draft-ietf-msec-mikey-dhhmac-01.txt   draft-ietf-msec-mikey-dhhmac-02.txt 
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
Internet Engineering Task Force M. Euchner Internet Engineering Task Force - MSEC WG
Internet Draft Siemens AG Internet Draft M. Euchner
Intended Category: Proposed Standard Intended Category: Proposed Standard
Expires: June 2003 January 2003 Expires: December 2003 June 2003
HMAC-authenticated Diffie-Hellman for MIKEY HMAC-authenticated Diffie-Hellman for MIKEY
(draft-ietf-msec-mikey-dhhmac-01.txt) <draft-ietf-msec-mikey-dhhmac-02.txt>
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Abstract Abstract
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Abstract Abstract
This document describes a point-to-point key management protocol This document describes a point-to-point key management protocol
variant for the multimedia Internet keying (MIKEY). In variant for the multimedia Internet keying (MIKEY). In particular,
particular, the classic Diffie-Hellman key agreement protocol is the classic Diffie-Hellman key agreement protocol is used for key
used for key establishment in conjunction with a keyed hash (HMAC- establishment in conjunction with a keyed hash (HMAC-SHA1) for
SHA1) for achieving mutual authentication and message integrity of HMAC-authenticated Diffie-Hellman for MIKEY June 2003
the key management messages exchanged. This MIKEY variant is
called the HMAC-authenticated Diffie-Hellmann (DHHMAC). It
addresses the security and performance constraints of multimedia
key management in MIKEY.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003 achieving mutual authentication and message integrity of the key
management messages exchanged. This MIKEY variant is called the
HMAC-authenticated Diffie-Hellmann (DHHMAC). It 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 Table of Contents
1. Introduction..................................................2 1. Introduction................................................3
1.1. Notational Conventions........................................4 1.1. Definitions.................................................5
1.2. Definitions...................................................4 1.2. Abbreviations...............................................5
1.3. Abbreviations.................................................4 2. Scenario....................................................6
2. Scenario......................................................5 2.1. Applicability...............................................7
3. DHHMAC Security Protocol......................................5 3. DHHMAC Security Protocol....................................7
3.1. TGK re-keying.................................................7 3.1. TGK re-keying...............................................9
4. DHHMAC payload formats........................................7 4. DHHMAC payload formats.....................................10
4.1. Common header payload (HDR) ..................................8 4.1. Common header payload (HDR)................................11
4.2. Key data transport payload (KEMAC) ...........................8 4.2. Key data transport payload (KEMAC).........................11
4.3. ID payload (ID) ..............................................9 4.3. ID payload (ID)............................................12
5. Security Considerations.......................................9 5. Security Considerations....................................12
5.1. Security environment..........................................9 5.1. Security environment.......................................13
5.2. Threat model..................................................9 5.2. Threat model...............................................13
5.3. Security features and properties.............................11 5.3. Security features and properties...........................16
5.4. Assumptions..................................................14 5.4. Assumptions................................................20
5.5. Residual risk................................................15 5.5. Residual risk..............................................21
6. IANA considerations..........................................15 IANA considerations.............................................22
7. Intellectual Property Rights.................................15 Intellectual Property Rights....................................22
8. Acknowledgements.............................................16 References......................................................23
9. Conclusions..................................................16 Normative References............................................23
10. Normative References.........................................17 Informative References..........................................23
11. Informative References.......................................17 Acknowledgments.................................................25
12. Author's Address.............................................18 Conclusions.....................................................25
13. Full Copyright Statement.....................................18 Full Copyright Statement........................................25
14. Expiration Date..............................................19 Expiration Date.................................................26
15. Revision History.............................................19 Revision History................................................26
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
Author's Addresses..............................................27
1. Introduction 1. Introduction
As pointed out in MIKEY (see [1]), secure real-time multimedia As pointed out in MIKEY (see [3]), secure real-time multimedia
applications demand a particular adequate key management scheme that applications demand a particular adequate key management scheme that
cares for how to securely and efficiently establish dynamic session cares for how to securely and efficiently establish dynamic session
keys in a conversational multimedia scenario. keys in a conversational multimedia scenario.
In general, MIKEY scenarios cover peer-to-peer, simple-one-to-many In general, MIKEY scenarios cover peer-to-peer, simple-one-to-many
and small-sized groups. MIKEY in particular, describes three key and small-sized groups. MIKEY in particular, describes three key
management schemes for the peer-to-peer case that all finish their management schemes for the peer-to-peer case that all finish their
task within one round trip: task within one round trip:
- a symmetric key distribution protocol based upon pre-shared - a symmetric key distribution protocol based upon pre-shared
master keys; master keys;
- a public-key encryption-based key distribution protocol - a public-key encryption-based key distribution protocol
assuming a public-key infrastructure with RSA-based (Rivest, assuming a public-key infrastructure with RSA-based (Rivest,
Shamir and Adleman) private/public keys and digital Shamir and Adleman) private/public keys and digital
certificates; certificates;
- and a Diffie-Hellman key agreement protocol deploying digital - and a Diffie-Hellman key agreement protocol deploying digital
signatures and certificates. signatures and certificates.
All these three key management protocols are designed such that they All these three key management protocols are designed such that they
complete their work within just one round trip. This requires complete their work within just one round trip. This requires
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
depending on loosely synchronized clocks and deploying timestamps depending on loosely synchronized clocks and deploying timestamps
within the key management protocols. within the key management protocols.
However, it is known [5] that each of the three key management However, it is known [7] that each of the three key management
schemes has its subtle constraints and limitations: schemes has its subtle constraints and limitations:
- The symmetric key distribution protocol is simple to - The symmetric key distribution protocol is simple to
implement but does not nicely scale in any larger implement but does not nicely scale in any larger
configuration of potential peer entities due to the need of configuration of potential peer entities due to the need of
mutually pre-assigned shared master secrets. mutually pre-assigned shared master secrets.
Moreover, the security provided does not achieve the property Moreover, the security provided does not achieve the property
of perfect forward secrecy; i.e. compromise of the shared of perfect forward secrecy; i.e. compromise of the shared
master secret would render past and even future session keys master secret would render past and even future session keys
susceptible to compromise. susceptible to compromise.
Further, the generation of the session key happens just at HMAC-authenticated Diffie-Hellman for MIKEY June 2003
the initiator. Thus, the responder has to fully trust the
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 initiator on choosing a good and secure session secret; the
responder neither is able to participate in the key responder neither is able to participate in the key generation
generation nor to influence that process. This is considered nor to influence that process. This is considered as a
as a specific limitation in less trusted environments. specific limitation in less trusted environments.
- The public-key encryption scheme depends upon a public-key - The public-key encryption scheme depends upon a public-key
infrastructure that certifies the private-public keys by infrastructure that certifies the private-public keys by
issuing and maintaining digital certificates. While such a issuing and maintaining digital certificates. While such a
key management scheme provides full scalability in large key management scheme provides full scalability in large
networked configurations, public-key infrastructures are networked configurations, public-key infrastructures are still
still not widely available and in general, implementations not widely available and in general, implementations are
are significantly more complex. significantly more complex.
Further additional round trips might be necessary for each Further additional round trips might be necessary for each
side in order to ascertain verification of the digital side in order to ascertain verification of the digital
certificates. certificates.
Finally, as in the symmetric case, the responder depends Finally, as in the symmetric case, the responder depends
completely upon the initiator choosing good and secure completely upon the initiator choosing good and secure session
session keys. keys.
- The third MIKEY key management protocol deploys the Diffie- - The third MIKEY key management protocol deploys the Diffie-
Hellman key agreement scheme and authenticates the exchange Hellman key agreement scheme and authenticates the exchange of
of the Diffie-Hellman half-keys in each direction by using a the Diffie-Hellman half-keys in each direction by using a
digital signature upon. As in the previous method, this digital signature upon. As in the previous method, this
introduces the dependency upon a public-key infrastructure introduces the dependency upon a public-key infrastructure
with its strength on scalability but also the limitations on with its strength on scalability but also the limitations on
computational costs in performing the asymmetric long-integer computational costs in performing the asymmetric long-integer
operations and the potential need for additional operations and the potential need for additional communication
communication for verification of the digital certificates. for verification of the digital certificates.
However, the Diffie-Hellman key agreement protocol is known However, the Diffie-Hellman key agreement protocol is known
for its subtle security strengths in that it is able to for its subtle security strengths in that it is able to
provide full perfect secrecy and further have both parties provide full perfect secrecy and further have both parties
actively involved in session key generation. actively involved in session key generation.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
This document describes a fourth key management scheme for MIKEY that This document describes a fourth key management scheme for MIKEY that
could somehow be seen as a synergetic optimization between the pre- could somehow be seen as a synergetic optimization between the pre-
shared key distribution scheme and the Diffie-Hellman key agreement. shared key distribution scheme and the Diffie-Hellman key agreement.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
The idea of that protocol is to apply the Diffie-Hellman key The idea of that protocol is to apply the Diffie-Hellman key
agreement but instead of deploying a digital signature for agreement but instead of deploying a digital signature for
authenticity of the exchanged keying material rather uses a keyed- authenticity of the exchanged keying material rather uses a keyed-
hash upon using symmetrically pre-assigned shared secrets. This hash upon using symmetrically pre-assigned shared secrets. This
combination of security mechanisms is called the HMAC-authenticated combination of security mechanisms is called the HMAC-authenticated
Diffie-Hellman (DH) key agreement for MIKEY (DHHMAC). 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 Like the MIKEY Diffie-Hellman protocol, DHHMAC does not scale beyond
a point-to-point constellation; thus, both MIKEY Diffie-Hellman a point-to-point constellation; thus, both MIKEY Diffie-Hellman
protocols do not support group-based keying for any group size larger protocols do not support group-based keying for any group size larger
than two entities. than two entities.
1.1. Notational Conventions 1.1. Definitions
The key word "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.
1.2. Definitions
The definitions and notations in this document are aligned with The definitions and notations in this document are aligned with
MIKEY, see [1] and [1] sections 1.2 - 1.3. MIKEY, see [3] and [3] sections 1.2 - 1.3.
All large integer computations in this document should be understood 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 as being mod p within some fixed group G for some large prime p; see
[1] section 3.3; however, the DHHMAC protocol is applicable in [3] section 3.3; however, the DHHMAC protocol is applicable in
general to other appropriate groups as well. general to other appropriate groups as well.
It is assumed that a pre-shared key s is known by both entities It is assumed that a pre-shared key s is known by both entities
(initiator and responder). The authentication key auth_key is (initiator and responder). The authentication key auth_key is
derived from the pre-shared secret s using the pseudo-random function derived from the pre-shared secret s using the pseudo-random function
PRF; see [1] sections 4.1.3 and 4.1.5. PRF; see [3] sections 4.1.3 and 4.1.5.
1.3. Abbreviations 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.
1.2. Abbreviations
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
auth_key pre-shared authentication key, PRF-derived from auth_key pre-shared authentication key, PRF-derived from
pre-shared key s. pre-shared key s.
DH Diffie-Hellman DH Diffie-Hellman
DHi public Diffie-Hellman half key g**(xi) of DHi public Diffie-Hellman half key g**(xi) of Initiatior
Initiatior
DHr public Diffie-Hellman half key g**(xr) of Responder DHr public Diffie-Hellman half key g**(xr) of Responder
DHHMAC HMAC-authenticated Diffie-Hellman DHHMAC HMAC-authenticated Diffie-Hellman
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
DoS Denial-of-service DoS Denial-of-service
G Diffie-Hellman group G Diffie-Hellman group
HDR MIKEY common header payload HDR MIKEY common header payload
HMAC keyed Hash Message Authentication Code HMAC keyed Hash Message Authentication Code
HMAC-SHA1 HMAC using SHA1 as hash function (160-bit result) HMAC-SHA1 HMAC using SHA1 as hash function (160-bit result)
HMAC-SHA-1-96 HMAC-SHA1 truncated to 96 bits HMAC-SHA1-96 HMAC-SHA1 truncated to 96 bits
IDi Identity of initiator IDi Identity of initiator
IDr Identity of receiver IDr Identity of receiver
IKE Internet Key Exchange IKE Internet Key Exchange
IPSEC Internet Protocol Security IPSec Internet Protocol Security
MIKEY Multimedia Internet KEYing MIKEY Multimedia Internet KEYing
p Diffie-Hellman prime modulus p Diffie-Hellman prime modulus
PRF MIKEY pseudo-random function PRF MIKEY pseudo-random function (see [3] section 4.1.3.)
(see [1] section 4.1.3.)
RSA Rivest, Shamir and Adleman RSA Rivest, Shamir and Adleman
s pre-shared key s pre-shared key
SOI Son-of-IKE SDP Session Description Protocol
SOI Son-of-IKE, IKEv2
SP MIKEY Security Policy (Parameter) Payload SP MIKEY Security Policy (Parameter) Payload
T timestamp T timestamp
TEK Traffic Encryption Key TEK Traffic Encryption Key
TGK MIKEY TEK Generation Key as the common Diffie- TGK MIKEY TEK Generation Key as the common Diffie-
Hellman shared secret Hellman shared secret
TLS Transport Layer Security TLS Transport Layer Security
xi secret, random Diffie-Hellman key of Initiator xi secret, random Diffie-Hellman key of Initiator
xr secret, random Diffie-Hellman key of Responder xr secret, random Diffie-Hellman key of Responder
2. Scenario 2. Scenario
The HMAC-authenticated Diffie-Hellman key agreement protocol (DHHMAC) The HMAC-authenticated Diffie-Hellman key agreement protocol (DHHMAC)
for MIKEY addresses the same scenarios and scope as the other three for MIKEY addresses the same scenarios and scope as the other three
key management schemes in MIKEY address. key management schemes in MIKEY address.
DHHMAC is applicable in a peer-to-peer group where no access to a DHHMAC is applicable in a peer-to-peer group where no access to a
public-key infrastructure can be assumed available. Rather, pre- public-key infrastructure can be assumed available. Rather, pre-
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
shared master secrets are assumed available among the entities in shared master secrets are assumed available among the entities in
such an environment. such an environment.
In a pair-wise group, it is assumed that each client will be setting In a pair-wise group, it is assumed that each client will be setting
up a session key for its outgoing links with it's peer using the DH- up a session key for its outgoing links with it's peer using the DH-
MAC key agreement protocol. MAC key agreement protocol.
As is the case for the other three MIKEY key management protocol, As is the case for the other three MIKEY key management protocol,
DHHMAC assumes loosely synchronized clocks among the entities in the DHHMAC assumes loosely synchronized clocks among the entities in the
small group. small group.
2.1. Applicability
MIKEY-DHHMAC as well as the other MIKEY key management protocols are
optimized and targeted for the purpose of multimedia applications
with application-level key management needs under 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 (see [5]) where the encoded MIKEY messages are
encapsulated and transported in SDP containers of the SDP
offer/answer handshake,
b) H.323 (see [22]) where the encoded MIKEY messages are transported
in the H.225.0 fast start call signaling handshake.
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 peculiar strengths (see section
5).
3. DHHMAC Security Protocol 3. DHHMAC Security Protocol
The following figure defines the security protocol for DHHMAC: The following figure defines the security protocol for DHHMAC:
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
Initiator Responder Initiator Responder
I_message = HDR, T, RAND, [IDi], I_message = HDR, T, RAND, [IDi],
{SP}, DHi, KEMAC {SP}, DHi, KEMAC
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
I_message I_message
-----------------------> R_message = HDR, T, -----------------------> R_message = HDR, T,
[IDr], IDi, DHr, DHi, [IDr], IDi, DHr,
KEMAC DHi, KEMAC
R_message R_message
<---------------------- <----------------------
TGK = g**(xi * yi) TGK = g**(xi * yi) TGK = g**(xi * yi) TGK = g**(xi * yi)
Figure 1: HMAC-authenticated Diffie-Hellman key based exchange, Figure 1: HMAC-authenticated Diffie-Hellman key based exchange,
where xi and xr are randomly chosen respectively where xi and xr are randomly chosen respectively
by the initiator and the responder. by the initiator and the responder.
The DHHMAC key exchange SHALL be done according to Figure 1. The The DHHMAC key exchange SHALL be done according to Figure 1. The
initiator chooses a random value xi, and sends an HMACed message initiator chooses a random value xi, and sends an HMACed message
including g**xi and a timestamp to the responder (optionally also including g**xi and a timestamp to the responder. It is
including its identity). recommended that the initiator SHOULD always include the identity
payload IDi within the I_message; unless the receiver can defer
the initiator's identity by some other means and IDi MAY
optionally be left out.
The group parameters (e.g., the group G) are a set of parameters The group parameters (e.g., the group G) are a set of parameters
chosen by the initiator. The responder chooses a random positive chosen by the initiator. The responder chooses a random positive
integer xr, and sends an HMACed message including g**xr and the integer xr, and sends an HMACed message including g**xr and the
timestamp to the initiator (optionally also providing its timestamp to the initiator. The responder SHALL always include the
identity). 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 reponder's identity by some
other means and IDr MAY optionally be left out.
Both parties then calculate the TGK, g**(xi * xr). Both parties then calculate the TGK, g**(xi * xr).
The HMAC authentication is due to provide authentication of the DH The HMAC authentication is due to provide authentication of the DH
half-keys, and is necessary to avoid man-in-the-middle attacks. half-keys, and is necessary to avoid man-in-the-middle attacks.
This approach is less expensive than digitally signed Diffie- This approach is less expensive than digitally signed Diffie-
Hellman. It requires first of all, that both sides compute one Hellman. It requires first of all, that both sides compute one
exponentiation and one HMAC, then one HMAC verification and exponentiation and one HMAC, then one HMAC verification and
finally another Diffie-Hellman exponentiation. finally another Diffie-Hellman exponentiation.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
With off-line pre-computation, the initial Diffie-Hellman half-key With off-line pre-computation, the initial Diffie-Hellman half-key
MAY be computed before the key management transaction and thereby MAY be computed before the key management transaction and thereby
MAY further reduce the overall round trip delay as well as reduce MAY further reduce the overall round trip delay as well as reduce
the risk of denial-of-service attacks. the risk of denial-of-service attacks.
Processing of the TGK SHALL be accomplished as described in MIKEY Processing of the TGK SHALL be accomplished as described in MIKEY
[1] chapter 4. [3] chapter 4.
The computed HMAC result SHALL be conveyed in the KEMAC payload The computed HMAC result SHALL be conveyed in the KEMAC payload
field where the MAC fields holds the HMAC result. The HMAC shall field where the MAC fields holds the HMAC result. The HMAC shall
be computed over the entire message using auth_key, see also be computed over the entire message using auth_key, see also
section 4.2. section 4.2.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
3.1. TGK re-keying 3.1. TGK re-keying
TGK re-keying for DHHMAC generally proceeds as described in [1] TGK re-keying for DHHMAC generally proceeds as described in [3]
section 4.5. Specifically, figure 2 provides the message fields section 4.5. Specifically, figure 2 provides the message fields
for DHHMAC update message. for DHHMAC update message.
Initiator Responder Initiator Responder
I_message = HDR, T, [IDi], I_message = HDR, T, [IDi],
{SP}, [DHi], KEMAC {SP}, [DHi], KEMAC
I_message I_message
-----------------------> R_message = HDR, T, -----------------------> R_message = HDR, T,
[IDr], IDi, [DHr, DHi], [IDr], IDi,
KEMAC [DHr, DHi], KEMAC
R_message R_message
<---------------------- <----------------------
[TGK = g**(xi * yi)] [TGK = g**(xi * yi)] [TGK = g**(xi * yi)] [TGK = g**(xi * yi)]
Figure 2: DHHMAC update message Figure 2: DHHMAC update message
TGK re-keying supports two procedures:
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
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.
4. DHHMAC payload formats 4. DHHMAC payload formats
This section specifies the payload formats and data type values for This section specifies the payload formats and data type values for
DHHMAC, see also [1] chapter 6 for a definition of the MIKEY DHHMAC, see also [3] chapter 6 for a definition of the MIKEY
payloads. payloads.
The following referenced MIKEY payloads are used for DH-MAC: The following referenced MIKEY payloads are used for DH-MAC:
* Common header payload (HDR), see section 4.1 and [1] section 6.1 * Common header payload (HDR), see section 4.1 and [3] section 6.1
* SRTP ID sub-payload, see [1] section 6.1.1, * SRTP ID sub-payload, see [3] section 6.1.1,
* Key data transport payload (KEMAC), see section 4.2 and [1] section * Key data transport payload (KEMAC), see section 4.2 and [3] section
6.2 6.2
* DH data payload, see [1] section 6.4 * DH data payload, see [3] section 6.4
* Timestamp payload, [1] section 6.6 * Timestamp payload, [3] section 6.6
* ID payload, [1] section 6.7 * ID payload, [3] section 6.7
* Security Policy payload (SP), [1] section 6.10 * Security Policy payload (SP), [3] section 6.10
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
* RAND payload (RAND), [1] section 6.11 * RAND payload (RAND), [3] section 6.11
* Error payload (ERR), [1] section 6.12 * Error payload (ERR), [3] section 6.12
* General Extension Payload, [1] section 6.15 * General Extension Payload, [3] section 6.15
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
4.1. Common header payload (HDR) 4.1. Common header payload (HDR)
Referring to [1] section 6.1, for DHHMAC the following data type Referring to [3] section 6.1, for DHHMAC the following data types
SHALL be used: SHALL be used:
Data type | Value | Comment Data type | Value | Comment
-------------------------------------------------------------
DHHMAC init | 7 | Initiator's DHHMAC exchange message DHHMAC init | 7 | Initiator's DHHMAC exchange message
DHHMAC resp | 8 | Responder's DHHMAC exchange message DHHMAC resp | 8 | Responder's DHHMAC exchange message
Error | 6 | Error message, see [1] section 6.12 Error | 6 | Error message, see [3] section 6.12
The next payload field shall be one of the following values: The next payload field shall be one of the following values:
Next payload| Value | Section Next payload| Value | Section
----------------------------------------------------------------
Last payload| 0 | - Last payload| 0 | -
KEMAC | 1 | section 4.2 and [1] section 6.2 KEMAC | 1 | section 4.2 and [3] section 6.2
DH | 3 | [1] section 6.4 DH | 3 | [3] section 6.4
T | 5 | [1] section 6.6 T | 5 | [3] section 6.6
ID | 6 | [1] section 6.7 ID | 6 | [3] section 6.7
SP | 10 | [1] section 6.10 SP | 10 | [3] section 6.10
RAND | 11 | [1] section 6.11 RAND | 11 | [3] section 6.11
ERR | 12 | [1] section 6.12 ERR | 12 | [3] section 6.12
General Ext.| 21 | [1] section 6.15 General Ext.| 21 | [3] section 6.15
Other defined next payload values defined in [1] SHALL not be Other defined next payload values defined in [3] SHALL not be
applied to DHHMAC. applied to DHHMAC.
The responder in case of a decoding error or of a failed HMAC The responder in case of a decoding error or of a failed HMAC
authentication verification SHALL apply the Error payload data authentication verification SHALL apply the Error payload data
type. type.
4.2. Key data transport payload (KEMAC) 4.2. Key data transport payload (KEMAC)
DHHMAC SHALL apply this payload for conveying the HMAC result DHHMAC SHALL apply this payload for conveying the HMAC result
along with the indicated authentication algorithm. KEMAC when used along with the indicated authentication algorithm. KEMAC when used
in conjunction with DHHMAC SHALL not convey any encrypted data; 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 thus Encr alg SHALL be set to 2 (NULL), Encr data len shall be set
to 0 and Encr data SHALL be left empty. to 0 and Encr data SHALL be left empty.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
For DHHMAC, this key data transport payload SHALL be the last For DHHMAC, this key data transport payload SHALL be the last
payload in the message. Note that the Next payload field SHALL be payload in the message. Note that the Next payload field SHALL be
set to Last payload. The HMAC is then calculated over the entire set to Last payload. The HMAC is then calculated over the entire
MIKEY message using auth_key as described in [1] section 5.2 and MIKEY message using auth_key as described in [3] section 5.2 and
then stored within MAC field. then stored within MAC field.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
MAC alg | Value | Comments MAC alg | Value | Comments
HMAC-SHA-1 | 0 | Mandatory, Default (see [SHA1]) ------------------------------------------------------------------
HMAC-SHA-1 | 0 | Mandatory, Default (see [4])
NULL | 1 | Very restricted use, see NULL | 1 | Very restricted use, see
| [1] section 4.2.4 | [3] section 4.2.4
HMAC-SHA-1-96 | 5 | Optional, HMAC-SHA1 truncated to the 96 HMAC-SHA-1-96 | 5 | Optional, HMAC-SHA1 truncated to the 96
| leftmost bits of the HMAC-SHA-1 result | leftmost bits of the HMAC-SHA-1 result
| when represented in network byte order. | when represented in network byte order.
HMAC-SHA-1 is the default hash function that MUST be implemented 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 as part of the DHHMAC. The length of the HMAC-SHA-1 result is 160
bits. bits.
HMAC-SHA-1-96 produces a slightly shorter HMAC result where the HMAC-SHA-1-96 produces a slightly shorter HMAC result where the
HMAC-SHA-1 result SHALL be truncated to the 96 leftmost bits when HMAC-SHA-1 result SHALL be truncated to the 96 leftmost bits when
represented in network byte order. This saves some bandwidth. represented in network byte order. This saves some bandwidth.
4.3. ID payload (ID) 4.3. ID payload (ID)
For DHHMAC, this payload SHALL only hold a non-certificate based For DHHMAC, this payload SHALL only hold a non-certificate based
identify. identity.
5. Security Considerations 5. Security Considerations
This document addresses key management security issues throughout. This document addresses key management security issues throughout.
For a comprehensive explanation of MIKEY security considerations, For a comprehensive explanation of MIKEY security considerations,
please refer to MIKEY [1] section 9. please refer to MIKEY [3] section 9.
In addition to that, this document addresses security issues In addition to that, this document addresses security issues
according to [6] where the following security considerations apply in according to [8] where the following security considerations apply in
particular to this document: particular to this document:
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
5.1. Security environment 5.1. Security environment
Generally, the DHHMAC security protocol described in this document Generally, the DHHMAC security protocol described in this document
focuses primarily on communication security; i.e. the security issues focuses primarily on communication security; i.e. the security issues
concerned with the MIKEY DHHMAC protocol. Nevertheless, some system concerned with the MIKEY DHHMAC protocol. Nevertheless, some system
security issues are of interest as well that are not explicitly security issues are of interest as well that are not explicitly
defined by the DHHMAC protocol, but should be provided locally in defined by the DHHMAC protocol, but should be provided locally in
practice. The system where the DHHMAC protocol entity runs upon practice.
shall provide the capability to generate random numbers as input to
the Diffie-Hellman operation (see [7], [15]). Further, the system The system where the DHHMAC protocol entity runs upon shall provide
shall be capable of storing the generated random data, secret data, the capability to generate random numbers as input to the Diffie-
keys and other secret security parameters securely (i.e. confidential Hellman operation (see [9], [15]). Further, the system shall be
and safe from unauthorized tampering). capable of storing the generated random data, secret data, keys and
other secret security parameters securely (i.e. confidential and safe
from unauthorized tampering).
5.2. Threat model 5.2. Threat model
The threat model that this documents adheres to covers the issues of The threat model that this document adheres to cover the issues of
end-to-end security in the Internet generally; without ruling out the end-to-end security in the Internet generally; without ruling out the
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
possibility that MIKEY DHHMAC be deployed in a corporate, closed IP possibility that MIKEY DHHMAC be deployed in a corporate, closed IP
environment. This also includes the possibility that MIKEY DHHMAC be environment. This also includes the possibility that MIKEY DHHMAC be
deployed on a hop-by-hop basis with some intermediate trusted "MIKEY deployed on a hop-by-hop basis with some intermediate trusted "MIKEY
DHHMAC proxies" involved. DHHMAC proxies" involved.
Since DHHMAC is a key management protocol, the following security Since DHHMAC is a key management protocol, the following security
threats are of concern: threats are of concern:
* Unauthorized interception of plain TGKs. * Unauthorized interception of plain TGKs.
This threat shall not occur. Nevertheless, for DHHMAC this threat This threat shall not occur. Nevertheless, for DHHMAC this threat
does not occur since the TGK is not actually transmitted on the does not occur since the TGK is not actually transmitted on the
wire (not even in encrypted fashion). wire (not even in encrypted fashion).
* Eavesdropping of other, transmitted keying information: * Eavesdropping of other, transmitted keying information:
DHHMAC protocol does not explicitly transmit the TGK at all. DHHMAC protocol does not explicitly transmit the TGK at all.
Rather, by the Diffie-Hellman "encryption" operation, that Rather, by the Diffie-Hellman "encryption" operation, that conceals
conceals the secret, random values, only partial information (i.e. the secret random values, only partial information (i.e. the DH-
the DH- half key) for construction of the TGK is transmitted. It half key) for construction of the TGK is transmitted. It is
is assumed that availability of such Diffie-Hellman half-keys to fundamentally assumed that availability of such Diffie-Hellman
an eavesdropper does not result in any risk; see 5.4. Further, half-keys to an eavesdropper does not result in any substantial
the DHHMAC carries other data such as timestamps, random values, security risk; see 5.4. Further, the DHHMAC carries other data
identification information or security policy parameters; such as timestamps, random values, identification information or
eavesdropping of any such data is considered not to yield any HMAC-authenticated Diffie-Hellman for MIKEY June 2003
significant security risk.
security policy parameters; eavesdropping of any such data is
considered not to yield any significant security risk.
* Masquerade of either entity: * Masquerade of either entity:
This security threat must be avoided and if a masquerade attack This security threat must be avoided and if a masquerade attack
would be attempted, appropriate detection means must be in place. would be attempted, appropriate detection means must be in place.
DHHMAC addresses this threat by providing mutual peer entity DHHMAC addresses this threat by providing mutual peer entity
authentication. authentication.
* Man-in-the-middle attacks: * Man-in-the-middle attacks:
Such attacks threaten the security of exchanged, non-authenticated Such attacks threaten the security of exchanged, non-authenticated
messages. Man-in-the-middle attacks usually come with masquerade messages. Man-in-the-middle attacks usually come with masquerade
and or loss of message integrity (see below). Man-in-the-middle and or loss of message integrity (see below). Man-in-the-middle
attacks must be avoided, and if present or attempted must be attacks must be avoided, and if present or attempted must be
detected appropriately. DHHMAC addresses this threat by providing detected appropriately. DHHMAC addresses this threat by providing
mutual peer entity authentication and message integrity. mutual peer entity authentication and message integrity.
* Loss of integrity: * Loss of integrity:
This security threats relates to unauthorized replay, deletion, This security threat relates to unauthorized replay, deletion,
insertion and manipulation of messages. While any such attacks insertion and manipulation of messages. While any such attacks
cannot be avoided they must be detected at least. DHHMAC addresses cannot be avoided they must be detected at least. DHHMAC addresses
this threat by providing message integrity. this threat by providing message integrity.
* Bidding-down attacks:
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 mechanism as described in [3] section 7.1,
where all key management protocol identifiers must be listed
within the MIKEY General Extension Payload. The protocol
identifier for DHHMAC shall be "mikeydhhmac". The General
Extension Payload must be integrity-protected with the HMAC using
the shared secret.
Some potential threats are not within the scope of this threat model: Some potential threats are not within the scope of this threat model:
* Passive and off-line cryptanalysis of the Diffie-Hellman algorithm: * Passive and off-line cryptanalysis of the Diffie-Hellman algorithm:
Under certain reasonable assumptions (see 5.4 below) it is widely Under certain reasonable assumptions (see 5.4 below) it is widely
believed that DHHMAC is sufficiently secure and that such attacks believed that DHHMAC is sufficiently secure and that such attacks
be infeasible although the possibility of a successful attack be infeasible although the possibility of a successful attack
cannot be ruled out completely. cannot be ruled out completely.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003 HMAC-authenticated Diffie-Hellman for MIKEY June 2003
* Non-repudiation of the receipt or of the origin of the message: * Non-repudiation of the receipt or of the origin of the message:
These are not requirements of this environment and thus related These are not requirements of this environment and thus related
countermeasures not provided at all. countermeasures are not provided at all.
* Denial-of-service or distributed denial-of-service attacks: * Denial-of-service or distributed denial-of-service attacks:
Some considerations are given on some of those attacks, but DHHMAC Some considerations are given on some of those attacks, but DHHMAC
does not claim to provide full countermeasure against any of those does not claim to provide full countermeasure against any of those
attacks. For example, stressing the availability of the entities attacks. For example, stressing the availability of the entities
are not thwarted by means of the key management protocol; some are not thwarted by means of the key management protocol; some
other local countermeasures should be applied. Further, some DoS other local countermeasures should be applied. Further, some DoS
attacks are not countered such as interception of a valid DH- attacks are not countered such as interception of a valid DH-
requests and its massive instant duplication. Such attacks might request and its massive instant duplication. Such attacks might at
at least be countered partially by some local means that are least be countered partially by some local means that are outside
outside the scope of this document. the scope of this document.
* Identity protection: * Identity protection:
Like MIKEY, identity protection is not a major design requirement Like MIKEY, identity protection is not a major design requirement
for MIKEY-DHHMAC either, see [1]. No security protocol is known for MIKEY-DHHMAC either, see [3]. No security protocol is known so
so far, that is able to provide the objectives of DHHMAC as stated far, that is able to provide the objectives of DHHMAC as stated in
in section 5.3 including identity protection within just a single section 5.3 including identity protection within just a single
roundtrip. As such, MIKEY-DHHMAC does not provide identity roundtrip. As such, MIKEY-DHHMAC does not provide identity
protection on its own but may inherit such property from a protection on its own but may inherit such property from a security
security protocol underneath that actually features identity protocol underneath that actually features identity protection. On
protection. On the other hand, it is expected that MIKEY-DHHMAC the other hand, it is expected that MIKEY-DHHMAC is typically being
is typically being deployed within SDP/SIP ([21], [22]); both deployed within SDP/SIP ([20], [5]); both those protocols do not
those protocols do not provide end-to-end identity protection. provide end-to-end 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.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
5.3. Security features and properties 5.3. Security features and properties
With the security threats in mind, this draft provides the following With the security threats in mind, this draft provides the following
security features and yields the following properties: security features and yields the following properties:
* Secure key agreement with the establishment of a TGK at both peers: * Secure key agreement with the establishment of a TGK at both peers:
This is achieved using an authenticated Diffie-Hellman key This is achieved using an authenticated Diffie-Hellman key
management protocol. management protocol.
* Peer-entity authentication (mutual): * Peer-entity authentication (mutual):
This authentication corroborates that the host/user is authentic This authentication corroborates that the host/user is authentic in
in that possession of a pre-assigned secret key is proven using that possession of a pre-assigned secret key is proven using keyed
keyed HMAC. The authentication occurs on the request and on the HMAC. The authentication occurs on the request and on the response
response message, thus authentication is mutual. message, thus authentication is mutual.
The HMAC computation corroborates for authentication and message The HMAC computation corroborates for authentication and message
integrity of the exchanged Diffie-Hellman half-keys and associated integrity of the exchanged Diffie-Hellman half-keys and associated
messages. The authentication is absolutely necessary in order to messages. The authentication is absolutely necessary in order to
avoid man-in-the-middle attacks on the exchanged messages in avoid man-in-the-middle attacks on the exchanged messages in
transit and in particular, on the otherwise non-authenticated transit and in particular, on the otherwise non-authenticated
exchanged Diffie-Hellman half keys. exchanged Diffie-Hellman half keys.
Note: This document does not address issues regarding Note: This document does not address issues regarding
authorization; this feature is not provided explicitly. However, authorization; this feature is not provided explicitly. However,
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
DHHMAC authentication means support and facilitate realization of DHHMAC authentication means support and facilitate realization of
authorization means (local issue). authorization means (local issue).
* Cryptographic integrity check: * Cryptographic integrity check:
The cryptographic integrity check is achieved using a message The cryptographic integrity check is achieved using a message
digest (keyed HMAC). It includes the exchanged Diffie-Hellman digest (keyed HMAC). It includes the exchanged Diffie-Hellman
half-keys but covers the other parts of the exchanged message as half-keys but covers the other parts of the exchanged message as
well. Both mutual peer entity authentication and message well. Both mutual peer entity authentication and message integrity
integrity provide effective countermeasure against man-in-the- provide effective countermeasure against man-in-the-middle attacks.
middle attacks.
The initiator may deploy a local timer that fires when the awaited The initiator may deploy a local timer that fires when the awaited
response message did not arrive timely. This is to detect response message did not arrive timely. This is to detect deletion
deletion of entire messages. of entire messages.
* Replay protection of the messages is achieved using * Replay protection of the messages is achieved using embedded
embeddedtimestamps. timestamps.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
* Limited DoS protection: * Limited DoS protection:
Rapid checking of the message digest allows verifying the Rapid checking of the message digest allows verifying the
authenticity and integrity of a message before launching CPU authenticity and integrity of a message before launching CPU
intensive Diffie-Hellman operations or starting other resource intensive Diffie-Hellman operations or starting other resource
consuming tasks. This protects against some denial-of-service consuming tasks. This protects against some denial-of-service
attacks: malicious modification of messages and spam attacks with attacks: malicious modification of messages and spam attacks with
(replayed or masqueraded) messages. DHHMAC probably does not (replayed or masqueraded) messages. DHHMAC probably does not
explicitly counter sophisticated distributed denial-of-service explicitly counter sophisticated distributed, large-scale denial-
attacks that compromise system availability for example. of-service attacks that compromise system availability for example.
* Perfect-forward secrecy (PFS): * Perfect-forward secrecy (PFS):
Other than the MIKEY pre-shared and public-key based key Other than the MIKEY pre-shared and public-key based key
distribution protocols, the Diffie-Hellman key agreement protocol distribution protocols, the Diffie-Hellman key agreement protocol
features a security property called perfect forward secrecy. That features a security property called perfect forward secrecy. That
is, that even if the long-term pre-shared key would be compromised 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 at some point in time, this would not render past or future session
session keys compromised. 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 but also digitally signed DH provides a far As such, DHHMAC but also digitally signed DH provides a far
superior security level over the pre-shared or public-key based superior security level over the pre-shared or public-key based key
key distribution protocol in that respect. distribution protocol in that respect.
* Fair, mutual key contribution: * Fair, mutual key contribution:
The Diffie-Hellman key management protocol is not a strict key The Diffie-Hellman key management protocol is not a strict key
distribution protocol per se with the initiator distributing a key distribution protocol per se with the initiator distributing a key
to its peers. Actually, both parties involved in the protocol to its peers. Actually, both parties involved in the protocol
exchange are able to equally contribute to the common Diffie- exchange are able to equally contribute to the common Diffie-
Hellman TEK traffic generating key. This reduces the risk of Hellman TEK traffic generating key. This reduces the risk of
either party cheating or unintentionally generating a weak session either party cheating or unintentionally generating a weak session
key. This makes the DHHMAC a fair key agreement protocol. 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 In order for Diffie-Hellman key agreement to be secure, each party
shall generate its xi or xr values using a strong, unpredictable shall generate its xi or xr values using a strong, unpredictable
pseudo-random generator. Further these values xi or xr shall be pseudo-random generator. Further, these values xi or xr shall be
kept private. It is recommended that these secret values be HMAC-authenticated Diffie-Hellman for MIKEY June 2003
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
destroyed once the common Diffie-Hellman shared secret key has kept private. It is recommended that these secret values be
been established. destroyed once the common Diffie-Hellman shared secret key has been
established.
* Efficiency and performance: * Efficiency and performance:
The DHHMAC key agreement protocol securely establishes a TGK Like the MIKEY-public key protocol, the MIKEY DHHMAC key agreement
within just one roundtrip. Other existing key management protocol securely establishes a TGK within just one roundtrip.
techniques like IPSEC-IKE [14], IPSEC-SOI and TLS [13] and other Other existing key management techniques like IPSEC-IKE [14],
schemes are not deemed adequate in addressing sufficiently those IPSEC-IKEv2 [21] and TLS [13] and other schemes are not deemed
real-time and security requirements; they all use more than a adequate in addressing sufficiently those real-time and security
single roundtrip. 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 Using HMAC in conjunction with a strong one-way hash function such
as SHA1 may be achieved more efficiently in software than as SHA1 may be achieved more efficiently in software than expensive
expensive public-key operations. This yields a particular public-key operations. This yields a particular performance
performance benefit of DHHMAC over signed DH or the public-key benefit of DHHMAC over signed DH or the public-key encryption
encryption protocol. protocol.
DHHMAC optionally features a variant where the HMAC-SHA-1 result DHHMAC optionally features a variant where the HMAC-SHA-1 result is
is truncated to 96-bit instead of 160 bits. It is believed that truncated to 96-bit instead of 160 bits. It is believed that
although the truncated HMAC appears significantly shorter, the although the truncated HMAC appears significantly shorter, the
security provided would not suffer; it appears even reasonable security provided would not suffer; it appears even reasonable that
that the shorter HMAC could provide increased security against the shorter HMAC could provide increased security against known-
known-plaintext crypt-analysis, see RFC 2104 for more details. In plaintext crypt-analysis, see RFC 2104 [6] for more details. In
any way, truncated DHHMAC is able to reduce the bandwidth during any way, truncated DHHMAC is able to reduce the bandwidth during
Diffie-Hellman key agreement and yield better round trip delay on Diffie-Hellman key agreement and yield better round trip delay on
low-bandwidth links. If a very high security level is desired for low-bandwidth links. If a very high security level is desired for
long-term secrecy of the negotiated Diffie-Hellman shared secret, long-term secrecy of the negotiated Diffie-Hellman shared secret,
longer hash values may be deployed such as SHA256, SHA384 or longer hash values may be deployed such as SHA256, SHA384 or SHA512
SHA512 provide, possibly in conjunction with stronger Diffie- provide, possibly in conjunction with stronger Diffie-Hellman
Hellman groups. This is left as for further study. groups. This is left as for further study.
For the sake of improved performance and reduced round trip delay For the sake of improved performance and reduced round trip delay
either party may off-line pre-compute its public Diffie-Hellman either party may off-line pre-compute its public Diffie-Hellman
half-key. half-key.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
On the other side and under reasonable conditions, DHHMAC consumes On the other side and under reasonable conditions, DHHMAC consumes
more CPU cycles than the MIKEY pre-shared key distribution more CPU cycles than the MIKEY pre-shared key distribution
protocol. The same might hold true quite likely for the MIKEY protocol. The same might hold true quite likely for the MIKEY
public-key distribution protocol (depending on choice of the public-key distribution protocol (depending on choice of the
private and public key lengths). private and public key lengths).
As such, it can be said that DHHMAC provides sound performance As such, it can be said that DHHMAC provides sound performance when
when compared with the other MIKEY protocol variants. 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: * Security infrastructure:
This document describes the HMAC-authenticated Diffie-Hellman key This document describes the HMAC-authenticated Diffie-Hellman key
agreement protocol that completely avoids digital signatures and agreement protocol that completely avoids digital signatures and
the associated public-key infrastructure as would be necessary for the associated public-key infrastructure as would be necessary for
the X.509 RSA public-key based key distribution protocol or the the X.509 RSA public-key based key distribution protocol or the
digitally signed Diffie-Hellman key agreement protocol as digitally signed Diffie-Hellman key agreement protocol as described
described in MIKEY. Public-key infrastructures may not always be in MIKEY. Public-key infrastructures may not always be available
available in certain environments nor may they be deemed adequate in certain environments nor may they be deemed adequate for real-
for real-time multimedia applications when taking additional steps time multimedia applications when taking additional steps for
HMAC-authenticated Diffie-Hellman for MIKEY January 2003 certificate validation and certificate revocation methods with
for certificate validation and certificate revocation methods with
additional round-trips into account. additional round-trips into account.
DHHMAC does not depend on PKI nor do implementations require PKI DHHMAC does not depend on PKI nor do implementations require PKI
standards and thus is believed to be much simpler than the more standards and thus is believed to be much simpler than the more
complex PKI facilities. complex PKI facilities.
DHHMAC is particularly attractive in those environments where DHHMAC is particularly attractive in those environments where
provisioning of a pre-shared key has already been accomplished. provisioning of a pre-shared key has already been accomplished.
* NAT/Firewall-friendliness: * NAT/Firewall-friendliness:
DHHMAC is able to operate smoothly through firewall/NAT devices as DHHMAC is able to operate smoothly through firewall/NAT devices as
long as the protected identity information of the end entity is long as the protected identity information of the end entity is not
not an IP /transport address. Of course, DHHMAC does not an IP /transport address. Of course, DHHMAC does not necessarily
necessarily require a firewall/NAT to operate. require a firewall/NAT to operate.
* Scalability: * Scalability:
Like the MIKEY signed Diffie-Hellman protocol, DHHMAC does not Like the MIKEY signed Diffie-Hellman protocol, DHHMAC does not
scale to any larger configurations beyond peer-to-peer groups. scale to any larger configurations beyond peer-to-peer groups.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
5.4. Assumptions 5.4. Assumptions
This document states a couple of assumptions upon which the security This document states a couple of assumptions upon which the security
of DHHMAC significantly depends. It is assumed, that of DHHMAC significantly depends. It is assumed, that
* the parameters xi, xr, s and auth_key are to be kept secret. * the parameters xi, xr, s and auth_key are to be kept secret.
* the pre-shared key s has sufficient entropy and cannot * the pre-shared key s has sufficient entropy and cannot be
beeffectively guessed. effectively guessed.
* the pseudo-random function (PRF) is secure, yields indeed * the pseudo-random function (PRF) is secure, yields indeed the
thepseudo-random property and maintains the entropy. pseudo-random property and maintains the entropy.
* a sufficiently large and secure Diffie-Hellman group is applied. * a sufficiently large and secure Diffie-Hellman group is applied.
* the Diffie-Hellman assumption holds saying basically that even with * the Diffie-Hellman assumption holds saying basically that even with
knowledge of the exchanged Diffie-Hellman half-keys and knowledge knowledge of the exchanged Diffie-Hellman half-keys and knowledge
of the Diffie-Hellman group, it is infeasible to compute the TGK of the Diffie-Hellman group, it is infeasible to compute the TGK or
or to derive the secret parameters xi or xr. The latter is also to derive the secret parameters xi or xr. The latter is also
called the discrete logarithm assumption. Please see [10], [11] called the discrete logarithm assumption. Please see [7], [11] or
or [12] for more background information regarding the Diffie- [12] for more background information regarding the Diffie-Hellman
Hellman problem and its computational complexity assumptions. problem and its computational complexity assumptions.
* the hash function (SHA1) is secure; i.e. that it is computationally * the hash function (SHA1) is secure; i.e. that it is computationally
infeasible to find a message which corresponds to a given message infeasible to find a message which corresponds to a given message
digest, or to find two different messages that produce the same digest, or to find two different messages that produce the same
message digest. message digest.
* the HMAC algorithm is secure and does not leak the auth_key. In * the HMAC algorithm is secure and does not leak the auth_key. In
particular, the security depends on the message authentication particular, the security depends on the message authentication
property of the compression function of the hash function H when property of the compression function of the hash function H when
applied to single blocks (see [2]). applied to single blocks (see [6]).
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
* A source capable of producing sufficiently many bits of * A source capable of producing sufficiently many bits of randomness
randomnessis available. is available.
* The systems upon which DHHMAC runs are sufficiently secure. * The systems upon which DHHMAC runs are sufficiently secure.
The assumptions MUST be met as far as they can be enforced. The assumptions MUST be met as far as they can be enforced.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
5.5. Residual risk 5.5. Residual risk
Although these detailed assumptions are non-negligible, security Although these detailed assumptions are non-negligible, security
experts generally believe that all these assumptions are reasonable experts generally believe that all these assumptions are reasonable
and that the assumptions made can be fulfilled in practice with and that the assumptions made can be fulfilled in practice with
little or no expenses. little or no expenses.
The mathematical and cryptographic assumptions upon the properties of The mathematical and cryptographic assumptions upon the properties of
the PRF, the Diffie-Hellman algorithm (discrete log-assumption), the the PRF, the Diffie-Hellman algorithm (discrete log-assumption), the
HMAC and SHA1 algorithms have not been proved yet nor have they been HMAC and SHA1 algorithms have not been proved yet nor have they been
disproved by the time of this writing. disproved by the time of this writing.
Thus, a certain residual risk remains, which might threaten the Thus, a certain residual risk remains, which might threaten the
overall security at some unforeseeable time in the future. overall security at some unforeseeable time in the future.
The DHHMAC would be compromised as soon as The DHHMAC would be compromised as soon as
* the discrete logarithm problem could be solved efficiently, * the discrete logarithm problem could be solved efficiently,
* the hash function could be subverted (efficient collisions * the hash function could be subverted (efficient collisions become
become feasible), feasible),
* the HMAC method be broken (leaking the auth_key), * the HMAC method be broken (leaking the auth_key),
* systematic brute force attacks are effective by which an attacker * systematic brute force attacks are effective by which an attacker
attempts to discover the shared secret. It is assumed that the attempts to discover the shared secret. It is assumed that the
shared secret yields sufficient entropy to make such attacks shared secret yields sufficient entropy to make such attacks
infeasible, infeasible,
* or some other yet unknown attacking technique will be discovered. * or some other yet unknown attacking technique will be discovered.
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 computational intensive. Neverthess,
elliptic curve Diffie-Hellman (ECDH) allows to cut-down key lengths
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
substantially (say 170 bits or more) while maintaining at least the
security level and providing even 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.
It is not recommended to deploy DHHMAC for any other usage than It is not recommended to deploy DHHMAC for any other usage than
depicted in section 2. Otherwise any such misapplication might lead depicted in section 2. Otherwise any such misapplication might lead
to unknown, undefined properties. to unknown, undefined properties.
6. IANA considerations IANA considerations
This document does not define its own new name spaces for DHHMAC, This document does not define its own new name spaces for DHHMAC,
rather additional values for DHHMAC and EC are defined as part of rather additional values for DHHMAC are defined as part of the MIKEY
the MIKEY fields. Thus, close alignment between DHHMAC values and fields. Thus, close alignment between DHHMAC values and MIKEY
MIKEY values shall be maintained; see also [1] section 10. values shall be maintained; see also [3] section 10.
7. Intellectual Property Rights
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
Intellectual Property Rights
This proposal is in full conformity with [RFC-2026]. This proposal is in full conformity with [RFC-2026].
The author is aware of related intellectual property rights The author is aware of related intellectual property rights
currently being held by Infineon. Pursuant to the provisions of currently being held by Infineon. Pursuant to the provisions of
[RFC-2026], the author represents that he has disclosed the [RFC-2026], the author represents that he has disclosed the
existence of any proprietary or intellectual property rights in existence of any proprietary or intellectual property rights in
the contribution that are reasonably and personally known to the the contribution that are reasonably and personally known to the
author. The author does not represent that he personally knows of author. The author does not represent that he personally knows of
all potentially pertinent proprietary and intellectual property all potentially pertinent proprietary and intellectual property
rights owned or claimed by the organizations he represents or rights owned or claimed by the organizations he represents or
third parties. third parties.
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described pertain to the implementation or use of the technology described
in this document or the extent to which any license under such in this document or the extent to which any license under such
rights might or might not be available; neither does it represent rights might or might not be available; neither does it represent
that it has made any effort to identify any such rights. that it has made any effort to identify any such rights.
Information on the IETF's procedures with respect to rights in Information on the IETF's procedures with respect to rights in
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
standards-track and standards-related documentation can be found standards-track and standards-related documentation can be found
in BCP-11. Copies of claims of rights made available for in BCP-11. Copies of claims of rights made available for
publication and any assurances of licenses to be made available, publication and any assurances of licenses to be made available,
or the result of an attempt made to obtain a general license or or the result of an attempt made to obtain a general license or
permission for the use of such proprietary rights by implementors permission for the use of such proprietary rights by implementors
or users of this specification can be obtained from the IETF or users of this specification can be obtained from the IETF
Secretariat. Secretariat.
8. Acknowledgements References
Normative References
This document incorporates kindly review feedback by Steffen Fries
and Fredrick Lindholm.
9. Conclusions
Key management for environments and applications with real-time and
performance constraints are becoming of interest. Existing key
management techniques like IPSEC-IKE [14] and IPSEC-SOI, TLS [13] and
other schemes are not deemed adequate in addressing sufficiently
those real-time and security requirements.
MIKEY defines three key management security protocols addressing
real-time constraints. DHHMAC described in this document defines a
fourth MIKEY variant aiming at the same target.
While each of the four key management protocols has its own merits [1] Bradner, S., "The Internet Standards Process -- Revision 3",
there are also certain limitations of each approach. As such there BCP 9, RFC 2026, October 1996.
is no single ideal solution and none of the variants is able to
subsume the other remaining variants.
It is concluded that DHHMAC features useful security and performance [2] Bradner, S., "Key words for use in RFCs to Indicate
properties that none of the other three MIKEY variants is able to Requirement Levels", BCP 14, RFC 2119, March 1997.
provide.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003 [3] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman;
"MIKEY: Multimedia Internet KEYing", Internet Draft <draft-ietf-
msec-mikey-06.txt>, Work in Progress (MSEC WG), IETF, February
2003.
10. Normative References [4] NIST, FIBS-PUB 180-1, "Secure Hash Standard", April 1995,
http://csrc.nist.gov/fips/fip180-1.ps.
[1] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman; [5] J. Arkko, E. Carrara et al: "Key Management Extensions for SDP
"MIKEY:Multimedia Internet KEYing", Internet Draft <draft-ietf-msec- and RTSP", Internet Draft <draft-ietf-mmusic-kmgmt-ext-07.txt>,
mikey-05.txt>, Work in Progress (MSEC WG) Work in Progress (MMUSIC WG), IETF, February 2003.
[2] H. Krawczyk, M. Bellare, R. Canetti; "HMAC: Keyed-Hashing for [6] H. Krawczyk, M. Bellare, R. Canetti; "HMAC: Keyed-Hashing for
Message Authentication", RFC 2104, February 1997. Message Authentication", RFC 2104, February 1997.
[3] NIST, FIBS-PUB 180-1, "Secure Hash Standard", April 1995, Informative References
http://csrc.nist.gov/fips/fip180-1.ps
[4] S. Bradner: "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
11. Informative References
[5] A.J. Menezes, P v. Oorschot, S. Vanstone: "Applied Cryptography",
CRC Press, 1996
[6] E. Rescorla, B. Korver: " Guidelines for Writing RFC Text on [7] A.J. Menezes, P. van Oorschot, S. A. Vanstone: "Handbook of
Security Considerations", Work in Progress <draft-iab-sec-cons-01.txt>, Applied Cryptography", CRC Press 1996.
October 2002.
[7] D. Eastlake, S. Crocker: "Randomness Recommendations for Security", [8] E. Rescorla, B. Korver: " Guidelines for Writing RFC Text on
1750, IETF, December 1994. Security Considerations", Work in Progress <draft-iab-sec-cons-
03.txt>, IETF, January 2003.
[8] S.M. Bellovin, J. I. Schiller: "Security Mechanisms for the [9] D. Eastlake, S. Crocker: "Randomness Recommendations for
Internet", Work in Progress <draft-iab-secmech-01.txt>, June 2002. HMAC-authenticated Diffie-Hellman for MIKEY June 2003
[9] S. Bradner: "The Internet Standards Process -- Revision 3", RFC Security", RFC 1750, IETF, December 1994.
2026, IETF, October 1996
[10] A.J. Menezes, P. van Oorschot, S. A. Vanstone: "Handbook of [10] S.M. Bellovin, C. Kaufman, J. I. Schiller: "Security
Applied Cryptography", CRC Press 1996. Mechanisms for the Internet", Work in Progress <draft-iab-
secmech-02.txt>, IETF, January 2003.
[11] Ueli M. Maurer, S. Wolf: "The Diffie-Hellman Protocol", Designs, [11] Ueli M. Maurer, S. Wolf: "The Diffie-Hellman Protocol",
Codes, and Cryptography, Special Issue Public Key Cryptography, Kluwer Designs, Codes, and Cryptography, Special Issue Public Key
Academic Publishers, vol. 19, pp. 147-171, 2000. Cryptography, Kluwer Academic Publishers, vol. 19, pp. 147-171,
ftp://ftp.inf.ethz.ch/pub/crypto/publications/MauWol00c.ps 2000. ftp://ftp.inf.ethz.ch/pub/crypto/publications/MauWol00c.ps
[12] Discrete Logarithms and the Diffie-Hellman Protocol; [12] Discrete Logarithms and the Diffie-Hellman Protocol;
http://www.crypto.ethz.ch/research/ntc/dldh/ http://www.crypto.ethz.ch/research/ntc/dldh/
[13] T. Dierks, C. Allen: "The TLS Protocol Version 1.0.", RFC 2246, [13] T. Dierks, C. Allen: "The TLS Protocol Version 1.0.", RFC 2246,
IETF, January 1999. IETF, January 1999.
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
[14] D. Harkins, D. Carrel: "The Internet Key Exchange (IKE).", RFC [14] D. Harkins, D. Carrel: "The Internet Key Exchange (IKE).", RFC
2409, IETF, November 1998. 2409, IETF, November 1998.
[15] Donald E. Eastlake, Jeffrey I. Schiller, Steve Crocker: [15] Donald E. Eastlake, Jeffrey I. Schiller, Steve Crocker:
"Randomness Requirements for Security"; <draft-eastlake-randomness2- "Randomness Requirements for Security"; <draft-eastlake-
03.txt>; Work in Progress, IETF, 7/2002. randomness2-03.txt>; Work in Progress, IETF, July 2002.
[16] J. Schiller: "Strong Security Requirements for Internet [16] J. Schiller: "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", RFC 3365, IETF; 2002. Engineering Task Force Standard Protocols", RFC 3365, IETF,
2002.
[17] C. Meadows: "Advice on Writing an Internet Draft Amenable to [17] C. Meadows: "Advice on Writing an Internet Draft Amenable to
Security Analysis", Work in Progress < draft-irtf-cfrg-advice-00.txt>, Security Analysis", Work in Progress <draft-irtf-cfrg-advice-
October 2002. 00.txt>, IRTF, October 2002.
[18] Steven M. Bellovin: "Security Mechanisms for the Internet", Work [18] T. Narten: "Guidelines for Writing an IANA Considerations
in Progress <draft-iab-secmech-01.txt>, June 2002. Section in RFCs", RFC 2434, IETF, October 1998.
[19] T. Narten: "Guidelines for Writing an IANA Considerations Section [19] J. Reynolds: "Instructions to Request for Comments (RFC)
in RFCs", RFC 2434, October 1998. Authors", Work in Progress, <draft-rfc-editor-rfc2223bis-
04.txt>, IETF, 24 February 2003.
[20] J. Reynolds: "Instructions to Request for Comments (RFC) Authors", [20] J. Rosenberg et all: "SIP: Session Initiation Protocol", RFC
Work in Progress, <draft-rfc-editor-rfc2223bis-03.txt>, IETF, 12 3261, IETF, June 2002.
October 2002.
[21] J. Rosenberg et all: "SIP: Session Initiation Protocol", RFC 3261, [21] Ch. Kaufman: "Internet Key Exchange (IKEv2) Protocol", Work in
June 2002. HMAC-authenticated Diffie-Hellman for MIKEY June 2003
[22] J. Arkko et al: "Key Management Extensions for SDP and RTSP", Work Progress (IPSEC WG), <draft-ietf-ipsec-ikev2-08.txt>, IETF, May
in Progress, <draft-ietf-mmusic-kmgmt-ext-05.txt>, June, 2002. 2003.
12. Author's Address [22] Draft ITU-T Recommendation H.235 Annex G: "Usage of the Secure
Real Time Transport Protocol (SRTP) in conjunction with the
MIKEY Key Management Protocol within H.235"; 5/2003.
Please address all comments to: Acknowledgments
Martin Euchner Siemens AG This document incorporates kindly review feedback by Steffen Fries
Email: martin.euchner@siemens.com ICN M SR 3 and Fredrick Lindholm and general feedback by the MSEC WG.
Phone: +49 89 722 55790 Hofmannstr. 51
Fax: +49 89 722 62366
81359 Munich, Germany Conclusions
13. Full Copyright Statement Key management for environments and applications with real-time and
performance constraints are becoming of interest. Existing key
management techniques like IPSEC-IKE [14] and IPSEC-IKEv2 [22], TLS
[13] and other schemes are not deemed adequate in addressing
sufficiently those real-time and security requirements.
MIKEY defines three key management security protocols addressing
real-time constraints. DHHMAC as described in this document defines
a fourth MIKEY variant aiming at the same target.
While each of the four key management protocols has its own merits
there are also certain limitations of each approach. As such there
is no single ideal solution and none of the variants is able to
subsume the other remaining variants.
It is concluded that DHHMAC features useful security and performance
properties that none of the other three MIKEY variants is able to
provide.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
HMAC-authenticated Diffie-Hellman for MIKEY January 2003
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than followed, or as required to translate it into languages other than
English. English.
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
14. Expiration Date Expiration Date
This Internet Draft expires on 30 June 2003. This Internet Draft expires on 30 December 2003.
15. Revision History Revision History
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÷.
HMAC-authenticated Diffie-Hellman for MIKEY June 2003
* a few editorials and nits.
* references adjusted and cleaned-up.
Changes against draft-ietf-msec-mikey-dhhmac-00.txt: Changes against draft-ietf-msec-mikey-dhhmac-00.txt:
* category set to proposed standard. * category set to proposed standard.
* identity protection clarified. * identity protection clarified.
* aligned with MIKEY-05 DH protocol, notation and with payload * aligned with MIKEY-05 DH protocol, notation and with payload
* some editorials and nits. * some editorials and nits.
Changes against draft-euchner-mikey-dhhmac-00.txt: Changes against draft-euchner-mikey-dhhmac-00.txt:
* made a MSEC WG draft * made a MSEC WG draft
* aligned with MIKEY-03 DH protocol, notation and with payload * aligned with MIKEY-03 DH protocol, notation and with payload
formats formats
* clarified that truncated HMAC actually truncates the HMAC result * clarified that truncated HMAC actually truncates the HMAC result
rather than the SHA1 intermediate value. rather than the SHA1 intermediate value.
* improved security considerations section completely rewritten in * improved security considerations section completely rewritten in
the spirit of [6]. the spirit of [8].
* IANA consideration section added * IANA consideration section added
* a few editorial improvements and corrections * a few editorial improvements and corrections
* IPR clarified and IPR section changed. * 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
 End of changes. 

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