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Delay-Tolerant Networking                                     E. Birrane
Internet-Draft                                               K. McKeever
Intended status: Standards Track                                 JHU/APL
Expires: August 10, 2020                                February 7, 2020


                 Bundle Protocol Security Specification
                        draft-ietf-dtn-bpsec-19

Abstract

   This document defines a security protocol providing data integrity
   and confidentiality services for the Bundle Protocol.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on August 10, 2020.

Copyright Notice

   Copyright (c) 2020 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
   (https://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.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Supported Security Services . . . . . . . . . . . . . . .   3
     1.2.  Specification Scope . . . . . . . . . . . . . . . . . . .   4
     1.3.  Related Documents . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Design Decisions  . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Block-Level Granularity . . . . . . . . . . . . . . . . .   7
     2.2.  Multiple Security Sources . . . . . . . . . . . . . . . .   8
     2.3.  Mixed Security Policy . . . . . . . . . . . . . . . . . .   8
     2.4.  User-Defined Security Contexts  . . . . . . . . . . . . .   9
     2.5.  Deterministic Processing  . . . . . . . . . . . . . . . .   9
   3.  Security Blocks . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Block Definitions . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Uniqueness  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Target Multiplicity . . . . . . . . . . . . . . . . . . .  11
     3.4.  Target Identification . . . . . . . . . . . . . . . . . .  12
     3.5.  Block Representation  . . . . . . . . . . . . . . . . . .  12
     3.6.  Abstract Security Block . . . . . . . . . . . . . . . . .  12
     3.7.  Block Integrity Block . . . . . . . . . . . . . . . . . .  15
     3.8.  Block Confidentiality Block . . . . . . . . . . . . . . .  16
     3.9.  Block Interactions  . . . . . . . . . . . . . . . . . . .  17
     3.10. Parameter and Result Identification . . . . . . . . . . .  18
     3.11. BSP Block Examples  . . . . . . . . . . . . . . . . . . .  19
       3.11.1.  Example 1: Constructing a Bundle with Security . . .  19
       3.11.2.  Example 2: Adding More Security At A New Node  . . .  20
   4.  Canonical Forms . . . . . . . . . . . . . . . . . . . . . . .  20
   5.  Security Processing . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  Bundles Received from Other Nodes . . . . . . . . . . . .  22
       5.1.1.  Receiving BCBs  . . . . . . . . . . . . . . . . . . .  22
       5.1.2.  Receiving BIBs  . . . . . . . . . . . . . . . . . . .  23
     5.2.  Bundle Fragmentation and Reassembly . . . . . . . . . . .  24
   6.  Key Management  . . . . . . . . . . . . . . . . . . . . . . .  24
   7.  Security Policy Considerations  . . . . . . . . . . . . . . .  24
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
     8.1.  Attacker Capabilities and Objectives  . . . . . . . . . .  26
     8.2.  Attacker Behaviors and BPSec Mitigations  . . . . . . . .  27
       8.2.1.  Eavesdropping Attacks . . . . . . . . . . . . . . . .  27
       8.2.2.  Modification Attacks  . . . . . . . . . . . . . . . .  28
       8.2.3.  Topology Attacks  . . . . . . . . . . . . . . . . . .  29
       8.2.4.  Message Injection . . . . . . . . . . . . . . . . . .  30
   9.  Security Context Considerations . . . . . . . . . . . . . . .  30
     9.1.  Mandating Security Contexts . . . . . . . . . . . . . . .  30
     9.2.  Identification and Configuration  . . . . . . . . . . . .  31
     9.3.  Authorship  . . . . . . . . . . . . . . . . . . . . . . .  32
   10. Defining Other Security Blocks  . . . . . . . . . . . . . . .  33
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34



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     11.1.  Bundle Block Types . . . . . . . . . . . . . . . . . . .  34
     11.2.  Security Context Identifiers . . . . . . . . . . . . . .  35
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     12.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   This document defines security features for the Bundle Protocol (BP)
   [I-D.ietf-dtn-bpbis] and is intended for use in Delay Tolerant
   Networks (DTNs) to provide security services between a security
   source and a security acceptor.  When the security source is the
   bundle source and when the security acceptor is the bundle
   destination, the security service provides end-to-end protection.

   The Bundle Protocol specification [I-D.ietf-dtn-bpbis] defines DTN as
   referring to "a networking architecture providing communications in
   and/or through highly stressed environments" where "BP may be viewed
   as sitting at the application layer of some number of constituent
   networks, forming a store-carry-forward overlay network".  The term
   "stressed" environment refers to multiple challenging conditions
   including intermittent connectivity, large and/or variable delays,
   asymmetric data rates, and high bit error rates.

   The BP might be deployed such that portions of the network cannot be
   trusted, posing the usual security challenges related to
   confidentiality and integrity.  However, the stressed nature of the
   BP operating environment imposes unique conditions where usual
   transport security mechanisms may not be sufficient.  For example,
   the store-carry-forward nature of the network may require protecting
   data at rest, preventing unauthorized consumption of critical
   resources such as storage space, and operating without regular
   contact with a centralized security oracle (such as a certificate
   authority).

   An end-to-end security service is needed that operates in all of the
   environments where the BP operates.

1.1.  Supported Security Services

   BPSec provides integrity and confidentiality services for BP bundles,
   as defined in this section.

   Integrity services ensure that changes to target data within a bundle
   can be discovered.  Data changes may be caused by processing errors,
   environmental conditions, or intentional manipulation.  In the



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   context of BPSec, integrity services apply to plain text in the
   bundle.

   Confidentiality services ensure that target data is unintelligible to
   nodes in the DTN, except for authorized nodes possessing special
   information.  This generally means producing cipher text from plain
   text and generating authentication information for that cipher text.
   Confidentiality, in this context, applies to the contents of target
   data and does not extend to hiding the fact that confidentiality
   exists in the bundle.

   NOTE: Hop-by-hop authentication is NOT a supported security service
   in this specification, for two reasons.

   1.  The term "hop-by-hop" is ambiguous in a BP overlay, as nodes that
       are adjacent in the overlay may not be adjacent in physical
       connectivity.  This condition is difficult or impossible to
       detect and therefore hop-by-hop authentication is difficult or
       impossible to enforce.

   2.  Networks in which BPSec may be deployed may have a mixture of
       security-aware and not-security-aware nodes.  Hop-by-hop
       authentication cannot be deployed in a network if adjacent nodes
       in the network have different security capabilities.

1.2.  Specification Scope

   This document defines the security services provided by the BPSec.
   This includes the data specification for representing these services
   as BP extension blocks, and the rules for adding, removing, and
   processing these blocks at various points during the bundle's
   traversal of the DTN.

   BPSec applies only to those nodes that implement it, known as
   "security-aware" nodes.  There might be other nodes in the DTN that
   do not implement BPSec.  While all nodes in a BP overlay can exchange
   bundles, BPSec security operations can only happen at BPSec security-
   aware nodes.

   BPSec addresses only the security of data traveling over the DTN, not
   the underlying DTN itself.  Furthermore, while the BPSec protocol can
   provide security-at-rest in a store-carry-forward network, it does
   not address threats which share computing resources with the DTN and/
   or BPSec software implementations.  These threats may be malicious
   software or compromised libraries which intend to intercept data or
   recover cryptographic material.  Here, it is the responsibility of
   the BPSec implementer to ensure that any cryptographic material,




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   including shared secret or private keys, is protected against access
   within both memory and storage devices.

   This specification addresses neither the fitness of externally-
   defined cryptographic methods nor the security of their
   implementation.  Different networking conditions and operational
   considerations require varying strengths of security mechanism such
   that mandating a cipher suite in this specification may result in too
   much security for some networks and too little security in others.
   It is expected that separate documents will be standardized to define
   security contexts and cipher suites compatible with BPSec, to include
   those that should be used to assess interoperability and those fit
   for operational use in various network scenarios.  A sample security
   context has been defined ([I-D.ietf-dtn-bpsec-interop-sc]) to support
   interoperability testing and serve as an exemplar for how security
   contexts should be defined for this specification.

   This specification does not address the implementation of security
   policy and does not provide a security policy for the BPSec.  Similar
   to cipher suites, security policies are based on the nature and
   capabilities of individual networks and network operational concepts.
   This specification does provide policy considerations when building a
   security policy.

   With the exception of the Bundle Protocol, this specification does
   not address how to combine the BPSec security blocks with other
   protocols, other BP extension blocks, or other best practices to
   achieve security in any particular network implementation.

1.3.  Related Documents

   This document is best read and understood within the context of the
   following other DTN documents:

   "Delay-Tolerant Networking Architecture" [RFC4838] defines the
   architecture for DTNs and identifies certain security assumptions
   made by existing Internet protocols that are not valid in a DTN.

   The Bundle Protocol [I-D.ietf-dtn-bpbis] defines the format and
   processing of bundles, defines the extension block format used to
   represent BPSec security blocks, and defines the canonical block
   structure used by this specification.

   The Concise Binary Object Representation (CBOR) format [RFC7049]
   defines a data format that allows for small code size, fairly small
   message size, and extensibility without version negotiation.  The
   block-specific-data associated with BPSec security blocks are encoded
   in this data format.



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   The Bundle Security Protocol [RFC6257] and Streamlined Bundle
   Security Protocol [I-D.birrane-dtn-sbsp] documents introduced the
   concepts of using BP extension blocks for security services in a DTN.
   The BPSec is a continuation and refinement of these documents.

1.4.  Terminology

   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.  .

   This section defines terminology either unique to the BPSec or
   otherwise necessary for understanding the concepts defined in this
   specification.

   o  Bundle Destination - the node which receives a bundle and delivers
      the payload of the bundle to an application.  Also, the Node ID of
      the Bundle Protocol Agent (BPA) receiving the bundle.  The bundle
      destination acts as the security acceptor for every security
      target in every security block in every bundle it receives.

   o  Bundle Source - the node which originates a bundle.  Also, the
      Node ID of the BPA originating the bundle.

   o  Cipher Suite - a set of one or more algorithms providing integrity
      and/or confidentiality services.  Cipher suites may define user
      parameters (e.g. secret keys to use) but do not provide values for
      those parameters.

   o  Forwarder - any node that transmits a bundle in the DTN.  Also,
      the Node ID of the BPA that sent the bundle on its most recent
      hop.

   o  Intermediate Receiver, Waypoint, or Next Hop - any node that
      receives a bundle from a Forwarder that is not the Bundle
      Destination.  Also, the Node ID of the BPA at any such node.

   o  Path - the ordered sequence of nodes through which a bundle passes
      on its way from Source to Destination.  The path is not
      necessarily known in advance by the bundle or any BPAs in the DTN.

   o  Security Acceptor - a bundle node that processes and dispositions
      one or more security blocks in a bundle.  Also, the Node ID of
      that node.

   o  Security Block - a BPSec extension block in a bundle.



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   o  Security Context - the set of assumptions, algorithms,
      configurations and policies used to implement security services.

   o  Security Operation - the application of a security service to a
      security target, notated as OP(security service, security target).
      For example, OP(confidentiality, payload).  Every security
      operation in a bundle MUST be unique, meaning that a security
      service can only be applied to a security target once in a bundle.
      A security operation is implemented by a security block.

   o  Security Service - a process that gives some protection to a
      security target.  For example, this specification defines security
      services for plain text integrity, plain text confidentiality, and
      cipher text integrity.

   o  Security Source - a bundle node that adds a security block to a
      bundle.  Also, the Node ID of that node.

   o  Security Target - the block within a bundle that receives a
      security service as part of a security operation.

2.  Design Decisions

   The application of security services in a DTN is a complex endeavor
   that must consider physical properties of the network, policies at
   each node, application security requirements, and current and future
   threat environments.  This section identifies those desirable
   properties that guide design decisions for this specification and are
   necessary for understanding the format and behavior of the BPSec
   protocol.

2.1.  Block-Level Granularity

   Security services within this specification must allow different
   blocks within a bundle to have different security services applied to
   them.

   Blocks within a bundle represent different types of information.  The
   primary block contains identification and routing information.  The
   payload block carries application data.  Extension blocks carry a
   variety of data that may augment or annotate the payload, or
   otherwise provide information necessary for the proper processing of
   a bundle along a path.  Therefore, applying a single level and type
   of security across an entire bundle fails to recognize that blocks in
   a bundle represent different types of information with different
   security needs.





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   For example, a payload block might be encrypted to protect its
   contents and an extension block containing summary information
   related to the payload might be integrity signed but unencrypted to
   provide waypoints access to payload-related data without providing
   access to the payload.

2.2.  Multiple Security Sources

   A bundle can have multiple security blocks and these blocks can have
   different security sources.  BPSec implementations MUST NOT assume
   that all blocks in a bundle have the same security operations applied
   to them.

   The Bundle Protocol allows extension blocks to be added to a bundle
   at any time during its existence in the DTN.  When a waypoint adds a
   new extension block to a bundle, that extension block MAY have
   security services applied to it by that waypoint.  Similarly, a
   waypoint MAY add a security service to an existing extension block,
   consistent with its security policy.

   When a waypoint adds a security service to the bundle, the waypoint
   is the security source for that service.  The security block(s) which
   represent that service in the bundle may need to record this security
   source as the bundle destination might need this information for
   processing.

   For example, a bundle source may choose to apply an integrity service
   to its plain text payload.  Later a waypoint node, representing a
   gateway to an insecure portion of the DTN, may receive the bundle and
   choose to apply a confidentiality service.  In this case, the
   integrity security source is the bundle source and the
   confidentiality security source is the waypoint node.

2.3.  Mixed Security Policy

   The security policy enforced by nodes in the DTN may differ.

   Some waypoints might not be security aware and will not be able to
   process security blocks.  Therefore, security blocks must have their
   processing flags set such that the block will be treated
   appropriately by non-security-aware waypoints.

   Some waypoints will have security policies that require evaluating
   security services even if they are not the bundle destination or the
   final intended acceptor of the service.  For example, a waypoint
   could choose to verify an integrity service even though the waypoint
   is not the bundle destination and the integrity service will be
   needed by other nodes along the bundle's path.



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   Some waypoints will determine, through policy, that they are the
   intended recipient of the security service and terminate the security
   service in the bundle.  For example, a gateway node could determine
   that, even though it is not the destination of the bundle, it should
   verify and remove a particular integrity service or attempt to
   decrypt a confidentiality service, before forwarding the bundle along
   its path.

   Some waypoints could understand security blocks but refuse to process
   them unless they are the bundle destination.

2.4.  User-Defined Security Contexts

   A security context is the union of security algorithms (cipher
   suites), policies associated with the use of those algorithms, and
   configuration values.  Different contexts may specify different
   algorithms, different polices, or different configuration values used
   in the implementation of their security services.  BPSec provides a
   mechanism to define security contexts.  Users may select from
   registered security contexts and customize those contexts through
   security context parameters.

   For example, some users might prefer a SHA2 hash function for
   integrity whereas other users might prefer a SHA3 hash function.
   Providing either separate security contexts or a single,
   parameterized security context allows users flexibility in applying
   the desired cipher suite, policy, and configuration when populating a
   security block.

2.5.  Deterministic Processing

   Whenever a node determines that it must process more than one
   security block in a received bundle (either because the policy at a
   waypoint states that it should process security blocks or because the
   node is the bundle destination) the order in which security blocks
   are processed must be deterministic.  All nodes must impose this same
   deterministic processing order for all security blocks.  This
   specification provides determinism in the application and evaluation
   of security services, even when doing so results in a loss of
   flexibility.

3.  Security Blocks

3.1.  Block Definitions

   This specification defines two types of security block: the Block
   Integrity Block (BIB) and the Block Confidentiality Block (BCB).




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      The BIB is used to ensure the integrity of its plain text security
      target(s).  The integrity information in the BIB MAY be verified
      by any node along the bundle path from the BIB security source to
      the bundle destination.  Security-aware waypoints add or remove
      BIBs from bundles in accordance with their security policy.  BIBs
      are never used for integrity protection of the cipher text
      provided by a BCB.

      The BCB indicates that the security target(s) have been encrypted
      at the BCB security source in order to protect their content while
      in transit.  The BCB is decrypted by security-aware nodes in the
      network, up to and including the bundle destination, as a matter
      of security policy.  BCBs additionally provide integrity
      protection mechanisms for the cipher text they generate.

3.2.  Uniqueness

   Security operations in a bundle MUST be unique; the same security
   service MUST NOT be applied to a security target more than once in a
   bundle.  Since a security operation is represented as a security
   block, this limits what security blocks may be added to a bundle: if
   adding a security block to a bundle would cause some other security
   block to no longer represent a unique security operation then the new
   block MUST NOT be added.

   A security operation may be removed from a bundle as part of
   processing a security block and, once removed, the same security
   operation may be re-applied by adding a new security block into the
   bundle.  In this case, conflicting security blocks never co-exist in
   the bundle at the same time.

   It is important to note that any cipher text integrity mechanism
   supplied by the BCB is considered part of the confidentiality service
   and, therefore, unique from the plain text integrity service provided
   by the BIB.

   If multiple security blocks representing the same security operation
   were allowed in a bundle at the same time, there would exist
   ambiguity regarding block processing order and the property of
   deterministic processing of blocks would be lost.

   Using the notation OP(service, target), several examples illustrate
   this uniqueness requirement.

   o  Signing the payload twice: The two operations OP(integrity,
      payload) and OP(integrity, payload) are redundant and MUST NOT
      both be present in the same bundle at the same time.




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   o  Signing different blocks: The two operations OP(integrity,
      payload) and OP(integrity, extension_block_1) are not redundant
      and both may be present in the same bundle at the same time.
      Similarly, the two operations OP(integrity, extension_block_1) and
      OP(integrity,extension_block_2) are also not redundant and may
      both be present in the bundle at the same time.

   o  Different Services on same block: The two operations OP(integrity,
      payload) and OP(confidentiality, payload) are not inherently
      redundant and may both be present in the bundle at the same time,
      pursuant to other processing rules in this specification.

3.3.  Target Multiplicity

   A single security block MAY represent multiple security operations as
   a way of reducing the overall number of security blocks present in a
   bundle.  In these circumstances, reducing the number of security
   blocks in the bundle reduces the amount of redundant information in
   the bundle.

   A set of security operations can be represented by a single security
   block when all of the following conditions are true.

   o  The security operations apply the same security service.  For
      example, they are all integrity operations or all confidentiality
      operations.

   o  The security context parameters for the security operations are
      identical.

   o  The security source for the security operations is the same,
      meaning the set of operations are being added by the same node.

   o  No security operations have the same security target, as that
      would violate the need for security operations to be unique.

   o  None of the security operations conflict with security operations
      already present in the bundle.

   When representing multiple security operations in a single security
   block, the information that is common across all operations is
   represented once in the security block, and the information which is
   different (e.g., the security targets) are represented individually.

   It is RECOMMENDED that if a node processes any security operation in
   a security block that it process all security operations in the
   security block.  This allows security sources to assert that the set
   of security operations in a security block are expected to be



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   processed by the same security acceptor.  However, the determination
   of whether a node actually is a security acceptor or not is a matter
   of the policy of the node itself.  In cases where a receiving node
   determines that it is the security acceptor of only a subset of the
   security operations in a security block, the node may choose to only
   process that subset of security operations.

3.4.  Target Identification

   A security target is a block in the bundle to which a security
   service applies.  This target must be uniquely and unambiguously
   identifiable when processing a security block.  The definition of the
   extension block header from [I-D.ietf-dtn-bpbis] provides a "Block
   Number" field suitable for this purpose.  Therefore, a security
   target in a security block MUST be represented as the Block Number of
   the target block.

3.5.  Block Representation

   Each security block uses the Canonical Bundle Block Format as defined
   in [I-D.ietf-dtn-bpbis].  That is, each security block is comprised
   of the following elements:

   o  block type code

   o  block number

   o  block processing control flags

   o  CRC type

   o  block-type-specific-data

   o  CRC field (if present)

   Security-specific information for a security block is captured in the
   block-type-specific-data field.

3.6.  Abstract Security Block

   The structure of the security-specific portions of a security block
   is identical for both the BIB and BCB Block Types.  Therefore, this
   section defines an Abstract Security Block (ASB) data structure and
   discusses the definition, processing, and other constraints for using
   this structure.  An ASB is never directly instantiated within a
   bundle, it is only a mechanism for discussing the common aspects of
   BIB and BCB security blocks.




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   The fields of the ASB SHALL be as follows, listed in the order in
   which they must appear.

   Security Targets:
         This field identifies the block(s) targeted by the security
         operation(s) represented by this security block.  Each target
         block is represented by its unique Block Number.  This field
         SHALL be represented by a CBOR array of data items.  Each
         target within this CBOR array SHALL be represented by a CBOR
         unsigned integer.  This array MUST have at least 1 entry and
         each entry MUST represent the Block Number of a block that
         exists in the bundle.  There MUST NOT be duplicate entries in
         this array.

   Security Context Id:
         This field identifies the security context used to implement
         the security service represented by this block and applied to
         each security target.  This field SHALL be represented by a
         CBOR unsigned integer.  The values for this Id should come from
         the registry defined in Section 11.2

   Security Context Flags:
         This field identifies which optional fields are present in the
         security block.  This field SHALL be represented as a CBOR
         unsigned integer whose contents shall be interpreted as a bit
         field.  Each bit in this bit field indicates the presence (bit
         set to 1) or absence (bit set to 0) of optional data in the
         security block.  The association of bits to security block data
         is defined as follows.

         Bit 0  (the least-significant bit, 0x01): Security Context
                Parameters Present Flag.

         Bit 1  (0x02): Security Source Present Flag.

         Bit >1 Reserved

         Implementations MUST set reserved bits to 0 when writing this
         field and MUST ignore the values of reserved bits when reading
         this field.  For unreserved bits, a value of 1 indicates that
         the associated security block field MUST be included in the
         security block.  A value of 0 indicates that the associated
         security block field MUST NOT be in the security block.

   Security Source (Optional):
         This field identifies the Endpoint that inserted the security
         block in the bundle.  If the security source field is not
         present then the source MUST be inferred from other



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         information, such as the bundle source, previous hop, or other
         values defined by security policy.  This field SHALL be
         represented by a CBOR array in accordance with
         [I-D.ietf-dtn-bpbis] rules for representing Endpoint
         Identifiers (EIDs).

   Security Context Parameters (Optional):
         This field captures one or more security context parameters
         that should be provided to security-aware nodes when processing
         the security service described by this security block.  This
         field SHALL be represented by a CBOR array.  Each entry in this
         array is a single security context parameter.  A single
         parameter SHALL also be represented as a CBOR array comprising
         a 2-tuple of the id and value of the parameter, as follows.

         *  Parameter Id.  This field identifies which parameter is
            being specified.  This field SHALL be represented as a CBOR
            unsigned integer.  Parameter Ids are selected as described
            in Section 3.10.

         *  Parameter Value.  This field captures the value associated
            with this parameter.  This field SHALL be represented by the
            applicable CBOR representation of the parameter, in
            accordance with Section 3.10.

         The logical layout of the parameters array is illustrated in
         Figure 1.


                   Figure 1: Security Context Parameters

   Security Results:
         This field captures the results of applying a security service
         to the security targets of the security block.  This field
         SHALL be represented as a CBOR array of target results.  Each
         entry in this array represents the set of security results for
         a specific security target.  The target results MUST be ordered
         identically to the Security Targets field of the security
         block.  This means that the first set of target results in this
         array corresponds to the first entry in the Security Targets
         field of the security block, and so on.  There MUST be one
         entry in this array for each entry in the Security Targets
         field of the security block.

         The set of security results for a target is also represented as
         a CBOR array of individual results.  An individual result is
         represented as a 2-tuple of a result id and a result value,
         defined as follows.



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         *  Result Id.  This field identifies which security result is
            being specified.  Some security results capture the primary
            output of a cipher suite.  Other security results contain
            additional annotative information from cipher suite
            processing.  This field SHALL be represented as a CBOR
            unsigned integer.  Security result Ids will be as specified
            in Section 3.10.

         *  Result Value.  This field captures the value associated with
            the result.  This field SHALL be represented by the
            applicable CBOR representation of the result value, in
            accordance with Section 3.10.

         The logical layout of the security results array is illustrated
         in Figure 2.  In this figure there are N security targets for
         this security block.  The first security target contains M
         results and the Nth security target contains K results.


                        Figure 2: Security Results

3.7.  Block Integrity Block

   A BIB is a bundle extension block with the following characteristics.

      The Block Type Code value is as specified in Section 11.1.

      The block-type-specific-data field follows the structure of the
      ASB.

      A security target listed in the Security Targets field MUST NOT
      reference a security block defined in this specification (e.g., a
      BIB or a BCB).

      The Security Context MUST utilize an authentication mechanism or
      an error detection mechanism.

      The EID of the security source MAY be present.  If this field is
      not present, then the security source of the block SHOULD be
      inferred according to security policy and MAY default to the
      bundle source.  The security source MAY be specified as part of
      security context parameters described in Section 3.10.

   Notes:

   o  It is recommended that designers carefully consider the effect of
      setting flags that either discard the block or delete the bundle
      in the event that this block cannot be processed.



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   o  Since OP(integrity, target) is allowed only once in a bundle per
      target, it is RECOMMENDED that users wishing to support multiple
      integrity signatures for the same target define a multi-signature
      security context.

   o  Security information MAY be checked at any hop on the way to the
      bundle destination that has access to the required keying
      information, in accordance with Section 3.9.

3.8.  Block Confidentiality Block

   A BCB is a bundle extension block with the following characteristics.

      The Block Type Code value is as specified in Section 11.1.

      The Block Processing Control flags value can be set to whatever
      values are required by local policy with the following exceptions.
      BCB blocks MUST have the "block must be replicated in every
      fragment" flag set if one of the targets is the payload block.
      Having that BCB in each fragment indicates to a receiving node
      that the payload portion of each fragment represents cipher text.
      BCB blocks MUST NOT have the "block must be removed from bundle if
      it can't be processed" flag set.  Removing a BCB from a bundle
      without decrypting its security targets removes information from
      the bundle necessary for their later decryption.

      The block-type-specific-data fields follow the structure of the
      ASB.

      A security target listed in the Security Targets field can
      reference the payload block, a non-security extension block, or a
      BIB.  A BCB MUST NOT include another BCB as a security target.  A
      BCB MUST NOT target the primary block.

      The Security Context MUST utilize a confidentiality cipher that
      provides authenticated encryption with associated data (AEAD).

      Additional information created by a cipher suite (such as an
      authentication tag) can be placed either in a security result
      field or in the generated cipher text.  The determination of where
      to place this information is a function of the cipher suite and
      security context used.

      The EID of the security source MAY be present.  If this field is
      not present, then the security source of the block SHOULD be
      inferred according to security policy and MAY default to the
      bundle source.  The security source MAY be specified as part of
      security context parameters described in Section 3.10.



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   The BCB modifies the contents of its security target(s).  When a BCB
   is applied, the security target body data are encrypted "in-place".
   Following encryption, the security target block-type-specific-data
   field contains cipher text, not plain text.

   Notes:

   o  It is RECOMMENDED that designers carefully consider the effect of
      setting flags that delete the bundle in the event that this block
      cannot be processed.

   o  The BCB block processing control flags can be set independently
      from the processing control flags of the security target(s).  The
      setting of such flags should be an implementation/policy decision
      for the encrypting node.

3.9.  Block Interactions

   The security block types defined in this specification are designed
   to be as independent as possible.  However, there are some cases
   where security blocks may share a security target creating processing
   dependencies.

   If a security target of a BCB is also a security target of a BIB, an
   undesirable condition occurs where a security aware waypoint would be
   unable to validate the BIB because one of its security target's
   contents have been encrypted by a BCB.  To address this situation the
   following processing rules MUST be followed.

   o  When adding a BCB to a bundle, if some (or all) of the security
      targets of the BCB also match all of the security targets of an
      existing BIB, then the existing BIB MUST also be encrypted.  This
      can be accomplished by either adding a new BCB that targets the
      existing BIB, or by adding the BIB to the list of security targets
      for the BCB.  Deciding which way to represent this situation is a
      matter of security policy.

   o  When adding a BCB to a bundle, if some (or all) of the security
      targets of the BCB match some (but not all) of the security
      targets of a BIB then that BIB MUST be altered in the following
      way.  Any security results in the BIB associated with the BCB
      security targets MUST be removed from the BIB and placed in a new
      BIB.  This newly created BIB MUST then be encrypted.  The
      encryption of the new BIB can be accomplished by either adding a
      new BCB that targets the new BIB, or by adding the new BIB to the
      list of security targets for the BCB.  Deciding which way to
      represent this situation is a matter of security policy.




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   o  A BIB MUST NOT be added for a security target that is already the
      security target of a BCB as this would cause ambiguity in block
      processing order.

   o  A BIB integrity value MUST NOT be checked if the BIB is the
      security target of an existing BCB.  In this case, the BIB data is
      encrypted.

   o  A BIB integrity value MUST NOT be checked if the security target
      associated with that value is also the security target of a BCB.
      In such a case, the security target data contains cipher text as
      it has been encrypted.

   o  As mentioned in Section 3.7, a BIB MUST NOT have a BCB as its
      security target.

   These restrictions on block interactions impose a necessary ordering
   when applying security operations within a bundle.  Specifically, for
   a given security target, BIBs MUST be added before BCBs.  This
   ordering MUST be preserved in cases where the current BPA is adding
   all of the security blocks for the bundle or whether the BPA is a
   waypoint adding new security blocks to a bundle that already contains
   security blocks.

   In cases where a security source wishes to calculate both a plain
   text integrity mechanism and encrypt a security target, a BCB with a
   security context that generates such signatures as additional
   security results MUST be used instead of adding both a BIB and then a
   BCB for the security target at the security source.

3.10.  Parameter and Result Identification

   Each security context MUST define its own context parameters and
   results.  Each defined parameter and result is represented as the
   tuple of an identifier and a value.  Identifiers are always
   represented as a CBOR unsigned integer.  The CBOR encoding of values
   is as defined by the security context specification.

   Identifiers MUST be unique for a given security context but do not
   need to be unique amongst all security contexts.

   An example of a security context can be found at
   [I-D.ietf-dtn-bpsec-interop-sc].








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3.11.  BSP Block Examples

   This section provides two examples of BPSec blocks applied to a
   bundle.  In the first example, a single node adds several security
   operations to a bundle.  In the second example, a waypoint node
   received the bundle created in the first example and adds additional
   security operations.  In both examples, the first column represents
   blocks within a bundle and the second column represents the Block
   Number for the block, using the terminology B1...Bn for the purpose
   of illustration.

3.11.1.  Example 1: Constructing a Bundle with Security

   In this example a bundle has four non-security-related blocks: the
   primary block (B1), two extension blocks (B4,B5), and a payload block
   (B6).  The bundle source wishes to provide an integrity signature of
   the plain text associated with the primary block, the second
   extension block, and the payload.  The bundle source also wishes to
   provide confidentiality for the first extension block.  The resultant
   bundle is illustrated in Figure 3 and the security actions are
   described below.


                   Figure 3: Security at Bundle Creation

   The following security actions were applied to this bundle at its
   time of creation.

   o  An integrity signature applied to the canonical form of the
      primary block (B1), the canonical form of the block-type-specific-
      data field of the second extension block (B5) and the canonical
      form of the payload block (B6).  This is accomplished by a single
      BIB (B2) with multiple targets.  A single BIB is used in this case
      because all three targets share a security source, security
      context, and security context parameters.  Had this not been the
      case, multiple BIBs could have been added instead.

   o  Confidentiality for the first extension block (B4).  This is
      accomplished by a BCB (B3).  Once applied, the block-type-
      specific-data field of extension block B4 is encrypted.  The BCB
      MUST hold an authentication tag for the cipher text either in the
      cipher text that now populates the first extension block or as a
      security result in the BCB itself, depending on which security
      context is used to form the BCB.  A plain text integrity signature
      may also exist as a security result in the BCB if one is provided
      by the selected confidentiality security context.





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3.11.2.  Example 2: Adding More Security At A New Node

   Consider that the bundle as it is illustrated in Figure 3 is now
   received by a waypoint node that wishes to encrypt the second
   extension block and the bundle payload.  The waypoint security policy
   is to allow existing BIBs for these blocks to persist, as they may be
   required as part of the security policy at the bundle destination.

   The resultant bundle is illustrated in Figure 4 and the security
   actions are described below.  Note that block IDs provided here are
   ordered solely for the purpose of this example and not meant to
   impose an ordering for block creation.  The ordering of blocks added
   to a bundle MUST always be in compliance with [I-D.ietf-dtn-bpbis].


                  Figure 4: Security At Bundle Forwarding

   The following security actions were applied to this bundle prior to
   its forwarding from the waypoint node.

   o  Since the waypoint node wishes to encrypt the block-type-specific-
      data field of blocks B5 and B6, it MUST also encrypt the block-
      type-specific-data field of the BIBs providing plain text
      integrity over those blocks.  However, BIB B2 could not be
      encrypted in its entirety because it also held a signature for the
      primary block (B1).  Therefore, a new BIB (B7) is created and
      security results associated with B5 and B6 are moved out of BIB B2
      and into BIB B7.

   o  Now that there is no longer confusion of which plain text
      integrity signatures must be encrypted, a BCB is added to the
      bundle with the security targets being the second extension block
      (B5) and the payload (B6) as well as the newly created BIB holding
      their plain text integrity signatures (B7).  A single new BCB is
      used in this case because all three targets share a security
      source, security context, and security context parameters.  Had
      this not been the case, multiple BCBs could have been added
      instead.

4.  Canonical Forms

   Security services require consistency and determinism in how
   information is presented to cipher suites at the security source and
   at a receiving node.  For example, integrity services require that
   the same target information (e.g., the same bits in the same order)
   is provided to the cipher suite when generating an original signature
   and when validating a signature.  Canonicalization algorithms are




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   used to construct a stable, end-to-end bit representation of a target
   block.

   Canonical forms are used to generate input to a security context for
   security processing at a security-aware node.

   BPSec operates on data fields within bundle blocks (e.g., the block-
   type-specific-data field).  In their canonical form, these fields
   MUST include their own CBOR encoding and MUST NOT include any other
   encapsulating CBOR encoding.  For example, the canonical form of the
   block-type-specific-data field is a CBOR byte string existing within
   the CBOR array containing the fields of the extension block.  The
   entire CBOR byte string is considered the canonical block-type-
   specific-data field.  The CBOR array framing is not considered part
   of the field.

   The canonical form of the primary block is specified in
   [I-D.ietf-dtn-bpbis].

   All non-primary blocks share the same block structure and are
   canonicalized as specified in [I-D.ietf-dtn-bpbis] with the following
   exceptions.

   o  If the service being applied is a confidentiality service, then
      the block type code, block number, block processing control flags,
      CRC type and CRC field (if present), and the length indication of
      the block-type-specific-data field MUST NOT be included in a
      canonical form.  Confidentiality services are used solely to
      convert block data in the block-type-specific-data field from
      plain text to cipher text.

   o  Reserved flags in the block processing control flags field MUST
      NOT be included in a canonical form as it is not known if those
      flags will change in transit.

   Cipher suites and security contexts MAY define their own
   canonicalization algorithms and require the use of those algorithms
   over the ones provided in this specification.  In the event of
   conflicting canonicalization algorithms, those algorithms take
   precedence over this specification.

5.  Security Processing

   This section describes the security aspects of bundle processing.







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5.1.  Bundles Received from Other Nodes

   Security blocks must be processed in a specific order when received
   by a security-aware node.  The processing order is as follows.

   o  When BIBs and BCBs share a security target, BCBs MUST be evaluated
      first and BIBs second.

5.1.1.  Receiving BCBs

   If a received bundle contains a BCB, the receiving node MUST
   determine whether it is the security acceptor for any of the security
   operations in the BCB.  If so, the node MUST process those operations
   and remove any operation-specific information from the BCB prior to
   delivering data to an application at the node or forwarding the
   bundle.  If processing a security operation fails, the target SHALL
   be processed according to the security policy.  A bundle status
   report indicating the failure MAY be generated.  When all security
   operations for a BCB have been removed from the BCB, the BCB MUST be
   removed from the bundle.

   If the receiving node is the destination of the bundle, the node MUST
   decrypt any BCBs remaining in the bundle.  If the receiving node is
   not the destination of the bundle, the node MUST process the BCB if
   directed to do so as a matter of security policy.

   If the security policy of a security-aware node specifies that a node
   should have applied confidentiality to a specific security target and
   no such BCB is present in the bundle, then the node MUST process this
   security target in accordance with the security policy.  It is
   recommended that the node remove the security target from the bundle.
   If the removed security target is the payload block, the bundle MUST
   be discarded.

   If an encrypted payload block cannot be decrypted (i.e., the cipher
   text cannot be authenticated), then the bundle MUST be discarded and
   processed no further.  If an encrypted security target other than the
   payload block cannot be decrypted then the associated security target
   and all security blocks associated with that target MUST be discarded
   and processed no further.  In both cases, requested status reports
   (see [I-D.ietf-dtn-bpbis]) MAY be generated to reflect bundle or
   block deletion.

   When a BCB is decrypted, the recovered plain text for each security
   target MUST replace the cipher text in each of the security targets'
   block-type-specific-data fields.  If the plain text is of different
   size than the cipher text, the CBOR byte string framing of this field
   must be updated to ensure this field remains a valid CBOR byte



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   string.  The length of the recovered plain text is known by the
   decrypting security context.

   If a BCB contains multiple security operations, each operation
   processed by the node MUST be treated as if the security operation
   has been represented by a single BCB with a single security operation
   for the purposes of report generation and policy processing.

5.1.2.  Receiving BIBs

   If a received bundle contains a BIB, the receiving node MUST
   determine whether it is the security acceptor for any of the security
   operations in the BIB.  If so, the node MUST process those operations
   and remove any operation-specific information from the BIB prior to
   delivering data to an application at the node or forwarding the
   bundle.  If processing a security operation fails, the target SHALL
   be processed according to the security policy.  A bundle status
   report indicating the failure MAY be generated.  When all security
   operations for a BIB have been removed from the BIB, the BIB MUST be
   removed from the bundle.

   A BIB MUST NOT be processed if the security target of the BIB is also
   the security target of a BCB in the bundle.  Given the order of
   operations mandated by this specification, when both a BIB and a BCB
   share a security target, it means that the security target must have
   been encrypted after it was integrity signed and, therefore, the BIB
   cannot be verified until the security target has been decrypted by
   processing the BCB.

   If the security policy of a security-aware node specifies that a node
   should have applied integrity to a specific security target and no
   such BIB is present in the bundle, then the node MUST process this
   security target in accordance with the security policy.  It is
   RECOMMENDED that the node remove the security target from the bundle
   if the security target is not the payload or primary block.  If the
   security target is the payload or primary block, the bundle MAY be
   discarded.  This action can occur at any node that has the ability to
   verify an integrity signature, not just the bundle destination.

   If a receiving node is not the security acceptor of a security
   operation in a BIB it MAY attempt to verify the security operation
   anyway to prevent forwarding corrupt data.  If the verification
   fails, the node SHALL process the security target in accordance to
   local security policy.  It is RECOMMENDED that if a payload integrity
   check fails at a waypoint that it is processed in the same way as if
   the check fails at the bundle destination.  If the check passes, the
   node MUST NOT remove the security operation from the BIB prior to
   forwarding.



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   If a BIB contains multiple security operations, each operation
   processed by the node MUST be treated as if the security operation
   has been represented by a single BIB with a single security operation
   for the purposes of report generation and policy processing.

5.2.  Bundle Fragmentation and Reassembly

   If it is necessary for a node to fragment a bundle payload, and
   security services have been applied to that bundle, the fragmentation
   rules described in [I-D.ietf-dtn-bpbis] MUST be followed.  As defined
   there and summarized here for completeness, only the payload block
   can be fragmented; security blocks, like all extension blocks, can
   never be fragmented.

   Due to the complexity of payload block fragmentation, including the
   possibility of fragmenting payload block fragments, integrity and
   confidentiality operations are not to be applied to a bundle
   representing a fragment.  Specifically, a BCB or BIB MUST NOT be
   added to a bundle if the "Bundle is a Fragment" flag is set in the
   Bundle Processing Control Flags field.

   Security processing in the presence of payload block fragmentation
   may be handled by other mechanisms outside of the BPSec protocol or
   by applying BPSec blocks in coordination with an encapsulation
   mechanism.  A node should apply any confidentiality protection prior
   to performing any fragmentation.

6.  Key Management

   There exist a myriad of ways to establish, communicate, and otherwise
   manage key information in a DTN.  Certain DTN deployments might
   follow established protocols for key management whereas other DTN
   deployments might require new and novel approaches.  BPSec assumes
   that key management is handled as a separate part of network
   management and this specification neither defines nor requires a
   specific key management strategy.

7.  Security Policy Considerations

   When implementing BPSec, several policy decisions must be considered.
   This section describes key policies that affect the generation,
   forwarding, and receipt of bundles that are secured using this
   specification.  No single set of policy decisions is envisioned to
   work for all secure DTN deployments.

   o  If a bundle is received that contains combinations of security
      operations that are disallowed by this specification the BPA must
      determine how to handle the bundle.  The bundle may be discarded,



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      the block affected by the security operation may be discarded, or
      one security operation may be favored over another.

   o  BPAs in the network must understand what security operations they
      should apply to bundles.  This decision may be based on the source
      of the bundle, the destination of the bundle, or some other
      information related to the bundle.

   o  If a waypoint has been configured to add a security operation to a
      bundle, and the received bundle already has the security operation
      applied, then the receiver must understand what to do.  The
      receiver may discard the bundle, discard the security target and
      associated BPSec blocks, replace the security operation, or some
      other action.

   o  It is recommended that security operations be considered for every
      block in a bundle and that the default behavior of a bundle agent
      is to use the security services defined in this specification.
      Designers should only deviate from the use of security operations
      when the deviation can be justified - such as when doing so causes
      downstream errors when processing blocks whose contents must be
      inspected or changed at one or more hops along the path.

   o  It is recommended that BCBs be allowed to alter the size of
      extension blocks and the payload block.  However, care must be
      taken to ensure that changing the size of the payload block while
      the bundle is in transit do not negatively affect bundle
      processing (e.g., calculating storage needs, scheduling
      transmission times).

   o  Adding a BIB to a security target that has already been encrypted
      by a BCB is not allowed.  If this condition is likely to be
      encountered, there are (at least) three possible policies that
      could handle this situation.

      1.  At the time of encryption, a security context can be selected
          which computes a plain text integrity signature and included
          as a security context result field.

      2.  The encrypted block may be replicated as a new block with a
          new block number and given integrity protection.

      3.  An encapsulation scheme may be applied to encapsulate the
          security target (or the entire bundle) such that the
          encapsulating structure is, itself, no longer the security
          target of a BCB and may therefore be the security target of a
          BIB.




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   o  It is recommended that security policy address whether cipher
      suites whose cipher text is larger than the initial plain text are
      permitted and, if so, for what types of blocks.  Changing the size
      of a block may cause processing difficulties for networks that
      calculate block offsets into bundles or predict transmission times
      or storage availability as a function of bundle size.  In other
      cases, changing the size of a payload as part of encryption has no
      significant impact.

8.  Security Considerations

   Given the nature of DTN applications, it is expected that bundles may
   traverse a variety of environments and devices which each pose unique
   security risks and requirements on the implementation of security
   within BPSec.  For these reasons, it is important to introduce key
   threat models and describe the roles and responsibilities of the
   BPSec protocol in protecting the confidentiality and integrity of the
   data against those threats.  This section provides additional
   discussion on security threats that BPSec will face and describes how
   BPSec security mechanisms operate to mitigate these threats.

   The threat model described here is assumed to have a set of
   capabilities identical to those described by the Internet Threat
   Model in [RFC3552], but the BPSec threat model is scoped to
   illustrate threats specific to BPSec operating within DTN
   environments and therefore focuses on man-in-the-middle (MITM)
   attackers.  In doing so, it is assumed that the DTN (or significant
   portions of the DTN) are completely under the control of an attacker.

8.1.  Attacker Capabilities and Objectives

   BPSec was designed to protect against MITM threats which may have
   access to a bundle during transit from its source, Alice, to its
   destination, Bob.  A MITM node, Mallory, is a non-cooperative node
   operating on the DTN between Alice and Bob that has the ability to
   receive bundles, examine bundles, modify bundles, forward bundles,
   and generate bundles at will in order to compromise the
   confidentiality or integrity of data within the DTN.  For the
   purposes of this section, any MITM node is assumed to effectively be
   security-aware even if it does not implement the BPSec protocol.
   There are three classes of MITM nodes which are differentiated based
   on their access to cryptographic material:

   o  Unprivileged Node: Mallory has not been provisioned within the
      secure environment and only has access to cryptographic material
      which has been publicly-shared.





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   o  Legitimate Node: Mallory is within the secure environment and
      therefore has access to cryptographic material which has been
      provisioned to Mallory (i.e., K_M) as well as material which has
      been publicly-shared.

   o  Privileged Node: Mallory is a privileged node within the secure
      environment and therefore has access to cryptographic material
      which has been provisioned to Mallory, Alice and/or Bob (i.e.
      K_M, K_A, and/or K_B) as well as material which has been publicly-
      shared.

   If Mallory is operating as a privileged node, this is tantamount to
   compromise; BPSec does not provide mechanisms to detect or remove
   Mallory from the DTN or BPSec secure environment.  It is up to the
   BPSec implementer or the underlying cryptographic mechanisms to
   provide appropriate capabilities if they are needed.  It should also
   be noted that if the implementation of BPSec uses a single set of
   shared cryptographic material for all nodes, a legitimate node is
   equivalent to a privileged node because K_M == K_A == K_B.  For this
   reason, sharing cryptographic material in this way is not
   recommended.

   A special case of the legitimate node is when Mallory is either Alice
   or Bob (i.e., K_M == K_A or K_M == K_B).  In this case, Mallory is
   able to impersonate traffic as either Alice or Bob, respectively,
   which means that traffic to and from that node can be decrypted and
   encrypted, respectively.  Additionally, messages may be signed as
   originating from one of the endpoints.

8.2.  Attacker Behaviors and BPSec Mitigations

8.2.1.  Eavesdropping Attacks

   Once Mallory has received a bundle, she is able to examine the
   contents of that bundle and attempt to recover any protected data or
   cryptographic keying material from the blocks contained within.  The
   protection mechanism that BPSec provides against this action is the
   BCB, which encrypts the contents of its security target, providing
   confidentiality of the data.  Of course, it should be assumed that
   Mallory is able to attempt offline recovery of encrypted data, so the
   cryptographic mechanisms selected to protect the data should provide
   a suitable level of protection.

   When evaluating the risk of eavesdropping attacks, it is important to
   consider the lifetime of bundles on a DTN.  Depending on the network,
   bundles may persist for days or even years.  Long-lived bundles imply
   that the data exists in the network for a longer period of time and,
   thus, there may be more opportunities to capture those bundles.



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   Additionally, bundles that are long-lived imply that the information
   stored within them may remain relevant and sensitive for long enough
   that, once captured, there is sufficient time to crack encryption
   associated with the bundle.  If a bundle does persist on the network
   for years and the cipher suite used for a BCB provides inadequate
   protection, Mallory may be able to recover the protected data either
   before that bundle reaches its intended destination or before the
   information in the bundle is no longer considered sensitive.

   NOTE: Mallory is not limited by the bundle lifetime and may retain a
   given bundle indefinitely.

   NOTE: Irrespective of whether BPSec is used, traffic analysis will be
   possible.

8.2.2.  Modification Attacks

   As a node participating in the DTN between Alice and Bob, Mallory
   will also be able to modify the received bundle, including non-BPSec
   data such as the primary block, payload blocks, or block processing
   control flags as defined in [I-D.ietf-dtn-bpbis].  Mallory will be
   able to undertake activities which include modification of data
   within the blocks, replacement of blocks, addition of blocks, or
   removal of blocks.  Within BPSec, both the BIB and BCB provide
   integrity protection mechanisms to detect or prevent data
   manipulation attempts by Mallory.

   The BIB provides that protection to another block which is its
   security target.  The cryptographic mechanisms used to generate the
   BIB should be strong against collision attacks and Mallory should not
   have access to the cryptographic material used by the originating
   node to generate the BIB (e.g., K_A).  If both of these conditions
   are true, Mallory will be unable to modify the security target or the
   BIB and lead Bob to validate the security target as originating from
   Alice.

   Since BPSec security operations are implemented by placing blocks in
   a bundle, there is no in-band mechanism for detecting or correcting
   certain cases where Mallory removes blocks from a bundle.  If Mallory
   removes a BCB, but keeps the security target, the security target
   remains encrypted and there is a possibility that there may no longer
   be sufficient information to decrypt the block at its destination.
   If Mallory removes both a BCB (or BIB) and its security target there
   is no evidence left in the bundle of the security operation.
   Similarly, if Mallory removes the BIB but not the security target
   there is no evidence left in the bundle of the security operation.
   In each of these cases, the implementation of BPSec must be combined
   with policy configuration at endpoints in the network which describe



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   the expected and required security operations that must be applied on
   transmission and are expected to be present on receipt.  This or
   other similar out-of-band information is required to correct for
   removal of security information in the bundle.

   A limitation of the BIB may exist within the implementation of BIB
   validation at the destination node.  If Mallory is a legitimate node
   within the DTN, the BIB generated by Alice with K_A can be replaced
   with a new BIB generated with K_M and forwarded to Bob.  If Bob is
   only validating that the BIB was generated by a legitimate user, Bob
   will acknowledge the message as originating from Mallory instead of
   Alice.  Validating a BIB indicates only that the BIB was generated by
   a holder of the relevant key; it does not provide any guarantee that
   the bundle or block was created by the same entity.  In order to
   provide verifiable integrity checks BCB should require an encryption
   scheme that is Indistinguishable under adaptive Chosen Ciphertext
   Attack (IND-CCA2) secure.  Such an encryption scheme will guard
   against signature substitution attempts by Mallory.  In this case,
   Alice creates a BIB with the protected data block as the security
   target and then creates a BCB with both the BIB and protected data
   block as its security targets.

8.2.3.  Topology Attacks

   If Mallory is in a MITM position within the DTN, she is able to
   influence how any bundles that come to her may pass through the
   network.  Upon receiving and processing a bundle that must be routed
   elsewhere in the network, Mallory has three options as to how to
   proceed: not forward the bundle, forward the bundle as intended, or
   forward the bundle to one or more specific nodes within the network.

   Attacks that involve re-routing the packets throughout the network
   are essentially a special case of the modification attacks described
   in this section where the attacker is modifying fields within the
   primary block of the bundle.  Given that BPSec cannot encrypt the
   contents of the primary block, alternate methods must be used to
   prevent this situation.  These methods may include requiring BIBs for
   primary blocks, using encapsulation, or otherwise strategically
   manipulating primary block data.  The specifics of any such
   mitigation technique are specific to the implementation of the
   deploying network and outside of the scope of this document.

   Furthermore, routing rules and policies may be useful in enforcing
   particular traffic flows to prevent topology attacks.  While these
   rules and policies may utilize some features provided by BPSec, their
   definition is beyond the scope of this specification.





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8.2.4.  Message Injection

   Mallory is also able to generate new bundles and transmit them into
   the DTN at will.  These bundles may either be copies or slight
   modifications of previously-observed bundles (i.e., a replay attack)
   or entirely new bundles generated based on the Bundle Protocol,
   BPSec, or other bundle-related protocols.  With these attacks
   Mallory's objectives may vary, but may be targeting either the bundle
   protocol or application-layer protocols conveyed by the bundle
   protocol.  The target could also be the storage and compute of the
   nodes running the bundle or application layer protocols (e.g., a
   denial of service to flood on the storage of the store-and-forward
   mechanism; or compute which would process the packets and perhaps
   prevent other activities).

   BPSec relies on cipher suite capabilities to prevent replay or forged
   message attacks.  A BCB used with appropriate cryptographic
   mechanisms may provide replay protection under certain circumstances.
   Alternatively, application data itself may be augmented to include
   mechanisms to assert data uniqueness and then protected with a BIB, a
   BCB, or both along with other block data.  In such a case, the
   receiving node would be able to validate the uniqueness of the data.

9.  Security Context Considerations

9.1.  Mandating Security Contexts

   Because of the diversity of networking scenarios and node
   capabilities that may utilize BPSec there is no one security context
   mandated for every possible BPSec implementation.  For example, a
   security context appropriate for a resource-constrained node with
   limited connectivity may be inappropriate for use in a well-
   resourced, well connected node.

   This does not mean that the use of BPSec in a particular network is
   meant to be used without security contexts for interoperability and
   default behavior.  Network designers must identify the minimal set of
   security contexts necessary for functions in their network.  For
   example, a default set of security contexts could be created for use
   over the terrestrial Internet and required by any BPSec
   implementation communicating over the terrestrial Internet.

   Implementations of BPSec MUST support the mandated security contexts
   of the networks in which they are applied.  If a node serves as a
   gateway amongst two or more networks, the BPSec implementation at
   that node MUST support the union of security contexts mandated in
   those networks.




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   BPSec has been designed to allow for a diversity of security contexts
   and for new contexts to be defined over time.  The use of different
   security contexts does not change the BPSec protocol itself and the
   definition of new security contexts MUST adhere to the requirements
   of such contexts as presented in this section and generally in this
   specification.

9.2.  Identification and Configuration

   Security blocks must uniquely define the security context for their
   services.  This context MUST be uniquely identifiable and MAY use
   parameters for customization.  Where policy and configuration
   decisions can be captured as parameters, the security context
   identifier may identify a cipher suite.  In cases where the same
   cipher suites are used with differing predetermined configurations
   and policies, users can define multiple security contexts that use
   the same cipher suite.

   Network operators must determine the number, type, and configuration
   of security contexts in a system.  Networks with rapidly changing
   configurations may define relatively few security contexts with each
   context customized with multiple parameters.  For networks with more
   stability, or an increased need for confidentiality, a larger number
   of contexts can be defined with each context supporting few, if any,
   parameters.

                         Security Context Examples

   +---------+------------+--------------------------------------------+
   | Context | Parameters |                 Definition                 |
   |    Id   |            |                                            |
   +---------+------------+--------------------------------------------+
   |    1    | Encrypted  |   AES-GCM-256 cipher suite with provided   |
   |         |  Key, IV   |       ephemeral key encrypted with a       |
   |         |            | predetermined key encryption key and clear |
   |         |            |        text initialization vector.         |
   |    2    |     IV     |       AES-GCM-256 cipher suite with        |
   |         |            |    predetermined key and predetermined     |
   |         |            |            key rotation policy.            |
   |    3    |    Nil     |   AES-GCM-256 cipher suite with all info   |
   |         |            |               predetermined.               |
   +---------+------------+--------------------------------------------+

                                  Table 1







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9.3.  Authorship

   Developers or implementers should consider the diverse performance
   and conditions of networks on which the Bundle Protocol (and
   therefore BPSec) will operate.  Specifically, the delay and capacity
   of delay-tolerant networks can vary substantially.  Developers should
   consider these conditions to better describe the conditions when
   those contexts will operate or exhibit vulnerability, and selection
   of these contexts for implementation should be made with
   consideration for this reality.  There are key differences that may
   limit the opportunity for a security context to leverage existing
   cipher suites and technologies that have been developed for use in
   traditional, more reliable networks:

   o  Data Lifetime: Depending on the application environment, bundles
      may persist on the network for extended periods of time, perhaps
      even years.  Cryptographic algorithms should be selected to ensure
      protection of data against attacks for a length of time reasonable
      for the application.

   o  One-Way Traffic: Depending on the application environment, it is
      possible that only a one-way connection may exist between two
      endpoints, or if a two-way connection does exist, the round- trip
      time may be extremely large.  This may limit the utility of
      session key generation mechanisms, such as Diffie-Hellman, as a
      two-way handshake may not be feasible or reliable.

   o  Opportunistic Access: Depending on the application environment, a
      given endpoint may not be guaranteed to be accessible within a
      certain amount of time.  This may make asymmetric cryptographic
      architectures which rely on a key distribution center or other
      trust center impractical under certain conditions.

   When developing security contexts for use with BPSec, the following
   information SHOULD be considered for inclusion in these
   specifications.

   o  Security Context Parameters.  Security contexts MUST define their
      parameter Ids, the data types of those parameters, and their CBOR
      encoding.

   o  Security Results.  Security contexts MUST define their security
      result Ids, the data types of those results, and their CBOR
      encoding.

   o  New Canonicalizations.  Security contexts may define new
      canonicalization algorithms as necessary.




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   o  Cipher-Text Size.  Security contexts MUST state whether their
      associated cipher suites generate cipher text (to include any
      authentication information) that is of a different size than the
      input plain text.

      If a security context does not wish to alter the size of the plain
      text it should place overflow bytes and authentication tags in
      security result fields.

   o  Block Header Information.  Security contexts SHOULD include block
      header information that is considered to be immutable for the
      block.  This information MAY include the block type code, block
      number, CRC Type and CRC field (if present or if missing and
      unlikely to be added later), and possibly certain block processing
      control flags.  Designers should input these fields as additional
      data for integrity protection when these fields are expected to
      remain unchanged over the path the block will take from the
      security source to the security acceptor.  Security contexts
      considering block header information MUST describe expected
      behavior when these fields fail their integrity verification.

10.  Defining Other Security Blocks

   Other security blocks (OSBs) may be defined and used in addition to
   the security blocks identified in this specification.  Both the usage
   of BIB, BCB, and any future OSBs can co-exist within a bundle and can
   be considered in conformance with BPSec if each of the following
   requirements are met by any future identified security blocks.

   o  Other security blocks (OSBs) MUST NOT reuse any enumerations
      identified in this specification, to include the block type codes
      for BIB and BCB.

   o  An OSB definition MUST state whether it can be the target of a BIB
      or a BCB.  The definition MUST also state whether the OSB can
      target a BIB or a BCB.

   o  An OSB definition MUST provide a deterministic processing order in
      the event that a bundle is received containing BIBs, BCBs, and
      OSBs.  This processing order MUST NOT alter the BIB and BCB
      processing orders identified in this specification.

   o  An OSB definition MUST provide a canonicalization algorithm if the
      default non-primary-block canonicalization algorithm cannot be
      used to generate a deterministic input for a cipher suite.  This
      requirement can be waived if the OSB is defined so as to never be
      the security target of a BIB or a BCB.




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   o  An OSB definition MUST NOT require any behavior of a BPSEC-BPA
      that is in conflict with the behavior identified in this
      specification.  In particular, the security processing
      requirements imposed by this specification must be consistent
      across all BPSEC-BPAs in a network.

   o  The behavior of an OSB when dealing with fragmentation must be
      specified and MUST NOT lead to ambiguous processing states.  In
      particular, an OSB definition should address how to receive and
      process an OSB in a bundle fragment that may or may not also
      contain its security target.  An OSB definition should also
      address whether an OSB may be added to a bundle marked as a
      fragment.

   Additionally, policy considerations for the management, monitoring,
   and configuration associated with blocks SHOULD be included in any
   OSB definition.

   NOTE: The burden of showing compliance with processing rules is
   placed upon the specifications defining new security blocks and the
   identification of such blocks shall not, alone, require maintenance
   of this specification.

11.  IANA Considerations

   This specification includes fields requiring registries managed by
   IANA.

11.1.  Bundle Block Types

   This specification allocates two block types from the existing
   "Bundle Block Types" registry defined in [RFC6255].

       Additional Entries for the Bundle Block-Type Codes Registry:

          +-------+-----------------------------+---------------+
          | Value |         Description         |   Reference   |
          +-------+-----------------------------+---------------+
          |  TBA  |    Block Integrity Block    | This document |
          |  TBA  | Block Confidentiality Block | This document |
          +-------+-----------------------------+---------------+

                                  Table 2

   The Bundle Block Types namespace notes whether a block type is meant
   for use in BP version 6, BP version 7, or both.  The two block types
   defined in this specification are meant for use with BP version 7.




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11.2.  Security Context Identifiers

   BPSec has a Security Context Identifier field for which IANA is
   requested to create and maintain a new registry named "BPSec Security
   Context Identifiers".  Initial values for this registry are given
   below.

   The registration policy for this registry is: Specification Required.

   The value range is: unsigned 16-bit integer.

                BPSec Security Context Identifier Registry

                  +-------+-------------+---------------+
                  | Value | Description |   Reference   |
                  +-------+-------------+---------------+
                  |   0   |   Reserved  | This document |
                  +-------+-------------+---------------+

                                  Table 3

12.  References

12.1.  Normative References

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
              Version 7", draft-ietf-dtn-bpbis-22 (work in progress),
              February 2020.

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

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC6255]  Blanchet, M., "Delay-Tolerant Networking Bundle Protocol
              IANA Registries", RFC 6255, DOI 10.17487/RFC6255, May
              2011, <https://www.rfc-editor.org/info/rfc6255>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.




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

12.2.  Informative References

   [I-D.birrane-dtn-sbsp]
              Birrane, E., Pierce-Mayer, J., and D. Iannicca,
              "Streamlined Bundle Security Protocol Specification",
              draft-birrane-dtn-sbsp-01 (work in progress), October
              2015.

   [I-D.ietf-dtn-bpsec-interop-sc]
              Birrane, E., "BPSec Interoperability Security Contexts",
              draft-ietf-dtn-bpsec-interop-sc-01 (work in progress),
              February 2020.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
              April 2007, <https://www.rfc-editor.org/info/rfc4838>.

   [RFC6257]  Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", RFC 6257,
              DOI 10.17487/RFC6257, May 2011,
              <https://www.rfc-editor.org/info/rfc6257>.

Appendix A.  Acknowledgements

   The following participants contributed technical material, use cases,
   and useful thoughts on the overall approach to this security
   specification: Scott Burleigh of the Jet Propulsion Laboratory, Amy
   Alford and Angela Hennessy of the Laboratory for Telecommunications
   Sciences, and Angela Dalton and Cherita Corbett of the Johns Hopkins
   University Applied Physics Laboratory.

Authors' Addresses

   Edward J. Birrane, III
   The Johns Hopkins University Applied
         Physics Laboratory
   11100 Johns Hopkins Rd.
   Laurel, MD  20723
   US

   Phone: +1 443 778 7423
   Email: Edward.Birrane@jhuapl.edu




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   Kenneth McKeever
   The Johns Hopkins University Applied
         Physics Laboratory
   11100 Johns Hopkins Rd.
   Laurel, MD  20723
   US

   Phone: +1 443 778 2237
   Email: Ken.McKeever@jhuapl.edu










































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