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DTN Research Group                                          S. Symington
Internet-Draft                                     The MITRE Corporation
Expires: August 22, 2010                                      S. Farrell
                                                  Trinity College Dublin
                                                                H. Weiss
                                                               P. Lovell
                                                            SPARTA, Inc.
                                                       February 18, 2010


                 Bundle Security Protocol Specification
                  draft-irtf-dtnrg-bundle-security-15

Abstract

   This document defines the bundle security protocol, which provides
   data integrity and confidentiality services for the bundle protocol.
   Separate capabilities are provided to protect the bundle payload and
   additional data that may be included within the bundle.  We also
   describe various bundle security considerations including policy
   options.

   This document is a product of the Delay Tolerant Networking Research
   Group and has been reviewed by that group.  No objections to its
   publication as an RFC were raised.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 22, 2010.



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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Related Documents  . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Security Blocks  . . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Abstract Security Block  . . . . . . . . . . . . . . . . .  9
     2.2.  Bundle Authentication Block  . . . . . . . . . . . . . . . 13
     2.3.  Payload Integrity Block  . . . . . . . . . . . . . . . . . 14
     2.4.  Payload Confidentiality Block  . . . . . . . . . . . . . . 16
     2.5.  Extension Security Block . . . . . . . . . . . . . . . . . 19
     2.6.  Parameters and Result Fields . . . . . . . . . . . . . . . 21
     2.7.  Key Transport  . . . . . . . . . . . . . . . . . . . . . . 23
     2.8.  PIB and PCB combinations . . . . . . . . . . . . . . . . . 23
   3.  Security Processing  . . . . . . . . . . . . . . . . . . . . . 26
     3.1.  Nodes as policy enforcement points . . . . . . . . . . . . 26
     3.2.  Processing order of security blocks  . . . . . . . . . . . 26
     3.3.  Security Regions . . . . . . . . . . . . . . . . . . . . . 29
     3.4.  Canonicalisation of bundles  . . . . . . . . . . . . . . . 31
     3.5.  Endpoint ID confidentiality  . . . . . . . . . . . . . . . 37
     3.6.  Bundles received from other nodes  . . . . . . . . . . . . 37
     3.7.  The At-Most-Once-Delivery Option . . . . . . . . . . . . . 39
     3.8.  Bundle Fragmentation and Reassembly  . . . . . . . . . . . 40
     3.9.  Reactive fragmentation . . . . . . . . . . . . . . . . . . 41
     3.10. Attack Model . . . . . . . . . . . . . . . . . . . . . . . 41
   4.  Mandatory Ciphersuites . . . . . . . . . . . . . . . . . . . . 43
     4.1.  BAB-HMAC . . . . . . . . . . . . . . . . . . . . . . . . . 43
     4.2.  PIB-RSA-SHA256 . . . . . . . . . . . . . . . . . . . . . . 44
     4.3.  PCB-RSA-AES128-PAYLOAD-PIB-PCB . . . . . . . . . . . . . . 45
     4.4.  ESB-RSA-AES128-EXT . . . . . . . . . . . . . . . . . . . . 49
   5.  Key Management . . . . . . . . . . . . . . . . . . . . . . . . 52
   6.  Default Security Policy  . . . . . . . . . . . . . . . . . . . 53
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 55
   8.  Conformance  . . . . . . . . . . . . . . . . . . . . . . . . . 56
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 57
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 58
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 58
     10.2. Informative References . . . . . . . . . . . . . . . . . . 58
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60












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1.  Introduction

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document defines security features for the bundle protocol
   [DTNBP] intended for use in delay tolerant networks, in order to
   provide DTN security services.

   The bundle protocol is used in DTNs which overlay multiple networks,
   some of which may be challenged by limitations such as intermittent
   and possibly unpredictable loss of connectivity, long or variable
   delay, asymmetric data rates, and high error rates.  The purpose of
   the bundle protocol is to support interoperability across such
   stressed networks.  The bundle protocol is layered on top of
   underlay-network-specific convergence layers, on top of network-
   specific lower layers, to enable an application in one network to
   communicate with an application in another network, both of which are
   spanned by the DTN.

   Security will be important for the bundle protocol.  The stressed
   environment of the underlying networks over which the bundle protocol
   will operate makes it important for the DTN to be protected from
   unauthorized use, and this stressed environment poses unique
   challenges for the mechanisms needed to secure the bundle protocol.
   Furthermore, DTNs may very likely be deployed in environments where a
   portion of the network might become compromised, posing the usual
   security challenges related to confidentiality, integrity and
   availability.

   Different security processing applies to the payload and extension
   blocks that may accompany it in a bundle, and different rules apply
   to various extension blocks.

   This document describes both the base Bundle Security Protocol (BSP)
   and a set of mandatory ciphersuites.  A ciphersuite is a specific
   collection of various cryptographic algorithms and implementation
   rules that are used together to provide certain security services.

   The Bundle Security Protocol applies, by definition, only to those
   nodes that implement it, known as "security-aware" nodes.  There MAY
   be other nodes in the DTN that do not implement BSP.  All nodes can
   interoperate with the exception that BSP security operations can only
   happen at security-aware nodes.






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1.1.  Related Documents

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

      The Delay-Tolerant Network Architecture [DTNarch] defines the
      architecture for delay-tolerant networks, but does not discuss
      security at any length.

      The DTN Bundle Protocol [DTNBP] defines the format and processing
      of the blocks used to implement the bundle protocol, excluding the
      security-specific blocks defined here.

1.2.  Terminology

   We introduce the following terminology for purposes of clarity:

      source - the bundle node from which a bundle originates

      destination - the bundle node to which a bundle is ultimately
      destined

      forwarder - the bundle node that forwarded the bundle on its most
      recent hop

      intermediate receiver or "next hop" - the neighboring bundle node
      to which a forwarder forwards a bundle.

      path - the ordered sequence of nodes through which a bundle passes
      on its way from source to destination

   In the figure below, which is adapted from figure 1 in the Bundle
   Protocol Specification, four bundle nodes (denoted BN1, BN2, BN3, and
   BN4) reside above some transport layer(s).  Three distinct transport
   and network protocols (denoted T1/N1, T2/N2, and T3/N3) are also
   shown.















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   +---------v-|   +->>>>>>>>>>v-+     +->>>>>>>>>>v-+   +-^---------+
   | BN1     v |   | ^   BN2   v |     | ^   BN3   v |   | ^  BN4    |
   +---------v-+   +-^---------v-+     +-^---------v-+   +-^---------+
   | T1      v |   + ^  T1/T2  v |     + ^  T2/T3  v |   | ^  T3     |
   +---------v-+   +-^---------v-+     +-^---------v +   +-^---------+
   | N1      v |   | ^  N1/N2  v |     | ^  N2/N3  v |   | ^  N3     |
   +---------v-+   +-^---------v +     +-^---------v-+   +-^---------+
   |         >>>>>>>>^         >>>>>>>>>>^         >>>>>>>>^         |
   +-----------+   +------------+      +-------------+   +-----------+
   |                     |                    |                      |
   |<--  An Internet --->|                    |<--- An Internet  --->|
   |                     |                    |                      |

   BN = "Bundle Node" as defined in the Bundle Protocol Specification

   Bundle Nodes Sit at the Application layer of the Internet Model.

                                 Figure 1

   Bundle node BN1 originates a bundle that it forwards to BN2.  BN2
   forwards the bundle to BN3, and BN3 forwards the bundle to BN4.  BN1
   is the source of the bundle and BN4 is the destination of the bundle.
   BN1 is the first forwarder, and BN2 is the first intermediate
   receiver; BN2 then becomes the forwarder, and BN3 the intermediate
   receiver; BN3 then becomes the last forwarder, and BN4 the last
   intermediate receiver, as well as the destination.

   If node BN2 originates a bundle (for example, a bundle status report
   or a custodial signal), which is then forwarded on to BN3, and then
   to BN4, then BN2 is the source of the bundle (as well as being the
   first forwarder of the bundle) and BN4 is the destination of the
   bundle (as well as being the final intermediate receiver).

   We introduce the following security-specific DTN terminology:

      security-source - a bundle node that adds a security block to a
      bundle

      security-destination - a bundle node that processes a security
      block of a bundle

      security path - the ordered sequence of security-aware nodes
      through which a bundle passes on its way from the security-source
      to the security-destination

   Referring to Figure 1 again:

   If the bundle that originates at BN1 as source is given a security



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   block by BN1, then BN1 is the security-source of this bundle with
   respect to that security block, as well as being the source of the
   bundle.

   If the bundle that originates at BN1 as source is given a security
   block by BN2, then BN2 is the security-source of this bundle with
   respect to that security block, even though BN1 is the source.

   If the bundle that originates at BN1 as source is given a security
   block by BN1 that is intended to be processed by BN3, then BN1 is the
   security-source and BN3 is the security destination with respect to
   this security block.  The security path for this block is BN1 to BN3.

   A bundle MAY have multiple security blocks.  The security-source of a
   bundle with respect to a given security block in the bundle MAY be
   the same as or different from the security-source of the bundle with
   respect to a different security block in the bundle.  Similarly, the
   security-destination of a bundle with respect to each of that
   bundle's security blocks MAY be the same or different.  Therefore the
   security paths for various blocks MAY be and often will be different.

   If the bundle that originates at BN1 as source is given a security
   block by BN1 that is intended to be processed by BN3, and BN2 adds a
   security block with security-destination BN4, the security paths for
   the two blocks overlap but not completely.  This problem is discussed
   further in Section 3.3.

   As required in [DTNBP], forwarding nodes MUST transmit blocks in a
   bundle in the same order in which they were received.  This
   requirement applies to all DTN nodes, not just ones which implement
   security processing.  Blocks in a bundle MAY be added or deleted
   according to the applicable specification, but those blocks which are
   both received and transmitted MUST be transmitted in the same order
   that they were received.

   If a node is not security-aware then it forwards the security blocks
   in the bundle unchanged unless the bundle's block processing flags
   specify otherwise.  If a network has some nodes that are not
   security-aware then the block processing flags SHOULD be set such
   that security blocks are not discarded at those nodes solely because
   they can not be processed there.  Except for this, the non-security-
   aware nodes are transparent relay points and are invisible as far as
   security processing is concerned.

   The block sequence also indicates the order in which certain
   significant actions have affected the bundle, and therefore the
   sequence in which actions MUST occur in order to produce the bundle
   at its destination.



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2.  Security Blocks

   There are four types of security block that MAY be included in a
   bundle.  These are the Bundle Authentication Block (BAB), the Payload
   Integrity Block (PIB), the Payload Confidentiality Block (PCB) and
   the Extension Security Block (ESB).

      The BAB is used to assure the authenticity and integrity of the
      bundle along a single hop from forwarder to intermediate receiver.
      Since security blocks are only processed at security-aware nodes,
      a "single hop" from a security-aware forwarder to the next
      security-aware intermediate receiver might be more than one actual
      hop.  This situation is discussed further below Section 2.2.

      The PIB is used to assure the authenticity and integrity of the
      payload from the PIB security-source, which creates the PIB, to
      the PIB security-destination, which verifies the PIB
      authenticator.  The authentication information in the PIB MAY (if
      the ciphersuite allows) be verified by any node in between the PIB
      security-source and the PIB security-destination that has access
      to the cryptographic keys and revocation status information
      required to do so.

      Since a BAB protects a bundle on a "hop-by-hop" basis and other
      security blocks MAY be protecting over several hops or end-to-end,
      whenever both are present the BAB MUST form the "outer" layer of
      protection - that is, the BAB MUST always be calculated and added
      to the bundle after all other security blocks have been calculated
      and added to the bundle.

      The PCB indicates that the payload has been encrypted, in whole or
      in part, at the PCB security-source in order to protect the bundle
      content while in transit to the PCB security-destination.

      PIB and PCB protect the payload and are regarded as "payload-
      related" for purposes of the security discussion in this document.
      Other blocks are regarded as "non-payload" blocks.  Of course, the
      primary block is unique and has separate rules.

      The ESB provides security for non-payload blocks in a bundle.  ESB
      therefore is not applied to PIB or PCBs, and of course is not
      appropriate for either the payload block or primary block.

   Each of the security blocks uses the Canonical Bundle Block Format as
   defined in the Bundle Protocol Specification.  That is, each security
   block is comprised of the following elements:





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      - Block type code

      - Block processing control flags

      - Block EID reference list (optional)

      - Block data length

      - Block-type-specific data fields

   Since the four security blocks have most fields in common, we can
   shorten the description of the Block-type-specific data fields of
   each security block if we first define an abstract security block
   (ASB) and then specify each of the real blocks in terms of the fields
   which are present/absent in an ASB.  Note that no bundle ever
   contains an actual ASB, which is simply a specification artifact.

2.1.  Abstract Security Block

   An ASB consists of the following mandatory and optional fields:

      - Block-type code (one byte) - as in all bundle protocol blocks
      except the primary bundle block.  The block types codes for the
      security blocks are:

         BundleAuthenticationBlock - BAB: 0x02

         PayloadIntegrityBlock - PIB: 0x03

         PayloadConfidentialityBlock - PCB: 0x04

         ExtensionSecurityBlock - ESB: 0x09

      - Block processing control flags (SDNV) - defined as in all bundle
      protocol blocks except the primary bundle block (as described in
      the Bundle Protocol [DTNBP]).  SDNV encoding is described in the
      bundle protocol.  There are no general constraints on the use of
      the block processing flags, and some specific requirements are
      discussed later.

      - EID references - composite field defined in [DTNBP] containing
      references to one or two EIDs.  Presence of the EID-reference
      field is indicated by the setting of the "block contains an EID-
      reference field" (EID_REF) bit of the block processing control
      flags.  If one or more references is present, flags in the
      ciphersuite ID field, described below, specify which.





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      If no EID fields are present then the composite field itself MUST
      be omitted entirely and the EID_REF bit MUST be unset.  A count
      field of zero is not permitted.

      The possible EIDs are:

         - (optional) Security-source - specifies the security source
         for the block.  If this is omitted, then the source of the
         bundle is assumed to be the security-source unless otherwise
         indicated.

         - (optional) Security-destination - specifies the security
         destination for the block.  If this is omitted, then the
         destination of the bundle is assumed to be the security-
         destination unless otherwise indicated.

      If two EIDs are present, security-source is first and security-
      destination comes second.

      - Block data length (SDNV) - as in all bundle protocol blocks
      except the primary bundle block.  SDNV encoding is described in
      the bundle protocol.

      - Block-type-specific data fields as follows:

         - Ciphersuite ID (SDNV)

         - Ciphersuite flags (SDNV)

         - (optional) Correlator - when more than one related block is
         inserted then this field MUST have the same value in each
         related block instance.  This is encoded as an SDNV.  See note
         in Section 3.8 with regard to correlator values in bundle
         fragments.

         - (optional) Ciphersuite parameters - compound field of next
         two items

            - Ciphersuite parameters length - specifies the length of
            the following Ciphersuite parameters data field and is
            encoded as an SDNV.

            - Ciphersuite parameters data - parameters to be used with
            the ciphersuite in use, e.g. a key identifier or
            initialization vector (IV).  See Section 2.6 for a list of
            potential parameters and their encoding rules.  The
            particular set of parameters that are included in this field
            are defined as part of the ciphersuite specification.



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         - (optional) Security result - compound field of next two items

            - Security result length - contains the length of the next
            field and is encoded as an SDNV.

            - Security result data - contains the results of the
            appropriate ciphersuite-specific calculation (e.g., a
            signature, MAC or ciphertext block key).

   Although the diagram hints at a 32-bit layout this is purely for the
   purpose of exposition.  Except for the "type" field, all fields are
   variable in length.

   +----------------+----------------+----------------+----------------+
   | type           |  flags (SDNV)  |  EID ref list(comp)             |
   +----------------+----------------+----------------+----------------+
   | length  (SDNV)                  |  ciphersuite (SDNV)             |
   +----------------+----------------+----------------+----------------+
   | ciphersuite flags (SDNV)        |  correlator  (SDNV)             |
   +----------------+----------------+----------------+----------------+
   |params len(SDNV)| ciphersuite params data                          |
   +----------------+----------------+----------------+----------------+
   |res-len  (SDNV) | security result data                             |
   +----------------+----------------+----------------+----------------+

   The structure of an abstract security block.

                                 Figure 2

   Some ciphersuites are specified in Section 4, which also specifies
   the rules which MUST be satisfied by ciphersuite specifications.
   Additional ciphersuites MAY be defined in separate specifications.
   Ciphersuite IDs not specified are reserved.  Implementations of the
   bundle security protocol decide which ciphersuites to support,
   subject to the requirements of Section 4.  It is RECOMMENDED that
   implementations that allow additional ciphersuites permit ciphersuite
   ID values at least up to and including 127, and they MAY decline to
   allow larger ID values.

   The structure of the ciphersuite flags field is shown in Figure 3.
   In each case the presence of an optional field is indicated by
   setting the value of the corresponding flag to one.  A value of zero
   indicates the corresponding optional field is missing.  Presently
   there are five flags defined for the field and for convenience these
   are shown as they would be extracted from a single-byte SDNV.  Future
   additions may cause the field to grow to the left so, as with the
   flags fields defined in [DTNBP], the description below numbers the
   bit positions from the right rather than the standard RFC definition



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   which numbers bits from the left.

      src - bit 4 indicates whether the EID-reference field of the ASB
      contains the optional reference to the security-source.

      dest - bit 3 indicates whether the EID-reference field of the ASB
      contains the optional reference to the security-destination.

      parm - bit 2 indicates whether the ciphersuite-parameters-length
      and ciphersuite parameters data fields are present or not.

      corr - bit 1 indicates whether or not the ASB contains an optional
      correlator.

      res - bit 0 indicates whether or not the ASB contains the security
      result length and security result data fields.

      bits 5-6 are reserved for future use.



   Ciphersuite flags
     Bit   Bit   Bit   Bit   Bit   Bit   Bit
      6     5     4     3     2     1     0
   +-----+-----+-----+-----+-----+-----+-----+
   | reserved  |src  |dest |parm |corr |res  |
   +-----+-----+-----+-----+-----+-----+-----+

                                 Figure 3

   A little bit more terminology: if the block is a PIB then when we
   refer to the "PIB-source", we mean the security source for the PIB as
   represented by the EID reference in the EID-references field.
   Similarly we may refer to the PCB-dest, meaning the security-
   destination of the PCB, again as represented by an EID reference.
   For example, referring to Figure 1 again, if the bundle that
   originates at BN1 as source is given a Confidentiality Block (PCB) by
   BN1 that is protected using a key held by BN3 and it is given a
   Payload Integrity Block (PIB) by BN1, then BN1 is both the PCB-source
   and the PIB-source of the bundle, and BN3 is the PCB-dest of the
   bundle.

   The correlator field is used to associate several related instances
   of a security block.  This can be used to place a BAB that contains
   the ciphersuite information at the "front" of a (probably large)
   bundle, and another correlated BAB that contains the security result
   at the "end" of the bundle.  This allows even very memory-constrained
   nodes to be able to process the bundle and verify the BAB.  There are



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   similar use cases for multiple related instances of PIB and PCB as
   will be seen below.

   The ciphersuite specification MUST make it clear whether or not
   multiple block instances are allowed, and if so, under what
   conditions.  Some ciphersuites can of course leave flexibility to the
   implementation, whereas others might mandate a fixed number of
   instances.

   For convenience, we use the term "first block" to refer to the
   initial block in a group of correlated blocks, or to the single block
   if there are no others in the set.  Obviously there can be several
   unrelated groups in a bundle, each containing only one block or more
   than one, and each has its own "first block".

2.2.  Bundle Authentication Block

   In this section we describe typical BAB field values for two
   scenarios - where a single instance of the BAB contains all the
   information and where two related instances are used, one "up front"
   which contains the ciphersuite and another following the payload
   which contains the security result (e.g. a MAC).

   For the case where a single BAB is used:

      The block-type code field value MUST be 0x02.

      The block processing control flags value can be set to whatever
      values are required by local policy.  Ciphersuite designers should
      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.

      The ciphersuite ID MUST be documented as a hop-by-hop
      authentication-ciphersuite which requires one instance of the BAB.

      The correlator field MUST NOT be present.

      The ciphersuite parameters field MAY be present, if so specified
      in the ciphersuite specification.

      An EID reference to the security-source MAY be present.  The
      security-source can also be specified as part of key information
      described in Section 2.6 or another block such as the Previous Hop
      Insertion Block [PHIB].  The security-source might also be
      inferred from some implementation-specific means such as the
      convergence layer.




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      An EID reference to the security-destination MAY be present and is
      useful to ensure that the bundle has been forwarded to the correct
      next-hop node.

      The security result MUST be present as it is effectively the
      "output" from the ciphersuite calculation (e.g. the MAC or
      signature) applied to the (relevant parts of) the bundle (as
      specified in the ciphersuite definition).

   For the case using two related BAB instances, the first instance is
   as defined above, except the ciphersuite ID MUST be documented as a
   hop-by-hop authentication ciphersuite that requires two instances of
   the BAB.  In addition, the correlator MUST be present and the
   security result length and security result fields MUST be absent.
   The second instance of the BAB MUST have the same correlator value
   present and MUST contain security result length and security result
   data fields.  The other optional fields MUST NOT be present.
   Typically, this second instance of a BAB will be the last block of
   the bundle.

   The details of key transport for BAB are specified by the particular
   ciphersuite.  In the absence of conflicting requirements, the
   following should be noted by implementors:

      - the key information item Section 2.6 is optional, and if not
      provided then the key should be inferred from the source-
      destination tuple, being the previous key used, a key created from
      a key-derivation function, or a pre-shared key

      - if all the nodes are security-aware, the capabilities of the
      underlying convergence layer might be useful for identifying the
      security-source

      - depending upon the key mechanism used, bundles can be signed by
      the sender, or authenticated for one or more recipients, or both.

2.3.  Payload Integrity Block

   A PIB is an ASB with the following additional restrictions:

      The block type code value MUST be 0x03.

      The block processing control flags value can be set to whatever
      values are required by local policy.  Ciphersuite designers should
      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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      The ciphersuite ID MUST be documented as an end-to-end
      authentication-ciphersuite or as an end-to-end error-detection-
      ciphersuite.

      The correlator MUST be present if the ciphersuite requires more
      than one related instance of a PIB be present in the bundle.  The
      correlator MUST NOT be present if the ciphersuite only requires
      one instance of the PIB in the bundle.

      The ciphersuite parameters field MAY be present.

      An EID reference to the security-source MAY be present.  The
      security-source can also be specified as part of key information
      described in Section 2.6.

      An EID reference to the security-destination MAY be present.

      The security result is effectively the "output" from the
      ciphersuite calculation (e.g. the MAC or signature) applied to the
      (relevant parts of) the bundle.  As in the case of the BAB, this
      field MUST be present if the correlator is absent.  If more than
      one related instance of the PIB is required then this is handled
      in the same way as described for the BAB above.

      The ciphersuite MAY process less than the entire original bundle
      payload.  This might be because it is defined to process some
      subset of the bundle, or perhaps because the the current payload
      is a fragment of an original bundle.  For whatever reason, if the
      ciphersuite processes less than the complete, original bundle
      payload, the ciphersuite parameters of this block MUST specify
      which bytes of the bundle payload are protected.

   For some ciphersuites, (e.g. those using asymmetric keying to produce
   signatures or those using symmetric keying with a group key), the
   security information can be checked at any hop on the way to the
   security destination that has access to the required keying
   information.  This possibility is further discussed in Section 3.6
   below.

   The use of a generally-available key is RECOMMENDED if custodial
   transfer is employed and all nodes SHOULD verify the bundle before
   accepting custody.

   Most asymmetric PIB-ciphersuites will use the PIB-source to indicate
   the signer and will not require the PIB-dest field because the key
   needed to verify the PIB authenticator will be a public key
   associated with the PIB-source.




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2.4.  Payload Confidentiality Block

   A typical confidentiality ciphersuite will encrypt the payload using
   a randomly generated bundle encrypting key (BEK) and will use a key
   information item in the PCB security parameters to carry the BEK
   encrypted with some long term key encryption key (KEK) or well-known
   public key.  If neither the destination nor security-destination
   resolves the key to use for decryption, the key information item in
   the ciphersuite parameters field can also be used to indicate the
   decryption key with which the BEK can be recovered.  If the bundle
   already contains PIBs and/or PCBs these SHOULD also be encrypted
   using this same BEK, as described just below for "super-encryption".
   The encrypted block is encapsulated into a new PCB that replaces the
   original block at the same place in the bundle.

   It is strongly RECOMMENDED that a data integrity mechanism be used in
   conjunction with confidentiality, and that encryption-only
   ciphersuites NOT be used.  AES-GCM satisfies this requirement.  The
   "authentication tag" or "integrity check value" is stored into
   security-result rather than being appended to the payload as is
   common in some protocols since, as described below, it is important
   that there be no change in the size of the payload.

   The payload is encrypted "in-place", that is, following encryption,
   the payload block payload field contains ciphertext, not plaintext.
   The payload block processing flags are unmodified.

   The "in-place" encryption of payload bytes is to allow bundle payload
   fragmentation and re-assembly, and custody transfer, to operate
   without knowledge of whether or not encryption has occurred and, if
   so, how many times.

   Fragmentation and reassembly and custody transfer are adversely
   affected by a change in size of the payload due to ambiguity about
   what byte range of the original payload is actually in any particular
   fragment.  Ciphersuites SHOULD place any payload expansion, such as
   authentication tags (integrity check values) and any padding
   generated by a block-mode cipher, into an "integrity check value"
   item in the security-result field (see Section 2.6) of the
   confidentiality block.

   Payload super-encryption is allowed; that is, encrypting a payload
   that has already been encrypted, perhaps more than once.
   Ciphersuites SHOULD define super-encryption such that, as well as re-
   encrypting the payload, it also protects the parameters of earlier
   encryption.  Failure to do so may represent a vulnerability in some
   circumstances.




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   Confidentiality is normally applied to the payload, and possibly to
   additional blocks.  It is RECOMMENDED to apply a Payload
   Confidentiality ciphersuite to non-payload blocks only if these
   should be super-encrypted with the payload.  If super-encryption of
   the block is not desired then protection of the block should be done
   using the Extension Security Block mechanism rather than PCB.

   Multiple related PCB instances are required if both the payload and
   PIBs and PCBs in the bundle are to be encrypted.  These multiple PCB
   instances require correlators to associate them with each other since
   the key information is provided only in the first PCB.

   There are situations where more than one PCB instance is required but
   the instances are not "related" in the sense which requires
   correlators.  One example is where a payload is encrypted for more
   than one security-destination so as to be robust in the face of
   routing uncertainties.  In this scenario the payload is encrypted
   using a BEK.  Several PCBs contain the BEK encrypted using different
   KEKs, one for each destination.  These multiple PCB instances, are
   not "related" and should not contain correlators.

   The ciphersuite MAY apply different rules to confidentiality for non-
   payload blocks.

   A PCB is an ASB with the following additional restrictions:

      The block type code value MUST be 0x04.

      The block processing control flags value can be set to whatever
      values are required by local policy, except that a PCB "first
      block" MUST have the "replicate in every fragment" flag set.  This
      flag SHOULD NOT be set otherwise.  Ciphersuite designers should
      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.

      The ciphersuite ID MUST be documented as a confidentiality-
      ciphersuite.

      The correlator MUST be present if there is more than one related
      PCB instance.  The correlator MUST NOT be present if there are no
      related PCB instances.

      If a correlator is present, the key information MUST be placed in
      the PCB "first block".

      Any additional bytes generated as a result of encryption and/or
      authentication processing of the payload SHOULD be placed in an



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      "integrity check value" field (see Section 2.6) in the security-
      result of the first PCB.

      The ciphersuite parameters field MAY be present.

      An EID reference to the security-source MAY be present.  The
      security-source can also be specified as part of key information
      described in Section 2.6.

      An EID reference to the security-destination MAY be present.

      The security result MAY be present and normally contains fields
      such as an encrypted bundle encryption key, authentication tag or
      the encrypted versions of bundle blocks other than the payload
      block.

   The ciphersuite MAY process less than the entire original bundle
   payload, either because the current payload is a fragment of the
   original bundle or just because it is defined to process some subset.
   For whatever reason, if the ciphersuite processes less than the
   complete, original bundle payload the "first" PCB MUST specify, as
   part of the ciphersuite parameters, which bytes of the bundle payload
   are protected.

   PCB ciphersuites MUST specify which blocks are to be encrypted.  The
   specification MAY be flexible and be dependent upon block type,
   security policy, various data values and other inputs but it MUST be
   deterministic.  The determination of whether a block is to be
   encrypted or not MUST NOT be ambiguous.

   As was the case for the BAB and PIB, if the ciphersuite requires more
   than one instance of the PCB, then the "first block" MUST contain any
   optional fields (e.g., security destination etc.) that apply to all
   instances with this correlator.  These MUST be contained in the first
   instance and MUST NOT be repeated in other correlated blocks.  Fields
   that are specific to a particular instance of the PCB MAY appear in
   that PCB.  For example, security result fields MAY (and probably
   will) be included in multiple related PCB instances, with each result
   being specific to that particular block.  Similarly, several PCBs
   might each contain a ciphersuite parameters field with an IV specific
   to that PCB instance.

   Put another way: when confidentiality will generate multiple blocks,
   it MUST create a "first" PCB with the required ciphersuite ID,
   parameters etc. as specified above.  Typically, this PCB will appear
   early in the bundle.  This "first" PCB contains the parameters that
   apply to the payload and also to the other correlated PCBs.  The
   correlated PCBs follow the "first" PCB and MUST NOT repeat the



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   ciphersuite parameters, security-source, or security-destination
   fields from the first PCB.  These correlated PCBs need not follow
   immediately after the "first" PCB, and probably will not do so.  Each
   correlated block, encapsulating an encrypted PIB or PCB, is at the
   same place in the bundle as the original PIB or PCB.

   A ciphersuite MUST NOT mix payload data and a non-payload block in a
   single PCB.

   Even if a to-be-encrypted block has the "discard" flag set, whether
   or not the PCB's "discard" flag is set is an implementation/policy
   decision for the encrypting node.  (The "discard" flag is more
   properly called the "discard if block cannot be processed" flag.)

   Any existing EID-list in the to-be-encapsulated original block
   remains exactly as-is, and is copied to become the EID-list for the
   replacing block.  The encapsulation process MUST NOT replace or
   remove the existing EID-list entries.  This is critically important
   for correct updating of entries at the security-destination.

   At the security-destination, either specific destination or the
   bundle destination, the processes described above are reversed.  The
   payload is decrypted in-place using the salt, IV and key values in
   the first PCB, including verification using the ICV.  These values
   are described below in Section 2.6.  Each correlated PCB is also
   processed at the same destination, using the salt and key values from
   the first PCB and the block-specific IV item.  The "encapsulated
   block" item in the security-result is decrypted and validated, using
   also the tag which SHOULD have been appended to the ciphertext of the
   original block data.  Assuming the validation succeeds, the resultant
   plaintext, which is the entire content of the original block,
   replaces the PCB at the same place in the bundle.  The block type
   reverts to that of the original block prior to encapsulation, and the
   other block-specific data fields also return to their original
   values.  Implementors are cautioned that this "replacement" process
   requires delicate stitchery, as the EID-list contents in the
   decapsulated block are invalid.  As noted above, the EID-list
   references in the original block were preserved in the replacing PCB,
   and will have been updated as necessary as the bundle has toured the
   dtn.  The references from the PCB MUST replace the references within
   the EID-list of the newly-decapsulated block.  Caveat implementor.

2.5.  Extension Security Block

   Extension security blocks provide protection for non-payload-related
   portions of a bundle.  ESBs MUST NOT be used for the primary block or
   payload, including payload-related security blocks (PIBs and PCBs).




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   It is sometimes desirable to protect certain parts of a bundle in
   ways other than those applied to the bundle payload.  One such
   example is bundle metadata that might specify the kind of data in the
   payload but not the actual payload detail, as described in [DTNMD].

   ESBs are typically used to apply confidentiality protection.  While
   it is possible to create an integrity-only ciphersuite, the block
   protection is not transparent and makes access to the data more
   difficult.  For simplicity, this discussion describes use of a
   confidentiality ciphersuite.

   The protection mechanisms in ESBs are similar to other security
   blocks with two important differences:

      - different key values are used (using same key as for payload
      would defeat the purpose)

      - the block is not encrypted or super-encrypted with the payload

   A typical ESB ciphersuite will encrypt the extension block using a
   randomly generated ephemeral key and will use the key information
   item in the security parameters field to carry the key encrypted with
   some long term key encryption key (KEK) or well-known public key.  If
   neither the destination nor security-destination resolves the key to
   use for decryption, the key information item in the ciphersuite
   parameters field can be used also to indicate the decryption key with
   which the BEK can be recovered.

   It is strongly RECOMMENDED that a data integrity mechanism be used in
   conjunction with confidentiality, and that encryption-only
   ciphersuites NOT be used.  AES-GCM satisfies this requirement.

   The ESB is placed in the bundle in the same position as the block
   being protected.  That is, the entire original block is processed
   (encrypted, etc) and encapsulated in a "replacing" ESB-type block,
   and this appears in the bundle at the same sequential position as the
   original block.  The processed data is placed in the security-result
   field.

   The process is reversed at the security destination with the
   recovered plaintext block replacing the ESB that had encapsulated it.
   Processing of EID-list entries, if any, is described above in
   Section 2.4 and this MUST be followed in order to correctly recover
   EIDs.

   An ESB is an ASB with the following additional restrictions:





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      Block type is 0x09.

      Ciphersuite flags indicate which fields are present in this block.
      Ciphersuite designers should 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.

      EID references MUST be stored in the EID reference list.

      Security-source MAY be present.  The security-source can also be
      specified as part of key information described in Section 2.6.  If
      neither is present then the bundle-source is used as the security-
      source.

      Security-destination MAY be present.  If not present, then the
      bundle-destination is used as the security-destination.

   The security-parameters MAY optionally contain a block-type field to
   indicate the type of the encapsulated block.  Since this replicates a
   field in the encrypted portion of the block, it is a slight security
   risk and its use is therefore OPTIONAL.

2.6.  Parameters and Result Fields

   Various ciphersuites include several items in the security-parameters
   and/or security-result fields.  Which items MAY appear is defined by
   the particular ciphersuite description.  A ciphersuite MAY support
   several instances of the same type within a single block.

   Each item is represented as type-length-value.  Type is a single byte
   indicating which item this is.  Length is the count of data bytes to
   follow, and is an SDNV-encoded integer.  Value is the data content of
   the item.

   Item types are

      0: reserved

      1: initialization vector (IV)

      2: reserved

      3: key information

      4: fragment range (offset and length as a pair of SDNVs)

      5: integrity signature




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      6: reserved

      7: salt

      8: PCB integrity check value (ICV)

      9: reserved

      10: encapsulated block

      11: block type of encapsulated block

      12 - 191: reserved

      192 - 250: private use

      251 - 255: reserved

   The folowing descriptions apply to usage of these items for all
   ciphersuites.  Additional characteristics are noted in the discussion
   for specific suites.

      - initialization vector(IV): random value, typically eight to
      sixteen bytes

      - key information: key material encoded or protected by the key
      management system, and used to transport an ephemeral key
      protected by a long-term key.  This item is discussed further
      below in Section 2.7

      - fragment range: pair of SDNV values (offset then length)
      specifying the range of payload bytes to which a particular
      operation applies.  This is termed "fragment range" since that is
      its typical use, even though sometimes it describes a subset range
      that is not a fragment.  The offset value MUST be the offset
      within the original bundle, which might not be the offset within
      the current bundle if the current bundle is already a fragment

      - integrity signature: result of BA or PI digest or signing
      operation.  This item is discussed further below in Section 2.7

      - salt: an IV-like value used by certain confidentiality suites

      - PCB integrity check value(ICV): output from certain
      confidentiality ciphersuite operations to be used at the
      destination to verify that the protected data has not been
      modified




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      - encapsulated block: result of confidentiality operation on
      certain blocks, contains the ciphertext of the block and MAY also
      contain an integrity check value appended to the ciphertext; MAY
      also contain padding if required by the encryption mode; used for
      non-payload blocks only

      - block type of encapsulated block: block type code for a block
      that has been encapsulated in ESB

2.7.  Key Transport

   This specification endeavours to maintain separation between the
   security protocol and key management.  However, these two interact in
   the transfer of key information, etc., from security-source to
   security-destination.  The intent of the separation is to facilitate
   use of a variety of key management systems without a necessity to
   tailor a ciphersuite to each individually.

   The key management process deals with such things as long-term keys,
   specifiers for long-term keys, certificates for long-term keys and
   integrity signatures using long-term keys.  The ciphersuite itself
   should not require a knowledge of these, and separation is improved
   if it treats these as opaque entities, to be handled by the key
   management process.

   The key management process deals specifically with the content of two
   of the items defined above in Section 2.6:- key information (item
   type 3) and integrity signature (item type 5).  The ciphersuite MUST
   define the details and format for these items.  To facilitate
   interoperability, it is strongly RECOMMENDED that the implementations
   use the appropriate definitions from Cryptographic Message Syntax
   (CMS) [RFC5652] and related RFCs.

   Many situations will require several pieces of key information.
   Again, ciphersuites MUST define whether they accept these packed into
   a single key information item and/or separated into multiple
   instances of key information.  For interoperability, it is
   RECOMMENDED that ciphersuites accept these packed into a single key-
   information item, and that they MAY additionally choose to accept
   them sent as separate items.

2.8.  PIB and PCB combinations

   Given the above definitions, nodes are free to combine applications
   of PIB and PCB in any way they wish - the correlator value allows for
   multiple applications of security services to be handled separately.
   Since PIB and PCB apply to the payload and ESB to non-payload blocks,
   combinations of ESB with PIB and/or PCB are not considered.



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   There are some obvious security problems that could arise when
   applying multiple services.  For example, if we encrypted a payload
   but left a PIB security result containing a signature in the clear,
   payload guesses could be confirmed.

   We cannot, in general, prevent all such problems since we cannot
   assume that every ciphersuite definition takes account of every other
   ciphersuite definition.  However, we can limit the potential for such
   problems by requiring that any ciphersuite which applies to one
   instance of a PIB or PCB, MUST be applied to all instances with the
   same correlator.

   We now list the PIB and PCB combinations which we envisage as being
   useful to support:

      Encrypted tunnels - a single bundle MAY be encrypted many times
      en-route to its destination.  Clearly it has to be decrypted an
      equal number of times, but we can imagine each encryption as
      representing the entry into yet another layer of tunnel.  This is
      supported by using multiple instances of PCB, but with the payload
      encrypted multiple times, "in-place".  Depending upon the
      ciphersuite defintion, other blocks can and should be encrypted,
      as discussed above and in Section 2.4 to ensure that parameters
      are protected in the case of super-encryption.

      Multiple parallel authenticators - a single security source might
      wish to protect the integrity of a bundle in multiple ways.  This
      could be required if the bundle's path is unpredictable, and if
      various nodes might be involved as security destinations.
      Similarly, if the security source cannot determine in advance
      which algorithms to use, then using all might be reasonable.  This
      would result in uses of PIB which presumably all protect the
      payload, and which cannot in general protect one another.  Note
      that this logic can also apply to a BAB, if the unpredictable
      routing happens in the convergence layer, so we also envisage
      support for multiple parallel uses of BAB.

      Multiple sequential authenticators - if some security destination
      requires assurance about the route that bundles have taken, then
      it might insist that each forwarding node add its own PIB.  More
      likely, however would be that outbound "bastion" nodes would be
      configured to sign bundles as a way of allowing the sending
      "domain" to take accountability for the bundle.  In this case, the
      various PIBs will likely be layered, so that each protects the
      earlier applications of PIB.

      Authenticated and encrypted bundles - a single bundle MAY require
      both authentication and confidentiality.  Some specifications



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      first apply the authenticator and follow this by encrypting the
      payload and authenticator.  As noted previously in the case where
      the authenticator is a signature, there are security reasons for
      this ordering.  (See the PCB-RSA-AES128-PAYLOAD-PIB-PCB
      ciphersuite defined later in Section 4.3.)  Others apply the
      authenticator after encryption, that is, to the ciphertext.  This
      ordering is generally recommended and minimizes attacks which, in
      some cases, can lead to recovery of the encryption key.

   There are no doubt other valid ways to combine PIB and PCB instances,
   but these are the "core" set supported in this specification.  Having
   said that, as will be seen, the mandatory ciphersuites defined here
   are quite specific and restrictive in terms of limiting the
   flexibility offered by the correlator mechanism.  This is primarily
   designed to keep this specification as simple as possible, while at
   the same time supporting the above scenarios.



































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3.  Security Processing

   This section describes the security aspects of bundle processing.

3.1.  Nodes as policy enforcement points

   All nodes are REQUIRED to have and enforce their own configurable
   security policies, whether these policies be explicit or default, as
   defined in Section 6.

   All nodes serve as Policy Enforcement Points (PEP) insofar as they
   enforce polices that MAY restrict the permissions of bundle nodes to
   inject traffic into the network.  Policies MAY apply to traffic
   originating at the current node, traffic terminating at the current
   node and traffic to be forwarded by the current node to other nodes.
   If a particular transmission request, originating either locally or
   remotely, satisfies the node's policy or policies and is therefore
   accepted, then an outbound bundle can be created and dispatched.  If
   not, then in its role as a PEP, the node will not create or forward a
   bundle.  Error handling for such cases is currently considered out of
   scope of this document.

   Policy enforcing code MAY override all other processing steps
   described here and elsewhere in this document.  For example, it is
   valid to implement a node which always attempts to attach a PIB.
   Similarly it is also valid to implement a node which always rejects
   all requests which imply the use of a PIB.

   Nodes MUST consult their security policy to determine the criteria
   that a received bundle ought to meet before it will be forwarded.
   These criteria MUST include a determination of whether or not the
   received bundle MUST include a valid BAB, PIB, PCB or ESB.  If the
   bundle does not meet the node's policy criteria, then the bundle MUST
   be discarded and processed no further; in this case, a bundle status
   report indicating the failure MAY be generated.

   The node's policy MAY call for the node to add or subtract some
   security blocks.  For example, it might require the node attempt to
   encrypt (parts of) the bundle for some security-destination, or that
   it add a PIB.  If the node's policy requires a BAB to be added to the
   bundle, it MUST be added last so that the calculation of its security
   result MAY take into consideration the values of all other blocks in
   the bundle.

3.2.  Processing order of security blocks

   The processing order of security actions for a bundle is critically
   important for the actions to complete successfully.  In general, the



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   actions performed at the originating node MUST be executed in the
   reverse sequence at the destination.  There are variations and
   exceptions, and these are noted below.

   The sequence is maintained in the ordering of security blocks in the
   bundle.  It is for this reason that blocks MUST not be rearranged at
   forwarding nodes, whether they support the security protocols or not.
   The only blocks that participate in this ordering are the primary and
   payload blocks, and the PIB and PCB security blocks themselves.  All
   other extension blocks, including ESBs, are ignored for purposes of
   determining the processing order.

   The security blocks are added to and removed from a bundle in a last-
   in-first-out (LIFO) manner, with the top of the stack immediately
   after the primary block.  A newly-created bundle has just the primary
   and payload blocks, and the stack is empty.  As security actions are
   requested for the bundle, security blocks are pushed onto the stack
   immediately after the primary block.  The early actions have security
   blocks close to the payload, later actions have blocks nearer to the
   primary block.  The actions deal with only those blocks in the bundle
   at the time so, for example, the first to be added processes only the
   payload and primary blocks, the next might process the first if it
   chooses and the payload and primary, and so on.  The last block to be
   added can process all the blocks.

   When the bundle is received, this process is reversed and security
   processing begins at the top of the stack, immediately after the
   primary block.  The security actions are performed and the block is
   popped from the stack.  Processing continues with the next security
   block until finally only the payload and primary blocks remain.

   The simplicity of this description is undermined by various real-
   world requirements.  Nonetheless it serves as a helpful initial
   framework for understanding the bundle security process.

   The first issue is a very common one and easy to handle.  The bundle
   may be sent indirectly to its destination, requiring several
   forwarding hops to finally arrive there.  Security processing happens
   at each node, assuming that the node supports bundle security.  For
   the following discussion, we assume that a bundle is created and that
   confidentiality, then payload integrity and finally bundle
   authentication are applied to it.  The block sequence would therefore
   be primary-BAB-PIB-PCB-payload.  Traveling from source to destination
   requires going through one intermediate node, so the trip consists of
   two hops.

   When the bundle is received at the intermediate node, the receive
   processing validates the BAB and pops it from the stack.  However the



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   PIBs and PCBs have the final destination as their security
   destination, so these can't be processed and removed.  The
   intermediate node then begins the send process with the four
   remaining blocks in the bundle.  The outbound processing adds any
   security blocks required by local policy, and these are pushed on the
   stack immediately after the primary block, ahead of the PIB.  In this
   example, the intermediate node adds a PIB as a signature that the
   bundle has passed through the node.

   The receive processing at the destination first handles the
   intermediate node's PIB and pops it, next is the originator's PIB,
   also popped, and finally the originator's confidentiality block which
   allows the payload to be decrypted and the bundle handled for
   delivery.

   DTNs in practice are likely to be more complex.  The security policy
   for a node specifies the security requirements for a bundle.  The
   policy will possibly cause one or more security operations to be
   applied to the bundle at the current node, each with its own
   security-destination.  Application of policy at subsequent nodes
   might cause additional security operations, each with a security-
   destination.  The list of security-destinations in the security
   blocks (BAB, PIB and PCB, not ESB) creates a partial-ordering of
   nodes that MUST be visited en route to the bundle destination.

   The bundle security scheme does not deal with security paths that
   overlap partially but not completely.  The security policy for a node
   MUST avoid specifying for a bundle a security-destination that causes
   a conflict with any existing security-destination in that bundle.
   This is discussed further below in Section 3.3.

   The second issue relates to the reversibility of certain security
   process actions.  In general, the actions fall into two categories:
   those which do not affect other parts of the bundle, and those which
   are fully reversible.  Creating a bundle signature, for example, does
   not change the bundle content except for the result.  The encryption
   performed as part of the confidentiality processing does change the
   bundle, but the reverse processing at the destination restores the
   original content.

   The third category is the one where the bundle content has changed
   slightly and in a non-destructive way, but there is no mechanism to
   reverse the change.  The simplest example is the addition of an EID-
   reference to a security block.  The addition of the reference causes
   the text to be added to the bundle's dictionary.  The text may be
   used also by other references so removal of the block and this
   specific EID-reference does not cause removal of the text from the
   dictionary.  This shortcoming is of no impact to the "sequential" or



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   "wrapping" security schemes described above, but does cause failures
   with "parallel" authentication mechanisms.  Solutions for this
   problem are implementation-specific and typically involve multi-pass
   processing such that blocks are added at one stage and the security
   results calculated at a later stage of the overall process.

   Certain ciphersuites have sequence requirements for their correct
   operation, most notably the bundle authentication ciphersuites.
   Processing for bundle authentication is required to happen after all
   other sending operations, and prior to any receive operations at the
   next hop node.  It follows therefore that BA blocks MUST always be
   pushed onto the stack after all others.

   Although we describe the security block list as a stack, there are
   some blocks which are placed after the payload and therefore are not
   part of the stack.  The BundleAuthentication ciphersuite #1 ("BA1")
   requires a second, correlated block to contain the security-result
   and this block is placed after the payload, usually as the last block
   in the bundle.  We can apply the stack rules even to these blocks by
   specifying that they be added to the end of the bundle at the same
   time that their "owner" or "parent" block is pushed on the stack.  In
   fact, they form a stack beginning at the payload but growing in the
   other direction.  Also, not all blocks in the main stack have a
   corresponding entry in the trailing stack.  The only blocks which
   MUST follow the payload are those mandated by ciphersuites as
   correlated blocks for holding a security-result.  No other blocks are
   required to follow the payload block and it is RECOMMENDED that they
   NOT do so.

   ESBs are effectively placeholders for the blocks they encapsulate
   and, since those do not form part of the processing sequence
   described above, ESBs themselves do not either.  ESBs MAY be
   correlated, however, so the "no reordering" requirement applies to
   them as well.

3.3.  Security Regions

   Each security block has a security path, as described in the
   discussion for Figure 1, and the paths for various blocks are often
   different.

   BA blocks are always for a single hop and these restricted paths
   never cause conflict.

   The paths for PIBs and PCBs are often from bundle source to bundle
   destination, to provide end-to-end protection.  A bundle-source-to-
   bundle-destination path likewise never causes a problem.




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   Another common scenario is for gateway-to-gateway protection of
   traffic between two sub-networks ("tunnel-mode").

   Looking at Figure 1 and the simplified version shown in Figure 4, we
   can regard BN2 and BN3 as gateways connecting the two subnetworks
   labeled "An internet".  As long as they provide security for the BN2-
   BN3 path, all is well.  Problems begin, for example, when BN2 adds
   blocks with BN4 as the security-destination, and originating node BN1
   has created blocks with BN3 as security-destination.  We now have two
   paths and neither is a subset of the other.

   This scenario should be prevented by node BN2's security policy being
   aware of the already-existing block with BN3 as the security
   destination.  This policy SHOULD NOT specify a security-dest that is
   further distant than any existing security-dest.


   +---------v-|   +->>>>>>>>>>v-+     +->>>>>>>>>>v-+   +-^---------+
   | BN1     v |   | ^   BN2   v |     | ^   BN3   v |   | ^  BN4    |
   +---------v-+   +-^---------v-+     +-^---------v-+   +-^---------+
             >>>>>>>>^         >>>>>>>>>>^         >>>>>>>>^

    <-------------  BN1 to BN3 path  ------------>

                       <-------------  BN2 to BN4 path  ------------>

   Overlapping security paths

                                 Figure 4

   Consider the case where the security concern is for data integrity,
   so the blocks are PIBs.  BN1 creates one ("PIa") along with the new
   bundle, and BN2 pushes its own PIB "PIb" on the stack, with security-
   destination BN4.  When this bundle arrives at BN3, the bundle blocks
   are

   primary - PIb - PIa - payload

   Block PIb is not destined for this node BN3 so has to be forwarded.
   This is the security-destination for block PIa so, after validation,
   it should be removed from the bundle.  But that will invalidate the
   PIb signature when the block is checked at the final destination.
   The PIb signature includes the primary block, PIb itself, PIa and the
   payload block, so PIa MUST remain in the bundle.  This is why
   security blocks are treated as a stack and add/remove operations are
   permitted only at the top-of-stack.

   The situation would be worse if the security concern is



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   confidentiality, and PCBs are employed, using the confidentiality
   ciphersuite #3 ("PC3") described in Section 4.3.  In this scenario,
   BN1 would encrypt the bundle with BN3 as security-destination, BN2
   would create an overlapping security path by super-encrypting the
   payload and encapsulating the PC3 block for security-destination BN4.
   BN3 forwards all the blocks without change.  BN4 decrypts the payload
   from its super-encryption and decapsulates the PC3 block, only to
   find that it should have been processed earlier.  Assuming that BN4
   has no access to BN3's key store, BN4 has no way to decrypt the
   bundle and recover the original content.

   As mentioned above, authors of security policy need to use care to
   ensure that their policies do not cause overlaps.  These guidelines
   should prove helpful:

      the originator of a bundle can always specify the bundle-dest as
      the security-dest, and should be cautious about doing otherwise

      in the "tunnel-mode" scenario where two sub-networks are connected
      by a tunnel through a network, the gateways can each specify the
      other as security-dest, and should be cautious about doing
      otherwise

      BAB is never a problem because it is always only a single hop

      PIB for a bundle without PCB will usually specify the bundle
      destination as security-dest

      PIB for a bundle containing a PCB should specify as its security-
      dest the security-dest of the PCB (outermost PCB if there are more
      than one)

3.4.  Canonicalisation of bundles

   In order to verify a signature or MAC on a bundle the exact same
   bits, in the exact same order, MUST be input to the calculation upon
   verification as were input upon initial computation of the original
   signature or MAC value.  Consequently, a node MUST NOT change the
   encoding of any URI in the dictionary field, e.g., changing the DNS
   part of some HTTP URL from lower case to upper case.  Because bundles
   MAY be modified while in transit (either correctly or due to
   implementation errors), a canonical form of any given bundle (that
   contains a BAB or PIB) MUST be defined.

   This section defines bundle canonicalisation algorithms used in the
   Section 4.1 and Section 4.2 ciphersuites.  Other ciphersuites can use
   these or define their own canonicalization procedures.




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3.4.1.  Strict canonicalisation

   The first algorithm that can be used permits no changes at all to the
   bundle between the security-source and the security-destination.  It
   is mainly intended for use in BAB ciphersuites.  This algorithm
   conceptually catenates all blocks in the order presented, but omits
   all security result data fields in blocks of this ciphersuite type.
   That is, when a BA ciphersuite specifies this algorithm then we omit
   all BAB security results for all BA ciphersuites, when a PIB
   ciphersuite specifies this algorithm then we omit all PIB security
   results for all PI ciphersuites.  All security result length fields
   are included, even though their corresponding security result data
   fields are omitted.

   Notes:

      - In the above we specify that security result data is omitted.
      This means that no bytes of the security result data are input.
      We do not set the security result length to zero.  Rather, we
      assume that the security result length will be known to the module
      that implements the ciphersuite before the security result is
      calculated, and require that this value be in the security result
      length field even though the security result data itself will be
      omitted.

      - The 'res' bit of the ciphersuite ID, which indicates whether or
      not the security result length and security result data field are
      present, is part of the canonical form.

      - The value of the block data length field, which indicates the
      length of the block, is also part of the canonical form.  Its
      value indicates the length of the entire bundle when the bundle
      includes the security result data field.

      - BABs are always added to bundles after PIBs, so when a PIB
      ciphersuite specifies this strict canonicalisation algorithm and
      the PIB is received with a bundle that also includes one or more
      BABs, application of strict canonicalisation as part of the PIB
      security result verification process requires that all BABs in the
      bundle be ignored entirely.

3.4.2.  Mutable canonicalisation

   This algorithm is intended to protect parts of the bundle which
   should not be changed in-transit.  Hence it omits the mutable parts
   of the bundle.

   The basic approach is to define a canonical form of the primary block



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   and catenate it with the security (PIBs and PCBs only) and payload
   blocks in the order that they will be transmitted.  This algorithm
   ignores all other blocks, including ESBs, because it cannot be
   determined whether or not they will change as the bundle transits the
   network.  In short, this canonicalization protects the payload,
   payload-related security blocks and parts of the primary block.

   Many fields in various blocks are stored as variable-length SDNVs.
   These are canonicalized in unpacked form, as eight-byte fixed-width
   fields in network byte order.  The size of eight bytes is chosen
   because implementations MAY handle larger values as invalid, as noted
   in [DTNBP].

   The canonical form of the primary block is shown in Figure 5.
   Essentially, it de-references the dictionary block, adjusts lengths
   where necessary and ignores flags that MAY change in transit.



































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   +----------------+----------------+----------------+----------------+
   |    Version     |      Processing flags (incl. COS and  SRR)       |
   +----------------+----------------+---------------------------------+
   |                Canonical primary block length                     |
   +----------------+----------------+---------------------------------+
   |                Destination endpoint ID length                     |
   +----------------+----------------+---------------------------------+
   |                                                                   |
   |                      Destination endpoint ID                      |
   |                                                                   |
   +----------------+----------------+---------------------------------+
   |                    Source endpoint ID length                      |
   +----------------+----------------+----------------+----------------+
   |                                                                   |
   |                        Source endpoint ID                         |
   |                                                                   |
   +----------------+----------------+---------------------------------+
   |                  Report-to endpoint ID length                     |
   +----------------+----------------+----------------+----------------+
   |                                                                   |
   |                      Report-to endpoint ID                        |
   |                                                                   |
   +----------------+----------------+----------------+----------------+
   |                                                                   |
   +                    Creation Timestamp (2 x SDNV)                  +
   |                                                                   |
   +---------------------------------+---------------------------------+
   |                             Lifetime                              |
   +----------------+----------------+----------------+----------------+

   The canonical form of the primary bundle block.

                                 Figure 5

   The fields shown in Figure 5 are:

      Version is the single-byte value in the primary block.

      Processing flags in the primary block is an SDNV, and includes the
      class-of-service (COS) and status report request (SRR) fields.
      For purposes of canonicalization, the SDNV is unpacked into a
      fixed-width field and some bits are masked out.  The unpacked
      field is ANDed with mask 0x0000 0000 0007 C1BE to set to zero all
      reserved bits and the "bundle is a fragment" bit.

      Length - a four-byte value containing the length (in bytes) of
      this structure, in network byte order.




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      Destination endpoint ID length and value - are the length (as a
      four byte value in network byte order) and value of the
      destination endpoint ID from the primary bundle block.  The URI is
      simply copied from the relevant part(s) of the dictionary block
      and is not itself canonicalised.  Although the dictionary entries
      contain null-terminators, the null-terminators are not included in
      the length or the canonicalization.

      Source endpoint ID length and value are handled similarly to the
      destination.

      Report-to endpoint ID length and value are handled similarly to
      the destination.

      Creation time (2 x SDNV) and Lifetime (SDNV) are simply copied
      from the primary block, with the SDNV values being represented as
      eight-byte unpacked values.

      Fragment offset and Total application data unit length are
      ignored, as is the case for the "bundle is a fragment" bit
      mentioned above.  If the payload data to be canonicalized is less
      than the complete, original bundle payload, the offset and length
      are specified in the security-parameters.

   For non-primary blocks being included in the canonicalization, the
   block processing flags value used for canonicalization is the
   unpacked SDNV value with reserved and mutable bits masked to zero.
   The unpacked value is ANDed with mask 0x0000 0000 0000 0077 to zero
   reserved bits and the "last block" flag.  The "last block" flag is
   ignored because BABs and other security blocks MAY be added for some
   parts of the journey but not others so the setting of this bit might
   change from hop to hop.

   Endpoint ID references in security blocks are canonicalized using the
   de-referenced text form in place of the reference pair.  The
   reference count is not included, nor is the length of the endpoint ID
   text.

   The block-length is canonicalized as an eight-byte unpacked value in
   network byte order.  If the payload data to be canonicalized is less
   than the complete, original bundle payload, this field contain the
   size of the data being canonicalized (the "effective block") rather
   that the actual size of the block.

   Payload blocks are generally canonicalized as-is with the exception
   that in some instances only a portion of the payload data is to be
   protected.  In such a case, only those bytes are included in the
   canonical form, and additional ciphersuite parameters are required to



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   specify which part of the payload is protected, as discussed further
   below.

   Security blocks are handled likewise, except that the ciphersuite
   will likely specify that the "current" security block security result
   field not be considered part of the canonical form.  This differs
   from the strict canonicalisation case since we might use the mutable
   canonicalisation algorithm to handle sequential signatures such that
   signatures cover earlier ones.

   ESBs MUST NOT be included in the canonicalization.

   Notes:

      - The canonical form of the bundle is not transmitted.  It is
      simply an artifact used as input to digesting.

      - We omit the reserved flags because we cannot determine if they
      will change in transit.  The masks specified above will have to be
      revised if additional flags are defined and they need to be
      protected.

      - Our URI encoding does not preserve the "null-termination"
      convention from the dictionary field, nor do we separate the
      scheme and the scheme-specific part (SSP) as is done there.

      - The URI encoding will cause errors if any node rewrites the
      dictionary content (e.g. changing the DNS part of an HTTP URL from
      lower-case to upper case).  This could happen transparently when a
      bundle is synched to disk using one set of software and then read
      from disk and forwarded by a second set of software.  Because
      there are no general rules for canonicalising URIs (or IRIs), this
      problem may be an unavoidable source of integrity failures.

      - All SDNV fields here are canonicalized as eight-byte unpacked
      values in network byte order.  Length fields are canonicalized as
      four-byte values in network byte order.  Encoding does not need
      optimization since the values are never sent over the network.

      If a bundle is fragmented before the PIB is applied then the PIB
      applies to a fragment and not the entire bundle.  However, the
      protected fragment could be subsequently further fragmented, which
      would leave the verifier unable to know which bytes were protected
      by the PIB.  Even in the absence of fragmentation the same
      situation applies if the ciphersuite is defined to allow
      protection of less than the entire, original bundle payload.





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      For this reason, PIB ciphersuites which support applying a PIB to
      less than the complete, original bundle payload MUST specify, as
      part of the ciphersuite parameters, which bytes of the bundle
      payload are protected.  When verification occurs, only the
      specified range of the payload bytes are input to PIB
      verification.  It is valid for a ciphersuite to be specified so as
      to only apply to entire bundles and not to fragments.  A
      ciphersuite MAY be specified to apply to only a portion of the
      payload, regardless of whether the payload is a fragment or the
      complete original bundle payload.

      The same fragmentation issue applies equally to PCB ciphersuites.
      Ciphersuites which support applying confidentiality to fragments
      MUST specify, as part of the ciphersuite parameters, which bytes
      of the bundle payload are protected.  When decrypting a fragment,
      only the specified bytes are processed.  It is also valid for a
      confidentiality ciphersuite to be specified so as to only apply to
      entire bundles and not to fragments.

   This definition of mutable canonicalization assumes that endpoint IDs
   themselves are immutable and is unsuitable for use in environments
   where that assumption might be violated.

   The canonicalization applies to a specific bundle and not a specific
   payload.  If a bundle is forwarded in some way, the recipient is not
   able to verify the original integrity signature since the the source
   EID will be different, and possibly other fields.

   The solution for either of these issues is to define and use a PIB
   ciphersuite having an alternate version of mutable canonicalization
   any fields from the primary block.

3.5.  Endpoint ID confidentiality

   Every bundle MUST contain a primary block that contains the source
   and destinations endpoint IDs, and others, and that cannot be
   encrypted.  If endpoint ID confidentiality is required, then bundle-
   in-bundle encapsulation can solve this problem in some instances.

   Similarly, confidentiality requirements MAY also apply to other parts
   of the primary block (e.g. the current-custodian) and that is
   supported in the same manner.

3.6.  Bundles received from other nodes

   Nodes implementing this specification SHALL consult their security
   policy to determine whether or not a received bundle is required by
   policy to include a BAB.  If the bundle has no BAB and one is not



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   required then BAB processing on the received bundle is complete and
   the bundle is ready to be further processed for PIB/PCB/ESB handling
   or delivery or forwarding.

   If the bundle is required to have a BAB but does not, then the bundle
   MUST be discarded and processed no further.  If the bundle is
   required to have a BAB but all of its BABs identify a different node
   other than the receiving node as the BAB security destination, then
   the bundle MUST be discarded and processed no further.

   If the bundle is required to have a BAB and has one or more BABs that
   identify the receiving node as the BAB security destination, or for
   which there is no security destination, then the value in the
   security result field(s) of the BAB(s) MUST be verified according to
   the ciphersuite specification.  If for all such BABs in the bundle
   either the BAB security source cannot be determined or the security
   result value check fails, the bundle has failed to authenticate and
   the bundle MUST be discarded and processed no further.  If any of the
   BABs present verify, or if a BAB is not required, the bundle is ready
   for further processing as determined by extension blocks and/or
   policy.

   BABs received in a bundle MUST be stripped before the bundle is
   forwarded.  New BABs MAY be added as required by policy.  This MAY
   require correcting the "last block" field of the to-be-forwarded
   bundle.

   Further processing of the bundle MUST take place in the order
   indicated by the various blocks from the primary block to the payload
   block, except as defined by an applicable specification.

   If the bundle has a PCB and the receiving node is the PCB destination
   for the bundle (either because the node is listed as the bundle's
   PCB-dest or because the node is listed as the bundle's destination
   and there is no PCB-dest), the node MUST decrypt the relevant parts
   of the bundle in accordance with the ciphersuite specification.  The
   PCB SHALL be deleted.  If the relevant parts of the bundle cannot be
   decrypted (i.e. the decryption key cannot be deduced or decryption
   fails), then the bundle MUST be discarded and processed no further;
   in this case a bundle deletion status report (see the Bundle Protocol
   [DTNBP]) indicating the decryption failure MAY be generated.  If the
   PCB security result included the ciphertext of a block other than the
   payload block, the recovered plaintext block MUST be placed in the
   bundle at the location from which the PCB was deleted.

   If the bundle has one or more PIBs for which the receiving node is
   the bundle's PIB destination (either because the node is listed in
   the bundle's PIB-dest or because the node is listed as the bundle's



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   destination and there is no PIB-dest), the node MUST verify the value
   in the PIB security result field(s) in accordance with the
   ciphersuite specification.  If all the checks fail, the bundle has
   failed to authenticate and the bundle SHALL be processed according to
   the security policy.  A bundle status report indicating the failure
   MAY be generated.  Otherwise, if the PIB verifies, the bundle is
   ready to be processed for either delivery or forwarding.  Before
   forwarding the bundle, the node SHOULD remove the PIB from the
   bundle, subject to the requirements of Section 3.2, unless it is
   likely that some downstream node will also be able to verify the PIB.

   If the bundle has a PIB and the receiving node is not the bundle's
   PIB-dest the receiving node MAY attempt to verify the value in the
   security result field.  If it is able to check and the check fails,
   the node SHALL discard the bundle and it MAY send a bundle status
   report indicating the failure.

   If the bundle has an ESB and the receiving node is the ESB
   destination for the bundle (either because the node is listed as the
   bundle's ESB-dest or because the node is listed as the bundle's
   destination and there is no ESB-dest), the node MUST decrypt and/or
   decapsulate the encapsulated block in accordance with the ciphersuite
   specification.  The decapsulated block replaces the ESB in the bundle
   block sequence, and the ESB is thereby deleted.  If the content
   cannot be decrypted (i.e., the decryption key cannot be deduced or
   decryption fails), then the bundle MAY be discarded and processed no
   further unless the security policy specifies otherwise.  In this case
   a bundle deletion status report (see the Bundle Protocol [DTNBP])
   indicating the decryption failure MAY be generated.

3.7.  The At-Most-Once-Delivery Option

   An application MAY request (in an implementation specific manner)
   that a node be registered as a member of an endpoint and that
   received bundles destined for that endpoint be delivered to that
   application.

   An option for use in such cases is known as "at-most-once-delivery".
   If this option is chosen, the application indicates that it wants the
   node to check for duplicate bundles, discard duplicates, and deliver
   at most one copy of each received bundle to the application.  If this
   option is not chosen, the application indicates that it wants the
   node to deliver all received bundle copies to the application.  If
   this option is chosen, the node SHALL deliver at most one copy of
   each received bundle to the application.  If the option is not
   chosen, the node SHOULD, subject to policy, deliver all bundles.

   To enforce this the node MUST look at the source/timestamp pair value



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   of each complete (reassembled, if necessary) bundle received and
   determine if this pair, which uniquely identifies a bundle, has been
   previously received.  If it has, then the bundle is a duplicate.  If
   it has not, then the bundle is not a duplicate.  The source/timestamp
   pair SHALL be added to the list of pair values already received by
   that node.

   Each node implementation MAY decide how long to maintain a table of
   pair value state.

   Additional discussion relevant to at-most-once-delivery is in the DTN
   Retransmission Block specification [DTNRB].

3.8.  Bundle Fragmentation and Reassembly

   If it is necessary for a node to fragment a bundle and security
   services have been applied to that bundle, the fragmentation rules
   described in [DTNBP] MUST be followed.  As defined there and repeated
   here for completeness, only the payload MAY be fragmented; security
   blocks, like all extension blocks, can never be fragmented.  In
   addition, the following security-specific processing is REQUIRED:

   The security policy requirements for a bundle MUST be applied
   individually to all the bundles resulting from a fragmentation event.

   If the original bundle contained a PIB, then each of the PIB
   instances MUST be included in some fragment.

   If the original bundle contained one or more PCBs, then any PCB
   instances containing a key information item MUST have the "replicate
   in every fragment" flag set, and thereby be replicated in every
   fragment.  This is to ensure that the canonical block-sequence can be
   recovered during reassembly.

   If the original bundle contained one or more correlated PCBs not
   containing a key information item, then each of these MUST be
   included in some fragment, but SHOULD NOT be sent more than once.
   They MUST be placed in a fragment in accordance with the
   fragmentation rules described in [DTNBP].

   Note: various fragments MAY have additional security blocks added at
   this or later stages and it is possible that correlators will
   collide.  In order to facilitate uniqueness, ciphersuites SHOULD
   include the fragment-offset of the fragment as a high-order component
   of the correlator.






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3.9.  Reactive fragmentation

   When a partial bundle has been received, the receiving node SHALL
   consult its security policy to determine if it MAY fragment the
   bundle, converting the received portion into a bundle fragment for
   further forwarding.  Whether or not reactive fragmentation is
   permitted SHALL depend on the security policy and the ciphersuite
   used to calculate the BAB authentication information, if required.
   (Some BAB ciphersuites, i.e., the mandatory BAB-HMAC ciphersuite
   defined in Section 4.1, do not accommodate reactive fragmentation
   because the security result in the BAB requires that the entire
   bundle be signed.  It is conceivable, however, that a BAB ciphersuite
   could be defined such that multiple security results are calculated,
   each on a different segment of a bundle, and that these security
   results could be interspersed between bundle payload segments such
   that reactive fragmentation could be accommodated.)

   If the bundle is reactively fragmented by the intermediate receiver
   and the BAB-ciphersuite is of an appropriate type (e.g. with multiple
   security results embedded in the payload), the bundle MUST be
   fragmented immediately after the last security result value in the
   partial payload that is received.  Any data received after the last
   security result value MUST be dropped.

   If a partial bundle is received at the intermediate receiver and is
   reactively fragmented and forwarded, only the part of the bundle that
   was not received MUST be retransmitted, though more of the bundle MAY
   be retransmitted.  Before retransmitting a portion of the bundle, it
   SHALL be changed into a fragment and, if the original bundle included
   a BAB, the fragmented bundle MUST also, and its BAB SHALL be
   recalculated.

   This specification does not currently define any ciphersuite which
   can handle this reactive fragmentation case.

   An interesting possibility is a ciphersuite definition such that the
   transmission of a follow-up fragment would be accompanied by the
   signature for the payload up to the restart point.

3.10.  Attack Model

   An evaluation of resilience to cryptographic attack necessarily
   depends upon the algorithms chosen for bulk data protection and for
   key transport.  The mandatory ciphersuites described in the following
   section use AES, RSA and SHA algorithms in ways that are believed to
   be reasonably secure against ciphertext-only, chosen-ciphertext,
   known-plaintext and chosen-plaintext attacks.




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   The design has been careful to preserve the resilience of the
   algorithms against attack.  For example, if a message is encrypted
   then any message integrity signature is also encrypted so that
   guesses cannot be confirmed.















































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4.  Mandatory Ciphersuites

   This section defines the mandatory ciphersuites for this
   specification.  There is currently one mandatory ciphersuite for use
   with each of the security block types BAB, PIB, PCB and ESB.  The BAB
   ciphersuite is based on shared secrets using HMAC.  The PIB
   ciphersuite is based on digital signatures using RSA with SHA256.
   The PCB and ESB ciphersuites are based on using RSA for key transport
   and AES for bulk encryption.

   The key transport mechanisms defined in Cryptographic Message Syntax
   [RFC5652] are suitable for the ciphersuites, with only minor
   adjustment as required by the ciphersuite characteristics.

4.1.  BAB-HMAC

   The BAB-HMAC ciphersuite has ciphersuite ID value 0x001.

   BAB-HMAC uses the strict canonicalisation algorithm in Section 3.4.1.

   Strict canonicalization supports digesting of a fragment-bundle.  It
   does not permit the digesting of only a subset of the payload, but
   only the complete contents of the payload of the current bundle,
   which might be a fragment.  The "fragment range" item for security-
   parameters is not used to indicate a fragment, as this information is
   digested within the primary block.

   The variant of HMAC to be used is HMAC-SHA1 as defined in [RFC2104].

   This ciphersuite requires the use of two related instances of the
   BAB.  It involves placing the first BAB instance (as defined in
   Section 2.2) just after the primary block.  The second (correlated)
   instance of the BAB MUST be placed after all other blocks (except
   possibly other BAB blocks) in the bundle.

   This means that normally, the BAB will be the second and last blocks
   of the bundle.  If a forwarder wishes to apply more than one
   correlated BAB pair, then this can be done.  There is no requirement
   that each application "wrap" the others, but the forwarder MUST
   insert all the "up front" BABs, and their "at back" "partners"
   (without any security result), before canonicalising.

   Inserting more than one correlated BAB pair would be useful if the
   bundle could be routed to more than one potential "next-hop" or if
   both an old or a new key were valid at sending time, with no
   certainty about the situation that will obtain at reception time.

   The security result is the output of the HMAC-SHA1 calculation with



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   input being the result of running the entire bundle through the
   strict canonicalisation algorithm.  Both required BAB instances MUST
   be included in the bundle before canonicalisation.

   Security parameters are optional with this scheme, but if used then
   the only field that can be present is key information (see
   Section 2.6).

   Implementations MUST support use of "AuthenticatedData" type as
   defined in [RFC5652] section 9.1, with RecipientInfo type
   KeyTransRecipientInfo containing the issuer and serial number of a
   suitable certificate.  They MAY support additional RecipientInfo
   types.  They MAY additionally use the "SignedData" type described in
   [RFC5652] Section 5.1.  In either case, the optional "eContent" field
   in EncapsulatedContentInfo MUST be omitted.  That is, the data itself
   is external, being the canonicalized form of the bundle.

   Because this ciphersuite requires that the security result be in the
   second, correlated BAB, the content of the
   "MessageAuthenticationCode" field in AuthenticatedData is ignored,
   although the field has to be present.

   In the absence of key information the receiver is expected to be able
   to find the correct key based on the sending identity.  The sending
   identity MAY be known from the security-source field or the content
   of a previous-hop block in the bundle.  It MAY also be determined
   using implementation-specific means such as the convergence layer.

4.2.  PIB-RSA-SHA256

   The PIB-RSA-SHA256 ciphersuite has ciphersuite ID value 0x02.

   If the bundle being signed has been fragmented before signing, then
   we have to specify which bytes were signed in case the signed bundle
   is subsequently fragmented for a second time.  If the bundle is a
   fragment, then the ciphersuite parameters MUST include a fragment-
   range field, as described in Section 2.6, specifying the offset and
   length of the signed fragment.  If the entire bundle is signed then
   these numbers MUST be omitted.

   Implementations MUST support use of "SignedData" type as defined in
   [RFC5652] section 5.1, with SignerInfo type SignerIdentifier
   containing the issuer and serial number of a suitable certificate.
   The optional "eContent" field in EncapsulatedContentInfo MUST be
   omitted.  That is, the data itself is external, being the
   canonicalized form of the bundle.

   Because the signature field in SignedData SignatureValue is a



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   security-result field, the entire key information item MUST be placed
   in the block's security-result field, rather than security-
   parameters.

   PIB-RSA-SHA256 uses the mutable canonicalisation algorithm
   Section 3.4.2, with the security-result data field for only the
   "current" block being excluded from the canonical form.  The
   resulting canonical form of the bundle is the input to the signing
   process.  This ciphersuite requires the use of a single instance of
   the PIB.

   RSA is used with SHA256 as specified for the id-sha256 PKCSv2.1
   signature scheme in [RFC4055].  The output of the signing process is
   the SignatureValue field for the PIB.

   "Commensurate strength" cryptography is generally held to be a good
   idea.  A combination of RSA with SHA256 is reckoned to require a 3076
   bit RSA key according to this logic.  Few implementations will choose
   this length by default (and probably some just won't support such
   long keys).  Since this is an experimental protocol, we expect that
   1024 or 2048 bit RSA keys will be used in many cases, and that that
   will be fine since we also expect that the hash function "issues"
   will be resolved before any standard would be derived from this
   protocol.

4.3.  PCB-RSA-AES128-PAYLOAD-PIB-PCB

   The PCB-RSA-AES128-PAYLOAD-PIB-PCB ciphersuite has ciphersuite ID
   value 0x003.

   This scheme encrypts PIBs, PCBs and the payload.  The key size for
   this ciphersuite is 128 bits.

   Encryption is done using the AES algorithm in Galois/Counter Mode
   (GCM) as described in [RFC5084] [Note: parts of the following
   description are borrowed from RFC 4106].

   The choice of GCM avoids expansion of the payload, which causes
   problems with fragmentation/reassembly and custody transfer.  GCM
   also includes authentication, essential in preventing attacks that
   can alter the decrypted plaintext or even recover the encryption key.

   GCM is a block cipher mode of operation providing both
   confidentiality and data integrity.  The GCM encryption operation has
   four inputs: a secret key, an initialization vector (IV), a
   plaintext, and an input for additional authenticated data (AAD) which
   is not used here.  It has two outputs, a ciphertext whose length is
   identical to the plaintext, and an authentication tag, also known as



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   the Integrity Check Value (ICV).

   For consistency with the description in [RFC5084], we refer to the
   GCM IV as a nonce.  The same key and nonce combination MUST NOT be
   used more than once.  The nonce has the following layout


   +----------------+----------------+----------------+----------------+
   |                               salt                                |
   +----------------+----------------+----------------+----------------+
   |                                                                   |
   |                      initialization vector                        |
   |                                                                   |
   +----------------+----------------+----------------+----------------+

   Nonce format.

                                 Figure 6

   The salt field is a four-octet value, usually chosen at random.  It
   MUST be the same for all PCBs which have the same correlator value.
   The salt need not be kept secret.

   The initialization vector (IV) is an eight-octet value, usually
   chosen at random.  It MUST be different for all PCBs which have the
   same correlator value.  The value need not be kept secret.

   The key (bundle encryption key, BEK) is a sixteen-octet (128 bits)
   value, usually chosen at random.  The value MUST be kept secret, as
   described below.

   The integrity check value is a sixteen-octet value used to verify
   that the protected data has not been altered.  The value need not be
   kept secret.

   This ciphersuite requires the use of a single PCB instance to deal
   with payload confidentiality.  If the bundle already contains PIBs or
   PCBs then the ciphersuite will create additional correlated blocks to
   protect these PIBs and PCBs.  These "additional" blocks replace the
   original blocks on a one-for-one basis, so the number of blocks
   remains unchanged.  All these related blocks MUST have the same
   correlator value.  The term "first PCB" in this section refers to the
   single PCB if there is only one or, if there are several, then to the
   one containing the key information.  This MUST be the first of the
   set.

   First PCB - the first PCB MAY contain a correlator value, and MAY
   specify security-source and/or security-destination in the EID-list.



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   If not specified, the bundle-source and bundle-destination
   respectively are used for these values, as with other ciphersuites.
   The block MUST contain security-parameters and security-result
   fields.  Each field MAY contain several items formatted as described
   in Section 2.6.

   Security-parameters

      key information

      salt

      IV (this instance applies only to payload)

      fragment offset and length, if bundle is a fragment

   Security-result

      ICV

   Subsequent PCBs MUST contain a correlator value to link them to the
   first PCB.  Security-source and security-destination are implied from
   the first PCB, however see the discussion in Section 2.4 concerning
   EID-list entries.  They MUST contain security-parameters and
   security-result fields as follows:

   Security-parameters

      IV for this specific block

   Security-result

      encapsulated block

   The security-parameters and security-result fields in the subsequent
   PCBs MUST NOT contain any items other than these two.  Items such as
   key and salt are supplied in the first PCB and MUST NOT be repeated.

   Implementations MUST support use of "Enveloped-data" type as defined
   in [RFC5652] section 6, with RecipientInfo type KeyTransRecipientInfo
   containing the issuer and serial number of a suitable certificate.
   They MAY support additional RecipientInfo types.  The optional
   "eContent" field in EncryptedContentInfo MUST be omitted.  That is,
   the data itself is external, being the payload of the bundle.

   The Integrity Check Value from the AES-GCM encryption of the payload
   is placed in the security-result field of the first PCB.




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   If the bundle being encrypted is a fragment-bundle we have to specify
   which bytes are encrypted in case the bundle is subsequently
   fragmented again.  If the bundle is a fragment the ciphersuite
   parameters MUST include a fragment-range field, as described in
   Section 2.6, specifying the offset and length of the encrypted
   fragment.  Note that this is not the same pair of fields which appear
   in the primary block as "offset and length".  The "length" in this
   case is the length of the fragment, not the original length.  If the
   bundle is not a fragment then this field MUST be omitted.

   The confidentiality processing for payload and other blocks is
   different, mainly because the payload might be fragmented later at
   some other node.

   For the payload, only the bytes of the bundle payload field are
   affected, being replaced by ciphertext.  The salt, IV and key values
   specified in the first PCB are used to encrypt the payload, and the
   resultant authentication tag (ICV) is placed in an ICV item in the
   security-result field of that first PCB.  The other bytes of the
   payload block, such as type, flags and length, are not modified.

   For each PIB or PCB to be protected, the entire original block is
   encapsulated in a "replacing" PCB.  This replacing PCB is placed in
   the outgoing bundle in the same position as the original block, PIB
   or PCB.  As mentioned above, this is one-for-one replacement and
   there is no consolidation of blocks or mixing of data in any way.

   The encryption process uses AES-GCM with the salt and key values from
   the first PCB, and an IV unique to this PCB.  The process creates
   ciphertext for the entire original block, and an authentication tag
   for validation at the security destination.  For this encapsulation
   process, unlike the processing of the bundle payload, the
   authentication tag is appended to the ciphertext for the block and
   the combination is stored into the "encapsulated block" item in
   security-result.

   The replacing block, of course, also has the same correlator value as
   the first PCB with which it is associated.  It also contains the
   block-specific IV in security-parameters, and the combination of
   original-block-ciphertext and authentication tag, stored as an
   "encapsulated block" item in security-result.

   If the payload was fragmented after encryption then all those
   fragments MUST be present and reassembled before decryption.  This
   process might be repeated several times at different destinations if
   multiple fragmentation actions have occurred.

   The size of the GCM counter field limits the payload size to 2^39 -



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   256 bytes, about half a terabyte.  A future revision of this
   specification will address the issue of handling payloads in excess
   of this size.

4.4.  ESB-RSA-AES128-EXT

   The ESB-RSA-AES128-EXT ciphersuite has ciphersuite ID value 0x004.

   This scheme encrypts non-payload-related blocks.  It MUST NOT be used
   to encrypt PIBs, PCBs or primary or payload blocks.  The key size for
   this ciphersuite is 128 bits.

   Encryption is done using the AES algorithm in Galois/Counter Mode
   (GCM) as described in [RFC5084] [Note: parts of the following
   description are borrowed from RFC 4106].

   GCM is a block cipher mode of operation providing both
   confidentiality and data origin authentication.  The GCM
   authenticated encryption operation has four inputs: a secret key, an
   initialization vector (IV), a plaintext, and an input for additional
   authenticated data (AAD) which is not used here.  It has two outputs,
   a ciphertext whose length is identical to the plaintext, and an
   authentication tag, also known as the Integrity Check Value (ICV).

   For consistency with the description in [RFC5084], we refer to the
   GCM IV as a nonce.  The same key and nonce combination MUST NOT be
   used more than once.  The nonce has the following layout


   +----------------+----------------+---------------------------------+
   |                               salt                                |
   +----------------+----------------+---------------------------------+
   |                                                                   |
   |                      initialization vector                        |
   |                                                                   |
   +----------------+----------------+---------------------------------+

   Nonce format.

                                 Figure 7

   The salt field is a four-octet value, usually chosen at random.  It
   MUST be the same for all ESBs which have the same correlator value.
   The salt need not be kept secret.

   The initialization vector (IV) is an eight-octet value, usually
   chosen at random.  It MUST be different for all ESBs which have the
   same correlator value.  The value need not be kept secret.



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   The data encryption key is a sixteen-octet (128 bits) value, usually
   chosen at random.  The value MUST be kept secret, as described below.

   The integrity check value is a sixteen-octet value used to verify
   that the protected data has not been altered.  The value need not be
   kept secret.

   This ciphersuite replaces each BP extension block to be protected
   with a "replacing" ESB, and each can be individually specified.

   If a number of related BP extension blocks are to be protected they
   can be grouped as a correlated set and protected using a single key.
   These blocks replace the original blocks on a one-for-one basis, so
   the number of blocks remains unchanged.  All these related blocks
   MUST have the same correlator value.  The term "first ESB" in this
   section refers to the single ESB if there is only one or, if there
   are several, then to the one containing the key or key-identifier.
   This MUST be the first of the set.  If the blocks are individually
   specified then there is no correlated set and each block is its own
   "first ESB".

   First ESB - the first ESB MAY contain a correlator value, and MAY
   specify security-source and/or security-destination in the EID-list.
   If not specified, the bundle-source and bundle-destination
   respectively are used for these values, as with other ciphersuites.
   The block MUST contain security-parameters and security-result
   fields.  Each field MAY contain several items formatted as described
   in Section 2.6.

   Security-parameters

      key information

      salt

      IV for this specific block

      block type of encapsulated block (optional)

   Security-result

      encapsulated block

   Subsequent ESBs MUST contain a correlator value to link them to the
   first ESB.  Security-source and security-destination are implied from
   the first ESB, however see the discussion in Section 2.4 concerning
   EID-list entries.  Subsequent ESBs MUST contain security-parameters
   and security-result fields as follows:



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   Security-parameters

      IV for this specific block

      block type of encapsulated block (optional)

   Security-result

      encapsulated block

   The security-parameters and security-result fields in the subsequent
   ESBs MUST NOT contain any items other than those listed.  Items such
   as key and salt are supplied in the first ESB and MUST NOT be
   repeated.

   Implementations MUST support use of "Enveloped-data" type as defined
   in [RFC5652] section 6, with RecipientInfo type KeyTransRecipientInfo
   containing the issuer and serial number of a suitable certificate.
   They MAY support additional RecipientInfo types.  The optional
   "eContent" field in EncryptedContentInfo MUST be omitted.  That is,
   the data itself is external, being the content of the block being
   protected.

   For each block to be protected, the entire original block is
   encapsulated in a "replacing" ESB.  This replacing ESB is placed in
   the outgoing bundle in the same position as the original block.  As
   mentioned above, this is one-for-one replacement and there is no
   consolidation of blocks or mixing of data in any way.

   The encryption process uses AES-GCM with the salt and key values from
   the first ESB, and an IV unique to this ESB.  The process creates
   ciphertext for the entire original block, and an authentication tag
   for validation at the security destination.  The authentication tag
   is appended to the ciphertext for the block and the combination is
   stored into the "encapsulated block" item in security-result.

   The replacing block, of course, also has the same correlator value as
   the first ESB with which it is associated.  It also contains the
   block-specific IV in security-parameters, and the combination of
   original-block-ciphertext and authentication tag, stored as an
   "encapsulated block" item in security-result.










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5.  Key Management

   Key management in delay tolerant networks is recognized as a
   difficult topic and is one that this specification does not attempt
   to solve.  However, solely in order to support implementation and
   testing, implementations SHOULD support:

      - The use of well-known RSA public keys for all ciphersuites.

      - Long-term pre-shared-symmetric keys for the BAB-HMAC
      ciphersuite.

   Since endpoint IDs are URIs and URIs can be placed in X.509 [RFC3280]
   public key certificates (in the subjectAltName extension)
   implementations SHOULD support this way of distributing public keys.
   Implementations SHOULD NOT be very strict in how they process X.509
   though, for example, it would probably not be correct to insist on
   Certificate Revocation List (CRL) checking in many DTN contexts.

































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6.  Default Security Policy

   Every node serves as a Policy Enforcement Point insofar as it
   enforces some policy that controls the forwarding and delivery of
   bundles via one or more convergence layer protocol implementation.
   Consequently, every node SHALL have and operate according to its own
   configurable security policy, whether the policy be explicit or
   default.  The policy SHALL specify:

      Under what conditions received bundles SHALL be forwarded.

      Under what conditions received bundles SHALL be required to
      include valid BABs.

      Under what conditions the authentication information provided in a
      bundle's BAB SHALL be deemed adequate to authenticate the bundle.

      Under what conditions received bundles SHALL be required to have
      valid PIBs and/or PCBs.

      Under what conditions the authentication information provided in a
      bundle's PIB SHALL be deemed adequate to authenticate the bundle.

      Under what conditions a BAB SHALL be added to a received bundle
      before that bundle is forwarded.

      Under what conditions a PIB SHALL be added to a received bundle
      before that bundle is forwarded.

      Under what conditions a PCB SHALL be added to a received bundle
      before that bundle is forwarded.

      Under what conditions an ESB SHALL be applied to one or more
      blocks in a received bundle before that bundle is forwarded.

      The actions that SHALL be taken in the event that a received
      bundle does not meet the receiving node's security policy
      criteria.

   This specification does not address how security policies get
   distributed to nodes.  It only REQUIRES that nodes have and enforce
   security policies.

   If no security policy is specified at a given node, or if a security
   policy is only partially specified, that node's default policy
   regarding unspecified criteria SHALL consist of the following:





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      Bundles that are not well-formed do not meet the security policy
      criteria.

      The mandatory ciphersuites MUST be used.

      All bundles received MUST have a BAB which MUST be verified to
      contain a valid security result.  If the bundle does not have a
      BAB, then the bundle MUST be discarded and processed no further; a
      bundle status report indicating the authentication failure MAY be
      generated.

      No received bundles SHALL be required to have a PIB; if a received
      bundle does have a PIB, however, the PIB can be ignored unless the
      receiving node is the PIB-dest, in which case the PIB MUST be
      verified.

      No received bundles SHALL be required to have a PCB; if a received
      bundle does have a PCB, however, the PCB can be ignored unless the
      receiving node is the PCB-dest, in which case the PCB MUST be
      processed.  If processing of a PCB yields a PIB, that PIB SHALL be
      processed by the node according to the node's security policy.

      A PIB SHALL NOT be added to a bundle before sourcing or forwarding
      it.

      A PCB SHALL NOT be added to a bundle before sourcing or forwarding
      it.

      A BAB MUST always be added to a bundle before that bundle is
      forwarded.

      If a destination node receives a bundle that has a PIB-dest but
      the value in that PIB-dest is not the EID of the destination node,
      the bundle SHALL be delivered at that destination node.

      If a destination node receives a bundle that has an ESB-dest but
      the value in that ESB-dest is not the EID of the destination node,
      the bundle SHALL be delivered at that destination node.

      If a received bundle does not satisfy the node's security policy
      for any reason, then the bundle MUST be discarded and processed no
      further; in this case, a bundle deletion status report (see the
      Bundle Protocol [DTNBP]) indicating the failure MAY be generated.








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7.  Security Considerations

   If a BAB ciphersuite uses digital signatures but doesn't include the
   security destination (which for a BAB is the next host), then this
   allows the bundle to be sent to some node other than the intended
   adjacent node.  Because the BAB will still authenticate, the
   receiving node might erroneously accept and forward the bundle.  When
   asymmetric BAB ciphersuites are used, the security destination field
   SHOULD therefore be included in the BAB.

   If a bundle's PIB-dest is not the same as its destination, then some
   node other than the destination (the node identified as the PIB-dest)
   is expected to validate the PIB security result while the bundle is
   en route.  However, if for some reason the PIB is not validated,
   there is no way for the destination to become aware of this.
   Typically, a PIB-dest will remove the PIB from the bundle after
   verifying the PIB and before forwarding it.  However, if there is a
   possibility that the PIB will also be verified at a downstream node,
   the PIB-dest will leave the PIB in the bundle.  Therefore, if a
   destination receives a bundle with a PIB that has a PIB-dest (which
   isn't the destination), this might, but does not necessarily,
   indicate a possible problem.

   If a bundle is fragmented after being forwarded by its PIB-source but
   before being received by its PIB-dest, the payload in the bundle MUST
   be reassembled before validating the PIB security result in order for
   the security result to validate correctly.  Therefore, if the PIB-
   dest is not capable of performing payload reassembly, its utility as
   a PIB-dest will be limited to validating only those bundles that have
   not been fragmented since being forwarded from the PIB-source.
   Similarly, if a bundle is fragmented after being forwarded by its
   PIB-source but before being received by its PIB-dest, all fragments
   MUST be received at that PIB-dest in order for the bundle payload to
   be able to be reassembled.  If not all fragments are received at the
   PIB-dest node, the bundle will not be able to be authenticated, and
   will therefore never be forwarded by this PIB-dest node.

   Specification of a security-destination other than the bundle
   destination creates a routing requirement that the bundle somehow be
   directed to the security-destination node on its way to the final
   destination.  This requirement is presently private to the
   ciphersuite, since routing nodes are not required to implement
   security processing.








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8.  Conformance

   As indicated above, this document describes both BSP and
   ciphersuites.  A conformant implementation MUST implement both BSP
   support and the four ciphersuites described in Section 4.  It MAY
   also support other ciphersuites.

   Implementations that support BSP but not all four mandatory
   ciphersuites MUST claim only "restricted compliance" with this
   specification, even if they provide other ciphersuites.

   All implementations are strongly RECOMMENDED to provide at least a
   BAB ciphersuite.  A relay node, for example, might not deal with end-
   to-end confidentiality and data integrity but it SHOULD exclude
   unauthorized traffic and perform hop-by-hop bundle verification.




































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9.  IANA Considerations

   None at this time.  If the bundle protocol becomes a standards track
   protocol, then we may want to consider having IANA establish a
   register of block types, and in particular for this specification a
   separate register of ciphersuite specifications.













































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10.  References

10.1.  Normative References

   [DTNBP]    Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [DTNMD]    Symington, S., "Delay-Tolerant Networking Metadata
              Extension Block",
              draft-irtf-dtnrg-bundle-metadata-block-00.txt , June 2007.

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

   [RFC2119]  Bradner, S. and J. Reynolds, "Key words for use in RFCs to
              Indicate Requirement Levels", RFC 2119, October 1997.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, August 2002.

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509  Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              June 2005.

   [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
              (GCM) in IPsec Encapsulating Security Payload (ESP)",
              RFC 4106, June 2005.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)",
              RFC 3852, July 2004.

10.2.  Informative References

   [DTNRB]    Symington, S., "Delay-Tolerant Network Retransmission
              Block",
              draft-irtf-dtnrg-bundle-retrans-00.txt, work-in-progress,
              April 2007.

   [DTNarch]  Cerf, V., Burleigh, S., Durst, R., Fall, K., Hooke, A.,
              Scott, K., Torgerson, L., and H. Weiss, "Delay-Tolerant



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              Network Architecture", RFC 4838, April 2007.

   [PHIB]     Symington, S., "Delay-Tolerant Networking Previous Hop
              Insertion Block", draft-irtf-dtnrg-bundle-previous-hop-
              block-11.txt, work-in-progress, February 2010.

   [RFC5084]  Housley, R., "Using AES-CCM and AES-GCM Authenticated
              Encryption in the Cryptographic Message Syntax (CMS)",
              RFC 5084, November 2007.










































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Authors' Addresses

   Susan Flynn Symington
   The MITRE Corporation
   7515 Colshire Drive
   McLean, VA  22102
   US

   Phone: +1 (703) 983-7209
   Email: susan@mitre.org
   URI:   http://mitre.org/


   Stephen Farrell
   Trinity College Dublin
   Distributed Systems Group
   Department of Computer Science
   Trinity College
   Dublin  2
   Ireland

   Phone: +353-1-608-1539
   Email: stephen.farrell@cs.tcd.ie


   Howard Weiss
   SPARTA, Inc.
   7110 Samuel Morse Drive
   Columbia, MD  21046
   US

   Phone: +1-443-430-8089
   Email: hsw@sparta.com


   Peter Lovell
   SPARTA, Inc.
   7110 Samuel Morse Drive
   Columbia, MD  21046
   US

   Phone: +1-443-430-8052
   Email: peter.lovell@sparta.com








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