Network Working Group Derrell Piper
INTERNET-DRAFT Network Alchemy
draft-ietf-ipsec-isakmp-gss-auth-02.txt December 14, 1998
A GSS-API Authentication Mode for IKE
<draft-ietf-ipsec-isakmp-gss-auth-02.txt>
Status of this Memo
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Distribution of this memo is unlimited. This draft will expire six
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1. Abstract
This document describes an alternate authentication method for IKE
which makes use of GSS-API to authenticate the Diffie-Hellman
exchange. The mechanism described here extends the authentication
modes defined in RFC-2409 without introducing any modifications to
the IKE key exchange protocol. This constraint however, necessarily
restricts the number of GSS-API exchanges and may limit the broader
applicability of this method. Additional work is needed to provide a
fully generalized solution. Such a solution will require IKE
protocol modifications.
For a list of changes since the previous version of this document,
please see Section 5.
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2. Terms and Definitions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC 2119].
2.1 Notation
RFC-2409 uses the following notation throughout that draft. That
notation is included here along with a few additions.
HDR is an ISAKMP header whose exchange type is the mode. When
written as HDR* it indicates payload encryption.
SA is an SA negotiation payload with one or more proposals. An
initiator MAY provide multiple proposals for negotiation; a
responder MUST reply with only one.
<P>_b indicates the body of payload <P>-- the ISAKMP generic
payload is not included.
SAi_b is the entire body of the SA payload (minus the ISAKMP
generic header)-- i.e. the DOI, situation, all proposals and all
transforms offered by the Initiator.
CKY-I and CKY-R are the Initiator's cookie and the Responder's
cookie, respectively, from the ISAKMP header.
g^xi and g^xr are the Diffie-Hellman public values of the
initiator and responder respectively.
g^xy is the Diffie-Hellman shared secret.
GIi and GIr are identity name strings for the GSS-API initiator
and responder GSS-API endpoints. These name strings are private
to GSS-API.
GSSi and GSSr are initiator and responder GSS-API tokens generated
by the local GSS-API's using GSS_Init_sec_context and
GSS_Accept_sec_context respectively.
GSSi2 and GSSr2 are optional tokens which may be included for an
additional GSS-API token exchange in IKE Main Mode.
GSSIi and GSSIr are the concatenation of optional GSS-API identity
strings and either GSSi or GSSr for the initiator and responder
respectively.
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KE is the key exchange payload which contains the public
information exchanged in a Diffie-Hellman exchange. There is no
particular encoding used for the data of a KE payload.
Nx is the nonce payload; x can be: i or r for the ISAKMP initiator
and responder respectively.
IDx is the identity payload for "x". x can be: "ii" or "ir" for
the ISAKMP initiator and responder respectively during phase one
negotiation; or "ui" or "ur" for the user initiator and responder
respectively during phase two. The ID payload format for the
Internet DOI is defined in RFC-2407.
HASH (and any derivative such as HASH(2) or HASH_I) is the hash
payload. The contents of the hash are specific to the
authentication method.
prf(key, msg) is the keyed pseudo-random function-- often a keyed
hash function-- used to generate a deterministic output that
appears pseudo-random. prf's are used both for key derivations
and for authentication (i.e. as a keyed MAC).
SKEYID is a string derived from secret material known only to the
active players in the exchange.
SKEYID_e is the keying material used by the ISAKMP SA to protect
it's messages.
SKEYID_a is the keying material used by the ISAKMP SA to
authenticate it's messages.
SKEYID_d is the keying material used to derive keys for non-ISAKMP
security associations.
<x>y indicates that "x" is encrypted with the key "y".
--> signifies "initiator to responder" communication (requests).
<-- signifies "responder to initiator" communication (replies).
| signifies concatenation of information-- e.g. X | Y is the
concatenation of X with Y.
[x] indicates that x is optional.
Payload encryption (when noted by a '*' after the ISAKMP header) MUST
begin immediately after the ISAKMP header. When communication is
protected, all payloads following the ISAKMP header MUST be
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encrypted. Encryption keys are generated from SKEYID_e in a manner
that is defined for each algorithm.
3. Discussion
The ISAKMP/Oakley resolution document (RFC-2409) defines a key
negotiation protocol that blends the Oakley key determination
protocol (RFC-2412) with ISAKMP (RFC-2408) to provide authenticated
cryptographic key exchange for use with IP security protocols (e.g.
AH/ESP). The IKE negotiation includes an authentication method
negotiation which is used to select a scheme to be used for
authenticating a Diffie-Hellman key exchange. There are currently
five defined authentication methods: pre-shared key, DSS signature,
RSA signature, and two forms of RSA encryption. This document
defines a new method that uses the Generic Security Services API
([Linn98]) to provide the necessary authentication.
The GSS-API abstraction is that a host operating system provides an
API to applications that request security services (e.g. integrity
protection or confidentiality) through a formal interface (e.g.,
[Wray98]). GSS-API provides opaque tokens to applications which are
responsible for sending the tokens to peer GSS-API implementations,
presumably on remote hosts. A by-product of any GSS-API exchange is
a one way or mutual authentication using whatever authentication
scheme the application chose to bind to when GSS-API was initialized
(or whatever was negotiated by SNEGO ([Pinkas98])). Typical
authentication packages include Kerberos and SSL.
The ISAKMP/Oakley resolution defines a Main Mode and an Aggressive
Mode for establishing Security Associations (SA's) between IPSEC
hosts. These modes have a fixed set of round-trips: 4.5 or 5 for
Main Mode and 1 or 1.5 for Aggressive (depending on whether the
Commit bit (RFC-2408, Section 3.1) is used by the responder).
When using GSS-API, there's a separate protocol being run by the GSS-
API packages on the initiator and on the responder. (Initiator and
responder are ISAKMP terms, both are GSS-API clients.) The basic
model is that the IKE initiator calls GSS_Init_sec_context (with
mutual_req_flag) to construct a GSS-API token and sends this along
with the KE and nonce in the second Main Mode exchange. The
responder calls GSS_Accept_sec_context on this token and sends the
output of GSS_Accept_sec_context (another token) back along with his
KE and his nonce. On receipt of the responder's token, the initiator
calls GSS_Init_sec_context a second time to complete the mutual
authentication. Finally, each side exchanges a HASH payload which
has been wrapped using GSS_Wrap. Successfully calling GSS_Unwrap to
unwrap the HASH payloads along with verfiying the hashes proves
possesion of the GSS-API shared secret and authenticates the Diffie-
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Hellman exchange.
GSS-API requires that a client identify the target GSS-API endpoint
by name. If the initiator does not already know the GSS-API endpoint
name of the ISAKMP target, a new Phase 1 attribute can be used to
exchange endpoint names during the first Main Mode round trip
(Section 4.3). Note that these name string are bound to the exchange
but otherwise unauthenticated. The GSS-API endpoint names are also
assumed to be opaque.
Since the GSS-API tokens are exchanged during Phase 1 along with the
KE payloads, they are not protected by the (yet to be formed) ISAKMP
SA. To prevent a cut/paste attack on the GSS-API tokens, it's
therefore necessary to include the tokens in the HASH_I and HASH_R
computation (Section 4.1). This binds the tokens to a particular
ISAKMP exchange. If used, the GSS Identity Name strings MUST also be
included in these hash calculations.
In addition, the output from the prf for each hash is wrapped using
GSS_Wrap. Upon receipt of either hash payload, each side MUST
successfully call GSS_Unwrap. This proves possession of the GSS-API
shared secret by each peer and prevents an active man-in-the-middle
attack from simply forwarding on the GSS-API tokens. The choice of
whether to use integrity protection only or integrity with
confidentiality is somewhat mechanism specific. However, since the
strength of the algorithm chosen necessarily determines the outcome
of the authentication for ISAKMP, the strongest possible protection
SHOULD be chosen. The following flags should be specified to
GSS_Init_sec_context on the initiating side:
Flag Requirement
---- -----------
mutual_req_flag MUST
integ_req_flag MUST
conf_req_flag SHOULD
If the GSS-API authentication cannot be completed in 1.5 round-trips,
the method described in this document will not work. To fully
generalize this extension, a new XCHG type will need to be created
that allows for any number of GSS-API exchanges but is otherwise
similar to the existing Main Mode exchange. A single Main Mode-like
XCHG type is probably sufficient since there would be little use for
an Aggressive Mode construct given the open ended nature of GSS-API.
The primary motivation for this work was to integrate Kerberos
authentication into IKE in an environment where the host operating
system did not expose the underlying Kerberos authentication services
except as a GSS-API package. Since the details of the host operating
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system's Kerberos package were known, the limitations described above
were addressed in a reasonable manner by simply failing the IKE
negotiation when the GSS-API's failed to converge in the requisite
number of round-trips. When implemented this way, this event SHOULD
be auditable and should clearly differentiate this type of
authentication failure from one caused by truly bad credentials.
4.1 SKEYID Generation for GSS-API
RFC-2409 defines several authentication methods for Main Mode or
Aggressive Mode -- digital signatures, authentication using public
key encryption, and pre-shared keys. This document introduces
another and defines the value of SKEYID for GSS-API authentication as
follows.
For GSS-API: SKEYID = prf(Ni_b | Nr_b, g^xy)
To authenticate either exchange the initiator of the protocol
generates HASH_I and the responder generates HASH_R where:
HASH_I = GSS_Wrap(prf(SKEYID, g^xi | g^xr | CKY-I | CKY-R | SAi_b |
IDii_b | GSSIi [ | GSSi2]))
HASH_R = GSS_Wrap(prf(SKEYID, g^xr | g^xi | CKY-R | CKY-I | SAi_b |
IDir_b | GSSIr [ | GSSr2]))
For authentication using GSS-API, the GSS-API package on either side
provides authentication of the GSS-API identities, and HASH_I and
HASH_R are used to bind the GSS-API identities and tokens to the Main
Mode exchange. The GSS_Wrap (and subsequent GSS_Unwrap) proves
possession of the GSS-API shared secret for each peer. The initiator
MUST specify the mutual_req_flag to request mutual authentication
between the two GSS-API packages. A provision is defined for the
GSS-API peers to exchange GSS-API identities during Main Mode, at the
expense of identity protection for the GSS-API endpoint identities.
The content of the HASH_I and HASH_R ISAKMP payloads are the output
tokens from GSS_Wrap. The input to GSS_Wrap is the output of the
negotiated IKE hash function (prf) over the specified data. In other
words, you take the data, hash it with the negotiated hash function,
and then call GSS_Wrap on the hash digest. The output of GSS_Wrap is
placed in the HASH_I and HASH_R payloads.
When the optional GSSi2 and GSSr2 tokens are sent in a Main Mode
exchange (see Section 4.2), they MUST be included in the subsequent
HASH_I/HASH_R calculations defined above.
4.2 IKE Phase 1 Authenticated With GSS-API
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Using GSS-API, the ancillary information exchanged during the second
roundtrip are GSS-API tokens; the exchange is authenticated in GSS-
API and the GSS-API tokens are bound to the exchange using HASH_I and
HASH_R.
If the GSS-API requires that the initiator and responder have prior
knowledge of the GSS-API endpoint names for each peer, this
information may be exchanged during the first round trip (by
including the GSS Identity Name attribute in the SA) at the expense
of identity protection for the GSS-API endpoints. When the GSS-API
requires the exchange of identity names, Aggressive Mode cannot be
used.
Additionally, the local GSS-API may choose to make use of two GSS-API
token exchanges, using the optional GSSi2 and GSSr2 tokens, based on
local criteria. For example, a GSS-API implementation using Kerberos
may choose to make use of the extra round-trip for clock
sychronization reasons. This extra round-trip can only be done in
Main Mode.
Main Mode using GSS-API is defined as
Initiator Responder
----------- -----------
HDR, SA -->
<-- HDR, SA
HDR, KE, Ni, GSSi -->
<-- HDR, KE, Nr, GSSr
HDR*, IDii, [GSSi2,]
HASH_I -->
<-- HDR*, IDir, [GSSr2,]
HASH_R
Aggressive mode using GSS-API is defined as
Initiator Responder
----------- -----------
HDR, SA, KE, Ni,
IDii, GSSi -->
<-- HDR, SA, KE, Nr,
IDir, GSSr, HASH_R
HDR, HASH_I -->
4.3 GSS-API Identifiers: Authentication Method, Attribute, and Payload
Implementations using the GSS-API Authentication Mode will need to
agree on the values for the following items, possibly after
exchanging recognizable ISAKMP Vendor ID payloads. These numbers are
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simply the beginning of the "private-use" range for each particular
list and provided for illustration.
- Authentication Method (IKE)
Authentication with GSS-API 65001
Attribute Classes
class value type
------------------------------------------------------------
GSS Identity Name 16384 B/V
- GSS Identity Name Attribute (IKE)
When using the GSS-API authentication mode, the GSS Identity Name
attribute may be used to pass the GSS-API endpoint names for the
initiator and responder. The format for these name strings are
private to GSS-API.
- GSS Token Payload (ISAKMP)
When using the GSS-API authentication mode, the GSS Token Payload
is used to pass the content of the GSSi[2] and GSSr[2] tokens. The
Next Payload value for the GSS Token Payload is 129.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Token Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: GSS-API Token Payload (ISAKMP)
o Next Payload (1 octet) - Identifier for the payload type of
the next payload in the message. If the current payload is
the last in the message, this field will be zero (0).
o RESERVED (1 octet) - Unused, must be zero (0).
o Payload Length (2 octets) - Length in octets of the current
payload, including the generic payload header.
o Token Data (variable length) - GSS-API token data (private
to the local GSS-API).
5. Change Log
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5.2 Changes from V1
o Add optional GSSi2 and GSSr2 token definitions to Section 2.1.
o Add optional GSSi2 and GSSr2 tokens to Main Mode diagram.
o Add GSS Token Payload Figure to Section 4.3.
o Updated document references to reflect IPSEC RFC status (!).
o Update most references to ISAKMP/Oakley to IKE.
5.1 Changes from V0
o GSSIi and GSSIr are required; removed optional brackets.
o Added text for GSS_Wrap/GSS_Unwrap over HASH_I and HASH_R.
6. Security Considerations
This entire draft pertains to a negotiated key management protocol,
combining Oakley (RFC-2412) with ISAKMP (RFC-2408), which negotiates
and derives keying material for security associations in a secure and
authenticated manner. Specific discussion of the various security
protocols and transforms identified in this document can be found in
the associated base documents, in the cipher references, and
throughout this document.
Acknowledgments
Thanks to Dan Harkins for reviewing the early drafts and for allowing
me to liberate the notation from RFC-2409. Special thanks to Bill
Sommerfeld, Ran Canetti, Pau-Chen Cheng, and Hugo Krawczyk for
pointing out problems in previous versions of this document. Brian
Swander also provided text and design for the GSSi2 and GSSr2
definition and the optional round-trip.
References
[Linn98] Linn, J., "Generic Security Service Application Program
Interface, Version 2, Update 1," draft-ietf-cat-rfc2078bis-07.txt.
[Pinkas98] Pinkas, D., Baize, E., "The Simple and Protected GSS-API
Negotiation Mechanism," draft-ietf-cat-snego-09.txt.
[Wray98] Wray, J., "Generic Security Service API Version 2 : C-
bindings," draft-ietf-cat-gssv2-cbind-08.txt (supercedes RFC-1509).
Author's Address:
Derrell Piper <ddp@network-alchemy.com>
Network Alchemy
1521.5 Pacific Ave
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Santa Cruz, California, 95060
United States of America
+1 831 460-3822
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