Status of this Memo

This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668.

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 to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on April 16, 2005.

Copyright Notice

Copyright (C) The Internet Society (2004).

Abstract

With the Timed Efficient Stream Loss-tolerant Authentication protocol (TESLA) a protocol for providing source authentication in multicast scenarios was introduced. A mapping for TESLA to the Secure Real-time Transport Protocol (SRTP) has been published which assumes that some TESLA parameters are made available by out-of-band mechanisms. This document describes payloads for bootstrapping these parameters with the help of the Multimedia Internet KEYing (MIKEY) protocol.

Table of Contents

1.  Introduction
2.  Terminology
3.  TESLA Parameter Overview
4.  Parameter encoding within MIKEY
    4.1  Security Policy payload (SP)
    4.2  TESLA policy
    4.3  Key data transport within MIKEY's General Extension Payload
5.  Security Considerations
6.  References
6.1  Normative References
6.2  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1. Introduction

[I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004. describes extensions for SRTP [RFC3711]Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, The Secure Real-time Transport Protocol (SRTP), March 2004. in order to support TESLA [I-D.ietf-msec-tesla-intro]Perrig, A., Canetti, R., Song, D., Tygar, D. and B. Briscoe, TESLA: Multicast Source Authentication Transform Introduction, August 2004. for source authentication in multicast scenarios. Therefore the cryptographic context needs to be enhanced with a set of TESLA parameters. It is necessary to provide these parameters before the actual multicast session starts. [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004. does not address the bootstrapping for these parameters.

This document details bootstrapping of TESLA using the Multimedia Internet Keying (MIKEY) [RFC3830]Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, MIKEY: Multimedia Internet KEYing, August 2004. protocol. MIKEY defines an authentication and key management framework that can be used for real-time applications (both for peer-to-peer communication and group communication). In particular, [RFC3830]Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, MIKEY: Multimedia Internet KEYing, August 2004. is defined in a way to support SRTP in the first place but is open to enhancements to be used for other purposes too.

The three authentication and key exchange protocols defined in [RFC3830]Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, MIKEY: Multimedia Internet KEYing, August 2004. as well as the fourth protocol provided by [I-D.ietf-msec-mikey-dhhmac]Euchner, M., HMAC-authenticated Diffie-Hellman for MIKEY, May 2004. may be used to provide also the TESLA parameters. The required TESLA parameters to be exchanged are already described in [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004., while this document describes their transport within MIKEY.

The following security requirements have to be placed on the exchange of TESLA parameters:

• Integrity MUST be provided when sending the TESLA parameters, especially for the initial key.
• Confidentiallity MAY be provided for the TESLA parameter

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]Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997..

3. TESLA Parameter Overview

According to [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004. the following transform dependent parameters need to be provided for proper TESLA operation:

1. An identifier for the PRF, f, implementing the one-way function F(x) in TESLA (to derive the keys in the chain), e.g. to indicate HMAC-SHA1.
   
   
2. A non-negative integer n_c, determining the length of the F output, i.e. the length of the keys in the chain (that is also the key disclosed in an SRTP packet).
   
   
3. An identifier for the PRF, f', implementing the one-way function F'(x) in TESLA (to derive the keys for the TESLA MAC, from the keys in the chain), e.g. to indicate HMAC-SHA1.
   
   
4. A non-negative integer n_f, determining the length of the output of F', i.e. of the key for the TESLA MAC.
   
   
5. An identifier for the TESLA MAC, that accepts the output of F'(x) as its key, e.g. to indicate HMAC-SHA1.
   
   
6. A non-negative integer n_m, determining the length of the output of the TESLA MAC.
   
   
7. The beginning of the session T_0,
   
   
8. The interval duration T_int (in msec),
   
   
9. The key disclosure delay d (in number of intervals)
   
   
10. Non-negative integer n_c, determining the length of the key chain, which is determined based up the expected duration of the stream.
    
    
11. The initial key of the chain to which the sender has committed himself.

Section 6.2 in [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004. provides information about the default value for the above-listed parameters.

4. Parameter encoding within MIKEY

As mentioned in Section 3TESLA Parameter Overview, TESLA parameters need to be transported before actually starting a session. MIKEY currently only defines a payload for transporting the SRTP policy (see Section 6.10 of [RFC3830]Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, MIKEY: Multimedia Internet KEYing, August 2004.). This section describes the enhancement of MIKEY to allow the transport of a TESLA policy and additionally the initial TESLA key.

4.1 Security Policy payload (SP)

The Security Policy payload defines a set of policies that apply to a specific security protocol. The definition here relies on the security policy payload definition in [RFC3830]Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, MIKEY: Multimedia Internet KEYing, August 2004..

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload  ! Policy no     ! Prot type     ! Policy param  ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ length (cont) ! Policy param                                  ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           

                    
   *  Next payload (8 bits): 
      Identifies the payload that is added after
      this payload. See Section 6.1 of [RFC3830] for 
      more details.
           

                    
   *  Policy no (8 bits): 
      Each security policy payload must be given a
      distinct number for the current MIKEY session by the
      local peer. This number is used to map a cryptographic session 
      to a specific policy (see also Section 6.1.1 of [RFC3830]).
           

                    
   *  Prot type (8 bits): 
      This value defines the security protocol.
      A second value needs to be defined as shown below:

      Prot type     | Value |
      ---------------------------
      SRTP          |     0 |
      TESLA         |     1 |
           

                    
   *  Policy param length (16 bits): 
      This field defines the total length of the
      policy parameters for the selected security protocol.
           

   *  Policy param (variable length): 
      This field defines the policy for the specific 
      security protocol.      
           

The Policy param part is built up by a set of Type/Length/Value (TLV) payloads. For each security protocol, a set of possible type/value pairs can be negotiated as defined.

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

   *  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.
           

4.2 TESLA policy

This policy specifies the parameters for TESLA. The types/values that can be negotiated are defined by the following table. The concrete default values are taken from [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004., but other values may also be used:

   Type | Meaning                                | Possible values
   ---------------------------------------------------------------
      1 | PRF identifier for f, realising F(x)   | see below
      2 | Length of PRF f output                 | 160
      3 | PRF identifier for f', realising F'(x) | see below
      4 | Length of PRF f' output                | 160
      5 | Identifier for the TESLA MAC           | see below
      6 | Length of TESLA MAC output             | 80 (trunkened)
      7 | Start of session                       | in bytes
      8 | Interval duration T_int (in msec)      | in bytes
      9 | Key disclosure delay d                 | in bytes
      10| Key chain length (numer of intervals)  | in bytes
           

                    
   For the PRF realising F(x), a one byte length is sufficient.  
   The currently defined possible values are:

     TESLA PRF F(x)  | Value
     -----------------------
     NULL            |  0
     HMAC-SHA1       |  1
           

                    
   For the PRF realising F'(x), a one byte length is enough.  
   The currently defined possible values are:

     TESLA PRF F'(x) | Value
     -----------------------
     NULL            |  0
     HMAC-SHA1       |  1
           

                    
   For the TESLA MAC, a one byte length is enough.  
   The currently defined possible values are:

     TESLA MAC       | Value
     -----------------------
     NULL            |  0
     HMAC-SHA1       |  1
           

4.3 Key data transport within MIKEY's General Extension Payload

The General Extensions Payload was defined to allow possible extensions to MIKEY without the need for defining a completely new payload each time. This payload can be used in any MIKEY message and is part of the authenticated/signed data part.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload  ! Type          ! Length                        !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Data                                                          ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           

                
   *  Next payload (8 bits): 
      Identifies the payload following this payload.
           

   *  Type (8 bits): 
      Identifies the type of general payload. MIKEY
      already defines the Values 0 and 1. 
      This document introduces a new value (2). 

      Type          | Value | Comments
      ----------------------------------------------------
      Vendor ID     |     0 | Vendor specific byte string
      SDP IDs       |     1 | List of SDP key mgmt IDs
      TESLA I-Key   |     2 | TESLA initial key   
                 

   *  Length (16 bits): 
      The length in bytes of the Data field.
           

   *  Data (variable length): 
      The general payload data.
           

5. Security Considerations

The security properties of multi-media data in a multicast environment depends on a number of building blocks.

SRTP-TESLA [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004. describes extensions for SRTP [RFC3711]Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, The Secure Real-time Transport Protocol (SRTP), March 2004. in order to support TESLA [I-D.ietf-msec-tesla-intro]Perrig, A., Canetti, R., Song, D., Tygar, D. and B. Briscoe, TESLA: Multicast Source Authentication Transform Introduction, August 2004. for source authentication in multicast scenarios. As such, security considerations described with TESLA (see [PCST]Perrig, A., Canetti, R., Song, D. and D. Tygar, "Efficient and Secure Source Authentication for Multicast", in Proc. of Network and Distributed System Security Symposium NDSS 2001, pp. 35-46, 2001. and [I-D.ietf-msec-tesla-intro]Perrig, A., Canetti, R., Song, D., Tygar, D. and B. Briscoe, TESLA: Multicast Source Authentication Transform Introduction, August 2004.), the TESLA SRTP mapping [I-D.ietf-msec-srtp-tesla]Baugher, M., The Use of TESLA in SRTP, July 2004. and SRTP [RFC3711]Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, The Secure Real-time Transport Protocol (SRTP), March 2004. itself are relevant in this context.

Furthermore, since this document details bootstrapping of TESLA using the Multimedia Internet Keying (MIKEY) [RFC3830]Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, MIKEY: Multimedia Internet KEYing, August 2004. protocol the security considerations of MIKEY are immediately applicable to this document.

As mentioned in Section 1Introduction the TESLA parameters described in Section 3TESLA Parameter Overview MUST be integrity protected and MAY be confidentiality protected. Integrity protection is necessary to avoid a man-in-the-middle adversary to modify parameters to mount a number of attacks. Confidentity protection, if desired, can be provided by a subset of the available MIKEY authentication and key exchange protocols, namely those providing public key encryption and symmetric key encryption. Without confidentiality protection an adversary might be able to learn the parameters later used to secure the end-to-end multi-media communication (if the adversary is located along the signaling path). This might be undesirable in high-security environments. Please note that the initial hash key, which is also one of the TESLA bootstrapping parameters, does not require confidentiality protection due to the properties of a hash chain. This aspect is described in great detail in the respective TESLA documents since hash chains represent a core concept of TESLA.

6. References

6.1 Normative References

6.2 Informative References

Authors' Addresses

Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.

Disclaimer of Validity

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Copyright Statement

Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

Acknowledgment

Funding for the RFC Editor function is currently provided by the Internet Society.