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Versions: 00 01

      Internet Engineering Task Force     Mark Baugher (Cisco Systems)
      MSEC Working Group                      Ran Canetti (IBM Watson)
      INTERNET-DRAFT                       Pau-Chen Cheng (IBM Watson)
                                           Pankaj Rohatgi (IBM Watson)
      EXPIRES: September 2003                               March 2003

             MESP: A Multicast Framework for the IPsec ESP

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   Multicast ESP (MESP) is a framework for multicast data-origin
   authentication using the IPsec Encapsulating Security Payload (ESP)
   protocol. The MESP framework combines group-secrecy, group-
   authentication, and source-authentication transforms in an ESP
   packet. MESP uses a message authentication code for group
   authentication to protect a digital signature, TESLA timed MAC, or
   other multicast source-authentication transform.

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1.0 Notational Conventions..........................................2
2.0 Introduction....................................................2
 2.1 Changes from the Previous Version..............................3
 2.2 Overview.......................................................3
3.0 IP Multicast Security Functionalities...........................3
 3.1 Composition of the Functionalities.............................4
 3.2 MESP Security Association Database.............................5
4.0  MESP Packet Format.............................................5
 4.1 MESP Transforms................................................7
   4.1.1 Group Secrecy..............................................7
   4.1.2 Internal Authentication....................................7
   4.1.3 External Authentication....................................7
 4.2 MESP Signaling.................................................7
   4.2.1 GDOI.......................................................7
   4.2.2 GSAKMP.....................................................8
   4.2.3 MIKEY......................................................8
5.0 Security Considerations.........................................8
 5.1 MESP Authentication............................................8
 5.2 MESP Denial-of-Service Protection..............................9
 5.3 MESP Encryption................................................9
6.0 IANA Considerations............................................10
7.0 Acknowledgements...............................................11
8.0 Author's Address...............................................11
9.0 References.....................................................11
 9.1 Normative References..........................................11
 9.2 Informative References........................................12

1.0 Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
   document are to be interpreted as described in [RFC2119].
   The terminology conforms to [RFC2828].

2.0 Introduction

   The IPsec Encapsulation Security Payload (ESP) provides a set of
   security services that include data origin authentication, which
   enables an IPsec receiver to validate that a received packet
   originated from a peer-sender in a pairwise security association
   [Section 3.1, RFC2401].  A Message Authentication Code (MAC), such
   as the hash-based message authentication code [RFC2104, RFC2404]
   (HMAC), is the common means to provide data-origin authentication
   for pairwise IPsec security associations.  For secure groups such as
   IP multicast groups, however, a MAC supports only "group
   authentication" and not data-origin authentication [CP]. This
   Internet-Draft document (I-D) defines a framework for ESP data-

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   origin authentication that is suitable for IP multicast groups of
   ESP receivers.

2.1 Changes from the Previous Version

   This version is not a protocol, unlike the previous version, but is
   a transform framework for ESP and realizes all of its functions
   within the ESP protocol.  MESP now imposes an additional structure
   and usage on IPsec ESP Initialization Vector (IV) and Integrity
   Check Value (ICV) fields [ESPbis].

   A smaller change that appears in this version of MESP is the
   requirement that TESLA authentication be protected by external
   authentication transform such as a MAC.

2.2 Overview

   This I-D assumes that the reader is familiar with the "Security
   Architecture for Internet Protocol" [RFC2401] and the "IP
   Encapsulating Security Payload (ESP)" [ESPBIS] RFCs. Section 3
   reviews the functionalities of group data-security transforms for
   applications such as media streaming, process control, and reliable
   multicast applications.  Section 3 describes the problem of
   authenticating the source when the data-authentication key is shared
   by more than two IPsec endpoints. Section 4 describes the MESP
   framework in terms of the extensions and use of the ESP IV and
   integrity-check-value fields. The three functionalities of the MESP
   framework are realized in cryptographic transforms that are secure
   for various uses, and Section 5 recites the security considerations
   for each MESP transform.  Section 5 considers IP multicast risks,
   the transforms that address a particular risk, and the suitability
   of a transform for various applications and environments.

3.0 IP Multicast Security Functionalities

   The security requirements for multicast have been discussed in [CP].
   In general, group security has different requirements and
   characteristics than pairwise security.  In particular, data-origin
   authentication using a MAC will not prevent one member from
   impersonating another when a group of three or more members share
   the symmetric authentication key.  There are three new
   functionalities needed to add data-origin authentication to ESP.

   a) Group Secrecy (GS)

   The GS functionality is ESP confidentiality applied to a group . It
   ensures that transmitted data are accessible only to group members
   (i.e. two or more hosts in possession of a shared symmetric key).
   This can also be viewed as a way to enforce access control. A
   typical realization of GS is to encrypt data using a key known only
   to group members. Essentially, the solution for multicast is the

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   same as the solution for unicast confidentiality.  It is important
   to note, however, that some encryption transforms have special
   considerations when a key is shared among multiple senders.  An MESP
   encryption or authentication transform SHOULD describe any potential
   risks of multicast operation and how those risks are averted.

   b) Group Authentication (GA)

   The GA functionality enables a group member to verify that
   the received data originated from someone in the group and was not
   modified en-route by a non-group member.  Note that group
   authentication by itself does not identify the source of the
   data.  For example, the data might have been forged by any malicious
   group member. GA can be efficiently realized using standard shared
   key authentication mechanisms such as Message Authentication Codes
   (MACs), e.g., CBC-MAC, HMAC, or UMAC.

   c) Source and Data Authentication (SrA)

   The SrA functionality enables a group member to verify that
   the received data originated from the claimed source and was not
   modified en-route by anyone (including other malicious group
   members). Unlike Group Authentication, SrA provides the IPsec data-
   origin authentication function [RFC2401, ESPbis].  Source and Data
   Authentication provides a much stronger security guarantee than
   Group Authentication in that a particular group member can be
   identified as a source of a packet. Group and multicast source
   authentication requires stronger cryptographic techniques such as
   digital signatures, stream signatures [GR], flow signatures [WL],
   hybrid signatures [R], timed MACs, e.g. TESLA [TESLA,
   Ch,PCTS],asymmetric MACs [CGIMNP], etc.

3.1 Composition of the Functionalities

   A secure multicast solution is likely to utilize all three of the
   basic functionalities. Due to the diversity of the various
   application and deployment scenarios for multicast, several issues
   arise with respect to the composition and ordering of these

   In ESP, encryption precedes authentication when both are applied and
   a "combined-mode" confidentiality/integrity operation is not used
   [Section 3.3.2 of ESPBIS]. Combined modes of encryption and
   authentication are supported in ESP [ESPbis] but are not considered
   in this version of MESP. Encryption first is an efficient ordering
   that allows the receiver to apply a message authentication code
   (MAC) before it runs a more computationally-intensive decryption;
   fast authentication before decryption offers a better defense
   against bogus packets from a denial of service attack.  In MESP,
   therefore, the group secrecy (GS) transform MUST precede group
   authentication (GA) when GS is used.  In other words, the sender

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   applies GS prior to GA and the receiver applies GA prior to GS.
   Krawczyk has shown that it is more secure to authenticate encrypted
   data rather than encrypt authenticated data [K], but this ordering
   does not provide non-repudiation. The latter is usually not needed
   or even desirable for IPsec applications.

   MESP uses the same ordering for SrA: SrA MUST follow GS. Digital
   signatures offer the simplest method for multicast source
   authentication (SrA) but are computationally expensive, greatly
   expand the packet size and impractical for many, if not most, packet
   flows.  Given the relatively high cost of signature verification, a
   digital signature leaves the receiver vulnerable to denial of
   service attack when an attacker succeeds in getting the receiver to
   perform signature validation of bad packets.

   MESP partially protects the receiver from denial-of-service attacks
   from bogus digitally-signed packets by applying a MAC to the packet
   after signing it.  MESP calls this MAC "external authentication" and
   applies it in an identical fashion to ESP.  The digital signature is
   called "internal authentication," which the sender applies to the
   packet payload before the MAC.  MAC authentication, therefore, is
   applied first by the receiver.  If the attacker is not a member of
   the group, or otherwise has not obtained the group key, the MAC will
   fail before the receiver incurs the burden of a signature

   SrA transforms such as TESLA timed-MAC can be more efficient than
   digital signatures for many applications. But like a digital
   signature, it is REQUIRED that TESLA and other SrA transforms use
   internal authentication in MESP and be protected by an external-
   authentication MAC.  Thus, a digital signature or TESLA MAC MUST be
   applied prior to GA at the sender and after GA at the receiver.
   MAC-based GA is an external-authentication transform that MUST be
   applied last at the sender and first at the receiver.  As in ESP,
   encryption (GS) is applied before any authentication and is

3.2 MESP Security Association Database

   The MESP framework applies up to three transforms to a multicast ESP
   packet in the order described above: Group Secrecy (GS) is OPTIONAL,
   Source Authentication (SrA) SHOULD be applied next, and Group
   Authentication (GA) SHOULD be applied last.  The IPsec SAD MUST be
   extended to store the additional transform information if MESP is to
   be supported.

   There are no changes to ESP SA lookup beyond what is specified for
   multicast SAs in the IPsec specifications [AHbis, ESPbis].

4.0  MESP Packet Format

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   The ESP Packet format is illustrated in Figure 4-1:

   * Internal Authentication Parameters (variable). The length and
   contents of this field are defined by the SrA transform.  Certain
   internal authentication transforms have a zero length SrA Transform
   Parameters fields (Section 5.1).

   * Internal Authentication Tag (Variable).  The length and contents
   of this field are defined by the internal authentication transform.
   Certain SrA transforms have a zero length Internal Authentication
   Tag field.

             Figure 4-1: MESP Transforms in an ESP 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
|               Security Parameters Index (SPI)                 |    ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
|                      Sequence Number                          |  ^ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  | |
~                 IV (variable & optional)                      ~  | |
+---------------------------------------------------------------+  | |
~   Internal Authentication Parameters (variable & optional)    ~  | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  | |
~                        Data (variable)                        ~^ I E
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+E N X
~               ~         Padding (0-255 bytes)                 |N T T
+-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+C | |
|                               |  Pad Length   | Next Header   |v v |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
~             Internal Authentication Tag  (variable)           ~    v
~                 Integrity Check Value (variable)              ~

   Encryption (ENC), when applied, covers the ESP data, padding and
   next header fields.  Internal Authentication (INT) covers the ESP
   sequence number through the next header field, inclusive. External
   authentication (EXT) covers the entire ESP packet except for the
   Integrity Check Value (ICV) field.

   A sender MAY use an encryption-transform (ENC) as done in any other
   ESP packet.

   When INT is applied to the packet, its output (if any) is placed in
   the Internal Authentication Tag.  Section 4.1 identifies the INT
   transforms, which the sender MUST perform prior to the encryption

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   A sender of an MESP packet SHOULD use an external-authentication
   transform (EXT).  Section 4.1 identifies the EXT transforms, which
   the sender MUST perform last (and the receiver performs first).

   The sender MUST perform the MESP transforms in the order of ENC,
   INT, and EXT while the receiver MUST perform them in the order of
   EXT, INT and ENC.

4.1 MESP Transforms

   This version of MESP defines a minimal number of transforms.  In the
   future, new transforms MAY be added through the publication of an
   Internet RFC.  The transforms of the MESP framework are listed below
   and classified according to MESP type.

4.1.1 Group Secrecy

   MESP supports 3DES, AES-CBC, and AES-CTR.

4.1.2 Internal Authentication

   MESP internal authentication is either RSA-SHA1 or TESLA.

4.1.3 External Authentication

   MESP external authentication uses HMAC-SHA1.

4.2 MESP Signaling

   MESP parameters are signaled through a key management protocol or
   interface such as GDOI, GSAKMP, GKMP, or SDP.

4.2.1 GDOI

   GDOI MUST signal use of the MESP framework using PROTO_ESP_MESP with
   the TRANSFORM ID set to the MESP transform ID value (see IANA
   Section below).  MESP extends the RFC 2407 attributes ["IPsec
   Security Association Attributes," IANA] with the three new classes,
   "ENC_Transform, "INT_Transform," and "EXT_Transform" (see IANA
   Section below).

   The ENC Transform MUST be one of the transforms from 4.1.1.
   Additional ENC transforms MAY be added to this suite through the
   publication of an Internet RFC.

   The INT Transform MUST be non-null and MUST be one of the values
   from 4.1.2. Additional INT transforms MAY be added to this suite
   through the publication of an Internet RFC.

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   The EXT Transform MUST be one of the transforms from 4.1.3.
   Additional EXT transforms MAY be added to this suite through the
   publication of an Internet RFC.

   The IANA Considerations section of this document describes value
   assignments for the EXT, INT and ENC attributes.

   The SA Life Type and Life Duration as defined in RFC 2407 [RFC2407,
   IANA] apply to all keys used for the session, including the
   Signature PubKey, which MUST NOT be sent if the INT Transform is not
   a digital signature algorithm.  The length of the encoding is
   determined by INT. {Editor:  Need to define the Signature PubKey
   format and should make it a GDOI KD payload item instead.}

4.2.2 GSAKMP


4.2.3 MIKEY


5.0 Security Considerations

   MESP is a framework for IPsec ESP authentication and encryption
   transforms to support IP multicast delivery.  IPsec ESP provides
   access control, rejection of replayed packets, confidentiality
   (encryption), limited traffic-flow confidentiality and
   connectionless integrity.  ESP supports a datagram environment where
   each IP packet is cryptographically independent of other IP packets
   and can be decrypted, authenticated, and integrity checked when
   delayed, reordered, or when other packets from the flow are lost.
   ESP provides rejection of replayed packets for pairwise security
   associations and for multicast security associations under certain
   circumstances [ESPbis].  MESP has the same replay mechanism for the
   single-sender case; the multi-sender case is for further study.  ESP
   provides data origin authentication for pairwise security
   associations using symmetric message authentication codes, which are
   not sufficient for group security applications where more than two
   members share a symmetric key.

5.1 MESP Authentication

   The MESP framework for ESP provides data-origin authentication
   services to multicast ESP packets.  Secure groups, such as secure
   multicast groups, cannot use a symmetric-key MAC to uniquely
   identify the sender; any group member that is in possession of the
   group authentication key is capable of replacing the source address
   of the packet with that of another group member and applying the
   symmetric key to authenticate the packet.  To uniquely identify a
   sender among a group of members, digital signature, public key

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   encryption, or specialized source-authentication techniques such as
   timed MACs are needed. This version of MESP defines a general ESP
   packet structure for accommodating a digital signature or TESLA
   timed MAC.

5.2 MESP Denial-of-Service Protection

   As discussed above, a group member cannot authenticate the source of
   the packet for a multicast group where multiple members share the
   MAC key.  Thus, a rogue member of the group has all the keying
   material needed to impersonate a sender of the group if that
   attacker is able to inject packets into the network using that
   sender's IP address.  The MESP framework addresses this problem by
   augmenting the MAC with an "internal authentication" transform,
   which MAY be an RSA-SHA1 digital signature or a TESLA timed MAC.
   Digital signatures leave multicast receivers vulnerable to denial-
   of-service attack if the receiver is duped into performing
   computationally-expensive signature validation of bogus packets.  An
   external message authentication SHOULD accompany a digital signature
   so as to limit the effectiveness of bogus digitally signed packets
   by non-group members.  TESLA is also vulnerable to a denial of
   service attack if the receiver is duped into storing bogus packets
   awaiting MAC verification.  An external message authentication
   transform SHOULD accompany a TESLA authentication transform so as to
   limit the effectiveness of bogus TESLA packets by non-group members.

   Unfortunately, group members are still capable of sending packets
   with a valid external-authenticating MAC and invalid digital
   signature, i.e. a group member can launch a DoS attack on the group
   using invalid digital signatures.  And group members are still
   capable of sending packets with a valid external-authentication MAC
   but an invalid TESLA MAC.  In both the TESLA and digital-signature
   cases, the external authentication will succeed only to have the
   internal authentication fail.  MESP includes the ESP sequence number
   in the internal authentication to protect against a replay attack by
   a group member.  When the RECOMMENDED external authentication code
   is use, however, the ESP receiver MUST validate both the internal
   authentication as well as the external authentication before
   updating the ESP replay window.

   The value of MESP authentication transforms is to enable the
   receiver to greatly reduce the effect of an attack by non-group
   members, to reduce the effects of a denial of service attack by a
   group member, to detect an attack by a group member, and to support
   the integration of multicast source-authentication transforms into
   IPsec ESP for data-origin authentication.

5.3 MESP Encryption

   The value of MESP encryption is to validate individual encryption
   transforms for multicast operation.  It is possible that a

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   particular cipher and mode are suitable for pairwise security
   associations but not for multicast security associations, such as
   one where multiple senders share the key.  For example, a stream or
   hybrid stream/block cipher has special risks of keystream reuse when
   its key is shared by multiple senders.  Although IPsec encryption
   transforms are generally suitable for multicast operation, many have
   not been evaluated, tested or used in IP multicast environments.
   This I-D has considered the suitability of several IPsec ESP
   encryption transforms for inclusion in the MESP framework.  It is
   RECOMMENDED that all future IPsec encryption transforms be analyzed
   as to their security for multicast and group security as well as for
   pairwise security.

6.0 IANA Considerations

   This I-D extends the RFC 2407 attributes for IPsec ESP,
   PROTO_IPSEC_ESP [RFC2407]. Within PROTO_IPSEC_ESP, MESP reserves the
   transform identifier value 13 [See IANA, "IPSEC ESP Transform
   Identifiers"].  MESP also adds new type/length/value attributes to
   RFC 2407.  For the MESP transform ID security association
   attributes, the ENC Transform has type number 11, the INT transform
   has type number 12, and the EXT transform has type number 13 [see
   IANA, "Security Association Attributes"].

         class               value           type
         ENC-Transform        11               B
         INT-Transform        12               B
         EXT-Transform        13               B

   The ENC-Transform has the following values.
         name                value
         ----                -----
         Reserved              0
         3DES                  1
         AES-CBC               2
         AES-CTR               3

   The INT-Transform has the following values.
         name                value
         ----                -----
         Reserved              0
         RSA-SHA               1
         TESLA                 2

   The EXT-Transform has the following values.
         name                value
         ----                -----
         Reserved              0
         HMAC-SHA1             1

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7.0 Acknowledgements

   The authors wish to thank Scott Fluhrer, Thomas Hardjono, Steve Kent
   and Brian Weis for their thoughtful comments and suggestions.

8.0 Author's Address

   Mark Baugher
   Cisco Systems, Inc.
   5510 SW Orchid Street
   Portland, Oregon

   Ran Canetti
   IBM T.J. Watson Research Center
   30 Saw Mill River Road
   Hawthorne, NY 10598, USA
   Tel: +1-914-784-6692

   Pau-Chen Cheng
   IBM T.J. Watson Research Center
   30 Saw Mill River Road
   Hawthorne, NY 10598, USA
   Tel: +1-914-784-6692

   Pankaj Rohatgi
   IBM T.J. Watson Research Center
   30 Saw Mill River Road
   Hawthorne, NY 10598, USA
   Tel: +1-914-784-6692

9.0 References

9.1 Normative References

   [AHBIS] S. Kent, IP Authentication Header (AH), IETF, Work in
   Progress, March 2003, http://www.ietf.org/internet-drafts/draft-

   [ESPBIS] S. Kent, IP Encapsulated Security Payload (ESP), IETF, Work
   in Progress, March 2003, http://www.ietf.org/internet-drafts/draft-

   [GDOI] M. Baugher, H. Harjono, H. Harney, B. Weis, The Group Domain
   of Interpretation, IETF, Work in Progress, October 2002,

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   [GSAKMP] H. Harney, A. Schuett, A. Colegrove, GSAKMP Light, IETF,
   Work in Progress, July 2002, http://www.ietf.org/internet-

   [IANA] http://www.iana.org/assignments/isakmp-registry

   [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Normann,
   MIKEY: Multimedia Internet KEYing, IETF, Work in Progress, September
   2002, http://www.ietf.org/internet-drafts/draft-ietf-msec-mikey-

   [PKCS1] PKCS #1 v2.1: RSA Cryptography Standard, RSA Laboratories,
   ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf, June 2002.

   [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, HMAC: Keyed-Hashing
   for Message Authentication, Internet Request for Comments 2104,
   IETF, November 1997, ftp://ftp.rfc-editor.org/in-notes/rfc2104.txt.

   [RFC2401] S.Kent, R.Atkinson, Security Architecture for the Internet
   Protocol, Internet Request for Comments 2401, IETF, November 1998,

   [RFC2404] C. Madson, R. Glenn, The Use of HMAC-SHA-1-96 within ESP
   and AH, Internet Request for Comments 2404, IETF, November 1998,

   [RFC2407] D. Piper, The Internet IP Security Domain of
   Interpretation for ISAKMP, Internet Request for Comments 2407, IETF,
   November 1998, http://www.ietf.org/rfc/rfc2407.txt.

   [RFC2451]  Pereira, R., and R. Adams, "The ESP CBC-Mode Cipher
   Algorithms", Internet Request For Comments 2451, IETF, November
   1998, ftp://ftp.rfc-editor.org/in-notes/rfc2451.txt.

   [RFC3376] B.Cain, S.Deering, B.Fenner, I. Kouvelas, A. Thyagarajan,
   Internet Group Management Protocol, Version 3, Internet Request for
   Comments 3376, IETF, October 2002,

   [SSM]H.Holbrook, B.Cain, Source Specific Multicast for IP,
   http://www.ietf.org/internet-drafts/draft-ietf-ssm-arch-00.txt, Work
   in Progress


9.2 Informative References

   [CGIMNP] Canetti R., J. Garay, G. Itkis, D. Micciancio, M. Naor, B.
   Pinkas, "Multicast Security: A Taxonomy and Efficient
   Authentication", INFOCOM '99.

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   [CP] R. Canetti, B. Pinkas, "A taxonomy of multicast security
   issues",draft-canetti-secure-multicast-taxonomy-01.txt, Nov. 1998.

   [Ch] S. Cheung, An Efficient Message Authentication Scheme for
   Link State Routing, Proceedings of the 13th Annual Computer
   Security Applications Conference, San Diego, December 8-12, 1997,

   [GR] R. Gennaro, P. Rohatgi, "How to Sign Digital Streams", Advances
   in Cryptology - Crypto '97, Springer-Verlag LNCS 1294, pp. 180-197,

   [K] H. Krawczyk, The order of encryption and authentication for
   protecting communications (or: How secure is SSL?). In J. Kilian,
   editor, CRYPTO 2001.

   [PCTS] A. Perrig, R. Canetti, D. Tygar, D. Song, Efficient
   Authentication and Signature of Multicast Streams over Lossy
   Channels, IEEE Symposium on Security and Privacy, Oakland, CA, May

   [R] P. Rohatgi,  A Compact and Fast Signature Scheme for Multicast
   Packet Authentication, In 6th ACM Computer and Communications
   Security Conference (CCS) , Nov 1999.

   [WL]  C. K. Wong and  S. S. Lam, Digital Signatures for Flows and
   Multicasts, IEEE ICNP '98.  See also University of Texas at Austin,
   Computer Science Technical report TR 98-15.

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