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Versions: 00 draft-ietf-msec-gdoi-update

MSEC Working Group                                               B. Weis
Internet-Draft                                                 S. Rowles
Expires: August 21, 2006                                   Cisco Systems
                                                       February 17, 2006

          Updates to the Group Domain of Interpretation (GDOI)

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo describes additional updates to the Group Domain of
   Interpretation (GDOI) [RFC3547] .  It provides clarification where
   the original text is unclear.  It also includes a discussion of
   algorithm agility within GDOI, and proposes several new algorithm
   attribute values.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements notation  . . . . . . . . . . . . . . . . . .  3

   2.  Cryptographic Algorithm agility  . . . . . . . . . . . . . . .  4
     2.1.  Phase 1 protocol . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  GROUPKEY-PUSH signature  . . . . . . . . . . . . . . . . .  4
     2.3.  IPsec TEK Integrity HMAC algorithms  . . . . . . . . . . .  4
     2.4.  Certificate Payload  . . . . . . . . . . . . . . . . . . .  5
     2.5.  POP Hash Function  . . . . . . . . . . . . . . . . . . . .  5

   3.  RFC 3547 Clarification . . . . . . . . . . . . . . . . . . . .  6
     3.1.  SA Payload . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  SIG Payload  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  SEQ Payload  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  POP Payload  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.5.  TEK Integrity Key  . . . . . . . . . . . . . . . . . . . .  7
     3.6.  PFS  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.7.  GCKS Authorization . . . . . . . . . . . . . . . . . . . .  8
     3.8.  Minimum defined attributes . . . . . . . . . . . . . . . .  9
     3.9.  Attribute behavour . . . . . . . . . . . . . . . . . . . . 10

   4.  New GDOI Attributes  . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Signature Hash Algorithm . . . . . . . . . . . . . . . . . 11
     4.2.  Support of AH  . . . . . . . . . . . . . . . . . . . . . . 11

   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14

   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15

   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 17

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

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1.  Introduction

   The Group Domain of Interpretation (GDOI) is a group key management
   protocol fitting into the Multicast Security Group Key Management
   Architecture [RFC4046].  GDOI is used to disseminate policy and
   corresponding secrets to a group of participants.  GDOI is
   implemented on hosts and intermediate systems to protect group IP
   communication (e.g., IP multicast packets) by encapsulating them with
   the IP Encapsulating Security Payload (ESP) [RFC4303] packets.
   However, implementation experience has revealed some inconsistencies
   in RFC 3547 needing clarification.  It also defines some additional
   GDOI algorithm attributes which are useful to GDOI applications.

   Algorithm agility, the ability to add new algorithms to namespaces,
   is an important consideration for any protocol.  This memo analyzes
   the state of algorithm agility within GDOI, and proposes some changes
   based upon that analysis.  In particular, methods for fully
   supporting the SHA-256 algorithm [FIPS.180-2.2002] as an alternative
   to SHA-1 and MD5 are described.

1.1.  Requirements notation

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

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2.  Cryptographic Algorithm agility

   Algorithm agility was a goal during the development of GDOI, and RFC
   3547 generally provides the ability to add new algorithms as
   necessary.  However, further analysis has shown that there are places
   where algorithm agility is not complete.  This section discusses the
   use cryptographic algorithms within GDOI, puts out variances in
   algorithm agility, and proposes clarifications without changing any
   payload formats within the protocol.

   Recent published attacks on the SHA-1 algorithm motivate its
   replacement as a cryptographic hash algorithm.  The ability for GDOI
   to move to the SHA-256 hash algorithm is explicitly discussed.  Later
   sections on this document propose some enhancements to the GDOI
   protocol to provide for an easier means of supporting this and
   additional hash functions.

2.1.  Phase 1 protocol

   GDOI is a "phase 2" protocol protected by a "phase 1" protocol.  The
   Phase 1 protocol defined in RFC 3547 is an IKEv1 Phase 1 protocol
   (Main Mode or Aggressive Mode).  The Phase 1 protocol provides
   confidentiality via an encryption cipher.  It also provides message
   integrity via a pseudo random function ("prf") (described in Section
   4 of [RFC2409], which is usually a hash algorithm using the HMAC
   [RFC2104] construction.  IKEv1 negotiates which encryption ciphers
   and hash algorithms are to be used.

   IKEv1 cipher algorithms come from the "Encryption Algorithm" list in
   the IANA IPsec registry [IPSEC-REG], and the hash algorithms come
   from the "Hash Algorithm" list in the same registry.  The IANA IPsec
   registry currently includes the SHA2-256, which is intended to be the
   SHA-256 hash algorithm.

2.2.  GROUPKEY-PUSH signature

   The GROUPKEY-PUSH message is protected by both an encryption cipher
   and a digital signature for message integrity.  The encryption cipher
   is described by the IANA GDOI registry as the KEK_ALGORITHM attribute
   [GDOI-REG].  The digital signature comprises both a hash algorithm
   defined by the GDOI SIG_HASH_ALGORITHM attribute and a public key
   signature algorithm defined by the SIG_ALGORITHM attribute.  This
   memo adds the SHA-256 algorithm to the SIG_HASH_ALGORITHM attribute
   in a later section.

2.3.  IPsec TEK Integrity HMAC algorithms

   IPsec SAs are distributed by GDOI.  An IPsec ESP SA can include an

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   encryption cipher for confidentiality and an algorithm for packet
   authentication.  The encryption ciphers are defined by the IPsec ESP
   Transform Identifiers defined in the IANA ISAKMP registry [ISAKMP-
   REG].  The packet authentication method is distributed via an
   "Authentication Algorithm" SA attribute.  SHA-256 may be chosen as
   the authentication algorithm with HMAC-SHA2-256.  Similarly, an IPsec
   AH SA is defined by choosing AH_SHA2-256 as the "AH Transform

2.4.  Certificate Payload

   Messages in the GROUPKEY-PULL and GROUPKEY-PUSH protocols may include
   a Certificate Payload (CERT).  Digital signatures, and the algorithm
   agility thereof, are outside the scope of this memo.

2.5.  POP Hash Function

   The GDOI Proof of Possession (POP) payload may be included in the
   GROUPKEY-PULL protocol.  It contains a digital signature over the
   hash of some nonces, which provides the current possession of the
   peer.  The digital signature algorithm is defined as the "POP
   Algorithm" in the IANA GDOI registry [GDOI-REG].  However, identity
   of the hash algorithm to create the signed data was omitted in RFC
   3547.  This memo remedies this omission in a clarification section

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3.  RFC 3547 Clarification

3.1.  SA Payload

   The units of the SIG_KEY_LENGTH value was unspecified in RFC 3547.
   The value is the length of the keys in bits.

   The GDOI_PROTO_IPSEC_ESP attribute is sometimes referred to by the
   truncated name PROTO_IPSEC_ESP.

   RFC3547 explicitly specifies that if a KEK cipher requires an IV,
   then the IV MUST precede the key in the KEK_ALGORITHM_KEY KD payload
   attribute.  However, it should be noted that this IV length is not
   included in the KEK_KEY_LEN SA payload attribute sent in the SA
   payload.  The KEK_KEY_LEN includes only the actual length of the
   cipher key.

   The Group Controller/Key Server (GCKS) adds the KEK_KEY_LEN attribute
   to the SA payload when distributing KEK policy to group members.  The
   group member verifies whether or not it has the capability of using a
   cipher key of that size.  If the cipher definition includes a fixed
   key length (e.g., KEK_ALG_3DES), the group member can make its
   decision solely using KEK_ALGORITHM attribute and does not need the
   KEK_KEY_LEN attribute.  Sending the KEK_KEY_LEN attribute in the SA
   payload is OPTIONAL if the KEK cipher has a fixed key length.

3.2.  SIG Payload

   The GROUPKEY-PUSH message SIG payload is further clarified here; the
   SIG payload is a signature of the entire GROUPKEY-PUSH message (not
   including the SIG payload) before it's been encrypted.  The HASH is
   taken over the string 'rekey', the GROUPKEY-PUSH HDR, SEQ, SA, KD,
   and optionally the CERT payload.  After the SIG payload is created
   using the signature of the above hash, the current KEK encryption key
   encrypts all the payloads following the GROUPKEY-PUSH HDR.

3.3.  SEQ Payload

   Each GROUPKEY-PUSH message contains a sequence number, which provides
   anti-replay protection for a KEK.  Thus, the GCKS returns a SEQ
   payload in the GROUPKEY-PULL exchange only if a KEK attribute also
   exists in the SA payload.

   A KEK sequence number is associated with a single SPI (i.e., the
   single set of cookie pair values sent in a GROUPKEY-PUSH ISAKMP HDR).
   When a new KEK is distributed by a GCKS, it contains a new SPI and
   resets the sequence number.

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   When a SEQ payload is included in the GROUPKEY-PULL exchange, it
   includes the most recently used sequence number for the group.  At
   the conclusion of a GROUPKEY-PULL exchange, the initiating group
   member MUST NOT accept any rekey message with both the KEK attribute
   SPI value and a sequence number less than or equal to the one
   received during the GROUPKEY-PULL.  When the first group member
   initiates a GROUPKEY-PULL exchange, the GCKS provides a Sequence
   Number of zero, since no GROUPKEY-PUSH messages have yet been sent.
   Note the sequence number increments only with GROUPKEY-PUSH messages.
   The GROUKEY-PULL exchange distributes the current sequence number to
   the group member.

   The sequence number resets to one with a new KEK attribute, as
   described in section 5.6 of RFC 3547: "Thus the first packet sent for
   a given Rekey SA will have a Sequence Number of 1".  The sequence
   number increments with each successive rekey.

3.4.  POP Payload

   RFC 3547 defines the Proof of Possession (POP) payload, which
   contains a digital signature over a hash.  Some RFC 3537 text
   erroneously describes it as a "prf()".

   RFC 3547 omitted including a GDOI SA attribute in which the hash
   function type could be passed between the GCKS and the group member.
   This results in no method for the hash algorithm to be specified
   within the GDOI protocol.  To remedy this omission, the hash
   algorithm passed in the SIG_HASH_ALGORITHM MUST be also used as the
   POP hash algorithm.

   Receivers of the POP payload need the sender's public key in order to
   validate the POP.  RFC 3547 does not provide for passing of the POP
   signature key.  Indeed, the public key will usually come from a
   certificate in the CERT payload.  However, if a CERT payload is not
   sent with a POP payload, or if the CERT is an attribute cert (not
   containing a public key), then the distribution of the public key is
   outside of the scope of this standard.

3.5.  TEK Integrity Key

   Regarding the integrity key pushed to the member, the SHA1 keys will
   consist of 160 bits, SHA256 keys will consist of 256 bits, and MD5
   keys will consist of 128 bits.

3.6.  PFS

   RFC 3547 provides an OPTIONAL additional protection for the KD
   payload during a GROUPKEY-PULL exchange called Perfect Forward

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   Secrecy (PFS).  If the GCKS and group member exchange KE payloads
   containing Diffie-Hellman public keys, the GCKS encrypts the KD
   payload with a secret obtained from the Diffie-Hellman shared number.
   This encryption precedes the encryption of the entire GROUPKEY-PULL

   The purpose of PFS in GDOI is to more carefully protect the keying
   material passed from the GCKS to the group member.  If a passive
   attacker captures the GROUPKEY-PULL exchange and performs an offline
   attack of the IKE Phase 1 confidentiality keys, it may eventually
   discover them.  If PFS is not used, the attacker can immediately use
   the recovered keys to decrypt data packets and GROUPKEY-PUSH
   messages, either live or stored.  Thus, the IKE Phase 1 keys are
   critical to the long-term confidentiality of the group.  PFS was
   added as an additional mechanism to hinder a passive attacker by
   requiring it to perform an additional cryptanalysis to recover the
   Diffie-Hellman shared number computed by the GCKS and group member.

   RFC 3547 Section 3.2.1 says "The GCKS responder will xor the DH
   secret with the KD payload and send it to the member Initiator, which
   recovers the KD by repeating this operation as in the Oakley IEXTKEY
   procedure [RFC2412]".  However, the IEXTKEY procedure does not xor
   the DH shared secret with an entire payload, and the DH shared secret
   is not likely to be long enough to cover the entire payload.
   Therefore, the following amended procedure MUST be used for PFS.

   1.  The leftmost bits in the DH shared secret are used as an
       encryption key.  The encryption key algorithm described in the
       KEK_ALGORITHM attribute is used.

   2.  The new key is used to encrypt the KD payload.  Note that the
       length of the KD payload may be larger due to cipher block
       padding.  If so, the KD payload length must be modified to
       reflect the actual length of the ciphertext.

3.7.  GCKS Authorization

   Meadows and Pavlovic have published a paper [MP04] describing a means
   by which a rogue GDOI device (i.e., GCKS or group member) can gain
   access to a group for which it is not a group member.  The rogue
   devices perpetrates a man-in-the-middle attack, which can occur if
   the following conditions are true:

   1.  The rogue GDOI participant convinces an authorized member of the
       group (i.e., victim group member) that it is a key server for
       that group.

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   2.  The victim group member, victim GCKS, and rogue group member all
       share IKEv1 authentication credentials.

   3.  The victim GCKS does not properly verify that the IKEv1
       authentication credentials used to protect a GROUPKEY-PULL
       protocol are authorized to be join the group.

   The actual attack is too detailed to explain in this memo, but it is
   important to recognize that it can be perpetrated whether or not the
   group policy requires the use of CERT and POP payloads.  In all
   cases, the attack can be stopped when the authorized GCKS performs
   authorization based on the IKEv1 authentication credentials.  A GDOI
   key server SHOULD perform one of the following authorization checks:

   1.  If the use of CERT and POP payloads are not mandated in group
       policy, the GCKS SHOULD maintain an list of authorized group
       members for each group, where the group member identity is its
       IKEv1 authentication credentials.  The authorization check SHOULD
       be made after receipt of the ID payload containing a group id the
       group member is requesting to join.

   2.  If the CERT and POP payloads are used for authorization, the GCKS
       SHOULD verify that the identify in the CERT payload refers to the
       same identity in the IKEv1 authentication credentials.  This
       stops a group member from authenticating to the GCKS with its own
       credential, yet including another group member's credentials and
       proof-of-possession in the CERT and POP payloads.

   Additionally, a GDOI group member SHOULD be configured with policy
   describing which IKEv1 identities are authorized to act as GCKS for a

3.8.  Minimum defined attributes

   Minimum attributes that must be sent as part of an SA KEK:
   KEK_ALGORITHM, KEK_KEY_LENGTH (if the cipher definition includes a
   variable length key), KEK_KEY_LIFETIME, SIG_HASH_ALGORITHM (except
   for DSA based algorithms), SIG_ALGORITHM.

   RFC 3547 states that all mandatory IPsec DOI attributes are mandatory
   in GDOI_PROTO_IPSEC_ESP.  However, no such list of mandatory IPsec
   DOI attributes can be found in RFC 2407.  This memo requires that the
   following attributes MUST be supported by an RFC 3547 implementation
   supporting the GDOI_PROTO_IPSEC_ESP SA TEK: SA Life Type, SA Life
   Duration, Encapsulation Mode, Authentication Algorithm (if the ESP
   transform includes authentication).

3.9.  Attribute behavour

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   An GDOI implementation MUST abort if it encounters and attribute or
   capability that it does not understand.

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4.  New GDOI Attributes

   This section contains new attributes to be are defined as part of

4.1.  Signature Hash Algorithm

   RFC 3547 defines two signature hash algorithms (MD5 and SHA-1).
   However, steady advances in technology have rendered both hash
   algorithms to be weak when used as a signature hash algorithm.

   The SHA-256 hash algorithm [FIPS.180-2.2002] has been made available
   by NIST as a replacement for SHA-1, and is its preferred replacement
   for both MD5 and SHA-1.  A new value for the GDOI SIG_HASH_ALGORITHM
   attribute is defined by this memo to represent the SHA-256 hash
   algorithm: SIG_HASH_SHA256.  Support for SIG_HASH_SHA256 is OPTIONAL.

4.2.  Support of AH

   RFC3547 only specifies data-security SAs for one security protocol,
   IPsec ESP.  Typically IPsec implementations use ESP and AH IPsec SAs.
   This document extends the capability of GDOI to support both ESP and
   AH.  The GROUPKEY-PULL mechanism will establish IPsec ESP SAs and
   IPsec AH SAs.  The GROUPKEY-PUSH will refresh the IPsec ESP SAs and
   the IPsec AH SAs.  Support for AH [RFC4302] will come with the
   introduction of a new SA_TEK Protocol-ID with the name

   The TEK Protocol-Specific payload for AH is as follows:

        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
       !    Protocol   !  SRC ID Type  !         SRC ID Port           !
       !SRC ID Data Len!          SRC Identification Data              ~
       ! DST ID Type   !         DST ID Port           !DST ID Data Len!
       ! DST Identification Data                                       ~
       ! Transform ID  !                        SPI                    !
       !      SPI      !       RFC 2407 SA Attributes                  ~

   The SAT Payload fields are defined as follows:

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   o  Protocol (1 octet) -- Value describing an IP protocol ID (e.g.,
      UDP/TCP).  A value of zero means that the Protocol field should be

   o  SRC ID Type (1 octet) -- Value describing the identity information
      found in the SRC Identification Data field.  Defined values are
      specified by the IPsec Identification Type section in the IANA
      ISAKMP Registry [ISAKMP-REG].

   o  SRC ID Port (2 octets) -- Value specifying a port associated with
      the source Id.  A value of zero means that the SRC ID Port field
      should be ignored.

   o  SRC ID Data Len (1 octet) -- Value specifying the length of the
      SRC Identification Data field.

   o  SRC Identification Data (variable length) -- Value, as indicated
      by the SRC ID Type.  Set to three bytes of zero for multiple-
      source multicast groups that use a common TEK for all senders.

   o  DST ID Type (1 octet) -- Value describing the identity information
      found in the DST Identification Data field.  Defined values are
      specified by the IPsec Identification Type section in the IANA
      ISAKMP Registry [ISAKMP-REG].

   o  DST ID Port (1 octet) -- Value describing an IP protocol ID (e.g.,
      UDP/TCP).  A value of zero means that the DST Id Port field should
      be ignored.

   o  DST ID Port (2 octets) -- Value specifying a port associated with
      the source Id.  A value of zero means that the DST ID Port field
      should be ignored.

   o  DST ID Data Len (1 octet) -- Value specifying the length of the
      DST Identification Data field.

   o  DST Identification Data (variable length) -- Value, as indicated
      by the DST ID Type.

   o  Transform ID (1 octet) -- Value specifying which AH transform is
      to be used.  The list of valid values is defined in the IPsec AH
      Transform Identifiers section of the IANA ISAKMP Registry [ISAKMP-

   o  SPI (4 octets) -- Security Parameter Index for AH.

   o  RFC 2407 Attributes -- AH Attributes from Section 4.5 of
      [RFC2407].  The GDOI supports all IPsec DOI SA Attributes for

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      GDOI_PROTO_IPSEC_AH excluding the Group Description, which MUST
      NOT be sent by a GDOI implementation and is ignored by a GDOI
      implementation if received.  The Authentication Algorithm
      attribute of the IPsec DOI is group authentication in GDOI.  The
      following RFC 2407 attributes MUST be sent as part of a
      GDOI_PROTO_IPSEC_AH attribute: SA Life Type, SA Life Duration,
      Encapsulation Mode.

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5.  IANA Considerations

   The SIG_HASH_ALGORITHM KEK Attribute should be assigned a new
   Algorithm Type value from the RESERVED space to represent the SHA-256
   hash algorithm as defined.  The new algorithm name should be

   A new SA_TEK type Protocol-ID type should be assigned from the
   RESERVED space.  The new algorithm id should be called
   GDOI_PROTO_IPSEC_AH, and refers to the IPsec AH encapsulation.

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6.  Security Considerations

   This memo describes additional clarification and protocol updates to
   the GDOI protocol.  The security considerations in RFC 3547 remain
   accurate, with the following additions.

   o  Several minor cryptographic hash algorithm agility issues are
      resolved, and the stronger SHA-256 cryptographic hash algorithm is

   o  Protocol analysis has revealed a man-in-the-middle attack when the
      GCKS does not authorize group members based on their IKEv1
      authentication credentials.  This is true even when a CERT and POP
      payloads are used for authorization.  Although suggested as an
      option in RFC 3547, a GDOI device (group member or GCKS) SHOULD
      NOT accept an identity in a CERT payload that does not match the
      IKEv1 identity used to authenticate the group member.

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7.  References

7.1.  Normative References

              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002, <http://

              Internet Assigned Numbers Authority, "Group Domain of
              Interpretation (GDOI) Payload Type Values", IANA Registry,
              December 2004,

              Internet Assigned Numbers Authority, "Internet Key
              Exchange (IKE) Attributes IKE Attributes", IANA Registry,
              December 2005,

              Internet Assigned Numbers Authority, "Internet Security
              Association and Key Management Protocol (ISAKMP)
              Identifiers ISAKMP Attributes", IANA Registry,
              January 2006,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2407]  Piper, D., "The Internet IP Security Domain of
              Interpretation for ISAKMP", RFC 2407, November 1998.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", RFC 3547, July 2003.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, April 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",

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              RFC 4303, December 2005.

7.2.  Informative References

   [MP04]     Meadows, C. and D. Pavlovic, "Deriving, Attacking, and
              Defending the GDOI Protocol", ESORICS 2004 pp. 53-72,
              September 2004.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

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Authors' Addresses

   Brian Weis
   Cisco Systems
   170 W. Tasman Drive
   San Jose, California  95134-1706

   Phone: +1-408-526-4796
   Email: bew@cisco.com

   Sheela Rowles
   Cisco Systems
   170 W. Tasman Drive
   San Jose, California  95134-1706

   Phone: +1-408-527-7677
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Internet-Draft                 GDOI Update                 February 2006

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