Internet Engineering Task Force J. Arkko
MSEC Working Group E. Carrara
INTERNET-DRAFT F. Lindholm
Expires: August 2002 M. Naslund
K. Norrman
Ericsson
February, 2002
MIKEY: Multimedia Internet KEYing
<draft-ietf-msec-mikey-01.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Abstract
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,
which makes it suitable 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.
Arkko, et al. [Page 1]
INTERNET-DRAFT msec-mikey-01 February 2002
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..................................... 9
2.5. Existing solutions...................................... 9
3. Basic Key Transport and Exchange Schemes.................. 9
3.1. Pre-shared key..........................................10
3.2. Public-key encryption...................................10
3.3. Diffie-Hellman key exchange.............................12
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 TEK from PMK...............................15
4.1.5. Generating keys from an envelope/pre-shared key.......16
4.1.6. Generating KEK from a DH-key..........................16
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..........................17
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
4.3. Policies................................................17
4.4. Indexing the Data SA....................................18
4.5. Re-keying and MCS updating..............................18
5. Behavior and message handling.............................19
5.1. General.................................................19
5.1.1. Capability discovery..................................19
5.1.2. Error handling........................................19
5.2. Creating a message......................................19
5.3. Parsing a message.......................................21
5.4. Replay handling.........................................21
5.5. Reliability.............................................22
6. Integration with session establishment protocols..........23
6.1. SDP integration.........................................23
6.2. MIKEY with SIP..........................................23
6.3. MIKEY with RTSP.........................................24
6.4. MIKEY Interface.........................................25
Arkko, et al. [Page 2]
INTERNET-DRAFT msec-mikey-01 February 2002
7. Groups....................................................26
7.1. Simple one-to-"a few"...................................26
7.2. Small-size interactive group............................27
8. Security Considerations...................................27
8.1. General.................................................27
8.2. Key lifetime............................................28
8.3. Timestamps..............................................29
8.4. Identity protection.....................................30
8.5. Denial of Service.......................................30
8.6. Session establishment...................................30
9. Conclusions...............................................30
10. Acknowledgments..........................................31
11. Author's Addresses.......................................31
12. References...............................................31
Appendix A - Payload Encoding................................34
A.1. Common header payload...................................34
A.1.1. SRTP ID...............................................36
A.2. Key data transport payload..............................37
A.3. Envelope data payload...................................38
A.4. DH data payload.........................................38
A.5. Signature payload.......................................39
A.6. Timestamp payload.......................................40
A.7. ID payload / Certificate payload........................40
A.8. Cert hash payload.......................................41
A.9. Ver msg payload.........................................41
A.10. Security Policy payload................................42
A.10.1. SRTPbasic policy.....................................42
A.10.2. SRTPext policy.......................................44
A.10.3. Re-key policy........................................45
A.11. Rand payload...........................................46
A.12. Error payload..........................................46
A.13. Key data payload.......................................47
A.14. Key validity data .....................................48
Appendix B. - Payload usage summary..........................49
Revision History.............................................50
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.
This document describes a key management solution, that address
multimedia scenarios (e.g. SIP calls and RTSP sessions). The focus is
Arkko, et al. [Page 3]
INTERNET-DRAFT msec-mikey-01 February 2002
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: 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 ID: within an MCS unique identifier for the Crypto
Session.
Multimedia Crypto Session (MCS): collection of one or more Crypto
Sessions, which has common Pre-Master Key and security parameters.
Multimedia Crypto Session ID: unique identifier for the MCS.
Security Association (SA): collection of information needed to secure
a Multimedia Crypto Session.
Pre-Master Key (PMK): a bit-string agreed upon by two or more
parties, associated with a SA (and consequently MCS). From the pre-
master 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 MCS's PMK.
Key-encryption key (KEK): a key to be used to protect other keys that
are to be sent between the sender and the receiver.
PMK re-keying: the process of re-negotiating the PMK (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.
H(x): a cryptographic hash function with argument x
Random(): a secure (pseudo-)random number generator
Arkko, et al. [Page 4]
INTERNET-DRAFT msec-mikey-01 February 2002
PRF(k,x): a keyed pseudo-random function
E(k,m): encryption of m with the key k
D(k,m): decryption of m with the key k
Sign(k,m): the signature of message m with key k
PK_x: the public key of x
SK_x: the secret key of x
Cert_x: Certificate of x
k_p: the PMK
[] an optional piece of information
|| 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 being the
most significant.
1.3. Abbreviations
AES Advanced Encryption Standard
CM Counter Mode
DH Diffie-Hellman
DoS Denial of Service
KEK Key-encrypting Key
MAC Message Authentication Code
MIKEY Multimedia Internet KEYing
PK Public-Key
PMK Pre-Master 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
1.4. Outline
Section 2 describes the basic scenario and the design goals that
MIKEY are based on. 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, re-keying, 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.
Arkko, et al. [Page 5]
INTERNET-DRAFT msec-mikey-01 February 2002
As MIKEY may be carried in SDP over SIP and RTSP, Section 6 describes
how to integrate and use MIKEY in these scenarios.
Section 7 focuses on how MIKEY is used in group scenarios.
The Security Considerations section (Section 8), gives a deeper
explanation on different security related topics.
All definitions of the payloads in MIKEY are described in Appendix A
and Appendix B includes a list of when the payloads MUST/MAY be used.
2. Basic Overview
2.1. Scenarios
MIKEY is intended to be used for peer-to-peer, simple one-to-many,
and small-size (interactive) groups. One of the main multimedia
scenarios is 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 multimedia
streams (e.g. SRTP streams).
We identify in the following some typical scenarios which involve the
multimedia applications we are dealing with (see also Figure 1.1.).
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
groups where each party may set up the security for its own
outgoing media.
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.
MIKEY addresses the peer-to-peer case, one-to-many (one-to-"a few")
and small-size interactive groups.
Arkko, et al. [Page 6]
INTERNET-DRAFT msec-mikey-01 February 2002
peer-to-peer/ many-to-many many-to-many
one-to-many (distributed) (centralized)
++++ ++++ ++++ ++++ ++++
|. | |A | |B | |A |---- ----|B |
--| ++++ | |----------| | | | \ / | |
++++ / ++|. | ++++ ++++ ++++ (S) ++++
|A |---------| ++++ \ / |
| | \ ++|B | \ / |
++++ \-----| | \ ++++ / ++++
++++ \|C |/ |C |
| | | |
++++ ++++
Figure 1.1: Examples of the four scenarios: peer-to-peer, one-to-
many, many-to-many without centralized server, and many-to-many with
a centralized server.
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 round-trips.
* Tunneling. Possibility to "tunnel" MIKEY in session establishment
protocols (e.g. SIP and RTSP).
* Independent of specific security functionality of the underlying
transport.
2.3. System Overview
One objective of MIKEY is to produce Data security protocol SA (Data
SA), including a traffic-encrypting key (TEK), which then can be used
as key input to a Security Protocol. MIKEY can also be used to
distribute a Group Re-key SA, including a key-encrypting key (KEK). A
re-key SA can be used as input for an external group re-key protocol
(see also [GKMARCH] for more information about group re-keying).
The procedure of setting up a Multimedia Crypto Session (MCS) and
creating a TEK (and Data SA), is done in accordance to Figure 2.1.:
Arkko, et al. [Page 7]
INTERNET-DRAFT msec-mikey-01 February 2002
1. A set of security parameters and Pre-Master Key(s) (PMK) are
created for the Multimedia Crypto Session (this is done by one of
the three alternative key transport/exchange mechanisms, see
Section 3).
2. The PMK(s) is used to derive (in a cryptographically secure way) a
TEK for each Crypto Session.
3. The TEK, together with the security protocol parameters represent
the Data SA, which is used as the input to the Security Protocol.
+-----------------+
| MCS | +-----------------+
| Key transport | | External Group |
| /exchange |--> Re-key SA -->| Re-key protocol |
+-----------------+ +-----------------+
| :
| PMK :
v :
+----------+ :
CS ID ->| TEK | : Security Protocol
|derivation| : Parameters
+----------+ :
TEK | :
v v
Data SA
|
v
+-------------------+
| Crypto Session |
|(Security Protocol)|
+-------------------+
Figure 2.1. Overview of the key management procedure.
The security protocol MAY 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.
Re-keying may be done by an external group re-key protocol using a
Re-key SA (in accordance to the group key management architecture
[GKMARCH]). However, a separate re-key protocol may be most useful
for large scale groups. MIKEY can be used to update the TEKs without
an external re-key protocol. This is then done by executing the
transport/exchange phase once again to derive a new PMK (and
consequently the TEKs).
Arkko, et al. [Page 8]
INTERNET-DRAFT msec-mikey-01 February 2002
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
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. Note that e.g. in a SIP-
initiated call, the sender may also be a receiver. As MIKEY address
small interactive groups, a member may dynamically change between
being a sender and receiver (or being both).
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 more suitable for demanding cases such as real-
time data over heterogeneous networks, and small interactive groups.
3. Basic Key Transport and Exchange Schemes
The following sections define three different ways to transport/
exchange a Pre-Master Key: with the use of a pre-shared key, public-
key encryption, and Diffie-Hellman (DH) key exchange. The two first
methods will be denoted key transport. In the following it is for
simplicity assumed unicast communication. In addition to the PMK, a
random "nonce", denoted Rand, is also transported. In all three
cases, the PMK and Rand values are then used to derive TEKs as
described in Section 4.1.4.
Note that in general, keys for encryption and signing should be
different, though for simplicity we use the same notation for both.
Note also that in the following protocol definitions, things like
security protocol parameters, headers etc., have intentionally been
left out. In practice, the messages sent are constructed by a set of
payloads (see Appendix A), wherein the different parameters may be
fitted. The signature/MAC is then computed over the entire message
(not only the specific values that are shown in the protocol
definition).
Arkko, et al. [Page 9]
INTERNET-DRAFT msec-mikey-01 February 2002
3.1. Pre-shared key
The pre-shared key case is done according to Figure 3.1. One or more
Pre-Master Keys (PMKs) are randomly and independently chosen by the
initiator together with zero or one randomly and independently chosen
KEK. These are then encrypted with the pre-shared key and sent to the
responder. A random bit-string, Rand, is added together with a
timestamp, T. The entire message is integrity protected by a Message
Authentication Code (MAC).
The pre-shared secret, 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.
A B
Initialization:
Rand, PMKs, KEK = Random ()
encr_key, auth_key = PRF(s,...||Rand)
Protocol execution:
K = [IDa],T, Rand, E(encr_key,PMKs[||KEK])
A = MAC(auth_key,K)
K, A
---------------------->
auth_key = PRF(s,..||Rand)
V=MAC(auth_key,IDa||IDb||T),[IDb]
[V]
<----------------------
Figure 3.1. Pre-shared key based transport mechanism.
Authentication of the peers is provided by the MAC(s). The responder
MAY return (if requested by Initiator) the verification message, V.
The verification message is created by applying the MAC function with
an authentication key on the IDs and timestamp.
As will be seen, 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.
3.2. Public-key encryption
Public-key cryptography can be used to create a scalable system. A
disadvantage with this approach is that it is more resource consuming
Arkko, et al. [Page 10]
INTERNET-DRAFT msec-mikey-01 February 2002
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).
A B
Initialization:
Rand, PMKs, KEK = Random ()
encr_key, auth_key = PRF(env_key,...||Rand)
Protocol execution:
I=(IDa|Cert_A)
O=E(encr_key,IDa||PMKs[||KEK])
P=MAC(auth_key,O)
K=E(PK_b,env_key),
O, P, T, Rand
[, I]
[, H(Cert_B)]
S=Sign(SK_a,H(K))
K,S
---------------------->
{retrieve env_key using SK_b}
auth_key = PRF(env_key,...||Rand)
V=MAC(auth_key,IDa||IDb||T),[IDb]
[V]
<----------------------
Figure 3.2. Key transport using public keys.
The key transport mechanism is according to Figure 3.2. The initiator
encrypts one or more PMKs, the IDa, and optionally a KEK. The
encrypted keys MUST also be integrity protected. The keys for
encryption (encr_key) of the keys and the MAC (auth_key) are derived
from an "envelope" key (see Section 4.1.5). The envelope key is then
encrypted using the responder's public key (which the initiator
already has). While any public key techniques could be used, proposed
encryption and signature transforms are described in Section 4.2. We
also refer to Section 4.2 for key-encryption algorithm and MAC
definitions.
The Initiator creates a message consisting of the encrypted PMKs and
KEK, a timestamp, a Rand, and optionally its ID/Certificate and a
hash of the certificate used to encrypt the envelope key. The entire
message is finally signed and sent to the responder.
Arkko, et al. [Page 11]
INTERNET-DRAFT msec-mikey-01 February 2002
As mentioned, the initiator MAY include a hash of the certificate of
the public key used to encrypt the envelope key, env_key. The
responder MUST then use the private key corresponding to the
specified certificate to decrypt the encrypted envelope key.
The responder MAY send a verification message, V, (as in the pre-
shared case) to the initiator. This message uses a MAC (e.g. HMAC),
with an authentication key, derived from the PMK according to Section
4.1.4.
It is possible to cache the envelope key, so that it can be used as a
pre-shared key. It is not recommended that this key should be cached
indefinitely (however it is up to the local policy to decide this).
This function may be very convenient during a Multimedia Crypto
Session, if a new crypto session needs to be added (or an old on
removed). Then, the pre-shared key can be used, instead of the public
keys (see also Section 4.5.).
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:
- party A typically has to find the certificate of B in order to
send the first message. If A doesn't have B's certificate
already, this may involve one or more roundtrips to a central
directory agent.
- it will be possible for A to omit its own certificate and rely on
B getting this certificate using other means. However, we
recommend doing this, only when it is reasonable to assume that
B can be expected to have cached the certificate from a previous
connection. Otherwise accessing the certificate would mean
additional roundtrips for B as well.
- 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
The possibility of using a Diffie-Hellman (DH) key exchange method is
also offered. Though, this approach in general has a higher resource
consumption (both computationally and in bandwidth) than the previous
ones. With this method only one key is created, i.e. the DH-key. This
may then be used either as a PMK or (indirectly) as a KEK.
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
Arkko, et al. [Page 12]
INTERNET-DRAFT msec-mikey-01 February 2002
parameters are given in Section 4.2.7. The other transforms below are
described in Section 4.2.
A B
Initialization:
Rand, x = Random () y = Random ()
Protocol execution:
I = (IDa|Cert_A)
K = g^x, T, Rand [,I]
S = Sign (SK_a,H(K))
K,S I' = (IDb|Cert_B)
-----------------> K' = g^y,T,IDa,g^x [,I']
S' = Sign (SK_b,H(K'))
K',S'
<-----------------
PMK=g^(xy) PMK=g^(xy)
Figure 3.3. Diffie-Hellman key based exchange, where x and y are
randomly chosen respectively by A and B.
The key exchange is done according to Figure 3.3. The initiator
chooses a random value x, and sends a signed message including g^x, a
Rand, and a timestamp to the responder (optionally also including its
certificate or identity).
The group parameters (e.g., the group G) are a set of parameters
chosen by the initiator. The responder chooses a random positive
integer y, and sends a signed message including g^y and the timestamp
to the initiator (optionally also providing its certificate). The
signature must also cover the Initiator's id and the g^x value.
Both parties then calculate the PMK, g^(xy). The authentication is
due to the signing of the DH values (and identities), and is
necessary to avoid man-in-the-middle attacks.
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.
This approach is the most expensive approach. It requires that both
sides compute one signature, one verification and two DH-
exponentiations.
Arkko, et al. [Page 13]
INTERNET-DRAFT msec-mikey-01 February 2002
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 should be
used to derive:
* TEKs from a PMK and the Rand,
* a KEK from the DH-key and the Rand,
* encryption, authentication, or salting key from a pre-shared/
envelope key and the Rand.
4.1.1. Assumptions
We assume that the following parameters are in place (to be exchanged
as security parameters, in connection to the actual key exchange):
PMK: a Pre-Master Key, which MUST be random and kept secret. Note
that there may be more than one PMK transported.
The following parameter MAY be sent in the clear:
mcs_id: Master Crypto Session 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.
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.
seed: a specific seed, 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.
4.1.2. Notation
Let HMAC be the SHA1 based message authentication function, see
[HMAC,SHA1]. Similar to [TLS], define:
P (s, seed, m) = HMAC (s, A_1 || seed) ||
HMAC (s, A_2 || seed) || ...
HMAC (s, A_m || seed)
Arkko, et al. [Page 14]
INTERNET-DRAFT msec-mikey-01 February 2002
where
A_0 = seed,
A_i = HMAC (s, A_(i-1)).
While this is the default, HMAC using other hash function MAY be
used, see Section 4.2.1.
4.1.3. PRF Description
The following procedure describes a pseudo-random function, denoted
PRF(inkey,seed), 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,seed) = P(s_1,seed,m) XOR P(s_2,seed,m) XOR ...
XOR P(s_n,seed,m).
4.1.4. Generating TEK from PMK
The key derivation method should be executed with the following
parameters:
inkey: PMK
seed: 0x2AD01C64 || cs_id || mcs_id || Rand
outkey_len: length of the output TEK.
Note, the cs_id is the id of the cs_id the TEK is supposed to be
derived for.
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.
Note that the 32-bit constant integers (i.e. 0x2AD01C64 and the once
replacing it) is taken from the decimal digits of e (i.e. 2.7182...),
Arkko, et al. [Page 15]
INTERNET-DRAFT msec-mikey-01 February 2002
and where each constant consist of nine decimals digits (e.g. the
first nine decimal digits 718281828 = 0x2AD01C64).
4.1.5. Generating keys from an envelope/pre-shared key
inkey: the envelope key or the pre-shared key
seed: 0x150533E1 || 0xFF || mcs_id || Rand (for encryption key)
or
0x2D22AC75 || 0xFF || mcs_id || Rand (for auth. key)
or
0x29B88916 || 0xFF || mcs_id || Rand (for salting key)
outkey_len: desired length of the authentication/encryption/salting
key.
4.1.6. Generating KEK from a DH-key
inkey: DH-key
seed: 0x39A2C14B || 0xFF || mcs_id || Rand
outkey_len: desired length of the KEK.
4.2 Pre-defined Transforms and Timestamp Formats
This section identifies standard transforms for MIKEY. The following
transforms SHALL be used in the respective case. New transforms MAY
be added in the future. It is however recommended to be sparse with
extensions as it usually only creates interoperability problems
between old and newer versions.
4.2.1 Hash functions
MIKEY SHALL use one of the following hash function: SHA-1 (see
[SHA1], MD5 (see [MD5]), SHA256, SHA384, or SHA512 (see [SHA256] for
the last three). SHA-1 is default and the only mandatory to implement
and support.
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].
For the key derivations, the PRF specified in Section 4.1. MUST be
supported. This PRF MAY be extended by using SHA-256 or SHA-512,
instead of SHA-1.
Arkko, et al. [Page 16]
INTERNET-DRAFT msec-mikey-01 February 2002
4.2.3 Key data transport encryption
The default and mandatory-to-support key transport encryption is AES
in counter mode, as defined in [SRTP, Section 4], using a key as
derived in Section 4.1.5, and using initialization vector
IV = [S XOR (0x0000 || MCS ID || T)] || 0x0000,
where S is a 112-bit salting key, also derived as in Section 4.1.5,
and where T is the timestamp.
Note: this restricts the maximum size of the transported key to 2^23
bits, which is still enough for all practical purposes.
4.2.4 MAC and Verification Message function
MIKEY SHALL use 160-bit authentication tags, generated by HMAC with
SHA-1 as the default and mandatory to implement method, see [HMAC].
Authentication keys SHALL be derived according to Section 4.1.5.
4.2.5 Envelope Key encryption
When RSA is used for the envelope encryption, MIKEY SHALL use
RSA/PKCS#1, see [PKCS1].
4.2.6 Digital Signatures
When RSA is used for the signatures, MIKEY SHALL use RSA/PKCS#1, see
[PKCS1]. The default hash function SHALL be SHA-1.
4.2.7 Diffie-Hellman Groups
Diffie-Hellman key exchange SHALL use one of the groups: OAKLEY 5,
OAKLEY 1, or, OAKLEY 2, see [OAKLEY], where OAKLEY 5 is default and
mandatory to support.
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
the counter will overflow approximately every 136th year. It is
RECOMMENDED that the time is always specified in UTC.
4.3. Policies
Included in the message exchange, policies for the Data security
protocol and/or the re-key protocol are transmitted. The policies are
defined in a separate payload and are specific to the security/re-key
protocol (see also Appendix A.10.). Together with the keys, the
Arkko, et al. [Page 17]
INTERNET-DRAFT msec-mikey-01 February 2002
validity period of theses SHOULD also be specified. This could either
be done with an SPI (e.g. when a re-key protocol is used) 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 (or
re-key protocol).
4.4. Indexing the Data SA
The indexing of a Data SA will depend on the security protocol as
different security protocols will have different characteristics. For
SRTP the SSRC (see [SRTP]) is one of those. However, the SSRC is not
sufficient. For the local lookup in the MIKEY SA data base, it is
RECOMMENDED that the MIKEY implementation support a lookup using
destination network address and port together with SSRC. Note that
MIKEY does not send network addresses or ports. One reason to 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 form either SIP or RTSP. MIKEY can then use
these addresses as the index for the Data SA lookup.
4.5. Re-keying and MCS updating
A re-keying mechanism is necessary, e.g. when a key is compromised,
when access control is desired, or simply when a key expires.
Therefore, re-keying MUST be supported to allow a smooth (continuos)
communication. In accordance to the GKMARCH, MIKEY supports the
possibility to use an external group re-key protocol, by the re-key
SA. However, an external group re-key protocol may not be necessary
in a small group. Therefore, it is also possible to update the MCS
(e.g. a TEK or a crypto session parameter) by using MIKEY.
The updating of the MCS is performed by executing MIKEY again e.g.
before a TEK expires, or a new crypto session is added to the MCS.
When MIKEY is executed again to update the MCS, it MAY not be
necessary to include certificates and other information that was
provided in the first exchange, i.e. all parameters that are static
or optional to include.
The new message exchange MUST use the same MCS ID as the initial
exchange, but a new timestamp. A new Rand MUST NOT be included in the
message exchange (the Rand will only have affect in the Initial
exchange). New Crypto Sessions may be added if desired in the update
message. Therefore, the new MIKEY message does not need to contain
keys.
As explained in Section 3.2., the envelope key may be "cached" as a
pre-shared key. If so, the "update message" SHOULD be a pre-shared
key message, not a public key message. If the public key message is
Arkko, et al. [Page 18]
INTERNET-DRAFT msec-mikey-01 February 2002
used, but the envelope key was 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.
A Multimedia Crypto Session MAY contain several Crypto Sessions. A
problem that then MAY occur is to synchronize the re-keying if an SPI
is not used. It is therefore recommended that an SPI is used, if more
than one Crypto Session is used.
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 tries to guess the responder's capabilities in terms of
security algorithms etc. If the guess is wrong, then the responder
may send back its own capabilities (negotiation) to let the initiator
choose a common set of parameters. Multiple attributes may be
provided in sequence. This is done to reduce the number of roundtrips
as much as possible.
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.).
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
Arkko, et al. [Page 19]
INTERNET-DRAFT msec-mikey-01 February 2002
payload, Security Protocol payload). The defined payloads and the
exact encoding of each payload are described in Appendix A.
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 example.
The process of generating a message consists of the following steps:
* Create a master payload starting with the Common header payload.
* Concatenate necessary payloads to the master payload (Appendix B
lists which payloads MUST/MAY be used for the different messages).
* As a last step (for messages that must be authenticated, this also
include the verification message), concatenate the payload
containing the MAC/signature, where the MAC/signature field is
initiated with zeros.
* Calculate the MAC/signature over the entire master payload and
update the MAC/signature field with the MAC/signature. In the case
of the verification message, the IDa || IDb || T MUST follow
directly after the master payload in the MAC calculation.
Note that all messages from the Initiator MUST use a new timestamp!
Arkko, et al. [Page 20]
INTERNET-DRAFT msec-mikey-01 February 2002
5.3. Parsing a message
In general, parsing 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. Also check the replay cache so that the message is not
replayed (see also Section 5.4).
* Extract ID and authentication algorithm (if not included, assume
default one).
* Verify the MAC/signature.
* If the authentication is NOT successful, an Auth failure Error
message MUST be sent to the initiator (if SIP is used, this should
be signaled to SIP as a rejection of the offer). The message MUST
then be discarded from further processing, and the event SHOULD be
logged.
* If the authentication is successful, the message SHOULD be
processed. Though how it is processed is implementation specific.
* If any unsupported parameters or errors occur during the
processing, these SHOULD be reported to the Initiator by sending an
error message. The processing SHOULD then be aborted. The error
message MAY also include payloads to describe the supported
parameters. If SIP is used, this should be signaled to SIP as a
rejection of the offer (see also Section 6.2.).
* If needed, a verification/response message is created and sent to
the Initiator.
5.4. Replay handling
* Each Responder MUST utilize a replay cache in order to remember the
messages presented within the allowable clock skew (see also
Section 8.3., timestamp considerations).
* Replayed messages MUST NOT be processed.
* A message SHOULD be deleted from the cache when it is outdated with
respect to the clock skew.
* Due to physical limitations, the replay cache SHOULD be set to
store up to a maximum number of messages (see below for more
details).
Arkko, et al. [Page 21]
INTERNET-DRAFT msec-mikey-01 February 2002
* If the host loses track of the incoming requests (e.g. due to
overload), it MUST reject all incoming requests until the clock
skew interval has passed.
For a client, the maximum number of messages it will recall 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.
The following is a recommendation of how the maximum size of the
replay cache may be calculated:
maxsize = Min (A, e*x) * block_size
where
A: maximum memory blocks possible to allocate (for simplicity: 1
memory block can contain the information from one message)
e: fault-tolerance value (MUST be >1)
x: #max expected messages per "clock skew"
block_size: size of the message to be cached (note that it will
probably not be needed to cache the entire message, instead a hash of
the message and the timestamp might be enough).
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
When MIKEY is integrated with a transporting protocol, the
reliability scheme of the latter may be applied. Otherwise, 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
Arkko, et al. [Page 22]
INTERNET-DRAFT msec-mikey-01 February 2002
6. 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 MUST information.
6.1. 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. The following two SDP
attributes MUST be used by MIKEY.
a=keymgmt-prot:<protocol>
a=keymgmt-data:<data>
The keymgmt-prot attribute indicates the key management protocol.
Therefore, it MUST be set to "MIKEY", i.e.
a=keymgmt-prot:MIKEY
The data part is used to transport the actual key management payload
message. Due to the text based nature of SDP, this part MUST be
base64 encoded to avoid illegal characters but in the same time
avoiding a too large message expansion.
a=keymgmt-data:<base64 encoded data>
Example
| a=keymgmt-prot:MIKEY
| a=keymgmt-data:uiSDF9sdhs727gheWsnDSJD...
MCS < CS 1 < m=audio 49000 RTP/SAVP 98
| a=rtpmap:98 AMR/8000
| CS 2 < m=video 2232 RTP/SAVP 31
In this example the multimedia crypto session consists of two crypto
sessions (one audio stream and one video stream) to be protected by
SRTP (as indicated by the "RTP/SAVP" profile).
6.2. MIKEY with SIP
In a basic SIP call between two parties (see Figure 6.1.), 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
Arkko, et al. [Page 23]
INTERNET-DRAFT msec-mikey-01 February 2002
answerer. SIP complies to the offer/answer model [OFFANS], to which
MIKEY over SIP MUST be compliant with as well.
--------- ---------
|A's SIP| <.......> |B's SIP|
|Server | SIP |Server |
--------- ---------
^ ^
. .
++++ SIP . . SIP ++++
| | <............. ..............> | |
| | | |
++++ <-------------------------------------------> ++++
SRTP
Fig 6.1.: SIP-based call example. The two parties uses 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 message SHOULD be included, and in the answer to the offer,
the MIKEY Responder message SHOULD be included.
If the MIKEY part of the offer is not accepted, a MIKEY error message
SHOULD be provided in the answer (following Section 5.1.). MIKEY MUST
always signal to SIP 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 MCS (e.g. TEK update) SHOULD only 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.).
6.3. 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.
Arkko, et al. [Page 24]
INTERNET-DRAFT msec-mikey-01 February 2002
In an RTSP scenario, the RTSP server and initiator will be the same
entity. The Initiator/RTSP server includes the MIKEY message in a SDP
description. When responding to this, the client uses the 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 chose 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 MCS update procedure, either
the ANNOUNCE message or the SET_PARAMETER message SHOULD be used to
send the updated MIKEY material. A disadvantage of using these, is
that they are not mandatory to implement. Note that the ANNOUNCE
method has the possibility to send SDP descriptions to update
previous ones (i.e. it is not needed to use the RTSP KeyMgmt header).
6.4. MIKEY Interface
The SDP, SIP, and RTSP processing is defined in [KMASDP]. However, it
is necessary that MIKEY can work properly with these protocols.
Therefore, the interface between MIKEY and these protocols MUST
provide certain functionality (however, exactly how the interface
looks like is very implementation dependent).
MIKEY MUST have an interface towards the SIP/SDP or RTSP/SDP
implementation that allows 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 provide to MIKEY.
* MIKEY to receive incoming MIKEY messages. This MUST also include
the possibility to return the status of the incoming message to
SIP/SDP or to RTSP/SDP, i.e. whether the MIKEY message was accepted
or not.
* SIP/SDP or RTSP/SDP to receive information from MIKEY, this include
the receiving the MCS ID, receiving the SSRCs for SRTP. It is also
RECOMMENDED that extra information about errors can be received.
* SIP/SDP or RTSP/SDP to receive outgoing MIKEY messages.
* tearing down a MIKEY MCS (e.g. if the SIP sessions is shutdown, the
MCS SHOULD also be shutdown)
Arkko, et al. [Page 25]
INTERNET-DRAFT msec-mikey-01 February 2002
Note that if a MCS has already been established, it is still valid
for the SIP/SDP or RSP/SDP 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).
7. Groups
What has been discussed up to now is not limited to single peer-to-
peer communication, but can be used in small-size groups and simple
one-to-many scenarios. This section describes how MIKEY is used in a
group scenario.
7.1. Simple one-to-"a few"
++++
|S |
| |
++++
|
--------+-------------- - -
| | |
v v v
++++ ++++ ++++
|A | |B | |C |
| | | | | |
++++ ++++ ++++
Figure 7.1. Simple one-to-many/"a few" scenario.
In the most simple one-to-many/"a few" scenario, a server is
streaming to a small group of clients. In this scenario RTSP or SIP
could be used for the registration and the key management set up. The
streaming server would act 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 PMK to all the clients
(which will result in common TEKs for the group).
Note, if the same PMK/TEK(s) should be used by all the group members,
the streaming server MUST specify the same MCS_ID and CS_ID(s) for
the session to all the group members. Security considerations arising
from using the same key for several streams in the underlying
security protocol MUST be considered.
Arkko, et al. [Page 26]
INTERNET-DRAFT msec-mikey-01 February 2002
7.2. Small-size interactive group
++++ ++++
|A | -------> |B |
| | <------- | |
++++ ++++
^ | | ^
| | | |
| | ++++ | |
| --->|C |<--- |
------| |------
++++
Figure 7.2. Small-size group without centralized controller.
As described in the overview section, for small-size 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 and
the public-key transport methods will be used.
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 may set up the security for its
outgoing stream(s) to the others.
As for the one-to-"a few" case, the streaming client MUST specify the
same MCS_ID and CS_ID(s) for its outgoing sessions if the same
PMK/TEK(s) should be used for all the group members. The same
security considerations for key-sharing also apply.
8. Security Considerations
8.1. 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.
Moreover, if the PMKs 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 PMK
generation.
Arkko, et al. [Page 27]
INTERNET-DRAFT msec-mikey-01 February 2002
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
acceptable as the normal "existential" collision probabilities would
be of secondary importance.
In a Multimedia Crypto Session, the Crypto Sessions (audio, video
etc) share the same PMK 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 Multimedia Crypto Session), 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, we use only one single hash function, whereas TLS
uses two different functions. Note that the use of the Rand nonce in
the key derivation is essential to protect against off-line time/
memory trade-off attacks.
In the pre-shared key and public-key schemes, the PMK 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 scheme MUST be used.
Forward/backward security: if the PMK 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 PMK.
8.2. Key lifetime
Even if the lifetime of a PMK 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. Each security protocol MUST define such maximum
amount and trigger a re-keying procedure before the 'exhaustion' of
the key. For SRTP the key MUST be changed at least for every 2^48
SRTP packet (i.e. every time the ROC + SEQ nr in SRTP wraps).
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
Arkko, et al. [Page 28]
INTERNET-DRAFT msec-mikey-01 February 2002
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.
8.3. Timestamps
Timestamp usage prevents against replay attacks under the following
assumptions:
* Each host MUST have a clock which is at least "loosely
synchronized" to the time of the other hosts.
* If the clocks are to be synchronized over the network, a secure
network clock synchronization protocol MUST be used.
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 ok). 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.
Servers may be the entities that will have the highest work load. It
is also recommended that the servers are the Initiators of MIKEY.
This will result in that the servers will not 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.
Practical experiences of Kerberos and other timestamp based system
indicates 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.
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 round-trip (i.e. two messages), but still
provide a reasonable replay protection. A (secure) challenge-response
based version would require at least three messages.
Arkko, et al. [Page 29]
INTERNET-DRAFT msec-mikey-01 February 2002
8.4. 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.
8.5. 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 be
solved by letting the protocol transporting MIKEY do an IP address
validity test.
8.6. 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 secure media streams. This however only
applies to groups (and is not really that specific to MIKEY only).
The threat is that one group member may re-direct a stream from one
group member to another group member. 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 scheme is applied for
the security protocol.
9. 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,
which make it suitable in the context of conversational multimedia in
a heterogeneous environment and small interactive groups. MIKEY was
designed to fulfill such requirements and optimized so that it also
may be integrated in other protocol 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.
Arkko, et al. [Page 30]
INTERNET-DRAFT msec-mikey-01 February 2002
10. Acknowledgments
The authors would like to thank Mark Baugher, Ran Canetti, the rest
of the MSEC WG, Pasi Ahonen (with his group), Rolf Blom, and Magnus
Westerlund, for their valuable feedback.
11. 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
12. References
[AES] Advanced Encryption Standard, www.nist.gov/aes
[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.
Arkko, et al. [Page 31]
INTERNET-DRAFT msec-mikey-01 February 2002
[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).
[HMAC] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[IKE] Harkins, D. and Carrel, D., "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[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).
[LV] Lenstra, A. K., and Verheul, E. R., "Suggesting Key Sizes for
Cryptosystems", http://www.cryptosavvy.com/suggestions.htm
[MD5] Rivest, R.,"MD5 Digest Algorithm", RFC 1321, April 1992.
[NAI] Aboba, B. and Beadles, M., "The Network Access Identifier",
IETF, RFC 2486, January 1999.
[NTP] Mills, D., "Network Time Protocol (Version 3) specification,
implementation and analysis", RFC 1305, March 1992.
[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).
[PKCS1] PKCS #1 - RSA Cryptography Standard,
http://www.rsalabs.com/pkcs/pkcs-1/
[RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[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.
[SDP] Handley, M., and Jacobson, V., "Session Description Protocol
(SDP), IETF, RFC2327
Arkko, et al. [Page 32]
INTERNET-DRAFT msec-mikey-01 February 2002
[SHA1] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
http://csrc.nist.gov/fips/fip180-1.ps
[SHA256] NIST, "Description of SHA-256, SHA-384, and SHA-512",
http://csrc.nist.gov/encryption/shs/sha256-384-512.pdf
[SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J.,
"SIP: Session Initiation Protocol", IETF, RFC2543.
[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).
[TLS] Dierks, T. and Allen, C., "The TLS Protocol - Version 1.0",
IETF, RFC 2246.
[TMMH] McGrew, D., "The Truncated Multi-Modular Hash Function
(TMMH)", Internet Draft, IETF, Work in Progress.
[URI] Berners-Lee. T., Fielding, R., Masinter, L., "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396
Arkko, et al. [Page 33]
INTERNET-DRAFT msec-mikey-01 February 2002
Appendix A - Payload Encoding
This appendix describes in detail all the payloads. For all encoding,
Network byte order MUST always be used.
Note that everything denoted Mandatory MUST be implemented, and
everything denoted Default MUST be assumed to be selected if nothing
else is stated.
A.1. Common header payload
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.
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 !R! PRF func !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MCS 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. If no more payload follows, it MUST be set to Last
payload.
Arkko, et al. [Page 34]
INTERNET-DRAFT msec-mikey-01 February 2002
Next payload | Value | Appendix
------------------------------
Last payload | 0 | -
Key data trnsp| 1 | A2
Env data | 2 | A3
DH data | 3 | A4
Signature | 4 | A5
Timestamp | 5 | A6
ID | 6 | A7
Certificate | 7 | A7
Cert hash | 8 | A8
Ver msg | 9 | A9
SP | 10 | A10
Rand | 11 | A11
Error | 12 | A12
Key data | 20 | A13
* R: flag to indicate whether a response is expected or not (this has
only meaning when it is set by the Initiator).
R = 0 ==> no response expected
R = 1 ==> response expected
* PRF func: Indicates the PRF function that has been/will be used for
key derivation etc.
Hash 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)
* MCS ID: A 32-bit integer to identify the MCS. It is RECOMMENDED
that it is chosen at random by the Initiator (the Initiator SHOULD
however check for collisions). The Responder MUST use the same MCS
ID in the response.
* #CS: Indicates the number of Crypto Sessions that will be handled.
Note that even though it is possible to use 256 CSs, this may not
always be likely.
* CS ID map type: specifies the method to uniquely map Crypto
Sessions to the security protocol sessions.
CS ID map type | Value | Comments
-------------------------------------
SRTP-ID | 0 | Mandatory
Arkko, et al. [Page 35]
INTERNET-DRAFT msec-mikey-01 February 2002
* 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 A.1.1.).
A.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 nr 1 ! SSRC 1 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SSRC 1 cont. ! ROC 1 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ROC 1 cont. ! Policy nr 2 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SSRC 2 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ROC 2 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Policy nr #CS ! SSRC #CS ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~SSRC #CS (cont)! ROC #CS ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ROC #CS (cont)!
+-+-+-+-+-+-+-+-+
* Policy 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.
* ROC x: Current roll-over 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: A stream using SSRC x will also have Crypto Session ID equal to
x (NOT to SSRC).
Arkko, et al. [Page 36]
INTERNET-DRAFT msec-mikey-01 February 2002
A.2. Key data transport payload
The Key data transport payload contains encrypted Key data payloads.
It may contain one or more Key data payloads each including a PMK or
a KEK. The last Key data payload MUST have its Next payload field set
to Last payload. For 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 MUST be the last payload in the message (note
that the Next payload field MUST be set to Last payload). The MAC is
then calculated over the entire message (as described in Section
5.2.).
If the transport method used is the public-key method, the
Initiator's identity MUST be 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 MUST still be 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, where the MAC field and the Next payload
field have been initiated with zeros.
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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Encr alg: The encryption algorithm used to encrypt the PMK.
Encr alg | Value | Comments
-------------------------------------------
AES-CM-128 | 1 | Mandatory (as defined in Section 4.2.3.)
* Encr len: Length of encrypted part (in bytes).
* Encr data: The encrypted PMK.
* MAC alg specifies the authentication algorithm used.
Arkko, et al. [Page 37]
INTERNET-DRAFT msec-mikey-01 February 2002
MAC alg | Value | Comments
--------------------------------------
HMAC-SHA1-160 | 0 | Mandatory (see Section 4.2.4.)
* MAC: The message authentication code of the entire message.
A.3. Envelope data payload
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.
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 should be cached
Cache for MCS | 2 | The envelope key should be cached, but only
| | to be used for the specific MCS.
* Data len: The length of the data field (in bytes).
* Data: The encrypted envelope key (padding and formatting MUST be
done according to RSA/PKCS#1 if RSA is used).
A.4. DH data payload
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-key len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ DH-value ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Type ! KV ! KV data (optional) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Arkko, et al. [Page 38]
INTERNET-DRAFT msec-mikey-01 February 2002
* next payload: identifies the payload that is added after this
payload.
* DH-Group: identifies the DH group used.
DH-Group | Value | Comments
--------------------------------------
OAKLEY 5 | 0 | Mandatory
OAKLEY 1 | 1 |
OAKLEY 2 | 2 |
* DH-key len: The length of the DH-value field (in bytes).
* DH-value: The public DH-value.
* Type: Indicates the type of the key included in the payload, i.e.
if the resulting DH-key will be used as a PMK or KEK (in the second
case, the DH-key is not used directly as a KEK, but is derived
according to Section 4.1.6). See also Appendix A.13. for pre-
defined values.
* KV: Indicates the type of key validity period specified. This may
be done by using an SPI or by providing an interval in which the
key is valid (e.g. in the latter case, for SRTP this will be the
SEQ nr range where the key is valid). See Appendix A.13. for pre-
defined values.
* KV data: This includes either the SPI or an interval (see Appendix
A.14.). If KV is NULL, this field is not included.
A.5. Signature payload
The Signature payload carries the signature and its related data. The
signature payload MUST always be the last payload in the PK transport
and DH exchange messages.
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 (padding and formatting MUST be done
according to RSA/PKCS#1 if RSA is used).
Arkko, et al. [Page 39]
INTERNET-DRAFT msec-mikey-01 February 2002
A.6. Timestamp payload
The timestamp payload carries the time 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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload. If no more payload follows, it MUST be set to Last
payload. See Appendix A.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.
A.7. ID payload / Certificate payload
The ID payload carries a uniquely-defined identifier.
The certificate payload contains an indicator of the certificate
provided as well as the certificate data.
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. If no more payload follows, it MUST be set to Last
payload. See Appendix A.1. for values.
* ID Type: specifies the identifier type used.
ID Type | Value | Comments
----------------------------------------------
NAI | 0 | Mandatory (see [NAI])
URI | 1 | Mandatory (see [URI])
* Cert Type: specifies the certificate type used.
Arkko, et al. [Page 40]
INTERNET-DRAFT msec-mikey-01 February 2002
Cert Type | Value | Comments
----------------------------------------------
X.509 | 0 | Mandatory
X.509 URL | 1 |
X.509 Sign | 2 | Mandatory
X.509 Encr | 3 | Mandatory
* ID/Cert len: The length of the ID or Certificate field (in bytes).
* ID/Certificate: The ID or Certificate data.
A.8. Cert hash payload
The Cert hash payload contains the hash of the certificate used. The
hash function used MUST be the one specified in the Common header
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 ! Hash func ! Hash ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload.
* Hash func: Indicates the hash function that has been/will be used
(see also Section 4.2.1.).
Hash func | Value
----------------------
SHA-1 | 0
SHA256 | 1
SHA384 | 2
SHA512 | 3
MD5 | 4
* Hash: The hash data. Note: the hash length is implicit from the
hash function used.
A.9. Ver msg payload
The Ver msg payload contains the calculated verification message in
the PS/PK transport. Note that the MAC is calculated over the entire
message as well as the IDs and Timestamp.
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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Arkko, et al. [Page 41]
INTERNET-DRAFT msec-mikey-01 February 2002
* next payload: identifies the payload that is added after this
payload. If no more payload follows, it MUST be set to Last
payload. See Appendix A.1. for values.
* Auth alg specified the authentication algorithm used for the
verification message.
Auth alg | Value | Comments
------------------------------------
HMAC-SHA1-160 | 0 | Mandatory
HMAC-SHA1-160 is HMAC using SHA-1 with a 160-bits tag length.
* Ver data: The verification message data. Note: the length is
implicit from the authentication algorithm used.
A.10. Security Policy payload
The Security Policy payload defines a set of policies that applies to
a specific security/re-key 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 nr ! Prot type ! Policy param ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload. If no more payload follows, it MUST be set to Last
payload. See Appendix A.1. for values.
* Policy nr: Each security policy payload must be given a distinct
number.
* Prot type: defines the security protocol or re-key protocol.
Prot type | Value |
---------------------------
SRTPbasic | 0 | see A.10.1.
SRTPext | 1 | see A.10.2.
Re-key | 2 | see A.10.3.
* Policy param defines the policy for the security/re-key protocol.
A.10.1. SRTPbasic policy
This policy specifies the policy for SRTP and SRTCP. All defined
transform applies to both SRTP and (if used) SRTCP.
Arkko, et al. [Page 42]
INTERNET-DRAFT msec-mikey-01 February 2002
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! encr alg ! encr key len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! auth alg ! auth key len ! auth tag len ! salt key len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SRTP PRF ! Key Der rate !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NOTE: SRTP was not finalized by the date for this draft's submission.
Therefore, these parameters might be an issue for update!
* encr alg specifies the desired encryption algorithm to be used in
SRTP (and SRTCP, if used by SRTP).
encr alg | Value | Comments
------------------------------------------
NULL | 0 | Mandatory
AES-CM-128 | 1 | Mandatory
AES-F8-128 | 2 |
AES-CM-128 is AES in CM with 128-bit block size.
AES-F8-128 is AES in f8 mode with 128-bit block size.
* encr key len: desired session encryption key length in bytes.
* auth alg specifies the desired authentication algorithm to be used.
auth alg | Value | Comments
-------------------------------------------
NULL | 0 | Mandatory
TMMH-16 | 1 | Mandatory
HMAC-SHA1 | 2 | Mandatory
* auth key len: desired session authentication key length in bytes.
* auth tag len: desired length in bytes of the output tag of the MAC.
* salt key len: The desired session salting key length in bytes.
Note: do not mix this with the master salt that are exchanged.
* PRF: Specifies the PRF used.
SRTP PRF | Value | Comments
-------------------------------------------
AES-CM | 0 | Mandatory
* Key Der rate: The 2-logarithm of the desired key derivation rate.
Note that this is possible as the key derivation rate must be a
power of 2 in the range [0..2^16].
Arkko, et al. [Page 43]
INTERNET-DRAFT msec-mikey-01 February 2002
A.10.2. SRTPext policy
This policy separates the SRTP and SRTCP policies.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SRTP EA ! SRTP EKL !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SRTP AA ! SRTP AKL ! SRTP ATL ! SRTP SKL !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SRTxP PRF ! SRTP KDR ! SRTCP EA ! SRTCP EKL !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SRTCP AA ! SRTCP AKL ! SRTCP ATL ! SRTCP SKL !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SRTCP KDR !
+-+-+-+-+-+-+-+-+
* SRTP EA: encryption algorithm for SRTP (see Appendix A.10.1. for
defined ciphers).
* SRTP EAL: encryption key length in bytes for SRTP.
* SRTP AA: authentication algorithm for SRTP (see Appendix A.10.1.
for defined transforms).
* SRTP AKL: authentication key length in bytes for SRTP.
* SRTP ATL: authentication tag length in bytes for SRTP.
* SRTP SKL: salting key length in bytes for SRTP.
* SRTxP PRF: pseudo-random function for SRTP and SRTCP (see Appendix
A.10.1. for defined PRFs).
* SRTP KDR: the 2-logarithm of the key derivation rate for SRTP (see
also Appendix A.10.1).
* SRTCP EA: encryption algorithm for SRTCP (see Appendix A.10.1. for
defined ciphers).
* SRTCP EAL: encryption key length in bytes for SRTCP.
* SRTCP AA: authentication algorithm for SRTCP (see Appendix A.10.1.
for defined transforms).
* SRTCP AKL: authentication key length in bytes for SRTCP.
* SRTCP ATL: authentication tag length in bytes for SRTCP.
Arkko, et al. [Page 44]
INTERNET-DRAFT msec-mikey-01 February 2002
* SRTCP SKL: salting key length in bytes for SRTCP.
* SRTCP KDR: the 2-logarithm of the key derivation rate for SRTCP
(see also Appendix A.10.1).
A.10.3. Re-key policy
The following attributes is supported according to GKMARCH.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KEK alg ! auth alg !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KEK key len ! auth key len !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! mm alg !
+-+-+-+-+-+-+-+-+
* KEK alg: The KEK ENCRYPTION ALGORITHM
KEK alg | Value
-----------------------
NULL | 0
3DES | 1
AES | 2
* auth alg: The AUTHENTICATION ALGORITHM
auth alg | Value
-----------------------
NULL | 0
HMAC-SHA1 | 1
HMAC-MD5 | 2
* KEK key len: The key length of the KEK
* auth key len: The key length of the authentication key
* mm alg: The MEMBERSHIP MANAGEMENT ALGORITHM
mm alg | Value
-----------------------
NULL | 0
LKH | 1
Arkko, et al. [Page 45]
INTERNET-DRAFT msec-mikey-01 February 2002
A.11. Rand payload
The Rand payload consist of a random bit-string. The Rand MUST be
chosen at random and per MCS (note that the if a MCS 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.
A.12. Error payload
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 nr ! Reserved !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* next payload: identifies the payload that is added after this
payload. If no more payload follows, it MUST be set to Last
payload. See Appendix A.1. for values.
* Error nr indicates the type of error that was encountered.
Error nr | Value | Comment
-------------------------------------------------------
Auth failure | 0 | Authentication failure
Invalid TS | 1 | Invalid timestamp
Invalid hash | 2 | PRF function NOT supported
Invalid MA | 3 | MAC algorithm NOT supported
Invalid DH | 4 | DH group NOT supported
Invalid ID | 5 | ID NOT supported
Invalid Cert | 6 | certificate NOT supported
Invalid SP | 7 | SP NOT supported
Invalid SPpar | 8 | SP parameters NOT supported
Arkko, et al. [Page 46]
INTERNET-DRAFT msec-mikey-01 February 2002
A.13. Key data payload
The key data payload contains PMKs and a optionally also a KEK. These
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 TEKs are not sent directly, but a PMK, which is then used to
derive the TEK (or TEKs if there are several crypto sessions).
Type | Value | Comments
-------------------------------------------
PMK | 0 | A Pre-master key (used to derive TEKs from)
PMK+SALT | 1 | A PMK + a salt key are included
KEK | 2 | A Key-encrypting key
* KV: Indicates the type of key validity period specified. This may
be done by using an SPI or by providing an interval in which the
key is valid (e.g. in the latter case, for SRTP this will be the
SEQ nr 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
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).
Arkko, et al. [Page 47]
INTERNET-DRAFT msec-mikey-01 February 2002
* Key data: The PMK data or the KEK 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 Appendix
A.14.). If KV is NULL, this field is not included.
A.14. Key validity data
The Key validity data is not a payload, but part of either the Key
data payload (see Appendix A.13.) or the DH payload (see Appendix
A.4.). The Key validity data gives a guideline of when the key should
be used. This can be done, using an SPI or a lifetime range.
SPI
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 for SRTP).
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, 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.
Arkko, et al. [Page 48]
INTERNET-DRAFT msec-mikey-01 February 2002
* Valid to: Sequence number, 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 PMK should be
specified with either an interval, where the VF/VT length is equal to
6 bytes, or with an SPI (in SRTP denoted as a Master Key Identifier,
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 SPI rather than the From-To method.
Appendix B. - Payload usage summary
Depending on the type of message, different payloads MUST and MAY be
included. There are five distinct types of messages:
* Pre-shared key transport message
* Public key transport message
* Verification message (for either pre-shared key or public key)
* DH exchange message (bi-directional)
* Error message
| Message Type
Payload type | PS | PK | DH | Ver | Error
-------------------------------------------------
Key data trnsp| M M# - - O+
Env data | - M - - -
DH data | - - M# - -
Ver msg | - - - M -
Error | - - - - M
Timestamp | M M M - O
ID | O M M O O
Signature | - M M - O+
Certificate | - O O - -
Cert hash | - O O - -
SP | O O O - O
Rand | M@ M@ M@ - -
# These messages are only mandatory for initial messages, i.e. for an
update message of a MCS these are optional to include (see also
Section 4.5.).
+ These messages may be included to authenticate the error message.
However, before the other peer has been correctly authenticated, it
Arkko, et al. [Page 49]
INTERNET-DRAFT msec-mikey-01 February 2002
is not recommended that the error messages are sent authenticated
(as this would open up for DoS attacks).
@ MUST only be included by the Initiator in the initial exchange.
When a payload is not included, the default values for the
information carried by it SHALL be used (when applicable). The
following table summarizes what messages may be included in a
specific message.
For the encrypted sub payloads in the Key data transport payload, the
following should hold:
| Message Type
Payload type | PS | PK
-----------------------------
Keydata/PMK | O O
Keydata/KEK | O O
ID | - M
Revision history
Changes from -00 draft:
* Support for Re-key SA including KEK transport for all methods.
* PK: Id included in the encrypted part to avoid "impersonation"
attacks.
* PK: Envelope approach for encryption of keys (as the size may
exceed the limit that can be encrypted with one public-key
operation).
* Message processing updated
* SDP, SIP and RTSP considerations updated
* Group section updated
* The use of Rand (instead of require a large and random MCS ID)
* SRTP policies etc updated
* Payload update (to support the above changes)
* general editorial changes
This Internet-Draft expires in August 2002.
Arkko, et al. [Page 50]
