Internet Engineering Task Force - MSEC WG
Internet Draft M. Euchner
Intended Category: Proposed Standard
Expires: October 2005 April 2005
HMAC-authenticated Diffie-Hellman for MIKEY
<draft-ietf-msec-mikey-dhhmac-11.txt>
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Abstract
This document describes a light-weight point-to-point key management
protocol variant for the multimedia Internet keying (MIKEY) protocol
MIKEY, as defined in RFC 3830. In particular, this variant deploys
the classic Diffie-Hellman key agreement protocol for key
establishment featuring perfect forward secrecy in conjunction with
a keyed hash message authentication code for achieving mutual
authentication and message integrity of the key management messages
exchanged. This protocol addresses the security and performance
constraints of multimedia key management in MIKEY.
Conventions used in this document
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 RFC-2119 [2].
Table of Contents
1. Introduction................................................3
1.1. Definitions...............................................6
1.2. Abbreviations.............................................7
2. Scenario....................................................8
2.1. Applicability.............................................9
2.2. Relation to GKMARCH.......................................9
3. DHHMAC Security Protocol...................................10
3.1. TGK re-keying............................................12
4. DHHMAC payload formats.....................................13
4.1. Common header payload (HDR)..............................13
4.2. Key data transport payload (KEMAC).......................14
4.3. ID payload (ID)..........................................15
4.4. General Extension Payload................................15
5. Security Considerations....................................16
5.1. Security environment.....................................16
5.2. Threat model.............................................16
5.3. Security features and properties.........................19
5.4. Assumptions..............................................23
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5.5. Residual risk............................................24
5.6. Authorization and Trust Model............................26
6. Acknowledgments............................................26
7. IANA considerations........................................26
8. References.................................................27
8.1 Normative References.......................................27
8.2 Informative References...................................27
Appendix A Usage of MIKEY-DHHMAC in H.235......................30
Full Copyright Statement........................................33
Expiration Date.................................................34
Revision History................................................34
Author's Addresses..............................................37
1.
Introduction
There is work done in IETF to develop key management schemes. For
example, IKE [14] is a widely accepted unicast scheme for IPsec, and
the MSEC WG is developing other schemes, addressed to group
communication [24], [25]. For reasons discussed below, there is
however a need for a scheme with low latency, suitable for demanding
cases such as real-time data over heterogeneous networks, and small
interactive groups.
As pointed out in MIKEY (see [3]), secure real-time multimedia
applications demand a particular adequate light-weight key management
scheme that cares for how to securely and efficiently establish
dynamic session keys in a conversational multimedia scenario.
In general, MIKEY scenarios cover peer-to-peer, simple-one-to-many
and small-sized groups. MIKEY in particular, describes three key
management schemes for the peer-to-peer case that all finish their
task within one round trip:
- a symmetric key distribution protocol (MIKEY-PS) based upon
pre-shared master keys;
- a public-key encryption-based key distribution protocol
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(MIKEY-PK) assuming a public-key infrastructure with RSA-based
(Rivest, Shamir and Adleman) private/public keys and digital
certificates;
- and a Diffie-Hellman key agreement protocol (MIKEY-DHSIGN)
deploying digital signatures and certificates.
All these three key management protocols are designed such that they
complete their work within just one round trip. This requires
depending on loosely synchronized clocks and deploying timestamps
within the key management protocols.
However, it is known [7] that each of the three key management
schemes has its subtle constraints and limitations:
- The symmetric key distribution protocol (MIKEY-PS) is simple
to implement, however, was not intended to scale to support
any configurations beyond peer-to-peer, simple one-to-many,
and small-size (interactive) groups, due to the need of
mutually pre-assigned shared master secrets.
Moreover, the security provided does not achieve the property
of perfect forward secrecy; i.e. compromise of the shared
master secret would render past and even future session keys
susceptible to compromise.
Further, the generation of the session key happens just at the
initiator. Thus, the responder has to fully trust the
initiator on choosing a good and secure session secret; the
responder neither is able to participate in the key generation
nor to influence that process. This is considered as a
specific limitation in less trusted environments.
- The public-key encryption scheme (MIKEY-PK) depends upon a
public-key infrastructure that certifies the private-public
keys by issuing and maintaining digital certificates. While
such a key management scheme provides full scalability in
large networked configurations, public-key infrastructures are
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still not widely available and in general, implementations are
significantly more complex.
Further, additional round trips and computational processing
might be necessary for each end system in order to ascertain
verification of the digital certificates. For example,
typical operations in the context of a public-key
infrastructure such as validating digital certificates (RFC
3029, [31]), ascertaining the revocation status of digital
certificates (RFC 2560, [30]) and asserting certificate
policies, construction of certification path(s) ([33]),
requesting and obtaining necessary certificates (RFC 2511,
[32]) and management of certificates for such purposes ([29])
may involve extra network communication handshakes with the
public-key infrastructure and with certification authorities
and may typically involve additional processing steps in the
end systems. Such steps and tasks all result in further delay
of the key agreement or key establishment phase among the end
systems, negatively impacting setup time. Any extra PKI
handshakes and processing are not in scope of MIKEY and since
this document deploys symmetric security mechanisms only,
aspects of PKI, digital certificates and related processing
are not further covered in this document.
Finally, as in the symmetric case, the responder depends
completely upon the initiator choosing good and secure session
keys.
- The third MIKEY-DHSIGN key management protocol deploys the
Diffie-Hellman key agreement scheme and authenticates the
exchange of the Diffie-Hellman half-keys in each direction by
using a digital signature. This approach has the same
advantages and deficiencies as described in the previous
section in terms of a public-key infrastructure.
However, the Diffie-Hellman key agreement protocol is known
for its subtle security strengths in that it is able to
provide full perfect forward secrecy (PFS) and further have
both parties actively involved in session key generation.
This special security property - despite the somewhat higher
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computational costs - makes Diffie-Hellman techniques
attractive in practice.
In order to overcome some of the limitations as outlined above, a
special need has been recognized for another efficient key agreement
protocol variant in MIKEY. This protocol variant aims to provide the
capability of perfect forward secrecy as part of a key agreement with
low latency without dependency on a public-key infrastructure.
This document describes such a fourth light-weight key management
scheme for MIKEY that could somehow be seen as a synergetic
optimization between the pre-shared key distribution scheme and the
Diffie-Hellman key agreement.
The idea of the protocol in this document is to apply the Diffie-
Hellman key agreement, but rather than deploying a digital signature
for authenticity of the exchanged keying material, instead uses a
keyed-hash upon using symmetrically pre-assigned shared secrets.
This combination of security mechanisms is called the HMAC-
authenticated Diffie-Hellman (DH) key agreement for MIKEY (DHHMAC).
The DHHMAC variant closely follows the design and philosophy of MIKEY
and reuses MIKEY protocol payload components and MIKEY mechanisms to
its maximum benefit and for best compatibility.
Like the MIKEY Diffie-Hellman protocol, DHHMAC does not scale beyond
a point-to-point constellation; thus, both MIKEY Diffie-Hellman
protocols do not support group-based keying for any group size larger
than two entities.
1.1. Definitions
The definitions and notations in this document are aligned with
MIKEY, see [3], sections 1.3 - 1.4.
All large integer computations in this document should be understood
as being mod p within some fixed group G for some large prime p; see
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[3] section 3.3; however, the DHHMAC protocol is applicable in
general to other appropriate finite, cyclical groups as well.
It is assumed that a pre-shared key s is known by both entities
(initiator and responder). The authentication key auth_key is
derived from the pre-shared secret s using the pseudo-random function
PRF; see [3] sections 4.1.3 and 4.1.5.
In this text, [X] represents an optional piece of information.
Generally throughout the text, X SHOULD be present unless certain
circumstance MAY allow X being optional and not be present thereby
resulting in weaker security potentially. Likewise [X, Y] represents
an optional compound piece of information where the pieces X and Y
SHOULD be either both present or MAY optionally be both absent. {X}
denotes zero or more occurrences of X.
1.2. Abbreviations
auth_key pre-shared authentication key, PRF-derived from
pre-shared key s.
DH Diffie-Hellman
DHi public Diffie-Hellman half key g^(xi) of the
Initiator
DHr public Diffie-Hellman half key g^(xr) of the
Responder
DHHMAC HMAC-authenticated Diffie-Hellman
DoS Denial-of-service
G Diffie-Hellman group
HDR MIKEY common header payload
HMAC keyed Hash Message Authentication Code
HMAC-SHA1 HMAC using SHA1 as hash function (160-bit result)
IDi Identity of initiator
IDr Identity of receiver
IKE Internet Key Exchange
IPsec Internet Protocol Security
MIKEY Multimedia Internet KEYing
MIKEY-DHHMAC MIKEY Diffie-Hellman key management protocol using
HMAC
MIKEY-DHSIGN MIKEY Diffie-Hellman key agreement protocol
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MIKEY-PK MIKEY public-key encryption-based key distribution
protocol
MIKEY-PS MIKEY pre-shared key distribution protocol
p Diffie-Hellman prime modulus
PKI Public-key Infrastructure
PRF MIKEY pseudo-random function (see [3] section
4.1.3.)
RSA Rivest, Shamir and Adleman
s pre-shared key
SDP Session Description Protocol
SOI Son-of-IKE, IKEv2
SP MIKEY Security Policy (Parameter) Payload
T timestamp
TEK Traffic Encryption Key
TGK MIKEY TEK Generation Key as the common Diffie-
Hellman shared secret
TLS Transport Layer Security
xi secret, (pseudo) random Diffie-Hellman key of the
Initiator
xr secret, (pseudo) random Diffie-Hellman key of the
Responder
2.
Scenario
The HMAC-authenticated Diffie-Hellman key agreement protocol (DHHMAC)
for MIKEY addresses the same scenarios and scope as the other three
key management schemes in MIKEY address.
DHHMAC is applicable in a peer-to-peer group where no access to a
public-key infrastructure can be assumed available. Rather, pre-
shared master secrets are assumed available among the entities in
such an environment.
In a pair-wise group, it is assumed that each client will be setting
up a session key for its outgoing links with its peer using the DH-
MAC key agreement protocol.
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As is the case for the other three MIKEY key management protocols,
DHHMAC assumes, at least, loosely synchronized clocks among the
entities in the small group.
To synchronize the clocks in a secure manner, some operational or
procedural means are recommended. MIKEY-DHHMAC does not define any
secure time synchronization measures, however, sections 5.4 and 9.3
of [3] provide implementation guidance on clock synchronization and
timestamps.
2.1. Applicability
MIKEY-DHHMAC, as well as the other MIKEY key management protocols, is
intended for application-level key management and is optimized for
multimedia applications with real-time session setup and session
management constraints.
As the MIKEY-DHHMAC key management protocol terminates in one
roundtrip, DHHMAC is applicable for integration into two-way
handshake session- or call signaling protocols such as
a) SIP/SDP where the encoded MIKEY messages are encapsulated and
transported in SDP containers of the SDP offer/answer [RFC 3264]
handshake as described in [5],
b) H.323 (see [22]) where the encoded MIKEY messages are transported
in the H.225.0 fast start call signaling handshake. Appendix A
outlines the usage of MIKEY-DHHMAC within H.235.
MIKEY-DHHMAC is offered as option to the other MIKEY key management
variants (MIKEY-pre-shared, MIKEY-public-key and MIKEY-DH-SIGN) for
all those cases where DHHMAC has its particular strengths (see
section 5).
2.2. Relation to GKMARCH
The Group key management architecture (GKMARCH) [26] describes a
generic architecture for multicast security group key management
protocols. In the context of this architecture, MIKEY-DHHMAC may
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operate as a registration protocol, see also [3] section 2.4. The
main entities involved in the architecture are a group
controller/key server (GCKS), the receiver(s), and the sender(s).
Due to the pair-wise nature of the Diffie-Hellman operation and
the 1-roundtrip constraint, usage of MIKEY-DHHMAC rules out any
deployment as a group key management protocol with more than two
group entities. Only the degenerate case with two peers is
possible where for example the responder acts as the group
controller.
Note that MIKEY does not provide re-keying in the GKMARCH sense,
only updating of the keys by normal unicast messages.
3.
DHHMAC Security Protocol
The following figure defines the security protocol for DHHMAC:
Initiator Responder
I_message = HDR, T, RAND, [IDi], IDr,
{SP}, DHi, KEMAC
-----------------------> R_message = HDR, T,
[IDr], IDi, DHr,
DHi, KEMAC
<----------------------
Figure 1: HMAC-authenticated Diffie-Hellman key based exchange,
where xi and xr are (pseudo) randomly chosen respectively
by the initiator and the responder.
The DHHMAC key exchange SHALL be done according to Figure 1. The
initiator chooses a (pseudo) random value xi, and sends an HMACed
message including g^(xi) and a timestamp to the responder. It is
recommended that the initiator SHOULD always include the identity
payloads IDi and IDr within the I_message; unless the receiver can
defer the initiator's identity by some other means, then IDi MAY
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optionally be omitted. The initiator SHALL always include the
recipient's identity.
The group parameters (e.g., the group G) are a set of parameters
chosen by the initiator. Note, that like in the MIKEY protocol,
both sender and receiver explicitly transmit the Diffie-Hellman
group G within the Diffie-Hellman payload DHi or DHr through an
encoding (e.g., OAKLEY group numbering, see [3] section 6.4); the
actual group parameters g and p however are not explicitly
transmitted but can be deduced from the Diffie-Hellman group G.
The responder chooses a (pseudo) random positive integer xr, and
sends an HMACed message including g^(xr) and the timestamp to the
initiator. The responder SHALL always include the initiator's
identity IDi regardless of whether the I_message conveyed any IDi.
It is RECOMMENDED that the responder SHOULD always include the
identity payload IDr within the R_message; unless the initiator
can defer the responder's identity by some other means, then IDr
MAY optionally be left out.
Both parties then calculate the TGK as g^(xi * xr).
The HMAC authentication provides authentication of the DH half-
keys, and is necessary to avoid man-in-the-middle attacks.
This approach is less expensive than digitally signed Diffie-
Hellman in that both sides compute first one exponentiation and
one HMAC, then one HMAC verification and finally another Diffie-
Hellman exponentiation.
With off-line pre-computation, the initial Diffie-Hellman half-key
MAY be computed before the key management transaction and thereby
MAY further reduce the overall round trip delay as well as reduce
the risk of denial-of-service attacks.
Processing of the TGK SHALL be accomplished as described in MIKEY
[3] chapter 4.
The computed HMAC result SHALL be conveyed in the KEMAC payload
field where the MAC fields holds the HMAC result. The HMAC SHALL
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be computed over the entire message excluding the MAC field using
auth_key, see also section 4.2.
3.1. TGK re-keying
TGK re-keying for DHHMAC generally proceeds as described in [3]
section 4.5. Specifically, figure 2 provides the message exchange
for the DHHMAC update message.
Initiator Responder
I_message = HDR, T, [IDi], IDr,
{SP}, [DHi], KEMAC
-----------------------> R_message = HDR, T,
[IDr], IDi,
[DHr, DHi], KEMAC
<----------------------
Figure 2: DHHMAC update message
TGK re-keying supports two procedures:
a) True re-keying by exchanging new and fresh Diffie-Hellman half-
keys. For this, the initiator SHALL provide a new, fresh DHi
and the responder SHALL respond with a new, fresh DHr and the
received DHi.
b) Non-key related information update without any Diffie-Hellman
half-keys included in the exchange. Such transaction does not
change the actual TGK but updates other information like
security policy parameters for example. To only update the
non-key related information, [DHi] and [DHr, DHi] SHALL be left
out.
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4.
DHHMAC payload formats
This section specifies the payload formats and data type values for
DHHMAC, see also [3] chapter 6 for a definition of the MIKEY
payloads.
This document does not define new payload formats but re-uses MIKEY
payloads for DHHMAC as referenced:
* Common header payload (HDR), see section 4.1 and [3] section 6.1
* SRTP ID sub-payload, see [3] section 6.1.1,
* Key data transport payload (KEMAC), see section 4.2 and [3] section
6.2
* DH data payload, see [3] section 6.4
* Timestamp payload, [3] section 6.6
* ID payload, [3] section 6.7
* Security Policy payload (SP), [3] section 6.10
* RAND payload (RAND), [3] section 6.11
* Error payload (ERR), [3] section 6.12
* General Extension Payload, [3] section 6.15
4.1. Common header payload (HDR)
Referring to [3] section 6.1, for DHHMAC the following data types
SHALL be used:
Data type | Value | Comment
-------------------------------------------------------------
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DHHMAC init | 7 | Initiator's DHHMAC exchange message
DHHMAC resp | 8 | Responder's DHHMAC exchange message
Error | 6 | Error message, see [3] section 6.12
Table 4.1.a
Note: A responder is able to recognize the MIKEY DHHMAC protocol
by evaluating the data type field as 7 or 8. This is how the
responder can differentiate between MIKEY and MIKEY DHHMAC.
The next payload field SHALL be one of the following values:
Next payload| Value | Section
----------------------------------------------------------------
Last payload| 0 | -
KEMAC | 1 | section 4.2 and [3] section 6.2
DH | 3 | [3] section 6.4
T | 5 | [3] section 6.6
ID | 6 | [3] section 6.7
SP | 10 | [3] section 6.10
RAND | 11 | [3] section 6.11
ERR | 12 | [3] section 6.12
General Ext.| 21 | [3] section 6.15
Table 4.1.b
Other defined next payload values defined in [3] SHALL not be
applied to DHHMAC.
The responder in case of a decoding error or of a failed HMAC
authentication verification SHALL apply the Error payload data
type.
4.2. Key data transport payload (KEMAC)
DHHMAC SHALL apply this payload for conveying the HMAC result
along with the indicated authentication algorithm. KEMAC when used
in conjunction with DHHMAC SHALL not convey any encrypted data;
thus Encr alg SHALL be set to 2 (NULL), Encr data len SHALL be set
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to 0 and Encr data SHALL be left empty. The AES key wrap method
(see [23]) SHALL not be applied for DHHMAC.
For DHHMAC, this key data transport payload SHALL be the last
payload in the message. Note that the Next payload field SHALL be
set to Last payload. The HMAC is then calculated over the entire
MIKEY message excluding the MAC field using auth_key as described
in [3] section 5.2 and then stored within the MAC field.
MAC alg | Value | Comments
------------------------------------------------------------------
HMAC-SHA-1 | 0 | Mandatory, Default (see [4])
NULL | 1 | Very restricted use, see
| [3] section 4.2.4
Table 4.2.a
HMAC-SHA-1 is the default hash function that MUST be implemented
as part of the DHHMAC. The length of the HMAC-SHA-1 result is 160
bits.
4.3. ID payload (ID)
For DHHMAC, this payload SHALL only hold a non-certificate based
identity.
4.4. General Extension Payload
For DHHMAC and to avoid bidding-down attacks, this payload SHALL
list all key management protocol identifiers of a surrounding
encapsulation protocol such as for example, SDP [5]. The General
Extension Payload SHALL be integrity-protected with the HMAC using
the shared secret.
Type | Value | Comments
SDP IDs | 1 | List of SDP key management IDs (allocated for
use in [5]); see also [3] section 6.15.
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Table 4.4.a
5.
Security Considerations
This document addresses key management security issues throughout.
For a comprehensive explanation of MIKEY security considerations,
please refer to MIKEY [3] section 9.
In addition to that, this document addresses security issues
according to [8] where the following security considerations apply in
particular to this document:
5.1. Security environment
Generally, the DHHMAC security protocol described in this document
focuses primarily on communication security; i.e. the security issues
concerned with the MIKEY DHHMAC protocol. Nevertheless, some system
security issues are of interest as well that are not explicitly
defined by the DHHMAC protocol, but should be provided locally in
practice.
The system that runs the DHHMAC protocol entity SHALL provide the
capability to generate (pseudo) random numbers as input to the
Diffie-Hellman operation (see [9], [15]). Furthermore, the system
SHALL be capable of storing the generated (pseudo) random data,
secret data, keys and other secret security parameters securely (i.e.
confidential and safe from unauthorized tampering).
5.2. Threat model
The threat model, to which this document adheres, covers the issues
of end-to-end security in the Internet generally, without ruling out
the possibility that MIKEY DHHMAC can be deployed in a corporate,
closed IP environment. This also includes the possibility that MIKEY
DHHMAC can be deployed on a hop-by-hop basis with some intermediate
trusted "MIKEY DHHMAC proxies" involved.
Since DHHMAC is a key management protocol, the following security
threats are of concern:
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* Unauthorized interception of plain TGKs:
For DHHMAC this threat does not occur since the TGK is not actually
transmitted on the wire (not even in encrypted fashion).
* Eavesdropping of other, transmitted keying information:
DHHMAC protocol does not explicitly transmit the TGK at all.
Instead, by using the Diffie-Hellman "encryption" operation, which
conceals the secret (pseudo) random values, only partial
information (i.e. the DH- half key) for construction of the TGK is
transmitted. It is fundamentally assumed that availability of such
Diffie-Hellman half-keys to an eavesdropper does not result in any
substantial security risk; see 5.4. Furthermore, the DHHMAC
carries other data such as timestamps, (pseudo) random values,
identification information or security policy parameters;
eavesdropping of any such data is considered not to yield any
significant security risk.
* Masquerade of either entity:
This security threat must be avoided and if a masquerade attack
would be attempted, appropriate detection means must be in place.
DHHMAC addresses this threat by providing mutual peer entity
authentication.
* Man-in-the-middle attacks:
Such attacks threaten the security of exchanged, non-authenticated
messages. Man-in-the-middle attacks usually come with masquerade
and or loss of message integrity (see below). Man-in-the-middle
attacks must be avoided, and if present or attempted must be
detected appropriately. DHHMAC addresses this threat by providing
mutual peer entity authentication and message integrity.
* Loss of integrity:
This security threat relates to unauthorized replay, deletion,
insertion and manipulation of messages. While any such attacks
cannot be avoided they must be detected at least. DHHMAC addresses
this threat by providing message integrity.
* Bidding-down attacks:
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When multiple key management protocols each of a distinct security
level are offered (e.g., such as is possible by SDP [5]), avoiding
bidding-down attacks is of concern. DHHMAC addresses this threat
by reusing the MIKEY General Extension Payload mechanism, where
all key management protocol identifiers are be listed within the
MIKEY General Extension Payload.
Some potential threats are not within the scope of this threat model:
* Passive and off-line cryptanalysis of the Diffie-Hellman algorithm:
Under certain reasonable assumptions (see 5.4 below) it is widely
believed that DHHMAC is sufficiently secure and that such attacks
are infeasible, although the possibility of a successful attack
cannot be ruled out.
* Non-repudiation of the receipt or of the origin of the message:
These are not requirements within the context of DHHMAC in this
environment and thus related countermeasures are not provided at
all.
* Denial-of-service or distributed denial-of-service attacks:
Some considerations are given on some of those attacks, but DHHMAC
does not claim to provide full countermeasure against any of those
attacks. For example, stressing the availability of the entities
are not thwarted by means of the key management protocol; some
other local countermeasures should be applied. Further, some DoS
attacks are not countered such as interception of a valid DH-
request and its massive instant duplication. Such attacks might at
least be countered partially by some local means that are outside
the scope of this document.
* Identity protection:
Like MIKEY, identity protection is not a major design requirement
for MIKEY-DHHMAC either, see [3]. No security protocol is known so
far, that is able to provide the objectives of DHHMAC as stated in
section 5.3 including identity protection within just a single
roundtrip. MIKEY-DHHMAC trades identity protection for better
security for the keying material and shorter roundtrip time. Thus,
MIKEY-DHHMAC does not provide identity protection on its own but
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may inherit such property from a security protocol underneath that
actually features identity protection.
The DHHMAC security protocol (see section 3) and the TGK re-keying
security protocol (see section 3.1) provide the option not to
supply identity information. This option is only applicable if
some other means are available of supplying trustworthy identity
information; e.g., by relying on secured links underneath of MIKEY
that supply trustworthy identity information otherwise. However,
it is understood that without identity information present, the
MIKEY key management security protocols might be subject to
security weaknesses such as masquerade, impersonation and
reflection attacks particularly in end-to-end scenarios where no
other secure means of assured identity information is provided.
Leaving identity fields optional if possible thus should not be
seen as a privacy method either, but rather as a protocol
optimization feature.
5.3. Security features and properties
With the security threats in mind, this draft provides the following
security features and yields the following properties:
* Secure key agreement with the establishment of a TGK at both peers:
This is achieved using an authenticated Diffie-Hellman key
management protocol.
* Peer-entity authentication (mutual):
This authentication corroborates that the host/user is authentic in
that possession of a pre-assigned secret key is proven using keyed
HMAC. Authentication occurs on the request and on the response
message, thus authentication is mutual.
The HMAC computation corroborates for authentication and message
integrity of the exchanged Diffie-Hellman half-keys and associated
messages. The authentication is absolutely necessary in order to
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avoid man-in-the-middle attacks on the exchanged messages in
transit and in particular, on the otherwise non-authenticated
exchanged Diffie-Hellman half keys.
Note: This document does not address issues regarding
authorization; this feature is not provided explicitly. However,
DHHMAC authentication means support and facilitate realization of
authorization means (local issue).
* Cryptographic integrity check:
The cryptographic integrity check is achieved using a message
digest (keyed HMAC). It includes the exchanged Diffie-Hellman
half-keys but covers the other parts of the exchanged message as
well. Both mutual peer entity authentication and message integrity
provide effective countermeasures against man-in-the-middle
attacks.
The initiator may deploy a local timer that fires when the awaited
response message did not arrive in a timely manner. This is to
detect deletion of entire messages.
* Replay protection of the messages is achieved using embedded
timestamps. In order to detect replayed messages it is essential
that the clocks among initiator and sender be roughly synchronized.
The reader is referred to [3] section 5.4 and [3] section 9.3 that
provide further considerations and give guidance on clock
synchronization and timestamp usage. Should the clock
synchronization be lost, then end systems cannot detect replayed
messages anymore resulting that the end systems cannot securely
establish keying material. This may result in a denial-of-service,
see [3] section 9.5.
* Limited DoS protection:
Rapid checking of the message digest allows verifying the
authenticity and integrity of a message before launching CPU
intensive Diffie-Hellman operations or starting other resource
consuming tasks. This protects against some denial-of-service
attacks: malicious modification of messages and spam attacks with
(replayed or masqueraded) messages. DHHMAC probably does not
explicitly counter sophisticated distributed, large-scale denial-
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of-service attacks that compromise system availability for example.
Some DoS protection is provided by inclusion of the initiator's
identity payload in the I_message. This allows the recipient to
filter out those (replayed) I_messages that are not targeted for
him and avoids the recipient from creating unnecessary MIKEY
sessions.
* Perfect-forward secrecy (PFS):
Other than the MIKEY pre-shared and public-key based key
distribution protocols, the Diffie-Hellman key agreement protocol
features a security property called perfect forward secrecy. That
is, that even if the long-term pre-shared key would be compromised
at some point in time, this would not render past or future session
keys compromised.
Neither the MIKEY pre-shared nor the MIKEY public-key protocol
variants are able to provide the security property of perfect-
forward secrecy. Thus, none of the other MIKEY protocols is able
to substitute the Diffie-Hellman PFS property.
As such, DHHMAC, as well as digitally signed DH, provides a far
superior security level over the pre-shared or public-key based key
distribution protocol in that respect.
* Fair, mutual key contribution:
The Diffie-Hellman key management protocol is not a strict key
distribution protocol per se with the initiator distributing a key
to its peers. Actually, both parties involved in the protocol
exchange are able to equally contribute to the common Diffie-
Hellman TEK traffic generating key. This reduces the risk of
either party cheating or unintentionally generating a weak session
key. This makes the DHHMAC a fair key agreement protocol. One may
view this property as an additional distributed security measure
that is increasing security robustness over the case where all the
security depends just on the proper implementation of a single
entity.
In order for Diffie-Hellman key agreement to be secure, each party
SHALL generate its xi or xr values using a strong, unpredictable
pseudo-random generator if a source of true randomness is not
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available. Further, these values xi or xr SHALL be kept private.
It is RECOMMENDED that these secret values be destroyed once the
common Diffie-Hellman shared secret key has been established.
* Efficiency and performance:
Like the MIKEY-public key protocol, the MIKEY DHHMAC key agreement
protocol securely establishes a TGK within just one roundtrip.
Other existing key management techniques like IPsec-IKE [14],
IPsec-IKEv2 [21] and TLS [13] and other schemes are not deemed
adequate in addressing sufficiently those real-time and security
requirements; they all use more than a single roundtrip. All the
MIKEY key management protocols are able to complete their task of
security policy parameter negotiation including key-agreement or
key distribution in one roundtrip. However, the MIKEY pre-shared
and the MIKEY public-key protocol both are able to complete their
task even in a half-round trip when the confirmation messages are
omitted.
Using HMAC in conjunction with a strong one-way hash function such
as SHA1 may be achieved more efficiently in software than expensive
public-key operations. This yields a particular performance
benefit of DHHMAC over signed DH or the public-key encryption
protocol.
If a very high security level is desired for long-term secrecy of
the negotiated Diffie-Hellman shared secret, longer hash values may
be deployed such as SHA256, SHA384 or SHA512 provide, possibly in
conjunction with stronger Diffie-Hellman groups. This is left as
for further study.
For the sake of improved performance and reduced round trip delay
either party may off-line pre-compute its public Diffie-Hellman
half-key.
On the other side and under reasonable conditions, DHHMAC consumes
more CPU cycles than the MIKEY pre-shared key distribution
protocol. The same might hold true quite likely for the MIKEY
public-key distribution protocol (depending on choice of the
private and public key lengths).
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As such, it can be said that DHHMAC provides sound performance when
compared with the other MIKEY protocol variants.
The use of optional identity information (with the constraints
stated in section 5.2) and optional Diffie-Hellman half-key fields
provides a means to increase performance and shorten the consumed
network bandwidth.
* Security infrastructure:
This document describes the HMAC-authenticated Diffie-Hellman key
agreement protocol that completely avoids digital signatures and
the associated public-key infrastructure as would be necessary for
the X.509 RSA public-key based key distribution protocol or the
digitally signed Diffie-Hellman key agreement protocol as described
in MIKEY. Public-key infrastructures may not always be available
in certain environments nor may they be deemed adequate for real-
time multimedia applications when taking additional steps for
certificate validation and certificate revocation methods with
additional round-trips into account.
DHHMAC does not depend on PKI nor do implementations require PKI
standards and thus is believed to be much simpler than the more
complex PKI facilities.
DHHMAC is particularly attractive in those environments where
provisioning of a pre-shared key has already been accomplished.
* NAT-friendliness:
DHHMAC is able to operate smoothly through firewall/NAT devices as
long as the protected identity information of the end entity is not
an IP /transport address.
* Scalability:
Like the MIKEY signed Diffie-Hellman protocol, DHHMAC does not
scale to any larger configurations beyond peer-to-peer groups.
5.4. Assumptions
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This document states a couple of assumptions upon which the security
of DHHMAC significantly depends. It is assumed, that
* the parameters xi, xr, s and auth_key are to be kept secret.
* the pre-shared key s has sufficient entropy and cannot be
effectively guessed.
* the pseudo-random function (PRF) is secure, yields indeed the
pseudo-random property and maintains the entropy.
* a sufficiently large and secure Diffie-Hellman group is applied.
* the Diffie-Hellman assumption holds saying basically that even with
knowledge of the exchanged Diffie-Hellman half-keys and knowledge
of the Diffie-Hellman group, it is infeasible to compute the TGK or
to derive the secret parameters xi or xr. The latter is also
called the discrete logarithm assumption. Please see [7], [11] or
[12] for more background information regarding the Diffie-Hellman
problem and its computational complexity assumptions.
* the hash function (SHA1) is secure; i.e. that it is computationally
infeasible to find a message which corresponds to a given message
digest, or to find two different messages that produce the same
message digest.
* the HMAC algorithm is secure and does not leak the auth_key. In
particular, the security depends on the message authentication
property of the compression function of the hash function H when
applied to single blocks (see [6]).
* a source capable of producing sufficiently many bits of (pseudo)
randomness is available.
* the system upon which DHHMAC runs is sufficiently secure.
5.5. Residual risk
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Although these detailed assumptions are non-negligible, security
experts generally believe that all these assumptions are reasonable
and that the assumptions made can be fulfilled in practice with
little or no expenses.
The mathematical and cryptographic assumptions of the properties of
the PRF, the Diffie-Hellman algorithm (discrete log-assumption), the
HMAC algorithm and SHA1 algorithms have been neither proven or
disproven at this time.
Thus, a certain residual risk remains, which might threaten the
overall security at some unforeseeable time in the future.
The DHHMAC would be compromised as soon as any of the listed
assumptions do not hold anymore.
The Diffie-Hellman mechanism is a generic security technique that is
not only applicable to groups of prime order or of characteristic
two. This is because of the fundamental mathematical assumption that
the discrete logarithm problem is also a very hard one in general
groups. This enables Diffie-Hellman to be deployed also for GF(p)*,
for sub-groups of sufficient size and for groups upon elliptic
curves. RSA does not allow such generalization, as the core
mathematical problem is a different one (large integer
factorization).
RSA asymmetric keys tend to become increasingly lengthy (1536 bits
and more) and thus very computationally intensive. Nevertheless,
elliptic curve Diffie-Hellman (ECDH) allows to cut-down key lengths
substantially (say 170 bits or more) while maintaining at least the
security level and providing even more significant performance
benefits in practice. Moreover, it is believed that elliptic curve
techniques provide much better protection against side channel
attacks due to the inherent redundancy in the projective coordinates.
For all these reasons, one may view elliptic-curve-based Diffie-
Hellman as being more "future-proof" and robust against potential
threats than RSA. Note, that an elliptic-curve Diffie-Hellman
variant of MIKEY remains for further study.
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It is not recommended to deploy DHHMAC for any other usage than
depicted in section 2. Otherwise any such misapplication might lead
to unknown, undefined properties.
5.6. Authorization and Trust Model
Basically, similar remarks on authorization as stated in [3] section
4.3.2. hold also for DHHMAC. However, as noted before, this key
management protocol does not serve full groups.
One may view the pre-established shared secret to yield some pre-
established trust relationship between the initiator and the
responder. This results in a much simpler trust model for DHHMAC
than would be the case for some generic group key management protocol
and potential group entities without any pre-defined trust
relationship. The common group controller in conjunction with the
assumption of a shared key simplifies the communication setup of the
entities.
One may view the pre-established trust relationship through the pre-
shared secret as some means for pre-granted, implied authorization.
This document does not define any particular authorization means but
leaves this subject to the application.
6. Acknowledgments
This document incorporates kindly valuable review feedback from
Steffen Fries, Hannes Tschofenig, Fredrick Lindholm, Mary Barnes and
Russell Housley and general feedback by the MSEC WG.
7. IANA considerations
This document does not define its own new name spaces for DHHMAC,
beyond the IANA name spaces that have been assigned for MIKEY, see
[3] section 10 and section 10.1, see also IANA MIKEY payload name
spaces [37].
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In order to align Table 4.1.a with [3] table 6.1.a, IANA is
requested to add the following entries to their MIKEY Payload Name
Space:
Data Type Value Reference
--------------- ----- ---------
DHHMAC init 7 [RFCxxxx]
DHHMAC resp 8 [RFCxxxx]
[Note to the RFC editor: Please replace RFCxxxx with the RFC number of
this document prior to publication.]
8. References
8.1 Normative References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[3] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman;
"MIKEY: Multimedia Internet KEYing", RFC 3830 IETF, August 2004.
[4] NIST, FIBS-PUB 180-1, "Secure Hash Standard", April 1995,
http://csrc.nist.gov/fips/fip180-1.ps.
[5] J. Arkko, E. Carrara et al: "Key Management Extensions for SDP
and RTSP", Internet Draft <draft-ietf-mmusic-kmgmt-ext-14.txt>,
Work in Progress (MMUSIC WG), IETF, March 2005.
[6] H. Krawczyk, M. Bellare, R. Canetti: "HMAC: Keyed-Hashing for
Message Authentication", RFC 2104, February 1997.
8.2 Informative References
[7] A.J. Menezes, P. van Oorschot, S. A. Vanstone: "Handbook of
Applied Cryptography", CRC Press 1996.
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[8] E. Rescorla, B. Korver: " Guidelines for Writing RFC Text on
Security Considerations", RFC 3552, IETF, July 2003.
[9] D. Eastlake, S. Crocker: "Randomness Recommendations for
Security", RFC 1750, IETF, December 1994.
[10] S.M. Bellovin, C. Kaufman, J. I. Schiller: "Security
Mechanisms for the Internet", RFC 3631, IETF, December 2003.
[11] Ueli M. Maurer, S. Wolf: "The Diffie-Hellman Protocol",
Designs, Codes, and Cryptography, Special Issue Public Key
Cryptography, Kluwer Academic Publishers, vol. 19, pp. 147-171,
2000. ftp://ftp.inf.ethz.ch/pub/crypto/publications/MauWol00c.ps
[12] Discrete Logarithms and the Diffie-Hellman Protocol;
http://www.crypto.ethz.ch/research/ntc/dldh/
[13] T. Dierks, C. Allen: "The TLS Protocol Version 1.0.", RFC 2246,
IETF, January 1999.
[14] D. Harkins, D. Carrel: "The Internet Key Exchange (IKE).", RFC
2409, IETF, November 1998.
[15] Donald E. Eastlake, Jeffrey I. Schiller, Steve Crocker:
"Randomness Requirements for Security"; <draft-eastlake-
randomness2-10.txt>; Work in Progress, IETF, January 2005.
[16] J. Schiller: "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", RFC 3365, IETF,
2002.
[17] C. Meadows: "Advice on Writing an Internet Draft Amenable to
Security Analysis", Work in Progress, <draft-irtf-cfrg-advice-
00.txt>, IRTF, October 2002.
[18] T. Narten: "Guidelines for Writing an IANA Considerations
Section in RFCs", RFC 2434, IETF, October 1998.
[19] J. Reynolds: "Instructions to Request for Comments (RFC)
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Authors", Work in Progress, <draft-rfc-editor-rfc2223bis-
08.txt>, IETF, August 2004.
[20] J. Rosenberg et all: "SIP: Session Initiation Protocol", RFC
3261, IETF, June 2002.
[21] Ch. Kaufman: "Internet Key Exchange (IKEv2) Protocol", Work in
Progress (IPSEC WG), <draft-ietf-ipsec-ikev2-17.txt>, Internet
Draft, Work in Progress (IPSEC WG).
[22] ITU-T Recommendation H.235 Annex G: "Usage of the MIKEY
Key Management Protocol for the Secure Real Time Transport
Protocol (SRTP) within H.235"; 1/2005.
[23] Schaad, J., Housley R.: "Advanced Encryption Standard (AES)
Key Wrap Algorithm", RFC 3394, IETF, September 2002.
[24] Baugher, M., Weis, B., Hardjono, T., Harney, H.: "The Group
Domain of Interpretation", RFC 3547, IETF, July 2003.
[25] Harney, H., Colegrove, A., Harder, E., Meth, U., Fleischer, R.:
"Group Secure Association Key Management Protocol", <draft-ietf-
msec-gsakmp-sec-08.txt>, Internet Draft, Work in Progress (MSEC
WG).
[26] Baugher, M., Canetti, R., Dondeti, L., and Lindholm, F.: "Group
Key Management Architecture", <draft-ietf-msec-gkmarch-08.txt>,
Internet Draft, Work in Progress (MSEC WG).
[27] Baugher, McGrew, Oran, Blom, Carrara, Naslund: "The Secure
Real-time Transport Protocol", RFC 3711, IETF, March 2004.
[28] ITU-T Recommendation H.235V3Amd1 Corr1, "Security and
encryption for H-series (H.323 and other H.245-based) multimedia
terminals", (01/2005).
[29] C. Adams et al: "Internet X.509 Public Key Infrastructure
Certificate Management Protocols"; draft-ietf-pkix-rfc2510bis-
09.txt, Internet Draft, Work in Progress (PKIX WG).
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[30] M. Myers et al: "X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP", RFC 2560, IETF, June
1999.
[31] C. Adams et al: "Internet X.509 Public Key Infrastructure Data
Validation and Certification Server Protocols", RFC 3029, IETF,
February 2001.
[32] M. Myers: "Internet X.509 Certificate Request Message Format",
RFC 2511, IETF, March 1999.
[33] M. Cooper et al: "Internet X.509 Public Key Infrastructure:
Certification Path Building", <draft-ietf-pkix-certpathbuild-
05.txt>, Internet Draft, Work in Progress (PKIX WG).
[34] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3978,
March 2005.
[35] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3979, March 2005.
[36] J. Rosenberg, H. Schulzrinne: "An Offer/Answer Model with the
Session Description Protocol (SDP)", RFC 3264, IETF, June 2002.
[37] IANA MIKEY Payload Name Spaces per [RFC3830], see
http://www.iana.org/assignments/mikey-payloads
Appendix A Usage of MIKEY-DHHMAC in H.235
This appendix provides informative overview how MIKEY-DHHMAC can be
applied in some H.323-based multimedia environments. Generally,
MIKEY is applicable for multimedia applications including IP
telephony. [22] describes various use cases of the MIKEY key
management protocols (MIKEY-PS, MIKEY-PK, MIKEY-DHSIGN and MIKEY-
DHHMAC) with the purpose to establish TGK keying material among
H.323 endpoints. The TGKs are then used for media encryption by
applying SRTP [27]. Addressed scenarios include point-to-point with
one or more intermediate gatekeepers (trusted or partially trusted)
in-between.
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One particular use case addresses MIKEY-DHHMAC to establish a media
connection from an endpoint B calling (through a gatekeeper) to
another endpoint A that is located within that same gatekeeper zone.
While EP-A and EP-B typically do not share any auth_key a priori,
some separate protocol exchange means are achieved outside the
actual call setup procedure to establish an auth_key for the time
while endpoints are being registered with the gatekeeper; such
protocols exist [22] but are not shown in this document. The
auth_key between the endpoints is being used to authenticate and
integrity protect the MIKEY-DHHMAC messages.
To establish a call, it is assumed that endpoint B has obtained
permission from the gatekeeper (not shown). Endpoint B as the
caller builds the MIKEY-DHHMAC I_message(see section 3) and sends
the I_message encapsulated within the H.323-SETUP to endpoint A. A
routing gatekeeper (GK) would forward this message to endpoint B; in
case of a non-routing gatekeeper, endpoint B sends the SETUP
directly to endpoint A. In either case, H.323 inherent security
mechanisms [28] are applied to protect the (encapsulation) message
during transfer. This is not depicted here. The receiving endpoint
A is able to verify the conveyed I_message and can compute a TGK.
Assuming that endpoint A would accept the call, EP-A then builds the
MIKEY-DHHMAC R_message and sends the response as part of the
CallProceeding-to-Connect message back to the calling endpoint B
(possibly through a routing gatekeeper). Endpoint B processes the
conveyed R_message to compute the same TGK as the called endpoint A.
1.) EP-B -> (GK) -> EP-A: SETUP(I_fwd_message [, I_rev_message])
2.) EP-A -> (GK) -> EP-B: CallProceeding-to-CONNECT(R_fwd_message [,
R_rev_message])
Notes: If it is necessary to establish directional TGKs for full-
duplex links in both directions B->A and A->B, then the
calling endpoint B instantiates the DHHMAC protocol twice:
once in the direction B->A using I_fwd_message and another
run in parallel in the direction A->B using I_rev_message.
In that case, two MIKEY-DHHMAC I_messages are encapsulated
within SETUP (I_fwd_message and I_rev_message) and two
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MIKEY-DHHMAC R_messages (R_fwd_message and R_rev_message)
are encapsulted within CallProceeding-to-CONNECT. The
I_rev_message corresponds with the I_fwd_message.
Alternatively, the called endpoint A may instantiate the
DHHMAC protocol in a separate run with endpoint B (not
shown); however, this requires a third handshake to
complete.
For more details on how the MIKEY protocols may be deployed
with H.235, please refer to [22].
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Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property Rights
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
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it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
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Expiration Date
This Internet Draft expires on 30 October 2005.
[Note to the RFC editor: Please remove the entire following section
prior to publication.]
Revision History
Changes against draft-ietf-msec-mikey-dhhmac-10.txt:
* A few editorial bugs removed.
* References updated.
Changes against draft-ietf-msec-mikey-dhhmac-09.txt:
*IESG review feedback incorporated; generally, only editorial
corrections.
* Section 2.1.1 moved into new Appendix A.
* IANA considerations section reworked and clarified.
Changes against draft-ietf-msec-mikey-dhhmac-08.txt:
* PKIX removed; some minor editorials.
Changes against draft-ietf-msec-mikey-dhhmac-07.txt:
* Feedback addressed from AD review.
* added considerations on the possible impact of PKIX protocols and
operations to end systems with real-time constraints (section 1).
* added note that DH group is transmitted explicitly but not the
parameters g and p; see section 3.
* added considerations on clock synchronization and timestamps in
section 2 and in section 5.3 in the view of consequences on replay
protection.
* references updated.
* editorial corrections and cleanup.
Changes against draft-ietf-msec-mikey-dhhmac-06.txt:
* Abstract reworded.
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* used new RFC boilerplate: changed/moved IPR statement (now at
the beginning), status of Memo, and Intellectual Property Rights
section in accordance with RFC 3667, RFC 3668.
* ID nits removal.
* References updated.
* Note added to section 4.1 explaining how to differentiate
between MIKEY and DHHMAC.
* New section 4.4 added that describes the use of the general
extension payload to avoid bidding-down attacks.
* Description of the bidding-down avoidance mechanism removed from
the threat model in section 5.2.
* IANA considerations section re-written and aligned with MIKEY.
* Open issue on KMID pointed in IANA considerations section.
* editorial clean-up.
Changes against draft-ietf-msec-mikey-dhhmac-05.txt:
* HMAC-SHA1-96 option removed (see section 1.2, 4.2, 5.3,). This
option does not really provide much gain; removal reduces
number
of options.
* IDr added to I_message for DoS protection of the recipient; see
section 3, 3.1, 5.3.
* References updated.
Changes against draft-ietf-msec-mikey-dhhmac-04.txt:
* Introduction section modified: PFS property of DH, requirement
for 4th MIKEY key management variant motivated.
* MIKEY-DHSIGN, MIKEY-PK and MIKEY-PS added to section 1.2
Abbreviations.
* Note on secure time synchronization added to section 2.0.
* New section 2.2 "Relation to GMKARCH" added.
* New section 2.1.1 "Usage in H.235" added: this section outlines
a use case of DHHMAC in the context of H.235.
* Trade-off between identity-protection and security & performance
added to section 5.1.
* New section 5.6 "Authorization and Trust Model" added.
* Some further informative references added.
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Changes against draft-ietf-msec-mikey-dhhmac-03.txt:
* RFC 3552 available; some references updated.
Changes against draft-ietf-msec-mikey-dhhmac-02.txt:
* text allows both random and pseudo-random values.
* exponentiation ** changed to ^.
* Notation aligned with MIKEY-07.
* Clarified that the HMAC is calculated over the entire MIKEY
message excluding the MAC field.
* Section 4.2: The AES key wrap method SHALL not be applied.
* Section 1: Relationship with other, existing work mentioned.
Changes against draft-ietf-msec-mikey-dhhmac-01.txt:
* bidding-down attacks addressed (see section 5.2).
* optional [X], [X, Y] defined and clarified (see section 1.1,
5.3).
* combination of options defined in key update procedure (see
section 3.1).
* ID payloads clarified (see section 3 and 5.2).
* relationship with MIKEY explained (roundtrip, performance).
* new section 2.1 on applicability of DHHMAC for SIP/SDP and
H.323 added.
* more text due to DH resolution incorporated in section 5.3
regarding PFS, security robustness of DH, generalization
capability of DH to general groups in particular EC and
"future-proofness".
* a few editorials and nits.
* references adjusted and cleaned-up.
Changes against draft-ietf-msec-mikey-dhhmac-00.txt:
* category set to proposed standard.
* identity protection clarified.
* aligned with MIKEY-05 DH protocol, notation and with payload
* some editorials and nits.
Euchner Expires - October 2005 [Page 36]
HMAC-authenticated Diffie-Hellman for MIKEY April 2005
Changes against draft-euchner-mikey-dhhmac-00.txt:
* made a MSEC WG draft
* aligned with MIKEY-03 DH protocol, notation and with payload
formats
* clarified that truncated HMAC actually truncates the HMAC result
rather than the SHA1 intermediate value.
* improved security considerations section completely rewritten in
the spirit of [8].
* IANA consideration section added
* a few editorial improvements and corrections
* IPR clarified and IPR section changed.
Author's Addresses
Martin Euchner
Email: martin_euchner@hotmail.com
Phone: +49 89 722 55790 Hofmannstr. 51
Fax: +49 89 722 62366
81359 Munich, Germany
Euchner Expires - October 2005 [Page 37]
