HIPRG H. Tschofenig
Internet-Draft Siemens
Expires: January 12, 2006 F. Muenz
FH-Landshut
M. Shanmugam
TUHH
July 11, 2005
Using SRTP transport format with HIP
draft-tschofenig-hiprg-hip-srtp-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The Host Identity Protocol is a signaling protocol which adds another
layer to the Internet model and (optionally) establishes IPsec ESP
SAs to protect subsequent data traffic. HIP is an end-to-end
authentication and key exchange protocol, which supports security and
mobility in a commendable manner. This draft explains a Secure Real
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Time Protocol (SRTP) based mechanism for transmission of user data
packets, to be used with the Host Identity Protocol (HIP).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Message Flow . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Base Exchange . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Rekeying . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3 Packet Format . . . . . . . . . . . . . . . . . . . . . . 10
4. Key management . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1 Normative References . . . . . . . . . . . . . . . . . . . 17
6.2 Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
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. Since different
sizes for the public key are possible, it uses the Host Identity Tag
(HIT), which is the hash of the public key, for operational
representation. The HIP header carries HIT (128 bits long), which is
similar to IPV6 addresses.
Transport connections and Security Associations between the
communicating HIP hosts are bound to the HITs only. 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, it uses a readdressing mechanism to update the current
location of the peer, thereby supporting mobility in a seamless
manner.
Session Initiation Protocol (SIP) is an application layer protocol,
which is capable of establishing modifying and terminating sessions
between the hosts. The SIP architecture uses URIs to uniquely
identify (maps) the user agents and has various infrastructure
components like proxy server, redirect server etc., to achieve
personal mobility.
SIP, when combined with RTP, can effectively handle multimedia
applications. SIP can also invite participants to already existing
sessions, such as multicast conferences. Media can be added to (and
removed from) an existing session. SIP relies on other security
protocols like TLS, IPsec, HTTP Digest mechanisms to protect the SIP
traffic.
HIP base exchange [I-D.ietf-hip-base] does not describe any transport
formats for user data. This document proposes extensions to HIP by
exporting the relevant parameters to support other key management
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scheme, like MIKEY. SRTP proposes MIKEY [RFC3830] as a key
management protocol. We propose to use the same key management
scheme in HIP. HIP combined with MIKEY alike scheme can be used for
SRTP as a key management protocol to exchange Master Keys and create
a Cryptographic Context (SRTP RFC chapter 3.2). HIP has to satisfy
the requirements of SRTP (SRTP RFC chapter 7/8) for a key management
protocol and has to support the appropriate cryptographic algorithms
within its transform parameters.
HIP base exchange provides a mutual authentication of the hosts, but
does not specify any mechanism for protecting data packets for the
actual communication. [I-D.ietf-hip-esp] draft proposes a way to use
IPsec ESP format with HIP. In this document, we specify the use of
SRTP for protecting user data traffic after the HIP base exchange.
SRTP mandates the use of a external key management protocol (like
MIKEY) 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.
This document explains the compatibility of HIP and SIP together with
the new KEY management scheme. Section 3 explains the revised base
exchange, and Section 4 explains the key derivation and future work.
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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 used the terminology defined in [I-D.ietf-hip-base] and
[RFC3261].
The term MKI refers to Master Key Identifier used in SRTP packets.
It is similar to SPIs in IPsec.
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3. Message Flow
This section explains the integration of SIP and HIP. The motivation
to combine HIP and SIP is defined in [I-D.ietf-hip-sip]. SIP uses
URIs, which bind to an IP address or a host name. When HIP is used,
SIP headers will make use of HITs instead of IPs i.e, SIP URIs will
be bound to HITs. A HIT is derived from HI (public key), which
identify users/hosts and IP addresses. The resolution of IP from HI/
HITs can be done via DNS or other mechanisms. Also, the HI/HITs can
be exchanged using SIP/SDP mechanism as desribed in [I-D.ietf-hip-
sip].
Initially the caller sends an INVITE message to its proxy server,
assuming that the caller is already connected. Then the caller proxy
server locates the callee proxy server, possibly by performing a
particular type of DNS (Domain Name Service) lookup (DNS SRV record).
DNS will return the HI/HIT of the callee together with one or more IP
addresses of SIP proxies responsible for the callee. After
resolution it forwards the message to a callee proxy server and adds
a new entry in the SIP header (for route record routability). The
callee proxy receiving the INVITE message consults a location
database via a location service to resolve the HIT of the callee to
current IP address. Finally, it forwards the INVITE to the callee.
There are several ways how to combine SIP messages with HIP base-
exchange. SIP and HIP messages could be combined to reduce
roundtrips or can be used separately.The latter will be explained in
detail in this section.
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+---------+ +---------+
+------+ |SIP Proxy| |SIP Proxy| +------+
|Caller| |Caller | |Callee | |Callee|
+------+ +---------+ +---------+ +------+
+---INVITE-------->| | |
| +---INVITE---------->| |
| | +---INVITE-------->|
| | |<--OK-------------+
| |<--OK---------------+ |
|<--OK-------------+ | |
+----------------------ACK-------------------------------->|
+---------+
+------+ |HIP RVS | +------+
|Caller| |Callee | |Callee|
+------+ +---------+ +------+
| | |
+-------------------I1-------------->+-------I1--------->|
|<------------------R1-----------------------------------+
+-------------------I2---------------------------------->|
|<------------------R2-----------------------------------+
| | |
|<==================TCP/UDP Session=====================>|
Figure 1: SIP and HIP Base Exchange
Session establishment works in known ways. First an INVITE is routed
from the caller to the callee using SIP proxies. The callee then
answers with 200 OK and the caller acknowledges with an ACK message
directly to the callee. However in this scenario, the SDP of the SIP
signalling traffic will not include any SRTP parameters (transforms),
which will be decoupled and delegated to HIP. SIP only serves as a
rendezvous protocol for HIP to exchange end-host IP addresses and
negotiate HIP as the used end-to-end authentication and key exchange
protocol.
3.1 Base Exchange
After HIP is chosen there are again two possibilities on how to
proceed. Firstly HIP base-exchange may run directly between
communication partners or secondly the callee might be using HIP
rendezvous server which is shown in figure 1.
As explained in the previous sections, HIP allows the use of other
key management protocols. Figure 2 explains how the new KEYING
parameters fit into the HIP base exchange:
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Initiator Responder
I1: ------Trigger exchange--------------------------------->
R1: <------ puzzle{HI(R),DH(R)}sig(R)------------------------
I2: -{Soln,DH(I), KEYING param.,MKI, enc{HI}keymat }sig(I)->
R2: <------ { KEYING param.,MKI, HMAC }sig(R) ---------------
Fig 2: Base Exchange
The Initiator starts the HIP connection by sending the trigger
message. This message is nothing but two IPs and HITs of the
Initiator and the Responder respectively. The Responder answers with
the R1 packet, the difference between the actual HIP exchange and the
proposed mechanism is the removal of the cipher suites, because the
transforms will be chosen via KEYING parameter. Since we have to
avoid the state creation, it sends a precomputed packet.
Context id = <SSRC, destination HIT/address, destination port> This
triple SHALL uniquely identiies a cryptographic context (SRTP RFC
chapter 3.2.3.). This context id together with MKI will be mapped to
the master key and cipher suites in KEYING management scheme to find
the session keys to process the packet.
Any specific transform parameters needed for the SRTP cryptographic
context will be exchanged by using SP parameter of KEYING parameter
in HIP.
Master Key - derived from Diffie-Hellmann value
Master Salt - RAND in the KEYING parameter
MKI - Master Key Identifier
Upon receiving it , the Initiator solves the puzzle and sends the I2
packet with the Diffie Hellmann value and its KEYING parameter. For
the explanation of KEYING parameter see above. The Initiator's HI is
encrypted by the keying material derived from the master key (Diffie
Hellmann value), so that the responder can also derive the same key
using the negotiated cipher suites and Diffie Hellmann value to
decrypt the HI. The key management and key derivation is up to the
KEY scheme. The whole packet is signed by the Initiator's public
key.
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The Responder receives the packet verifies the solution, derives the
key using the Diffie Hellmann value and the KEYING parameter,
decrypts the HI, using the keying material , and verifies the
Signature. The Responder derives all the encryption and
authentication keys from the Initiator's master (Diffie Hellmann) and
salt key (KEYING parameter RAND). The reason for this is that both
the Initiator and Responder have the same key pairs for providing
confidentiality for the data traffic.
Next, the Responder sends its KEYING parameter , the same time stamp,
random no, the selected cipher suites and HMAC of the whole packet,
the key for HMAC is derived from the Master Key. It sends its MKI to
identify the incoming packet. The Initiator will check the HMAC and
also the Signature to verify the integrity and authenticity of the
packet. After this, the HIP association is established and both the
hosts use their respective master key and it derived keys for
protecting the traffic. The Master Key is 128 bit long, which can be
exchanged using the Diffie Hellmann parameter.
3.2 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 -Update([seq,REA],DH(R),KEYING param., MKI,HMAC)Sig(I) ----->
Seq <-Update seq([ack, REA],DH(I),KEYING param.,MKI,HMAC)Sig(R)-
Update ---------------Update ACK( ack, HMAC)Sig(I)----------------->
ACK
Fig 3:Rekeying mechanism
Figure 3 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. The key for HMAC has been derived from
the old Master key. It also sends a new MKI value to identify the
incoming packet.
The Responder chooses its Diffie Hellmann value, verifies the HMAC
and Signature. The other parameters are explained in [I-D.ietf-hip-
base] draft. The Responder checks the return routability by sending
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the Update seq message containing its 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.
3.3 Packet Format
This section explains the packet format for the KEYING parameter in
more detail.
KEYING parameter contains
T: The timestamp, used mainly to prevent replay attacks.
RAND: Random/pseudo-random byte-string, RAND(nonce) is used as a
freshness value for the key generation (salts).
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.
MKI : It is similar to SPI i.e, to identify the Master key and
also security associations.
Master Key and its length - obtained from Diffie Hellmann key
exchange
session keys is derived using Master key and SP
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encr. transf | Auth transf | Encr length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth Length | tag length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FEC| Reserved | SRTP prefix length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRTP prefix length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| SRTP-packets-max-lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key derivation rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRTCP-packets-max-lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig 4: Key management parameters
Type: 40000 (experimental identifier range)
Length: 256 bit
Value: Type/Meaning | Possible values
-------------------------------------------------------
SRTP and SRTCP encr transf | see below
SRTP and SRTCP auth transf. | see below
tag length | 80
SRTP prefix_length | variable (default 0)
Key derivation PRF | see below
encr session key length | 128
auth session key length | 160
key derivation rate | variable (default 0)
SRTP-packets-max-lifetime | variable
SRTCP-packets-max-lifetime | variable
Forward Error Control | 2-bits
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SRTP and SRTCP encr transf. | Value
---------------------------------
NULL | 0
AES_CM | 1
AES_f8 | 2
SRTP and SRTCP auth transf. | Value
---------------------------------
NULL | 0
HMAC-SHA1 | 1
Key derivation PRF | Value
---------------------------------
NULL | 0
AES_CM | 1
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 5: SRTP MKI parameter
Type: 40001 (experimental identifier range)
Length: variable
Value: Master Key Identifier (MKI)
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4. 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 6: Example Implementation Architecture
Figure 7 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 7: 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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5. Security Considerations
Security is considered throughout this document
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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6. References
6.1 Normative References
[I-D.ietf-hip-base]
Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", draft-ietf-hip-base-02 (work in
progress), February 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
6.2 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.
[RFC3711] Baugher, M., Carrara, E., McGrew, D., Naslund, M., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", March 2004.
[RFC3830] Arrko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing",
draft-ietf-hip-base-02 (work in progress), August 2004.
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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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