MSEC WG L. Dondeti
Internet-Draft QUALCOMM
Expires: April 2, 2006 J. Xiang
Nortel Networks
September 29, 2005
GKDP: Group Key Distribution Protocol
draft-ietf-msec-gkdp-00
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Copyright (C) The Internet Society (2005).
Abstract
This document specifies a group key distribution protocol (GKDP)
based on IKEv2, the IPsec key management protocol; the new protocol
is similar to IKEv2 in message and payload formats, and message
semantics to a large extent. The protocol in conformance with MSEC
key management architecture contains two components: member
registration and group rekeying, and downloads a group security
association from the GCKS to a member. This protocol is independent
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of IKEv2 except in its likeness.
Conventions Used In This Document
This document recommends, as policy, what specifications for Internet
protocols -- and, in particular, IETF standards track protocol
documents -- should include as normative language within them. The
capitalized keywords "SHOULD", "MUST", "REQUIRED", etc. are used in
the sense of how they would be used within other documents with the
meanings as specified in BCP 14, RFC 2119 [RFC2119].
Table of Contents
1. Revision History . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction and Overview . . . . . . . . . . . . . . . . . . 3
2.1. Why do we need another GSA management protocol? . . . . . 3
2.2. GKDP usage scenarios . . . . . . . . . . . . . . . . . . . 4
3. GKDP protocol . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Member registration and secure channel establishment . . . 4
3.1.1. Initial exchange:GSA_INIT_EXCH . . . . . . . . . . . . 4
3.1.2. Authenticated exchange:GSA_AUTH_EXCH . . . . . . . . . 6
3.2. GSA maintenance channel . . . . . . . . . . . . . . . . . 9
3.2.1. GSA rekey protocol . . . . . . . . . . . . . . . . . . 9
3.3. Informational exchange . . . . . . . . . . . . . . . . . . 10
3.3.1. Notify exchange . . . . . . . . . . . . . . . . . . . 10
3.3.2. Error message . . . . . . . . . . . . . . . . . . . . 10
4. GKDP protocol design details . . . . . . . . . . . . . . . . . 10
5. Header and payload formats . . . . . . . . . . . . . . . . . . 10
5.1. GKDP header . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Revision History
GKDP-xx: Draft tag and title changed to gkdp-xx
Version 01: The protocol has been renamed GKDP for Group Key
Distribution Protocol as per discussions at the MSEC meeting at
IETF-60 and mailing list discussions. The name GDOIv2 will be
used for a revision of GDOI which may retain the DOI concept and
build upon RFC 3547.
Version 02: This is a major revision with the following additions to
the specification:
*
2. Introduction and Overview
Security encapsulation protocols such as IPsec and SRTP provide
confidentiality, message integrity, replay protection, and in some
instances access control, and data origin authentication. These
security services require state establishment, maintenance, and
teardown for correct operation. While these security associations
can be managed manually, automatic key management protocols are
essential for efficient and scalable operation. In case of point-to-
point security associations, IKE and its successor IKEv2 are widely
used for IPsec SAs, and MIKEY for SRTP associations. For multi-point
SAs or group SAs (GSA), GDOI, GSAKMP, and MIKEY have been specified
by the MSEC WG. GKDP is designed to be a counterpart - for GSA
distribution and maintenance - to IKEv2 so we can reuse the work put
in to its design and analysis, and of course implementation.
2.1. Why do we need another GSA management protocol?
Given the collection of key management protocols mentioned above,
there is a question on the need for yet another group key management
protocol. First a look back at history: So far, we have two
experimental RFCs, viz., RFC 1949 [RFC1949] and RFC 2093 [RFC2093],
and a standards track RFC, RFC 3547 [RFC3547] specifying or
describing group key management protocols. Furthermore there is
GSAKMP, currently a standards track MSEC I-D, which borrows quite a
few concepts from IKEv2, but not quite similar to IKEv2. The
protocol we propose is mainly to reuse as much as the IKEv2 codebase,
similar to GDOI reusing payload and message formats of IKE [RFC2409]
and ISAKMP [RFC2408] . Consequently, GKDP requires fewer messages
compared to GDOI, specifically 4 in most cases, compared to 10 in
main mode and 7 in aggressive mode of GDOI. We discuss the
advantages of GKDP, the shortcomings and remedies to address those
shortcomings.
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2.2. GKDP usage scenarios
GKDP is a key download protocol. Key download as opposed to key
negotiation has several interesting use cases.
o The first application is multicast security. As with GDOI, the
current version of the GKDP spec limits the scope to single sender
multicast applications.
o The second intended application is point to point data security
associations facilitated by a centralized group key server.
o Others to be listed!
3. GKDP protocol
3.1. Member registration and secure channel establishment
The first of two components in GSA establishment and maintenance is
member registration.
3.1.1. Initial exchange:GSA_INIT_EXCH
The first step in the registration protocol is to establish a secure
channel with the group controller and key server (GCKS). This
exchange is similar to IKE_SA_INIT exchange of IKEv2. The
registering member proposes various combinations of algorithms in
SAi1 to constitute the secure channel, along with a nonce, Ni, and a
DH exponent, KEi. The GCKS has several options:
o In the first, it honors the member's request for registration and
sends the necessary information to complete the DH exchange: it
selects and specifies the parameters of the secure channel, and
includes a nonce Nr, and a public DH value of its own, KEr.
o The second option is for the GCKS to consider if the request for
secure channel establishment is spurious. The GCCKS has no way to
tell except to throttle such requests by making the initiator do
some work before it invests any computing resources. We refer to
this mode as the denial-of-service or DoS protection mode
specified in detail in Section 3.1.1.1 .
o Finally, if none of the proposals are acceptable to the GCKS, it
may reject the initial exchange itself.
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GSA_INIT_EXCH message is as follows:
Member->GCKS: M1: HDR, SAi1, KEi, Ni
GCKS->Member: M2: HDR, SAr1, KEr, Nr, [CERTREQ]
Figure 1: Secure channel establishment
3.1.1.1. DoS protection mode
In typical deployments of multicast or group security services, the
GCKS address is well-known, which allows adversaries to launch a DoS
attack by sending bogus GSA_INIT_EXCH messages. In the normal mode
of operation, the GCKS responds and needs to maintain state
(including storing Messages 1 and 2) corresponding to each exchange
in progress. Notice that this process might result in the GCKS
storing unnecessary state about bogus exchanges. To avoid this
attack, the GCKS may first choose to verify whether the Intiator is
live and responding to and processing GKDP messages.
The GCKS verifies whether a prospective member (or the initiatior of
the key exchange protocol) is live using the following procedure.
The GCKS responds to the Initiator's message, by sending a challenge
- a notify message (see Section 3.3), containing a a random value or
generally referred to as a COOKIE; the GCKS MUST choose the COOKIE
size between 1 and 64 octets. The Intiator is expected to include
the received COOKIE as part of modified Message 1, which we refer to
as "Response Message." (see Figure 2).
The GCKS may choose to store the COOKIE and other relevant additional
information such as Initiator's identity (thus reducing the amount of
state to be stored, but not entirely eliminating it), to verify that
the Initiator indeed used the COOKIE that was sent by the GCKS.
Alternatively, it may generate the COOKIE following a local procedure
(that the Initiator cannot repeat to generate another valid cookie)
to encode the Initiator's identity, Message 1 etc. For instance the
IKEv2 specification suggests the following derivation to generate
cookies:
COOKIE = VersionIDofGCKS-Secret | Hash(Ni | IPi | SPIi | GCKS-secret)
The GCKS may use (TBD) method to expand or truncate the above value
to generate the COOKIE of size (MUST be between 1-64 octets) based on
local policy.
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DoS protection exchange is as follows:
Member->GCKS: M1: HDR(A,0), SAi1, KEi, Ni
GCKS->Member: CM: HDR(A,0), N(COOKIE)
Member->GCKS: RM: HDR(A,0), N(COOKIE), SAi1, KEi, Ni
GCKS->Member: M2: HDR(A,B), SAr1, KEr, Nr, [CERTREQ]
CM: Challenge Message from the GCKS
RM: Challenge-Response Message from the Member
Figure 2: DoS protection mode of GSA_INIT_EXCH
3.1.2. Authenticated exchange:GSA_AUTH_EXCH
The GSA_INIT_EXCH (2 message or 4 message version) establishes an
unauthenticated secure channel between a prospective member and the
GCKS. The next step is for the member to request the GCKS to join a
group; the GCKS evaluates the request and based on the evaluation a)
accept the request and send the corresponding GSA
GSA_AUTH_EXCH message is as follows:
Member->GCKS: M3: HDR, SK{ G-ID, IDi, [ID_CERT,] [ID_CERTREQ,] AUTH,
[IDr,] [GM_CERT,] [GM_CERTREQ,] [POP_I] }
GCKS->Member: M4: HDR, SK{ IDr, [ID_CERT,] AUTH, GSA, [,KD] [,SEQ]
[GCKS_CERT,] [,POP_R]}
Figure 3: Authenticated Exchange
The various payloads in the GSA_AUTH_EXCH messages have the following
purposes:
o G-ID: The group identity payload constructed using the IKEv2
Identification Payload specifies the secure group that M3 wants to
join.
o ID_CERT: The optional ID_CERT payload contains a certificate(s)
asserting the GCKS's or a member's claimed identity as in IDi or
IDr payloads.
o GM_CERT: The optional GM_CERT payload contains a certificate
asserting the group member's authorization to join the group G-ID
as member.
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o GCKS_CERT: The optional GCKS_CERT payload contains a certificate
asserting the GCKS's authorization to serve the role of a group
controller and key server for the group G-ID.
o AUTH: The AUTH payload constitues the "authenticated" portion of
the 4 or 6 message AKE. In other words, the member in M3 and the
GCKS in M4 prove that they are indeed the entities that sent M1
and M2 respectively. A pre-established shared secret or a
certificate (optionally specified in the CERT payload) may be used
for entity authentication.
o POP: Similar to the AUTH payload's use in providing host/entity
authentication, the POP payload is for member/GCKS authorization
to assume their claimed roles. The GM_CERT or GCKS_CERT is used
to sign a block of data, specified below, to constitute the POP
payload.
o GSA: The GSA payload contains the rekey and data security SA
payloads. Note that this SA is not negotiated; the GCKS simply
sends this SA.
o KD: The KD payload contains the secret keys corresponding the
rekey and the data security SAs included in the GSA payload.
o SEQ: The optional SEQ payload MUST be included if the GSA payload
contains a rekey SA. The SEQ payload contains a SEQ number for
replay protection of the rekey messages.
3.1.2.1. Key material computation
The key material computation and the AUTH payload are identical to
that described in the IKEv2 specification.
Key material and registration SA keys are computed as follows:
SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr }
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ), where
prf+ is defined as follows:
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
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Figure 4: Registration SA key material computation
3.1.2.2. Member and GCKS authentication and authorization
GKDP requires mutual authentication between each member and a GCKS,
as well as mutual authorization. First the member and the GCKS
authenticate to each other using pre-shared keys or certificates
prior to establishing a secure channel. M3 and M4 contain AUTH
payloads that essentially protect against man-in-the-middle attacks
against the DH exchange in M1 and M2. The member and the GCKS
construct AUTH payloads by computing an HMAC over or signing a block
of data containing the message M1 or M2 they sent earlier, the other
party's nonce payload, and a prf over own identity. More formally,
the block of data for HMAC or signature is as follows:
Auth payload computation:
Auth payload in M3 is computed over:
auth-block-M3: M1 || Nr-Payload || prf(SK_pi, IDi-Payload)
Auth payload in M4 is computed over:
auth-block-M4: M2 || Ni-Payload || prf(SK_pr, IDr-Payload)
For shared secret based host authentication AUTH payload is
computed as follows:
AUTH = prf(prf(Shared Secret,"KeyPad:GKDP-AUTH-MX"),
<auth-block-MX>)
Figure 5: Auth payload computation
3.1.2.2.1. Use of asymmetric authentication methods
GKDP also allows the member and the GCKS to use different
authentication methods, similar to TLS and IKEv2. More specifically,
the GCKS uses a cert to authenticate itself and establish a secure
channel, and the member uses EAP to send its authentication
information via the secure channel.
Members may also use EAP to prove their authorization to join a
secure group. For instance, consider a use case where a member may
use a SIM card for authentication, or a pre-paid SIM card to pay for
content distributed to a secure group. In these cases, the
authentication or authorization information can be sent via EAP.
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3.1.2.2.2. Proof of possession
Proof of possession payload (POP) provides a mechanism so that
members and/or GCKS can prove to the other party that they are indeed
authorized to be a member or the GCKS, respectively. For POP payload
derivation in GKDP, the member or the GCKS first constructs a message
block, POP-HASH, containing the two nonces exchanged in GSA_INIT_EXCH
and the prf over the ID payload as defined in the AUTH payload
construction. Next, the member or the GCKS signs the POP-HASH value.
POP-HASH construction is as follows:
POP payload :
POP payload in M3 is constructed over the following message block:
POP-HASH-M3: "KeyPad:GKDP-POP-M3" ||
Ni-Payload || Nr-Payload || prf(SK_pi, IDi-Payload)
POP payload in M4 is computed over:
POP-HASH-M4: "KeyPad:GKDP-POP-M4" ||
Ni-Payload || Nr-Payload || prf(SK_pr, IDr-Payload)
Figure 6: POP payload computation block
3.2. GSA maintenance channel
3.2.1. GSA rekey protocol
GSA rekey protocol is optional to implement, but it plays a crucial
role for large and dynamic groups.
The GCKS is responsible for rekeying of the secure group as per the
group policy. The GCKS uses multicast or multi-unicast to transport
the rekey message. When multi-unicast is used, it may be appropriate
in some scenarios to have a reply message from the member(s) to the
GCKS. The reply message is optional.
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Rekey message is as follows:
Multicast:
GCKS->Member: HDR, SK {[N], SEQ, GSA, KD, [GCKS_CERT,] SIG}
Unicast:
GCKS->Member: HDR, SK {N, SEQ, GSA, KD, [GCKS_CERT,] SIG}
[Member->GCKS]: [HDR, SK {N, SEQ, AUTH}]
Figure 7: Rekey message
3.3. Informational exchange
3.3.1. Notify exchange
3.3.2. Error message
4. GKDP protocol design details
5. Header and payload formats
GKDP payload design is based on IKEv2 payloads, to allow reuse of the
IKEv2 payload processing code. Furthermore, we draw on the GDOI
design specified in RFC3547, where possible and appropriate to avoid
reinvention.
5.1. GKDP header
GKDP messages use UDP ports GKDP-PORT and GKDP-NAT-PORT (TBA-IANA),
with one GKDP message per datagram. The source and destination IP
addresses from the IP header are used with role reversal to send the
response messages. GKDP messages sent/received on UDP port GKDP-PORT
follow the format of a UDP header followed by a GDKP header. GKDP
messages sent/received on UDP port GKDP-NAT-PORT have four octets of
zero immediately following the UDP header; the GKDP header follows
the zeros. The zeros are not part of part of the GKDP message and
therefore not part of the payload length fields. All GKDP messages
begin with the GKDP header.
Following the GKDP header -denoted by HDR in GKDP messages - are one
or more GKDP payloads each identified by a "Next Payload" field in
the preceding payload. Payloads are processed in the order in which
they appear in an GKDP message by invoking the appropriate processing
routine according to the "Next Payload" field in the IKE header and
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subsequently according to the "Next Payload" field in the IKE payload
itself until a "Next Payload" field of zero indicates that no
payloads follow. If a payload of type "Encrypted" is found, that
payload is decrypted and its contents parsed as additional payloads.
An Encrypted payload MUST be the last payload in a packet and an
encrypted payload MUST NOT contain another encrypted payload.
IPsecbis multicast group address or the destination address in the IP
header and the Recipient SPI in the GKDP header identifies an
instance of an GKDP security association.
The format of the GKDP header is shown in Figure Figure 8:
1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! GKDP Initiator's SPI !
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! GKDP Responder's SPI !
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! MjVer ! MnVer ! Exchange Type ! Flags !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Message ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: GKDP Header Format
6. Security considerations
TBD
7. IANA Considerations
TBD
8. Acknowledgments
GKDP is based on IKEv2 and GDOI. Several sections of this document
are quite identical to IKEv2 and GDOI specifications; in some cases
the text may be identical to the text in those specifications. We
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included the text for completeness of this specification. We
appreciate the efforts of the contributors and editors of those
protocols.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[I-D.ietf-ipsec-ikev2]
Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress),
October 2004.
9.2. Informative References
[RFC1949] Ballardie, T., "Scalable Multicast Key Distribution",
RFC 1949, May 1996.
[RFC2093] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Specification", RFC 2093, July 1997.
[RFC2408] Maughan, D., Schneider, M., and M. Schertler, "Internet
Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[I-D.ipsec-rfc2401bis]
"Security Architecture for the Internet Protocol",
draft-ipsec-rfc2401bis-00 (work in progress),
October 2003.
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Authors' Addresses
Lakshminath Dondeti
QUALCOMM
5775 Morehouse Drive
San Diego, CA 92121
US
Phone: +1 858 845 1267
Email: ldondeti@qualcomm.com
Jing Xiang
Nortel Networks
600 Technology Park drive
Billerica, MA 01821
US
Phone: +1 978 288 8985
Email: jxiang@nortel.com
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