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NETWORK WORKING GROUP M. Short
Internet-Draft L. Zhu
Updates: 4178 (if approved) K. Damour
Intended status: Standards Track D. McPherson
Expires: July 7, 2011 Microsoft Corporation
January 3, 2011
SPNEGO Extended Negotiation (NEGOEX) Security Mechanism
draft-zhu-negoex-04
Abstract
This document defines the SPNEGO Extended Negotiation (NEGOEX)
Security Mechanism. NEGOEX enhances the capabilities of SPNEGO by
providing a security mechanism which can be negotiated by the SPNEGO
protocol as defined in RFC4178.
The NEGOEX protocol itself is a security mechanism negotiated by
SPNEGO. When the NEGOEX security mechanism is selected by SPNEGO,
NEGOEX provides a method allowing selection of a common
authentication protocol based on factors beyond just the fact that
both client and server support a given security mechanism. NEGOEX
OPTIONALLY adds a pair of meta-data messages for each negotiated
security mechanism. The meta-data exchange allows security
mechanisms to exchange auxiliary information such as trust
configurations, thus NEGOEX provides more flexibility than just
exchanging security mechanism OIDs in SPNEGO.
NEGOEX preserves the optimistic token semantics of SPNEGO and applies
that recursively. Consequently a context establishment mechanism
token can be included in the initial NEGOEX message, and NEGOEX does
not require an extra round-trip when the initiator's optimistic token
is accepted by the target.
Similar to SPNEGO, NEGOEX defines a few new GSS-API extensions that a
security mechanism MUST support in order to be negotiated by NEGOEX.
This document defines these GSS-API extensions.
Unlike SPNEGO however, NEGOEX defines its own way for signing the
protocol messages in order to protect the protocol negotiation. The
NEGOEX message signing or verification can occur before the security
context for the negotiated real security mechanism is fully
established.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 7, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Short, et al. Expires July 7, 2011 [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 6
3. Presentation Language and Primitive Data Types . . . . . . . . 7
3.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . . 7
3.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Constants . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Enum Types . . . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Typedef Declarations . . . . . . . . . . . . . . . . . . . 8
3.7. Array Types . . . . . . . . . . . . . . . . . . . . . . . 8
3.8. Constructed Types . . . . . . . . . . . . . . . . . . . . 8
4. Vector Types . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. NEGOEX Messages . . . . . . . . . . . . . . . . . . . . . . . 11
6. Cryptographic Computations . . . . . . . . . . . . . . . . . . 12
7. The NEGOEX Protocol . . . . . . . . . . . . . . . . . . . . . 12
7.1. High-level NEGOEX Message Flow . . . . . . . . . . . . . . 12
7.2. NEGOEX Supported Security Mechanisms . . . . . . . . . . . 13
7.3. ConversationID . . . . . . . . . . . . . . . . . . . . . . 13
7.4. Generation of the Initiator Initial Token . . . . . . . . 13
7.5. Receipt of the Initial Initiator Token and Generation
of the Initial Acceptor Response . . . . . . . . . . . . . 15
7.6. Receipt of the Acceptor Initial Response and
Completion of Authentication after the Negotiation
Phrase . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.7. Finalizing Negotiation . . . . . . . . . . . . . . . . . . 16
8. Supporting GSS-API Extensions . . . . . . . . . . . . . . . . 17
8.1. GSS_Query_meta_data . . . . . . . . . . . . . . . . . . . 17
8.2. GSS_Exchange_meta_data . . . . . . . . . . . . . . . . . . 18
8.3. GSS_Query_mechanism_info . . . . . . . . . . . . . . . . . 19
8.4. GSS_Inquire_context . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12. Normative References . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Protocol Data Structures and Constant Values . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
If more than one GSS-API mechanism is shared between the initator and
the acceptor, the Simple and Protected (GSS-API) Negotiation
Mechanism (SPNEGO) as defined in [RFC4178] can be deployed to choose
a mutually preferred one. This pseudo mechanism does well in the
most basic scenarios but suffers from a couple of drawbacks, notably:
o Since the SPNEGO negotiation is based on purely on exchanging
security mechanism OIDs, security mechanisms can be selected which
cannot successfully authenticate the initator. Just because an
initator and acceptor support the same security mechanism does not
mean that they have a mutually trusted authentication authority.
In such cases, the authentication will fail with the preferred
security mechanism, but might succeed with another common
mechanism.
o Secondly, the SPNEGO negotiation model is inadequate when the
choice cannot be made by the acceptor in the initial response. In
SPNEGO, the negotiation information is sent one-way from the
initiator for the acceptor to make a choice, and the acceptor must
choose one when it makes the initial response. This negotiation
model is counter intuitive. The selection of a security mechanism
is typically the result of selecting one type of credentials from
the available set, and the initiator typically does not wish to
reveal credentials information often associated with user
identities. In practice, in order to operate in this model, the
Kerberos GSS-API mechanism [RFC4121] must acquire the context
establishment token in the initial call to GSS_Init_sec_context().
If the initiator fails to acquire the initial Kerberos GSS-API
context token, it must not offer Kerberos; otherwise the SPNEGO
context negotiation will fail without being able to select the
next available mechanism that could work. Obtaining the initial
Kerberos GSS-API context token may require multiple round-trips of
network calls and the cost of the operation can be substantial.
It is suboptimal when multiple GSS-API mechanisms have to add the
extra cost that would not exist if the negotiated security
mechanism were selected based on configuration.
The SPNEGO Extended Negotiation (NEGOEX) Security Mechanism is
designed to address these concerns. NEGOEX is a security mechanism
that is negotiated by SPNEGO, and when negotiated, it can recursively
negotiate other security mechanisms.
Any security mechanism negotiated by NEGOEX MUST support integrity
protection and addition GSS-API interfaces specified in Section 8.
The basic form of NEGOEX works as follows:
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1. The initiator proposes a list of mechanisms in decreasing
preference order. For each of these mechanism, NEGOEX OPTIONALLY
includes a mechanism specific meta-data token. GSS-API
extensions are defined later in this document for NEGOEX to query
the meta-data token for inclusion in the NEGOEX message.
2. The acceptor then passes the meta-data token from the initiator
to the intended security mechanism. A meta-data token for a
security mechanism not supported on the acceptor side is ignored.
New GSS-API extensions are defined later in this document for a
security mechanism to consume the meta-data token. When
processing the received meta-data tokens, a security mechanism
that reports a failure is removed from the set of mutually
supported mechanisms. The acceptor then responds with the list
of mutually supported mechanisms in decreasing preference order.
For each of these mechanism, NEGOEX again OPTIONALLY supplies a
mechanism specific meta-data token in the response which it
obtains from each remaining supported mechanism via the new GSS-
API extensions described in the initial step.
3. The initiator then passes the meta-data tokens to the intended
security mechanisms by invoking the new GSS-API extensions. When
processing the received meta-data token, a security mechanism
that reports a failure is removed from the set of mutually
supported mechanisms for this negotiation context. The initiator
then selects one from the set of mutually-supported mechanisms.
If more than one security mechanism is available, unless
otherwise specified, the highest one in the acceptor's preference
order SHOULD be selected. Later when the common security
mechanism is identified, the security mechanism may also
negotiate mechanism-specific options during its context
establishments. This will be inside the mechanism tokens, and
invisible to the NEGOEX protocol during step 5.
4. The selected security mechanism provides keying materials to
NEGOEX via new GSS-API extensions which defined later in this
document. NEGOEX signs and verifies the negotiation NEGOEX
messages to protect the negotiation.
5. The initiator and the acceptor proceed to exchange tokens until
the GSS-API context for selected security mechanism is
established. Once the security context is established, the per-
message tokens are generated and verified in accordance with the
selected security mechanism.
NEGOEX does not work outside of SPNEGO. When negotiated by SPNEGO,
NEGOEX uses the concepts developed in the GSS-API specification
[RFC2743]. The negotiation data is encapsulated in context-level
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tokens. Therefore, callers of the GSS-API do not need to be aware of
the existence of the negotiation tokens but only of the SPNEGO
pseudo-security mechanism.
In its basic form NEGOEX requires at least one extra round-trip.
Network connection setup is a critical performance characteristic of
any network infrastructure and extra round trips over WAN links,
packet radio networks, etc. really make a difference. In order to
avoid such an extra round trip the initial security token of the
preferred mechanism for the initiator may be embedded in the initial
NEGOEX token. The optimistic mechanism token may be accompanied by
the meta-data tokens and the optimistic mechanism token MUST be that
of the first mechanism in the list of the mechanisms proposed by the
initiator. The NEGOEX MESSAGE_TYPE_INITIATOR_NEGO message that
contains signatures for protecting the NEGOEX negotiation may also
accompany the optimistic mechanism token. If the target preferred
mechanism matches the initiator's preferred mechanism, and when the
NEGOEX negotiation protection messages are included along with the
mechanism token, no additional round trips are incurred by using the
NEGOEX protocol with SPNEGO.
NEGOEX does not update the ASN.1 structures of SPNEGO [RFC4178]
because a widely deployed SPNEGO implementation does not have the
ASN.1 extensibility marker in the message definition. There is no
change to the SPNEGO messages.
NEGOEX uses a C-like definition language to describe message formats.
The rest of the document is organized as follows:
o Section 3 defines the encoding of NEGOEX data structures and all
the primitive data types.
o Section 6 describes the cryptographic framework required by the
NEGOEX for protecting the NEGOEX negotiation.
o Section 7 defines the NEGOEX messages and the NEGOEX protocol.
o Section 8 defines the new GSS-API extensions that a security
mechanism MUST support in order to be negotiated by NEGOEX.
o Section 9 contains the security considerations for NEGOEX.
o Appendix A contains all the protocol constructs and constants.
2. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Presentation Language and Primitive Data Types
The following very basic and somewhat casually defined presentation
syntax will be used in all NEGOEX messages. Although it resembles
the programming language "C" in its syntax, it would be risky to draw
too many parallels. The purpose of this presentation language is to
document NEGOEX only; it has no general application beyond that
particular goal.
This section also defines all the primitive data types. The
semantics of the data types is explained in the next section.
3.1. Basic Block Size
The representation of all data items is explicitly specified. The
basic data block size is one octet. Multiple octet data items are
concatenations of octets, from left to right, from top to bottom
Unless otherwise specific a multi-octet numeric is in little endian
order with the least significant octet first.
3.2. Miscellaneous
Comments start with "//"' and continue until the end of the line.
3.3. Constants
Constants are denoted using "#define" followed by the symbolic name
and then the constant value.
3.4. Numbers
UCHAR is the data type for a one-octet number.
ULONG is the data type for a 4-octet number encoded in little endian.
USHORT is the data type for a 2-octet number encoded in little
endian.
ULONG64 is the data type for a 8-octet number encoded in little
endian.
GUID is the data type for a 16-octet number encoded in little endian.
3.5. Enum Types
An enum type is the data type for a number with a small number of
permissible values. An instance of an enum type is a 4-octet number
encoded in little endian.
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The definition of an enum type follows the simple "C" convention.
MESSAGE_TYPE is an enum type defined as follows:
enum
{
MESSAGE_TYPE_INITIATOR_NEGO = 0,
MESSAGE_TYPE_ACCEPTOR_NEGO,
MESSAGE_TYPE_INITIATOR_META_DATA,
MESSAGE_TYPE_ACCEPTOR_META_DATA,
MESSAGE_TYPE_CHALLENGE,
// an exchange message from the acceptor
MESSAGE_TYPE_AP_REQUEST,
// an exchange message from the initiator
MESSAGE_TYPE_VERIFY,
MESSAGE_TYPE_ALERT,
} MESSAGE_TYPE;
MESSAGE_TYPE_INITIATOR_NEGO has the value 0, and MESSAGE_TYPE_ALERT
has the value 7.
3.6. Typedef Declarations
A typedef creates a synonym for the type. This is used to create
more meaningful names for existing types.
The following two type synonyms are defined.
typedef GUID AUTH_SCHEME;
typedef GUID CONVERSATION_ID;
3.7. Array Types
Arrays are a data structure which holds multiple variables of the
same data type consecutively and the number of elements is fixed. An
array is declared using "C" convention. The following defines an
array of 32 octets.
UCHAR Random[32];
3.8. Constructed Types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type. The
syntax for definition is much like that of C.
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struct {
T1 f1;
T2 f2;
...
Tn fn;
} T;
Structure definitions may be embedded.
The following types are defined as constructed types:
struct
{
ULONG ExtensionType; // negative extensions are critical
BYTE_VECTOR ExtensionValue;
} EXTENSION;
An extension has two fields. The ExtensionType field indicates how
the extension data should be interpreted. The ExtensionValue field
contains the extension data.
//
// schemes defined for the checksum in the VERIFY message
//
struct
{
ULONG cbHeaderLength;
ULONG ChecksumScheme;
ULONG ChecksumType; // in the case of RFC3961 scheme, this is
// the RFC3961 checksum type
BYTE_VECTOR ChecksumValue;
} CHECKSUM;
The CHECKSUM structure contains 4 fields. The cbHeaderLength length
contains the length of the structure defintion in octets, and this
field has a value of 20.
The ChecksumScheme field describes how checksum is computed and
verified. Currently only one value is defined.
#define CHECKSUM_SCHEME_RFC3961 1
When the value of the ChecksumScheme field is 1
(CHECKSUM_SCHEME_RFC3961), the ChecksumValue field contains a
sequence of octets computed according to [RFC3961] and the
ChecksumType field contains the checksum type value defined according
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to [RFC3961].
4. Vector Types
Vectors are a data structure which holds multiple variables of the
same data type consecutively and the number of elements is not fixed.
A vector contains a fixed length header followed by a variable length
payload. The header of a vector structure contains the count of
elements and the offset to the payload. In this document all the
offset fields are relative to the beginning of the containing NEGOEX
message. The size of each element is specified by the vector type
definition.
The following vector types are defined.
struct
{
ULONG ByteArrayOffset; // each element contains an octet/byte
ULONG ByteArrayLength;
} BYTE_VECTOR;
BYTE_VECTOR encapsulates a variable length array of octets (or bytes)
that are stored consecutively. Each element in is a byte (8 bits).
struct
{
ULONG AuthSchemeArrayOffset;
// each element contains an AUTH_SCHEME
USHORT AuthSchemeCount;
} AUTH_SCHEME_VECTOR;
AUTH_SCHEME_VECTOR encapsulates a variable length array of
AUTH_SCHEMEs that are stored consecutively. Each element is a
structure of the type AUTH_SCHEME.
struct
{
ULONG ExtensionArrayOffset;
// each element contains an EXTENSION
USHORT ExtensionCount;
} EXTENSION_VECTOR;
EXTENSION_VECTOR encapsulates a variable length array of EXTENSIONs
that are stored consecutively. Each element is a structure of the
type EXTENSION.
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5. NEGOEX Messages
The following structure is the MESSAGE_HEADER:
struct
{
ULONG64 Signature; // contains MESSAGE_SIGNATURE
MESSAGE_TYPE MessageType;
ULONG SequenceNum; // the message sequence number of this,
// conversation, starting with 0 and sequentially
// incremented
ULONG cbHeaderLength; // the header length of this message,
// including the message specific header, excluding the
// payload
ULONG cbMessageLength; // the length of this message
CONVERSATION_ID ConversationId;
} MESSAGE_HEADER;
The following structure is the NEGO_MESSAGE:
struct
{
MESSAGE_HEADER Header;
// MESSAGE_TYPE_INITIATOR_NEGO for the initiator,
// MESSAGE_TYPE_ACCEPTOR_NEGO for the acceptor
UCHAR Random[32];
ULONG64 ProtocolVersion;
// version of the protocol, this contains 0
AUTH_SCHEME_VECTOR AuthSchemes;
EXTENSION_VECTOR Extensions;
} NEGO_MESSAGE;
The following structure is the EXCHANGE_MESSAGE:
struct
{
MESSAGE_HEADER Header;
// MESSAGE_TYPE_CHALLENGE for the acceptor,
// or MESSAGE_TYPE_AP_REQUEST for the initiator
// MESSAGE_TYPE_INITIATOR_META_DATA for
// the initiator metadata
// MESSAGE_TYPE_ACCEPTOR_META_DATA for
// the acceptor metadata
AUTH_SCHEME AuthScheme;
BYTE_VECTOR Exchange;
// contains the opaque handshake message for the
// authentication scheme
} EXCHANGE_MESSAGE;
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6. Cryptographic Computations
The message signing and verification in NEGOEX is based on [RFC3961].
[RFC3961] is used here as a generic framework and this application is
not Kerberos specific.
A security mechanism MUST support [RFC3961] in order to be negotiated
by NEGOEX.
7. The NEGOEX Protocol
This section describes the NEGOEX protocol and it defines NEGOEX
messages in the order that the messages can appear on the wire. The
enum type MESSAGE_TYPE defined in Section 3.5 lists all NEGOEX
message types. A GSS-API context token for NEGOEX consists of one or
more NEGOEX messages. If there is more than one NEGOEX message,
these messages are concatenated together. The smallest data unit for
NEGOEX to compute the checksum for negotiation protection is s NEGOEX
message. Note that NEGOEX is not a GSS-API mechanism itself and the
initial NEGOEX context establishment token does not follow the
mechanism-independent token format defined in Section 3.1 of
[RFC2743].
The object identifier of the NEGOEX within SPNEGO is iso(1)
identified-organization(3) dod(6) internet(1) private(4)
enterprise(1) microsoft (311) security(2) mechanisms(2) negoex(30).
7.1. High-level NEGOEX Message Flow
The following text art summarizes the protocol message flow:
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Initiator Acceptor
INITIATOR_NEGO
+*INITIATOR_META_DATA
*AP_REQUEST
--------->
ACCEPTOR_NEGO
ACCEPTOR_META_DATA*+
<--------- CHALLENGE*
.
.
*AP_REQUEST --------->
<--------- CHALLENGE*
.
.
*AP_REQUEST
VERIFY --------->
CHALLENGE*
<--------- VERIFY
* Indicates optional or situation-dependent messages that are
not always sent.
+ Indicates there can be more than one instance.
7.2. NEGOEX Supported Security Mechanisms
NEGOEX maintains an ordered list of supported security mechanisms
names to determine priority of security mechanisms. A security
mechanism negotiable by NEGOEX is identified by a unique identifier
of data type AUTH_SCHEME defined in Section 3.5. Supported security
mechanisms are referenced by their corresponding authentication
scheme IDs. The authentication scheme ID of a security mechanism is
returned to NEGOEX by calling GSS_Query_mechanism_info() with the
name of the security mechnism as defined in Section 8.3.
7.3. ConversationID
Both initiator and acceptor must keep protocol state in the form of a
GUID, which will be referred to hereafter as the ConversationID.
7.4. Generation of the Initiator Initial Token
The GSS-API initiator makes the first call to GSS_Init_sec_context()
with no input token, and the output token will be a NEGO_MESSAGE
message with the MESSAGE_TYPE_INITIATOR_NEGO message followed by zero
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or more EXCHANGE_MESSAGE messages containing meta-data tokens,
followed by zero or one AP_REQUEST messages containing an optimistic
initial context token.
The initiator generates a cryptographic strength random 16 byte
value, stores it as the ConversationID, then sets the MESSAGE_HEADER
header field with the same name to that value. The ConversationID in
subsequent NEGOEX messages MUST remain the same. The initiator also
fills the Random field using a secure random number generator. The
initiator fills the AuthSchemes with available security mechanisms
supported by the initiator in decreasing preference order.
The extensions field contains NEGOEX extensions for future
extensibility. There are no extensions defined in this document.
All negative extension types (the highest bit is set to 1) are
critical. If the receiver does not understand a critical extension,
the authentication attempt must be rejected.
The initiator can OPTIONALLY include a meta-data token, one for each
available security mechanism.
A meta-data token is returned to NEGOEX for a security mechanism
using GSS_Query_meta_data() extension as defined in Section 8.1. If
a non-empty meta-data token is returned, then the meta-data token is
encapsulated in an EXCHANGE message with the message type
MESSAGE_TYPE_INITIATOR_META_DATA. On GSS_Query_meta_data call
failure, NEGOEX SHOULD remove the security mechanism from the set of
authentication schemes to be negotiated.
The AuthScheme field signifies the security mechanism for which the
EXCHANGE message is targeted. If a security mechanism fails to
produce the metadata token, it should be removed from the list of
supported security mechanism for this negotiation context.
If there is more than one exchange message, the order in which the
exchange message is included bears no significance. In other words,
the exchange messages are in an unordered set. The NEGO_MESSAGE MAY
be followed by a set of MESSAGE_TYPE_INITIATOR_META_DATA messages as
described above, in which case all the NEGOEX messages concatenated
are returned as a single output token.
The first mechanism in the initiator proposed list can OPTIONALLY
include its initial context token in an AP_REQUEST message.
Both an AP_REQUEST(short for MESSAGE_TYPE_AP_REQUEST) message and a
INITIATOR_META_DATA(short for MESSAGE_TYPE_INITIATOR_META_DATA)
message are instances of the EXCHANGE_MESSAGE structure with
different message type values. An AP_REQUEST message contains the
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type MESSAGE_TYPE_AP_REQUEST while an INITIATOR_META_DATA message
contains the type MESSAGE_TYPE_INITIATOR_META_DATA.
7.5. Receipt of the Initial Initiator Token and Generation of the
Initial Acceptor Response
Upon receipt of the NEGO_MESSAGE from the initiator, the acceptor
verifies the NEGO_MESSAGE to make sure it is well-formed. The
acceptor extracts the ConversationID from the NEGO_MESSAGE and stores
it as the ConversationID for the context handle. The acceptor then
computes the list of authentication schemes that are mutually
supported by examining the set of security mechanisms proposed by the
initiator and the meta-data tokens from the initiator. The meta-data
tokens are passed to the security mechanism via
GSS_Exchange_meta_data() as defined in Section 8.2. On
GSS_Exchange_meta_data call failure, NEGOEX SHOULD remove the
security mechanism from the set of authentication schemes to be
negotiated.
The acceptor MUST examine the NEGOEX extensions in the NEGO_MESSAGE.
If there is an unknown critical extension, the authentication must be
rejected.
The acceptor's output token is a NEGO_MESSAGE but with the the
Header.MessageType set to MESSAGE_TYPE_ACCEPTOR_NEGO followed by zero
or more EXCHANGE_MESSAGE containing meta-data tokens. The
AuthSchemes field contains the list of mutually supported security
mechanism in decreasing preference order of the acceptor. The
acceptor does not need to honor the preference order proposed by the
initiator when computing its preference list.
As with the initiator, the acceptor can OPTIONALLY include a meta-
data token, one for each available security mechanism.
A meta-data token is obtained by NEGOEX for a security mechanism
using GSS_Query_meta_data() extension as defined in Section 8.1. If
a non-empty meta-data token is returned, then the meta-data token is
encapsulated in an EXCHANGE message with the message type
MESSAGE_TYPE_ACCEPTOR_META_DATA. For a given security mechanism if a
meta-token is received from the initiator, GSS_Query_meta_data() MUST
be invoked on the acceptor side for that security mechanism, and the
output meta-data token, if present, MUST be included in the NEGOEX
reply. On GSS_Query_meta_data call failure, NEGOEX SHOULD remove the
security mechanism from the set of authentication schemes to be
negotiated.
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7.6. Receipt of the Acceptor Initial Response and Completion of
Authentication after the Negotiation Phrase
Upon receipt of the initial response token from the acceptor, the
application calls GSS_Init_sec_context with the response token. The
initiator verifies the NEGOEX message received to make sure it is
well-formed. The initiator ensures the correct context handle by
verifying that the ConversationID of the context handle matches the
conversation ID in the NEGOEX message received. The initiator then
computes the list of authentication schemes that are mutually
supported by examining the set of security mechanisms returned by the
acceptor and the meta-data tokens from the acceptor The meta-data
tokens are passed to the security mechanism via
GSS_Exchange_meta_data() as defined in Section 8.2. On
GSS_Exchange_meta_data call failure, NEGOEX SHOULD remove the
security mechanism from the set of authentication schemes to be
negotiated.
The initiator MUST examine the NEGOEX extensions in the NEGO_MESSAGE.
If there is an unknown critical extension, the authentication must be
rejected.
After the initial exchange of NEGO_MESSAGE messages, the initiator
MUST choose the negotiated security mechanism. The negotiated
security mechanism cannot be changed once it is selected.
The initiator and the acceptor can then proceed to exchange handshake
messages by returning GSS_S_CONTINUE_NEEDED to the calling
application as determined by the negotiated security mechanism until
its authentication context is established. The context tokens of the
negotiated security mechanism are encapsulated in an
EXCHANGE_MESSAGE. If the context token is from the initiator, the
EXCHANGE_MESSAGE message has the message type
MESSAGE_TYPE_AP_REQUEST; otherwise, the message type is
MESSAGE_TYPE_CHALLENGE.
7.7. Finalizing Negotiation
After the security mechanism has been selected, the initiator and
acceptor can use GSS_Inquire_context to obtain the Negoex_Verify_key
as defined in Section 8.4 to determine if there is a shared key for
the VERIFY message. When there is a shared key established returned
by GSS_Inquire_context as defined in Section 8.4, a VERIFY message is
produced using the required checksum mechanism per RFC 3961 and
included in the output token. The returned protocol key is used as
the base key in the parlance of RFC3961 to sign all the NEGOEX
messages in the negotiation context.
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A VERIFY message is a VERIFY_MESSAGE structure. The AuthScheme field
signifies from which security mechanism the protocol key was
obtained. The checksum is computed based on RFC3961 and the key
usage number is 23 for the message signed by the initiator, 25
otherwise. The checksum is performed over all the previous NEGOEX
messages in the context negotiation.
struct
{
MESSAGE_HEADER Header; // MESSAGE_TYPE_VERIFY
AUTH_SCHEME AuthScheme;
CHECKSUM Checksum;
// contains the checksum of all the previously
// exchanged messages in the order they were sent.
} VERIFY_MESSAGE;
Note that the VERIFY_MESSAGE message can be included before the
security context for the negotiated security mechanism is fully
established.
8. Supporting GSS-API Extensions
This section defined all the required GSS-API extensions required by
NEGOEX which must be supported by security mechanisms usable with
NEGOEX.
8.1. GSS_Query_meta_data
Inputs:
o input_context_handle CONTEXT HANDLE
o targ_name INTERNAL NAME, optional
o deleg_req_flag BOOLEAN,
o mutual_req_flag BOOLEAN,
o replay_det_req_flag BOOLEAN,
o sequence_req_flag BOOLEAN,
o conf_req_flag BOOLEAN,
o integ_req_flag BOOLEAN,
Outputs:
o metadata OCTET STRING,
o output_context_handle CONTEXT HANDLE
Return major_status codes:
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o GSS_S_COMPLETE indicates that the context referenced by the
input_context_handle argument is valid, and that the output
metadata value represents the security mechanism's provided
metadata. A security mechanism may return empty metadata.
o GSS_S_NO_CONTEXT indicates that no valid context was recognized
for the input context_handle provided. Return values other than
major_status and minor_status are undefined.
o GSS_S_NO_CRED indicates that no metadata could be returned about
the referenced credentials either because the input cred_handle
was invalid or the caller lacks authorization to access the
referenced credentials.
o GSS_S_UNAVAILABLE indicates that the authentication security
service does not support this operation.
o GSS_S_FAILURE indicates that the requested operation failed for
reasons unspecified at the GSS-API level. Return values other
than major_status and minor_status are undefined.
GSS_Query_meta_data is used to retrieve a security mechanism's
metadata.
8.2. GSS_Exchange_meta_data
Inputs:
o input_context_handle CONTEXT HANDLE
o cred_handle CREDENTIAL HANDLE, optional
o targ_name INTERNAL NAME, optional
o deleg_req_flag BOOLEAN,
o mutual_req_flag BOOLEAN,
o replay_det_req_flag BOOLEAN,
o sequence_req_flag BOOLEAN,
o conf_req_flag BOOLEAN,
o integ_req_flag BOOLEAN,
o metadata OCTET STRING,
Outputs:
o output_context_handle CONTEXT HANDLE
Return major_status codes:
o GSS_S_COMPLETE indicates that the metadata was provided to the
security mechanism.
o GSS_S_NO_CONTEXT indicates that no valid context was recognized
for the input context_handle provided. Return values other than
major_status and minor_status are undefined.
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o GSS_S_NO_CRED indicates that the metadata passed requested
credentials not available via this credential handle.
o GSS_S_UNAVAILABLE indicates that the security mechanism does not
support this operation.
o GSS_S_FAILURE indicates that the requested operation failed for
reasons unspecified at the GSS-API level. Return values other
than major_status and minor_status are undefined.
GSS_Exchange_meta_data is used to provide the metadata to each
security mechanism.
8.3. GSS_Query_mechanism_info
Inputs:
o SecMechName STRING,
Outputs:
o AuthScheme AUTH_SCHEME
Return major_status codes:
o GSS_S_COMPLETE indicates that the authentication scheme value
represents the security mechanism's AUTH_SCHEME.
o GSS_S_FAILURE indicates that the security mechanism does not
support NEGOEX. Return values other than major_status and
minor_status are undefined.
GSS_Query_mechanism_info returns a security mechanism's
authentication scheme value.
8.4. GSS_Inquire_context
The following output is added to GSS_Inquire_context as defined in
[RFC2743].
Outputs:
o Negoex_Verify_key OCTET STRING
This new output is the key to be used by NEGOEX for the VERIFY
message.
9. Security Considerations
Security mechanism SHOULD support providing VERIFY key material.
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This ensures that VERIFY messages are generated to make NEGOEX safe
from downgrade attacks.
10. Acknowledgements
TBD.
11. IANA Considerations
There is no action required for IANA.
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[RFC4178] Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
Simple and Protected Generic Security Service Application
Program Interface (GSS-API) Negotiation Mechanism",
RFC 4178, October 2005.
Appendix A. Protocol Data Structures and Constant Values
This section compiles all the protocol data structures and constant
values.
#define MESSAGE_SIGNATURE 0x535458454f47454ei64
// "NEGOEXTS"
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struct
{
ULONG ByteArrayOffset; // each element contains a byte
ULONG ByteArrayLength;
} BYTE_VECTOR;
struct
{
ULONG AuthSchemeArrayOffset;
// each element contains an AUTH_SCHEME
USHORT AuthSchemeCount;
} AUTH_SCHEME_VECTOR;
struct
{
ULONG ExtensionArrayOffset;
// each element contains an EXTENSION
USHORT ExtensionCount;
} EXTENSION_VECTOR;
struct
{
ULONG ExtensionType; // negative extensions are critical
BYTE_VECTOR ExtensionValue;
} EXTENSION;
//
// schemes defined for the checksum in the VERIFY message
//
#define CHECKSUM_SCHEME_RFC3961 1
struct
{
ULONG cbHeaderLength;
ULONG ChecksumScheme;
ULONG ChecksumType; // in the case of RFC3961 scheme, this is
// the RFC3961 checksum type
BYTE_VECTOR ChecksumValue;
} CHECKSUM;
typedef GUID AUTH_SCHEME;
typedef GUID CONVERSATION_ID;
enum
{
MESSAGE_TYPE_INITIATOR_NEGO = 0,
MESSAGE_TYPE_ACCEPTOR_NEGO,
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MESSAGE_TYPE_INITIATOR_META_DATA,
MESSAGE_TYPE_ACCEPTOR_META_DATA,
MESSAGE_TYPE_CHALLENGE,
// an exchange message from the acceptor
MESSAGE_TYPE_AP_REQUEST,
// an exchange message from the initiator
MESSAGE_TYPE_VERIFY,
MESSAGE_TYPE_ALERT,
} MESSAGE_TYPE;
struct
{
ULONG64 Signature; // contains MESSAGE_SIGNATURE
MESSAGE_TYPE MessageType;
ULONG SequenceNum; // the message sequence number of this,
// conversation, starting with 0 and sequentially
// incremented
ULONG cbHeaderLength; // the header length of this message,
// including the message specific header, excluding the
// payload
ULONG cbMessageLength; // the length of this message
CONVERSATION_ID ConversationId;
} MESSAGE_HEADER;
struct
{
MESSAGE_HEADER Header;
// MESSAGE_TYPE_INITIATOR_NEGO for the initiator,
// MESSAGE_TYPE_ACCEPTOR_NEGO for the acceptor
UCHAR Random[32];
ULONG64 ProtocolVersion;
// version of the protocol, this contains 0
AUTH_SCHEME_VECTOR AuthSchemes;
EXTENSION_VECTOR Extensions;
} NEGO_MESSAGE;
struct
{
MESSAGE_HEADER Header;
// MESSAGE_TYPE_CHALLENGE for the acceptor,
// or MESSAGE_TYPE_AP_REQUEST for the initiator
// MESSAGE_TYPE_INITiATOR_META_DATA for
// the initiator metadata
// MESSAGE_TYPE_ACCEPTOR_META_DATA for
// the acceptor metadata
AUTH_SCHEME AuthScheme;
BYTE_VECTOR Exchange;
// contains the opaque handshake message for the
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// authentication scheme
} EXCHANGE_MESSAGE;
struct
{
MESSAGE_HEADER Header; // MESSAGE_TYPE_VERIFY
AUTH_SCHEME AuthScheme;
CHECKSUM Checksum;
// contains the checksum of all the previously
// exchanged messages in the order they were sent.
} VERIFY_MESSAGE;
struct
{
ULONG AlertType;
BYTE_VECTOR AlertValue;
} ALERT;
//
// alert types
//
#define ALERT_TYPE_PULSE 1
//
// reason codes for the heartbeat message
//
#define ALERT_VERIFY_NO_KEY 1
struct
{
ULONG cbHeaderLength;
ULONG Reason;
} ALERT_PULSE;
struct
{
ULONG AlertArrayOffset; // the element is an ALERT
USHORT AlertCount; // contains the number of alerts
} ALERT_VECTOR;
struct
{
MESSAGE_HEADER Header;
AUTH_SCHEME AuthScheme;
ULONG ErrorCode; // an NTSTATUS code
ALERT_VECTOR Alerts;
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} ALERT_MESSAGE;
Authors' Addresses
Michiko Short
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: michikos@microsoft.com
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
Kevin Damour
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: kdamour@microsoft.com
Dave McPherson
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: davemm@microsoft.com
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