HIPRG H. Tschofenig
Internet-Draft Siemens
Expires: April 26, 2006 F. Muenz
FH-Landshut
M. Shanmugam
TUHH
October 23, 2005
Using SRTP transport format with HIP
draft-tschofenig-hiprg-hip-srtp-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The Host Identity Protocol (HIP) is a signaling protocol which adds a
new layer between the traditional Transport and Network layer. HIP
is an end-to-end authentication and key exchange protocol, which
supports security and mobility in a commendable manner. The HIP base
specification is genralized and purported to support different key
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exchange mechanisms in order to provide confidentiality protection
for the subsequent data traffic. In some cases it might not be
desirable to establish IPsec security associations for protection of
media traffic. This draft explains how keying material and
parameters for usage with the Secure Real Time Protocol (SRTP) can be
established using HIP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. The Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. SRTP in HIP . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.1. Setting up an SRTP Association . . . . . . . . . . . . 7
4.1.2. Rekeying . . . . . . . . . . . . . . . . . . . . . . . 8
5. Parameter and Packet Formats . . . . . . . . . . . . . . . . . 9
5.1. Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Pseudo-random byte-string (RAND) . . . . . . . . . . . . . 9
5.3. Security Policies (SP) . . . . . . . . . . . . . . . . . . 10
5.4. Master Key Identifier (MKI) . . . . . . . . . . . . . . . 12
6. Key management . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
The Host Identity Protocol (HIP) [I-D.ietf-hip-base] provides a way
to separate the dual role of IP (end point identifier and locator) by
adding a new layer between the traditional Network and Transport
layer. This separation helps the end host to achieve mobility,
furthermore, HIP provides better security features (like end-to-end
authentication, confidentiality for the data traffic etc) than other
multi6 proposals [I-D.ietf-hip-multi6].
HIP is based on public key cryptography. All HIP hosts have a
public/private key pair. HIP introduces a new name space called Host
Identity. It is nothing but the public key of an asymmetric key
pair. It provides a rapid exchange of host identities (public keys)
between communicating hosts and (optionally) establishes IPsec SAs to
protect subsequent data traffic. It is a four-way handshake
protocol, which supports end-to-end authentication and the data
traffic may experience IPsec ESP encapsulation.
Transport connections and security associations between the
communicating HIP hosts are bound to the HITs and IP addresses are
used for routing purposes only. Therefore, changes to IP addresses
do not change the connections or associations. So, when any of the
peers move (mobility scenarios), it uses a readdressing mechanism to
update the current location of the peer, thereby supporting mobility
in a seamless manner.
The HIP base exchange provides mutual authentication of the hosts,
but does not specify any mechanism for protecting data packets.
[I-D.ietf-hip-esp] draft proposes a way to use IPsec ESP format with
HIP.
Secure Real Time Protocol (SRTP) is a profile for Real Time Protocol
(RTP), which provides a framework for providing encryption,
integrity, message authentication, confidentiality and protection
against replay attacks for the real-time data traffic.
SRTP mandates the use of a external key management protocol to
exchange keys and cryptographic parameters, which are used to derive
keys (like cipher suites, random number etc.,). This draft proposes
a way to exchange the SRTP relevant parameters during the HIP base
exchange. Besides this, we inherited the key derivation procedure of
SRTP to show how the keys will be manipulated and maintained for the
data traffic. Appendix A describes one possible use case to support
this document.
This document is organized as follows. Section 3 explains the
revised base exchange, Section 4 explains the rekeying scenario,
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Section 5 presents the packet format and Section 6 explains the key
derivation, and future work.
This document was developed in the context of investigating the
benefits of using HIP for SIP.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This draft uses the terminology defined in [I-D.ietf-hip-base] and
[RFC3261].
The term MKI, an optionl parameter, refers to Master Key Identifier
used in SRTP packets.
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3. Goal
The HIP base exchange is used to set up a HIP association between two
hosts. The base exchange provides two-way host authentication and
key material generation, but it does not provide any means for
protecting data communication between the hosts. In this document,
we specify the use of SRTP for protecting user data traffic after the
HIP base exchange. Note that we did not consider the key management
issues in this draft.
To facilitate the use of SRTP, the HIP base exchange messages require
some minor additions to the parameters transported. In the R1
packet, the responder adds the possible KEYING Parameter before
sending it to the Initiator. The Initiator gets the proposed
transforms, selects one of those proposed transforms, and adds it to
the I2 packet in the corresponding KEYING Parameter.
In this context, the goal of our proposal is to,
o define new parameter exchange for the relevant SRTP parameters.
o define the relevant packets structure and parameters.
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4. The Protocol
In this section, the protocol for setting up an SRTP association to
be used with HIP association is described.
4.1. SRTP in HIP
4.1.1. Setting up an SRTP Association
Setting up an SRTP Association between hosts using HIP consists of
two messages passed between the hosts. The parameters are included
in R1 and I2 messages during base exchange.
Initiator Responder
I1
---------------------------------->
R1: T, KEYING PARAMS
<----------------------------------
I2: T, KEYING PARAMS
---------------------------------->
R2
<----------------------------------
The integration of HIP and SRTP requires some changes, as mentioned
earlier, in the HIP parameters. The changes are (will be) adding,
T: The timestamp, used mainly to prevent replay attacks.
KEYING parameter contains
RAND: Random/pseudo-random byte-string, RAND(nonce) is used as a
fresh value for the key generation.
SP: The security policies for the data security protocol. (eg.
Algorithms and transforms and PRFs supported by the peers). The
cipher suites can be negotiated from R1/I2 packet.
MKI : to identify the Master key and Master salt.
The R1 message contains the KEYING PARAMS, in which the sending host
defines the possible Algorithms and transforms, random number and
optionally MKI it is willing to use for the SRTP association.
The I2 message contains the response to an KEYING PARAMS received in
the R1 message. The sender must select one of the proposed
transforms from the SP parameter in the R1 message and include the
selected one in the SP parameter in the I2 packet. In addition to
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the transform, the host includes the RAND parameter, containing the
random value (and optionally MKI) to be used as a salt by the peer
host. In the R2 message, HIP exchange is finalized.
4.1.2. Rekeying
Rekeying can be supported using the UPDATE packet of HIP. The peer
which wants to rekey should use the UPDATE packet with the
appropriate parameters. The mechanism is explained below:
Initiator Responder
UPDATE: KEYING PARAMS [, DIFFIE_HELLMAN]
----------------------------------------------->
UPDATE: KEYING PARAMS [, DIFFIE_HELLMAN]
<-----------------------------------------------
Fig 2:Rekeying mechanism
Figure 2 depicts the rekeying scenario. Here, assume that the
Initiator wants to rekey after the Initial exchange. It can send the
rekeying parameters in the Update packet. The same mechanism is
followed here, the Initiator chooses its Diffie-Hellmann value and
sends it to the Responder. It may send a new MKI value to identify
the incoming packet.
The other parameters are explained in [I-D.ietf-hip-base]. The
Responder checks the return routability by sending the Update seq
message containing its Diffie-Hellmann value and relevant parameters
for the rekeying. After receiving the packet, the Initiator sends
the ACK thereby both the peers concluding the rekeying procedure and
now, both of the peers expect to receive the traffic in the new
keying material.
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5. Parameter and Packet Formats
This section explains the relationship between the SRTP and KEYING
parameter and presents the proposed packet format.
Master Key - derived from Diffie-Hellmann value
Master Salt - RAND in the KEYING parameter
MKI - Master Key Identifier
Master Key and its length - obtained from Diffie-Hellmann key
exchange
Session keys are derived using Master key, Master salt and SP and the
details are up to the key managament protocol.
As discussed previously, KEYING parameters contains four element:
5.1. Timestamp
The timestamp, used mainly to prevent replay attacks. Like in the
SRTP packet format a 32-bit value is used to store the timestamp.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (T) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 4: Timestamp parameter
Type: 40001 (experimental identifier range)
Length: 4
Value: Timestamp
5.2. Pseudo-random byte-string (RAND)
The RAND or master salt parameter is used as a fresh value for the
key generation. The RAND parameter is a 112 bit quantity.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Pseudo-random byte-string (RAND) |
| |
| |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 5: Pseudo-random byte-string parameter
Type: 40002 (experimental identifier range)
Length: 14
Value: Pseudo-random byte-string
5.3. Security Policies (SP)
The security policies for the data security protocol. (eg. algorithms
and transforms and PRFs supported by the peers). The cipher suites
can be negotiated from I2/R2 packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Security policy parameters ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 6: Security policy parameters parameters
Type: 40004 (experimental identifier range)
Length: variable
Value: See below
The security policy parameters themselves are built up by a set of
Type/Length/Value fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value ~
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (8 bits): specifies the type of the parameter.
Length (8 bits): specifies the length of the Value field (in bytes).
Value (variable length): specifies the value of the parameter.
Type | Length | Meaning | Value
-----+--------+----------------------------+--------------------
1 | 1 | SRTP and SRTCP encr transf | see below
2 | 2 | Encr session key length | 128
3 | 1 | SRTP and SRTCP auth transf | see below
4 | 2 | Auth session key length | 160
5 | 2 | Tag length | 80
6 | 4 | SRTP prefix_length | var(default 0)
7 | 1 | Key derivation PRF | see below
8 | 8 | Key derivation rate | var(default 0)
9 | 8 | SRTP-packets-max-lifetime | var
10 | 8 | SRTCP-packets-max-lifetime | var
11 | 1 | Forward Error Control | 2-bits
For the Encryption transforms, a one byte length is enough. The
currently defined possible values are:
SRTP and SRTCP encr transf | Value
---------------------------+------
NULL | 0
AES-CM | 1
AES-F8 | 2
where AES-CM is AES in CM, and AES-F8 is AES in f8 mode [RFC3711].
For the Authentication transforms, a one byte length is enough. The
currently defined possible Values are:
SRTP and SRTCP auth transf | Value
----------------------------+------
NULL | 0
HMAC-SHA-1 | 1
For the Key derivation PRF, a one byte length is enough. The
currently defined possible values are:
Key derivation PRF | Value
------------------------+-------
NULL | 0
AES_CM | 1
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5.4. Master Key Identifier (MKI)
The MKI identifies the master key and master salt from which the
session key(s) were derived that authenticate and/or encrypt the
particular packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ MKI (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 7: SRTP MKI parameter
Type: 40001 (experimental identifier range)
Length: variable
Value: Master Key Identifier (MKI)
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6. Key management
This section explains how the key management scheme can be used for
the data traffic. After the initial base exchange, both peers have
the same master key, salt and agreed crypto transforms (including
pseudo random function). When the application receives the data
traffic after the base exchange, an API is invoked and asks the HIP
daemon for the appropriate key to process the data packet.
The SRTP based key derivation helps to generate the session keys for
both peers, so that they have the same keys in possession for
encrypting/decrypting the incoming packets. It generates three keys
namely encryption key to provide confidentiality for the data
packets, authentication key for providing integrity and salt key for
the AES counter mode. For that, it uses the master key, salt and
crypto transforms together with the packet index.
Figure 6 depicts the example implementation architecture of the
proposed mechanism:
+------------+
-------------+ API | KEY ENGINE |
Application |<------------->| |
| | |
| | |
| | HIP daemon |
| +------------+
|
User space |
-------------+
PF_INET || || PF_RAW
==================||==========||=============
|| ||
Kernel space
+--------------+
| TCP|UDP / IP |
+--------------+
Fig 5: Example Implementation Architecture
Figure 6 depicts the key derivation, for example, when the peer
receives a packet it gets the packet index, MKI, which is used for
identifying the relevant master key and transforms. Then, the key
derivation function, which is explained below, will generate the
required keys.
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packet index ---+
|
v
+-----------+ master +--------+ session encr_key
| ext | key | |---------->
| key mgmt |-------->| key | session auth_key
| (optional | | deriv |---------->
| rekey) |-------->| | session salt_key
| | master | |---------->
+-----------+ salt +--------+
Fig 6: SRTP Key Derivation
For single key derivation (key_derivation_rate = 0), we define x for
later use in calculating keys using PRF and length of PRF bit string
output like shown in the following table:
X ROC || SEQ Usage PRF output length n
---------------------------------------------------------------
0x00 000000000000 SRTP encryption 128 bit
0x01 000000000000 SRTP message auth. 160 bit
0x02 000000000000 SRTP salting key 112 bit
0x03 000000000000 SRTCP encryption 128 bit
0x04 000000000000 SRTCP message auth. 160 bit
0x05 000000000000 SRTCP salting key 112 bit
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PRF_n (master_key, x)
For multiple key derivation (key_derivation_rate = 1,2,...2E24)
x must be calculated according to the following sequence:
r = index / key_derivation_rate
(with "/" defines r = 0 for key_derivation_rate = 0)
with index is a 48-bit concatenation of the 32 bit Roll Over Counter
(ROC) and the 16 bit sequence number of the SRTP packet given in the
SRTP header (ROC||SEQ)
r must be the same length like index, which results in leading zeros.
Next concatenate an 8-bit label for selecting the usage with r
key_id = <label> concatenated with r.
where <label> is one of the following>
0x00 for SRTP encryption
0x01 for SRTP message authentication
0x02 for SRTP salting key
0x03 for SRTCP encryption key
0x04 for SRTCP authentication key
0x05 for SRTCP salting key
Finally, x is calculated by performing key_id XOR master_salt,
where key_id and master_salt are aligned so that their least
significant bits agree (right-alignment).
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7. Security Considerations
The initial keying material is generated using using Diffie-Hellman
procedure. This document extends the usage of UDPATE packet, defined
in the base specification, for rekeying. The hosts may rekey for the
generation of new keying material using Diffie-Hellman procedure.
This mechanism enjoys the security protection provided by base
exchange using HMAC and signature verifications.
In this approach, we have tried to extend the HIP base exchange to
support SRTP based key management scheme. We have listed the
following security mechanisms that are incorporated with this idea:
DoS: This approach enjoys the merits of HIP like resisting cpu and
memory exhaustive DoS attacks by forcing the caller to calculate
the solution for a cryptographic puzzle. This provides only a
basic DoS protection for the callee.
MitM: HIP uses authenticated Diffie-Hellmann key exchange, which
prevents the man-in-the-middle (MitM) attacks.
Eavesdropping : Since the data traffic is encrypted, it is
unreadable for the attackers.
Authentication: Both peers are authenticated using asymmetric key
(signature verification) cryptography assuming that public keys
can be acquired by secure ways.
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8. Acknowledgements
The authors would like to gratefully acknowlege Pekka Nikander and
Jari Arkko for their comments to this document.
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9. References
9.1. Normative References
[I-D.ietf-hip-base]
Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", draft-ietf-hip-base-03 (work in
progress), June 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC3711] Baugher, M., Carrara, E., McGrew, D., Naslund, M., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", March 2004.
9.2. Informative References
[I-D.ietf-hip-esp]
Moskowitz, R., Nikander, P., and P. Jokela, "Host Identity
Protocol", draft-ietf-hip-esp-00 (work in progress),
June 2005.
[I-D.ietf-hip-multi6]
Tschofenig, H. and A. Nagarajan, "Comparative Analysis of
Multi6 Proposals using a Locator/Identifier Split",
October 2004.
[I-D.ietf-hip-sip]
Tschofenig, H., Schulzrinne, H., Henderson, T., Torvinen,
V., Camarillo, G., and J. ott, "Exchanging Host Identities
in SIP", October 2004.
[RFC3261] Schulzrinne, H., Camarillo, G., Rosenberg, J., Peterson,
J., Sparks, R., Handley, M., and E. Schooler, "Session
Initiation Protocol", February 2005.
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Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
Email: Hannes.Tschofenig@siemens.com
Franz Muenz
University of Applied Sciences
Lurzenhof 1
Landshut, Bayern 84036
Germany
Email: franz.muenz@fh-landshut.de
Murugaraj Shanmugam
Technical University Hamburg-Harburg
Schwarzenbergstrasse 95
Harburg, Hamburg 21075
Germany
Email: murugaraj.shanmugam@tuhh.de
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