Network Working Group J. Mattsson
Internet-Draft Ericsson
Updates: 3830 (if approved) T. Tian
Intended status: Informational ZTE
Expires: August 3, 2010 January 30, 2010
MIKEY-TICKET: An Additional Mode of Key Distribution
in Multimedia Internet KEYing (MIKEY)
draft-mattsson-mikey-ticket-01
Abstract
The Multimedia Internet KEYing (MIKEY) specification describes a key
management scheme for real-time applications. In this document, we
note that the currently defined MIKEY modes are insufficient to
address deployment scenarios built around a centralized key
management service. Such deployments are gaining in interest.
Therefore, a new MIKEY mode that works well in such scenarios is
defined. The new mode uses a trusted key management service and a
ticket concept, similar to that in Kerberos. The new mode also
supports features required by many existing applications, e.g. so
called forking where the exact identity of the other endpoint may not
be known at the start of the communication session.
Status of this Memo
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This Internet-Draft will expire on August 3, 2010.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definitions and Notation . . . . . . . . . . . . . . . . . 5
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Payloads . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Design Considerations . . . . . . . . . . . . . . . . . . . . 7
4. A New Mode: MIKEY-TICKET . . . . . . . . . . . . . . . . . . . 9
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Modes . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Ticket Request . . . . . . . . . . . . . . . . . . . . 13
4.2.2. Ticket Transfer . . . . . . . . . . . . . . . . . . . 17
4.2.3. Ticket Resolve . . . . . . . . . . . . . . . . . . . . 19
5. Key Management Functions . . . . . . . . . . . . . . . . . . . 22
5.1. Key Derivation . . . . . . . . . . . . . . . . . . . . . . 22
5.1.1. Deriving Forked Keys . . . . . . . . . . . . . . . . . 24
5.1.2. Deriving Keys from an Envelope/Pre-Shared Key/MPK . . 25
5.2. Deriving Keys from a TGK . . . . . . . . . . . . . . . . . 26
5.2.1. Deriving Keys from a GTGK . . . . . . . . . . . . . . 26
5.3. CSB Updating . . . . . . . . . . . . . . . . . . . . . . . 26
5.4. Ticket Reuse . . . . . . . . . . . . . . . . . . . . . . . 27
5.5. MAC/Signature Coverage . . . . . . . . . . . . . . . . . . 28
6. Payload Encoding . . . . . . . . . . . . . . . . . . . . . . . 28
6.1. Common Header Payload (HDR) . . . . . . . . . . . . . . . 29
6.2. Key Data Transport Payload (KEMAC) . . . . . . . . . . . . 30
6.3. Timestamp Payload (T) . . . . . . . . . . . . . . . . . . 31
6.4. Timestamp Payload with Role Indicator (TR) . . . . . . . . 31
6.5. ID Payload (ID) . . . . . . . . . . . . . . . . . . . . . 32
6.6. ID Payload with Role Indicator (IDR) . . . . . . . . . . . 32
6.7. Cert Hash Payload (CHASH) . . . . . . . . . . . . . . . . 33
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6.8. Error Payload (ERR) . . . . . . . . . . . . . . . . . . . 33
6.9. Key Data Sub-Payload . . . . . . . . . . . . . . . . . . . 34
6.10. Ticket Payload (TICKET) . . . . . . . . . . . . . . . . . 34
6.11. Ticket Policy Payload (TP) . . . . . . . . . . . . . . . . 35
7. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 37
8. Group Communication . . . . . . . . . . . . . . . . . . . . . 37
8.1. Key Forking . . . . . . . . . . . . . . . . . . . . . . . 38
9. Signaling Between Different KMSs . . . . . . . . . . . . . . . 39
10. Adding New Ticket Types to MIKEY-TICKET . . . . . . . . . . . 40
11. Security Considerations . . . . . . . . . . . . . . . . . . . 40
11.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 41
11.2. Denial of Service . . . . . . . . . . . . . . . . . . . . 42
11.3. Replay . . . . . . . . . . . . . . . . . . . . . . . . . . 42
11.4. Forking . . . . . . . . . . . . . . . . . . . . . . . . . 43
11.5. Group Key Management . . . . . . . . . . . . . . . . . . . 43
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 44
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
14.1. Normative References . . . . . . . . . . . . . . . . . . . 46
14.2. Informative References . . . . . . . . . . . . . . . . . . 47
Appendix A. MIKEY Base Ticket . . . . . . . . . . . . . . . . . . 47
A.1. Components of the Ticket Data . . . . . . . . . . . . . . 48
A.2. Deriving Keys from a TPK . . . . . . . . . . . . . . . . . 48
A.3. Deriving MPKi and MPKr . . . . . . . . . . . . . . . . . . 49
A.4. Ticket Header Payload (THDR) . . . . . . . . . . . . . . . 49
Appendix B. Alternative Use Cases . . . . . . . . . . . . . . . . 50
B.1. Compatibility Mode . . . . . . . . . . . . . . . . . . . . 50
B.2. Distribution of Pre-Encrypted Content . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 51
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1. Introduction
Key management systems are either based on negotiation and exchange
directly between peers (e.g. Diffie-Hellman based schemes), pre-
distribution of user credentials (shared secrets/certificates), or
availability of a trusted Key Management Service (KMS). The modes
described in the Multimedia Internet KEYing (MIKEY) specification
[RFC3830] and its extensions [RFC4650] [RFC4738] are all variants of
the first two alternatives.
In security systems serving a large number of users, a solution based
on a key management service is often preferred. With such a service
in place, there is no need to pre-distribute credentials that
directly can be used to establish security associations between peers
for protected communication, as users can request such credentials
when needed. Solutions based on a trusted key management service
also scale well when the number of users grows.
This document introduces a set of new MIKEY modes that go under the
common name MIKEY-TICKET. It supports a ticket concept, similar to
that in Kerberos [RFC4120], which is used to identify and deliver
keys. A high level outline of MIKEY-TICKET as defined herein is that
the Initiator requests keys and a ticket from the KMS and sends the
ticket containing a reference to the keys, or the enveloped keys, to
the Responder. The Responder then sends the ticket to the KMS, which
returns the appropriate keys.
MIKEY-TICKET is primarily designed to be used for media plane
security in the 3GPP IP Multimedia Subsystem (IMS) [3GPP.33.328].
This implies that some extensions to the basic Kerberos concept are
needed. For instance, the Initiator may not always know the exact
identity of the Responder when the communication with the key
management server is initiated; SIP forking (see [RFC3261]) is one
such situation.
This document updates [RFC3830] with the MIKEY-TICKET mode. It
defines a signaling framework enabling peers to request, transfer,
and resolve various Ticket Types using a key management service. A
default Ticket Type is also defined. To allow the use of 256-bit
keys for users with high security requirements, additional
encryption, authentication, and pseudo-random functions are defined.
2. Terminology
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 [RFC2119].
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Definitions of terms and notation will, unless otherwise stated, be
as defined in [RFC3830].
2.1. Definitions and Notation
Forking: In SIP, forking is the delivery of a request (e.g. INVITE)
to multiple endpoints.
Key forking: When used in conjunction with forking, key forking
refers to the process of modifying keys, making them
cryptographically unique for each responder targeted by the forking.
(Media) session: The communication session intended to be secured by
the MIKEY-TICKET provided key(s).
Session information: Information related to the security protocols
used to protect the media session: keys, salts, algorithms, etc.
Ticket: A Kerberos-like object used to identify and deliver keys over
an untrusted network.
Ticket Data: Ticket part with information intended only for the party
that resolves the ticket (e.g. keys).
Ticket Request: Exchange used by the Initiator to request keys and a
ticket from a trusted KMS.
Ticket Transfer: Exchange used to transfer the ticket as well as
session information from the Initiator to the Responder.
Ticket Resolve: Exchange used by the Responder to request the KMS to
return the keys encoded in a ticket.
Ticket Policy: Policy for ticket generation and resolution,
authorized applications, key derivation, etc.
Ticket Type: Defines ticket format and processing. May further have
subtype and version.
Solid arrows (----->) indicate mandatory messages.
Dashed arrows (- - ->) indicate optional messages.
E(k, p) Encryption of p with the key k
PKx Public Key of entity x
k' The forked key k
[p] p is optional
{p} Zero or more occurrences of p
(p) One or more occurrences of p
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|| Concatenation
| OR (selection operator)
2.2. Abbreviations
3GPP: 3rd Generation Partnership Project
AAA: Authentication, Authorization, and Accounting
ACL: Access Control List
CA: Certificate Authority
CS: Crypto Session
CSB: Crypto Session Bundle
DDoS: Distributed Denial of Service
DoS: Denial of Service
EKT: Encrypted Key Transport
IMS: IP Multimedia Subsystem
GTGK: Group TGK
KDC: Key Distribution Center
KMS: Key Management Service
KTC: Key Translation Center
MAC: Message Authentication Code
MIKEY: Multimedia Internet KEYing
NSPS: National Security and Public Safety
MKI: Master Key Identifier
MPK: MIKEY Protection Key
NTP: Network Time Protocol
PET: Privacy Enhancing Technologies
PK: Public-Key
PRF: Pseudo-Random Function
PRNG: Pseudo-Random Number Generator
PSK: Pre-Shared Key
RTSP: Real-Time Streaming Protocol
SDP: Session Description Protocol
SIP: Session Initiation Protocol
SPI: Security Parameters Index
TEK: Traffic Encryption Key
TGK: TEK Generation Key
TPK: Ticket Protection Key
UTC: Coordinated Universal Time
2.3. Payloads
CERTx: Certificate of entity x
CHASH: Hash of the certificate used
HDR: Common Header payload
ID: Identity payload
IDRx: Identity of entity x
IDRpsk: Identifier for pre-shared key
IDRapp: Identifier for application/service
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KEMAC: Key data transport payload
PKE: Encrypted envelope key
RANDx: Random value generated by entity x
SIGNx: Signature created using entity x's private key
SP: Security Policy payload
T: Timestamp payload
TRy: Timestamp payload with role indicator y
THDR: Ticket Header payload
TICKET: Ticket payload
TP: Ticket Policy payload
V: Verification payload
where
x is in the set {i, r, kms} (Initiator, Responder, KMS) and
y is in the set {i, s, e, r} (time of Issue, Start time, End time,
Reykeying interval).
The IDR, TR, TICKET, and TP payloads are defined in Section 6. Note
that in [RFC3830], there is defined both a V payload (carrying the
authentication tag) and a V flag in the HDR payload (indicating
whether a response message is expected or not).
3. Design Considerations
As mentioned in the introduction, none of the previously defined
MIKEY modes are based on a KMS. The pre-shared key method and the
public-key encryption method defined in [RFC3830] are examples of
systems based on pre-distribution of user credentials. The Diffie-
Hellman method [RFC3830] is an example of a system based on
negotiation and exchange directly between peers.
In SIP, forking is the delivery of a request (e.g. INVITE) to
multiple endpoints. This happens when a responder is registered on
several devices (e.g. mobile phone, fixed phone, and computer) or
when an invite is being made to addresses of the type
somebody@company.example, a group of users where only one is supposed
to answer. To prevent any form of eavesdropping, only the endpoint
that answers should get access to the session keys. The naive
application of [RFC3830] where all endpoints share the same pre-
shared/private key is not secure when it comes to forking as all
endpoints get access to the session keys. Conversely, having per-
user unique pre-shared keys/certificates creates more fundamental
problems with forking, as the initiator does not know which pre-
shared key/certificate to use at session initiation. Forking is
described in [RFC5479] and the applicability of different MIKEY modes
is discussed in [RFC5197].
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Deferred delivery of end-to-end protected content excludes all key
management schemes that are based on some type of direct online
negotiation between peers (e.g. Diffie-Hellman based schemes) as the
responder cannot rely on contacting the initiator to get access to
keys.
In security systems serving a large number of users, a solution based
on a key management service is often preferred. With such a service
in place, there is no need to pre-distribute credentials that
directly can be used to establish security associations between peers
for protected communication, as users can request such credentials
when needed. In many applications, e.g. National Security and
Public Safety (NSPS), the controlling organization wants to enforce
policies on the use of keys. A trusted KMS fits these applications
well as it makes it easier to enforce policies centrally. Solutions
based on a trusted KMS also scale well when the number of users
grows. A KMS based on symmetric keys has particular advantages as
symmetric key algorithms are generally much less computationally
intensive than asymmetric key algorithms.
Systems based on a key management service require a signaling
mechanism that allows peers to retrieve other peers' credentials. A
convenient way to implement such a signaling scheme is to use a
ticket concept, similar to that in Kerberos [RFC4120], to identify
and deliver keys. The ticket can be forwarded in the signaling
associated with the session setup. The initiator requests a ticket
from the key management service and sends the ticket to the
responder. The responder forwards the ticket to the key management
service, which returns the corresponding keys. It should here be
noted that Kerberos typically does not require that the responder
also contacts the key management service. However, in order to
support also the aforementioned forking scenarios it becomes
necessary that the ticket is not bound to the exact identity (or
credentials) of the responder until the final responder becomes fully
determined. Group and forking communication scenarios can also be
improved from access control point of view if authorization to access
the keys can be enforced with higher granularity at the responder
side.
The ticket can contain a reference to keys held by the key management
system or it can hold the keys itself. In the latter case, the
ticket needs to be confidentiality and integrity protected
(enveloped). In the following, the term encoded keys will be used to
describe both cases as well as keys derived from such keys.
By using different Ticket Types and ticket policies, some allowing
the initiator or responder to create or resolve the tickets without
assistance from the KMS, a wide range of different security levels
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and use cases can be supported. This has a number of advantages as
it offers a framework which is flexible enough to satisfy users with
a broad range of security and functional needs. The authorization
function in the KMS could also be used to help solve the key access
problem in forking and retargeting scenarios. The problems with
retargeting are similar to forking. The use of a ticket based system
may also help in the handling of keys for deferred delivery of end-
to-end protected content to currently off-line users.
At the same time, it is also important to be aware that (centralized)
key management services may introduce a single point of (security)
failure. The security requirements on the implementation and
protection of the KMS may therefore in high security applications be
more or less equivalent to the requirements of an AAA
(Authentication, Authorization, and Accounting) server or a
Certificate Authority (CA).
4. A New Mode: MIKEY-TICKET
4.1. Overview
All previously defined MIKEY modes consist of a single (or half)
round-trip between two peers. MIKEY-TICKET differs from these modes
as it consists of up to three different round-trips (Ticket Request,
Ticket Transfer, and Ticket Resolve) involving three parties
(Initiator, Responder, and KMS). Since the number of round-trips and
order of messages may vary, MIKEY-TICKET is actually the common name
for a set of modes, all revolving around a ticket concept. The third
party, the KMS, is only involved in some of the MIKEY exchanges and
not at all in the resulting secure media session. The Ticket Request
and Ticket Resolve exchanges are meant to be used in combination with
the Ticket Transfer exchange and not on their own. In Figure 1, the
signaling for the full three round-trip MIKEY-TICKET mode is
depicted.
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+---+ +-----+ +---+
| I | | KMS | | R |
+---+ +-----+ +---+
REQUEST_INIT
-------------------------------->
REQUEST_RESP
<--------------------------------
TRANSFER_INIT
---------------------------------------------------------------->
RESOLVE_INIT
<--------------------------------
RESOLVE_RESP
-------------------------------->
TRANSFER_RESP
<----------------------------------------------------------------
Figure 1: Full three round-trip signaling
The Initiator (I) wants to establish a secure media session with the
Responder (R). The Initiator and the Responder do not share any
credentials, instead they trust a third party, the KMS, with which
they both have or can establish shared credentials. The assumed
trust model is illustrated in Figure 2.
Pre-established trust relation Pre-established trust relation
<------------------------------> <------------------------------>
+---+ +-----+ +---+
| I | | KMS | | R |
+---+ +-----+ +---+
<--------------------------------------------------------------->
Security association based on ticket
Figure 2: Trust model
Note that rather than a single KMS, multiple KMSs may be involved,
e.g. one for the Initiator and one for the Responder; this is
discussed in Section 9.
The Initiator requests keys and a ticket (encoding the same keys)
from the KMS by sending a REQUEST_INIT message. The REQUEST_INIT
message includes session information (e.g. identities of the
authorized responders) and is integrity protected by a MAC based on a
pre-shared key or by a signature (similar to the pre-shared key and
public-key encryption modes in [RFC3830]). If the request is
authorized, the KMS generates the requested keys, encodes them in a
ticket, and returns the keys and the ticket in a REQUEST_RESP
message. The Ticket Request exchange is optional (depending on the
Ticket Type), and MAY be omitted if the Initiator can create the
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ticket without assistance from the KMS.
The Initiator next includes the ticket in a TRANSFER_INIT message,
which is sent to the Responder. The TRANSFER_INIT message is
protected by a MAC based on a MPK (MIKEY Protection Key) encoded in
the ticket. If the Responder finds the proposed Ticket Policy and
session security policies acceptable, the Responder forwards the
ticket to the KMS. This is done with a RESOLVE_INIT message, which
asks the KMS to return the keys encoded in the ticket. The
RESOLVE_INIT message is protected by a MAC based on a pre-shared key
(between Responder and KMS) or by a signature. The Ticket Resolve
exchange is optional (depending on the Ticket Policy), and SHOULD
only be used when the Responder is unable to resolve the ticket
without assistance from the KMS.
The KMS resolves the ticket. If the Responder is authorized to
receive the keys encoded in the ticket, the KMS retrieves the keys
and other information. If key forking is used, the keys are modified
(bound to the Responder) by the KMS, see Section 5.1.1. The keys and
additional information are then sent in a RESOLVE_RESP message to the
Responder. The Responder then sends a TRANSFER_RESP message to the
Initiator as verification. The TRANSFER_RESP message might include
information used for further key derivation.
The use case and signaling described above is the full three exchange
mode but other modes are allowed, see Section 4.1.1. Group
communication is discussed in Section 8 and signaling between
different KMSs is discussed in Section 9. Some alternative use cases
are discussed in Appendix B.
MIKEY-TICKET offers a framework which is flexible enough to satisfy
users with a broad range of security and functional needs. The
framework consists of the three exchanges for which different Ticket
Types can be defined. The ticket consists of a Ticket Policy as well
as Ticket Data. The Ticket Policy contains information intended for
all parties involved, whereas the Ticket Data is only intended for
the party that resolves the ticket. The Ticket Data could be a
reference to information (keys etc.) stored by the key management
service, it could contain all the information itself, or it could be
a combination of the two alternatives. The format of the Ticket Data
depends on the Ticket Type signaled in the Ticket Policy. The Ticket
Data corresponding to the Ticket Type called MIKEY base ticket is
given in Appendix A and requirements regarding new Ticket Types are
given in Section 10.
As MIKEY-TICKET is based on [RFC3830], the same terminology,
processing and considerations still apply unless otherwise stated.
Just like in [RFC3830], the messages are integrity protected and
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encryption is only applied to the keys and not to the entire
messages. Depending on the mode, the KMS might operate as a KDC (Key
Distribution Center) and supply the keys, as a KTC (Key Translation
Center) and re-encode and forward keys supplied by the Initiator, or
as a combination of the two (see [HBOAC]).
4.1.1. Modes
Depending on the Ticket Type and the Ticket Policy, some of the
exchanges might be optional or not used at all, see Figure 3. If the
ticket protection is based on a key known only by the KMS, both the
Initiator and the Responder have to contact the KMS to request/
resolve tickets (mode 1). If the key used to protect the ticket is
shared between the KMS and the Responder, the Ticket Resolve exchange
can be omitted (similar to Kerberos), as the Responder can resolve
the ticket without assistance from the KMS (mode 2).
+---+ +-----+ +---+
| I | | KMS | | R |
+---+ +-----+ +---+
Ticket Request
(1) <----------------------------> Ticket Transfer
<--------------------------------------------------------->
<---------------------------->
Ticket Resolve
Ticket Request
(2) <----------------------------> Ticket Transfer
<--------------------------------------------------------->
Ticket Transfer
(3) <--------------------------------------------------------->
<---------------------------->
Ticket Resolve
Ticket Transfer
(4) <--------------------------------------------------------->
Figure 3: Modes
If the key protecting the ticket is shared between the Initiator and
the KMS, the Ticket Request exchange can be omitted (similar to the
Otway-Rees protocol [Otway-Rees]), as the Initiator can create the
ticket without assistance from the KMS (mode 3). If the key
protecting the ticket is shared between the Initiator and the
Responder, both the Ticket Request and Ticket Resolve exchanges can
be omitted (mode 4). This can be seen as a variation of the pre-
shared key method of [RFC3830] with mutual key freshness guarantee.
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In mode 1 and 2 the Ticket Request can be omitted if the tickets and
the corresponding keys are distributed in some other way.
4.2. Exchanges
4.2.1. Ticket Request
This exchange is used by the Initiator to request keys and a ticket
from a trusted KMS, with which the Initiator has pre-shared
credentials. The request contains information (e.g. participant
identities, etc.) describing the session the ticket is intended to
protect. A full round-trip is required for the Initiator to receive
the ticket. The initial message REQUEST_INIT comes in two variants.
The first variant corresponds to the pre-shared key (PSK) method of
[RFC3830].
Initiator KMS
REQUEST_INIT_PSK = ---->
HDR, T, RANDi, [IDRi],
[IDRkms], TP, [KEMAC], <---- REQUEST_RESP =
[IDRpsk], V HDR, T, [IDRkms],
TICKET, KEMAC, V
The second variant corresponds to the public-key (PK) method of
[RFC3830].
Initiator KMS
REQUEST_INIT_PK = ---->
HDR, T, RANDi, [IDRi], {CERTi},
[IDRkms], TP, [KEMAC], <---- REQUEST_RESP =
[CHASH], PKE, SIGNi HDR, T, [IDRkms],
TICKET, KEMAC, V
As the REQUEST_INIT message MUST ensure the identity of the Initiator
to the KMS, it SHALL be integrity protected by a MAC based on a pre-
shared key or by a signature. The response message REQUEST_RESP is
the same for the two variants and SHALL be protected by using the
pre-shared/envelope key indicated in the REQUEST_INIT message.
In addition to the ticket, the Initiator receives keys, which it does
not already know. The ticket contains both session information and
information needed to resolve the ticket later, see Section 6.10.
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4.2.1.1. Common Components of the REQUEST_INIT Messages
The REQUEST_INIT message MUST always include the Header (HDR),
Timestamp (T), and RANDi payloads.
In HDR the CSB ID (Crypto Session Bundle ID) SHALL be assigned as in
[RFC3830]. The V flag MUST be set to '1' but SHALL be ignored by the
KMS as a response is MANDATORY. As Crypto Sessions (CS) SHALL NOT be
handled, the #CS MUST be set to 0 and the CS ID map type SHALL be the
"Empty map" as defined in [RFC4563].
IDRi contains the identity of the Initiator. This identity SHOULD be
included in the granted Ticket Policy (TP).
IDRkms contains the identity of the KMS. It SHOULD be included, but
it MAY be left out when it can be expected that the KMS has a single
identity.
TP contains the desired Ticket Policy (see Section 6.11). It
includes for instance, the identities of authorized responders.
The KEMAC payload is used by the Initiator to indicate the number of
requested keys, specify other key information (key type, key length,
KV (key validity) data [RFC3830]), and specify the Key Data itself.
Initiator specified Key Data in a KMS generated ticket SHOULD NOT be
used unless the Initiator has pre-encrypted content and specific TEKs
(Traffic Encryption Keys) need to be included in the ticket. See
Section 6.2 and Appendix B.2 for details.
4.2.1.2. Components of the REQUEST_INIT_PSK Message
The IDRi payload SHOULD be included but MAY be left out when it can
be expected that the KMS can identify the Initiator by other means.
The KEMAC payload SHOULD use the NULL authentication algorithm, as a
MAC is included in the V payload. The encryption key (encr_key) and
salting key used to encrypt the KEMAC SHALL be derived from the pre-
shared key (Initiator-KMS) (see Section 5.1.2 for key derivation
specification). The salting key goes into the IV as defined in
[RFC3830]. The KEMAC is hence constructed as follows:
KEMAC = E(encr_key, (TGK|TEK))
The IDRpsk payload is used to indicate the pre-shared key used. It
MAY be omitted if the KMS can find the pre-shared key by other means.
The last payload SHALL be a Verification payload (V) where the
authentication key (auth_key) is derived from the pre-shared key
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(Initiator-KMS) (see Section 5.1.2 for key derivation specification).
The MAC SHALL cover the entire message as well as the identities of
the involved parties (see Section 5.5 for the exact definition).
4.2.1.3. Components of the REQUEST_INIT_PK Message
The identity IDRi and certificate CERTi SHOULD be included, but they
MAY be left out when it can be expected that the KMS can obtain the
certificate in some other manner. If a certificate chain is to be
provided, each certificate in the chain SHOULD be included in a
separate CERT payload.
PKE contains the encrypted envelope key: PKE = E(PKkms, env_key). It
is encrypted using the KMS's public key (PKkms). If the KMS
possesses several public keys, the Initiator can indicate the key
used in the CHASH payload.
The KEMAC payload MUST include an identity payload (IDRi) and a MAC
calculated over the KEMAC. The identity MUST be equal to the
identity specified in the certificate. The reason to bind the
identity to the keys is to stop a man-in-the-middle-attack where an
attacker includes the KEMAC and PKE payloads in a new REQUEST_INIT
message with herself as an authorized responder. The encr_key,
salt_key, and auth_key SHALL be derived from the envelope key (see
Section 5.1.2 for key derivation specification). The salting key
goes into the IV as defined in [RFC3830]. The KEMAC is hence
constructed as follows:
KEMAC = E(encr_key, IDRi || (TGK|TEK)) || MAC
SIGNi is a signature covering the entire MIKEY message, using the
Initiator's signature key (see Section 5.5 for the exact definition).
4.2.1.4. Processing the REQUEST_INIT Message
If the KMS can verify the integrity of the received message, the
message can be correctly parsed, and the Initiator is authorized to
receive the requested ticket, possibly with a modified Ticket Policy,
the KMS MUST send a REQUEST_RESP message. Otherwise the KMS SHOULD
send an appropriate Error message. In case of a REQUEST_INIT_PK
message, the KMS MUST ensure that the identity in the KEMAC payload
is equal to the identity specified in the certificate.
4.2.1.5. Components of the REQUEST_RESP Message
The Header payload SHOULD be identical to the Header payload in the
REQUEST_INIT message with the exception of data type, next payload,
and V flag. The V flag has no meaning in this context. It SHALL be
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set to 0 by the KMS and ignored by the Initiator.
The timestamp type and value SHALL be identical to the one used in
the REQUEST_INIT message.
The TICKET payload carries the granted TP payload and Ticket Data
(see Section 6.10). As the KMS decides which Ticket Policy to use,
this may not be the same Ticket Policy as the Initiator requested.
The Ticket Type and the Ticket Data depend on the granted Ticket
Policy.
The KEMAC payload SHOULD use the NULL authentication algorithm, as a
MAC is included in the V payload. Depending on the type of
REQUEST_INIT message, either the pre-shared key or the envelope key
SHALL be used to derive the encr_key and salt_key. The salting key
goes into the IV as defined in [RFC3830]. If the REQUEST_INIT
message does not contain a KEMAC, it is RECOMMENDED that the KMS's
default KEMAC includes a single TGK. The KEMAC SHALL include a MPK
(MIKEY Protection Key), MPKi, used as a pre-shared key to protect the
messages in the Ticket Transfer exchange. If key forking (see
Section 5.1.1) is used (determined by the Ticket Policy) a second
MPK, MPKr, SHALL be included in the KEMAC. Then MPKi SHALL be used
to protect the TRANSFER_INIT message and MPKr SHALL be used to verify
the TRANSFER_RESP message. The KEMAC is hence constructed as
follows:
KEMAC = E(encr_key, MPKi || [MPKr] || (TGK|TEK))
The last payload SHALL be a Verification payload (V). Depending on
the type of REQUEST_INIT message, either the pre-shared key or the
envelope key SHALL be used to derive the auth_key. The MAC SHALL
cover the entire message as well as the INIT message (see Section 5.5
for the exact definition).
4.2.1.6. Processing the REQUEST_RESP Message
If the Initiator can verify the integrity of the received message and
the message can be correctly parsed, the ticket and the associated
session information SHOULD be stored. Otherwise the Initiator SHOULD
silently discard the message.
Before using the received ticket, the Initiator SHOULD check that the
granted Ticket Policy is acceptable. If not, the Initiator SHALL
either silently discard or send a new REQUEST_INIT message suggesting
a different Ticket Policy than before.
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4.2.2. Ticket Transfer
This exchange is used to transfer a ticket as well as session
information from the Initiator to a Responder. The exchange is
modeled after the pre-shared key mode [RFC3830], and the session
information is forwarded in the same way with the exception that the
keys are encoded in a TICKET payload instead of a KEMAC payload. As
the motive for this exchange is to setup a shared secret key between
Initiator and Responder, the Responder cannot check the authenticity
of the message before the ticket is resolved (by KMS or Responder).
A full round-trip is required if Responder key confirmation and
freshness guarantee are needed. The messages are preferably included
in the session setup signaling (e.g. SIP INVITE and 200 OK).
Initiator Responder
TRANSFER_INIT = ---->
HDR, T, RANDi, [IDRi], [IDRr],
{SP}, TICKET, V < - - TRANSFER_RESP =
HDR, T, [RANDr],
[IDRr], [RANDkms],
{SP}, [KEMAC], V
4.2.2.1. Components of the TRANSFER_INIT Message
The TRANSFER_INIT message MUST always include the Header (HDR),
Timestamp (T), and RANDi payloads.
In HDR, the CSB ID (Crypto Session Bundle ID) SHALL be assigned as in
[RFC3830]. The value of the V flag SHALL agree with the Ticket
Policy (TP) and it SHALL be ignored by the Responder.
The IDRi and IDRr payloads SHOULD be included but they MAY be left
out when it can be expected that the Responder has a single identity
and can identify the Initiator by other means.
The use of the SP (Security Policy) payload is identical to that in
[RFC3830].
The TICKET payload contains the Ticket Policy to be applied when
resolving the ticket as well as the Ticket Data.
The last payload SHALL be a Verification payload (V) where the
authentication key (auth_key) is derived from the MPKi (see
Section 5.1.2 for key derivation specification). The MAC SHALL cover
the entire message as well as the identities of the involved parties
(see Section 5.5 for the exact definition).
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4.2.2.2. Processing the TRANSFER_INIT Message
As the Initiator and Responder do not have any pre-shared keys, the
Responder cannot check the authenticity of the message before the
ticket is resolved. The Responder SHOULD however check that both the
Ticket Policy (TP) and the security policy (SP) are acceptable. If
they are not, the Responder SHOULD reject without contacting the KMS.
This is an early reject mechanism to avoid unnecessary KMS signaling
when the Responder can conclude from the information at hand that it
will not accept the connection. After the ticket has been resolved
the parsing of the TRANSFER_INIT message continues and SHALL be done
as in [RFC3830].
4.2.2.3. Components of the TRANSFER_RESP Message
The Header payload SHOULD be identical to the Header payload in the
TRANSFER_INIT message with the exception that the V flag has no
meaning in this context. It SHALL be set to 0 by the Responder and
ignored by the Initiator.
The timestamp type and value SHALL be identical to the one used in
the TRANSFER_INIT message.
If indicated by the Ticket Policy, the Responder SHALL generate a
fresh (pseudo-)random byte string RANDr. RANDr is used to produce
Responder freshness guarantee in key derivations.
If the Responder receives an IDRr payload in the RESOLVE_RESP
message, the same identity MUST be sent in an IDRr payload in the
TRANSFER_RESP message. The identity sent in the IDRr payload in the
TRANSFER_RESP message (e.g. user1@company.example) MAY differ from
the one sent in the IDRr payload in the TRANSFER_INIT message (e.g.
somebody@company.example).
If the Responder receives a RANDkms payload in the RESOLVE_RESP
message, the same RAND MUST be sent in a RANDkms payload in the
TRANSFER_RESP message. The RANDkms payload MUST be placed after the
RANDr payload to avoid ambiguity.
The SP and KEMAC payloads SHALL be processed exactly as if they were
received in a later CSB update Section 5.3.
The last payload SHALL be a Verification payload (V) where the
authentication key (auth_key) is derived from MPKi or MPKr'
(depending on if key forking is used). The MAC SHALL cover the
entire message as well as the TRANSFER_INIT message (see Section 5.5
for the exact definition).
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4.2.2.4. Processing the TRANSFER_RESP Message
If the Initiator cannot verify the integrity of the received message
or the message cannot be parsed the Initiator SHOULD silently discard
the message.
4.2.3. Ticket Resolve
This exchange is used by the Responder to request the KMS to return
the keys encoded in a ticket. The KMS does not need to be the same
KMS that originally issued the ticket, see Section 9. A full round-
trip is required for the Responder to receive the keys. The Ticket
Resolve exchange is optional (depending on the Ticket Policy), and
SHOULD only be used when the Responder is unable to resolve the
ticket without assistance from the KMS. The initial message
RESOLVE_INIT comes in two variants (independent from the used
REQUEST_INIT variant). The first variant corresponds to the pre-
shared key (PSK) method of [RFC3830].
Responder KMS
RESOLVE_INIT_PSK = ---->
HDR, T, RANDr, [IDRr],
[IDRkms], TICKET, <---- RESOLVE_RESP
[IDRpsk], V HDR, T, [IDRkms], KEMAC,
[IDRr], [RANDkms], V
The second variant corresponds to the public-key (PK) method of
[RFC3830].
Responder KMS
RESOLVE_INIT_PK = ---->
HDR, T, RANDr, [IDRr], {CERTr},
[IDRkms], TICKET, <---- RESOLVE_RESP
[CHASH], PKE, SIGNr HDR, T, [IDRkms], KEMAC,
[IDRr], [RANDkms], V
As the RESOLVE_INIT message MUST ensure the identity of the Responder
to the KMS, it SHALL be protected by a MAC based on a pre-shared key
or by a signature. The response message RESOLVE_RESP is the same for
the two variants and SHALL be protected by using the pre-shared/
envelope key indicated in the RESOLVE_INIT message.
Upon receiving the RESOLVE_INIT message, the KMS verifies that the
Responder is authorized to resolve the ticket based on ticket and KMS
policies. The KMS extracts the session information from the ticket
and returns this to the Responder. Since the KMS resolved the
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ticket, the Responder is assured of the integrity of the Ticket
Policy (TP), which contains the identity of the peer that requested
or created the ticket. The Responder can complete the session
information it got from the Initiator with the additional session
information received from the KMS.
4.2.3.1. Common Components of the RESOLVE_INIT Messages
The RESOLVE_INIT message MUST always include the Header (HDR),
Timestamp (T), and RANDr payloads.
The CSB ID (Crypto Session Bundle ID) SHALL be assigned as in
[RFC3830]. The V flag MUST be set to '1' but SHALL be ignored by the
KMS as a response is MANDATORY. As crypto sessions SHALL NOT be
handled, the #CS MUST be set to 0 and the CS ID map type SHALL be the
"Empty map" as defined in [RFC4563].
IDRkms SHOULD be included, but it MAY be left out when it can be
expected that the KMS has a single identity.
The TICKET payload contains the Ticket Policy and Ticket Data that
the Responder wants to have resolved.
4.2.3.2. Components of the RESOLVE_INIT_PSK Message
IDRr contains the identity of the Responder. IDRr SHOULD be
included, but it MAY be left out when it can be expected that the KMS
can identify the Responder in some other manner.
The IDRpsk payload is used to indicate the pre-shared key used. It
MAY be omitted if the KMS can find the pre-shared key by other means.
The last payload SHALL be a Verification payload (V) where the
authentication key (auth_key) is derived from the pre-shared key.
The MAC SHALL cover the entire message as well as the identities of
the involved parties (see Section 5.5 for the exact definition).
4.2.3.3. Components of the RESOLVE_INIT_PK Message
The identity IDRr and certificate CERTr SHOULD be included, but they
MAY be left out when it can be expected that the KMS can obtain the
certificate in some other manner. If a certificate chain is to be
provided, each certificate in the chain SHOULD be included in a
separate CERT payload.
PKE contains the encrypted envelope key: PKE = E(PKkms, env_key). It
is encrypted using PKkms. If the KMS possesses several public keys,
the Responder can indicate the key used in the CHASH payload.
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SIGNr is a signature covering the entire MIKEY message, using the
Responder's signature key (see Section 5.5 for the exact definition).
4.2.3.4. Processing the RESOLVE_INIT Message
If the KMS can verify the integrity of the received message and
Ticket Policy, the message can be correctly parsed, and the Responder
is authorized to resolve the ticket, the KMS MUST send an
RESOLVE_RESP message. Otherwise the KMS SHOULD send an appropriate
Error message.
4.2.3.5. Components of the RESOLVE_RESP Message
The Header payload SHOULD be identical to the Header payload in the
RESOLVE_INIT message with the exception of data type, next payload,
and V flag. The V flag has no meaning in this context. It SHALL be
set to 0 by the KMS and ignored by the Responder.
The timestamp type and value SHALL be identical to the one used in
the RESOLVE_INIT message.
The KEMAC payload SHOULD use the NULL authentication algorithm, as a
MAC is included in the V payload. Depending on the type of
RESOLVE_INIT message, either the pre-shared key or the envelope key
SHALL be used to derive the encr_key and salt_key. The salting key
goes into the IV as defined in [RFC3830]. The KEMAC SHALL include a
MPK (MPKi), used as a pre-shared key to protect the messages in the
Ticket Transfer exchange. The KEMAC is hence constructed as follows:
KEMAC = E(encr_key, MPKi || [MPKr'] || (TGK|TEK))
If key forking (see Section 5.1.1) is used (determined by the Ticket
Policy) a second MPK (MPKr') SHALL be included in the KEMAC. Then
MPKi SHALL be used to verify the TRANSFER_INIT message and MPKr'
SHALL be used to protect the TRANSFER_RESP message. The KMS SHALL
also fork the MPKr and the TGKs. The modifier used to derive the
forked keys SHALL be included in the IDRr and RANDkms payloads, where
IDRr is the identity of the endpoint that answered and RANDkms is a
fresh (pseudo-)random byte string generated by the KMS. The reason
that the KMS MAY adjust the Responder's identity is so that it
matches an identity encoded in the ticket.
The last payload SHALL be a Verification payload (V). Depending on
the type of RESOLVE_INIT message, either the pre-shared key or the
envelope key SHALL be used to derive the auth_key. The MAC SHALL
cover the entire message as well as the INIT message (see Section 5.5
for the exact definition).
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4.2.3.6. Processing the RESOLVE_RESP Message
If the Responder can verify the integrity of the received message and
the message can be correctly parsed, the Responder SHOULD verify the
TRANSFER_INIT message with the MPK received from the KMS. Otherwise
the Responder SHOULD silently discard the message.
If the Responder cannot verify the integrity of the received message
or the message cannot be parsed the Responder SHOULD silently discard
the message.
5. Key Management Functions
5.1. Key Derivation
For all messages in the Ticket Request and Ticket Resolve exchanges,
the keys used to protect the MIKEY messages are derived from the pre-
shared key or the envelope key. As crypto sessions SHALL NOT be
handled, further keying material (i.e. TEKs) SHALL NOT be derived.
In the Ticket Transfer exchange, the keys used to protect the MIKEY
messages are derived from a MPK. If key forking is used, the KMS and
the Initiator SHALL fork the MPKr and the TGKs based on a modifier,
and different MPKs (MPKi and MPKr') SHALL be used to protect the
TRANSFER_INIT and TRANSFER_RESP messages. In addition, the Responder
MAY generate a RAND used to give Responder key freshness guarantee.
The key hierarchy and its dependencies on TRANSFER_INIT message
contents for the case without key forking and with a single RAND are
illustrated in Figure 4. The KEMAC shown is the KEMAC sent from the
KMS. The illustrated key derivations are done by the Initiator and
the Responder.
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+------+------------------+-----+------+
KEMAC | MPKi |..................| TGK | SALT |
+--+---+------------------+--+--+--+---+
| MPKi | |
v | |
csb_id ----- auth_key ----- | |
+---------->| PRF |------------->| MAC | | |
| cs_id ----- ----- | |
| ^ MAC | | |
| | RANDi v | |
+--+--+-------+---+---+--+--------+--+---+ | |
TRANSFER_INIT | HDR |.......| RANDi |..| TICKET |..| V | | |
+--+--+-------+---+---+--+--------+--+---+ | |
| | RANDi | |
| v | |
| csb_id ----- TGK | |
+---------->| PRF |<---------------------+ |
cs_id ----- |
| |
v TEK SALT v
---------------------------------------
| Security Protocol e.g. SRTP |
---------------------------------------
Figure 4: Key hierarchy without key forking and with a single RAND
The key hierarchy and its dependencies on TRANSFER_RESP message
contents for the case with key forking and two RANDs are illustrated
in Figure 5. The KEMAC shown is the KEMAC sent from the KMS to the
Initiator. MOD is the modifier (IDRr, RANDkms). The two key
derivations that produce forked keys are done by the Initiator and
the KMS, and the remaining two key derivations are done by the
Initiator and the Responder. The random value RANDi from the
TRANSFER_INIT message is also used as input to the derivation of the
auth_key and the TEK, but this is omitted from the figure. The
protection of the TRANSFER_INIT message is done as in Figure 4.
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+------+---------------------------+-----+------+
KEMAC | MPKr |...........................| TGK | SALT |
+--+---+---------------------------+--+--+--+---+
| MPKr | |
v | |
----- MPKr' | |
| PRF |------------+ | |
----- | | |
^ v | |
| csb_id ----- auth_key | |
+--------)----------->| PRF |----------+ | |
| | cs_id ----- v | |
| | ^ ----- | |
| | | | MAC | | |
| | IDRr, | RANDr ----- | |
| | RANDkms | MAC | | |
| | | v | |
+--+--+--+--+--+--------+---+---+-------+---+ | |
TRANSFER_RESP | HDR |..| MOD |........| RANDr |.......| V | | |
+--+--+--+--+--+--------+---+---+-------+---+ | |
| | ----- | | |
| +---->| PRF |<--)------------------+ |
| ----- | TGK |
| TGK' | | |
| v | RANDr |
| csb_id ----- | |
+------------->| PRF |<--+ |
cs_id ----- |
| |
v TEK SALT v
---------------------------------------
| Security Protocol e.g. SRTP |
---------------------------------------
Figure 5: Key hierarchy with key forking and two RANDs
5.1.1. Deriving Forked Keys
When key forking is used (determined by the Ticket Policy), the MPKr
and TGKs SHALL be forked. The TEKs and GTGKs (Group TGKs), however,
SHALL NOT be forked. This key forking is done by the KMS and the
Initiator using the PRF (Pseudo-Random Function) indicated in the
Ticket Policy (TP). The parameters for the default PRF are:
inkey: : MPKr or TGK
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x02 ||
length ID Data || ID Data || length RANDkms || RANDkms
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outkey_len : desired bit length of the outkey (MPKr', TGK')
where ID Data is taken from the IDRr payload sent in the RESOLVE_RESP
and TRANSFER_RESP messages. Length ID Data is the length of ID Data
in bytes as a 16-bit integer. Length RANDkms is the length of
RANDkms in bytes as an 8-bit integer. The constant depends on the
derived key type as summarized below.
Derived key | Constant
------------+-----------
MPKr' | 0x2B288856
TGK' | 0x1512B54A
Table 5.1: Constants for forking key derivation
The constants are taken from the decimal digits of e as described in
[RFC3830].
5.1.2. Deriving Keys from an Envelope/Pre-Shared Key/MPK
This derivation is to form the keys used to protect the MIKEY
messages. For the Ticket Request and Ticket Resolve exchanges, the
keys used to protect the MIKEY messages are derived from the pre-
shared key or the envelope key. For the Ticket Transfer exchange,
the keys are derived from a MPK. If key forking is used, different
MPKs (MPKi and MPKr') SHALL be used to protect the TRANSFER_INIT and
TRANSFER_RESP messages. The initial messages SHALL be protected with
the keys derived using the parameters given below.
inkey: : envelope key, pre-shared key, or MPKi
inkey_len : bit length of the inkey
label : constant || 0xFF || csb_id || 0x03 ||
length RANDi || RANDi
outkey_len : desired bit length of the output key
where the constants are as defined in [RFC3830]. The parameters for
the response messages are given below.
inkey: : envelope key, pre-shared key, MPKi, or MPKr'
inkey_len : bit length of the inkey
label : constant || 0xFF || csb_id || 0x04 ||
length RANDi || RANDi || length RANDr || [RANDr]
outkey_len : desired bit length of the output key
RANDr SHALL be included in the derivation of the keys used to protect
TRANSFER_RESP if the Ticket Policy determines that it shall be
present in the TRANSFER_RESP message. Length RANDi is the length of
RANDi in bytes as an 8-bit integer. Length RANDr is the length of
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RANDr in bytes as an 8-bit integer. If RANDr is omitted, lenght
RANDr SHALL be 0.
5.2. Deriving Keys from a TGK
This only affects the Ticket Transfer exchange. In the following, we
describe how keying material is derived from a TGK. If key forking
is used, the forked key TGK' SHALL be used. The key derivation
method SHALL be executed using the PRF indicated in the HDR payload.
The parameters for the default PRF are given below.
inkey: : TGK or TGK'
inkey_len : bit length of the inkey
label : constant || cs_id || csb_id || 0x05 ||
length Application ID || Application ID ||
length RANDi || RANDi || length RANDr || [RANDr]
outkey_len : bit length of the outkey
where the constants are as defined in Section 4.1.3. of [RFC3830].
Application ID is an identifier for the security protocol in which
the derived keys will be used. Currently, the only defined
Application ID is "SRTP". To support other security protocols in
MIKEY-TICKET, new Application IDs have to be defined. If the Ticket
Policy includes one or more IDRapp payloads, Application ID MUST
correspond to one of them. RANDr SHALL be included if the Ticket
Policy determines that it shall be present in the TRANSFER_RESP
message. Length ID Data is the length of ID Data in bytes as a 16-
bit integer. Length RANDi is the length of RANDi in bytes as an
8-bit integer. Length RANDr is the length of RANDr in bytes as an
8-bit integer. If RANDr is omitted, lenght RANDr SHALL be 0.
Note that the ticket may carry a salt. A security protocol in need
of a salting key SHALL use the salting key carried in the ticket when
present. If a salt is not included, it is possible to derive a
salting key by using the constant defined in [RFC3830].
5.2.1. Deriving Keys from a GTGK
The same key derivation as in Section 5.2 SHALL be done, with the
exceptions that the GTGK SHALL NOT be forked and the label SHALL NOT
include RANDr.
5.3. CSB Updating
Similar to [RFC3830], MIKEY-TICKET provides a means of updating the
CSB (Crypto Session Bundle), e.g. transporting new TGK/TEK or adding
new crypto sessions. The CSB updating is done by executing the
Ticket Transfer exchange again, e.g. before a TEK expires or when a
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new crypto session is needed. The CSB updating MAY be started by
either the Initiator or the Responder.
Initiator Responder
TRANSFER_INIT = ---->
HDR, T, [IDRi], [IDRr],
{SP}, [KEMAC], V < - - TRANSFER_RESP =
HDR, T, [IDRr], V
Responder Initiator
TRANSFER_INIT = ---->
HDR, T, [IDRr], [IDRi],
{SP}, [KEMAC], V < - - TRANSFER_RESP =
HDR, T, [IDRi], V
The new message exchange MUST use the same CSB ID as the initial
exchange, but MUST use a new timestamp. The ticket and other static
payloads that were provided in the initial exchange SHOULD NOT be
included. New RANDs MUST NOT be included in the message exchange
(the RANDs will only have effect in the initial exchange). The
reason that new RANDs SHALL NOT be used is that if several TGKs are
used, the peers would need to keep track of which RANDs to use for
each TGK. This adds unnecessary complexity.
New keying material SHOULD be sent in a KEMAC payload. The KEMAC
SHOULD use the NULL authentication algorithm, as a MAC is included in
the V payload. Both messages SHALL be protected with the keys that
protected the TRANSFER_RESP message in the initial exchange. If a
new TGK is exchanged in a CSB update, the new TGK SHALL NOT be
forked. The KEMAC is hence constructed as follows:
KEMAC = E(encr_key, (TGK|TEK))
5.4. Ticket Reuse
MIKEY-TICKET includes features aiming to offload the KMS from
receiving ticket requests. One such feature is that tickets may be
reused. This means that a user may request a ticket for another user
and then for a specified time period use this ticket to protect calls
to that user.
When reusing a ticket that has been used in a previous Ticket
Transfer exchange, a new Ticket Transfer exchange is executed. The
new exchange MUST use a new CSB ID, a new timestamp, and new RANDs
(RANDi, RANDr). If the Responder has resolved the ticket before, the
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Responder does not need to resolve the ticket again. In that case,
the same modifier (IDRr, RANDkms) SHALL be used. If the Ticket
Policy forbids reuse, the ticket MUST NOT be reused. Note that such
reuse cannot be detected by a stateless KMS. When group keys are
used, ticket reuse leaves the Initiator responsible to ensure that
group membership has not changed since the ticket was last used.
(Otherwise, unauthorized responders may gain access to the group
communication.) Thus, if group dynamics are difficult to verify, the
Initiator SHOULD NOT initiate ticket reuse.
When key forking is used, only the user that requested the ticket has
access to the encoded master keys (MPKr, TGKs). Because of this, no
one else can initiate a Ticket Transfer exchange using the ticket.
5.5. MAC/Signature Coverage
The MAC/Signature in the V/SIGN payloads covers the entire MIKEY
message, except the MAC/Signature field itself. For initial
messages, the identities (not whole payloads) of the parties involved
MUST directly follow the MIKEY message in the Verification MAC/
Signature calculation. Note that in the Transfer Exchange,
Identity_r in TRANSFER_RESP (e.g. user1@company.example) MAY differ
from that appearing in TRANSFER_INIT (e.g. somebody@company.example).
For response messages, the entire initial message (including the MAC/
Signature field) MUST directly follow the MIKEY message in the
Verification MAC/Signature calculation (the identities are implicitly
covered as they are covered by the initial message's MAC/Signature).
Message type | MAC/Signature coverage
--------------+--------------------------------------------
REQUEST_INIT | REQUEST_INIT || Identity_i || Identity_kms
REQUEST_RESP | REQUEST_RESP || REQUEST_INIT
TRANSFER_INIT | TRANSFER_INIT || Identity_i || Identity_r
TRANSFER_RESP | TRANSFER_RESP || TRANSFER_INIT
RESOLVE_INIT | RESOLVE_INIT || Identity_r || Identity_kms
RESOLVE_RESP | RESOLVE_RESP || RESOLVE_INIT
Table 5.2: MAC/Signature coverage
6. Payload Encoding
This section does not describe all the payloads that are used in the
new message types. It describes in detail the new TR, IDR, TICKET,
and TP payloads. For the other payloads, only the additions and
changes compared to [RFC3830] are described. For a detailed
description of the other MIKEY payloads, see [RFC3830].
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6.1. Common Header Payload (HDR)
For the Common Header Payload, new values are added to the Data Type
and the Next Payload name spaces.
* Data Type (8 bits): describes the type of message.
Data Type | Value | Comment
-----------------+-------+-------------------------------------
REQUEST_INIT_PSK | TBD1 | Ticket request initial message (PSK)
REQUEST_INIT_PK | TBD2 | Ticket request initial message (PK)
REQUEST_RESP | TBD3 | Ticket request response message
| |
TRANSFER_INIT | TBD4 | Ticket transfer initial message
TRANSFER_RESP | TBD5 | Ticket transfer response message
| |
RESOLVE_INIT_PSK | TBD6 | Ticket resolve initial message (PSK)
RESOLVE_INIT_PK | TBD7 | Ticket resolve initial message (PK)
RESOLVE_RESP | TBD8 | Ticket resolve response message
Table 6.1: Data Type (Additions)
* Next Payload (8 bits): identifies the payload that is added after
this payload.
Next Payload | Value | Section
-------------+-------+--------
TR | TBD9 | 6.4
IDR | TBD10 | 6.6
TICKET | TBD11 | 6.10
TP | TBD12 | 6.11
Table 6.2: Next Payload (Additions)
* V (1 bits): flag to indicate whether a response message is
expected or not. The V flag SHALL be set to 1 in the REQUEST_INIT
and RESOLVE_INIT messages and agree with the Ticket Policy in the
TRANSFER_INIT message. It SHALL be set to 0 in the response
messages. It SHALL be ignored in all messages.
* PRF func (7 bits): indicates the PRF function that has been/will
be used for key derivation. Besides the PRFs already defined in
[RFC3830] the following additional PRF may be used.
PRF func | Value | Comments
-----------------+-------+---------
PRF-HMAC-SHA-256 | TBD13 |
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Table 6.3: PRF func (Additions)
The new PRF SHALL be constructed as described in Section 4.1.2 of
[RFC3830] with the differences that HMAC-SHA-256 (see Section 6.2)
SHALL be used instead of HMAC-SHA-1 and the value "256" SHALL be
used instead of "160". This corresponds to the full output length
of SHA-256.
* #CS (8 bits): indicates the number of crypto sessions that will be
handled within the CBS. It SHALL be set to 0 in the Ticket
Request and Ticket Resolve exchanges, as crypto sessions SHALL NOT
be handled.
* CS ID map type (8 bits): specifies the method of uniquely mapping
crypto sessions to the security protocol sessions. In the Ticket
Request and Ticket Resolve exchanges, the CS ID map type SHALL be
the "Empty map" (defined in [RFC4563]) as crypto sessions SHALL
NOT be handled.
6.2. Key Data Transport Payload (KEMAC)
The key data transport payload contains encrypted key data sub-
payloads. The keys MAY be supplied by the Initiator or the KMS. The
number of keys with Key Data supplied by the Initiator SHALL be
indicated in the Ticket Policy (see Section 6.11). Such keys SHALL
be placed last, after all keys with Key Data supplied by the KMS.
If the Initiator require something else than the default, the
Initiator may use the KEMAC in the REQUEST_INIT to indicate the
number of keys, specify other key information (key type, key length,
KV data), and specify the Key Data itself. Initiator specified Key
Data in a KMS generated ticket SHOULD NOT be used unless the
Initiator has pre-encrypted content and specific TEKs must be
included in the ticket. For keys where the KMS should supply Key
Data, the Key Data field SHALL be set to 0 by the Initiator and
ignored by the KMS.
* Encr alg (8 bits): besides the algorithms already defined in
[RFC3830], this specification defines the following additional
encryption algorithm that may be used to encrypt the Encr data
field.
Encr alg | Value | Comment
-----------+-------+---------------------------
AES-CM-256 | TDB14 | AES-CM using a 256-bit key
Table 6.4: Encr alg (Additions)
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The new encryption algorithm is defined as described in Section
4.2.3 of [RFC3830] with the only difference that a 256-bit key
SHALL be used.
* MAC alg (8 bits): besides the algorithms already defined in
[RFC3830], this specification defines that the following
additional authentication algorithm may be used.
MAC alg | Value | Length (bits)
-----------------+-------+--------------
HMAC-SHA-256-256 | TBD15 | 256
Table 6.5: MAC alg (Additions)
The new authentication algorithm is Hash-based Message
Authentication Code (HMAC) [RFC2104] in conjunction with SHA-256
[FIPS.180-3]. It SHOULD be used with a 256-bit authentication
key. Note that the MAC coverage depends on the method used.
6.3. Timestamp Payload (T)
For the timestamp payload, a new type of timestamp is defined. The
new type is intended to be used when defining validity periods, where
fractions of seconds seldom matter. The NTP-UTC-32 string contain
four bytes, in the same format as the first four bytes in the NTP
timestamp format, defined in [RFC4330]. This represents the number
of seconds since 0h on 1 January 1900 with respect to the Coordinated
Universal Time (UTC). On 7 February 2036 the time value will
overflow. [RFC4330] describes a procedure to extend the time to 2104
and this procedure is MANDATORY to support.
* TS Type (8 bits): specifies the timestamp type used.
TS Type | Value | Length of TS value
-----------+-------+-------------------
NTP-UTC-32 | TDB16 | 32-bits
Table 6.6: TS Type (Additions)
Note: NTP-UTC-32 SHALL be padded to a 64-bit NTP-UTC timestamp (with
zeroes in the fractional second part) when used as input for a PRF
requiring a 64-bit timestamp.
6.4. Timestamp Payload with Role Indicator (TR)
The TR payload uses all the fields from the standard timestamp
payload (T) but expands it with a new field describing the role of
the timestamp. Whereas the TS Type describes the type of the TS
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Value, the TS Role describes the meaning of the timestamp itself.
The TR payload is intended to eliminate ambiguity when a MIKEY
message contains several timestamp payloads.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! TS Role ! TS Type ! TS Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* TS Role (8 bits): specifies the sort of timestamp.
TS Role | Value
-------------------------------+------
Time of issue (TRi) | 1
Start of validity period (TRs) | 2
End of validity period (TRe) | 3
Reykeying interval (TRr) | 4
Table 6.7: TS Role
6.5. ID Payload (ID)
For the ID payload, a new ID Type byte string is defined. The byte
string type is intended to be used when the ID payload is used to
identify a pre-shared key.
* ID Type (8 bits): specifies the identifier type used.
ID Type | Value
------------+------
Byte string | TDB17
Table 6.8: ID Type (Additions)
6.6. ID Payload with Role Indicator (IDR)
The IDR payload uses all the fields as the standard identity payload
(ID) but expands it with a new field describing the role of the ID
payload. Whereas the ID Type describes the type of the ID Data, the
ID Role describes the meaning of the identity itself. The IDR
payload is intended to eliminate ambiguity when a MIKEY message
contains several identity payloads. The IDR payload MUST be used
instead of the ID payload in all MIKEY-TICKET messages.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! ID Role ! ID Type ! ID len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ID len (cont) ! ID Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* ID Role (8 bits): specifies the sort of identity.
ID Role | Value
------------------------+------
Initiator (IDRi) | 1
Responder (IDRr) | 2
KMS (IDRkms) | 3
Pre-Shared Key (IDRpsk) | 4
Application (IDRapp) | 5
Table 6.9: ID Role
IDRapp is intended to specify the authorized Application IDs (see
Section 5.2 and Section 6.11)
6.7. Cert Hash Payload (CHASH)
* Hash func (8 bits): besides the algorithms already defined in
[RFC3830], this specification defines that the following hah
function algorithm may be used.
Hash func | Value | Hash Length (bits)
----------+-------+-------------------
SHA-256 | TDB18 | 256
Table 6.10: Hash func (Additions)
The SHA-256 hash function is defined in [FIPS.180-3].
6.8. Error Payload (ERR)
For the key data sub-payload, new types of errors are defined.
* Error no (8 bits): indicates the type of error that was
encountered.
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Error no | Value | Comments
------------------+-------+----------------------------
Invalid TICKET | TBD19 | Ticket Type not supported
Invalid TPpar | TBD20 | TP parameters not supported
Table 6.11: Error no (Additions)
6.9. Key Data Sub-Payload
For the key data sub-payload, new types of keys are defined. The
Group TGK (GTGK) is used as a regular TGK, with the differences that
it SHALL NOT be forked and that RANDr SHALL NOT be used when deriving
keys from it. It is intended to enable the establishment of a group
TGK when key forking is used. The MIKEY Protection Key (MPK) is used
to protect the MIKEY messages in the Ticket Transfer exchange. The
MPK is used as the pre-shared key in the pre-shared key method of
[RFC3830], it is however not known by the Responder before the ticket
has been resolved.
* Type (4 bits): indicates the type of key included in the payload.
Type | Value | Comments
----------+-------+---------------------
GTGK | TBD21 | Group TGK
GTGK+SALT | TBD22 | Group TGK + SALT
MPK | TBD23 | MIKEY Protection Key
Table 6.12: Key Data Type (Additions)
6.10. Ticket Payload (TICKET)
The ticket payload contains a TP payload (see Section 6.11 ) as well
as Ticket Data (see Appendix A for an example). The Ticket Policy
contains information intended for all parties involved whereas the
Ticket Data is only intended for the party that resolves the ticket.
The Ticket Type provided in the Ticket Data is indicated in the TP
payload. The Next Payload field in the TP sub-payload SHALL be set
to Last payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! TP length ! TP ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Ticket Data length ! Ticket Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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* Next Payload (8 bits): identifies the payload that is added after
this payload.
* TP length (16 bits): the length of the TP field (in bytes).
* TP (variable length): the granted Ticket Policy.
* Ticket Data length (16 bits): the length of the Ticket Data field
(in bytes).
* Ticket Data (variable length): The Ticket Data.
6.11. Ticket Policy Payload (TP)
The Ticket Policy payload contains a desired or granted Ticket
Policy. Note that the flags are not independent as D implies C, F
implies B, and F implies C.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Ticket Type ! Subtype !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Version ! #IGenKeys ! PRF Func !A!B!C!D!E!F!G!H!I!
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! First Payload ! TP Data len ! TP Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next Payload (8 bits): identifies the payload that is added after
this payload.
* Ticket Type (16 bits): specifies the Ticket Type used.
Ticket Type | Value | Comments
------------------+-------+----------------------
MIKEY Base Ticket | 1 | Defined in Appendix A
3GPP Base Ticket | 2 |
Table 6.13: Ticket Type
* Subtype (8 bits): specifies the ticket subtype used.
* Version (8 bits): specifies the ticket subtype version used.
* #IGenKeys (8 bits): specifies the number of traffic keys (i.e. not
MPK) with Key Data supplied by the Initiator encoded in the Ticket
Data. The value '255' means 255 or more keys. This is included
to give the Responder possibility to reject Initiator generated
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keys.
* PRF Func (7 bit): specifies the PRF that SHALL be used for key
forking.
* A (1 bit): flag to indicate whether the ticket was generated by
the KMS ('1') or by the Initiator ('0').
* B (1 bit): flag to indicate whether the Ticket Resolve exchange is
MANDATORY ('1') or if the Responder MAY resolve the ticket ('0').
* C (1 bit): flag to indicate whether the TRANSFER_RESP message
SHALL be sent ('1') or if it SHALL NOT be sent ('0').
* D (1 bit): flag to indicate whether the Responder SHALL generate
RANDr ('1') or if the Responder SHALL NOT generate RANDr ('0').
* E (1 bit): flag to indicate whether the ticket MAY be reused ('1')
and therefore MAY be cached or if it SHALL NOT be reused ('0').
* F (1 bit): flag to indicate whether key forking SHALL be used
('1') or if key forking SHALL NOT be used ('0').
* G (1 bit): flag to indicate whether the KMS changed the desired
Ticket Policy or the desired KEMAC ('1') or if it did not ('0').
* H (1 bit): flag to indicate whether an Initiator following this
specification can initiate a TRANSFER_INIT message using the
ticket ('1') or if additional processing is required ('0'). If
the flag is set to ('0') the Initiator SHOULD follow the
processing in the specification of the received Ticket Type.
* I (1 bit): flag to indicate whether a Responder following this
specification can process a TRANSFER_INIT message containing the
ticket ('1') or if additional processing is required ('0'). If
the flag is set to ('0') the Responder SHOULD follow the
processing in the specification of the received Ticket Type.
* First Payload (8 bits): identifies the first payload in TP Data.
* TP Data len (16 bits): length of TP Data.
* TP Data (variable length): [IDRkms], [IDRi], [TRs], [TRe], [TRr],
{IDRapp}, (IDRr)
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IDRkms contains the identity of a KMS that can resolve the ticket.
IDRi contains the identity of the peer that requested or created
the ticket.
TRs is the start of the validity period. TRs SHALL be interpreted
as being in the range 1968-2104 as described in [RFC4330]. An
omitted TRs means that the validity period has defined beginning.
TRe is the end of the validity period. TRe SHALL be interpreted
as being in the range 1968-2104 as described in [RFC4330]. An
omitted TRe means that the validity period has no defined end.
TRr indicates how often rekeying SHOULD be done. How the rekeying
is done is implementation specific.
IDRapp is an identifier for an authorized application ID. Zero
IDRapp payloads means that all application IDs are authorized.
IDRr is the identity of a responder or a group of responders that
are authorized to resolve the ticket. If there is more than one
responder identity, each responder identity SHALL be included in a
separate IDR payload.
7. Transport Protocols
As the Ticket Transfer exchange terminates in at most one full round-
trip, it is applicable for integration into two-way handshake session
or call signaling protocols such as SIP/SDP and RTSP. Such
integration of MIKEY within SIP/SDP and RTSP is defined in [RFC4567].
Although any such transport protocol defined for general MIKEY
messages can be used for MIKEY-TICKET, it may not be suitable for the
MIKEY-TICKET exchanges that do not establish keying material for
media sessions (Ticket Request and Ticket Resolve), in which case it
has to be defined how MIKEY is transported over the transport
protocol in question.
8. Group Communication
What has been discussed up to now can also be used to distribute
group keys for small-size interactive groups. The MIKEY signaling
for multi-party sessions can either be centralized as illustrated in
Figure 6
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+---+ +---+ +---+
| A | | B | | C |
+---+ +---+ +---+
Ticket Transfer
<----------------------------> Ticket Transfer
<--------------------------------------------------------->
Figure 6: Centralized signaling
or decentralized as illustrated in Figure 7.
+---+ +---+ +---+
| A | | B | | C |
+---+ +---+ +---+
Ticket Transfer
<----------------------------> Ticket Transfer
<---------------------------->
Figure 7: Decentralized signaling
In the decentralized scenario, B's and C's identities SHALL be used
in the second Ticket Transfer exchange.
If a (G)TGK is used a group key, the same RANDi MUST be used in all
TRANSFER_INIT messages and in the case of a TGK, RANDr MUST NOT be
used in the TRANSFER_RESP message. Note also caveats with ticket
reuse in group communication settings as discussed in Section 5.4.
8.1. Key Forking
When key forking is used, the MIKEY signaling MUST be centralized to
the party that initially requested the ticket. Decentralized
signaling does not work, as only the user that requested the ticket
could initiate the Ticket Transfer exchange, see Section 5.4.
+---+ +---+ +---+
| A | | B | | C |
+---+ +---+ +---+
Ticket Transfer
<----------------------------> Ticket Transfer
<--------------------------------------------------------->
Rekeying
-----------------------------> Rekeying
---------------------------------------------------------->
Figure 8: Multi-party rekeying
Another consideration is that different users get different TEKs if
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TGKs (not GTGKs) are used, so if the media mixing (e.g. in a voice
conference) is decentralized, a new (non-forked) group key MUST be
distributed before the session starts, see Figure 8. The rekeying
does not need to be done with a CSB Updating exchange (see
Section 5.3); it can be done with any appropriate rekeying mechanism,
e.g. EKT (Encrypted Key Transport) [I-D.mcgrew-srtp-ekt].
Using a group key might also be preferred when centralized media
mixing is used; the mixer does not have to re-encrypt, which
minimizes CPU and memory use, and means that an untrusted
conferencing server can be used.
9. Signaling Between Different KMSs
A user can in general only be expected to have a trust relation with
a single KMS. Different users might therefore use tickets issued by
different KMSs using only locally known keys. Thus, if users with
trust relations to different KMSs are to be able to establish a
secure session with each other, the KMSs involved have to cooperate
and there has to be a trust relation between them. The KMSs SHALL be
mutually authenticated and signaling between them SHALL be integrity
protected. The technical means for the inter-KMS security is however
outside the scope of this specification. Under these assumptions,
the following approach MAY be used.
+---+ +---+ +-------+ +-------+
| I | | R | | KMS R | | KMS I |
+---+ +---+ +-------+ +-------+
TRANSFER_INIT
--------------------> RESOLVE_INIT
- - - - - - - - - - -> RESOLVE_INIT
- - - - - - - - - - ->
RESOLVE_RESP
RESOLVE_RESP <- - - - - - - - - - -
TRANSFER_RESP < - - - - - - - - - -
<--------------------
Figure 9: Routing of resolve messages
If the Responder cannot directly resolve a ticket, the ticket SHOULD
be included in a RESOLVE_INIT message sent to a KMS. If the
Responder does not have a shared credential with the KMS that issued
the ticket (KMS I) or if the Responder does not know which KMS that
issued the ticket, the Responder SHOULD send the RESOLVE_INIT message
to one of the Responder's trusted KMS (KMS R). If KMS R did not
issue the ticket, KMS R would normally be unable to directly resolve
the ticket and must hence ask another KMS to resolve it (typically
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the issuing KMS).
The signaling between different KMSs MAY be done with a Ticket
Resolve exchange as illustrated in Figure 9. The IDRr and TICKET
payloads from the previous RESOLVE_INIT message SHOULD be reused.
Note that IDRr cannot be used to look up the pre-shared key/
certificate.
10. Adding New Ticket Types to MIKEY-TICKET
The Ticket Data (in the TICKET payload) could be a reference to
information (keys etc.) stored by the key management service, it
could contain all the information itself, or it could be a
combination of the two alternatives. For systems serving many users,
it is not ideal to use the reference-only ticket approach as this
would force the key management service to keep state of all issued
tickets that are still valid. Tickets may carry many different types
of information helping to enforce usage policies. The policies may
be group policies or per-user policies.
Tickets may either be transparent, meaning they can be resolved
without contacting the KMS that generated them, or opaque, meaning
that the original KMS must be contacted. The ticket information
SHOULD typically be integrity protected and certain fields need
confidentiality protection, in particular the keys if explicitly
included. Other types of information may also require
confidentiality protection due to privacy reasons. In mode 2 (see
Section 4.1.1) it may be preferable to include several encrypted
ticket protection keys (similar to S/MIME) as this may allow multiple
peers to resolve the ticket.
The Ticket Data MUST include information so that the resolving party
can retrieve an encoded KEMAC. It MUST also be possible to verify
the integrity of the TICKET payload. It is RECOMMENDED that future
specifications use the recommended payload order and do not add any
additional payloads or processing. New Ticket Types SHOULD not
change the processing for the Responder. If a new Ticket Type
requires additional processing, it MUST be indicated in the TP
payload. New specifications MUST specify which modes are supported
and if any additional security considerations apply.
11. Security Considerations
Unless otherwise stated, the security considerations in [RFC3830]
still apply and contain notes on the security properties of the MIKEY
protocol, key derivation functions, and other components. As some
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security properties depend on the specific Ticket Type, only generic
security considerations concerning the MIKEY-TICKET framework are
discussed.
11.1. General
In addition to the Ticket Policy the KMS MAY have its own set of
policies (authorized key lengths, algorithms, etc.) that in some way
are shared with the peers. The KMS MAY also provide keying material
to authorized intermediate nodes performing various network functions
(e.g. transcoding services, recording services, conference bridges).
The key management service can enforce end-to-end security by only
distributing the keys to authorized end-users. As in [RFC3830] the
user identities are not confidentiality protected. If user privacy
is needed some kind of Privacy Enhancing Technologies (PET) like
anonymous or temporary credentials MAY be used.
In the standard MIKEY modes [RFC3830], the keys are generated by the
Initiator (or by both peers in the Diffie-Hellman scheme). If a bad
random number generator is used, this is likely to make any key
management protocol sensitive to different kinds of attacks, and
MIKEY is no exception. As the choice of the random number generator
is implementation specific, the easiest (and often bad) choice is to
use the PRNG (Pseudo-Random Number Generator) supplied by the
operating system. In MIKEY-TICKET's default mode of operation, the
key generation is mostly done by the KMS, which can be assumed to be
less likely to use a bad random number generator. All keys
(including keys used to protect the ticket) MUST have adequate
strength/length, i.e. 128 bits or more.
The use of random nonces (RANDs) in the key derivation is of utmost
importance to counter offline pre-computation attacks and other
generic attacks. A key of length n, using RANDs of length r, has
effective key entropy of (n + r) / 2 against a birthday attack.
Therefore, the length of RAND generated by the Initiator MUST at
least be equal to the length of the pre-shared key/envelope key and
the sum of the lengths of the RANDs (RANDi, RANDr) MUST at least be
equal to the key size of the longest TGK.
Note that the CSB Updating messages reuse the old RANDs. This means
that the total effective key entropy (relative to pre-computation
attacks) for k consecutive key updates, assuming the TGKs are each n
bits long, is still no more than n bits. In other words, a 2^n work
enables an attacker to get all k n-bit keys. While this might seem
like a defect, this is in practice (for all reasonable values of k)
not better than brute force, which on average requires k * 2^(n-1)
work (even if different RANDs would be used). A birthday attack
would only require 2^(n/2) work, but would need access to 2^(n/2)
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sessions protected with equally many different keys using a single
pair of RANDs. This is, for typical values of n, clearly totally
infeasible. The success probability of such an attack can be
controlled by limiting the number of updates correspondingly. As
stated in [RFC3830], the fact that more than one key can be
compromised in a single attack is inherent to any solution using
secret- or public-key algorithms. An attacker always gets access to
all the exchanged keys by doing an exhaustive search on the pre-
shared key/envelope key/MPK. This requires 2^m work, where m is the
effective size of the key.
As the Responder MAY generate a RAND, The Ticket Transfer exchange
can provide mutual freshness guarantee for all derived keys.
11.2. Denial of Service
This protocol is resistant to Denial of Service attacks against the
KMS in the sense that it does not construct any state (at the key
management protocol level) before it has authenticated the Initiator
or Responder. Typical prevention methods such as rate-limiting and
ACL (Access Control List) capability SHOULD be implemented in the KMS
as well as the clients. The types and amount of prevention needed
depends on how critical the system is and may vary depending on the
Ticket Type.
Since the Responder in general cannot verify the validity of a
TRANSFER_INIT message without first contacting the KMS, Denial of
Service may be launched against the Responder and/or the KMS via the
Responder. The Responder SHOULD therefore implement additional
protection such as early abort if the Initiator's identity is
suspicious, if the policy is not acceptable, etc., before attempting
a RESOLVE_INIT with the KMS.
11.3. Replay
In a replay attack an attacker may intercept and later retransmit the
whole or part of a MIKEY message, attempting to trick the receiver
(Responder or KMS) into undesired operations, leading e.g. to lack of
key freshness. MIKEY-TICKET implements several mechanisms to prevent
and detect such attacks. Timestamps together with a replay cache
efficiently stop the replay of entire MIKEY messages. Parts of the
received messages (or their hashes) can be saved in the replay cache
until their timestamp is outdated. To prevent replay attacks, the
sender's (Initiator or Responder) and the receiver's identity
(Responder or KMS) is always (explicitly or implicitly) included in
the MAC/Signature calculation.
An attacker may also attempt to replay a ticket by inserting it into
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a new MIKEY message. A possible scenario is that Alice and Bob first
communicate based on a ticket, which an attacker Mallory intercepts.
Later, Mallory (acting as herself) invites Bob by inserting the
ticket into her own TRANSFER_INIT message. Unless Mallory has
knowledge of the MPK encoded in the ticket, such replays will be
detected when Bob has resolved the ticket. If Mallory has knowledge
of the MPK (i.e. she is authorized to resolve the ticket) and key
forking is used together with a TGK, Mallory will not be able to
communicate with Bob due to her inability to deduce the session keys.
If key forking is not used or a TEK or GTGK is used, the session key
is a group key and there is no attack. For the reasons explained
above, it is RECOMMENDED to use key forking and TGKs unless required
by the use case.
11.4. Forking
Forking occurs when a Responder is registered on several devices
(e.g. mobile phone, fixed phone, and computer) or when an invite is
being made to addresses of the type somebody@company.example, a group
of users where only one is supposed to answer. The Initiator may not
even always know exactly who the authorized group members are. To
prevent all forms of eavesdropping, only the endpoint that answers
should get access to the session keys.
When key forking is used together with TGKs, the keys are modified,
making them cryptographically unique for each responder targeted by
the forking. As only the Initiator and the KMS have access to the
master TGKs, it is infeasible for anyone else to derive the session
keys.
11.5. Group Key Management
In a group scenario, only authorized group members must have access
to the keys. In some situation, the communication may be initiated
by the Initiator using a group identity and the Initiator may not
even know exactly who the authorized group members are. Moreover,
group membership may change over time due to leaves/joins. In such a
situation, it is foremost the responsibility of the KMS to reject
ticket resolution requests from unauthorized responders, implying
that the KMS needs to be able to map an individual's identity
(carried in the RESOLVE_INIT message) to group membership (where the
group identity is carried in the ticket).
As noted, reuse of tickets, which bypasses the KMS, is NOT
RECOMMENDED when the Initiator is not fully ensured about group
membership status.
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12. Acknowledgements
The authors would like to thank Fredrik Ahlqvist, Rolf Blom, Yi
Cheng, Lakshminath Dondeti, Vesa Lehtovirta, Fredrik Lindholm, Mats
Naslund, Karl Norrman, Oscar Olsson, Brian Rosenberg, Bengt Sahlin,
Wei Yinxing, and Zhu Yunwen for their support and valuable comments.
13. IANA Considerations
This document defines several new values for the namespaces Data
Type, Next Payload, PRF func, Encr alg, MAC alg, TS Type, ID Type,
Hash func, Error no, and Key Data Type defined in [RFC3830]. The
following IANA assignments were added to the MIKEY Payload registry
(in bracket is a reference to the table containing the registered
values):
o Data Type (see Table 6.1)
o Next Payload (see Table 6.2)
o PRF func (see Table 6.3)
o Encr alg (see Table 6.4)
o MAC alg (see Table 6.5)
o TS Type (see Table 6.6)
o ID Type (see Table 6.8)
o Hash func (see Table 6.10)
o Error no (see Table 6.11)
o Key Data Type (see Table 6.12)
The TR payload defines an 8-bit TS Role field for which IANA is to
create and maintain a new namespace in the MIKEY Payload registry.
Assignments consist of a TS Role name and its associated value.
Values in the range 1-239 SHOULD be approved by the process of
Specification Required, values in the range 240-254 are for Private
Use, and the values 0 and 255 are Reserved according to [RFC5226].
The initial contents of the registry should be as follows:
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Value TS Role
------- ------------------------------
0 Reserved
1 Time of issue (TRi)
2 Start of validity period (TRs)
3 End of validity period (TRe)
4 Reykeying interval (TRr)
5-239 Unassigned
240-254 Private Use
255 Reserved
The IDR payload defines an 8-bit ID Role field for which IANA is to
create and maintain a new namespace in the MIKEY Payload registry.
Assignments consist of an ID Role name and its associated value.
Values in the range 1-239 SHOULD be approved by the process of
Specification Required, values in the range 240-254 are for Private
Use, and the values 0 and 255 are Reserved according to [RFC5226].
The initial contents of the registry should be as follows:
Value ID Role
------- -----------------------
0 Reserved
1 Initiator (IDRi)
2 Responder (IDRr)
3 KMS (IDRkms)
4 Pre-Shared Key (IDRpsk)
5 Application (IDRapp)
6-239 Unassigned
240-254 Private Use
255 Reserved
The TP payload defines an 16-bit Ticket Type field for which IANA is
to create and maintain a new namespace in the MIKEY Payload registry.
Assignments consist of a Ticket Type name and its associated value.
Values in the range 1-61439 SHOULD be approved by the process of
Specification Required, values in the range 61440-65534 are for
Private Use, and the values 0 and 65535 are Reserved according to
[RFC5226]. The initial contents of the registry should be as
follows:
Value Ticket Type
----------- -----------------
0 Reserved
1 MIKEY base ticket
2 3GPP base ticket
3-61439 Unassigned
61440-65534 Private Use
65535 Reserved
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14. References
14.1. Normative References
[FIPS.180-3]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-3, October 2008, <http:
//csrc.nist.gov/publications/fips/fips180-3/
fips180-3_final.pdf>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 4330, January 2006.
[RFC4563] Carrara, E., Lehtovirta, V., and K. Norrman, "The Key ID
Information Type for the General Extension Payload in
Multimedia Internet KEYing (MIKEY)", RFC 4563, June 2006.
[RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, July 2006.
[RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for
Multimedia Internet KEYing (MIKEY)", RFC 4650,
September 2006.
[RFC4738] Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-
RSA-R: An Additional Mode of Key Distribution in
Multimedia Internet KEYing (MIKEY)", RFC 4738,
November 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
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14.2. Informative References
[3GPP.33.328]
3GPP, "IP Multimedia Subsystem (IMS) media plane
security", 3GPP TS 33.328 9.0.0, December 2009.
[HBOAC] Menezes, A., Van Oorschot, P., and Vanstone S., "Handbook
of Applied Cryptography", CRC Press , August 2001.
[I-D.mcgrew-srtp-ekt]
McGrew, D., Andreasen, F., Wing, D., and L. Dondeti,
"Encrypted Key Transport for Secure RTP",
draft-mcgrew-srtp-ekt-06 (work in progress), October 2009.
[Otway-Rees]
Otway, D., and O. Rees, "Efficient and Timely Mutual
Authentication", ACM SIGOPS Operating Systems Review v.21
n.1, p.8-10, January 1987.
[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.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC5197] Fries, S. and D. Ignjatic, "On the Applicability of
Various Multimedia Internet KEYing (MIKEY) Modes and
Extensions", RFC 5197, June 2008.
[RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet,
"Requirements and Analysis of Media Security Management
Protocols", RFC 5479, April 2009.
Appendix A. MIKEY Base Ticket
The MIKEY base ticket MAY be used in any of the modes described in
Section 4.1.1. The Ticket Data SHALL be constructed as a MIKEY
message protected by a MAC based on a pre-shared Ticket Protection
Key (TPK). The parties that shares the TPK depends on the mode.
Ticket Data =
THDR, T, RAND, KEMAC, [IDRpsk], V
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A.1. Components of the Ticket Data
The Ticket Data MUST always begin with a Ticket Header payload
(THDR). The ticket header is a new payload type, for definition see
Appendix A.4.
T is a timestamp containing the time of issue or a counter. It MAY
be used in the IV (Initialization Vector) formation (e.g. Section
4.2.3 of [RFC3830]).
RAND is used as input to the key derivation function when keys are
derived from the TPK and the MPK (see Sections A.2 and A.3).
The KEMAC payload SHOULD use the NULL authentication algorithm, as a
MAC is included in the V payload. The encryption key (encr_key) and
salting key SHALL be derived from the TPK (see Appendix A.2). If CSB
ID is needed in the IV formation it SHALL be set to 0xFFFFFFFF. The
KEMAC is hence constructed as follows:
KEMAC = E(encr_key, MPK || (TGK|TEK))
MPKi and MPKr are derived from the MPK as defined in Appendix A.3.
IDRpsk contains an identifier that enables the KMS/Responder to
retrieve the TPK. It MAY be omitted when the TPK can be retrieved
anyhow.
The last payload SHALL be a Verification payload (V) where the
authentication key (auth_key) is derived from the TPK. The MAC SHALL
be calculated over the entire TICKET payload except the Next Payload
field (in the TICKET payload) and the MAC field itself.
A.2. Deriving Keys from a TPK
In the following, we describe how keying material is derived from a
TPK. The key derivation method SHALL be executed using the PRF
indicated in the Ticket Policy (TP). The parameters for the default
PRF are given below.
inkey: : TPK
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x00 ||
length RAND || RAND
outkey_len : desired bit length of the outkey
Length RAND is the length of RAND in bytes as an 8-bit integer. The
constants are as defined in Section 4.1.4 of [RFC3830]. The key
derivation and its dependencies on Ticket Data contents are
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illustrated in Figure 10. The illustrated key derivation is done by
the party that creates the ticket (KMS or Initiator) and by the party
that resolves the ticket (KMS or Responder). The encryption key and
salting key are used to encrypt the KEMAC.
+-----+ TPK ----- auth_key -----
| TPK |-------->| PRF |---------------------------->| MAC |
+-----+ | |----------------------+ -----
^ ----- encr_key, salt_key | |
: identify ^ | | MAC
: | RAND v v
Ticket +---+----+-------+--+---+-----------------+-------+---+---+
Data | IDRpsk |.......| RAND |.................| KEMAC |...| V |
+--------+-------+------+-----------------+-------+---+---+
Figure 10: Deriving keys from a TPK
A.3. Deriving MPKi and MPKr
In the following, we describe how MPKi and MPKr are derived from the
MPK in the KEMAC payload. The key derivation method SHALL be
executed using the PRF indicated in the Ticket Policy (TP). The
parameters for the default PRF are given below.
inkey: : MPK
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x01 ||
length RAND || RAND
outkey_len : desired bit length of the outkey
Length RAND is the length of RAND in bytes as an 8-bit integer. The
constant depends on the derived key type as summarized below.
Derived key | Constant
------------+-----------
MPKi | 0x220E99A2
MPKr | 0x1F4D675B
Table A.1: Constants for MPK key derivation
The constants are taken from the decimal digits of e as described in
[RFC3830].
A.4. Ticket Header Payload (THDR)
The ticket header payload contains an indicator of the next payload
as well as implementation specific data.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! THDR Data length ! THDR Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next Payload (8 bits): identifies the payload that is added after
this payload.
* THDR Data length (16 bits): the length of the THDR Data field (in
bytes).
* THDR Data (variable length): implementation specific data that
SHOULD be ignored if it is not expected.
Appendix B. Alternative Use Cases
B.1. Compatibility Mode
MIKEY-TICKET can be used to define a Ticket Type compatible with
[RFC3830] or any other half-round-trip key management protocol. The
Initiator requests and gets a ticket from the KMS where the Ticket
Data is a [RFC3830] message protected with a pre-shared key (KMS-
Responder) or with the Responder's certificate. The Ticket Data is
then sent to the Responder according to [RFC3830]. In this way the
Initiator can communicate with a Responder that only supports
[RFC3830] and with whom the Initiator do not have any shared
credentials.
+---+ +-----+ +---+
| I | | KMS | | R |
+---+ +-----+ +---+
REQUEST_INIT
-------------------------------->
REQUEST_RESP
<--------------------------------
3830 MIKEY
---------------------------------------------------------------->
Figure 11: Compatibility mode
B.2. Distribution of Pre-Encrypted Content
The default setting is that the KMS operates as a KDC (Key
Distribution Center) and supplies keys. This is not possible if the
Initiator has pre-encrypted content (e.g. Video on Demand). In this
case the KMS has to operate as a KTC (Key Translation Center) and re-
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encode and forward the keys that the Initiator supplied.
In such use cases, the exchange is typically reversed and MAY be
carried out as follows. The Responder sends a message (e.g. SIP
INVITE) to the Initiator requesting delivery of certain content. The
Initiator includes the TEKs used to protect the requested content in
a REQUEST_INIT message, which is sent to the KMS. The KMS encodes
the TEKs in a ticket and replies with a REQUEST_RESP message
containing the requested ticket, which is forwarded to the Responder
in a TRANSFER_INIT message.
+---+ +-----+ +---+
| I | | KMS | | R |
+---+ +-----+ +---+
Media request
<----------------------------------------------------------------
REQUEST_INIT {KEMAC}
-------------------------------->
REQUEST_RESP
<--------------------------------
TRANSFER_INIT
---------------------------------------------------------------->
Figure 12: Distribution of pre-encrypted content
Authors' Addresses
John Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden
Phone: +46 10 71 43 501
Email: john.mattsson@ericsson.com
Tian Tian
ZTE Corpoporation
4F,RD Building 2,Zijinghua Road
Yuhuatai District,Nanjing 210012
P.R.China
Phone: +86-025-5287-7867
Email: tian.tian1@zte.com.cn
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