MSEC S. Fries
Internet-Draft Siemens
Expires: November 17, 2006 D. Ignjatic
Polycom
May 16, 2006
On the applicability of various MIKEY modes and extensions
draft-ietf-msec-mikey-applicability-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
Multimedia Internet Keying - MIKEY - is a key management scheme that
can be used for real-time applications. In particular, it has been
defined focusing on the support of the Secure Real-time Transport
Protocol. MIKEY itself defines four key distribution schemes.
Moreover, it is defined to allow extensions of the protocol. As
MIKEY becomes more and more accepted, extensions to the base protocol
arose, especially in terms of additional key distribution schemes,
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but also in terms of payload enhancements.
This document provides an overview about MIKEY in general as well as
the existing extensions in MIKEY, which have been defined or are in
the process of definition.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
3. MIKEY Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Symmetric key distribution . . . . . . . . . . . . . . . . 6
3.2. Asymmetric key distribution . . . . . . . . . . . . . . . 6
3.3. Diffie-Hellman key agreement protected with digital
signatures . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Unprotected key distribution . . . . . . . . . . . . . . . 8
4. MIKEY Extensions . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Diffie-Hellman key agreement protected with pre-shared
secrets . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. SAML assisted DH-key agreement . . . . . . . . . . . . . . 9
4.3. Asymmetric key distribution with in-band certificate
exchange . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. ECC algorithms support . . . . . . . . . . . . . . . . . . 11
4.4.1. Elliptic Curve Integrated Encryption Scheme
application in MIKEY . . . . . . . . . . . . . . . . . 12
4.4.2. Elliptic Curve Menezes-Qu-Vanstone Scheme
application in MIKEY . . . . . . . . . . . . . . . . . 12
4.5. New Payload for bootstrapping TESLA . . . . . . . . . . . 12
4.6. New Key ID information type . . . . . . . . . . . . . . . 13
4.7. Supporting Integrity Transform carrying the Rollover
Counter . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.8. OMA BCAST MIKEY General Extension Payload Specification . 14
5. Selection and interworking of MIKEY modes . . . . . . . . . . 14
5.1. MIKEY and Early Media . . . . . . . . . . . . . . . . . . 15
5.2. MIKEY and Forking . . . . . . . . . . . . . . . . . . . . 16
5.3. MIKEY and Call Transfer . . . . . . . . . . . . . . . . . 16
6. Transport of MIKEY messages . . . . . . . . . . . . . . . . . 16
7. Summary of MIKEY related IANA Registrations . . . . . . . . . 17
8. MIKEY alternatives for SRTP security parameter negotiation . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
Key distribution describes the process of delivering cryptographic
keys to the required parties. MIKEY [RFC3830], the Multimedia
Internet Keying, has been defined focusing on support for the
establishment of security context for the Secure Real-time Transport
Protocol [RFC3711]. Note that MIKEY is not restricted to be used for
SRTP only, as it features a generic approach and allows for
extensions to the key distribution schemes, but also for the payload
associated with the protocol using the distributed security context.
MIKEY defines four key distribution schemes as there are:
o Symmetric key distribution
o Asymmetric key distribution
o Diffie-Hellman key agreement protected by digital signatures
o Unprotected key distribution
There have been scenarios where none of the above schemes fits
perfectly, so extensions have been defined. These extensions
comprise new key distribution schemes, algorithm enhancements, new
payload definitions supporting other protocols than SRTP. The key
distribution scheme extensions are defined in the following
documents:
o Diffie-Hellman key agreement protected by symmetric pre-shared
keys as defined in [I-D.ietf-msec-mikey-dhhmac]
o SAML assisted Diffie-Hellman key agreement as defined [Reference
to draft-moskowitz-MIKEY-SAML-DH]
o Asymmetric key distribution (based on asymmetric encryption) with
in-band certificate provision as defined in [I-D.ietf-msec-mikey-
rsa-r]
Algorithm extensions are defined in the following document:
o ECC algorithms for MIKEY as defined in [I-D.ietf-msec-mikey-ecc]
Payload extensions are defined in the following documents:
o Bootstrapping TESLA, defining a new payload for the Timed
Efficient Stream Loss-tolerant Authentication protocol [RFC4082]
as defined in [RFC4442]
o The Key ID information type for the general extension payload as
defined in [I-D.ietf-msec-newtype-keyid]
o Integrity Transform Carrying Roll-over Counter for SRTP, as
defined in [I-D.lehtovirta-srtp-rcc]
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o OMA BCAST MIKEY General Extension Payload Specification, as
defined in [I-D.dondeti-msec-mikey-genext-oma]
This document provides an overview about MIKEY and the relations to
the different extensions to provide a framework when using MIKEY. It
is intended as additional source of information for developers or
architects to provide more insight in use case scenarios and
motivations as well as advantages and disadvantages for the different
key distribution schemes. This document may be enhanced as soon as
new extensions to MIKEY appear. It has been seen that enhancing the
overview document requires much less effort than enhancing an
established standard.
2. Terminology and Definitions
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 [RFC2119].
The following definitions have mostly been taken from [RFC3830]:
(Data) Security Protocol: the security protocol used to protect the
actual data traffic. Examples of security protocols are IPsec and
SRTP.
Data Security Association (Data SA): information for the security
protocol, including a TEK and a set of parameters/policies.
Crypto Session (CS): uni- or bi-directional data stream(s), protected
by a single instance of a security protocol.
Crypto Session Bundle (CSB): collection of one or more Crypto
Sessions, which can have common TGKs (see below) and security
parameters.
Crypto Session ID: unique identifier for the CS within a CSB.
Crypto Session Bundle ID (CSB ID): unique identifier for the CSB.
TEK Generation Key (TGK): a bit-string agreed upon by two or more
parties, associated with CSB. From the TGK, Traffic-encrypting Keys
can then be generated without needing further communication.
Traffic-Encrypting Key (TEK): the key used by the security protocol
to protect the CS (this key may be used directly by the security
protocol or may be used to derive further keys depending on the
security protocol). The TEKs are derived from the CSB's TGK.
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TGK re-keying: the process of re-negotiating/updating the TGK (and
consequently future TEK(s)).
Initiator: the initiator of the key management protocol, not
necessarily the initiator of the communication.
Responder: the responder in the key management protocol.
Salting key: a random or pseudo-random (see [RAND, HAC]) string used
to protect against some off-line pre-computation attacks on the
underlying security protocol.
HDR: denotes the protocol header
PRF(k,x): a keyed pseudo-random function
E(k,m): encryption of m with the key k
RAND: Random value
T: Timestamp
CERTx: the certificate of x
SIGNx: the signature from x using the private key of x
PKx: the public key of x
IDx: the identity of x
[] an optional piece of information
{} denotes zero or more occurrences
|| concatenation
| OR (selection operator)
^ exponentiation
XOR exclusive or
3. MIKEY Overview
This section will provide an overview about the MIKEY base document.
The focus lies here on the key distribution schemes as well as the
discussion about advantages and disadvantages of the different
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schemes. Note that the MIKEY key distribution schemes rely on
loosely synchronized clocks. Thus should be realized by a secure
network clock synchronization protocol. MIKEY recommends for this
the ISO time synchronization protocol [ISO_sec_time]. The format
applied to the timestamps submitted in the MIKEY have to match the
NTP format described in [RFC1305]. In other cases, such as of a SIP
endpoint clock synchronization by deriving time from a trusted
outbound proxy may be appropriate.
3.1. Symmetric key distribution
This option of the key management uses a pre-shared secret key to
derive key material for integrity protection and encryption to
protect the actual exchange of key material. Note that the pre-
shared secret is agreed upon before the session, e.g., by out-of-band
means. The response message is optional and may be used for mutual
authentication or error signaling.
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi],[IDr],
{SP}, KEMAC --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
The advantages of this approach lay in the fact that there is no
dependency on a PKI (Public Key Infrastructure), the solution
consumes low bandwidth and enables high performance, and is all in
all a simple straightforward master key provisioning. The
disadvantages are that no perfect forward secrecy is provided and key
generation is just performed by the initiator. Furthermore, the
approach is not scaleable to larger configurations but acceptable in
small-sized groups. Note, according to [RFC3830] this option is
mandatory to implement.
3.2. Asymmetric key distribution
Using the asymmetric option of the key management, the initiator
generates the key material to be transmitted and sends it encrypted
with the responder's public key. Additionally a so-called envelope
key is transmitted, encrypted with the receiver's public key. The
envelope key env-key, which is a random number, is used to derive the
auth-key and the enc-key. Moreover, the envelope key is used to
protect the signaling message and may be used as a pre-shared key to
establish further crypto sessions. The response message is optional
and may be used for mutual authentication or error signaling.
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Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
[IDr], {SP}, KEMAC, [CHASH],
PKE, SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
An advantage of this approach are that the usage of self-signed
certificates can avoid PKI. Note that using self-signed certificates
may result in limited scalability and complex provisioning. The
disadvantages comprise the necessity of a PKI for fully scalability,
the performance of the key generation just by the initiator, and no
provision of perfect forward secrecy. Furthermore, the verification
of certificates may not be done in real-time. This could be the case
in scenarios where the revocation status of certificates is checked
through a further component. Note, according to [RFC3830] this
option is mandatory to implement.
3.3. Diffie-Hellman key agreement protected with digital signatures
The Diffie-Hellman option of the key management enables a shared
secret establishment between initiator and responder in a way where
both parties contribute to the shared secret. The Diffie-Hellman key
agreement is authenticated (and integrity protected) using digital
signatures.
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
[IDr], {SP}, DHi, SIGNi --->
R_MESSAGE =
<--- HDR, T, [IDr|CERTr],
IDi, DHr, DHi, SIGNr
[RFC3830] does not mandate any specific asymmetric algorithm for the
signature calculation. The algorithm used for signature or public
key encryption is rather defined by, and dependent on the certificate
used. Besides the use of X.509v3 certificates it is mandatory to
support the Diffie-Hellmann group "OAKLEY5" [RFC2412]. The
advantages of this approach are a fair, mutual key agreement, perfect
forward secrecy, and the option to use self-signed certificates to
avoid PKI (would result in limited scalability and more complex
provisioning). Negatively to remark is that this approach just
scales to point-to-point groups and depends on PKI for full
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scalability. Moreover, it has a limited performance since expensive,
non-real time certificate validation has to be done.
3.4. Unprotected key distribution
MIKEY also supports a mode to provide a key in an unprotected manner.
This is based on the pre-shared key option depicted in Section 3.1
and may be compared with the plain approach in sdescriptions
[I-D.ietf-mmusic-sdescriptions]. This MIKEY scheme is based on the
symmetric key distribution approach described in Section 3.1, but is
used with the NULL encryption and the NULL authentication algorithm.
It completely relies on the security of the underlying layer, e.g.,
provided by TLS This option should be used with caution as it does
not protect the key management.
4. MIKEY Extensions
This section will provide an overview about the MIKEY extensions for
key distribution schemes, crypto algorithms and payloads which have
been defined in several documents.
4.1. Diffie-Hellman key agreement protected with pre-shared secrets
This is an additional option which has been defined in [I-D.ietf-
msec-mikey-dhhmac]. In contrast to the method described in
Section 3.3 here the Diffie-Hellmann key agreement is authenticated
(and integrity protected) using a pre-shared secret and keyed hash
function.
Initiator Responder
I_MESSAGE =
3D HDR, T, RAND, [IDi],
IDr, {SP}, DHi, KEMAC --->
R_MESSAGE =
<--- 3D HDR, T,[IDr], IDi,
DHr, DHi, KEMAC
TGK =3D g^(xi * yi) TGK =3D g^(xi * yi)
For the integrity protection of the Diffie-Hellman key agreement
[I-D.ietf-msec-mikey-dhhmac] mandates the use of HMAC SHA-1.
Regarding Diffie-Hellman groups [RFC3830] is referenced. Thus, it is
mandatory to support the Diffie-Hellman group "OAKLEY5" [RFC2412].
This option has also several advantages, as there are the fair mutual
key agreement, the perfect forward secrecy, and no dependency on a
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PKI and PKI standards. Moreover, this scheme has a sound performance
and reduced bandwidth requirements and provides a simple and
straightforward master key provisioning. The scalability of this
approach just to point-to-point groups is a disadvantage.
This mode of operation provides an efficient scheme in deployments
where there is a central trusted server that is provisioned with
shared secrets for many clients. Such setups could for example be
enterprise PBXs, service provider proxies, etc.
4.2. SAML assisted DH-key agreement
This document [Reference to draft-moskowitz-MIKEY-SAML-DH] is
targeted to fulfill the general requirements on key management
approaches repeated here:
1. Mutual authentication of involved parties
2. Both parties involved contribute to the session key generation
3. Provide perfect forward secrecy
4. Support distribution of group session keys
5. Provide liveliness tests when involved parties do not have a
reliable clock
6. Support of limited parties involved
To fullfill all of the requirements, the document proposes the use of
a classic Diffie-Hellman key agreement protocol for key establishment
in conjunction with UA's SIP server signed element authenticating the
Diffie-Hellman key and the ID using the SAML (Security Association
Markup Language, [SAML_overview]) approach. Here the client's public
Diffie-Hellman-credentials are signed by the server to form a SAML
assertion [CRED], which may be used for later sessions with other
clients. This assertion needs at least to convey the ID, public DH
key, expiry, and the signature from the server. This provides the
involved clients with mutual authentication and message integrity of
the key management messages exchanged.
Initiator Responder
I_MESSAGE =
HDR, T, RAND1, [CREDi],
IDr, {SP} --->
R_MESSAGE =
<--- HDR, T, [CREDr], IDi, DHr,
RAND2, (SP)
TGK = HMACx(RAND1|RAND2), where x = g^(xi * xr).
Additionally the document proposes a second roundtrip to avoid the
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dependence on synchronized clocks and provide liveliness checks.
This is achieved by exchanging nonces, protected with the session
key. This second roundtrip can also be used for distribution of
group keys or for the leverage of a weak DH key for a stronger
session key. The trigger for the second round trip would be handled
via SP, the Security Policy communicated via MIKEY.
Initiator Responder
I_MESSAGE =
HDR, SIGN(ENC(RAND3)) --->
R_MESSAGE =
<--- SIGN(ENC(RAND4))
Note if group keys are to be provided RAND would be substituted by
that group key.
With the second roundtrip, this approach also provides an option for
all of the other key distribution methods, when liveliness checks are
needed. The drawback of the second roundtrip is that these messages
need to be integrated into the call flow of the signaling protocol.
In straight forward call one roundtrip may be enough to setup a
session. Thus this second roundtrip would require additional
messages to be exchanged.
4.3. Asymmetric key distribution with in-band certificate exchange
This is an additional option which has been defined in [I-D.ietf-
msec-mikey-rsa-r]. It describes the asymmetric key distribution with
optional in-band certificate exchange.
Initiator Responder
I_MESSAGE =
HDR, T, [IDi|CERTi], [IDr],
{SP}, [RAND], SIGNi --->
R_MESSAGE =
<--- HDR, [GenExt(CSB-ID)], T,
RAND, [IDr|CERTr], [SP],
KEMAC, SIGNr
This option has some advantages compared to the asymmetric key
distribution stated in Section 3.2. Here, the sender and receiver do
not need to know the certificate of the other peer in advance as it
may be sent in the MIKEY initiator message. Thus, the receiver of
this message can utilize the received key material to encrypt the
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session parameter and send them back as part of the MIKEY response
message. The certificate check may be done depending on the signing
authority. If the certificate is signed by an publicly accepted
authority the certificate validation is done on the common base. In
the other case additional steps may be necessary. The disadvantage
is that no perfect forward secrecy is provided.
This mode is meant to provide a low cost solution when PKI is present
and/or required. Specifically in SIP, session invitations can be
retargeted or forked. MIKEY modes that require the Initiator to
target a single well known Responder may be impractical here as they
may require multiple roundtrips to do key negotiation. By allowing
the Responder to generate secret material used for key derivation
this mode allows for an efficient key delivery scheme. Note that the
Initiator can contribute to the material the key is derived from
through CSB-ID and RAND payloads in unicast use cases. This mode is
also useful in multicast scenarios where multiple clients are
contacting a known server and are downloading the key. Server
workload is significantly reduced in these scenarios compared to
MIKEY in public key mode. Examples of deployments where this mode
can be used are enterprises with PKI, service provider setups where
the service provider decides to provision certificates to its users,
etc.
4.4. ECC algorithms support
[I-D.ietf-msec-mikey-ecc] proposes extensions to the authentication,
encryption and digital signature methods described for use in MIKEY,
employing elliptic-curve cryptography (ECC). These extensions are
defined to align MIKEY with other ECC implementations and standards.
The motivation for supporting ECC within the MIKEY stems from the
following advantages:
o ECC support is generally added to security protocols
o ECC support requires considerably smaller keys by keeping the same
security level compared to other asymmetric techniques (like RSA).
Elliptic curve algorithms are capable of providing security
consistent with AES keys of 128, 192, and 256 bits without
extensive growth in asymmetric key sizes.
o As stated in [I-D.ietf-msec-mikey-ecc] implementations have shown
that elliptic curve algorithms can significantly improve
performance and security-per-bit over other recommended
algorithms.
These advantages make the usage of ECC especially interesting for
embedded devices, which may have only limited performance and storage
capabilities.
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[I-D.ietf-msec-mikey-ecc] proposes several ECC based mechanisms to
enhance the MIKEY key distribution schemes, as there are:
o Use of ECC methods with public-key encryption (MIKEY-RSA); ECDSA
o Use of Elliptic Curve Integrated Encryption Scheme (MIKEY-ECIES)
o Use of ECC methods with Diffie-Hellman key exchange (MIKEY-DHSIGN)
o Use of Elliptic Curve Menezes-Qu-Vanstone (MIKEY-MQV)
The following subsections will provide more detailed information
about the message exchanges for MIKEY-ECIES and MIKEY-MQV.
4.4.1. Elliptic Curve Integrated Encryption Scheme application in MIKEY
The following figure shows the message exchange for the MIKEY-ECIES
scheme:
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
[IDr], {SP}, ECCPT, KEMAC,
[CHASH], SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
4.4.2. Elliptic Curve Menezes-Qu-Vanstone Scheme application in MIKEY
The following figure shows the message exchange for the MIKEY-MQV
scheme:
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
[IDr], {SP}, ECCPT, KEMAC,
[CHASH], SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
4.5. New Payload for bootstrapping TESLA
TESLA [RFC4082] is a protocol for providing source authentication in
multicast scenarios. TESLA is an efficient protocol with low
communication and computation overhead, which scales to large numbers
of receivers, and also tolerates packet loss. TESLA is based on
loose time synchronization between the sender and the receivers.
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Source authentication is realized in TESLA by using Message
Authentication Code (MAC) chaining. The use of TESLA within the
Secure Real-time Transport Protocol (SRTP) has been published in
[RFC4383] targeting multicast authentication in scenarios, where SRTP
is applied to protect the multimedia data. This solution assumes
that TESLA parameters are made available by out-of-band mechanisms.
[RFC4442] specifies payloads for MIKEY to bootstrap TESLA for source
authentication of secure group communications using SRTP. TESLA may
be bootstrapped using one of the MIKEY key management approaches
described above sent via unicast, multicast or broadcast. This
approach provides the necessary parameter payload extensions for the
usage of TESLA in SRTP but is not limited to this.
4.6. New Key ID information type
This extension specifies a new Type (the Key ID Information Type) for
the General Extension Payload. This is used in, e.g., the Multimedia
Broadcast/Multicast Service (MBMS) specified in the 3rd Generation
Partnership Project (3GPP). MBMS requires the use of MIKEY to convey
the keys and related security parameters needed to secure the
multimedia that is multicast or broadcast.
One of the requirements that MBMS puts on security is the possibility
to perform frequent updates of the keys. The rationale behind this
is that it should be inconvenient for subscribers to publish the
decryption keys enabling non-subscribers to view the content. To
implement this, MBMS uses a three level key management, to distribute
group keys to the clients, and be able to re-key by pushing down a
new group key. MBMS has the need to identify, which types of key are
involved in the MIKEY message, and their identity.
[I-D.ietf-msec-newtype-keyid] specifies a new Type for the General
Extension Payload in MIKEY, to identify the type and identity of
involved keys.
4.7. Supporting Integrity Transform carrying the Rollover Counter
The document [I-D.lehtovirta-srtp-rcc] defines a new integrity
transform for SRTP [RFC3711] providing the option to also transmit
the Roll Over Counter (ROC) as part of dedicated SRTP packets. This
extension has been defined for the use in the 3GPP multicast/
broadcast service. While the communicating parties did agree on a
starting ROC, in some cases the receiver will not be able to
synchronize his ROC with the one used by the sender even if it is
signaled to him out of band. Here the new extension provides the
possibility for the receiver to re-synchronize to the sender's ROC.
To signal the use of the new integrity transform new definitions for
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certain MIKEY payloads need to be made. These MIKEY new definition
comprise the integrity transform s and new integrity transform
parameter. Moreover, the document specifies integrity parameter, to
enable the usage of different integrity transforms for SRTP and
SRTCP.
4.8. OMA BCAST MIKEY General Extension Payload Specification
The document [I-D.dondeti-msec-mikey-genext-oma] specifies a new
general extension payload type for use in the Open Mobile Alliance's
(OMA) Browser and Content Broadcast (BCAST) group. OMA BCAST's
service and content protection specification uses short term key
message and long term key message payloads that in certain broadcast
distribution systems are carried in MIKEY. The document defines the
payloads for both, short term and long term key messages as part of
the general extension payload. Note, that only a parameter
description is included, but no key information.
5. Selection and interworking of MIKEY modes
While MIKEY and its extensions provide plenty of choice in terms of
modes of operation an implementation may choose to simplify its
behavior. This can be achieved by operating in a single mode of
operation when in Initiator's role. Where PKI is available and/or
required an implementation may choose for example to start all
sessions in RSA-R mode but it would be trivial for it to act as a
Responder in public key mode. If envelope keys are cached it can
then also choose to do re-keying in shared key mode. In general,
modes of operation where the Initiator generates keying material are
useful when two peers are aware of each other before the MIKEY
communication takes place. If an implementation chooses not to
operate in shared key mode its behavior may be identical to a peer
that does but lacks the shared key. Similarly, if a peer chooses not
to operate in the public key mode it may reject the certificate of
the Initiator. The same applies to peers that choose to operate in
one of the DH modes exclusively.
Forward MIKEY modes like public key or shared key mode when used in
SIP/SDP may lead to complications in some calls scenarios, for
example forking scenarios key derivation material gets distributed to
multiple parties. As mentioned earlier this may be impractical as
some of the destinations may not have the resources to validate the
message and may cause the initiator to drop the session invitation.
Even in the case all parties involved have all the prerequisites for
interpreting the MIKEY message received there is a possible problem
with multiple responders starting media sessions using the same key.
While the SSRCs will be different in most of the cases they are only
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sixteen bits long and there is a high probability of a two time pad
problem. As suggested earlier forward modes are most useful when the
two peers are aware of each other before the communication takes
place (as is the case in key renewal scenarios when costly public key
operations can be avoided by using the envelope key).
Choosing between the different modes of MIKEY depends strongly on the
use case. This document may discuss further scenarios to argue for
preferred modes. The following call scenarios provide a list of
potential call scenarios and are matter of discussion:
o Early Media
o Forking
o Call Transfer
5.1. MIKEY and Early Media
In early media scenarios, SRTP data may be received before the answer
over the SIP signaling arrives. The two MIKEY modes, which only
require one message to be transported (Section 3.1 and Section 3.2),
work nicely in early media situations, as both, sender and receiver
have all the necessary parameters in place before actually sending/
receiving encrypted data. The other modes, featuring either Diffie-
Hellman key agreement (Section 3.3, Section 4.1, and Section 4.2) or
the enhanced asymmetric variant (Section 4.3) suffer from the
requirements that the initiator has to wait for the response before
being able to decrypt the incoming SRTP media. In fact, even if
early media is not used, in other words if media is not sent before
the SDP answer a similar problem may arise from the fact that SIP/SDP
signaling has to traverse multiple proxies on its way back and media
may arrive before the SDP answer. It is expected that this delay
would be significantly shorter than in the case of early media
though.
It is worth mentioning here that security descriptions ([I-D.ietf-
mmusic-sdescriptions]) have the same problem as the initiating end
needs the SDP answer before it can start decrypting SRTP media.
To cope with the early media problem there are further approaches to
describe security preconditions [I-D.ietf-mmusic-
securityprecondition], i.e., certain preconditions need to be met to
enable voice data encryption. One example is for instance that a
scenario where a provisional response, containing the required MIKEY
parameter, is sent before encrypted media is processed.
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5.2. MIKEY and Forking
In SIP forking scenarios a SIP proxy server sends an INVITE request
to more than one location. This means that also the MIKEY payload,
which is part of the SDP is sent to several (different) locations.
MIKEY modes supporting signatures may be used in forking scenarios
(Section 3.3 and Section 4.3) as here the receiver can validate the
signature. There are limitations with the symmetric key encryption
as well as the asymmetric key encryption modes (Section 3.1 and
Section 3.2). This is due to the fact that in symmetric encryption
the recipient needs to possess the symmetric key before handling the
MIKEY data. For asymmetric MIKEY modes, if the sender is aware of
the forking he may not know in advance to which location the INVITE
is forked and thus may not use the right receiver certificate to
encrypt the MIKEY envelope key. Note, the sender may include several
MIKEY containers into the same INVITE message to cope with forking,
but this requires the knowledge of all forking targets in advance and
also requires the possession of the target certificates. It is out
of the scope of MIKEY to specify behavior in such a case. DH modes
or the Section 4.3 do not have this problem. In scenarios, where the
sender is not aware of forking, only the intended receiver is able to
decrypt the MIKEY container.
If forking is combined with early media the situation gets
aggravated. If MIKEY modes requiring full roundtrip are used, like
the signed Diffie-Hellman, multiple responses may overload the end
device. An example is forking to 30 destinations (group pickup),
while MIKEY is used with the signed Diffie-Hellman mode together with
security preconditions. Here, every target would answer with a
provisional response, leading to 30 signature validations and Diffie-
Hellman calculations at the senders site. This may lead to a
prolonged media setup delay.
5.3. MIKEY and Call Transfer
In a SIP environment MIKEY exchange is tied to SDP offer/answer and
irrespective of the implementation model used for call transfer the
same properties and limitations of MIKEY modes apply as in a normal
call setup scenarios.
6. Transport of MIKEY messages
MIKEY defines message formats to transport key information and
security policies between communicating entities. It does not define
the embedding of these messages into the used signaling protocol.
This definition is provided in separate documents, depending on the
used signaling protocol.
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Several IETF defined protocols utilize the Session Description
Protocol (SDP, [RFC2327]) to transport the session parameters.
Examples are the Session Initiation Protocol (SIP, [RFC3261] or the
Gateway Control Protocol (GCP, [RFC3525]). The transport of MIKEY
messages as part of SDP is described in [I-D.ietf-mmusic-kmgmt-ext].
Here, the complete MIKEY message is base64 encoded and transmitted as
part of the SDP part of the signaling protocol message. Note, as
several key distribution messages may be transported within one SDP
container, [I-D.ietf-mmusic-kmgmt-ext] also comprises an integrity
protection regarding all supplied key distribution attempts. Thus,
bidding down attacks will be recognized.
MIKEY is also applied in ITU-T protocols like H.323, which is used to
establish communication sessions similar to SIP. For H.323 a
security framework exists, which is defined in H.235. Within this
framework H.235.7 [H.235.7] describes the usage of MIKEY and SRTP in
the context of H.323. In contrast to SIP H.323 uses ASN.1 (Abstract
Syntax Notation). Thus there is no need to encode the MIKEY
container as base64. Within H.323 the MIKEY container is binary
encoded.
7. Summary of MIKEY related IANA Registrations
For MIKEY and the extensions to MIKEY IANA registrations have been
made. Here only a link to the appropriate IANA registration is
provided to avoid inconsistencies. The IANA registrations for MIKEY
payloads can be found under
http://www.iana.org/assignments/mikey-payloads These registrations
comprise the MIKEY base registrations as well as registrations made
by MIKEY extensions regarding the payload.
The IANA registrations for MIKEY port numbers can be found under
http://www.iana.org/assignments/port-numbers (search for MIKEY).
8. MIKEY alternatives for SRTP security parameter negotiation
Besides MIKEY there exists several approaches to handle the security
parameter establishment. This is due to the fact, that some
limitations in certain scenarios have been seen. Examples are early
media and forking situations as described in Section 5. The
following list provides a short summary about currently discussed
alternatives:
o sdescription - [I-D.ietf-mmusic-sdescriptions] describes a key
management scheme, which uses SDP for transport and completly
relies on underlying protocol security. For transport the
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documents defines a SDP attribute transmitting all necessary SRTP
parameter in clear. For security it references TLS and S/MIME.In
contrast to MIKEY in the message from the initiator to the
responder the SRTP parameter for the direction initiator to
responder is sent rather than vice versa. This may lead to
problems in early media scenarios.
o sdescription with early media support - [I-D.wing-mmusic-sdes-
early-media] enhances the above scheme with the possibility to
also be usable in early media scenarios, when security
preconditions is not used.
o Encrypted Key Transport for Secure RTP - [Reference to
draft-mcgrew-srtp-ekt] is an extension to SRTP that provides for
the secure transport of SRTP master keys, Rollover Counters, and
other information, within SRTCP. This facility enables SRTP to
work for decentralized conferences with minimal control, and to
handle situations caused by SIP forking and early media.
o Diffie Hellman support in SDP - [I-D.baugher-mmusic-sdp-dh]
defines a new SDP attribute for exchanging Diffie-Hellman public
keys. The attribute is an SDP session-level attribute for
describing DH keys, and there is a new media-level parameter for
describing public keying material for SRTP key generation.
o DTLS/SRTP compatibility mode - is described as part of
[I-D.tschofenig-avt-rtp-dtls] and provides for using DTLS as key
management approach in conjunction with partial encryption
targeted for low bandwidth connections.
o SRTP extensions for DTLS - [Reference to draft-mcgrew-dtls-srtp]
describes a method of using DTLS key management for SRTP by using
a new extension that indicates that SRTP is to be used for data
protection, and which establishes SRTP keys.
o ZRTP - [I-D.zimmermann-avt-zrtp] This document defines ZRTP as RTP
header extensions for a Diffie-Hellman exchange to agree on a
session key and parameters for establishing SRTP sessions. The
ZRTP protocol is completely self-contained in RTP and does not
require support in the signaling protocol or assume a PKI.
9. Security Considerations
This document does not define extensions to existing protocols. It
rather provides an overview about the set of MIKEY and available
extensions. Thus, the reader is referred to the original documents
defining the base protocol and the extensions for the security
considerations.
10. IANA Considerations
This document does not require any IANA registration.
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11. Acknowledgments
The authors would like to thank Lakshminath Dondeti for his document
reviews and for his guidance.
12. References
12.1. Normative References
[I-D.baugher-mmusic-sdp-dh]
Baugher, M. and D. McGrew, "Diffie-Hellman Exchanges for
Multimedia Sessions", draft-baugher-mmusic-sdp-dh-00 (work
in progress), February 2006.
[I-D.dondeti-msec-mikey-genext-oma]
Dondeti, L. and D. Castleford, "OMA BCAST MIKEY General
Extension Payload Specification",
draft-dondeti-msec-mikey-genext-oma-00 (work in progress),
April 2006.
[I-D.ietf-mmusic-kmgmt-ext]
Arkko, J., "Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", draft-ietf-mmusic-kmgmt-ext-15 (work in
progress), June 2005.
[I-D.ietf-mmusic-securityprecondition]
Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol Media Streams",
draft-ietf-mmusic-securityprecondition-01 (work in
progress), October 2005.
[I-D.ietf-msec-mikey-dhhmac]
Euchner, M., "HMAC-authenticated Diffie-Hellman for
MIKEY", draft-ietf-msec-mikey-dhhmac-11 (work in
progress), April 2005.
[I-D.ietf-msec-mikey-ecc]
Milne, A., "ECC Algorithms For MIKEY",
draft-ietf-msec-mikey-ecc-00 (work in progress),
February 2006.
[I-D.ietf-msec-mikey-rsa-r]
Ignjatic, D., "An additional mode of key distribution in
MIKEY: MIKEY-RSA-R", draft-ietf-msec-mikey-rsa-r-04 (work
in progress), April 2006.
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[I-D.ietf-msec-newtype-keyid]
Carrara, E., "The Key ID Information Type for the General
Extension Payload in MIKEY",
draft-ietf-msec-newtype-keyid-05 (work in progress),
March 2006.
[I-D.lehtovirta-srtp-rcc]
Lehtovirta, V., "Integrity Transform Carrying Roll-over
Counter", draft-lehtovirta-srtp-rcc-01 (work in progress),
February 2006.
[I-D.tschofenig-avt-rtp-dtls]
Tschofenig, H. and E. Rescorla, "Real-Time Transport
Protocol (RTP) over Datagram Transport Layer Security
(DTLS)", draft-tschofenig-avt-rtp-dtls-00 (work in
progress), March 2006.
[I-D.wing-mmusic-sdes-early-media]
Raymond, R. and D. Wing, "Security Descriptions Extension
for Early Media", draft-wing-mmusic-sdes-early-media-00
(work in progress), October 2005.
[I-D.zimmermann-avt-zrtp]
Zimmermann, P., "ZRTP: Extensions to RTP for Diffie-
Hellman Key Agreement for SRTP",
draft-zimmermann-avt-zrtp-01 (work in progress),
March 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4442] Fries, S. and H. Tschofenig, "Bootstrapping Timed
Efficient Stream Loss-Tolerant Authentication (TESLA)",
RFC 4442, March 2006.
12.2. Informative References
[H.235.7] ""ITU-T Recommendation H.235.7: Usage of the MIKEY Key
Management Protocol for the Secure Real Time Transport
Protocol (SRTP) within H.235"", 2005.
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[I-D.ietf-mmusic-sdescriptions]
Andreasen, F., "Session Description Protocol Security
Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-12 (work in progress),
September 2005.
[ISO_sec_time]
""ISO/IEC 18014 Information technology - Security
techniques - Time-stamping services, Part 1-3."", 2002.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol",
RFC 2412, November 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
"Gateway Control Protocol Version 1", RFC 3525, June 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
[RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient
Stream Loss-Tolerant Authentication (TESLA) in the Secure
Real-time Transport Protocol (SRTP)", RFC 4383,
February 2006.
[SAML_overview]
Huges, J. and E. Maler, ""Security Assertion Markup
Language (SAML) 2.0 Technical Overview, Working Draft"",
2005.
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Authors' Addresses
Steffen Fries
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: steffen.fries@siemens.com
Dragan Ignjatic
Polycom
1000 W. 14th Street
North Vancouver, BC V7P 3P3
Canada
Email: dignjatic@polycom.com
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