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RFC 8052

Proposed STD
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Group Domain of Interpretation (GDOI) Protocol Support for IEC 62351 Security Services

 


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Internet Engineering Task Force (IETF)                           B. Weis
Request for Comments: 8052                                    M. Seewald
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                                  H. Falk
                                                                   SISCO
                                                               June 2017


             Group Domain of Interpretation (GDOI) Protocol
                Support for IEC 62351 Security Services

Abstract

   The IEC 61850 power utility automation family of standards describes
   methods using Ethernet and IP for distributing control and data
   frames within and between substations.  The IEC 61850-90-5 and IEC
   62351-9 standards specify the use of the Group Domain of
   Interpretation (GDOI) protocol (RFC 6407) to distribute security
   transforms for some IEC 61850 security protocols.  This memo defines
   GDOI payloads to support those security protocols.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8052.

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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  IEC 61850 Protocol Information  . . . . . . . . . . . . . . .   5
     2.1.  ID Payload  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  SA TEK Payload  . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  KD Payload  . . . . . . . . . . . . . . . . . . . . . . .  11
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Example ID, SA TEK, and KD Payloads for IEC 61850  .  19
   Appendix B.  Implementation Considerations  . . . . . . . . . . .  23
     B.1.  DER Length Fields . . . . . . . . . . . . . . . . . . . .  23
     B.2.  Groups with Multiple Senders  . . . . . . . . . . . . . .  23
   Appendix C.  Data Attribute Format  . . . . . . . . . . . . . . .  23
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   Power substations use Generic Object Oriented Substation Events
   (GOOSE) protocol [IEC-61850-8-1] to distribute control information to
   groups of devices using a multicast strategy.  Sources within the
   power substations also distribute IEC 61850-9-2 sampled values data
   streams [IEC-61850-9-2].  The IEC 62351-9 standard [IEC-62351-9]
   describes key management methods for the security methods protecting
   these IEC 61850 messages, including methods of device authentication
   and authorization, and methods of policy and keying material

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   agreement for IEC 61850 message encryption and data integrity
   protection.  These key management methods include the use of GDOI
   [RFC6407] to distribute the security policy and session keying
   material used to protect IEC 61850 messages when the messages are
   sent to a group of devices.

   The protection of the messages is defined in IEC 62351-6
   [IEC-62351-6], IEC 61850-8-1 [IEC-61850-8-1], and IEC 61850-9-2
   [IEC-61850-9-2].  Protected IEC 61850 messages typically include the
   output of a Message Authentication Code (MAC) and may also be
   encrypted using a symmetric cipher such as the Advanced Encryption
   Standard (AES).

   Section 5.5.2 of RFC 6407 specifies that the following information
   needs to be provided in order to fully define a new security
   protocol:

   o  The Protocol-ID for the particular security protocol

   o  The SPI Size

   o  The method of SPI generation

   o  The transforms, attributes, and keys needed by the security
      protocol

   This document defines GDOI payloads to distribute policy and keying
   material to protect IEC 61850 messages and defines the necessary
   information to ensure interoperability between IEC 61850
   implementations.

   This memo extends RFC 6407 in order to define extensions needed by
   IEC 62351-9.  With the current IANA registry rules set up by RFC
   6407, this requires "Standards Action" [RFC5226] by the IETF; this
   document satisfies that requirement.  As the relevant IEC
   specifications are not available to the IETF community, it is not
   possible for this RFC to fully describe the security considerations
   that apply.  Therefore, implementers need to depend on the security
   analysis within the IEC specifications.  As two different Standards
   Development Organizations are involved here, and since group key
   management is inherently complex, it is possible that some security
   issues have not been identified, so additional analysis of the
   security of the combined set of specifications may be advisable.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174]  when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   The following key terms are used throughout this document:

   Generic Object Oriented Substation Events:  Power substation control
         model defined as per IEC 61850.

   IEC 61850 message:  A message in the IEC 61850 family of protocols
         carrying control or data frames between substation devices.

1.3.  Acronyms

   The following acronyms are used throughout this document:

   AES     Advanced Encryption Standard

   GCKS    Group Controller/Key Server

   GDOI    Group Domain of Interpretation

   GM      Group Member

   GOOSE   Generic Object Oriented Substation Events

   KD      Key Download

   KEK     Key Encryption Key

   MAC     Message Authentication Code

   SA      Security Association

   SPI     Security Parameter Index

   TEK     Traffic Encryption Key

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2.  IEC 61850 Protocol Information

   The following subsections describe the GDOI payload extensions that
   are needed in order to distribute security policy and keying material
   for the IEC 62351 Security Services.  The Identification (ID) Payload
   is used to describe an IEC 62351 GDOI group.  The Security
   Association (SA) Traffic Encryption Key (TEK) payload is used to
   describe the policy defined by a Group Controller/Key Server (GCKS)
   for a particular IEC 62351 traffic selector.  No changes are required
   to the Key Download (KD) Payload, but a mapping of IEC 62351 keys to
   the KD payload key types is included.

   All multi-octet fields are in network byte order.

2.1.  ID Payload

   The ID payload in a GDOI GROUPKEY-PULL exchange allows the Group
   Member (GM) to declare the group it would like to join.  A group is
   defined by an ID payload as defined in GDOI [RFC6407] and reproduced
   in Figure 1.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Next Payload  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !    ID Type    !      DOI-Specific ID Data = 0                 !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ~                       Identification Data                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                 Figure 1: RFC 6407 Identification Payload

   An ID Type name of ID_OID (value 13) is defined in this memo to
   specify an Object Identifier (OID) [ITU-T-X.683] encoded using
   Distinguished Encoding Rules (DER) [ITU-T-X.690].  Associated with
   the OID may be an OID-Specific Payload DER encoded as further
   defining the group.  Several OIDs are specified in [IEC-62351-9] for
   use with IEC 61850.  Each OID represents a GOOSE or Sampled Value
   protocol, and in some cases IEC 61850 also specifies a particular
   multicast destination address to be described in the OID-Specific
   Payload field.  The format of the ID_OID Identification Data is
   specified as shown in Figure 2.

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID Length   !                       OID                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID-Specific Payload Length  !     OID-Specific Payload      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                   Figure 2: ID_OID Identification Data

   The ID_OID Identification Data fields are defined as follows:

   o  OID Length (1 octet) -- Length of the OID field.

   o  OID (variable) -- An ASN.1 ObjectIdentifier encoded using DER
      [ITU-T-X.690].

   o  OID-Specific Payload Length (2 octets) -- Length of the OID-
      Specific payload.  Set to zero if the OID does not require an OID-
      Specific payload.

   o  OID-Specific Payload (variable) -- OID-specific selector encoded
      in DER.  If OID-Specific Payload Length is set to zero, this field
      does not appear in the ID payload.

2.2.  SA TEK Payload

   The SA TEK payload contains security attributes for a single set of
   policy associated with a group TEK.  The type of policy to be used
   with the TEK is described by a Protocol-ID field included in the SA
   TEK.  As shown in Figure 3 reproduced from RFC 6407, each Protocol-ID
   describes a particular TEK Protocol-Specific Payload definition.

      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  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Protocol-ID   !       TEK Protocol-Specific Payload           ~
     +-+-+-+-+-+-+-+-+                                               ~
     ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                     Figure 3: RFC 6407 SA TEK Payload

   The Protocol-ID name of GDOI_PROTO_IEC_61850 (value 3) is defined in
   this memo for the purposes of distributing IEC 61850 policy.  A
   GDOI_PROTO_IEC_61850 SA TEK includes an OID and (optionally) an OID-

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   Specific payload that together define the selectors for the network
   traffic.  The selector fields are followed by security policy fields
   indicating how the specified traffic is to be protected.  The
   GDOI_PROTO_IEC_61850 TEK Protocol-Specific Payload is defined as
   shown in Figure 4.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID Length   !                       OID                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID-Specific Payload Length  !     OID-Specific Payload      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                              SPI                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !           Auth Alg            !            Enc Alg            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                    Remaining Lifetime Value                   !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                      SA Data Attributes                       ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                    Figure 4: IEC 61850 SA TEK Payload

   The GDOI_PROTO_IEC_61850 SA TEK payload fields are defined as
   follows:

   o  OID Length (1 octet) -- Length of the OID field.

   o  OID (variable) -- An ASN.1 ObjectIdentifier encoded using DER.
      OIDs defined in IEC 61850 declare the type of IEC 61850 message to
      be protected, as defined by [IEC-62351-9].

   o  OID-Specific Payload Length (2 octets) -- Length of the OID-
      Specific payload.  This field is set to zero if the policy does
      not include an OID-Specific payload.

   o  OID-Specific Payload (variable) -- The traffic selector (e.g.,
      multicast address) specific to the OID encoded using DER.  Some
      OID policy settings do not require the use of an OID-Specific
      payload, in which case this field is not included in the TEK and
      the OID-Specific Payload Length is set to zero.

   o  SPI (4 octets) -- Identifier for the Current Key. This field
      represents an SPI.

   o  Auth Alg (2 octets) -- Authentication Algorithm ID.  Valid values
      are defined in Section 2.2.2.

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   o  Enc Alg (2 octets) -- Confidentiality Algorithm ID.  Valid values
      are defined in Section 2.2.3.

   o  Remaining Lifetime value (4 octets) -- The number of seconds
      remaining before this TEK expires.  A value of zero (0) shall
      indicate that the TEK does not have an expire time.

   o  SA Data Attributes (variable length) -- Contains zero or more
      attributes associated with this SA.  Section 2.2.4 defines
      attributes.

2.2.1.  Selectors

   The OID and (optionally) an OID-Specific payload together define the
   selectors for the network traffic.  While they may match the OID and
   OID-Specific payload that the GM had previously requested in the ID
   payload, there is no guarantee that this will be the case.  Including
   selectors in the SA TEK is important for at least the following
   reasons:

   o  The Key Server (KS) policy may direct the KS to return multiple
      TEKs, each representing different traffic selectors, and it is
      important that every GM receiving the set of TEKs explicitly
      identify the traffic selectors associated with the TEK.

   o  The KS policy may include the use of a GDOI GROUPKEY-PUSH message,
      which distributes new or replacement TEKs to group members.  Since
      the GROUPKEY-PUSH message does not contain an ID payload, the TEK
      definition must include the traffic selectors.

2.2.2.  Authentication Algorithms

   This memo defines the following authentication algorithms for use
   with this TEK.  These algorithms are defined in [IEC-TR-61850-90-5],
   including requirements on one or more algorithms defined as mandatory
   to implement.

   o  NONE.  Specifies that an authentication algorithm is not required,
      or when the accompanying confidentiality algorithm includes
      authentication (e.g., AES-GCM-128).  See Section 3 for cautionary
      notes regarding using this value without any confidentiality
      algorithm.

   o  HMAC-SHA256-128.  Specifies the use of SHA-256 [FIPS180-4]
      combined with HMAC [RFC2104].  The output is truncated to 128
      bits, as per [RFC2104].  The key size is the size of the hash
      value produced by SHA-256 (256 bits).

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   o  HMAC-SHA256.  Specifies the use of SHA-256 [FIPS180-4] combined
      with HMAC [RFC2104].  The key size is the size of the hash value
      produced by SHA-256 (256 bits).

   o  AES-GMAC-128.  Specifies the use of AES [FIPS197] in the Galois
      Message Authentication Code (GMAC) mode [SP.800-38D] with a
      128-bit key size.

   o  AES-GMAC-256.  Specifies the use of AES [FIPS197] in the Galois
      Message Authentication Code (GMAC) mode [SP.800-38D] with a
      256-bit key size.

2.2.3.  Confidentiality Algorithms

   This memo defines the following confidentiality algorithms for use
   with this TEK.  These algorithms are defined in [IEC-TR-61850-90-5],
   including requirements on one or more algorithms defined as mandatory
   to implement.

   o  NONE.  Specifies that confidentiality is not required.  Note: See
      Section 3 for guidance on cautionary notes regarding using this
      value.

   o  AES-CBC-128.  Specifies the use of AES [FIPS197] in the Cipher
      Block Chaining (CBC) mode [SP.800-38A] with a 128-bit key size.
      This encryption algorithm does not provide authentication and MUST
      NOT be used with the NONE authentication algorithm.

   o  AES-CBC-256.  Specifies the use of AES [FIPS197] in the Cipher
      Block Chaining (CBC) mode [SP.800-38A] with a 256-bit key size.
      This encryption algorithm does not provide authentication and MUST
      NOT be used with the NONE authentication algorithm.

   o  AES-GCM-128.  Specifies the use of AES [FIPS197] in the Galois/
      Counter Mode (GCM) mode [SP.800-38D] with a 128-bit key size.
      This encryption algorithm provides authentication and is used with
      a NONE authentication algorithm.

   o  AES-GCM-256.  Specifies the use of AES [FIPS197] in the Galois/
      Counter Mode (GCM) mode [SP.800-38D] with a 256-bit key size.
      This encryption algorithm provides authentication and is used with
      a NONE authentication algorithm.

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2.2.4.  SA Attributes

   The following attributes may be present in an SA TEK.  The attributes
   must follow the format described in Appendix C).

2.2.4.1.  SA Time Activation Delay (SA_ATD)

   A GCKS will sometimes distribute an SA TEK in advance of when it is
   expected to be used.  This is communicated to group members using the
   SA Activation Time Delay (SA_ATD) attribute.  When a GM receives an
   SA_TEK with this attribute, it waits for the number of seconds
   contained within the attribute before installing it for either
   transmitting or receiving.

   This Activation Time Delay attribute applies only this SA, and MAY be
   used in either a GROUPKEY-PULL or GROUPKEY-PUSH exchange.  RFC 6407
   also describes an ACTIVATION_TIME_DELAY attribute for the Group
   Associated Policy (GAP) payload, which is applied to all Security
   Associations and is restricted to use in a GROUPKEY-PUSH message.  If
   both attributes are included in a GROUPKEY-PUSH payload, the value
   contained in SA_ATD will be used.

2.2.4.2.  Key Delivery Assurance (SA_KDA)

   Group policy can include notifying a multicast source ("Publisher")
   of an indication of whether multicast receivers ("Subscribers") have
   previously received the SA TEK.  This notification allows a Publisher
   to set a policy as to whether to activate the new SA TEK or not based
   on the percentage of Subscribers that are able to receive packets
   protected by the SA TEK.  The attribute value is a number between 0
   and 100 (inclusive).

2.2.5.  SPI Discussion

   As noted in Section 1, RFC 6407 requires that characteristics of an
   SPI must be defined.  An SPI in a GDOI_PROTO_IEC_61850 SA TEK is
   represented as a Key Identifier (KeyID).  The SPI size is 4 octets.
   The SPI is unilaterally chosen by the GCKS using any method chosen by
   the implementation.  However, an implementation needs to take care
   not to duplicate an SPI value that is currently in use for a
   particular group.

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2.3.  KD Payload

   The KD payload contains group keys for the policy specified in the SA
   Payload.  It is comprised of a set of Key Packets, each of which hold
   the keying material associated with an SPI (i.e., an IEC 61850 Key
   Identifier).  The RFC 6407 KD payload format is reproduced in
   Figure 5.

      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  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Number of Key Packets         !            RESERVED2          !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ~                    Key Packets                                ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                           Figure 5: KD Payload

   Each Key Packet holds the keying material associated with a
   particular IEC 61850 Key Identifier, although GDOI refers to it as an
   SPI.  The keying material is described in a set of attributes
   indicating an encryption key, integrity key, etc., in accordance with
   the security policy of the group as defined by the associated SA
   Payload.  Each Key Packet has the following format, reproduced in
   Figure 6.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   KD Type     !   RESERVED    !       Key Packet Length       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !    SPI Size   !                   SPI (variable)              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ~                    Key Packet Attributes                      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                           Figure 6: Key Packet

   No changes are needed to GDOI in order to distribute IEC 61850 keying
   material, but the keys MUST be distributed as defined in Section 5.6
   of RFC 6407.  The KD Type MUST be TEK (1).

   A key associated with an IEC 61850 authentication algorithm
   (distributed in the Auth Alg field) MUST be distributed as a
   TEK_INTEGRITY_KEY attribute.  The value of the attribute is
   interpreted according to the type of key distributed in the SA TEK:

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   o  HMAC-SHA256-128, HMAC-SHA256.  The value is 32 octets.

   o  AES-GMAC-128.  The value is 20 octets.  The first 16 octets are
      the 128-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

   o  AES-GMAC-256.  The value is 36 octets.  The first 32 octets are
      the 256-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

   A key associated with an IEC 61850 confidentiality algorithm
   (distributed in the Enc Alg SA TEK field) MUST be distributed as a
   TEK_ALGORITHM_KEY attribute.  The value of the attribute is
   interpreted according to the type of key distributed in the SA TEK:

   o  AES-CBC-128.  The value is 16 octets.

   o  AES-CBC-256.  The value is 32 octets.

   o  AES-GCM-128.  The value is 20 octets.  The first 16 octets are the
      128-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

   o  AES-GCM-256.  The value is 36 octets.  The first 32 octets are the
      256-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

3.  Security Considerations

   GDOI is a Security Association (SA) management protocol for groups of
   senders and receivers.  This protocol performs authentication of
   communicating protocol participants (Group Member, Group Controller/
   Key Server).  GDOI provides confidentiality of key management
   messages, and it provides source authentication of those messages.
   GDOI includes defenses against man-in-middle, connection-hijacking,
   replay, reflection, and denial-of-service (DOS) attacks on unsecured
   networks.  GDOI assumes that the network is not secure and may be
   under the complete control of an attacker.  The Security
   Considerations described in RFC 6407 are relevant to the distribution
   of GOOSE and sampled values policy as defined in this memo.

   Message Authentication is an optional property for IEC 62351 Security
   Services; however, when encryption is used, authentication MUST also
   be provided by using an authenticated encryption algorithm such as
   AES-GCM-128 or by using a specific authentication algorithm such as
   HMAC-SHA-256.  Setting the authentication algorithm to NONE but
   setting the confidentiality algorithm to an algorithm that does not

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   include authentication (i.e., is marked with an N in the
   "Authenticated Encryption" column of the "IEC 62351-9 Confidentiality
   Values" registry) is not safe and MUST NOT be done.

   When Message Authentication is used, a common practice is to truncate
   the output of a MAC and include some of the bits in the integrity
   protection field of the data security transform.  Current guidance in
   [RFC2104] is to truncate no less than half of the length of the hash
   output.  The authentication algorithm HMAC-SHA256-128 defined in this
   memo truncates the output to exactly half of the output, which
   follows this guidance.

   Confidentiality is an optional security property for IEC 62351
   Security Services.  Confidentiality Algorithm IDs SHOULD be included
   in the IEC 61850 SA TEK payload if the IEC 61850 messages are
   expected to traverse public network links and are not protected by
   another level of encryption (e.g., an encrypted Virtual Private
   Network).  Current cryptographic advice indicates that the use of
   AES-CBC-128 for confidentiality is sufficient for the foreseeable
   future [SP.800-131A], but some security policies may require the use
   of AES-CBC-256.

   IEC 62351 Security Services describe a variety of policy choices for
   protecting network traffic, including the option of specifying no
   protection at all.  This is enabled with the use of NONE as an
   authentication algorithm and/or confidentiality algorithm.  The
   following guidance is given regarding the use of NONE.

   o  Setting both the authentication algorithm and confidentiality
      algorithm to NONE is possible but NOT RECOMMENDED.  Setting such a
      policy is sometimes necessary during a migration period, when
      traffic is being protected incrementally and some traffic has not
      yet been scheduled for protection.  Alternatively, site security
      policy for some packet flows requires inspection of packet data on
      the private network followed by network-layer encryption before
      delivery to a public network.

   o  Setting the confidentiality algorithm to NONE but setting the
      authentication algorithm to a MAC can be an acceptable policy in
      the following conditions: the disclosed information in the data
      packets is comprised of raw data values and the disclosure of the
      data files is believed to be of no more value to an observer than
      traffic analysis on the frequency and size of packets protected
      for confidentiality.  Alternatively, site security policy for some
      packet flows requires inspection of packet data on the private
      network followed by network-layer encryption before delivery to a
      public network.

Top      ToC       Page 14 
   o  Setting the authentication algorithm to NONE but setting the
      confidentiality algorithm to an algorithm that does not include
      authentication is not safe and MUST NOT be done.

4.  IANA Considerations

   The "Group Domain of Interpretation (GDOI) Payloads" registry
   [GDOI-REG] has been updated as described below.  The terms "Expert
   Review", "Reserved", and "Private Use" are used as defined in
   [RFC5226].

   o  GDOI_PROTO_IEC_61850 (value 3) has been added to the "SA TEK
      Payload Values - Protocol-ID" registry.

   o  A new "IEC 62351-9 Authentication Values" registry has been
      created.  This registry defines Auth Alg values.  Initial values
      for the registry are given below; future assignments are to be
      made through "Expert Review" [RFC5226].

      Name                         Value
      ----                         -----
      Reserved                       0
      NONE                           1
      HMAC-SHA256-128                2
      HMAC-SHA256                    3
      AES-GMAC-128                   4
      AES-GMAC-256                   5
      Unassigned                  6-61439
      Reserved for Private Use  61440-65535

   o  A new "IEC 62351-9 Confidentiality Values" registry has been
      created.  This registry defines Enc Alg values.  Initial values
      for the registry are given below; future assignments are to be
      made through "Expert Review" [RFC5226].

      Name                         Value     Authenticated Encryption
      ----                         -----     ------------------------
      Reserved                       0
      NONE                           1
      AES-CBC-128                    2                 N
      AES-CBC-256                    3                 N
      AES-GCM-128                    4                 Y
      AES-GCM-256                    5                 Y
      Unassigned                  6-61439
      Reserved for Private Use  61440-65535

Top      ToC       Page 15 
   o  A new "GDOI SA TEK Attributes" registry has been created.  This
      registry defines SA TEK attributes.  Initial values for the
      registry are given below; future assignments are to be made
      through "Expert Review" [RFC5226].  In the table, attributes that
      are defined as Type/Value (TV) are marked as Basic (B); attributes
      that are defined as Type/Length/Value (TLV) are marked as Variable
      (V).

      Attribute                    Value           Type
      ---------                    -----           ----
      Reserved                       0
      SA_ATD                         1               V
      SA_KDA                         2               B
      Unassigned                  3-28671
      Reserved for Private Use   28672-32767

   o  A new "ID Types" registry has been created for the Identification
      Payload when the DOI is GDOI.  This registry is taken from the
      "IPSEC Identification Type" registry for the IPsec DOI
      [IPSEC-DOI-REG].  Values 1-12 are defined identically to the
      equivalent values in the "IPSEC Identification Type" registry.
      Value 13 (ID_OID) is defined in this memo.  Initial values for the
      registry are given below; future assignments are to be made
      through "Expert Review" [RFC5226].

      Name                          Value
      ----                          -----
      Reserved                        0
      ID_IPV4_ADDR                    1
      ID_FQDN                         2
      ID_USER_FQDN                    3
      ID_IPV4_ADDR_SUBNET             4
      ID_IPV6_ADDR                    5
      ID_IPV6_ADDR_SUBNET             6
      ID_IPV4_ADDR_RANGE              7
      ID_IPV6_ADDR_RANGE              8
      ID_DER_ASN1_DN                  9
      ID_DER_ASN1_GN                  10
      ID_KEY_ID                       11
      ID_LIST                         12
      ID_OID                          13
      Unassigned                   14-61439
      Reserved for Private Use   61440-65535

Top      ToC       Page 16 
5.  References

5.1.  Normative References

   [IEC-62351-9]
              International Electrotechnical Commission, "Power systems
              management and associated information exchange - Data and
              communications security - Part 9: Cyber security key
              management for power system equipment", IEC 62351-9:2017,
              May 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <http://www.rfc-editor.org/info/rfc6407>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

5.2.  Informative References

   [FIPS180-4]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [FIPS197]  National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", FIPS PUB 197, November 2001,
              <http://csrc.nist.gov/publications/fips/fips197/
              fips-197.pdf>.

   [GDOI-REG]
              IANA, "Group Domain of Interpretation (GDOI) Payloads",
              <http://www.iana.org/assignments/gdoi-payloads>.

Top      ToC       Page 17 
   [IEC-61850-8-1]
              International Electrotechnical Commission, "Communication
              networks and systems for power utility automation - Part
              8-1: Specific communication service mapping (SCSM) -
              Mappings to MMS (ISO 9506-1 and ISO 9506-2) and to ISO/IEC
              8802-3", IEC 61850-8-1, June 2011.

   [IEC-61850-9-2]
              International Electrotechnical Commission, "Communication
              networks and systems for power utility automation - Part
              9-2: Specific communication service mapping (SCSM) -
              Sampled values over ISO/IEC 8802-3", IEC 61850-2,
              September 2011.

   [IEC-62351-6]
              International Electrotechnical Commission, "Power systems
              management and associated information exchange - Data and
              communications security - Part 6: Security for IEC 61850",
              IEC 62351-6, June 2007.

   [IEC-TR-61850-90-5]
              International Electrotechnical Commission, "Communication
              networks and systems for power utility automation - Part
              90-5: Use of IEC 61850 to transmit synchrophasor
              information according to IEEE C37.118", IEC TR 62351-90-5,
              May 2012.

   [IPSEC-DOI-REG]
              IANA, "'Magic Numbers' for ISAKMP Protocol",
              <http://www.iana.org/assignments/isakmp-registry>.

   [ITU-T-X.683]
              International Telecommunications Union, "Information
              technology - Abstract Syntax Notation One (ASN.1):
              Parameterization of ASN.1 specifications",
              ITU-T Recommendation X.683, August 2015,
              <https://www.itu.int/rec/T-REC-X.683-201508-I/en>.

   [ITU-T-X.690]
              International Telecommunications Union, "Information
              technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, August 2015,
              <https://www.itu.int/rec/T-REC-X.690-201508-I/en>.

Top      ToC       Page 18 
   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [SP.800-131A]
              Barker, E. and A. Roginsky, "Transitions: Recommendation
              for Transitioning the Use of Cryptographic Algorithms and
              Key Lengths", NIST Special Publication 800-131A,
              DOI 10.6028/NIST.SP.800-131Ar1, November 2015,
              <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-131Ar1.pdf>.

   [SP.800-38A]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Methods and Techniques", NIST Special
              Publication 800-38A, DOI 10.6028/NIST.SP.800-38A, December
              2001, <http://nvlpubs.nist.gov/nistpubs/Legacy/SP/
              nistspecialpublication800-38a.pdf>.

   [SP.800-38D]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC", NIST
              Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D,
              November 2007,
              <http://nvlpubs.nist.gov/nistpubs/Legacy/SP/
              nistspecialpublication800-38d.pdf>.

Top      ToC       Page 19 
Appendix A.  Example ID, SA TEK, and KD Payloads for IEC 61850

   An Intelligent Electronic Device (IED) begins a GROUPKEY-PULL
   exchange and requests keys and security policy for
   61850_UDP_ADDR_GOOSE (OID = 1.2.840.10070.61850.8.1.2 as defined in
   [IEC-61850-9-2]) and IP multicast address 233.252.0.1 encoded as
   specified in [IEC-61850-9-2].

   OID and OID-Specific Payload protocol fields are variable-length
   fields.  To improve readability, their representations in Figures 7
   and 8 are "compressed", as indicated by a trailing "~" for these
   fields.  Implementations should be aware that because these fields
   are variably sized, some payload fields may not be conveniently
   aligned on an even octet.

   Note: The actual DER for the OID-Specific Payload field is defined in
   [IEC-62351-6].

      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  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! ID Type=13    !     DOI-Specific ID Data = 0                  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID Len=13    ! OID=<06 0B 2A 86 48 CE 56 83 E3 1A 08 01 02>  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID-Specific Payload Len      ! OID SP=<DER for 233.252.0.1>  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                  Figure 7: Sample Identification Payload

   The Key Server responds with the following SA TEK payload including
   two GDOI_PROTO_IEC_61850 Protocol-Specific TEK payloads in the second
   GROUPKEY-PULL message.  The first one is to be activated immediately
   and has a lifetime of 3600 seconds (0x0E10) remaining.  The second
   has a lifetime of 12 hours (0xA8C0) and should be activated in 3300
   seconds (0x0CE4), which gives a 5-minute (300-second) overlap of the
   two SAs.

Top      ToC       Page 20 
      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  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                             DOI = 2                           !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                         Situation = 0                         !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! SA Attr NP=16 (SA TEK)        !          RESERVED2            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! NP=16 (SA TEK)!   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Prot-ID=3     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID Len=13    ! OID=<06 0B 2A 86 48 CE 56 83 E3 1A 08 01 02>  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID-Specific Payload Len      !OID SP=<DER for 233.252.0.1>   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=1                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  AuthAlg=1 (HMAC-SHA256-128)  !    EncAlg=2  (AES-CBC-128)    !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !              Remaining Lifetime=0x0E01                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! SA Attr NP=16 (SA TEK)        !          RESERVED2            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! NP=0          !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Prot-ID=3     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID Len=13    ! OID=<06 0B 2A 86 48 CE 56 83 E3 1A 08 01 02>  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID-Specific Payload Len      !OID SP=<DER for 233.252.0.1>   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=2                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !       AuthAlg=0 (NONE)        !    EncAlg=4 (AES-GCM-128)     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !              Remaining Lifetime=0xA8C0                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !       Type=1 (SA_ATD)         !           Length=4            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                        Value=0x0CE4                           !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                   Figure 8: Sample IEC 61850 SA Payload

Top      ToC       Page 21 
   The IED acknowledges that it is capable and willing to use this
   policy in the third GROUPKEY-PULL message.  In response, the KS sends
   a KD payload to the requesting IED.  This concludes the GROUPKEY-PULL
   exchange.

Top      ToC       Page 22 
      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  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Number of Key Packets=2       !            RESERVED2          !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   KD Type=1   !   RESERVED    !        Key Packet Length      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   SPI Size=4  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=1                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! TYPE=TEK_INTEGRITY_KEY (2)    ! LENGTH=32 (256-bit key)       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                                                               !
     !                                                               !
     !                                                               !
     !                        HMAC-SHA256 Key                        !
     !                                                               !
     !                                                               !
     !                                                               !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! TYPE=TEK_ALGORITHM_KEY (1)    ! LENGTH=16                     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                                                               !
     !                        AES-CBC-128 Key                        !
     !                                                               !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   KD Type=1   !   RESERVED    !        Key Packet Length      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   SPI Size=4  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=2                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! TYPE=TEK_ALGORITHM_KEY (1)    ! LENGTH=20                     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                                                               !
     !                    AES-GCM-128 Key & Salt                     !
     !                                                               !
     !                                                               !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                        Figure 9: Sample KD Payload

Top      ToC       Page 23 
Appendix B.  Implementation Considerations

   Several topics have been suggested as useful for implementers.

B.1.  DER Length Fields

   The ID and SA TEK payloads defined in this memo include explicit
   lengths for fields formatted as DER.  This includes the OID Length
   and OID-Specific Payload Length fields shown in Figures 2 and 4.
   Strictly speaking, these lengths are redundant since the length of
   the DER value is also encoded within the DER fields.  It would be
   possible to determine the lengths of the fields from those encoded
   values.  However, many implementations will find the explicit length
   fields convenient when constructing and sanity checking the GDOI
   messages including these payloads.  Implementations will thus be
   spared from manipulating the DER itself when performing activities
   that do not otherwise require parsing in order to obtain values
   therein.

B.2.  Groups with Multiple Senders

   GCKS policy may specify more than one protected type of IEC 61850
   message within a GDOI group.  This is represented within a GDOI SA
   Payload by the presence of an SA TEK payload for each multicast group
   that is protected as part of group policy.  The OID contained in each
   of the SA TEK payloads may be identical, but the value of each OID-
   Specific Payload would be unique.  Typically, the OID-Specific
   payload defines a destination address, and there is typically a
   single sender to that destination address.

Appendix C.  Data Attribute Format

   Data attributes attached to an SA TEK following the data attribute
   format are described in this section.  Data attributes can be in
   Type/Value (TV) format (useful when a value is defined to be less
   than two octets in size) or in Type/Length/Value (TLV) form.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !A!       Attribute Type        !    AF=0  Attribute Length     !
   !F!                             !    AF=1  Attribute Value      !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                   AF=0  Attribute Value                       .
   .                   AF=1  Not Transmitted                       .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 10: Data Attributes

Top      ToC       Page 24 
   The Data Attributes fields are defined as follows:

   o  Attribute Type (2 octets) -- Unique identifier for each type of
      attribute.  These attributes are defined as part of the DOI-
      specific information.  The most significant bit, or Attribute
      Format (AF), indicates whether the data attributes follow the
      Type/Length/Value (TLV) format or a shortened Type/Value (TV)
      format.  If the AF bit is a zero (0), then the data attributes are
      of the Type/Length/Value (TLV) form.  If the AF bit is a one (1),
      then the data attributes are of the Type/Value form.

   o  Attribute Length (2 octets) -- Length in octets of the Attribute
      Value.  When the AF bit is a one (1), the Attribute Value is only
      2 octets, and the Attribute Length field is not present.

   o  Attribute Value (variable length) -- Value of the attribute
      associated with the DOI-specific Attribute Type.  If the AF bit is
      a zero (0), this field has a variable length defined by the
      Attribute Length field.  If the AF bit is a one (1), the Attribute
      Value has a length of 2 octets.

Acknowledgements

   The authors thank Sean Turner, Steffen Fries, Yoav Nir, Vincent Roca,
   Dennis Bourget, and David Boose for their thoughtful reviews, each of
   which resulted in substantial improvements to this memo.  Joe Salowey
   provided valuable guidance as document shepherd during the
   publication process.  The authors are indebted to Kathleen Moriarty
   for her agreement to sponsor the publication of the document.

Top      ToC       Page 25 
Authors' Addresses

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

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


   Maik Seewald
   Cisco Systems
   Am Soeldnermoos 17
   D-85399 Hallbergmoos
   Germany

   Phone: +49 619 6773 9655
   Email: maseewal@cisco.com


   Herb Falk
   SISCO
   6605 19-1/2 Mile Road
   Sterling Heights, MI  48314
   United States of America

   Phone: +1 586 254 0020 x105
   Email: herb@sisconet.com