Internet Engineering Task Force (IETF) J. Mattsson

Internet Engineering Task Force (IETF) J. Mattsson
Request for Comments: 6043 Ericsson
Category: Informational T. Tian
ISSN: 2070-1721 ZTE
March 2011

MIKEY-TICKET: Ticket-Based Modes of Key Distribution
in Multimedia Internet KEYing (MIKEY)

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. Interest in such deployments is increasing.
Therefore, a set of new MIKEY modes that work well in such scenarios
are defined. The new modes use a trusted key management service and
a ticket concept, similar to that in Kerberos. The new modes also
support features used by many existing applications, where the exact
identity of the other endpoint may not be known at the start of the
communication session.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6043.

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Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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. 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 ....................................16
4.2.3. Ticket Resolve .....................................19
5. Key Management Functions .......................................23
5.1. Key Derivation ............................................23
5.1.1. Deriving Forked Keys ...............................25
5.1.2. Deriving Keys from an Envelope Key/PSK/MPK .........26
5.1.3. Deriving Keys from a TGK/GTGK ......................27
5.2. CSB Updating ..............................................28
5.3. Ticket Reuse ..............................................29
5.4. Error Handling ............................................29
5.5. MAC/Signature Coverage ....................................30
6. Payload Encoding ...............................................31
6.1. Common Header Payload (HDR) ...............................31
6.1.1. The GENERIC-ID Map Type ............................33
6.2. Key Data Transport Payload (KEMAC) ........................34
6.2.1. Key Data Sub-Payload ...............................35
6.3. Timestamp Payload (T) .....................................36
6.4. Timestamp Payload with Role Indicator (TR) ................36
6.5. ID Payload (ID) ...........................................37
6.6. ID Payload with Role Indicator (IDR) ......................37

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6.7. Cert Hash Payload (CHASH) .................................38
6.8. RAND Payload with Role Indicator (RANDR) ..................38
6.9. Error Payload (ERR) .......................................39
6.10. Ticket Policy Payload (TP) / Ticket Payload (TICKET) .....39
7. Transport Protocols ............................................43
8. Pre-Encrypted Content ..........................................43
9. Group Communication ............................................44
9.1. Key Forking ...............................................45
10. Signaling between Different KMSs ..............................45
11. Adding New Ticket Types to MIKEY-TICKET .......................46
12. Security Considerations .......................................47
12.1. General ..................................................47
12.2. Key Forking ..............................................48
12.3. Denial of Service ........................................49
12.4. Replay ...................................................49
12.5. Group Key Management .....................................50
13. Acknowledgements ..............................................50
14. IANA Considerations ...........................................50
15. References ....................................................53
15.1. Normative References .....................................53
15.2. Informative References ...................................53
Appendix A. MIKEY Base Ticket ....................................55
A.1. Components of the Ticket Data .............................55
A.2. Key Derivation ............................................56
A.2.1. Deriving Keys from a TPK ..............................56
A.2.2. Deriving MPKi and MPKr ................................57
A.3. Ticket Header Payload (THDR) ..............................57
Appendix B. Alternative Use Cases ................................58
B.1. Compatibility Mode ........................................58

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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. The new modes support 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 to the Responder. The ticket contains a reference
to the keys, or the enveloped keys. 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 3rd Generation Partnership Project (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.

This document 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 pseudorandom functions are
defined. And to eliminate the limitations with the existing SRTP-ID
map type, a new CS ID map type called GENERIC-ID is 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: The delivery of a request to multiple endpoints (multiple
devices owned by a single user or multiple users).

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
AES: Advanced Encryption Standard
CA: Certification Authority
CS: Crypto Session
CSB: Crypto Session Bundle
IMS: IP Multimedia Subsystem
GTGK: Group TGK
HMAC: Hash-based Message Authentication Code
KMS: Key Management Service
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: Pseudorandom Function
PRNG: Pseudorandom Number Generator
PSK: Pre-Shared Key
RTSP: Real Time Streaming Protocol
SDP: Session Description Protocol
SHA: Secure Hash Algorithm
SIP: Session Initiation Protocol
SPI: Security Parameters Index
SRTP: Secure Realtime Transport Protocol
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: Identifier for entity x
IDRpsk: Identifier for pre-shared key
IDRapp: Identifier for application/service
KEMAC: Key data transport payload

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PKE: Encrypted envelope key
RAND: RAND payload
RANDRx: 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,
Rekeying interval).

The IDR, RANDR, 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 or not a response message is expected).

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 some situations, a request may be delivered to multiple endpoints.
The endpoints may be multiple devices owned by a single user (e.g.,
mobile phone, fixed phone, and computer), or multiple users (e.g.,
[email protected], a group of users where only one is supposed
to answer). In the following, the term "forking" will be used to
describe all such cases. One example of delivery to multiple
endpoints is forking and retargeting in SIP [RFC3261]. 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. SIP-signaled media protection is described in
[RFC5479] and the applicability of different MIKEY modes is discussed
in [RFC5197].

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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 KMS 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 KMS and sends the
ticket to the responder. The responder forwards the ticket to the
KMS, which returns the corresponding keys.

It should be noted that Kerberos does not require that the responder
also contact the KMS. 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
mechanism specified in this document is useful for any system where
the initial message may be transferred to arbitrarily many potential
responders and where the set of responders may change at any time.
In addition to being able to meet the requirements just described,
the mechanism specified in this document also supports group key
management.

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 that is flexible enough to satisfy users with a
broad range of security and functional needs.

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 offline users. Such scenarios exclude 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.

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
Certification Authority (CA).

4. 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. These pre-
established trust relations are used to establish a security
association between I and R. 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 10.

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

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message. The Ticket Request exchange is OPTIONAL (depending on the
Ticket Type), and MAY be omitted if the Initiator can create the
ticket without assistance from the KMS (see mode 3 of Section 4.1.1).

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 an MPK (MIKEY Protection Key) encoded in
the ticket. If the Responder finds the 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 (see mode 2 of Section 4.1.1).

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 round-
trip mode, but other modes are allowed, see Section 4.1.1. Pre-
encrypted content is discussed in Section 8, group communication is
discussed in Section 9, and signaling between different KMSs is
discussed in Section 10. An alternative use case is discussed in
Appendix B.

The session keys are normally generated/supplied by the KMS (encoded
in the ticket), but in certain use cases (see Section 8) the session
key may be supplied by the Initiator or Responder (sent in a separate
KEMAC protected with keys derived from the MPK).

MIKEY-TICKET offers a framework that 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

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depends on the Ticket Type signaled in the Ticket Policy. The Ticket
Data corresponding to the default Ticket Type, called MIKEY base
ticket, is defined in Appendix A and requirements regarding new
Ticket Types are given in Section 11.

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
encryption is only applied to the keys and not to the entire
messages.

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

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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 a mutual key-freshness guarantee.

In modes 1 and 2, the Ticket Request exchange can be omitted if the
tickets and the corresponding keys are distributed from the KMS to
the Initiator in some other way. In addition, as tickets may be
reused (see Section 5.3), a single Ticket Request exchange may be
followed by several Ticket Transfer exchanges.

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, RANDRi, [IDRi],
[IDRkms], TP, <---- 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, RANDRi, [IDRi],
{CERTi}, [IDRkms], TP, <---- 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 using the pre-
shared/envelope key indicated in the REQUEST_INIT message.

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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.

4.2.1.1. Common Components of the REQUEST_INIT Messages

The REQUEST_INIT message MUST always include the Header (HDR),
Timestamp (T), and RANDRi 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 (CSs) 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.

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.

The Ticket Policy payload (TP) contains the desired Ticket Policy.
It includes for instance, the ticket's validity period, the number of
requested keys, and the identities of authorized responders (see
Section 6.10).

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 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
shared by the Initiator and the KMS (see Section 5.1.2 for key
derivation specification). The MAC SHALL cover the entire
REQUEST_INIT_PSK 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

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separate CERT payload. The Initiator's certificate MUST come first.
Each following certificate MUST directly certify the one preceding
it.

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.

SIGNi is a signature covering the entire REQUEST_INIT_PK 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 and the
message can be correctly parsed, the KMS MUST check the Initiator's
authorization. If the Initiator is authorized to receive the
requested ticket, possibly with a modified Ticket Policy, the KMS
MUST send a REQUEST_RESP message. Unexpected payloads in the
REQUEST_INIT message SHOULD be ignored. Errors are handled as
described in Section 5.4.

4.2.1.5. Components of the REQUEST_RESP Message

The version, PRF func and CSB ID, #CS, and CS ID map type fields in
the HDR payload SHALL be identical to the corresponding fields in the
REQUEST_INIT message. The V flag has no meaning in this context. It
SHALL be set to '0' by the KMS and ignored by the Initiator.

If one of the NTP timestamp types is used, the KMS SHALL generate a
fresh timestamp value (unlike [RFC3830]), which may be used for clock
synchronization. If the COUNTER timestamp type (see Section 6.6 of
[RFC3830]) is used, the timestamp value MAY be equal to the one in
the REQUEST_INIT message.

The TICKET payload carries the granted Ticket Policy and the 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 SHALL 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). Depending on
the encryption algorithm, the salting key may go into the IV (see
[RFC3830]). If the TP payload in the REQUEST_INIT message does not

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contain a KEMAC, it is RECOMMENDED that the KMS's default KEMAC
include a single TGK. The KEMAC SHALL include an 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] || {TEK|TGK|GTGK})

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 REQUEST_RESP message as well as the REQUEST_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 SHALL be stored. Unexpected payloads in the
REQUEST_RESP message SHOULD be ignored. Errors are handled as
described in Section 5.4.

Before using the received ticket, the Initiator MUST check that the
granted Ticket Policy is acceptable. If not, the Initiator SHALL
discard and MAY send a new REQUEST_INIT message suggesting a
different Ticket Policy than before.

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], but instead of a
pre-shared key, an MPK encoded in the ticket is used. The session
keys are also encoded in the TICKET payload, but in some use cases
(see Section 8) they need to be sent in a separate KEMAC payload.
The session information may be sent from the Initiator to the
Responder (similar to [RFC3830]) or from the Responder to the
Initiator (similar to [RFC4738]). 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.

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Initiator Responder

TRANSFER_INIT = ---->
HDR, T, RANDRi, [IDRi],
[IDRr], {SP}, TICKET, < - - TRANSFER_RESP =
[KEMAC], V HDR, T, [RANDRr],
[IDRr], [RANDRkms],
{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 RANDRi 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 F flag in
the Ticket Policy and it SHALL be ignored by the Responder.

The IDRi and IDRr payloads SHOULD be included, but IDRi MAY be left
out if the Responder can identify the Initiator by other means, and
IDRr MAY be left out when it can be expected that the Responder has a
single identity.

Multiple SP payloads MAY be used both to indicate supported security
policies for a specific crypto session (similar to [RFC4738]) and to
specify security policies for different crypto sessions (similar to
[RFC3830]).

The ticket payload (see Section 6.10) contains the Ticket Policy (see
Section 6.10), Ticket Data (the default ticket type is defined in
Appendix A), and Initiator 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 Type provided in the Ticket Data is indicated in the Ticket
Policy. The Initiator Data authenticates the Initiator when key
forking (I flag) is used.

The KEMAC payload is handled in the same way as if it were sent in a
later CSB update (see Section 5.2), with the only difference that the
encr_key is always derived from MPKi and therefore accessible by all
responders authorized to resolve the ticket. Initiator-specified
keys MAY be used if the Initiator has pre-encrypted content and
specific TEKs (Traffic Encryption Keys) need to be used (see
Section 8). If indicated by the Ticket Policy (L flag), a KEMAC
payload SHALL NOT be included.

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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 TRANSFER_INIT message as well as the identities of the
involved parties (see Section 5.5 for the exact definition).

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 SHALL however check that both the
Ticket Policy and the security policies are acceptable. If they are
not, the Responder SHALL 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. Unexpected payloads
in the TRANSFER_INIT message SHOULD be ignored. Errors are handled
as described in Section 5.4. If the F flag in the Ticket Policy is
set, the Responder MUST send a TRANSFER_RESP message.

4.2.2.3. Components of the TRANSFER_RESP Message

The version, PRF func and CSB ID fields in the HDR payload SHALL be
identical to the corresponding fields in the TRANSFER_INIT message.
The V flag has no meaning in this context. It SHALL be set to '0' by
the Responder and ignored by the Initiator. The Responder SHALL
update the CS ID map info so that each crypto session has exactly one
security policy indicated. The Responder MUST provide Session Data
(at least for SRTP) and SPI for each crypto session for which the
Initiator has not supplied Session Data and SPI. If needed, the
Responder MAY update Session Data and SPI provided by the Initiator.
If the Responder adds crypto sessions, the #CS SHALL be updated.

If one of the NTP timestamp types is used, the Responder SHALL
generate a fresh timestamp value (unlike [RFC3830]). If the COUNTER
timestamp type (see Section 6.6 of [RFC3830]) is used, the timestamp
value MAY be equal to the one in the TRANSFER_INIT message.

If indicated by the Ticket Policy (G flag), the Responder SHALL
generate a fresh (pseudo-)random byte string RANDRr. RANDRr 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

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TRANSFER_RESP message (e.g., [email protected]) MAY differ from the
one sent in the IDRr payload in the TRANSFER_INIT message (e.g.,
[email protected]).

If the Responder receives a RANDRkms payload in the RESOLVE_RESP
message, the same RAND MUST be sent in a RANDRkms payload in the
TRANSFER_RESP message.

The Responder MAY provide additional Security Policy payloads. The
Responder SHOULD NOT resend SP payloads, which the Initiator
supplied.

The KEMAC payload SHALL be handled exactly as if it was sent in a
later CSB update, see Section 5.2. Responder-specified keys MAY be
used if Responder has pre-encrypted content and specific TEKs
(Traffic Encryption Keys) need to be used (see Section 8). If
indicated by the Ticket Policy (M flag), a KEMAC payload SHALL NOT be
included.

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 TRANSFER_RESP message as well as the TRANSFER_INIT message
(see Section 5.5 for the exact definition).

4.2.2.4. Processing the TRANSFER_RESP Message

If the Initiator can verify the integrity of the received message and
the message can be correctly parsed, the Initiator MUST check that
any Responder-generated security policies are acceptable. If not,
the Initiator SHALL discard and MAY send a new TRANSFER_INIT message
to indicate supported security policies. Unexpected payloads in the
TRANSFER_RESP message SHOULD be ignored. Errors are handled as
described in Section 5.4.

4.2.3. Ticket Resolve

This exchange is used by the Responder to request that the KMS return
the keys encoded in a ticket. The KMS does not need to be the same
KMS that originally issued the ticket, see Section 10. 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].

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Responder KMS

RESOLVE_INIT_PSK = ---->
HDR, T, RANDRr, [IDRr],
[IDRkms], TICKET, <---- RESOLVE_RESP
[IDRpsk], V HDR, T, [IDRkms], KEMAC,
[IDRr], [RANDRkms], V

The second variant corresponds to the public-key (PK) method of
[RFC3830].

Responder KMS

RESOLVE_INIT_PK = ---->
HDR, T, RANDRr, [IDRr],
{CERTr}, [IDRkms], TICKET, <---- RESOLVE_RESP
[CHASH], PKE, SIGNr HDR, T, [IDRkms], KEMAC,
[IDRr], [RANDRkms], 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
ticket, the Responder is assured of the integrity of the Ticket
Policy, which contains the identity of the peer that requested or
created the ticket. If key forking is used (I flag), the Responder
is also assured that the peer that requested or created the ticket
also sent the TRANSFER_INIT message. 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 RANDRr 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].

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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
shared by the Responder and the KMS. The MAC SHALL cover the entire
RESOLVE_INIT_PSK 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. The Responder's certificate MUST come first.
Each following certificate MUST directly certify the one preceding
it.

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.

SIGNr is a signature covering the entire RESOLVE_INIT_PK 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, the
message can be correctly parsed, and the Responder is authorized to
resolve the ticket, the KMS MUST send a RESOLVE_RESP message. If key
forking is used (I flag), the KMS SHALL also verify the integrity of
the Initiator Data field in the TICKET payload. Unexpected payloads
in the RESOLVE_INIT message SHOULD be ignored. Errors are handled as
described in Section 5.4.

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4.2.3.5. Components of the RESOLVE_RESP Message

The version, PRF func and CSB ID, #CS, and CS ID map type fields in
the HDR payload SHALL be identical to the corresponding fields in the
RESOLVE_INIT message. The V flag has no meaning in this context. It
SHALL be set to '0' by the KMS and ignored by the Responder.

If one of the NTP timestamp types is used, the KMS SHALL generate a
fresh timestamp value (unlike [RFC3830]), which may be used for clock
synchronization. If the COUNTER timestamp type (see Section 6.6 of
[RFC3830]) is used, the timestamp value MAY be equal to the one in
the RESOLVE_INIT message.

The KEMAC payload SHALL 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). Depending on
the encryption algorithm, the salting key may go into the IV (see
[RFC3830]). The KEMAC SHALL include an 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'] || {TEK|TGK|GTGK})

If key forking (see Section 5.1.1) is used (determined by the I flag
in 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 RANDRkms
payloads, where IDRr is the identity of the endpoint that answered
and RANDRkms 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 RESOLVE_RESP message as well as the RESOLVE_INIT
message (see Section 5.5 for the exact definition).

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 MUST verify the
TRANSFER_INIT message with the MPKi received from the KMS. If key
forking is used, the Responder SHALL also verify that the MAC field
in the V payload in the TRANSFER_INIT message is identical to the MAC

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field in the Vi payload in the Initiator Data field in the TICKET
payload. Unexpected payloads in the RESOLVE_RESP message SHOULD be
ignored. Errors are handled as described in Section 5.4.

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 a pre-
shared key or an envelope key. As crypto sessions SHALL NOT be
handled, further keying material (i.e., TEKs) does not have to be
derived.

In the Ticket Transfer exchange, the keys used to protect the MIKEY
messages are derived from an MPK. If key forking is used, the KMS
and the Initiator SHALL fork the MPKr and the TGKs (encoded in the
ticket) 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 RANDRr are illustrated
in Figure 4. The KEMAC shown is the KEMAC sent from the KMS to the
Initiator and the Responder. 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 |------------>| AUTH | | |
| ----- ------ | |
| ^ MAC | | |
| | RAND v | |
+--+--+------+----+---+--+--------+--+---+ | |
TRANSFER_INIT | HDR |......| RANDRi |..| TICKET |..| V | | |
+--+--+------+----+---+--+--------+--+---+ | |
| | RAND | |
| v | |
| CS ID ----- TGK | |
+---------->| PRF |<---------------------+ |
----- |
| TEK SALT |
v v
---------------------------------------
| Security Protocol, e.g., SRTP |
---------------------------------------

Figure 4: Key hierarchy without key forking and RANDRr

The key hierarchy and its dependencies on TRANSFER_RESP message
contents for the case with key forking and RANDRr are illustrated in
Figure 5. The KEMAC shown is the KEMAC sent from the KMS to the
Initiator. MOD is the modifier (IDRr, RANDRkms). 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 RANDRi from the
TRANSFER_INIT message is used as input to the derivation of the
auth_key and may be used as input to the derivation of 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 |-------+ TGK | |
----- | | |
^ v | |
CSB ID | ----- auth_key ------ | |
+---------)------>| PRF |--------->| AUTH | | |
| | ----- ------ | |
| | ID Data ^ MAC | | |
| | RAND | RAND v | |
+--+--+---+--+--+---+---+----+----------+---+ | |
TRANSFER_RESP | HDR |...| MOD |...| RANDRr |..........| V | | |
+--+--+---+--+--+---+---+----+----------+---+ | |
| | | RAND v |
| | | ID Data ----- |
| +----------)------------------>| PRF | |
| | RAND ----- |
| v | |
| CS ID ----- TGK' | |
+---------------->| PRF |<------------------+ |
----- |
| TEK SALT |
v v
---------------------------------------
| Security Protocol, e.g., SRTP |
---------------------------------------

Figure 5: Key hierarchy with key forking and RANDRr

The labels in the key derivations SHALL NOT include entire RANDR
payloads, only the fields RAND length and RAND from the corresponding
payload.

5.1.1. Deriving Forked Keys

When key forking is used (determined by the I flag in the Ticket
Policy), the MPKr and TGKs (encoded in the ticket) 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
(Pseudorandom Function) indicated in the Ticket Policy. The
parameters for the PRF are:

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inkey: : MPKr or TGK
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x00 ||
length ID Data || ID Data || length RANDRkms || RANDRkms
outkey_len : desired bit length of the outkey (MPKr', TGK')
SHALL be equal to inkey_len

where the ID Data field is taken from the IDRr payload sent in the
RESOLVE_RESP and TRANSFER_RESP messages. Length ID Data is the
length of the ID Data field in bytes as a 16-bit unsigned integer.
Length RANDRkms is the length of RANDRkms in bytes as an 8-bit
unsigned 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 Key/PSK/MPK

This derivation is used 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 a pre-shared
key or an envelope key. For the Ticket Transfer exchange, the keys
are derived from an 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
keys derived using the following parameters:

inkey: : pre-shared key, envelope key, or MPKi
inkey_len : bit length of the inkey
label : constant || 0xFF || CSB ID || 0x01 ||
length RANDRi || [RANDRi] || length RANDRr || [RANDRr]
outkey_len : desired bit length of the outkey (encr_key,
auth_key, salt_key)

The response messages SHALL be protected with keys derived using the
following parameters:

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inkey: : pre-shared key, envelope key, MPKi, or MPKr'
inkey_len : bit length of the inkey
label : constant || 0xFF || CSB ID || 0x02 ||
length RANDRi || [RANDRi] || length RANDRr || [RANDRr]
outkey_len : desired bit length of the outkey (encr_key,
auth_key, salt_key)

The constant depends on the derived key type as defined in Section
4.1.4 of [RFC3830]. The 32-bit CSB ID field is taken from the HDR
payload. RANDRi SHALL be included in the derivation of keys used to
protect the Ticket Request and Ticket Transfer exchanges. RANDRr
SHALL be included in the derivation of keys used to protect the
Ticket Resolve exchange and in the derivation of keys used to protect
TRANSFER_RESP if the Ticket Policy determines that it shall be
present in the TRANSFER_RESP message (G flag). Length RANDRi is the
length of RANDRi in bytes as an 8-bit unsigned integer, and Length
RANDRr is the length of RANDRr in bytes as an 8-bit unsigned integer.
If RANDRi is omitted, length RANDRi SHALL be 0 and if RANDRr is
omitted, length RANDRr SHALL be 0. Note that at least one of RANDRi
and RANDRr is always used.

5.1.3. Deriving Keys from a TGK/GTGK

This only affects the Ticket Transfer exchange. In the following, we
describe how keying material is derived from a TGK/GTGK. If key
forking is used, any TGK encoded in the ticket SHALL be forked, and
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 PRF are:

inkey: : TGK, TGK', or GTGK
inkey_len : bit length of the inkey
label : constant || CS ID || 0xFFFFFFFF || 0x03 ||
length RANDRi || [RANDRi] || length RANDRr || [RANDRr]
outkey_len : desired bit length of the outkey (TEK, encr_key,
auth_key, salt_key)

The constant depends on the derived key type as defined in Section
4.1.3 of [RFC3830]. If a salting key is present in the key data sub-
payload, a security protocol in need of a salting key SHALL use this
salting key and a new salting key SHALL NOT be derived. The 8-bit CS
ID field is given by the CS ID map info field in the HDR payload.
RANDRi SHALL be included if the Ticket Policy determines that it
shall be used (H flag). RANDRr SHALL be included if the Ticket
Policy determines that it shall be present in the TRANSFER_RESP
message (G flag). Length RANDRi is the length of RANDRi in bytes as
an 8-bit unsigned integer, and Length RANDRr is the length of RANDRr

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in bytes as an 8-bit unsigned integer. If RANDRi or RANDRr is
omitted the corresponding length SHALL be 0. Note that at least one
of RANDRi and RANDRr MUST be used.

5.2. CSB Updating

Similar to [RFC3830], MIKEY-TICKET provides a means of updating the
CSB (Crypto Session Bundle), e.g., transporting a new TEK/TGK/GTGK 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 new crypto session is needed. The CSB updating can be started
by the Initiator:

Initiator Responder

TRANSFER_INIT = ---->
HDR, T, [IDRi], [IDRr],
{SP}, [KEMAC], V < - - TRANSFER_RESP =
HDR, T, [IDRr],
{SP}, [KEMAC], V

The CSB updating can also be started by the Responder:

Responder Initiator

TRANSFER_INIT = ---->
HDR, T, [IDRr], [IDRi],
{SP}, [KEMAC], V < - - TRANSFER_RESP =
HDR, T, [IDRi],
{SP}, [KEMAC], V

The new message exchange MUST use the same CSB ID as the initial
exchange but MUST use new timestamps. The crypto sessions
negotiation (#CS field, CS ID map info field, and SP payloads) are
handled as in the initial exchange. In the TRANSFER_INIT message the
V flag SHALL be used to indicate whether or not a response message is
expected. Static payloads such as RANDRi, RANDRr, RANDRkms, and
TICKET that were provided in the initial exchange SHOULD NOT be
included unless they are needed by a specific use case. New RANDs or
TICKETs MUST NOT be included. 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. Both messages SHALL be protected with the same keys
(derived from MPKi or MPKr') that protected the last message
(TRANSFER_INIT or TRANSFER_RESP) in the initial exchange.

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New keying material MAY be sent in a KEMAC payload. If indicated by
the Ticket Policy (L and M flags), KEMAC payloads SHALL NOT be
included. In the TRANSFER_RESP message, a session key MUST be
provided for each crypto session. The KEMAC SHALL use the NULL
authentication algorithm, as a MAC is included in the V payload. The
encr_key (and salt_key) SHALL be derived from the MPK (MPKi or
MPKr'). Depending on the encryption algorithm, the salting key may
go into the IV (see [RFC3830]). If a new TGK is exchanged, it SHALL
NOT be forked. The KEMAC is hence constructed as follows:

KEMAC = E(encr_key, (TEK|TGK|GTGK))

5.3. 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 media
sessions with another user and then under the ticket's validity
period use this ticket to protect several media sessions with 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
(RANDRi, RANDRr). If the Responder has resolved the ticket before,
the Responder does not need to resolve the ticket again. In that
case, the same modifier (IDRr, RANDRkms) SHALL be used. If the
Ticket Policy forbids reuse (J flag), 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.

5.4. Error Handling

If a fatal error occurs during the parsing of a message, the message
SHOULD be discarded, and an Error message SHOULD be sent to the other
party (Initiator, Responder, KMS). If a failure is due to the
inability to authenticate the peer, the message SHALL be discarded,
the Error message is OPTIONAL, and the caveats in Section 5.1.2 of
[RFC3830] apply. Error messages may be used to report errors in both
initial and response messages, but not in Error messages.

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In the Ticket Request and Ticket Resolve exchanges, the Error message
MAY be authenticated with a MAC or a signature. The Error message is
hence constructed as follows:

Error message = HDR, T, (ERR), [V|SIGNx]

where x is in the set {i, r, kms} (Initiator, Responder, KMS).
Unexpected payloads in the Error message SHOULD be ignored.

In the Ticket Transfer exchange, the Error message MAY be
authenticated with a MAC. If the suggested security policies are not
supported, the Error message SHOULD include the supported parameters.
The Error message is hence constructed as follows:

Error message = HDR, T, (ERR), {SP}, [V]

In Error messages, the version, PRF func, and CSB ID fields in the
HDR payload SHALL be identical to the corresponding fields in the
message where the error occurred. The V field SHALL be set to '0'
and be ignored.

If one of the NTP timestamp types is used, a fresh timestamp value
SHALL be used. If the COUNTER timestamp type (see Section 6.6 of
[RFC3830]) is used, the timestamp value MAY be equal to the one in
the message where the error occurred.

The MAC/Signature in the V/SIGN payloads covers the entire Error
message, except the MAC/Signature field itself. The auth_key SHALL
be the same as in the message where the error occurred.

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. In the TRANSFER_INIT message, the MAC SHALL
NOT cover the Initiator Data length and Initiator Data fields in the
TICKET payload. Note that in the Transfer Exchange, Identity_r in
TRANSFER_RESP (e.g., [email protected]) MAY differ from that
appearing in TRANSFER_INIT (e.g., [email protected]). 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).

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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
Error message | Error message

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, RANDR,
TP, and TICKET 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]. Note that
the fields with variable length are byte aligned and not 32-bit
aligned.

6.1. Common Header Payload (HDR)

For the Common Header Payload, new values are added to the Data Type,
Next Payload, PRF func, and CS ID map type name spaces.

* Data Type (8 bits): describes the type of message.

Data Type | Value | Comment
-----------------+-------+-------------------------------------
REQUEST_INIT_PSK | 11 | Ticket request initial message (PSK)
REQUEST_INIT_PK | 12 | Ticket request initial message (PK)
REQUEST_RESP | 13 | Ticket request response message
| |
TRANSFER_INIT | 14 | Ticket transfer initial message
TRANSFER_RESP | 15 | Ticket transfer response message
| |
RESOLVE_INIT_PSK | 16 | Ticket resolve initial message (PSK)
RESOLVE_INIT_PK | 17 | Ticket resolve initial message (PK)
RESOLVE_RESP | 18 | Ticket resolve response message

Table 6.1: Data Type (Additions)

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* Next Payload (8 bits): identifies the payload that is added after
this payload.

Next Payload | Value | Section
-------------+-------+--------
TR | 13 | 6.4
IDR | 14 | 6.6
RANDR | 15 | 6.8
TP | 16 | 6.10
TICKET | 17 | 6.10

Table 6.2: Next Payload (Additions)

* V (1 bit): flag to indicate whether a response message is expected
('1') or not ('0'). It MUST be set to '0' and ignored in all
messages except TRANSFER_INIT messages used for CSB updating (see
Section 5.2).

* 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
-----------------+------
PRF-HMAC-SHA-256 | 1

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 in the CS ID
map info.

* CS ID map type (8 bits): specifies the method of uniquely mapping
crypto sessions to the security protocol sessions. In the Ticket
Transfer exchange the new GENERIC-ID map type, which is intended
to eliminate the limitations with the existing SRTP-ID map type,
SHOULD be used. The map type SRTP-ID SHALL NOT be used.

CS ID map type | Value
----------------------
GENERIC-ID | 2

Table 6.4: CS ID map type (Additions)

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* CS ID map info (variable length): identifies and maps the crypto
sessions to the security protocol sessions for which security
associations should be created.

6.1.1. The GENERIC-ID Map Type

For the GENERIC-ID map type, the CS ID map info consists of #CS
number of blocks, each mapping policies, session data (e.g., SSRC),
and key to a specific crypto session.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! CS ID ! Prot type !S! #P ! Ps (OPTIONAL) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Session Data Length ! Session Data (OPTIONAL) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SPI Length ! SPI (OPTIONAL) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

* CS ID (8 bits): defines the CS ID to be used for the crypto
session.

* Prot type (8 bits): defines the security protocol to be used for
the crypto session. Allowed values are the ones defined for the
Prot type field in the SP payload (see Section 6.10 of [RFC3830]).

* S (1 bit): flag that MAY be used by the Session Data.

* #P (7 bits): indicates the number of security policies provided
for the crypto session. In response messages, #P SHALL always be
exactly 1. So if #P = 0 in an initial message, a security profile
MUST be provided in the response message. If #P > 0, one of the
suggested policies SHOULD be chosen in the response message. If
needed (e.g., in group communication, see Section 9), the
suggested policies MAY be changed.

* Ps (variable length): lists the policies for the crypto session.
It SHALL contain exactly #P policies, each having the specified
Prot type.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Policy_no_1 ! Policy_no_2 ! ... ! Policy_no_#P !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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* Policy_no_i (8 bits): a policy_no that corresponds to the
policy_no of a SP payload. In response messages, the policy_no
may refer to a SP payload in the initial message.

* Session Data Length (16 bits): the length of Session Data (in
bytes). For the Prot type SRTP, Session Data MAY be omitted in
the initial message (length = 0), but it MUST be provided in the
response message.

* Session Data (variable length): contains session data for the
crypto session. The type of Session Data depends on the specified
Prot type. The Session Data for the Prot type SRTP is defined
below. The S flag is used to indicate whether the ROC and SEQ
fields are provided ('1') or if they are omitted ('0').

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SSRC !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ROC (OPTIONAL) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SEQ (OPTIONAL) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

* SSRC (32 bits): specifies the SSRC that MUST be used for the
crypto session. Note that unlike [RFC3830], an SSRC field set
to '0' has no special meaning.

* ROC (32 bits): current/initial rollover counter. If the
session has not started, this field is set to '0'.

* SEQ (16 bits): current/initial sequence number.

* SPI Length (8 bits): the length of SPI (in bytes). SPI MAY be
omitted in the initial message (length = 0), but it MUST be
provided in the response message.

* SPI (variable length): the SPI (or MKI) corresponding to the
session key to (initially) be used for the crypto session. This
does not exclude other keys to be used. All keys MUST belong to
the crypto session bundle.

6.2. Key Data Transport Payload (KEMAC)

For the KEMAC payload, new encryption and authentication algorithms
are defined.

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* Encr alg (8 bits): the encryption algorithm used to encrypt the
Encr data field. Besides the algorithms already defined in
[RFC3830], the following additional encryption algorithm may be
used.

Encr alg | Value | Comment
-----------+-------+---------------------------
AES-CM-256 | 3 | AES-CM using a 256-bit key

Table 6.5: Encr alg (Additions)

The new encryption algorithm is defined as described in Section 4.2.3
of [RFC3830] with the only difference being that a 256-bit key SHALL
be used.

* MAC alg (8 bits): specifies the authentication algorithm used.
Besides the algorithms already defined in [RFC3830], the following
additional authentication algorithm may be used.

MAC alg | Value | Length
-----------------+-------+---------
HMAC-SHA-256-256 | 2 | 256 bits

Table 6.6: MAC alg (Additions)

The new authentication algorithm is Hash-based Message Authentication
Code (HMAC) [RFC2104] in conjunction with SHA-256 [FIPS.180-3]. It
SHALL be used with a 256-bit authentication key.

6.2.1. 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 difference that
it SHALL NOT be forked. 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]; however, it is not known by the Responder
before the ticket has been resolved.

An SPI (or MKI) MUST be specified for each key (see Section 6.13 of
[RFC3830]).

* Type (4 bits): indicates the type of key included in the payload.

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Type | Value | Comments
----------+-------+---------------------
GTGK | 4 | Group TGK
GTGK+SALT | 5 | Group TGK + SALT
MPK | 6 | MIKEY Protection Key

Table 6.7: Key Data Type (Additions)

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 contains
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
-----------+-------+--------
NTP-UTC-32 | 3 | 32 bits

Table 6.8: TS Type (Additions)

NTP-UTC-32 SHALL be padded to a 64-bit NTP-UTC timestamp (with zeroes
in the fractional second part) when a 64-bit timestamp is required
(e.g. IV creation in AES-CM-128 and AES-CM-256).

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
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 (e.g., in the Ticket
Policy).

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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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* 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
Rekeying interval (TRr) | 4

Table 6.9: 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. Contrary to the previously defined ID
Types (URI, Network Access Identifier), the byte string does not have
any encoding rules.

* ID Type (8 bits): specifies the identifier type used.

ID Type | Value
------------+------
Byte string | 2

Table 6.10: ID Type (Additions)

6.6. ID Payload with Role Indicator (IDR)

The IDR payload uses all the fields from 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.

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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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* 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 6.11: ID Role

IDRapp is intended to specify the authorized Application IDs (see
Sections 5.1.3 and 6.10)

6.7. Cert Hash Payload (CHASH)

* Hash func (8 bits): indicates the hash function that is used.
Besides the hash functions already defined in [RFC3830], the
following hash function may be used.

Hash func | Value | Hash Length
----------+-------+------------
SHA-256 | 2 | 256 bits

Table 6.12: Hash func (Additions)

The SHA-256 hash function is defined in [FIPS.180-3].

6.8. RAND Payload with Role Indicator (RANDR)

The RANDR payload uses all the fields from the standard RAND payload
(RAND) but expands it with a new field describing the role (the
generating entity) of the RAND. The RANDR payload is intended to
eliminate ambiguity when a MIKEY message contains several RAND
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 ! RAND Role ! RAND length ! RAND ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

* RAND Role (8 bits): specifies the entity that generated the RAND.

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RAND Role | Value
-------------------+------
Initiator (RANDRi) | 1
Responder (RANDRr) | 2
KMS (RANDRkms) | 3

Table 6.13: RAND Role

6.9. 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.

Error no | Value | Comments
---------------+-------+----------------------------
Invalid TICKET | 14 | Ticket Type not supported
Invalid TPpar | 15 | TP parameters not supported

Table 6.14: Error no (Additions)

6.10. Ticket Policy Payload (TP) / Ticket Payload (TICKET)

Note that the Ticket Policy payload (TP) and the Ticket Payload
(TICKET) are two different payloads (having different payload
identifiers). However, as they share much of the payload structure,
they are described in the same section.

The Ticket Policy payload contains a desired Ticket Policy and does
not include the Ticket Data length, Ticket Data, Initiator Data
length, or Initiator Data fields. The ticket payload contains the
granted Ticket Policy as well as Ticket Data (the default ticket type
is defined in Appendix A). 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 Ticket Policy. When
key forking is used (I flag), the Initiator Data authenticates the
Initiator.

Note that the flags are not independent: NOT D implies L, G implies
F, NOT G implies H, NOT H implies G, I implies E, K implies D, and M
implies F. The F flag SHALL be set to '1' when the I flag (key
forking) is set to '1' and a TGK is encoded in the ticket.

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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 ! Ticket Type ! Subtype !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Version ! PRF Func !D!E!F!G!H!I!J!K!L!M!N!O! Res !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! TP Data length ! TP Data ~
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
! Ticket Data length ! Ticket Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Initiator Data length ! Initiator Data (OPTIONAL) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

* 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 | Used and specified by 3GPP

Table 6.15: Ticket Type

Subtype = 0x01 and Version = 0x01 refers to MIKEY Base Ticket as
defined in this document.

* Subtype (8 bits): specifies the ticket subtype used.

* Version (8 bits): specifies the ticket subtype version used.

* PRF Func (7 bits): specifies the PRF that SHALL be used for key
forking.

* D (1 bit): flag to indicate whether the ticket was generated by
the KMS ('1') or by the Initiator ('0').

* E (1 bit): flag to indicate whether the Ticket Resolve exchange is
MANDATORY ('1') or if the Responder MAY resolve the ticket ('0').

* F (1 bit): flag to indicate whether the TRANSFER_RESP message
SHALL be sent ('1') or if it SHALL NOT be sent ('0').

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* G (1 bit): flag to indicate whether the Responder SHALL generate
RANDRr ('1') or if the Responder SHALL NOT generate RANDRr ('0').

* H (1 bit): flag to indicate whether RANDRi SHALL be used when
deriving keys from a TGK/GTGK ('1') or if RANDRi SHALL NOT be used
('0').

* I (1 bit): flag to indicate whether key forking SHALL be used
('1') or if key forking SHALL NOT be used ('0').

* J (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').

* K (1 bit): flag to indicate whether the KMS changed the desired
Ticket Policy or the desired KEMAC ('1') or if it did not ('0').
In the TP payload, it SHALL be set to '0' by the Initiator and
ignored by the KMS.

* L (1 bit): flag to indicate whether the Initiator MAY supply
session keys ('1') or if the Initiator SHALL NOT supply session
keys ('0').

* M (1 bit): flag to indicate whether the Responder MAY supply
session keys ('1') or if the Responder SHALL NOT supply session
keys ('0').

* N (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.

* O (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.

* Res (5 bits): reserved for future use.

* TP Data length (16 bits): length of TP Data (in bytes).

* TP Data (variable length): The first 8 bits identify the first
payload. The rest of TP Data SHALL be constructed of MIKEY
payloads. Unexpected payloads in the TP Data SHOULD be ignored.

TP Data = First Payload, [IDRkms], [IDRi], [TRs],
[TRe], [TRr], [KEMAC], {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 no 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 MUST be done. TS Type SHALL be
NTP-UTC-32 and the time between two rekeyings SHALL NOT be longer
than the number of seconds in the integer part of the timestamp.
How the rekeying is done is implementation specific.

The KEMAC payload may be used to indicate the number of requested
keys and specify other key information (key type, key length, and
KV (key validity) data). The KEMAC payload SHALL use the NULL
encryption algorithm and the NULL authentication algorithm, as a
MAC is included in the V payload. The KEMAC is hence constructed
as follows:

KEMAC = {TEK|TGK|GTGK}

The Key Data fields SHALL be set to '0' by the Initiator and ignored
by the KMS. The KEMAC SHALL NOT be present in the granted Ticket
Policy.

IDRapp is an identifier for an authorized application ID. The
application IDs are implementation specific. If no IDRapp
payloads are supplied, 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.

* Ticket Data length (16 bits): the length of the Ticket Data field
(in bytes). Not present in the TP payload.

* Ticket Data (variable length): contains the Ticket Data. Not
present in the TP payload.

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* Initiator Data length (16 bits): the length of the Initiator Data
field (in bytes). Not present in the TP payload.

* Initiator Data (variable length): Not present in the TP payload.
SHALL be inserted by the Initiator if and only if key forking is
used (I flag). The first 8 bits identifies the first payload.
The rest of Initiator Data SHALL be constructed of MIKEY payloads.
Unexpected payloads in the Initiator Data SHOULD be ignored.

Initiator Data = First Payload, Vi, Vr

The Vi payload SHALL be identical to the V payload in the
TRANSFER_INIT message.

The last payload (Vr) SHALL be a Verification payload where the
MAC SHALL cover the entire Initiator Data field except the MAC
field itself. The authentication algorithm SHALL be the same as
used for the Vi payload. The authentication key (auth_key) SHALL
be derived from MPKr (not forked) using the following parameters:

inkey: : MPKr
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x04
outkey_len : desired bit length of the outkey (encr_key,
auth_key, salt_key)

The constant depends on the derived key type as defined in Section
4.1.4 of [RFC3830].

7. Transport Protocols

MIKEY messages are not tied to any specific transport protocols. In
[RFC4567], extensions for SDP and RTSP to carry MIKEY messages (and
therefore MIKEY-TICKET messages) are defined. The messages in the
Ticket Transfer exchange (TRANSFER_INIT, TRANSFER_RESP) are
preferably included in the session setup signaling (e.g., SIP INVITE
and 200 OK). However, it may not be suitable for the MIKEY-TICKET
exchanges that do not establish keying material for media sessions
(Ticket Request and Ticket Resolve) to be carried in SDP or RTSP. If
SDP or RTSP is not used, the transport protocol needs to be defined.
In [3GPP.33.328], it is defined how the Ticket Request and Ticket
Resolve exchanges are carried over HTTP.

8. Pre-Encrypted Content

The default setting is that the KMS supplies the session keys
(encoded in the ticket). This is not possible if the content is pre-
encrypted (e.g., Video on Demand). In such use cases, the key

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exchange is typically reversed and MAY be carried out as follows.
The Initiator sends a ticket without encoded session keys to the
Responder in a TRANSFER_INIT message. The Responder has access to
the TEKs used to protect the requested content, but may not be
streaming the content. The Responder includes the TEK in the
TRANSFER_RESP message, which is sent to the Initiator.

+---+ +---+
| I | | R |
+---+ +---+
TRANSFER_INIT
---------------------------------------------------------------->
TRANSFER_RESP {KEMAC}
<----------------------------------------------------------------

Figure 6: Distribution of pre-encrypted content

9. Group Communication

What has been discussed up to now can also be used for group
communication. The MIKEY signaling for multi-party sessions can be
centralized as illustrated in Figure 7.

+---+ +---+ +---+
| A | | B | | C |
+---+ +---+ +---+
Ticket Transfer
<-------------------------------> Ticket Transfer
<--------------------------------------------------------------->

Figure 7: Centralized signaling around party A

or decentralized as illustrated in Figure 8.

+---+ +---+ +---+
| A | | B | | C |
+---+ +---+ +---+
Ticket Transfer
<-------------------------------> Ticket Transfer
<------------------------------->

Figure 8: Decentralized signaling

In the decentralized scenario, the identities of B and C SHALL be
used in the second Ticket Transfer exchange. Independent of the how
the MIKEY signaling is done, a group key may be used as session key.

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If a group key is used, the group key and session information may be
pushed to all group members (similar to [RFC3830]), or distributed
when requested (similar to [RFC4738]). If a TGK/GTGK is used as a
group key, the same RANDs MUST be used to derive the session keys in
all Ticket Transfer exchanges. Also note caveats with ticket reuse
in group communication settings as discussed in Section 5.3.

9.1. Key Forking

When key forking is used, only the user that requested the ticket can
initiate a Ticket Transfer exchange using that ticket, see
Section 5.3. So if a group key is to be distributed, the MIKEY
signaling MUST be centralized to the party that initially requested
the ticket, or different tickets needs to be used in each Ticket
Transfer exchange and the group key needs to be sent in a KEMAC.

Another consideration is that different users get different session
keys if TGKs (encoded in the ticket) are used.

10. 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
and confidentiality 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

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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 issued
the ticket, the Responder SHOULD send the RESOLVE_INIT message to one
of the Responder's trusted KMSs (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 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.

11. 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 Secure/Multipurpose Internet Mail
Extensions (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

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requires additional processing, it MUST be indicated in the Ticket
Policy (N and O flags). New specifications MUST specify which modes
are supported and if any additional security considerations apply.

12. 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
security properties depend on the specific Ticket Type, only generic
security considerations concerning the MIKEY-TICKET framework are
discussed.

This specification includes a large number of optional features,
which adds complexity to the general case. Protocol designers are
strongly encouraged to establish strict profiles defining MIKEY-
TICKET options (e.g., exchanges or message fields) that SHOULD or
MUST be supported. Such profiles should preclude unexpected
consequences from compliant implementations with wildly differing
option sets.

12.1. 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
PRNG (Pseudorandom 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 PRNG is
implementation specific, the easiest (and often bad) choice is to use
the PRNG 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.

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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 sum of the lengths of RANDRi and RANDRr MUST at least
be equal to the size of the longest pre-shared key/envelope key/MPK/
TGK/GTGK, RANDRkms MUST at least be as long as the longest MPKr/TGK,
and the RAND in the MIKEY base ticket MUST at least be as long as the
longest of TPK and MPK.

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, the time
and memory needed by an attacker to get all k n-bit keys are
proportional to 2^n. 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) 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.

The new algorithms PRF-HMAC-SHA-256, AES-CM-256, and HMAC-SHA-256-256
use 256-bit keys and offer a higher security level than the
previously defined algorithms. If one of the 256-bit algorithms are
supported, the other two algorithms SHALL also be supported. The
256-bit algorithms SHOULD be used together, and they SHALL NOT be
mixed with algorithms using key sizes less than 256 bits. If session
keys (TEK/TGK/GTGK) longer than 128 bits are used, 128-bit algorithms
SHALL NOT be used.

12.2. Key Forking

In some situations, the TRANSFER_INIT message may be delivered to
multiple endpoints. For example, 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

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[email protected], 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,
entities other than the endpoint that answers MUST NOT get access to
the session keys.

When key forking is not used, keys are accessible by everyone that
can resolve the ticket. When key forking is used, some keys (MPKr
and TGKs encoded in the ticket) 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.

When key forking is used, some keys (MPKi and TEKs and GTGK encoded
in the ticket) are still accessible by everyone that can resolve the
ticket and should be used with this in mind. This also concerns
session keys transferred in a KEMAC in the first TRANSFER_INIT (as
they are protected with MPKi).

12.3. 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. 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. Typical prevention methods such as rate-
limiting and ACL (Access Control List) capability SHOULD therefore be
implemented in the KMS as well as the clients. If something in the
signaling is suspicious, the Responder SHOULD abort before attempting
a RESOLVE_INIT with the KMS. The types and amount of prevention
needed depends on how critical the system is and may vary depending
on the Ticket Type.

12.4. 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, e.g., leading
to a 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 (Responder or KMS) identity is always (explicitly or
implicitly) included in the MAC/Signature calculation.

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An attacker may also attempt to replay a ticket by inserting it into
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. If key forking is used,
such replays will always be detected when Bob has resolved the
ticket. If key forking is not used, such replays will be detected
unless Mallory has knowledge of the MPKi. And if Mallory has
knowledge of the MPKi (i.e., she is authorized to resolve the ticket)
and key forking is not used, there is no attack. For the reasons
explained above, it is RECOMMENDED to use key forking.

12.5. 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.

13. Acknowledgements

The authors would like to thank Fredrik Ahlqvist, Rolf Blom, Yi
Cheng, Lakshminath Dondeti, Vesa Lehtovirta, Fredrik Lindholm, Mats
Naslund, Karl Norrman, Oscar Ohlsson, Brian Rosenberg, Bengt Sahlin,
Wei Yinxing, and Zhu Yunwen for their support and valuable comments.

14. IANA Considerations

This document defines several new values for the namespaces Data
Type, Next Payload, PRF func, CS ID map type, 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 parentheses is a reference to the table
containing the registered values):

o Data Type (see Table 6.1)

o Next Payload (see Table 6.2)

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o PRF func (see Table 6.3)

o CS ID map type (see Table 6.4)

o Encr alg (see Table 6.5)

o MAC alg (see Table 6.6)

o TS Type (see Table 6.7)

o ID Type (see Table 6.9)

o Hash func (see Table 6.11)

o Error no (see Table 6.13)

o Key Data Type (see Table 6.14)

The TR payload defines an 8-bit TS Role field for which IANA has
created and will 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 Reserved
for Private Use, and the values 0 and 255 are Reserved according to
[RFC5226]. The initial contents of the registry are as follows:

Value TS Role
------- ------------------------------
0 Reserved
1 Time of issue (TRi)
2 Start of validity period (TRs)
3 End of validity period (TRe)
4 Rekeying interval (TRr)
5-239 Unassigned
240-254 Reserved for Private Use
255 Reserved

The IDR payload defines an 8-bit ID Role field for which IANA has
created and will 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 Reserved
for Private Use, and the values 0 and 255 are Reserved according to
[RFC5226]. The initial contents of the registry are as follows:

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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 Reserved for Private Use
255 Reserved

The RANDR payload defines an 8-bit RAND Role field for which IANA has
created and will maintain a new namespace in the MIKEY Payload
registry. Assignments consist of a RAND 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 Reserved
for Private Use, and the values 0 and 255 are Reserved according to
[RFC5226]. The initial contents of the registry are as follows:

Value RAND Role
------- ------------------
0 Reserved
1 Initiator (RANDRi)
2 Responder (RANDRr)
3 KMS (RANDRkms)
4-239 Unassigned
240-254 Reserved for Private Use
255 Reserved

The TP/TICKET payload defines a 16-bit Ticket Type field for which
IANA has created and will 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 Reserved for Private Use, and the values 0 and 65535 are
Reserved according to [RFC5226]. The initial contents of the
registry are as follows:

Value Ticket Type
----------- -----------------
0 Reserved
1 MIKEY base ticket
2 3GPP base ticket
3-61439 Unassigned
61440-65534 Reserved for Private Use
65535 Reserved

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15. References

15.1. 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.

[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.

15.2. Informative References

[3GPP.33.328] 3GPP, "IP Multimedia Subsystem (IMS) media plane
security", 3GPP TS 33.328 9.3.0, December 2010.

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[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.

[RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for
Multimedia Internet KEYing (MIKEY)", RFC 4650,
September 2006.

[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.

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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 of MIKEY
payloads and SHALL be protected by a MAC based on a pre-shared Ticket
Protection Key (TPK). The parties that shares the TPK depends on the
mode. Unexpected payloads in the Ticket Data SHOULD be ignored.

Ticket Data = THDR, T, RAND, KEMAC, [IDRpsk], V

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 the definition,
see Appendix A.3.

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 Appendices A.2.1 and A.2.2).

The KEMAC payload SHALL use the NULL authentication algorithm, as a
MAC is included in the V payload. The encryption key (encr_key) and
salting key (salt_key) SHALL be derived from the TPK (see
Appendix A.2.1). Depending on the encryption algorithm, the salting
key be used in the IV creation (see Section 4.2.3 of [RFC3830]). If
CSB ID is needed in the IV creation it SHALL be set to '0xFFFFFFFF'.
The KEMAC is hence constructed as follows:

KEMAC = E(encr_key, MPK || {TEK|TGK|GTGK})

MPKi and MPKr are derived from the MPK as defined in Appendix A.2.2.

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), the Initiator Data length field, the
Initiator Data field, and the MAC field itself.

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A.2. Key Derivation

The labels in the key derivations SHALL NOT include entire RAND
payloads, only the fields RAND length and RAND from the corresponding
payload.

A.2.1. 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. The parameters for the PRF are:

inkey: : TPK
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x05 ||
length RAND || RAND
outkey_len : desired bit length of the outkey (encr_key,
auth_key, salt_key)

Length RAND is the length of RAND in bytes as an 8-bit unsigned
integer. The constants are as defined in Section 4.1.4 of [RFC3830].
The key derivation and its dependencies on Ticket Data contents when
AES-CM is used are illustrated in Figure 10. The 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 the IV are used to encrypt the KEMAC.

----- auth_key ------
----- TPK | |----------------------->| AUTH |
| TPK |----------->| | encr_key ------
----- | PRF |--------------------+ |
^ +-->| | salt_key | |
: | | |----------------+ | |
: | ----- | | |
: | v | |
identify : RAND | TS value ---- | | MAC
: | +------------>| IV | | |
: | | ---- | |
: | | IV | | |
: | | v v v
Ticket +---+----+---+--+---+---+-+-+------------+-------+---+---+
Data | IDRpsk |...| RAND |...| T |............| KEMAC |...| V |
+--------+---+------+---+---+------------+-------+---+---+

Figure 10: Deriving keys from a TPK

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RFC 6043 MIKEY-TICKET March 2011

A.2.2. 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. The
parameters for the PRF are:

inkey: : MPK
inkey_len : bit length of the inkey
label : constant || 0xFF || 0xFFFFFFFF || 0x06 ||
length RAND || RAND
outkey_len : desired bit length of the outkey (MPKi, MPKr)
SHALL be equal to inkey_len

Length RAND is the length of RAND in bytes as an 8-bit unsigned
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.3. Ticket Header Payload (THDR)

The ticket header payload contains an indicator of the next payload
as well as implementation-specific data.

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.

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RFC 6043 MIKEY-TICKET March 2011

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

Authors' Addresses

John Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden

Phone: +46 10 71 43 501
EMail: [email protected]

Tian Tian
ZTE Corporation
4F, RD Building 2, Zijinghua Road
Yuhuatai District, Nanjing 210012
P.R. China

Phone: +86-025-5287-7867
EMail: [email protected]

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