Network Working Group L. Howard
Internet-Draft PADL
Intended status: Experimental April 5, 2020
Expires: October 7, 2020
A Simple Anonymous GSS-API Mechanism
draft-howard-gss-sanon-01
Abstract
This document defines protocols, procedures and conventions for a
Generic Security Service Application Program Interface (GSS-API)
security mechanism that provides key agreement without authentication
of either party.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Authentication . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Application Services . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
3. Discovery and Negotiation . . . . . . . . . . . . . . . . . . 3
4. Naming . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. GSS Name Types . . . . . . . . . . . . . . . . . . . . . . 4
4.1.1. GSS_C_NT_USER_NAME . . . . . . . . . . . . . . . . . . . 4
4.1.2. GSS_C_NT_HOSTBASED_SERVICE . . . . . . . . . . . . . . . 4
4.1.3. GSS_C_NT_DOMAINBASED_SERVICE . . . . . . . . . . . . . . 5
4.1.4. GSS_C_NT_ANONYMOUS . . . . . . . . . . . . . . . . . . . 5
4.2. Canonicalization . . . . . . . . . . . . . . . . . . . . . 5
5. Mechanism Attributes . . . . . . . . . . . . . . . . . . . . 5
6. Definitions and Token Formats . . . . . . . . . . . . . . . . 6
6.1. Context Establishment Tokens . . . . . . . . . . . . . . . 6
6.1.1. Initial context token . . . . . . . . . . . . . . . . . . 6
6.1.2. Acceptor context token . . . . . . . . . . . . . . . . . 6
6.1.3. Initiator context completion . . . . . . . . . . . . . . 7
6.2. Per-Message Tokens . . . . . . . . . . . . . . . . . . . . 7
6.3. Context Deletion Tokens . . . . . . . . . . . . . . . . . . 7
6.4. Exported Name Tokens . . . . . . . . . . . . . . . . . . . 7
7. Key derivation . . . . . . . . . . . . . . . . . . . . . . . 7
8. Pseudo-Random Function . . . . . . . . . . . . . . . . . . . 8
9. NegoEx . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
13. Normative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Generic Security Service Application Program Interface (GSS-API)
[RFC2743] provides a framework for authentication and message
protection services through a common programming interface.
The Simple Anonymous mechanism described in this document (hereafter
SAnon) is a simple protocol based on the X25519 elliptic curve
Diffie-Hellman (ECDH) key agreement scheme defined in [RFC7748]. No
authentication of initiator or acceptor is provided. A potential use
of SAnon is to provide a degree of privacy when bootstrapping unkeyed
entities.
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1.1. Authentication
The GSS-API protocol involves a client, known as the initiator,
sending an initial security context token of a chosen GSS-API
security mechanism to a peer, known as the acceptor. The two peers
subsequently exchange, synchronously, as many security context tokens
as necessary to complete the authentication or fail. The specific
number of context tokens exchanged varies by security mechanism: in
the case of the SAnon mechanism, it is two (i.e. a single round
trip). Once authentication is complete, the initiator and acceptor
share a security context which can be used for integrity or
confidentiality, protecting subsequent application messages.
1.2. Application Services
GSS-API provides a number of a services to the calling application:
GSS_Wrap() integrity and optional confidentiality for a message
GSS_GetMIC() integrity for a message sent separately
GSS_Pseudo_random() shared key derivation (e.g., for keying external
confidentiality and integrity layers)
These services are used with security contexts having a shared
session key to protect application-layer messages.
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Discovery and Negotiation
The means of discovering GSS-API peers and their supported mechanisms
is out of this specification's scope. However, when the Simple and
Protected Negotiation mechanism (SPNEGO) defined in [RFC4178] is
used, the mechanism MUST be negotiated under [I-D.zhu-negoex]. This
provides improved interoperability with Microsoft Windows, which has
limited support for negotiating new mechanisms directly under SPNEGO.
SAnon may also be negotiated by a GSS-API peer that requests it
explicitly.
To avoid multiple negotiation layers and implementation complexity,
this specification is deliberately not crypto-agile. A future
variant using a different key exchange algorithm would be assigned a
different mechanism OID and authentication scheme identifier.
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The selection of SAnon is subject to local policy. If anonymity is
not desired, mechanisms that provide authentication SHOULD be
preferred. Initiators use GSS_C_ANON_FLAG or the well known
anonymous credential to indicate that anonymous authentication is
desired. Either party can test for the presence of GSS_C_ANON_FLAG
to check if anonymous authentication was performed.
4. Naming
The GSS-API provides a rich security principal naming model. At its
most basic the query forms of names consist of a user-entered/
displayable string and a "name-type". Name-types are constants with
names prefixed with "GSS_C_NT_" in the GSS-API.
Many deployed applications do not have explicit support for anonymous
authentication. To ease deployment, we recommend allowing anonymous
authentication to be requested by the initiator acquiring a
credential with a well known anonymous name. This may allow the end-
user to request anonymous authentication directly, without requiring
the application be modified to support GSS_C_ANON_FLAG. The well
known anonymous name has the same display form as in Kerberos
[RFC8062], allowing acceptors to perform name-based authorization in
a mechanism-agnostic manner.
This approach may, however, disadvantage applications that wish to
use GSS_C_ANON_FLAG to select anonymous authentication, as importing
a non-anonymous initiator name will fail with this approach. We
consider this an acceptable compromise given the limited deployment
of GSS_C_ANON_FLAG in existing implementations.
4.1. GSS Name Types
4.1.1. GSS_C_NT_USER_NAME
This name type is supported when the input name string is the well
known anonymous name string, WELLKNOWN/ANONYMOUS@WELLKNOWN:ANONYMOUS.
In all other cases, importing the name SHOULD fail.
4.1.2. GSS_C_NT_HOSTBASED_SERVICE
This name type identifies a host-based service and is generally used
by acceptors. To allow existing applications to work unmodified with
SAnon, it is useful to allow anonymous acceptor credentials to be
acquired regardless of the service name. (It follows from SAnon not
authenticating the acceptor that the acceptor identity is
meaningless.) When importing a name of this type the name string
SHOULD be ignored.
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4.1.3. GSS_C_NT_DOMAINBASED_SERVICE
The [RFC5179] name type is treated identically to
GSS_C_NT_HOSTBASED_SERVICE.
4.1.4. GSS_C_NT_ANONYMOUS
When importing a name of this type the name string MUST be ignored.
Functions that return a name type to the caller MUST always return
this name type. The display form is the well known anonymous name
string, WELLKNOWN/ANONYMOUS@WELLKNOWN:ANONYMOUS.
4.2. Canonicalization
The SAnon GSS-API mechanism has a single anonymous identity, the well
known anonymous name. The canonical form is the well known anonymous
name string with the GSS_C_NT_ANONYMOUS name type.
5. Mechanism Attributes
The [RFC5587] mechanism attributes for this mechanism are:
GSS_C_MA_MECH_CONCRETE
GSS_C_MA_ITOK_FRAMED
GSS_C_MA_AUTH_INIT_ANON
GSS_C_MA_AUTH_TARG_ANON
GSS_C_MA_INTEG_PROT
GSS_C_MA_CONF_PROT
GSS_C_MA_MIC
GSS_C_MA_WRAP
GSS_C_MA_REPLAY_DET
GSS_C_MA_OOS_DET
GSS_C_MA_CBINDINGS
GSS_C_MA_PFS
GSS_C_MA_CTX_TRANS
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6. Definitions and Token Formats
6.1. Context Establishment Tokens
6.1.1. Initial context token
The initial context token is framed per Section 1 of [RFC2743]:
GSS-API DEFINITIONS ::=
BEGIN
MechType ::= OBJECT IDENTIFIER
-- representing SAnon mechanism
GSSAPI-Token ::=
[APPLICATION 0] IMPLICIT SEQUENCE {
thisMech MechType,
innerToken ANY DEFINED BY thisMech
-- 32 byte initiator public key
}
END
On the first call to GSS_Init_sec_context(), the mechanism checks
whether the caller set anon_req_flag (GSS_C_ANON_FLAG), or if the
claimant_cred_handle identity is the well known anonymous name. If
neither is the case then the call MUST fail with GSS_S_UNAVAILABLE,
so that anonymous context establishment is not attempted without the
initiator requesting it.
If proceeding, the initiator generates a secret and public key per
Section 6.1 of [RFC7748] and returns GSS_S_CONTINUE_NEEDED indicating
that a subsequent context token from the acceptor is expected. The
innerToken field of the output_token contains the initiator's 32 byte
public key.
6.1.2. Acceptor context token
Upon receiving a context token from the initiator, the acceptor
validates that the token is well formed and contains a public key of
the requisite length. The acceptor generates its own public key, to
be be sent to the initiator in the output token without any
additional framing. A session key is computed as specified in
Section 7.
The acceptor then returns GSS_S_COMPLETE, setting src_name to the
well known anonymous name. The reply_det_state (GSS_C_REPLAY_FLAG),
sequence_state (GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG),
integ_avail (GSS_C_INTEG_FLAG) and anon_state (GSS_C_ANON_FLAG)
security context flags are set to TRUE. The context is ready to use.
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6.1.3. Initiator context completion
Upon receiving the acceptor context token and verifying it is well
formed, the initiator computes the session key per Section 7. The
initiator returns GSS_S_COMPLETE to the caller, to indicate the
initiator is authenticated and the context is ready for use. No
output token is emitted. Security context flags are set as for the
acceptor context.
6.2. Per-Message Tokens
The per-message tokens definitions are imported from [RFC4121]
Section 4.2. The base key used to derive specific keys for signing
and sealing messages is the session key defined in Section 7. The
[RFC3961] encryption and checksum algorithms use the aes128-cts-hmac-
sha256-128 encryption type defined in [RFC8009].
6.3. Context Deletion Tokens
Context deletion tokens are empty in this mechanism. The behavior of
GSS_Delete_sec_context() [RFC2743] is as specified in [RFC4121]
Section 4.3.
6.4. Exported Name Tokens
The exported name token format for the SAnon GSS-API mechanism is the
same as the display form, plus the standard exported name token
format header mandated by the GSS-API [RFC2743].
7. Key derivation
The ECDH shared secret k is computed by calling the X25519 function
with the initiator (or acceptor's) secret key and the acceptor (or
initiator's) public key, as specified in Section 6.1 of [RFC7748].
The context session key K1 is computed using a key derivation
function from Section 5.1 of [SP800-108] with HMAC as the PRF:
K1 = HMAC-SHA-256(key, 0x00000001 | label | 0x00 | context | k)
where:
k the X25519 shared secret computed above
0x00000001 the iteration count from Section 5.1 of [SP800-108]
label the string "sanon-x25519" (without quotation marks)
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context the concatenation of the initiator and acceptor public
keys, along with the channel binding application data
(if present)
The inclusion of channel bindings in the key derivation function
means that the acceptor cannot ignore initiator channel bindings;
this differs from some other mechanisms.
The session key K1 is used as the base key for the generation of per-
message tokens and the GSS-API PRF. The encryption type of K1 is
aes128-cts-hmac-sha256-128 as defined in [RFC8009].
8. Pseudo-Random Function
The [RFC4401] GSS-API pseudo-random function for this mechanism
imports the definitions from [RFC8009], using the context session key
as the base key for both GSS_C_PRF_KEY_FULL and GSS_C_PRF_KEY_PARTIAL
usages.
9. NegoEx
The NegoEx authentication scheme identifier for this mechanism is
DEE384FF-1086-4E86-BE78-B94170BFD376.
The initiator and acceptor keys for NegoEx checksum generation and
verification are derived using the PRF from the previous section,
with the input data "sanon-x25519-initiator-negoex-key" and "sanon-
x25519-acceptor-negoex-key" (without quotation marks).
No NegoEx metadata is specified. Any metadata present MUST be
ignored.
10. Security Considerations
This document defines a GSS-API security mechanism, and therefore
deals in security and has security considerations text embedded
throughout. This section only addresses security considerations
associated with the SAnon mechanism described in this document. It
does not address security considerations associated with the GSS-API
itself.
This mechanism provides only for key agreement. It does not
authenticate or identify either party.
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11. IANA Considerations
IANA is requested to assign an OID for this GSS-API mechanism in the
SMI numbers registry, with the prefix of
iso.org.dod.internet.security.mechanisms (1.3.6.1.5.5) and to
reference this specification in the registry.
12. Acknowledgements
AuriStor, Inc funded the design of this protocol, along with an
implementation for the Heimdal GSS-API library.
Jeffrey Altman, Greg Hudson, Simon Josefsson, and Nicolas Williams
provided valuable feedback on this document.
13. Normative References
[I-D.zhu-negoex]
Short, M., Zhu, L., Damour, K., and D. McPherson, "SPNEGO
Extended Negotiation (NEGOEX) Security Mechanism", draft-
zhu-negoex-04 (work in progress), January 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
2005, <https://www.rfc-editor.org/info/rfc3961>.
[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,
DOI 10.17487/RFC4121, July 2005,
<https://www.rfc-editor.org/info/rfc4121>.
[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, DOI 10.17487/RFC4178, October 2005,
<https://www.rfc-editor.org/info/rfc4178>.
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[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401,
DOI 10.17487/RFC4401, February 2006,
<https://www.rfc-editor.org/info/rfc4401>.
[RFC5179] Williams, N., "Generic Security Service Application
Program Interface (GSS-API) Domain-Based Service Names
Mapping for the Kerberos V GSS Mechanism", RFC 5179,
DOI 10.17487/RFC5179, May 2008,
<https://www.rfc-editor.org/info/rfc5179>.
[RFC5587] Williams, N., "Extended Generic Security Service Mechanism
Inquiry APIs", RFC 5587, DOI 10.17487/RFC5587, July 2009,
<https://www.rfc-editor.org/info/rfc5587>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8009] Jenkins, M., Peck, M., and K. Burgin, "AES Encryption with
HMAC-SHA2 for Kerberos 5", RFC 8009, DOI 10.17487/RFC8009,
October 2016, <https://www.rfc-editor.org/info/rfc8009>.
[RFC8062] Zhu, L., Leach, P., Hartman, S., and S. Emery, Ed.,
"Anonymity Support for Kerberos", RFC 8062,
DOI 10.17487/RFC8062, February 2017,
<https://www.rfc-editor.org/info/rfc8062>.
[SP800-108]
Chen, L., "Recommendation for Key Derivation Using
Pseudorandom Functions (Revised)", October 2009.
Author's Address
Luke Howard
PADL Software Pty Ltd
PO Box 59
Central Park, VIC 3145
Australia
Email: lukeh@padl.com
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