Internet Engineering Task Force J. Arkko
MSEC Working Group E. Carrara
INTERNET-DRAFT F. Lindholm
Expires: March 2003 M. Naslund
K. Norrman
Ericsson
September, 2002
MIKEY: Multimedia Internet KEYing
<draft-ietf-msec-mikey-04.txt>
Status of this memo
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Abstract
Security protocols for real-time multimedia applications have started
to appear. This has brought forward the need for a key management
solution to support these protocols. Such a solution has to be
suitable to be used in the context of conversational multimedia in a
heterogeneous environment.
This document describes a key management scheme that can be used for
real-time applications (both for peer-to-peer communication and group
communication), and shows how it may work together with protocols
such as SIP and RTSP. In particular, its use to support the Secure
Real-time Transport Protocol, [SRTP], is described in detail.
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TABLE OF CONTENTS
1. Introduction.....................................................3
1.1. Notational Conventions.........................................4
1.2. Definitions....................................................4
1.3. Abbreviations..................................................5
1.4. Outline........................................................5
2. Basic Overview...................................................6
2.1. Scenarios......................................................6
2.2. Design Goals...................................................7
2.3. System Overview................................................7
2.4. Relation to GKMARCH............................................8
2.5. Existing solutions.............................................9
3. Basic Key Transport and Exchange Methods.........................9
3.1. Pre-shared key................................................10
3.2. Public-key encryption.........................................11
3.3. Diffie-Hellman key exchange...................................13
4. Key Management..................................................14
4.1. Key Calculation...............................................14
4.1.1. Assumptions.................................................14
4.1.2. Notation....................................................14
4.1.3. PRF Description.............................................15
4.1.4. Generating keys from TGK....................................15
4.1.5. Generating keys from an envelope/pre-shared key.............15
4.2 Pre-defined Transforms and Timestamp Formats...................16
4.2.1 Hash functions...............................................16
4.2.2 Pseudo random number generator and PRF.......................16
4.2.3 Key data transport encryption................................16
4.2.4 MAC and Verification Message function........................17
4.2.5 Envelope Key encryption......................................17
4.2.6 Digital Signatures...........................................17
4.2.7 Diffie-Hellman Groups........................................17
4.2.8. Timestamps..................................................17
4.2.9. Adding new parameters to MIKEY..............................18
4.3. Policies......................................................18
4.4. Retrieving the Data SA........................................18
4.5. TGK re-keying and CSB updating................................19
5. Behavior and message handling...................................20
5.1. General.......................................................20
5.1.1. Capability Discovery........................................20
5.1.2. Error Handling..............................................21
5.2. Creating a message............................................21
5.3. Parsing a message.............................................23
5.4. Replay handling and timestamp usage...........................23
5.5. Reliability...................................................25
6. Payload Encoding................................................25
6.1. Common header payload (HDR)...................................25
6.1.1. SRTP ID.....................................................28
6.2. Key data transport payload (KEMAC)............................28
6.3. Envelope data payload (PKE)...................................30
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6.4. DH data payload (DH)..........................................30
6.5. Signature payload (SIGN)......................................31
6.6. Timestamp payload (T).........................................31
6.7. ID payload (ID) / Certificate payload (CERT)..................32
6.8. Cert hash payload (CHASH).....................................33
6.9. Ver msg payload (V)...........................................33
6.10. Security Policy payload (SP).................................34
6.10.1. SRTP policy................................................35
6.11. RAND payload (RAND)..........................................36
6.12. Error payload (ERR)..........................................36
6.13. Key data sub-payload.........................................37
6.14. Key validity data............................................38
6.15. General Extension Payload....................................39
7. Integration with session establishment protocols................40
7.1. SDP integration...............................................40
7.2. MIKEY within SIP..............................................40
7.3. MIKEY with RTSP...............................................41
7.4. MIKEY Interface...............................................42
8. Groups..........................................................43
8.1. Simple one-to-many............................................43
8.2. Small-size interactive group..................................43
9. Security Considerations.........................................44
9.1. General.......................................................44
9.2. Key lifetime..................................................46
9.3. Timestamps....................................................46
9.4. Identity protection...........................................47
9.5. Denial of Service.............................................47
9.6. Session establishment.........................................47
10. IANA considerations............................................48
11. Conclusions....................................................49
12. Acknowledgments................................................50
13. Author's Addresses.............................................50
14. References.....................................................50
14.1. Normative References.........................................50
14.2. Informative References.......................................51
Appendix A. - MIKEY - SRTP relation................................53
Revision history...................................................53
1. Introduction
There has recently been work to define a security protocol for the
protection of real-time applications running over RTP, [SRTP].
However, a security protocol needs a key management solution to
exchange keys, security parameters, etc. There are some fundamental
properties that such a key management scheme has to fulfil with
respect to the kind of real-time applications (streaming, unicast,
groups, multicast, etc.) and to the heterogeneous nature of the
scenarios dealt with.
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This document describes a key management solution, that address
multimedia scenarios (e.g. SIP calls and RTSP sessions). The focus is
on how to set up key management for secure multimedia sessions such
that requirements in a heterogeneous environment are fulfilled.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119.
1.2. Definitions
Crypto Session (CS): uni- or bi-directional data stream(s), protected
by a single instance of a security protocol. E.g. when SRTP is used,
the Crypto Session may contain two streams, an RTP stream and the
corresponding RTCP as they are both protected by a single instance of
SRTP (i.e. they share key and some other parameters).
Crypto Session Bundle (CSB): collection of one or more Crypto
Sessions, which can have common TEK Generation Keys and security
parameters.
Crypto Session ID: unique identifier for the Crypto Session within an
CSB.
Crypto Session Bundle ID: unique identifier for the CSB.
TEK Generation Key (TGK): a bit-string agreed upon by two or more
parties, associated with CSB. From the TEK Generation Key, Traffic-
encrypting Keys can then be generated without need of further
communication.
Traffic-encrypting Key (TEK): the key used by the security protocol
to protect the crypto session (this key may be used directly by the
security protocol or may be used to derive further keys depending on
the security protocol). The TEKs are derived from the CSB's TGK.
TGK re-keying: the process of re-negotiating/updating the TGK (and
consequently future TEK(s)).
Initiator: the Initiator of the key management protocol, not
necessarily the Initiator of the communication.
Responder: the Responder in the key management protocol.
Data SA: information for the security protocol, including a TEK and a
set of parameters/policies.
PRF(k,x): a keyed pseudo-random function.
E(k,m): encryption of m with the key k.
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PKx: the public key of x
[] an optional piece of information
{} denotes zero or more occurrences
|| concatenation
| OR (selection operator)
^ exponentiation
XOR binary exclusive or
Bit and byte ordering: throughout the document bits and bytes are as
usual indexed from left to right, with the leftmost bits/bytes being
the most significant.
1.3. Abbreviations
AES Advanced Encryption Standard
CM Counter Mode
CS Crypto Session
CSB Crypto Session Bundle
DH Diffie-Hellman
DoS Denial of Service
MAC Message Authentication Code
MIKEY Multimedia Internet KEYing
PK Public-Key
PS Pre-Shared key
RTP Real-time Transport Protocol
RTSP Real Time Streaming Protocol
SDP Session Description Protocol
SIP Session Initiation Protocol
SRTP Secure RTP
TEK Traffic-encrypting key
TGK TEK Generation Key
1.4. Outline
Section 2 describes the basic scenarios and the design goals for
which MIKEY is intended. It also gives a brief overview of the entire
solution and its relation to the group key management architecture
[GKMARCH].
The basic key transport/exchange mechanisms are explained in detail
in Section 3. The key derivation, and other general key management
procedures are described in Section 4.
Section 5 describes the expected behavior of the involved parties.
This also includes message creation and parsing.
All definitions of the payloads in MIKEY are described in Section 6.
As MIKEY can be carried in SDP over SIP or RTSP, Section 7 describes
how to integrate and use MIKEY in these scenarios.
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Section 8 focuses on how MIKEY is used in group scenarios.
The Security Considerations section (Section 9), gives a deeper
explanation on different security related topics.
2. Basic Overview
2.1. Scenarios
MIKEY is mainly intended to be used for peer-to-peer, simple one-to-
many, and small-size (interactive) groups. One of the main multimedia
scenarios considered when designing MIKEY has been the conversational
multimedia scenario, where users may interact and communicate in
real-time. In these scenarios it can be expected that peers set up
multimedia sessions between each other, where a multimedia session
may consist of one or more secured multimedia streams (e.g. SRTP
streams).
peer-to-peer/ many-to-many many-to-many
simple one-to-many (distributed) (centralized)
++++ ++++ ++++ ++++ ++++
|. | |A | |B | |A |---- ----|B |
--| ++++ | |----------| | | | \ / | |
++++ / ++|. | ++++ ++++ ++++ (S) ++++
|A |---------| ++++ \ / |
| | \ ++|B | \ / |
++++ \-----| | \ ++++ / ++++
++++ \|C |/ |C |
| | | |
++++ ++++
Figure 2.1: Examples of the four scenarios: peer-to-peer, simple one-
to-many, many-to-many without centralized server (also denoted as
small interactive group), and many-to-many with a centralized server.
We identify in the following some typical scenarios which involve the
multimedia applications we are dealing with (see also Figure 2.1).
a) peer-to-peer (unicast), e.g. a SIP-based [SIP] call between two
parties where it may be desirable that the security is either set
up by mutual agreement or that each party sets up the security
for its own outgoing streams.
b) many-to-many, without a centralized control unit, e.g. for small-
size interactive groups where each party may set up the security
for its own outgoing media.
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c) many-to-many, with a centralized control unit, e.g. for larger
groups with some kind of Group Controller that sets up the
security.
d) simple one-to-many (multicast), e.g. real-time presentations,
where the sender is in charge of setting up the security.
The key management solutions may be different in the above scenarios.
When designing MIKEY, the main focus has been on case a, b, and d.
2.2. Design Goals
The key management protocol is designed to have the following
characteristics:
* End-to-end security. Only the participants have access to the
generated key(s).
* Simplicity.
* Efficiency. Designed to have:
- low bandwidth consumption,
- low computational workload,
- small code size, and
- minimal number of roundtrips.
* Tunneling. Possibility to "tunnel"/integrate MIKEY in session
establishment protocols (e.g. SIP and RTSP).
* Independent of any specific security functionality of the
underlying transport.
2.3. System Overview
One objective of MIKEY is to produce a Data security protocol SA
(Data SA), including a traffic-encrypting key (TEK), which is used as
the input to the security protocol.
MIKEY supports the possibility to negotiate keys and parameters for
more than one security protocol at the same time. Therefore, the
concept of Crypto Session Bundle (CSB) is used, which is a collection
of one or more Crypto Sessions that can have common TEK Generation
Keys and security parameters.
The procedure of setting up a CSB and creating a TEK (and Data SA),
is done in accordance with Figure 2.2:
1. A set of security parameters and TEK Generation Key(s) (TGK) are
agreed upon for the Crypto Session Bundle (this is done by one of
the three alternative key transport/exchange mechanisms, see
Section 3).
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2. The TGK(s) is used to derive (in a cryptographically secure way) a
TEK for each Crypto Session.
3. The TEK, together with the security protocol policy parameters
represent the Data SA, which is used as the input to the Security
Protocol.
+-----------------+
| CSB |
| Key transport |
| /exchange |
+-----------------+
| :
| TGK :
v :
+----------+ :
CS ID ->| TEK | : Security protocol
|derivation| : parameters (policies)
+----------+ :
TEK | :
v v
Data SA
|
v
+-------------------+
| Crypto Session |
|(Security Protocol)|
+-------------------+
Figure 2.2: Overview of the key management procedure.
The security protocol can then either use the TEK directly, or, if
supported, derive further session keys from the TEK (e.g. see SRTP
[SRTP]). It is however up to the security protocol to define how the
TEK is used.
MIKEY can be used to update TEKs and the Crypto Sessions in a current
Crypto Session Bundle (see Section 4.5). This is done by executing
the transport/exchange phase once again to derive a new TGK (and
consequently the TEKs) or to update some other specific Crypto
Session parameters.
2.4. Relation to GKMARCH
The Group key management architecture (GKMARCH) [GKMARCH] describes a
general architecture for group key management protocols. MIKEY is a
part of this architecture, and can be used as a so called
Registration protocol. The main entities involved in the architecture
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are a group controller/key server (GCKS), the receiver(s), and the
sender(s).
In MIKEY the GCKS and the sender can be viewed as the same entity,
which pushes down keys to the receiver(s). Note that e.g., in a SIP-
initiated call, the sender may also be a receiver. As MIKEY addresses
small interactive groups, a member may dynamically change between
being a sender and receiver (or being both simultaneously).
2.5. Existing solutions
There is work done in IETF to develop key management schemes. For
example, IKE [IKE] is a widely accepted unicast scheme for IPsec, and
the MSEC WG is developing other schemes, addressed to group
communication [GDOI, GSAKMP]. For reasons discussed, there is however
a need for a scheme with low latency, suitable for demanding cases
such as real-time data over heterogeneous networks, and small
interactive groups.
3. Basic Key Transport and Exchange Methods
The following sub-sections define three different methods to
transport/exchange a TEK Generation Key (TGK): with the use of a pre-
shared key, public-key encryption, and Diffie-Hellman (DH) key
exchange. The two first methods are of key transport type. In the
following we for simplicity assume unicast communication. In addition
to the TGK, a random "nonce", denoted RAND, is also transported. In
all three cases, the TGK and RAND values are then used to derive TEKs
as described in Section 4.1.4.
The pre-shared case is, by far, the most efficient way to handle the
key transport due to the use of symmetric cryptography only. This
approach has also the advantage that only a small amount of data has
to be exchanged. Of course, the problematic issue is scalability.
Public-key cryptography can be used to create a scalable system. A
disadvantage with this approach is that it is more resource consuming
than the pre-shared key approach. Another disadvantage is that in
most cases a PKI (Public Key Infrastructure) is needed to handle the
distribution of public keys. Of course, it is possible to use public
keys as pre-shared keys (e.g. by using self-signed certificates).
The Diffie-Hellman (DH) key exchange method has in general a higher
resource consumption (both computationally and in bandwidth) than the
previous ones. However, it has the advantage of providing perfect
forward secrecy (PFS).
Note that by using the DH method, the two involved parties will
generate a unique random key (which neither of the parties are likely
to significantly affect the outcome of). Therefore, it is not
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possible to use this DH method to establish a group TEK (as the
different parties in the group would end up with different TEKs). It
is not the intention of the DH method to work in this scenario, but
be a good alternative in the special peer-to-peer case.
The following general notation is used:
HDR: The general MIKEY header, which includes MIKEY CSB related data
(e.g. CSB ID) and information mapping to the specific security
protocol used. See Section 6.1 for payload definition.
T: The timestamp. See Section 6.6 for payload definition and also
Section 5.4 for other timestamp related information.
IDx: The identity of x. See Section 6.7 for payload definition.
RAND: Random bit-string, which is always included in the first
message from the Initiator. It is not included in update
messages of a CSB. See Section 6.11 for payload definition.
SP: The security policies for the data security protocol. See
Section 6.10 for payload definition.
3.1. Pre-shared key
In this method, the pre-shared secret key, s, is used to derive key
material for both the encryption (encr_key) and the integrity
protection (auth_key) as described in Section 4.1.5. The encryption
and authentication transforms are described in Section 4.2.
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi],
{SP}, KEMAC --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
The main objective of the Initiator's message is to transport one or
more TGKs and a set of data protocol parameters to the Responder in a
secure manner. As the verification message from the Responder is
optional, the Initiator indicates in the HDR whether it requires a
verification message or not from the Responder.
KEMAC = E(encr_key, {TGK}) || MAC(auth_key, I_MESSAGE).
The KEMAC payload contains a set of encrypted sub-payloads and a MAC.
Each sub-payload includes a, by the Initiator, randomly and
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independently chosen TGK (and possible other related parameters,
e.g., the key lifetime). The MAC is a Message Authentication Code
covering the entire MIKEY message (with the exception of the MAC
field) using the authentication key, auth_key. See Section 6.2 for
payload definition and Section 5.2 for exact definition of the MAC
calculation.
The main objective of the verification message from the Responder is
to obtain mutual authentication.
V = MAC(auth_key, R_MESSAGE||IDi||IDr||T).
The verification, V, is a MAC computed over the Responder's entire
message (with the exception of the MAC field), the timestamp (that
was included in the Initiator's message), and the two parties
identities, using the authentication key. See also Section 5.2 for
the exact definition of the MAC calculation and Section 6.9 for
payload definition.
3.2. Public-key encryption
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi], {SP},
[CHASH], KEMAC, PKE, SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
The main objective of the Initiator's message is to transport one or
more TGKs and a set of data protocol parameters to the Responder in a
secure manner. This is done using an envelope approach where the TGKs
are encrypted (and integrity protected) with keys derived from a
randomly chosen "envelope key". The envelope key is sent to the
Responder encrypted with the public key of the Responder.
As the verification message from the Responder is optional, the
Initiator indicates in the HDR whether it requires a verification
message or not from the Responder.
KEMAC = K || M
K = E(encr_key, IDi || {TGK})
M = MAC(auth_key, K).
The KEMAC contains a set of encrypted sub-payloads and a MAC. The
first sub-payload is the identity of the Initiator (not a
certificate, but generally the same ID as the one specified in the
certificate). Each of the following sub-payloads includes a, by the
Initiator, randomly and independently chosen TGK (and possible other
related parameters, e.g., the key lifetime). The encrypted part is
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then followed by a MAC, which is calculated over the KEMAC payload
(except the MAC field). The encr_key and the auth_key is derived from
the envelope key, env_key (see Section 4.1.5). See also Section 6.2
for payload definition.
PKE = E(PKr, env_key)
The PKE contains the encrypted envelope key. It is encrypted using
the Responder's public key. If the Responder posses several public
keys, the Initiator can use CHASH to indicate the key used.
The SIGNi is a signature covering the entire MIKEY message,
I_MESSAGE, using the Initiator's signature key.
The main objective of the verification message from the Responder is
to obtain mutual authentication. It is calculated in the same way as
for the one in the pre-shared key mode (see also Section 5.2 for the
exact definition). See Section 6.9 for payload definition.
Note that there will be one encrypted IDi and possibly also one
unencrypted IDi. The encrypted one is needed to avoid certain man-in-
the-middle attacks, while the unencrypted is always useful for the
Responder to immediately identify the Initiator.
It is possible to cache the envelope key, so that it can be used as a
pre-shared key. It is not recommended to cache this key indefinitely
(however it is up to the local policy to decide this). This function
may be very convenient during the life-time of a Crypto Session
Bundle, if a new crypto session needs to be added (or an expired one
removed). Then, the pre-shared key can be used, instead of the public
keys (see also Section 4.5). If the Initiator indicates that the
envelope key should be cached, the key is at least to be cached
during the life-time of the entire CSB.
Certificate handling may involve a number of additional tasks not
shown here, and effect the inclusion of certain parts of the message.
The following observations can, however, be made:
* the Initiator typically has to find the certificate of the
Responder in order to send the first message. If the Initiator
does not have the Responder's certificate already, this may
involve one or more roundtrips to a central directory agent.
* it will be possible for the Initiator to omit its own certificate
and rely on the Responder getting this certificate using other
means. However, we recommend doing this, only when it is
reasonable to expect that the Responder has cached the certificate
from a previous connection. Otherwise accessing the certificate
would mean additional roundtrips for the Responder as well.
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* verification of the certificates using Certificate Revocation Lists
(CRLs) or an on-line verification protocol may mean additional
roundtrips for both parties. If a small number of roundtrips is
required for acceptable performance, it may be necessary to omit
some of these checks.
3.3. Diffie-Hellman key exchange
For a fixed, agreed upon, group, (G,*), for g in G and a natural
number x, we let g^x denote g*g*..*g (x times). Choices for the
parameters are given in Section 4.2.7. The other transforms below are
described in Section 4.2.
With this method only one key is created, i.e. the DH-key, which is
used as the TGK.
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
{SP}, DHi, SIGNi --->
R_MESSAGE =
<--- HDR, T, [IDr|CERTr], IDi,
DHr, DHi, SIGNr
The main objective of the Initiator's message is to, in a secure way,
provide the Responder with its DH value (i.e., DHi = g^xi, where xi
is randomly and secretly chosen) and a set of data protocol
parameters.
The SIGNi is a signature covering the Initiator's MIKEY message,
I_MESSAGE, using the Initiator's signature key.
The main objective of the Responder's message is to, in a secure way,
provide the Initiator with its own DH value (i.e., DHr = g^xr, where
xr is randomly and secretly chosen).
The SIGNr is a signature covering the Responder's MIKEY message,
R_MESSAGE, using the Responder's signature key.
The group parameters (e.g., the group G) are a set of parameters
chosen by the Initiator. Both parties calculate the TGK, g^(xi*xr)
from the exchanged DH-values.
Note that this approach does not require that the Initiator has to
posses any of the responder's certificate before the setup. Instead,
it is sufficient that the responder includes it's signing certificate
in the response.
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4. Key Management
4.1. Key Calculation
We define in the following a general method (pseudo random function)
to derive one or more keys from a "master" key. This method is used
to derive:
* TEKs from a TGK and the RAND value,
* encryption, authentication, or salting key from a pre-shared/
envelope key and the RAND value.
4.1.1. Assumptions
We assume that the following parameters are in place:
csb_id: Crypto Session Bundle ID (32-bits unsigned integer)
cs_id: The Crypto Session ID (8-bits unsigned integer)
RAND: An (at least) 128-bit random bit-string sent by the Initiator
in the initial exchange.
The key derivation method has the following input parameters:
inkey: the input key to the derivation function.
inkey_len: the length in bits of the input key.
label: a specific label, dependent on the type of the key to be
derived, the RAND, and the session IDs.
outkey_len: desired length in bits of the output key.
The key derivation method has the following output:
outkey: the output key of desired length.
4.1.2. Notation
Let HMAC be the SHA1 based message authentication function, see
[HMAC,SHA1]. Similar to [TLS], define:
P (s, label, m) = HMAC (s, A_1 || label) ||
HMAC (s, A_2 || label) || ...
HMAC (s, A_m || label)
where
A_0 = label,
A_i = HMAC (s, A_(i-1)).
While SHA-1 is the default, HMAC using other hash function MAY be
used, see Section 4.2.2.
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4.1.3. PRF Description
The following procedure describes a pseudo-random function, denoted
PRF(inkey,label), applied to compute the output key, outkey:
* let n = inkey_len / 512, rounded up to the nearest integer
* split the inkey into n blocks, inkey = s_1 || ... || s_n, where all
s_i, except possibly s_n, are 512 bits each
* let m = outkey_len / 160, rounded up to the nearest integer
If another hash function than SHA1 is used, "512" and "160" MUST be
replaced by the appropriate input/output block-sizes of that
function.
Then, the output key, outkey, is obtained as the outkey_len most
significant bits of
PRF(inkey, label) = P(s_1, label, m) XOR P(s_2, label, m) XOR ...
XOR P(s_n, label, m).
4.1.4. Generating keys from TGK
The key derivation method should be executed with the following
parameters to generate a TEK:
inkey: TGK
inkey_len: length of TGK
label: 0x2AD01C64 || cs_id || csb_id || RAND
outkey_len: length of the output TEK.
If the security protocol does not support key derivation for
authentication and encryption itself from the TEK, separate
authentication and encryption keys MAY directly be created for the
security protocol by replacing 0x2AD01C64 with 0x1B5C7973 and
0x15798CEF respectively, and outkey_len by the desired key-length(s)
in each case.
A salt key can be derived from the TGK as well. This is done by using
the constant 0x39A2C14B.
Note that the 32-bit constant integers (i.e. 0x2AD01C64 or the one
replacing it) are taken from the decimal digits of e (i.e.
2.7182...), and where each constant consist of nine decimals digits
(e.g. the first nine decimal digits 718281828 = 0x2AD01C64). The
strings of nine decimal digits are not chosen at random, but as
consecutive "chunks" from the decimal digits of e.
4.1.5. Generating keys from an envelope/pre-shared key
This derivation is to form the symmetric encryption key (and salting
key) for the encryption of the TGK in the pre-shared key and public
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key methods. This is also used to derive the symmetric key used for
the message authentication code in these messages (and the
corresponding verification messages). Hence, this derivation is
needed in order to get different keys for the encryption and the MAC
(and in the case of the pre-shared key, it will result in fresh key
material for each new CSB).
inkey: the envelope key or the pre-shared key
inkey_len: the length of inkey
label: 0x150533E1 || 0xFF || csb_id || RAND (for encryption key)
or
0x2D22AC75 || 0xFF || csb_id || RAND (for auth. key)
or
0x29B88916 || 0xFF || csb_id || RAND (for salting key)
outkey_len: desired length of the authentication/encryption/salting
key.
4.2 Pre-defined Transforms and Timestamp Formats
This section identifies standard transforms for MIKEY. The following
transforms are mandatory to implement and support in the respective
case. New transforms can be added in the future (see Section 4.2.9
for further guidelines).
4.2.1 Hash functions
In MIKEY, SHA-1 is the default hash function that is mandatory to
implement.
4.2.2 Pseudo random number generator and PRF
A cryptographically secure pseudo random number generator MUST be
used for the generation of the keying material and nonces, e.g.
[BMGL]. However, it is implementation specific which one to use (as
the choice will not affect the interoperability).
For the key derivations, the PRF specified in Section 4.1, using SHA-
1 is mandatory to implement. This PRF MAY be extended by using SHA-
256, SHA-384, or SHA-512, instead of SHA-1. However, it is not
mandatory to support these.
4.2.3 Key data transport encryption
The default and mandatory-to-implement key transport encryption is
AES in counter mode, as defined in [SRTP], using a key as derived in
Section 4.1.5, and using initialization vector
IV = [S XOR (0x0000 || CSB ID || T)] || 0x0000,
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where S is a 112-bit salting key, also derived as in Section 4.1.5,
and where T is the timestamp sent by the Initiator.
Note: this restricts the maximum size of the transported key to 2^23
bits, which is still enough for all practical purposes.
The NULL encryption algorithm (i.e., no encryption) can be used (but
is not mandatory to implement). Note that this MUST NOT be used
unless the underlying protocols can guarantee the security. The main
reason for including this is for certain specific SIP scenarios,
where SDP is protected end-to-end. For this scenario, MIKEY MAY be
used with the pre-shared key method and the NULL encryption and
authentication algorithm while relying on the security of SIP. Use
this option with caution!
4.2.4 MAC and Verification Message function
MIKEY uses a 160-bit authentication tag, generated by HMAC with SHA-1
as the mandatory to implement method, see [HMAC]. Authentication keys
are derived according to Section 4.1.5.
The NULL authentication algorithm (i.e., no MAC) can be used together
with the NULL encryption algorithm (but is not mandatory to
implement). Note that this MUST NOT be used unless the underlying
protocols can guarantee the security. The main reason for including
this is for certain specific SIP scenarios, where SDP is protected
end-to-end. For this scenario, MIKEY MAY be used with the pre-shared
key method and the NULL encryption and authentication algorithm while
relying on the security of SIP. Use this option with caution!
4.2.5 Envelope Key encryption
The public key encryption algorithm applied is defined by, and
dependent on the certificate used.
4.2.6 Digital Signatures
The signature algorithm applied is defined by, and dependent on the
certificate used.
4.2.7 Diffie-Hellman Groups
The Diffie-Hellman key exchange uses OAKLEY 5 [OAKLEY] as mandatory
to implement. Both OAKLEY 1 and OAKLEY 2 MAY be used (but these are
not mandatory to implement).
4.2.8. Timestamps
The current defined timestamp is as defined in NTP [NTP], i.e. a 64-
bit number in seconds relative to 0h on 1 January 1900. An
implementation must be aware of (and take into account) the fact that
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the counter will overflow approximately every 136th year. It is
RECOMMENDED that the time is always specified in UTC.
4.2.9. Adding new parameters to MIKEY
There are two different parameter sets that can be added to MIKEY.
The first is a set of MIKEY transforms (needed for the exchange
itself), and the second is the data security protocol policies/
parameters.
New transforms and parameters SHALL be added by registering a new
number for the payload, and also if necessary, document how the new
transform/parameter is used. Sometimes it might be enough to point to
an already specified document for the usage, e.g., when adding a new
already standardized hash function.
When adding support for a new data security protocol, the following
MUST be specified:
* A map sub payload (see Section 6.1). This is used to be able to map
a crypto session to the right instance of the data security
protocol and possibly also to provide individual parameters for
each data security protocol.
* a policy payload, i.e., specification of parameters and supported
values.
* general guidelines of usage.
4.3. Policies
Included in the message exchange, policies for the Data security
protocol are transmitted. The policies are defined in a separate
payload and are specific to the security protocol (see also Section
6.10). Together with the keys, the validity period of these can also
be specified. This can be done e.g., with an SPI (or SRTP MKI) or
with an Interval (e.g. a sequence number interval for SRTP). Whether
an SPI or an Interval should be used, depends on the security
protocol.
New parameters can be added to a policy by documenting how they
should be interpreted by MIKEY and also by registering new values in
the appropriate name space. If a completely new policy is needed, see
Section 4.2.9 for guidelines.
4.4. Retrieving the Data SA
The retrieval of a Data SA will depend on the security protocol as
different security protocols will have different characteristics.
When adding support for a security protocol to MIKEY, some interface
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of how the security protocol retrieves the Data SA from MIKEY MUST be
specified (together with policies that can be negotiated etc.).
For SRTP the SSRC (see [SRTP]) is one of the parameters used to
retrieve the Data SA. However, the SSRC is not sufficient. For the
retrieval of the Data SA from MIKEY, it is RECOMMENDED that the MIKEY
implementation supports a lookup using destination network address
and port together with SSRC. Note that MIKEY does not send network
addresses or ports. One reason for this is that they may not be known
in advance, as well as if a NAT exists in-between, problems may
arise. When SIP or RTSP is used, the local view of the destination
address and port can be obtained from either SIP or RTSP. MIKEY can
then use these addresses as the index for the Data SA lookup.
4.5. TGK re-keying and CSB updating
MIKEY provides the means to update the CSB (e.g. transporting a new
TGK/TEK or adding a new Crypto Session to the CSB). The updating of
the CSB is done by the Initiator and performed by executing MIKEY
again e.g. before a TEK expires, or when a new Crypto Session is
added to the CSB. Note that MIKEY does not provide re-keying in the
GKMARCH sense, only updating of the keys by normal unicast messages.
When MIKEY is executed again to update the CSB, it is not necessary
to include certificates and other information that was provided in
the first exchange, i.e. all payloads that are static or optional to
include may be left out (see Figure 4.1).
The new message exchange uses the same CSB ID as the initial
exchange, but a new timestamp. A new RAND is NOT included in the
message exchange (the RAND will only have affect in the Initial
exchange). New Crypto Sessions are added if desired in the update
message. Note that a MIKEY update message does not need to contain
new keying material (i.e., new TGK). In this case the crypto session
continues to use the previously established keying material, while
updating the new information.
As explained in Section 3.2, the envelope key can be "cached" as a
pre-shared key (this is indicated by the Initiator in the first
message sent). If so, the update message is a pre-shared key message
(with the cached envelope key as the pre-shared key), i.e., it MUST
NOT be a public key message. If the public key message is used, but
the envelope key is not cached, the Initiator MUST provide a new
encrypted envelope key that can be used in the verification message.
However, the Initiator does not need to provide any other keys.
Figure 4.1 visualizes the update messages that can be sent, including
the optional parts. The big differences from the original message is
mainly that it is optional to include TGKs (or DH values in the DH
method).
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By definition, a Crypto Session Bundle can contain several Crypto
Sessions. A problem that then might occur is to synchronize the TGK
re-keying if an SPI (or similar functionality, e.g., MKI) is not
used. It is therefore recommended that an SPI or MKI is used, if more
than one Crypto Session is used.
Initiator Responder
Pre-shared key method:
I_MESSAGE =
HDR, T, [IDi], {SP}, KEMAC --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
Public key method:
I_MESSAGE =
HDR, T, [IDi|CERTi], {SP}, [CHASH],
[KEMAC], PKE, SIGNi --->
R_MESSAGE =
[<---] HDR, T, [IDr], V
DH method:
I_MESSAGE =
HDR, T, [IDi|CERTi], {SP},
[DHi], SIGNi --->
R_MESSAGE =
<--- HDR, T, [IDr|CERTr], IDi,
[DHr, DHi], SIGNr
Figure 4.1: Update messages.
5. Behavior and message handling
Each message that is sent by the Initiator or the Responder, is built
by a set of payloads. This section describes how messages are created
and also when they can be used.
5.1. General
5.1.1. Capability Discovery
The initiator indicates the security policy to use (i.e. in terms of
security protocol algorithms etc). If the Responder does not support
it (for some reason), the Responder can together with an error
message (indicating that it does not support the parameters), send
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back its own capabilities (negotiation) to let the Initiator choose a
common set of parameters. This is done by including one or more
security policy payloads. Multiple attributes can be provided in
sequence in the response. This is done to reduce the number of
roundtrips as much as possible (i.e. in most cases, where the policy
is accepted the first time, one roundtrip is enough). If the
Responder does not accept the offer, the Initiator must go out with a
new MIKEY message.
If the Responder is not willing/capable to provide security or the
parties simply cannot agree, it is up to the parties' policies how to
behave, i.e. accept an insecure communication or reject it.
Note that it is not the intention of this protocol to have a very
broad variety of options, as it is assumed that it should not be too
common that an offer is denied.
5.1.2. Error Handling
All errors due to the key management protocol SHOULD be reported to
the peer(s) by an error message. The Initiator SHOULD therefore
always be prepared to receive such message back from the Responder.
If the Responder does not support the set of parameters suggested by
the Initiator, the error message SHOULD include the supported
parameters (see also Section 5.1.2).
The error message should be formed as:
HDR, T, {ERR}, [V|SIGNr]
Note that if the failure is due to the inability to authenticate the
peer, the error message is OPTIONAL, and does not need to be
authenticated. It is up to the local policy how to treat this kind of
messages. However, if a signed error message in response to a failed
authentication is returned this can be used for DoS purposes.
Similarly, an unauthenticated error message could be sent to the
Initiator in order to fool her to tear down the CSB. The local policy
MUST take this into consideration. One advice would be not to
authenticate such an error message, and when receiving an
unauthenticated error message only see it as a recommendation of what
may have gone wrong.
5.2. Creating a message
To create a MIKEY message, a Common header payload is first created.
This payload is then followed, depending on the message type, by a
set of information payloads (e.g. DH-value payload, Signature
payload, Security Protocol payload). The defined payloads and the
exact encoding of each payload are described in Section 6.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! version ! data type ! next payload ! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... +
~ Common Header... ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! next payload ! Payload 1 ... !
+-+-+-+-+-+-+-+-+ +
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: : :
: : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! next payload ! Payload x ... !
+-+-+-+-+-+-+-+-+ +
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MAC/Signature ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5.1. MIKEY payload message example. Note that the payloads are
byte aligned and not 32-bit aligned.
The process of generating a MIKEY message consists of the following
steps:
* Create an initial MIKEY message starting with the Common header
payload.
* Concatenate necessary payloads to the MIKEY message (see the
exchange definitions for payloads that may be included and
recommended order).
* As a last step (for messages that must be authenticated, this also
include the verification message), create and concatenate the
MAC/signature payload without the MAC/signature field filled in
(if a Next payload field is included in this payload, it is set to
Last payload).
* Calculate the MAC/signature over the entire MIKEY message, except
the MAC/Signature field, and add put the MAC/signature in the
field. In the case of the verification message, the IDi || IDr ||
T MUST follow directly after the MIKEY message in the MAC
calculation.
In the public key case, the Key data transport payload is generated
by concatenating the IDi with the TGKs. This is then encrypted and
placed in the data field. The MAC is calculated over the entire Key
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data transport payload except the MAC field. Before calculating the
MAC, the Next payload field is set to zero.
Note that all messages from the Initiator MUST use a unique
timestamp. The Responder does not create a new timestamp, but uses
the timestamp used by the Initiator.
5.3. Parsing a message
In general, parsing of a MIKEY message is done by extracting payload
by payload and checking that no errors occur (the exact procedure is
implementation specific). However, for the Responder, it is
RECOMMENDED that the following procedure is followed:
* Extract the Timestamp and check that it is within the allowable
clock skew (if not, discard the message). Also check the replay
cache so that the message is not replayed (see also Section 5.4).
If the message is replayed, discard it.
* Extract ID and authentication algorithm (if not included, assume
the default one).
* Verify the MAC/signature.
* If the authentication is not successful, an Auth failure Error
message is possibly sent to the Initiator (if SIP is used, this is
signaled to SIP as a rejection of the offer). The message is then
discarded from further processing. See also Section 5.1.2 for
treatment of errors.
* If the authentication is successful, the message is processed.
Though how it is processed is implementation specific.
* If any unsupported parameters or errors occur during the
processing, these are reported to the Initiator by sending an
error message. The processing is then aborted. The error message
can also include payloads to describe the supported parameters. If
SIP is used, this is signaled to SIP as a rejection of the offer
(see also Section 7.2).
* If the processing was successful and if needed, a verification/
response message is created and sent to the Initiator.
5.4. Replay handling and timestamp usage
MIKEY does not use a challenge-response mechanism for replay
handling, instead timestamps are used. This requires that the clocks
are synchronized. The required synchronization is dependent on the
number of messages that can be cached. If we could assume an
unlimited cache, the terminals would not need to be synchronized at
all (as the cache could then contain all previously messages).
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However, if there are restrictions on the size of the replay cache,
the clocks will need to be synchronized to some extent. In short, one
can in general say that it is a tradeoff between the size of the
replay cache and the required synchronization.
Timestamp usage prevents against replay attacks under the following
assumptions:
* Each host have a clock which is at least "loosely synchronized" to
the clocks of the other hosts.
* If the clocks are to be synchronized over the network, a secure
network clock synchronization protocol is used.
* Each Responder utilize a replay cache in order to remember the
messages presented within an allowable clock skew (which is set by
the local policy).
* Replayed and outdated messages, i.e., messages that can be found in
the replay cache or which have an outdated timestamp, are
discarded and not processed.
* If the host loses track of the incoming requests (e.g. due to
overload), it rejects all incoming requests until the clock skew
interval has passed.
In a client-server scenario, servers may be the entities that will
have the highest work load. It is therefore RECOMMENDED that the
servers are the Initiators of MIKEY. This will result in that the
servers will not need to manage any significant replay cache as they
will refuse all incoming messages that are not a response to an
already (by the server) sent message.
In general, a client may not expect a very high load of incoming
messages and may therefore allow the degree of looseness to be on the
order of minutes (5-10 minutes are believed to be acceptable). If a
DoS attack is launched and the replay cache grows too large, MIKEY
MAY dynamically decrease the looseness so that the replay cache
becomes manageable.
The maximum number of messages that a client will need to cache may
vary depending on the capacity of the client itself and the network,
but also the number of expected messages should be taken into
account.
For example, assume that we can at most spend 6kB on a replay cache.
Assume further that we need to store 30 bytes for each incoming
message (the hash of the message is 20 bytes). This implies that it
is possible to cache approximately 204 messages. If the expected
number of messages per minute can be estimated, the clock skew can
easily be calculated. E.g., in a SIP scenario where the client is
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expected in the most extreme case, a few calls per minute (assume 10
at most in this example), the clock skew that can be used is
approximately 20 minutes.
In a more extreme case, where the maximum number of incoming messages
are assumed to be on the order of 120 messages per minute, and a
requirement that the clock skew is on the order of 10 minutes, a 48kB
replay cache would be required.
One recommendation is to fix a size for the replay cache, and let the
allowable clock skew be large. As the replay cache grows, the clock
skew is decreased depending on how many percent of the replay cache
that are used. Note that this is locally handled, which will not
require interaction with the peer (even though it may indirectly
affect the peer). Exactly how to implement such functionality is
however out of the scope of this document and considered
implementation specific.
In case of a DoS attack, the client will in most cases be able to
handle the replay cache. A bigger problem will probably be to process
the messages (verify signatures/MACs), due to the computational
workload this implies.
5.5. Reliability
If MIKEY is sent on an unreliable transport, the basic processing
applied to ensure protocol reliability is the following.
The transmitting entity (Initiator or Responder) MUST:
* Set a timer and initialize a retry counter
* If the timer expires, the message is resent and the retry counter
is decreased.
* If the retry counter reaches zero (0), the event MAY be logged in
the appropriate system audit file.
6. Payload Encoding
This section describes in detail all the payloads. For all encoding,
Network byte order is always used.
6.1. Common header payload (HDR)
The Common header payload MUST always be present as the first payload
in each message. The common header includes general description of
the exchange message.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! version ! data type ! next payload !V! PRF func !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! CSB ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! #CS ! CS ID map type! CS ID map info ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The common header contains the following information:
* version: the version number of MIKEY.
version = 1
* data type: describes the type of message (e.g. public-key transport
message, verification message, error message).
Data type | Value | Comment
--------------------------------------
Pre-shared | 0 | Initiator's pre-shared key message
PS ver msg | 1 | Verification message of a Pre-shared
| | key message
Public key | 2 | Initiator's public-key transport message
PK ver msg | 3 | Verification message of a public-key
| | message
D-H init | 4 | Initiator's DH exchange message
D-H resp | 5 | Responder's DH exchange message
Error | 6 | Error message
* next payload: identifies the payload that is added after this
payload.
Next payload | Value | Section
------------------------------
Last payload | 0 | -
KEMAC | 1 | 6.2
PKE | 2 | 6.3
DH | 3 | 6.4
SIGN | 4 | 6.5
T | 5 | 6.6
ID | 6 | 6.7
CERT | 7 | 6.7
CHASH | 8 | 6.8
V | 9 | 6.9
SP | 10 | 6.10
RAND | 11 | 6.11
ERR | 12 | 6.12
Key data | 20 | 6.13
General Ext. | 21 | 6.15
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Note that some of the payloads cannot possibly come right after
the header (such as "Last payload", "Signature", etc.). However,
the Next payload field is generic for all payloads. Therefore, a
value is allocated for each payload.
* V: flag to indicate whether a verification message is expected or
not (this has only meaning when it is set by the Initiator).
V = 0 ==> no response expected
V = 1 ==> response expected
* PRF func: Indicates the PRF function that has been/will be used for
key derivation etc.
PRF func | Value | Comments
--------------------------------------------------------
MIKEY-1 | 0 | Mandatory, Default (see Section 4.1.2-3)
MIKEY-256 | 1 | (as MIKEY-1 but using a HMAC with SHA256)
MIKEY-384 | 2 | (as MIKEY-1 but using a HMAC with SHA384)
MIKEY-512 | 3 | (as MIKEY-1 but using a HMAC with SHA512)
* CSB ID: A 32-bit integer to identify the CSB. It is RECOMMENDED
that it is chosen at random by the Initiator. This ID MUST be
unique between each Initiator-Responder pair, i.e., not globally
unique. An Initiator MUST check for collisions when choosing the
ID (if the Initiator already has one or more established CSB with
the Responder). The Responder uses the same CSB ID in the
response.
* #CS: Indicates the number of Crypto Sessions that will be handled.
Note that even though it is possible to use 255 CSs, it is not
likely that a CSB will include this many CSs. The integer 0 is
interpreted as no CS included. This may be the case in an initial
setup message.
* CS ID map type: specifies the method to uniquely map Crypto
Sessions to the security protocol sessions.
CS ID map type | Value
-----------------------
SRTP-ID | 0
* CS ID map info: Identifies the crypto session(s) that the SA should
be created for. The currently defined map type is the SRTP-ID
(defined in Section 6.1.1).
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6.1.1. SRTP ID
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 ! SSRC 1 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SSRC 1 (cont) ! ROC 1 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ROC 1 (cont) ! Policy no 2 ! SSRC 2 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SSRC 2 (cont) ! ROC 2 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ROC 2 (cont) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
: : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Policy no #CS ! SSRC #CS ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~SSRC #CS (cont)! ROC #CS ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ROC #CS (cont)!
+-+-+-+-+-+-+-+-+
* Policy no x: The policy applied for the stream with SSRC x. The
same policy may apply for all CSs.
* SSRC x: specifies the SSRC that MUST be used for the SRTP streams.
Note that it is the sender of the streams who chooses the SSRC.
Therefore, it might be that the Initiator of MIKEY can not fill in
all fields. In this case, SSRCs that are not chosen by the
Initiator are set to zero and the Responder fills in these field
in the response message. It is in general RECOMMENDED or required
to use unique SSRCs (both to avoid RTP SSRC collision, and from an
SRTP perspective, to avoid two-time pad problems if the same TEK
is used for more than one stream).
* ROC x: Current rollover counter used in SRTP. If the SRTP session
has not started, this field is set to 0. This field is used to be
able for a member to join and synchronize to an already started
stream.
NOTE: The stream using SSRC x will also have Crypto Session ID equal
to x (NOT to SSRC).
6.2. Key data transport payload (KEMAC)
The Key data transport payload contains encrypted Key data payloads
(see Section 6.13 for definition of Key data payloads). It may
contain one or more Key data payloads each including a TGK. The last
Key data payload has its Next payload field set to Last payload. For
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an update message (see also Section 4.5), it is allowed to skip the
Key data payloads (which will result in that the Encr data len is
equal to 0).
If the transport method used is the pre-shared key method, this Key
data transport payload is the last payload in the message (note that
the Next payload field is set to Last payload). The MAC is then
calculated over the entire MIKEY message (as described in Section
5.2).
If the transport method used is the public-key method, the
Initiator's identity is added in the encrypted data. This is done by
adding the ID payload as the first payload, which then are followed
by the Key data payloads. Note that for an update message, the ID is
still sent encrypted to the Responder (this is to avoid certain re-
direction attacks) even though no Key data payloads is added after.
The MAC field is in the public-key case calculated only over the Key
data transport payload except the MAC field and where the Next
payload field has been set to zero (see also Section 5.2).
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 ! Encr alg ! Encr data len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Encr data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Mac alg ! MAC ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload (see Section 6.1 for defined values).
* Encr alg: The encryption algorithm used to encrypt the TGK.
Encr alg | Value | Comments
-------------------------------------------
AES-CM | 1 | Mandatory (as defined in Section 4.2.3)
NULL | 2 | Very restricted usage, see Section 4.2.3!
* Encr len: Length of encrypted part (in bytes).
* Encr data: The encrypted TGK sub-payloads (see Section 6.13).
* MAC alg specifies the authentication algorithm used.
MAC alg | Value | Comments
--------------------------------------
HMAC-SHA1-160 | 0 | Mandatory (see Section 4.2.4)
NULL | 1 | Very restricted usage, see Section 4.2.4!
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* MAC: The message authentication code of the entire message.
6.3. Envelope data payload (PKE)
The Envelope data payload contains the encrypted envelope key that is
used in the public-key transport to protect the data in the Key data
transport payload. The encryption algorithm used is implicit from the
certificate/public key used.
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 ! C ! Data len ! Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload.
* C: Envelope key cache indicator (see also Section 3.2, for more
information of the usage).
Cache type | Value | Comments
--------------------------------------
No cache | 0 | The envelope key MUST NOT be cached
Cache | 1 | The envelope key MUST be cached
Cache for CSB | 2 | The envelope key MUST be cached, but only
| | to be used for the specific CSB.
* Data len: The length of the data field (in bytes).
* Data: The encrypted envelope key (if nothing else stated in the
certificate, padding and formatting is done according to
RSA/PKCS#1 if RSA is used).
6.4. DH data payload (DH)
The DH data payload carries the DH-value and indicates the DH-group
used.
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 ! DH-Group ! DH-value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Reserv! KV ! KV data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload.
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* DH-Group: identifies the DH group used.
DH-Group | Value | Comments
--------------------------------------
OAKLEY 5 | 0 | Mandatory
OAKLEY 1 | 1 |
OAKLEY 2 | 2 |
* DH-value: The public DH-value (the length is implicit from the
group used).
* KV: Indicates the type of key validity period specified. This may
be done by using an SPI (alternatively an MKI) or by providing an
interval in which the key is valid (e.g. in the latter case, for
SRTP this will be the index range where the key is valid). See
Section 6.13 for pre-defined values.
* KV data: This includes either the SPI/MKI or an interval (see
Section 6.14). If KV is NULL, this field is not included.
6.5. Signature payload (SIGN)
The Signature payload carries the signature and its related data. The
signature payload is always the last payload in the PK transport and
DH exchange messages. The signature algorithm used is implicit from
the certificate/public key used.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Signature len ! Signature ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Signature len: The length of the signature field (in bytes).
* Signature: The signature (if nothing else stated in the
certificate, padding and formatting is done according to
RSA/PKCS#1 if RSA is used).
6.6. Timestamp payload (T)
The timestamp payload carries the timestamp information.
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 type ! TS value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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* next payload: identifies the payload that is added after this
payload. If no more payload follows, it MUST be set to Last
payload. See Section 6.1 for values.
* TS type: specifies the timestamp type used.
TS type | Value | Comments
-------------------------------------
NTP-UTC | 0 | Mandatory (64-bits)
NTP | 1 | Mandatory (64-bits)
* TS-value: The timestamp value of the specified TS type.
6.7. ID payload (ID) / Certificate payload (CERT)
Note that the ID payload and the Certificate payload are two
completely different payloads (having different payload identifiers).
However, as they share the same payload structure they are described
in the same section.
The ID payload carries a uniquely-defined identifier.
The certificate payload contains an indicator of the certificate
provided as well as the certificate data. If a certificate chain are
to be provided, each certificate in the chain should be included in a
separate CERT payload.
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/Cert Type ! ID/Cert len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID/Certificate Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload. See Section 6.1 for values.
If the payload is an ID payload the following values applies for the
ID type field:
* ID Type: specifies the identifier type used.
ID Type | Value | Comments
----------------------------------------------
NAI | 0 | Mandatory (see [NAI])
URI | 1 | Mandatory (see [URI])
If the payload is an Certificate payload the following values applies
for the Cert type field:
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* Cert Type: specifies the certificate type used.
Cert Type | Value | Comments
----------------------------------------------
X.509v3 | 0 | Mandatory
X.509v3 URL | 1 | plain ASCII URL to the location of the Cert
X.509v3 Sign | 2 | Mandatory (used for signatures only)
X.509v3 Encr | 3 | Mandatory (used for encryption only)
* ID/Cert len: The length of the ID or Certificate field (in bytes).
* ID/Certificate: The ID or Certificate data. The X.509 [X.509]
certificates are included as a bytes string using DER encoding as
specified in X.509.
6.8. Cert hash payload (CHASH)
The Cert hash payload contains the hash of the certificate used.
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 ! Hash func ! Hash ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload.
* Hash func: Indicates the hash function that is used (see also
Section 4.2.1).
Hash func | Value
----------------------
SHA-1 | 0 Mandatory
SHA256 | 1
SHA384 | 2
SHA512 | 3
MD5 | 4
* Hash: The hash data. Note: the hash length is implicit from the
hash function used.
6.9. Ver msg payload (V)
The Ver msg payload contains the calculated verification message in
the pre-shared key and the public-key transport methods. Note that
the MAC is calculated over the entire MIKEY message as well as the
IDs and Timestamp (see also Section 5.2).
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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 ! Auth alg ! Ver data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload. If no more payload follows, it is set to Last payload.
See Section 6.1 for values.
* Auth alg: specifies the MAC algorithm used for the verification
message. See Section 6.2 for defined (MAC field) for defined
values.
* Ver data: The verification message data. Note: the length is
implicit from the authentication algorithm used.
6.10. Security Policy payload (SP)
The Security Policy payload defines a set of policies that applies to
a specific security protocol.
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 ! Policy no ! Prot type ! Policy param ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ length (cont) ! Policy param ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload: identifies the payload that is added after this
payload. See Section 6.1 for values.
* Policy no: Each security policy payload must be given a distinct
number.
* Prot type: defines the security protocol.
Prot type | Value |
---------------------------
SRTP | 0 |
* Policy param length: defines the total length of the policy
parameters for the specific security protocol.
* Policy param: defines the policy for the specific security
protocol.
The Policy param part is built up by a set of Type/Length/Value
fields. For each security protocol, a set of possible types/values
that can be negotiated are defined.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Type ! Length ! Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Type: specifies the type of the parameter.
* Length: specifies the length of the Value field (in bytes).
* Value: specifies the value of the parameter.
6.10.1. SRTP policy
This policy specifies the parameters for SRTP and SRTCP. The
types/values that can be negotiated are defined by the following
table:
Type | Meaning | Possible values
----------------------------------------------------
0 | Encryption algorithm | see below
1 | Session Encr. key length | depends on cipher used
2 | Authentication algorithm | see below
3 | Session Auth. key length | depends on MAC used
4 | Session Salt key length | see [SRTP] for recommendations
5 | SRTP Pseudo Random Function | see below
6 | Key derivation rate | see [SRTP] for recommendations
7 | SRTP encryption off/on | 0 if off, 1 if on
8 | SRTCP encryption off/on | 0 if off, 1 if on
9 | FEC order | see below
10 | SRTP authentication off/on | 0 if off, 1 if on
11 | Authentication tag length | in bytes
12 | SRTP prefix length | in bytes
Note that if a Type/Value is not set, the default one is used
(according to SRTPs own criteria).
For the Encryption algorithm, it is enough with a one byte length and
the currently defined possible Values are:
SRTP encr alg | Value
---------------------
NULL | 0
AES-CM | 1
AES-F8 | 2
where AES-CM is AES in CM and AES-F8 is AES in f8 mode.
For the Authentication algorithm, it is enough with a one byte length
and the currently define possible Values are:
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SRTP auth alg | Value
---------------------
NULL | 0
HMAC-SHA1 | 1
For the SRTP pseudo random function, it is also enough with a one
byte length and the currently define possible Values are:
SRTP PRF | Value
---------------------
AES-CM | 0
If FEC is used at the same time as SRTP is used, MIKEY can negotiate
the order in which these should be applied.
FEC order | Value | Comments
--------------------------------
FEC-SRTP | 0 | First FEC, then SRTP
SRTP-FEC | 1 | First SRTP, then FEC
SPLIT | 2 | SRTP encr., then FEC, finally SRTP auth
6.11. RAND payload (RAND)
The RAND payload consist of a random bit-string. The RAND MUST be
chosen at random and per CSB (note that the if a CSB has several
members, the Initiator MUST use the same RAND to all the members).
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 len ! RAND ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload: identifies the payload that is added after this
payload.
* RAND len: Length of the RAND (in bytes). SHOULD be at least 16.
* RAND: a randomly chosen bit-string.
6.12. Error payload (ERR)
The Error payload is used to specify the error(s) that may have
occurred.
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 ! Error no ! Reserved !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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* next payload: identifies the payload that is added after this
payload. If no more payload follows, it is set to Last payload.
See Section 6.1 for values.
* Error no indicates the type of error that was encountered.
Error no | Value | Comment
-------------------------------------------------------
Auth failure | 0 | Authentication failure
Invalid TS | 1 | Invalid timestamp
Invalid PRF | 2 | PRF function not supported
Invalid MAC | 3 | MAC algorithm not supported
Invalid EA | 4 | Encryption algorithm not supported
Invalid HA | 5 | Hash function not supported
Invalid DH | 6 | DH group not supported
Invalid ID | 7 | ID not supported
Invalid Cert | 8 | Certificate not supported
Invalid SP | 9 | SP type not supported
Invalid SPpar | 10 | SP parameters not supported
6.13. Key data sub-payload
The Key data payload contains TGKs. The Key data payloads are never
included in clear, but as an encrypted part of the Key data transport
payload.
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 ! Type ! KV ! Key data len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Key data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Salt len (optional) ! Salt data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KV data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload: identifies the payload that is added after this
payload.
* Type: Indicates the type of the key included in the payload. Note
that generally TEKs are not sent directly, but a TGK, which is
then used to derive the TEK (or TEKs if there are several crypto
sessions) as described in Section 4.1.4.
Type | Value | Comments
---------------------------------------
TGK | 0 | A TGK (used to derive TEKs from)
TGK+SALT | 1 | A TGK + a salt key are included
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TEK | 2 | A plain TEK
TEK+SALT | 3 | A plain TEK + a salt key are included
Note that the possibility to include a TEK (instead of using the
TGK is provided). However, if this is used, the TEK can generally
not be shared between more than one Crypto Session. The
recommended use of a TEK instead of a TGK is when pre-encrypted
material exist and therefore, the TEK must be known in advance.
* KV: Indicates the type of key validity period specified. This may
be done by using an SPI/MKI or by providing an interval in which
the key is valid (e.g., in the latter case, for SRTP this will be
the index range where the key is valid).
KV | Value | Comments
-------------------------------------------
Null | 0 | No specific usage rule (e.g. a TEK
| | that has no specific lifetime)
SPI | 1 | The key is associated with the SPI/MKI
Interval | 2 | The key has a start and expiration time
| | (e.g. an SRTP TEK)
Note that when NULL is specified, any SPI or Interval is valid.
For an Interval this means that the key is valid from the first
observed sequence number until the key is replaced (or the
security protocol is shutdown).
* Key data len: The length of the Key data field (in bytes).
* Key data: The TGK data.
* Salt len: The salt key length in bytes. Note that this field is
only included if the salt is specified in the Type-field.
* Salt data: The salt key data. Note that this field is only included
if the salt is specified in the Type-field. (For SRTP, this is the
so-called master salt.)
* KV data: This includes either the SPI or an interval (see Section
6.14). If KV is NULL, this field is not included.
6.14. Key validity data
The Key validity data is not a standalone payload, but part of either
the Key data payload (see Section 6.13) or the DH payload (see
Section 6.4). The Key validity data gives a guideline of when the key
should be used. This can be done, using an SPI/MKI or a lifetime
range.
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SPI/MKI
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SPI Length ! SPI ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* SPI Length: The length of the SPI (or MKI) in bytes.
* SPI: The SPI (or MKI) value.
Interval
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! VF Length ! Valid from ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! VT Length ! Valid to (expires) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* VF Length: Length of the Valid From field in bytes.
* Valid From: Sequence number, index, timestamp, or other start value
that the security protocol uses to identify the start position of
the key usage.
* VT Length: Length of the Valid To field in bytes.
* Valid to: Sequence number, index, timestamp, or other expiration
value that the security protocol can use to identify the
expiration of the key usage.
Note that for SRTP usage, the key validity period for a TGK should be
specified with either an interval, where the VF/VT length is equal to
6 bytes (i.e., the size of the index), or, with an MKI. It is
RECOMMENDED that if more than one SRTP stream is sharing the same
keys and key update/re-keying is desired, this is handled using MKI
rather than the From-To method.
6.15. General Extension Payload
The General extensions payload is included to allow possible
extensions to MIKEY without the need to define a complete new payload
each time. This payload can be used in any MIKEY message. Currently
the only use defined, is to transport Vendor Id. Support of the
Vendor ID is OPTIONAL.
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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 ! Type ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Next payload: identifies the payload that is added after this
payload.
* Type: identifies the type of the general payload.
Type | Value | Comments
---------------------------------------
Vendor ID | 0 | Vendor specific byte string
* Length: the length in bytes of the Data field.
* Data: the general payload data.
7. Integration with session establishment protocols
This section describes how MIKEY should be integrated with SDP, SIP
and RTSP. It is based on [KMASDP], which describes extensions to SDP
and SIP to carry key management protocol information.
7.1. SDP integration
SDP descriptions [SDP] can be carried by several protocols, such as
SIP and RTSP. Both SIP and RTSP often use SDP to describe the media
sessions. Therefore, it is also convenient to be able to integrate
the key management in the session description it is supposed to
protect. [KMASDP] describes attributes that should be used by a key
management protocol that is integrated in SDP. We refer to [KMASDP]
for both definitions and examples. Note that MIKEY uses the name
"mikey" as a protocol name in SDP and RTSP. The key management data
that is placed in SDP or RTSP MUST be base64 encoded.
7.2. MIKEY within SIP
In e.g., a basic SIP call between two parties (see Figure 7.1.), SIP
(Session Initiation Protocol, [SIP]) is used as a session
establishment protocol between two or more parties. In general an
offer is made, whereby it is either accepted or rejected by the
answerer. SIP complies to the offer/answer model [OFFANS], to which
MIKEY over SIP MUST be compliant with as well.
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--------- ---------
|A's SIP| <.......> |B's SIP|
|Server | SIP/MIKEY |Server |
--------- ---------
^ ^
. .
++++ SIP/MIKEY . . SIP/MIKEY ++++
| | <............. ..............> | |
| | | |
++++ <-------------------------------------------> ++++
SRTP
Fig 7.1.: SIP-based call example. The two parties uses MIKEY over SIP
to set up an SRTP stream between A and B.
The SIP offerer will be the MIKEY Initiator and the SIP answerer will
be the MIKEY Responder. This implies that in the offer, the MIKEY
Initiator's message is included, and in the answer to the offer, the
MIKEY Responder's message is included.
If the MIKEY part of the offer is not accepted, a MIKEY error message
is provided in the answer (following Section 5.1.2). The MIKEY
implementation signals to the SIP implementation whether the MIKEY
message was an acceptable offer or not.
It may be assumed that the offerer knows the identity of the
answerer. However, unless the Initiator's identity can be derived
from SIP itself, the Initiator (caller) MUST provide the identity to
the callee. It is RECOMMENDED to use the same identity for both SIP
and MIKEY.
Updating of the CSB (e.g. TEK update) is only supposed to be seen as
a new offer. Note that it might not be necessary to send all
information, such as the certificate, due to the already established
call (see also Section 4.5).
7.3. MIKEY with RTSP
The Real Time Streaming Protocol (RTSP) [RTSP] is used to control
media streaming from a server. The media session is typically
obtained via an SDP description, received by a DESCRIBE message, or
by other means (e.g., HTTP). To be able to pass the MIKEY messages in
RTSP messages which does not contain an SDP description, the RTSP
KeyMgmt header (defined in [KMASDP]) is used. This header includes
basically the same fields as the SDP extensions. As for SDP, "mikey"
is used as the protocol identifier.
In an RTSP scenario, the RTSP server and the MIKEY Initiator will be
the same entity. The Initiator/RTSP server includes the MIKEY message
in an SDP description. When responding to this, the client uses the
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defined RTSP header to send back the answer (included in the SETUP
message).
Note that it is the server that will be the Initiator of MIKEY in
this case. This has some advantages. First, the server will always be
able to choose the key for the content it distributes. Secondly, it
will then have the possibility to use the same key for the same
content that are streamed/sent to more than one client.
To be able to have a server-initiated CSB update procedure, the
ANNOUNCE message is used to send the updated MIKEY material. Note
that the ANNOUNCE method has the ability to send SDP descriptions to
update previous ones (i.e., it is not required to use the RTSP
KeyMgmt header from server to client).
7.4. MIKEY Interface
The SDP, SIP, and RTSP processing is defined in [KMASDP]. However, it
is necessary that MIKEY can work properly with these protocols. This
subsection describes some aspects which implementers SHOULD consider.
If the MIKEY implementation is separate from the SDP/SIP/RTSP, an
application programming interface (API) between MIKEY and these
protocols is needed with certain functionality (however, exactly
what it looks like is implementation dependent).
Implementers of MIKEY are RECOMMENDED to consider providing at least
the following functionality:
* the possibility for MIKEY to receive information about the sessions
negotiated. This is to some extent implementation dependent. But
it is RECOMMENDED that, in the case of SRTP streams, the number of
SRTP streams are included (and the direction of these). The
destination addresses and ports is also RECOMMENDED to be provided
to MIKEY.
* the possibility for MIKEY to receive incoming MIKEY messages and
return a status code from/to the SIP/RTSP application.
* the possibility for the SIP or RTSP applications to receive
information from MIKEY. This would typically include the receiving
of the CSB ID or the SSRCs for SRTP. It is also RECOMMENDED that
extra information about errors can be received.
* the possibility for the SIP or RTSP application to receive outgoing
MIKEY messages.
* the possibility to tear down a MIKEY CSB (e.g. if the SIP session
is closed, the CSB SHOULD also be closed).
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Note that if a CSB has already been established, it is still valid
for the SIP or RTSP implementation to request a new message from
MIKEY, e.g. when a new offer is issued. MIKEY SHOULD then send an
update message to the Responder (see also Section 4.5).
8. Groups
What has been discussed up to now is not limited to single peer-to-
peer communication (except for the DH method), but can be used to
distribute group keys for small-size interactive groups and simple
one-to-many scenarios. This section describes how MIKEY is used in a
group scenario.
8.1. Simple one-to-many
++++
|S |
| |
++++
|
--------+-------------- - -
| | |
v v v
++++ ++++ ++++
|A | |B | |C |
| | | | | |
++++ ++++ ++++
Figure 8.1. Simple one-to-many scenario.
In the simple one-to-many scenario, a server is streaming to a small
group of clients. RTSP or SIP is used for the registration and the
key management set up. The streaming server acts as the Initiator of
MIKEY. In this scenario the pre-shared key or public key transport
mechanism will be appropriate to use to transport the same TGK to all
the clients (which will result in common TEKs for the group).
Note, if the same TGK/TEK(s) should be used by all the group members,
the streaming server MUST specify the same CSB_ID and CS_ID(s) for
the session to all the group members.
8.2. Small-size interactive group
As described in the overview section, for small-size interactive
groups, one may expect that each client will be in charge for setting
up the security for its outgoing streams. In these scenarios, the
pre-shared key or the public-key transport method is used.
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++++ ++++
|A | -------> |B |
| | <------- | |
++++ ++++
^ | | ^
| | | |
| | ++++ | |
| --->|C |<--- |
------| |------
++++
Figure 8.2. Small-size group without centralized controller.
One scenario may then be that the client sets up a three-part call,
using SIP. Due to the small size of the group, unicast SRTP is used
between the clients. Each client sets up the security for its
outgoing stream(s) to the others.
As for the simple one-to-many case, the streaming client specifies
the same CSB_ID and CS_ID(s) for its outgoing sessions if the same
TGK/TEK(s) is used for all the group members.
9. Security Considerations
9.1. General
No chain is stronger than its weakest link. The cryptographic
functions protecting the keys during transport/exchange SHOULD offer
a security at least corresponding to the (symmetric) keys they
protect. For instance, with current state of the art, see [LV],
protecting a 128-bit AES key by a 512-bit RSA [RSA] key offers an
overall security below 64-bits. On the other hand, protecting a 64-
bit symmetric key by a 2048-bit RSA key appears to be an "overkill",
leading to unnecessary time delays. Therefore, key size for the key-
exchange mechanism SHOULD be weighed against the size of the
exchanged key. We refer to [LV] for concrete key size
recommendations.
Moreover, if the TGKs are not random, a brute force search may be
facilitated, again lowering the effective key size. Therefore, care
MUST be taken when designing the (pseudo) random generators for TGK
generation.
For the selection of the hash function, SHA-1 with 160-bit output is
the default one. In general, hash sizes should be twice the "security
level", indicating that SHA1-256, [SHA256], should be used for the
default 128-bit level. However, due to the real-time aspects in the
scenarios we are treating, hash size slightly below 256 are
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acceptable as the normal "existential" collision probabilities would
be of secondary importance.
In a Crypto Session Bundle, the Crypto Sessions can share the same
TGK as discussed earlier. From a security point of view, the
criterion to be satisfied is that the encryption of the individual
Crypto Sessions are performed "independently". In MIKEY this is
accomplished by having unique Crypto Session identifiers (see also
Section 4.1). The TEK derivation method assures this by providing
cryptographically independent TEKs to distinct Crypto Sessions
(within the Crypto Session Bundle), regardless of the security
protocol used.
Specifically, the key derivations are implemented by a pseudo-random
function. The one used here is a simplified version of that used in
TLS [TLS]. Here, only one single hash function is used, whereas TLS
uses two different functions. This choice is motivated by the high
confidence in the SHA-1 hash function, and, by efficiency and
simplicity of design (complexity does not imply security). Indeed, as
shown in [DBJ], if one of the two hashes is severely broken, the TLS
PRF is actually less secure than if a single hash had been used on
the whole key. Thus, the construction does not meet its goals.
In the pre-shared key and public-key schemes, the TGK is generated by
a single party (Initiator). This makes MIKEY more sensitive if the
Initiator uses a bad random number generator. It should also be noted
that neither the pre-shared nor the public-key scheme provides
perfect forward secrecy. If mutual contribution or perfect forward
secrecy is desired, the Diffie-Hellman method is to be used.
Forward/backward security: if the TGK is exposed, all TEKs generated
from it are compromised. However, under the assumption that the
derivation function is a pseudo-random function, disclosure of an
individual TEK does not compromise other (previous or later) TEKs
derived from the same TGK.
The use of random nonces (RANDs) in the key derivation is of utmost
importance to counter off-line pre-computation attacks. Note however
that update messages re-use the old RAND. This means that the total
effective key entropy (relative to pre-computation attacks) for k
consecutive key updates, assuming the TGKs and RAND are each n bits
long, is about L = n*(k+1)/2 bits, compared to the theoretical
maximum of n*k bits. In other words, a 2^L work effort MAY enable an
attacker to get all k keys. While this might seem as a defect, first
note that for proper choice of n, the 2^L complexity of the attack is
way out of reach. Moreover, the fact that more than one key can be
compromised in a single attack is inherent to the key exchange
problematic. Consider for instance a user who, using say a fixed
1024-bit RSA key, exchanges keys and communicates during one or two
years life-time of the public key. Breaking this single RSA key will
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enable access to all exchanged keys and consequently the entire
communication of that user over the whole period.
All the pre-defined transforms in MIKEY use state-of-the-art
algorithms that has undergone large amounts of public evaluation.
9.2. Key lifetime
Even if the lifetime of a TGK (or TEK) is not specified, it MUST be
taken into account that the encryption transform in the underlying
security protocol can in some way degenerate after a certain amount
of encrypted data. It is not possible to here state general key life-
time bounds, universally applicable; each security protocol should
define such maximum amount and trigger a re-keying procedure before
the "exhaustion" of the key. E.g., according to SRTP [SRTP] the TEK
MUST be changed at least every 2^48 SRTP packet (i.e. every time the
ROC + SEQ no in SRTP wraps).
Still, the following can be said as a rule of thumb. If the security
protocol uses an "ideal" b-bit block cipher (in CBC mode, counter
mode, or a feedback mode with full b-bit feedback), degenerate
behavior in the crypto stream, possibly useful for an attacker, is
(with constant probability) expected to occur after a total of
roughly 2^(b/2) encrypted b-bit blocks (using random IVs). For
security margin, re-keying MUST be triggered well in advance compared
to the above bound. See [BDJR] for more details.
For use of a dedicated stream cipher, we refer to the analysis and
documentation of said cipher in each specific case.
9.3. Timestamps
The use of timestamps instead of challenge-response requires the
systems to have synchronized clocks. Of course, if two clients are
not synchronized, they will have difficulties with setting up the
security. The current timestamp based solution has been selected to
allow a maximum of one roundtrip (i.e., two messages), but still
provide a reasonable replay protection. A (secure) challenge-response
based version would require at least three messages. For a detailed
description of the timestamp and replay handling in MIKEY, see
Section 5.4.
Practical experiences of Kerberos and other timestamp based systems
indicate that it is not always necessary to synchronize the terminals
over the network. Manual configuration could be a feasible
alternative in many cases (especially in scenarios where the degree
of looseness is high). However, the choice must be carefully based
with respect to the usage scenario.
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9.4. Identity protection
Identity protection was not a main design goal for MIKEY. Such
feature will add more complexity to the protocol and was therefore
chosen not to be included. As MIKEY is anyway proposed to be
transported over e.g. SIP, the identity may be exposed by this.
However, if the transporting protocol is secured and also provides
identity protection, MIKEY might inherit the same feature. How this
should be done is for future study.
9.5. Denial of Service
This protocol is resistant to Denial of Service attacks in the sense
that a Responder does not construct any state (at the key management
protocol level) before it has authenticated the Initiator. However,
this protocol, like many others, is open to attacks that use spoofed
IP addresses to create a large number of fake requests. This may
e.g., be solved by letting the protocol transporting MIKEY do an IP
address validity test. For example, the SIP protocol can provide this
using the anonymous authentication challenge mechanism (specified in
Section 22.1 of [SIP]).
As also discussed in Section 5.4, the tradeoff between time
synchronization and the size of the replay cache, may be affected in
case of e.g., a flooding type of DoS attack. However, if the
recommendations of using a dynamic size of the replay cache are
followed, it is believed that the client will in most cases be able
to handle the replay cache. Of course, as the replay cache decreases
in size, the required time synchronization is more restricted.
However, a bigger problem during such attack would probably be to
process the messages (e.g., verify signatures/MACs), due to the
computational workload this implies.
9.6. Session establishment
It should be noted that if the session establishment protocol is
insecure there may be attacks on this that will have indirect
security implications on the secured media streams. This however only
applies to groups (and is not specific to MIKEY). The threat is that
one group member may re-direct a stream from one group member to
another. This will have the same implication as when a member tries
to impersonate another member, e.g. by changing its IP address. If
this is seen as a problem, it is RECOMMENDED that a Source Origin
Authentication (SOA) scheme (e.g., digital signatures) is applied to
the security protocol.
Re-direction of streams can of course be done even if it is not a
group. However, the effect will not be the same compared to a group
where impersonation can be done if SOA is not used. Instead, re-
direction will only deny the receiver the possibility to receive (or
just delay) the data.
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10. IANA considerations
This document defines several new name spaces associated with the
MIKEY payloads. This section summarize the name spaces for which IANA
is requested to manage the allocation of values.
IANA is requested to record the pre-defined values defined in the
given sections for each name space. IANA is also requested to manage
the definition of additional values in the future. Unless explicitly
stated otherwise, values in the range 0-240 for each name space
should be approved by the process of IETF consensus and values in the
range 241-255 are reserved for Private Use.
The name spaces for the following fields in the Common header payload
(from Section 6.1) are requested to be managed by IANA:
* version
* data type
* Next payload
* PRF func. This name space is between 0-127 where values between 0-
111 should be approved by the process of IETF consensus and values
between 112-127 are reserved for Private Use.
* CS ID map type
The name spaces for the following fields in the Key data transport
payload (from Section 6.2) are requested to be managed by IANA:
* Encr alg
* MAC alg
The name spaces for the following fields in the DH data payload (from
Section 6.4) are requested to be managed by IANA:
* DH-Group
The name spaces for the following fields in the Timestamp payload
(from Section 6.6) are requested to be managed by IANA:
* TS type
The name spaces for the following fields in the ID payload and the
Certificate payload (from Section 6.7) are requested to be managed by
IANA:
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* ID type
* Cert type
The name spaces for the following fields in the Cert hash payload
(from Section 6.8) are requested to be managed by IANA:
* Hash func
The name spaces for the following fields in the Security policy
payload (from Section 6.10) are requested to be managed by IANA:
* Prot type
From Section 6.10.1.
* SRTP Type
* SRTP encr alg
* SRTP auth alg
* SRTP PRF
* FEC order
The name spaces for the following fields in the Error payload (from
Section 6.12) are requested to be managed by IANA:
* Error no
The name spaces for the following fields in the Key data payload
(from Section 6.13) are requested to be managed by IANA:
* Type. This name space is between 0-16 which should be approved by
the process of IETF consensus.
* KV. This name space is between 0-16 which should be approved by the
process of IETF consensus.
The name spaces for the following fields in the General Extensions
payload (from Section 6.15) are requested to be managed by IANA:
* Type
11. Conclusions
Work for securing real-time applications have started to appear. This
has brought forward the need for a key management solution to support
the security protocol. The key management has to fulfil requirements,
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which make it suitable in the context of conversational multimedia in
a heterogeneous environment and small interactive groups. MIKEY is
designed to fulfill such requirements and optimized so that it also
may be integrated in other protocols such as SIP and RTSP.
MIKEY is designed to be used in scenarios for peer-to-peer
communication, simple one-to-many, and for small-size interactive
groups without a centralized group server.
12. Acknowledgments
The authors would like to thank Mark Baugher, Ran Canetti, Martin
Euchner, the rest of the MSEC WG, Pasi Ahonen (with his group), Rolf
Blom, and Magnus Westerlund, for their valuable feedback.
13. Author's Addresses
Jari Arkko
Ericsson
02420 Jorvas Phone: +358 40 5079256
Finland Email: jari.arkko@ericsson.com
Elisabetta Carrara
Ericsson Research
SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@era.ericsson.se
Fredrik Lindholm
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58531705
Sweden EMail: fredrik.lindholm@era.ericsson.se
Mats Naslund
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@era.ericsson.se
Karl Norrman
Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@era.ericsson.se
14. References
14.1. Normative References
[AES] Advanced Encryption Standard (AES), Federal Information
Processing Standard Publications (FIPS PUBS) 197, November 2001.
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[HMAC] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[KMASDP] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
Norrman, K., "Key Management Extensions for SDP and RTSP", Internet
Draft, Work in Progress (MMUSIC WG).
[NAI] Aboba, B. and Beadles, M., "The Network Access Identifier",
IETF, RFC 2486, January 1999.
[OAKLEY] Orman, H., "The Oakley Key Determination Protocol", RFC
2412, November 1998.
[OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with
SDP", Internet Draft, IETF, Work in progress (MMUSIC).
[RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[SDP] Handley, M., and Jacobson, V., "Session Description Protocol
(SDP), IETF, RFC2327
[SHA1] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
http://csrc.nist.gov/fips/fip180-1.ps
[SIP] Rosenberg, J. et al, "SIP: Session Initiation Protocol", IETF,
RFC3261.
[SRTP] Baugher, M., Blom, R., Carrara, E., McGrew, D., Naslund, M,
Norrman, K., and Oran, D., "The Secure Real Time Transport Protocol",
Internet Draft, IETF, Work in Progress (AVT WG).
[URI] Berners-Lee. T., Fielding, R., Masinter, L., "Uniform Resource
Identifiers (URI): Generic Syntax", IETF, RFC 2396.
[X.509] Housley, R., Polk, W., Ford, W., and Solo, D., "Internet
X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", IETF, RFC 3280.
14.2. Informative References
[BDJR] Bellare, M., Desai, A., Jokipii, E., and Rogaway, P., "A
Concrete Analysis of Symmetric Encryption: Analysis of the DES Modes
of Operation", in Proceedings of the 38th Symposium on Foundations of
Computer Science, IEEE, 1997, pp. 394-403.
[BMGL] Hastad, J. and Naslund, M.: "Practical Construction and
Analysis of Pseduo-randomness Primitives", Proceedings of
Asiacrypt'01, Lecture Notes in Computer Science vol 2248, pp. 442-
459.
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[DBJ] Johnson, D.B., "Theoretical Security Concerns with TLS use of
MD5", Contribution to ANSI X9F1 WG, 2001.
[GKMARCH] Baugher, M., Canetti, R., Dondeti, L., and Lindholm, F.,
"Group Key Management Architecture", Internet Draft, Work in Progress
(MSEC WG).
[GDOI] Baugher, M., Hardjono, T., Harney, H., Weis, B., "The Group
Domain of Interpretation", Internet Draft, Work in Progress (MSEC
WG).
[GSAKMP] Harney, H., Colegrove, A., Harder, E., Meth, U., Fleischer,
R., "Group Secure Association Key Management Protocol", Internet
Draft, Work in Progress (MSEC WG).
[IKE] Harkins, D. and Carrel, D., "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[LV] Lenstra, A. K., and Verheul, E. R., "Suggesting Key Sizes for
Cryptosystems", http://www.cryptosavvy.com/suggestions.htm
[NTP] Mills, D., "Network Time Protocol (Version 3) specification,
implementation and analysis", RFC 1305, March 1992.
[PKCS1] PKCS #1 - RSA Cryptography Standard,
http://www.rsalabs.com/pkcs/pkcs-1/
[RSA] Rivest, R., Shamir, A., and Adleman, L. "A Method for Obtaining
Digital Signatures and Public-Key Cryptosystems". Communications of
the ACM. Vol.21. No.2. pp.120-126. 1978.
[SHA256] NIST, "Description of SHA-256, SHA-384, and SHA-512",
http://csrc.nist.gov/encryption/shs/sha256-384-512.pdf
[TLS] Dierks, T. and Allen, C., "The TLS Protocol - Version 1.0",
IETF, RFC 2246.
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Appendix A. - MIKEY - SRTP relation
The terminology in MIKEY differs from the one used in SRTP as MIKEY
needs to be more general. Therefore it might be hard to see the
relations between keys and parameters generated in MIKEY and the ones
used by SRTP. This section provides some hints on their relation.
MIKEY | SRTP
-------------------------------------------------
Crypto Session | SRTP stream
Data SA | input to SRTP's crypto context
TEK | SRTP master key
The Data SA is built up by a TEK and the security policy exchanged.
SRTP may use a MKI to index the TEK. The TEK is then derived from the
TGK that have the corresponding MKI.
Revision history
Changes from -01 draft:
* Removed: Support for Re-key SA including KEK transport for all
methods.
* Timestamp required explicitly in the verification message
* Renamed R flag in Common header to V (for verification)
* Change of notation
- Pre-Master Key (PMK) --> TEK Generation Key (TGK)
- Multimedia Crypto Session (MCS) --> Crypto Session Bundle (CSB)
- Some payloads have also had their name changed.
- Seed (in the PRF definition) --> Label
* General extensions payload added.
* Possibility to send a TEK only (instead of a TGK) is provided for
pre-encryption purposes.
* General updates of all sections (trying to address all comments
received from the list).
* IANA considerations added
Changes from -02 draft:
* General editorial updates
* Clarifications about replay cache added in Section 5.4
* Clarification about replay/timestamps usage vs DoS added in
Section 9.5
This Internet-Draft expires in March 2003.
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