
requested service, it should do so even if additional services must
be enabled in order to provide the requested service. If the
mechanism is incapable of providing a requested service, it should
proceed without the service leaving the application to abort the
context establishment process if it considers the requested service
to be mandatory.
Some mechanisms may specify that support for some services is
optional, and that implementors of the mechanism need not provide it.
This is most commonly true of the confidentiality service, often
because of legal restrictions on the use of data-encryption, but may
apply to any of the services. Such mechanisms are required to send
at least one token from acceptor to initiator during context
establishment when the initiator indicates a desire to use such a
service, so that the initiating GSS-API can correctly indicate
whether the service is supported by the acceptor"s GSS-API.
3.1. Delegation
The GSS-API allows delegation to be controlled by the initiating
application via the requestCredDeleg method before the first call to
init has been issued. Some mechanisms do not support delegation, and
for such mechanisms attempts by an application to enable delegation
are ignored.
The acceptor of a security context, for which the initiator enabled
delegation, can check if delegation was enabled by using the
getCredDelegState method of the GSSContext interface. In cases when
it is, the delegated credential object can be oBTained by calling the
getDelegCred method. The obtained GSSCredential object may then be
used to initiate subsequent GSS-API security contexts as an agent or
delegate of the initiator. If the original initiator"s identity is
"A" and the delegate"s identity is "B", then, depending on the
underlying mechanism, the identity embodied by the delegated
credential may be either "A" or "B acting for A".
For many mechanisms that support delegation, a simple boolean does
not provide enough control. Examples of additional ASPects of
delegation control that a mechanism might provide to an application
are duration of delegation, network addresses from which delegation
is valid, and constraints on the tasks that may be performed by a
delegate. Such controls are presently outside the scope of the GSS-
API. GSS-API implementations supporting mechanisms offering
additional controls should provide extension routines that allow
these controls to be exercised (perhaps by modifying the initiator"s
GSS-API credential object prior to its use in establishing a
context). However, the simple delegation control provided by GSS-API
should always be able to over-ride other mechanism-specific
delegation controls. If the application instructs the GSSContext
object that delegation is not desired, then the implementation must
not permit delegation to occur. This is an exception to the general
rule that a mechanism may enable services even if they are not
requested - delegation may only be provided at the explicit request
of the application.
3.2. Mutual Authentication
Usually, a context acceptor will require that a context initiator
authenticate itself so that the acceptor may make an access-control
decision prior to performing a service for the initiator. In some
cases, the initiator may also request that the acceptor authenticate
itself. GSS-API allows the initiating application to request this
mutual authentication service by calling the requestMutualAuth method
of the GSSContext interface with a "true" parameter before making the
first call to init. The initiating application is informed as to
whether or not the context acceptor has authenticated itself. Note
that some mechanisms may not support mutual authentication, and other
mechanisms may always perform mutual authentication, whether or not
the initiating application requests it. In particular, mutual
authentication may be required by some mechanisms in order to support
replay or out-of-sequence message detection, and for such mechanisms
a request for either of these services will automatically enable
mutual authentication.
3.3. Replay and Out-of-Sequence Detection
The GSS-API may provide detection of mis-ordered messages once a
security context has been established. Protection may be applied to
messages by either application, by calling either getMIC or wrap
methods of the GSSContext interface, and verified by the peer
application by calling verifyMIC or unwrap for the peer"s GSSContext
object.
The getMIC method calculates a cryptographic checksum of an
application message, and returns that checksum in a token. The
application should pass both the token and the message to the peer
application, which presents them to the verifyMIC method of the
peer"s GSSContext object.
The wrap method calculates a cryptographic checksum of an application
message, and places both the checksum and the message inside a single
token. The application should pass the token to the peer
application, which presents it to the unwrap method of the peer"s
GSSContext object to extract the message and verify the checksum.
Either pair of routines may be capable of detecting out-of-sequence
message delivery, or duplication of messages. Details of such mis-
ordered messages are indicated through supplementary query methods of
the MessageProp object that is filled in by each of these routines.
A mechanism need not maintain a list of all tokens that have been
processed in order to support these status codes. A typical
mechanism might retain information about only the most recent "N"
tokens processed, allowing it to distinguish duplicates and missing
tokens within the most recent "N" messages; the receipt of a token
older than the most recent "N" would result in the isOldToken method
of the instance of MessageProp to return "true".
3.4. Anonymous Authentication
In certain situations, an application may wish to initiate the
authentication process to authenticate a peer, without revealing its
own identity. As an example, consider an application providing
access to a database containing medical information, and offering
unrestricted access to the service. A client of such a service might
wish to authenticate the service (in order to establish trust in any
information retrieved from it), but might not wish the service to be
able to obtain the client"s identity (perhaps due to privacy concerns
about the specific inquiries, or perhaps simply to avoid being placed
on mailing-lists).
In normal use of the GSS-API, the initiator"s identity is made
available to the acceptor as a result of the context establishment
process. However, context initiators may request that their identity
not be revealed to the context acceptor. Many mechanisms do not
support anonymous authentication, and for such mechanisms the request
will not be honored. An authentication token will still be
generated, but the application is always informed if a requested
service is unavailable, and has the option to abort context
establishment if anonymity is valued above the other security
services that would require a context to be established.
In addition to informing the application that a context is
established anonymously (via the isAnonymous method of the GSSContext
class), the getSrcName method of the acceptor"s GSSContext object
will, for such contexts, return a reserved internal-form name,
defined by the implementation.
The toString method for a GSSName object representing an anonymous
entity will return a printable name. The returned value will be
syntactically distinguishable from any valid principal name supported
by the implementation. The associated name-type object identifier
will be an oid representing the value of NT_ANONYMOUS. This name-
type oid will be defined as a public, static Oid object of the
GSSName class. The printable form of an anonymous name should be
chosen such that it implies anonymity, since this name may appear in,
for example, audit logs. For example, the string "<anonymous>" might
be a good choice, if no valid printable names supported by the
implementation can begin with "<" and end with ">".
When using the equal method of the GSSName interface, and one of the
operands is a GSSName instance representing an anonymous entity, the
method must return "false".
3.5. Confidentiality
If a GSSContext supports the confidentiality service, wrap method may
be used to encrypt application messages. Messages are selectively
encrypted, under the control of the setPrivacy method of the
MessageProp object used in the wrap method.
3.6. Inter-process Context Transfer
GSS-API V2 provides functionality which allows a security context to
be transferred between processes on a single machine. These are
implemented using the export method of GSSContext and a byte array
constructor of the same class. The most common use for such a
feature is a client-server design where the server is implemented as
a single process that accepts incoming security contexts, which then
launches child processes to deal with the data on these contexts. In
such a design, the child processes must have access to the security
context object created within the parent so that they can use per-
message protection services and delete the security context when the
communication session ends.
Since the security context data structure is expected to contain
sequencing information, it is impractical in general to share a
context between processes. Thus GSSContext interface provides an
export method that the process, which currently owns the context, can
call to declare that it has no intention to use the context
subsequently, and to create an inter-process token containing
information needed by the adopting process to successfully re-create
the context. After successful completion of export, the original
security context is made inaccessible to the calling process by GSS-
API and any further usage of this object will result in failures.
The originating process transfers the inter-process token to the
adopting process, which creates a new GSSContext object using the
byte array constructor. The properties of the context are equivalent
to that of the original context.
The inter-process token may contain sensitive data from the original
security context (including cryptographic keys). Applications using
inter-process tokens to transfer security contexts must take
appropriate steps to protect these tokens in transit.
Implementations are not required to support the inter-process
transfer of security contexts. Calling the isTransferable method of
the GSSContext interface will indicate if the context object is
transferable.
3.7. The Use of Incomplete Contexts
Some mechanisms may allow the per-message services to be used before
the context establishment process is complete. For example, a
mechanism may include sufficient information in its initial context-
level tokens for the context acceptor to immediately decode messages
protected with wrap or getMIC. For such a mechanism, the initiating
application need not wait until subsequent context-level tokens have
been sent and received before invoking the per-message protection
services.
An application can invoke the isProtReady method of the GSSContext
class to determine if the per-message services are available in
advance of complete context establishment. Applications wishing to
use per-message protection services on partially-established contexts
should query this method before attempting to invoke wrap or getMIC.
4. Calling Conventions
Java provides the implementors with not just a syntax for the
language, but also an operational environment. For example, memory
is automatically managed and does not require application
intervention. These language features have allowed for a simpler API
and have led to the elimination of certain GSS-API functions.
Moreover, the JCA defines a provider model which allows for
implementation independent access to security services. Using this
model, applications can seamlessly switch between different
implementations and dynamically add new services. The GSS-API
specification leverages these concepts by the usage of providers for
the mechanism implementations.
4.1. Package Name
The classes and interfaces defined in this document reside in the
package called "org.ietf.jgss". Applications that wish to make use
of this API should import this package name as shown in section 7.
4.2. Provider Framework
The Java security API"s use a provider architecture that allows
applications to be implementation independent and security API
implementations to be modular and extensible. The
java.security.Provider class is an abstract class that a vendor
extends. This class maps various properties that represent different
security services that are available to the names of the actual
vendor classes that implement those services. When requesting a
service, an application simply specifies the desired provider and the
API delegates the request to service classes available from that
provider.
Using the Java security provider model insulates applications from
implementation details of the services they wish to use.
Applications can switch between providers easily and new providers
can be added as needed, even at runtime.
The GSS-API may use providers to find components for specific
underlying security mechanisms. For instance, a particular provider
might contain components that will allow the GSS-API to support the
Kerberos v5 mechanism and another might contain components to support
the SPKM mechanism. By delegating mechanism specific functionality
to the components obtained from providers the GSS-API can be extended
to support an arbitrary list of mechanism.
How the GSS-API locates and queries these providers is beyond the
scope of this document and is being deferred to a Service Provider
Interface (SPI) specification. The availability of such a SPI
specification is not mandatory for the adoption of this API
specification nor is it mandatory to use providers in the
implementation of a GSS-API framework. However, by using the provider
framework together with an SPI specification one can create an
extensible and implementation independent GSS-API framework.
4.3. Integer types
All numeric values are declared as "int" primitive Java type. The
Java specification guarantees that this will be a 32 bit two"s
complement signed number.
Throughout this API, the "boolean" primitive Java type is used
wherever a boolean value is required or returned.
4.4. Opaque Data types
Java byte arrays are used to represent opaque data types which are
consumed and produced by the GSS-API in the forms of tokens. Java
arrays contain a length field which enables the users to easily
determine their size. The language has automatic garbage collection
which alleviates the need by developers to release memory and
simplifies buffer ownership issues.
4.5. Strings
The String object will be used to represent all textual data. The
Java String object, transparently treats all characters as two-byte
Unicode characters which allows support for many locals. All
routines returning or accepting textual data will use the String
object.
4.6. Object Identifiers
An Oid object will be used to represent Universal Object Identifiers
(Oids). Oids are ISO-defined, hierarchically globally-interpretable
identifiers used within the GSS-API framework to identify security
mechanisms and name formats. The Oid object can be created from a
string representation of its dot notation (e.g. "1.3.6.1.5.6.2") as
well as from its ASN.1 DER encoding. Methods are also provided to
test equality and provide the DER representation for the object.
An important feature of the Oid class is that its instances are
immutable - i.e. there are no methods defined that allow one to
change the contents of an Oid. This property allows one to treat
these objects as "statics" without the need to perform copies.
Certain routines allow the usage of a default oid. A "null" value
can be used in those cases.
4.7. Object Identifier Sets
The Java bindings represents object identifiers sets as arrays of Oid
objects. All Java arrays contain a length field which allows for
easy manipulation and reference.
In order to support the full functionality of RFC2743, the Oid class
includes a method which checks for existence of an Oid object within
a specified array. This is equivalent in functionality to
gss_test_oid_set_member. The use of Java arrays and Java"s automatic
garbage collection has eliminated the need for the following
routines: gss_create_empty_oid_set, gss_release_oid_set, and
gss_add_oid_set_member. Java GSS-API implementations will not
contain them. Java"s automatic garbage collection and the immutable
property of the Oid object eliminates the complicated memory
management issues of the C counterpart.
When ever a default value for an Object Identifier Set is required, a
"null" value can be used. Please consult the detailed method
description for details.
4.8. Credentials
GSS-API credentials are represented by the GSSCredential interface.
The interface contains several constructs to allow for the creation
of most common credential objects for the initiator and the acceptor.
Comparisons are performed using the interface"s "equals" method. The
following general description of GSS-API credentials is included from
the C-bindings specification:
GSS-API credentials can contain mechanism-specific principal
authentication data for multiple mechanisms. A GSS-API credential is
composed of a set of credential-elements, each of which is applicable
to a single mechanism. A credential may contain at most one
credential-element for each supported mechanism. A credential-
element identifies the data needed by a single mechanism to
authenticate a single principal, and conceptually contains two
credential-references that describe the actual mechanism-specific
authentication data, one to be used by GSS-API for initiating
contexts, and one to be used for accepting contexts. For mechanisms
that do not distinguish between acceptor and initiator credentials,
both references would point to the same underlying mechanism-specific
authentication data.
Credentials describe a set of mechanism-specific principals, and give
their holder the ability to act as any of those principals. All
principal identities asserted by a single GSS-API credential should
belong to the same entity, although enforcement of this property is
an implementation-specific matter. A single GSSCredential object
represents all the credential elements that have been acquired.
The creation"s of an GSSContext object allows the value of "null" to
be specified as the GSSCredential input parameter. This will
indicate 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:
For the initiator side of the context:
1) If there is only a single principal capable of initiating
security contexts for the chosen mechanism that the application
is authorized to act on behalf of, then that principal shall be
used, otherwise
2) If the platform maintains a concept of a default network-
identity for the chosen mechanism, 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
3) If the platform maintains a concept of a default local
identity, and provides a means to map local identities into
network-identities for the chosen mechanism, 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 using the chosen mechanism, then
the principal corresponding to that identity shall be used,
otherwise
4) A user-configurable default identity should be used.
and for the acceptor side of the context
1) If there is only a single authorized principal identity capable
of accepting security contexts for the chosen mechanism, then
that principal shall be used, otherwise
2) If the mechanism can determine the identity of the target
principal by examining the context-establishment token
processed during the accept method, and if the accepting
application is authorized to act as that principal for the
purpose of accepting security contexts using the chosen
mechanism, then that principal identity shall be used,
otherwise
3) If the mechanism supports context acceptance by any principal,
and if mutual authentication was not requested, any principal
that the application is authorized to accept security contexts
under using the chosen mechanism may be used, otherwise
4) 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
whenever possible. Applications requesting default behavior are
likely to be more portable across mechanisms and implementations than
ones that instantiate an GSSCredential object representing a specific
identity.
4.9. Contexts
The GSSContext interface is used to represent one end of a GSS-API
security context, storing state information appropriate to that end
of the peer communication, including cryptographic state information.
The instantiation of the context object is done differently by the
initiator and the acceptor. After the context has been instantiated,
the initiator may choose to set various context options which will
determine the characteristics of the desired security context. When
all the application desired characteristics have been set, the
initiator will call the initSecContext method which will produce a
token for consumption by the peer"s acceptSecContext method. It is
the responsibility of the application to deliver the authentication
token(s) between the peer applications for processing. Upon
completion of the context establishment phase, context attributes can
be retrieved, by both the initiator and acceptor, using the accessor
methods. These will reflect the actual attributes of the established
context. At this point the context can be used by the application to
apply cryptographic services to its data.
4.10. Authentication tokens
A token is a caller-opaque type that GSS-API uses to maintain
synchronization between each end of the GSS-API security context.
The token is a cryptographically protected octet-string, generated by
the underlying mechanism at one end of a GSS-API security context for
use by the peer mechanism at the other end. Encapsulation (if
required) within the application protocol and transfer of the token
are the responsibility of the peer applications.
Java GSS-API uses byte arrays to represent authentication tokens.
Overloaded methods exist which allow the caller to supply input and
output streams which will be used for the reading and writing of the
token data.
4.11. Interprocess tokens
Certain GSS-API routines are intended to transfer data between
processes in multi-process programs. These routines use a caller-
opaque octet-string, generated by the GSS-API in one process for use
by the GSS-API in another process. The calling application is
responsible for transferring such tokens between processes. Note
that, while GSS-API implementors are encouraged to avoid placing
sensitive information within interprocess tokens, or to
cryptographically protect them, many implementations will be unable
to avoid placing key material or other sensitive data within them.
It is the application"s responsibility to ensure that interprocess
tokens are protected in transit, and transferred only to processes
that are trustworthy. An interprocess token is represented using a
byte array emitted from the export method of the GSSContext
interface. The receiver of the interprocess token would initialize
an GSSContext object with this token to create a new context. Once a
context has been exported, the GSSContext object is invalidated and
is no longer available.
4.12. Error Reporting
RFC2743 defined the usage of major and minor status values for
signaling of GSS-API errors. The major code, also called GSS status
code, is used to signal errors at the GSS-API level independent of
the underlying mechanism(s). The minor status value or Mechanism
status code, is a mechanism defined error value indicating a
mechanism specific error code.
Java GSS-API uses exceptions implemented by the GSSException class to
signal both minor and major error values. Both mechanism specific
errors and GSS-API level errors are signaled through instances of
this class. The usage of exceptions replaces the need for major and
minor codes to be used within the API calls. GSSException class also
contains methods to obtain textual representations for both the major
and minor values, which is equivalent to the functionality of
gss_display_status.
4.12.1. GSS status codes
GSS status codes indicate errors that are independent of the
underlying mechanism(s) used to provide the security service. The
errors that can be indicated via a GSS status code are generic API
routine errors (errors that are defined in the GSS-API
specification). These bindings take advantage of the Java exceptions
mechanism, thus eliminating the need for calling errors.
A GSS status code indicates a single fatal generic API error from the
routine that has thrown the GSSException. Using exceptions announces
that a fatal error has occurred during the execution of the method.
The GSS-API operational model also allows for the signaling of
supplementary status information from the per-message calls. These
need to be handled as return values since using exceptions is not
appropriate for informatory or warning-like information. The methods
that are capable of producing supplementary information are the two
per-message methods GSSContext.verifyMIC() and GSSContext.unwrap().
These methods fill the supplementary status codes in the MessageProp
object that was passed in.
GSSException object, along with providing the functionality for
setting of the various error codes and translating them into textual
representation, also contains the definitions of all the numeric
error values. The following table lists the definitions of error
codes:
Table: GSS Status Codes
Name Value Meaning
BAD_MECH 1 An unsupported mechanism
was requested.
BAD_NAME 2 An invalid name was supplied.
BAD_NAMETYPE 3 A supplied name was of an
unsupported type.
BAD_BINDINGS 4 Incorrect channel bindings were
supplied.
BAD_STATUS 5 An invalid status code was
supplied.
BAD_MIC 6 A token had an invalid MIC.
NO_CRED 7 No credentials were supplied, or
the credentials were unavailable
or inaccessible.
NO_CONTEXT 8 Invalid context has been
supplied.
DEFECTIVE_TOKEN 9 A supplied token was invalid.
DEFECTIVE_CREDENTIAL 10 A supplied credential was
invalid.
CREDENTIALS_EXPIRED 11 The referenced credentials
have expired.
CONTEXT_EXPIRED 12 The context has expired.
FAILURE 13 Miscellaneous failure,
unspecified at the GSS-API level.
BAD_QOP 14 The quality-of-protection
requested could not be provided.
UNAUTHORIZED 15 The operation is forbidden by
local security policy.
UNAVAILABLE 16 The operation or option is
unavailable.
DUPLICATE_ELEMENT 17 The requested credential
element already exists.
NAME_NOT_MN 18 The provided name was not a
mechanism name.
OLD_TOKEN 19 The token"s validity period has
expired.
DUPLICATE_TOKEN 20 The token was a duplicate of an
earlier version.
The GSS major status code of FAILURE is used to indicate that the
underlying mechanism detected an error for which no specific GSS
status code is defined. The mechanism-specific status code can
provide more details about the error.
The different major status codes that can be contained in the
GSSException object thrown by the methods in this specification are
the same as the major status codes returned by the corresponding
calls in RFC2743 [GSSAPIv2-UPDATE].
4.12.2. Mechanism-specific status codes
Mechanism-specific status codes are communicated in two ways, they
are part of any GSSException thrown from the mechanism specific layer
to signal a fatal error, or they are part of the MessageProp object
that the per-message calls use to signal non-fatal errors.
A default value of 0 in either the GSSException object or the
MessageProp object will be used to represent the absence of any
mechanism specific status code.
4.12.3. Supplementary status codes
Supplementary status codes are confined to the per-message methods of
the GSSContext interface. Because of the informative nature of these
errors it is not appropriate to use exceptions to signal them.
Instead, the per-message operations of the GSSContext interface
return these values in a MessageProp object.
The MessageProp class defines query methods which return boolean
values indicating the following supplementary states:
Table: Supplementary Status Methods
Method Name Meaning when "true" is returned
isDuplicateToken The token was a duplicate of an
earlier token.
isOldToken The token"s validity period has
expired.
isUnseqToken A later token has already been
processed.
isGapToken An expected per-message token was
not received.
"true" return value for any of the above methods indicates that the
token exhibited the specified property. The application must
determine the appropriate course of action for these supplementary
values. They are not treated as errors by the GSS-API.
4.13. Names
A name is used to identify a person or entity. GSS-API authenticates
the relationship between a name and the entity claiming the name.
Since different authentication mechanisms may employ different
namespaces for identifying their principals, GSS-API"s naming support
is necessarily complex in multi-mechanism environments (or even in
some single-mechanism environments where the underlying mechanism
supports multiple namespaces).
Two distinct conceptual representations are defined for names:
1) A GSS-API form represented by implementations of the GSSName
interface: A single GSSName object may contain multiple names from
different namespaces, but all names should refer to the same
entity. An example of such an internal name would be the name
returned from a call to the getName method of the GSSCredential
interface, when applied to a credential containing credential
elements for multiple authentication mechanisms employing
different namespaces. This GSSName object will contain a distinct
name for the entity for each authentication mechanism.
For GSS-API implementations supporting multiple namespaces,
GSSName implementations must contain sufficient information to
determine the namespace to which each primitive name belongs.
2) Mechanism-specific contiguous byte array and string forms:
Different GSSName initialization methods are provided to handle
both byte array and string formats and to accommodate various
calling applications and name types. These formats are capable of
containing only a single name (from a single namespace).
Contiguous string names are always accompanied by an object
identifier specifying the namespace to which the name belongs, and
their format is dependent on the authentication mechanism that
employs that name. The string name forms are assumed to be
printable, and may therefore be used by GSS-API applications for
communication with their users. The byte array name formats are
assumed to be in non-printable formats (e.g. the byte array
returned from the export method of the GSSName interface).
A GSSName object can be converted to a contiguous representation by
using the toString method. This will guarantee that the name will be
converted to a printable format. Different initialization methods in
the GSSName interface are defined allowing support for multiple
syntaxes for each supported namespace, and allowing users the freedom
to choose a preferred name representation. The toString method
should use an implementation-chosen printable syntax for each
supported name-type. To obtain the printable name type,
getStringNameType method can be used.
There is no guarantee that calling the toString method on the GSSName
interface will produce the same string form as the original imported
string name. Furthermore, it is possible that the name was not even
constructed from a string representation. The same applies to name-
space identifiers which may not necessarily survive unchanged after a
journey through the internal name-form. An example of this might be
a mechanism that authenticates X.500 names, but provides an
algorithmic mapping of Internet DNS names into X.500. That
mechanism"s implementation of GSSName might, when presented with a
DNS name, generate an internal name that contained both the original
DNS name and the equivalent X.500 name. Alternatively, it might only
store the X.500 name. In the latter case, the toString method of
GSSName would most likely generate a printable X.500 name, rather
than the original DNS name.
The context acceptor can obtain a GSSName object representing the
entity performing the context initiation (through the usage of
getSrcName method). Since this name has been authenticated by a
single mechanism, it contains only a single name (even if the
internal name presented by the context initiator to the GSSContext
object had multiple components). Such names are termed internal
mechanism names, or "MN"s and the names emitted by GSSContext
interface in the getSrcName and getTargName are always of this type.
Since some applications may require MNs without wanting to incur the
overhead of an authentication operation, creation methods are
provided that take not only the name buffer and name type, but also
the mechanism oid for which this name should be created. When
dealing with an existing GSSName object, the canonicalize method may
be invoked to convert a general internal name into an MN.
GSSName objects can be compared using their equal method, which
returns "true" if the two names being compared refer to the same
entity. This is the preferred way to perform name comparisons
instead of using the printable names that a given GSS-API
implementation may support. Since GSS-API assumes that all primitive
names contained within a given internal name refer to the same
entity, equal can return "true" if the two names have at least one
primitive name in common. If the implementation embodies knowledge
of equivalence relationships between names taken from different
namespaces, this knowledge may also allow successful comparisons of
internal names containing no overlapping primitive elements.
When used in large access control lists, the overhead of creating an
GSSName object on each name and invoking the equal method on each
name from the ACL may be prohibitive. As an alternative way of
supporting this case, GSS-API defines a special form of the
contiguous byte array name which may be compared directly (byte by
byte). Contiguous names suitable for comparison are generated by the
export method. Exported names may be re-imported by using the byte
array constructor and specifying the NT_EXPORT_NAME as the name type
object identifier. The resulting GSSName name will also be a MN.
The GSSName interface defines public static Oid objects representing
the standard name types. Structurally, an exported name object
consists of a header containing an OID identifying the mechanism that
authenticated the name, and a trailer containing the name itself,
where the syntax of the trailer is defined by the individual
mechanism specification. Detailed description of the format is
specified in the language-independent GSS-API specification
[GSSAPIv2-UPDATE].
Note that the results obtained by using the equals method will in
general be different from those obtained by invoking canonicalize and
export, and then comparing the byte array output. The first series
of operation determines whether two (unauthenticated) names identify
the same principal; the second whether a particular mechanism would
authenticate them as the same principal. These two operations will
in general give the same results only for MNs.
It is important to note that the above are guidelines as how GSSName
implementations should behave, and are not intended to be specific
requirements of how names objects must be implemented. The mechanism
designers are free to decide on the details of their implementations
of the GSSName interface as long as the behavior satisfies the above
guidelines.
4.14. Channel Bindings
GSS-API supports the use of user-specified tags to identify a given
context to the peer application. These tags are intended to be used
to identify the particular communications channel that carries the
context. Channel bindings are communicated to the GSS-API using the
ChannelBinding object. The application may use byte arrays to
specify the application data to be used in the channel binding as
well as using instances of the InetAddress. The InetAddress for the
initiator and/or acceptor can be used within an instance of a
ChannelBinding. ChannelBinding can be set for the GSSContext object
using the setChannelBinding method before the first call to init or
accept has been performed. Unless the setChannelBinding method has
been used to set the ChannelBinding for a GSSContext object, "null"
ChannelBinding will be assumed. InetAddress is currently the only
address type defined within the Java platform and as such, it is the
only one supported within the ChannelBinding class. Applications
that use other types of addresses can include them as part of the
application specific data.
Conceptually, the GSS-API concatenates the initiator and acceptor
address information, and the application supplied byte array to form
an octet string. The mechanism calculates a MIC over this octet
string and binds the MIC to the context establishment token emitted
by init method of the GSSContext interface. The same bindings are
set by the context acceptor for its GSSContext object and during
processing of the accept method a MIC is calculated in the same way.
The calculated MIC is compared with that found in the token, and if
the MICs differ, accept will throw a GSSException with the major
code set to BAD_BINDINGS, and the context will not be established.
Some mechanisms may include the actual channel binding data in the
token (rather than just a MIC); applications should therefore not use
confidential data as channel-binding components.
Individual mechanisms may impose additional constraints on addresses
that may appear in channel bindings. For example, a mechanism may
verify that the initiator address field of the channel binding
contains the correct network address of the host system. Portable
applications should therefore ensure that they either provide correct
information for the address fields, or omit setting of the addressing
information.
4.15. Stream Objects
The context object provides overloaded methods which use input and
output streams as the means to convey authentication and per-message
GSS-API tokens. It is important to note that the streams are
expected to contain the usual GSS-API tokens which would otherwise be
handled through the usage of byte arrays. The tokens are expected to
have a definite start and an end. The callers are responsible for
ensuring that the supplied streams will not block, or expect to block
until a full token is processed by the GSS-API