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Network Working Group                                           S. FriesRequest for Comments: 4442                                 H. TschofenigCategory: Standards Track                                        Siemens                                                              March 2006BootstrappingTimed Efficient Stream Loss-Tolerant Authentication (TESLA)Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   TESLA, the Timed Efficient Stream Loss-tolerant Authentication   protocol, provides source authentication in multicast scenarios.   TESLA is an efficient protocol with low communication and computation   overhead that scales to large numbers of receivers and also tolerates   packet loss.  TESLA is based on loose time synchronization between   the sender and the receivers.  Source authentication is realized in   TESLA by using Message Authentication Code (MAC) chaining.  The use   of TESLA within the Secure Real-time Transport Protocol (SRTP) has   been published, targeting multicast authentication in scenarios where   SRTP is applied to protect the multimedia data.  This solution   assumes that TESLA parameters are made available by out-of-band   mechanisms.   This document specifies payloads for the Multimedia Internet Keying   (MIKEY) protocol for bootstrapping TESLA for source authentication of   secure group communications using SRTP.  TESLA may be bootstrapped   using one of the MIKEY key management approaches, e.g., by using a   digitally signed MIKEY message sent via unicast, multicast, or   broadcast.Fries & Tschofenig          Standards Track                     [Page 1]

RFC 4442                  Bootstrapping TESLA                 March 2006Table of Contents1. Introduction ....................................................32. Terminology .....................................................43. TESLA Parameter Overview ........................................44. Parameter Encoding within MIKEY .................................64.1. Security Policy (SP) Payload ...............................64.2. TESLA Policy ...............................................74.3. Time Synchronization .......................................8      4.4. Key Data Transport within MIKEY's General           Extension Payload .........................................105. Security Considerations ........................................115.1. Man-in-the-Middle Attack ..................................115.2. Downgrading Attack ........................................125.3. Denial of Service Attack ..................................125.4. Replay Attack .............................................135.5. Traffic Analysis ..........................................136. IANA Considerations ............................................147. Acknowledgements ...............................................158. References .....................................................168.1. Normative References ......................................168.2. Informative References ....................................16Fries & Tschofenig          Standards Track                     [Page 2]

RFC 4442                  Bootstrapping TESLA                 March 20061.  Introduction   In many multicast, broadcast, and unicast communication scenarios, it   is necessary to guarantee that a received message has been sent from   a dedicated source and has not been altered in transit.  In unicast   communication, commonly a pairwise security association exists that   enables the validation of message integrity and data origin.  The   approach in group-based communication is different, as here a key is   normally shared between the members of a group and thus may not be   used for data origin authentication.  As in some applications a   dedicated identification of a sender is required, there exists the   requirement to support data origin authentication also in multicast   scenarios.  One of the methods supporting this is TESLA [RFC4082].   TESLA provides source authentication in multicast scenarios by using   MAC chaining.  It is based on loose time synchronization between the   sender and the receivers.   [RFC4383] describes extensions for SRTP [RFC3711] in order to support   TESLA [RFC4082] for source authentication in multicast scenarios.   SRTP needs dedicated cryptographic context describing the security   parameter and security policy per multimedia session to be protected.   This 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.  [RFC4383] does not address the   bootstrapping for these parameters.   This document details bootstrapping of TESLA parameters in terms of   parameter distribution for TESLA policy as well as the initial key,   using the Multimedia Internet Keying (MIKEY) [RFC3830] 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] is   defined in a way that is intended to support SRTP in the first place   but is open to enhancements to be used for other purposes too.   Following the description in [RFC3830], MIKEY is targeted for point-   to-point as well as group communication.  In the context of group   communication, an administrator entity can distribute session keys to   the associated entities participating in the communication session.   This scenario is also applicable for TESLA where one entity may   provide information to many others in a way that the integrity of the   communicated information can be assured.  The combination of MIKEY   and TESLA supports this group-based approach by utilizing the MIKEY   framework to distribute TESLA parameter information to all involved   entities.  Note that this document focuses only on the distribution   of the parameters, not on the generation of those parameters.   MIKEY [RFC3830] itself describes three authentication and key   exchange protocols (symmetric key encryption, public key encryption,Fries & Tschofenig          Standards Track                     [Page 3]

RFC 4442                  Bootstrapping TESLA                 March 2006   and signed Diffie-Hellman).  Extensions to the MIKEY key exchange   methods have been defined.  A fourth key distribution method is   provided by [DHHMAC] and describes a symmetrically protected Diffie-   Hellman key agreement.  A further option has been proposed in [RSA-R]   that describes an enhanced asymmetric exchange variant, also   supporting inband certificate exchange.  All the different key   management schemes mentioned above may be used to provide the TESLA   parameters.  The required TESLA parameters to be exchanged are   already described in [RFC4383], while this document describes their   transport within MIKEY.   The following security requirements have to be placed on the exchange   of TESLA parameters:   o  Authentication and Integrity MUST be provided when sending the      TESLA parameters, especially for the initial key.   o  Confidentiality MAY be provided for the TESLA parameters.   These security requirements apply to the TESLA bootstrapping   procedure only.  Security requirements for applications using TESLA   are beyond the scope of this document.  Security aspects that relate   to TESLA itself are described in [RFC4082], and security issues for   TESLA usage for SRTP are covered in [RFC4383].   It is important to note that this document is one piece of a complete   solution.  Assuming that media traffic is to be secured using TESLA   as described in [RFC4383], then (a) keying material and (b)   parameters for TESLA are required.  This document contributes the   parameters and the authentication methods used in MIKEY to provide   the keying material.  The parameter exchange for TESLA also needs to   be secured against tampering.  This protection is also provided by   MIKEY.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 inRFC 2119 [RFC2119].3.  TESLA Parameter Overview   According to [RFC4383], a number of transform-dependent parameters   need to be provided for proper TESLA operation.  The complete list of   parameters can be found inSection 4.3 of [RFC4383].  Note that   parameter 10 of [RFC4383], describing the lag of the receiver clock   relative to the sender clock, is omitted in this document since it   can be computed.Fries & Tschofenig          Standards Track                     [Page 4]

RFC 4442                  Bootstrapping TESLA                 March 2006   MIKEY already requires synchronized clocks, which also provides for   synchronization for TESLA.  Moreover,Section 4.3 states an option to   use MIKEY for clock drift determination between the sender and   receiver.  Thus, this parameter does not need to be transmitted in   MIKEY directly.   The information in brackets provides the default values as specified   inSection 6.2 of [RFC4383].   1.   An identifier for the PRF (TESLA PRF), implementing the one-way        function F(x) in TESLA (to derive the keys in the chain), and        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 the        keyed hash function (default HMAC-SHA1).   2.   A non-negative integer, determining the length of the F output,        i.e., the length of the keys in the chain, which is also the key        disclosed in an SRTP packet if TESLA is used in the SRTP context        (default 160 bit).   3.   A non-negative integer, determining the length of the output of        F', i.e., the length of the key for the TESLA MAC (default 160        bit).   4.   An identifier for the TESLA MAC that accepts the output of F'(x)        as its key, e.g., to indicate a keyed hashing function (default        HMAC-SHA1).   5.   A non-negative integer, determining the length of the output of        the TESLA MAC (default 80 bit).   6.   The beginning of the session for which a key will be applied.   7.   The interval duration (in milliseconds) for which a dedicated        key will be used.   8.   The key disclosure delay (in number of intervals) characterizes        the period after which the key will be sent to the involved        entities (e.g., as part of SRTP packets).   9.   Non-negative integer, determining the length of the key chain,        which is determined based on the expected duration of the        stream.   10.  The initial key of the chain to which the sender has committed        himself.Fries & Tschofenig          Standards Track                     [Page 5]

RFC 4442                  Bootstrapping TESLA                 March 20064.  Parameter Encoding within MIKEY   As mentioned inSection 3, TESLA parameters need to be transported   before actually starting a session.  MIKEY currently only defines a   payload for transporting the SRTP policy (seeSection 6.10 of   [RFC3830]).  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 (SP) Payload   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].    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.  SeeSection 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 alsoSection 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:         (MIKEY already defines the value 0.)         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.Fries & Tschofenig          Standards Track                     [Page 6]

RFC 4442                  Bootstrapping TESLA                 March 2006      *  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 [RFC4383], but other values   may also be used:      Type | Meaning                                | Possible values      ---------------------------------------------------------------         1 | PRF identifier for f and f', realising | see below             F(x) and F'(x)         2 | Length of PRF f' output                | 160         3 | Identifier for the TESLA MAC           | see below         4 | Length of TESLA MAC output             | 80 (truncated)         5 | Start of session                       | in bytes         6 | Interval duration (in msec)            | in bytes         7 | Key disclosure delay                   | in bytes         8 | Key chain length (number of intervals) | in bytes         9 | Local timestamp media receiver         | see below   The time values stated in items 5 and 9 SHALL be transported in NTP-   UTC format, which is one of the three options described inSection6.6 of [RFC3830].  A four-byte integer value for policy item 6 and a   two-byte integer value for policy item 7 are RECOMMENDED, carrying   interval duration and key disclosure delay.  Policy type 9 statedFries & Tschofenig          Standards Track                     [Page 7]

RFC 4442                  Bootstrapping TESLA                 March 2006   above is optional and SHOULD be used if the time synchronization   described inSection 4.3, point two, is used.  Otherwise, it SHOULD   be omitted.      For the PRF realizing F(x) and F'(x), a one-byte length is      sufficient.  The currently defined possible values are:        TESLA PRF F(x), F'(x)  | Value        ------------------------------        HMAC-SHA1              |  0      For the TESLA MAC, a one-byte length is enough.      The currently defined possible values are:        TESLA MAC       | Value        -----------------------        HMAC-SHA1       |  04.3.  Time Synchronization   MIKEY as well as TESLA require the time synchronization of the   communicating peers.  MIKEY requires time synchronization to provide   timestamp-based replay protection for the one-roundtrip   authentication and key exchange protocols.  TESLA, on the other hand,   needs this information to determine the clock drift between the   senders and the receivers in order to release the disclosed key   appropriately.  Two alternatives are available for time   synchronization:   1.  Usage of out-of-band synchronization using NTP [RFC1305].  This       approach is already recommended within [RFC3830].  The advantage       of this approach is the option to use the MIKEY key management       variants that perform within a half-roundtrip.  The disadvantage       is the required time synchronization via an additional protocol.   2.  [RFC4082] also sketches a possible inband synchronization inSection 3.3.1.  This approach is summarized here in the context       of MIKEY.  Note that here the actual TESLA policy payload is       transmitted as part of the MIKEY responder message.       *  The data receiver, which would be the MIKEY initiator, sets          the local time parameter t_r and sends it as part of the          timestamp payload as described in [RFC3830].  This value t_r          needs to be stored locally.       *  Upon receipt of the MIKEY initiator message, the data sender          replies with the MIKEY responder message, setting the local          time stamp at data receiver (parameter 11) to the value t_rFries & Tschofenig          Standards Track                     [Page 8]

RFC 4442                  Bootstrapping TESLA                 March 2006          received in the MIKEY initiator message, and sets his local          time as a 64-bit UTC value t_s in the timestamp payload as          described in [RFC3830].           MIKEY initiator message           [MIKEY parameter incl. local timestamp (t_r)]           ------------------>           MIKEY responder message           [MIKEY parameter incl. local timestamp (t_s), TESLA policy            payload, received local time stamp t_r]           <------------------       *  Upon receiving the MIKEY responder message the data receiver          sets D_t = t_s - t_r + S, where S is an estimated bound on the          clock drift throughout the duration of the session.       This approach has the advantage that it does not require an       additional time synchronization protocol.  The disadvantage is       the necessity to perform a full MIKEY handshake, to enable       correct parameter transport.  Moreover this approach is direction       dependent, as it may only be applied if the media receiver is       also the MIKEY initiator.   Out-of-band synchronization using NTP (i.e., alternative 1) is the   RECOMMENDED approach for clock synchronization.  In scenarios where   the media receiver is also the MIKEY initiator piggybacking timestamp   information in MIKEY (i.e., alternative 2) MAY be used to allow for   inband determination of the clock drift between sender and receiver.Fries & Tschofenig          Standards Track                     [Page 9]

RFC 4442                  Bootstrapping TESLA                 March 20064.4.  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.Fries & Tschofenig          Standards Track                    [Page 10]

RFC 4442                  Bootstrapping TESLA                 March 20065.  Security Considerations   The security properties of multi-media data in a multicast   environment depends on a number of building blocks.   SRTP-TESLA [RFC4383] describes extensions for SRTP [RFC3711] in order   to support TESLA [RFC4082] for source authentication in multicast   scenarios.  As such, security considerations described with TESLA   (see [PCST] and [RFC4082]), the TESLA SRTP mapping [RFC4383], and   SRTP [RFC3711] itself are relevant in this context.   Furthermore, since this document details bootstrapping of TESLA using   the Multimedia Internet Keying (MIKEY) [RFC3830] protocol, the   security considerations of MIKEY are applicable to this document.   As a summary, in order for a multi-media application to support   TESLA, the following protocol interactions (in relationship to this   document) are necessary:   o  MIKEY [RFC3830] is executed between the desired entities to      perform authentication and a secure distribution of keying      material.  In order to subsequently use TESLA, the parameters      described in this document are distributed using MIKEY.  MIKEY      itself uses another protocol for parameter transport, namely, the      Session Description Protocol (SDP) [RFC2327].  SDP might again be      used within Session Initiation Protocol (SIP, [RFC3261]) to set up      a session between the desired entities.   o  After the algorithms, parameters, and session keys are available      at the respective communication entities, data traffic protection      via SRTP-TESLA [RFC4383] can be used.  SRTP-TESLA itself applies      TESLA to the SRTP protocol, and as such the processing guidelines      of TESLA need to be followed.5.1.  Man-in-the-Middle Attack   Threat:      The exchange of security-related parameters and algorithms without      mutual authentication of the two peers can allow an adversary to      perform a man-in-the-middle attack.  The mechanisms described in      this document do not themselves provide such an authentication and      integrity protection.   Countermeasures:      Throughout the document, it is assumed that the parameter exchange      is secured using another protocol, i.e., the exchange parametersFries & Tschofenig          Standards Track                    [Page 11]

RFC 4442                  Bootstrapping TESLA                 March 2006      and algorithms are part of a authentication and key exchange      protocol (namely, MIKEY).  Source authentication of group and      multicast communication cannot be provided for the data traffic if      the prior signaling exchange did not provide facilities to      authenticate the source.  Using an authentication protocol that      does not provide session keys as part of a successful protocol      exchange will make it impossible to derive the necessary      parameters required by TESLA.  MIKEY provides session key      establishment.  Additionally, the exchange of parameters and      algorithms MUST be authenticated and integrity protected.  The      security protection of the parameter exchange needs to provide the      same level or a higher level of security.5.2.  Downgrading Attack   Threat:      The exchange of security-related parameters and algorithms is      always subject to downgrading whereby an adversary modifies some      (or all) of the provided parameters.  For example, a few      parameters require that a supported hash algorithm be listed.  To      mount an attack, the adversary has to modify the list of provided      algorithms and to select the weakest one.   Countermeasures:      TESLA parameter bootstrapping MUST be integrity protected to      prevent modification of the parameters and their values.      Moreover, since unmodified parameters from an unknown source are      not useful, authentication MUST be provided.  This functionality      is not provided by mechanisms described in this document.      Instead, the capabilities of the underlying authentication and key      exchange protocol (MIKEY) are reused for this purpose.5.3.  Denial of Service Attack   Threat:      An adversary might want to modify parameters exchanged between the      communicating entities in order to establish different state      information at the respective communication entities.  For      example, an adversary might want to modify the key disclosure      delay or the interval duration in order to disrupt the      communication at a later state since the TESLA algorithm assumes      that the participating communication entities know the same      parameter set.Fries & Tschofenig          Standards Track                    [Page 12]

RFC 4442                  Bootstrapping TESLA                 March 2006   Countermeasures:      The exchanged parameters and the parameters and algorithms MUST be      integrity protected to allow the recipient to detect whether an      adversary attempted to modify the exchanged information.      Authentication and key exchange algorithms provided by MIKEY offer      this protection.5.4.  Replay Attack   Threat:      An adversary who is able to eavesdrop on one or multiple protocol      exchanges (MIKEY exchanges with the parameters described in this      document) might be able to replay the payloads in a later protocol      exchange.  If the recipients accept the parameters and algorithms      (or even the messages that carry these payloads), then a denial of      service, downgrading, or a man-in-the-middle attack might be the      consequence (depending on the entire set of replayed attributes      and messages).   Countermeasures:      In order to prevent replay attacks, a freshness guarantee MUST be      provided.  As such, the TESLA bootstrapping message exchange MUST      be unique and fresh, and the corresponding authentication and key      exchange protocol MUST provide the same properties.  In fact, it      is essential to derive a unique and fresh session key as part of      the authentication and key exchange protocol run that MUST be      bound to the protocol session.  This includes the exchanged      parameters.5.5.  Traffic Analysis   Threat:      An adversary might be able to learn parameters and algorithms if      he is located along the signaling path.  This information can then      later be used to mount attacks against the end-to-end multimedia      communication.  In some high-security and military environments,      it might even be desirable not to reveal information about the      used parameters to make it more difficult to launch an attack.   Countermeasures:      Confidentiality protection can be provided by a subset of the      available MIKEY authentication and key exchange protocols, namely,      those providing public key encryption and symmetric keyFries & Tschofenig          Standards Track                    [Page 13]

RFC 4442                  Bootstrapping TESLA                 March 2006      encryption.  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.6.  IANA Considerations   This document requires an IANA registration for the following   attributes.  The registries are provided by MIKEY [RFC3830].   Prot Type:      This attribute specifies the protocol type for the security      protocol as described inSection 4.1.   Type:      Identifies the type of the general payload.  The General      Extensions Payload was defined to allow possible extensions to      MIKEY without the need for defining a completely new payload each      time.Section 4.4 describes this attribute in more detail.   Following the policies outlined in [RFC3830], the values in the range   up to 240 (including 240) for the above attributes are assigned after   expert review by the MSEC working group or its designated successor.   The values in the range from 241 to 255 are reserved for private use.   The IANA has added the following attributes and their respective   values to an existing registry created in [RFC3830]:   Prot Type:            Prot Type     | Value | Description            -----------------------------------------------------            TESLA         |     1 | TESLA as a security protocol   The value of 1 for the 'Prot Type' must be added to the 'Prot type'   registry created by [RFC3830].   Type:            Type          | Value | Description            -------------------------------------------            TESLA I-Key   |     2 | TESLA initial key   The value of 2 for the 'Type' must be added to the 'Type' registry   created by [RFC3830].  The values of 0 and 1 are already registered   in [RFC3830].Fries & Tschofenig          Standards Track                    [Page 14]

RFC 4442                  Bootstrapping TESLA                 March 2006   Also, the IANA has created two new registries:   TESLA-PRF: Pseudo-random Function (PRF) used in the TESLA policy:      This attribute specifies values for pseudo-random functions used      in the TESLA policy (seeSection 4.2).   TESLA-MAC: MAC Function used in TESLA:      This attribute specifies values for pseudo-random functions used      in the TESLA policy (seeSection 4.2).   Following the policies outlined in [RFC2434], the values for the   TESLA-PRF and the TESLA-MAC registry in the range up to 240   (including 240) for the above attributes are assigned after expert   review by the MSEC working group or its designated successor.  The   values in the range from 241 to 255 are reserved for private use.   IANA has added the following values to the TESLA-PRF and the   TESLA-MAC registry:   TESLA-PRF:            PRF Function     | Value            --------------------------            HMAC-SHA1        |  0   TESLA-MAC:            MAC Function     | Value            --------------------------            HMAC-SHA1        |  07.  Acknowledgements   The authors would like to thank Mark Baugher and Ran Canetti for the   discussions in context of time synchronization.  Additionally, we   would like to thank Lakshminath Dondeti, Russ Housley, and Allison   Mankin for their document reviews and for their guidance.Fries & Tschofenig          Standards Track                    [Page 15]

RFC 4442                  Bootstrapping TESLA                 March 20068.  References8.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an              IANA Considerations Section in RFCs",BCP 26,RFC 2434,              October 1998.   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.              Norrman, "MIKEY: Multimedia Internet KEYing",RFC 3830,              August 2004.   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.              Briscoe, "Timed Efficient Stream Loss-Tolerant              Authentication (TESLA): Multicast Source Authentication              Transform Introduction",RFC 4082, June 2005.   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient              Stream Loss-Tolerant Authentication (TESLA) in the Secure              Real-time Transport Protocol (SRTP)",RFC 4383,              February 2006.8.2.  Informative References   [DHHMAC]   Euchner, M., "HMAC-authenticated Diffie-Hellman for              MIKEY", Work in Progress, April 2005.   [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.   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)              Specification, Implementation",RFC 1305, March 1992.   [RFC2327]  Handley, M. and V. Jacobson, "SDP: Session Description              Protocol",RFC 2327, April 1998.   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,              A., Peterson, J., Sparks, R., Handley, M., and E.              Schooler, "SIP: Session Initiation Protocol",RFC 3261,              June 2002.Fries & Tschofenig          Standards Track                    [Page 16]

RFC 4442                  Bootstrapping TESLA                 March 2006   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.              Norrman, "The Secure Real-time Transport Protocol (SRTP)",RFC 3711, March 2004.   [RSA-R]    Ignjatic, D., "An additional mode of key distribution in              MIKEY: MIKEY-RSA-R", Work in Progress, February 2006.Authors' Addresses   Steffen Fries   Siemens   Otto-Hahn-Ring 6   Munich, Bavaria  81739   Germany   EMail: steffen.fries@siemens.com   Hannes Tschofenig   Siemens   Otto-Hahn-Ring 6   Munich, Bavaria  81739   Germany   EMail: Hannes.Tschofenig@siemens.comFries & Tschofenig          Standards Track                    [Page 17]

RFC 4442                  Bootstrapping TESLA                 March 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Fries & Tschofenig          Standards Track                    [Page 18]

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