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DTN Research Group                                          S. Symington
Internet-Draft                                     The MITRE Corporation
Expires: September 4, 2006                                    S. Farrell
                                                  Trinity College Dublin
                                                                H. Weiss
                                                            SPARTA, Inc.
                                                           March 3, 2006


                 Bundle Security Protocol Specification
                  draft-irtf-dtnrg-bundle-security-01

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   This Internet-Draft will expire on September 4, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines the bundle security protocol, which provides
   data integrity and confidentiality services.  We also describe
   various bundle security considerations including policy options.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Related Documents  . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Security Headers . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Abstract Security Header . . . . . . . . . . . . . . . . .  7
     2.2.  Bundle Authentication Header . . . . . . . . . . . . . . . 10
     2.3.  Payload Security Header  . . . . . . . . . . . . . . . . . 11
     2.4.  Confidentiality Header . . . . . . . . . . . . . . . . . . 11
     2.5.  PSH and CH combinations  . . . . . . . . . . . . . . . . . 14
   3.  Security Processing  . . . . . . . . . . . . . . . . . . . . . 16
     3.1.  Nodes as policy enforcement points . . . . . . . . . . . . 16
     3.2.  Canonicalisation of bundles  . . . . . . . . . . . . . . . 16
     3.3.  Source confidentiality . . . . . . . . . . . . . . . . . . 20
     3.4.  Bundles received from other nodes  . . . . . . . . . . . . 21
     3.5.  The At-Most-Once-Delivery Option . . . . . . . . . . . . . 22
     3.6.  Bundle Fragmentation and Reassembly  . . . . . . . . . . . 23
     3.7.  Reactive fragmentation . . . . . . . . . . . . . . . . . . 23
   4.  Mandatory Ciphersuites . . . . . . . . . . . . . . . . . . . . 25
     4.1.  BAH-HMAC . . . . . . . . . . . . . . . . . . . . . . . . . 25
     4.2.  PSH-RSA-SHA256 . . . . . . . . . . . . . . . . . . . . . . 26
     4.3.  CH-RSA-AES-PAYLOAD-PSH . . . . . . . . . . . . . . . . . . 26
   5.  Key Management . . . . . . . . . . . . . . . . . . . . . . . . 28
   6.  Default Security Policy  . . . . . . . . . . . . . . . . . . . 29
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 32
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 33
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 33
   Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
   Intellectual Property and Copyright Statements . . . . . . . . . . 37


















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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 [1].

   This document defines security features for the bundle protocol [2]
   intended for use in delay tolerant networks, in order to provide the
   DTN security services as described in the DTN Security Overview and
   Motivations document [9].

   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 that the DTN be protected from
   unauthorized use, and this stressed environment poses unique
   challenges on 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.

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 [3] defines the
      architecture for delay-tolerant networks, but does not discuss
      security at any length.

      The DTN Bundle Protocol [2] defines the format and processing of
      the headers used to implement the bundle protocol, excluding the
      security-specific headers defined here.

      The Delay-Tolerant Networking Security Overview [9] provides an
      informative overview and high-level description of DTN security.




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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.

   [Comment.1]

   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.




   +---------v-|   +->>>>>>>>>>v-+     +->>>>>>>>>>v-+   +-^---------+
   |BN1      v |   | ^   BN2   v |     | ^   BN3   v |   | ^  BN4    |
   +---------v-+   +-^---------v-+     +-^---------v-+   +-^---------+
   |Trans1   v |   + ^  T1/T2  v |     + ^  T2/T3  v |   | ^  Trans3 |
   +---------v-+   +-^---------v-+     +-^---------v +   +-^---------+
   |Net1     v |   | ^  N1/N2  v |     | ^  N2/N3  v |   | ^  Net3   |
   +---------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.



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   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 BPA4 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 header to a
      bundle

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

   Referring to figure 1 again:

   If the bundle that originates at BN1 as source is given a security
   header by BN1, then BN1 is the security-source of this bundle with
   respect to that security header, as well as being the source of the
   bundle.

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

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

   A bundle may have multiple security headers.  The security-source of
   a bundle with respect to a given security header in the bundle may be
   the same as or different from the security-source of the bundle with
   respect to a different security header in the bundle.  Similarly, the
   security-destination of a bundle with respect to each of that
   bundle's security headers may be the same or different.










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2.  Security Headers

   There are three types of security headers that MAY be included in a
   bundle.  These are the Bundle Authentication Header (BAH), the
   Payload Security Header (PSH), and the Confidentiality Header (CH).

      The BAH is used to assure the authenticity of the bundle along a
      single hop from forwarder to intermediate receiver.

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

      Since a BAH protects on a "hop-by-hop" basis and a PSH protects on
      a (sort of) "end-to-end" basis, whenever both are present the BAH
      MUST form the "outer" layer of protection - that is, the BAH MUST
      always be calculated and added to the bundle after the PSH has
      been calculated and added to the bundle.

      The CH indicates that some parts of the bundle have been encrypted
      while en route between the CH security-source and the CH security-
      destination.

   Each of the security headers uses the Canonical Bundle Header Format
   as defined in the Bundle Protocol Specification.  That is, each
   security header is comprised of the following elements:

      - Header type code

      - Header processing control flags

      - Header data length

      - Header-type-specific data fields

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





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2.1.  Abstract Security Header

   An ASH consists of the following mandatory and optional fields:

      - Header-type code (one byte) - as in all bundle protocol headers
      except the primary bundle header.  The header types codes for the
      security headers are:

         BAH: 0x02

         PSH: 0x03

         CH: 0x04

      - Header processing control flags (one byte) - as in all bundle
      protocol headers except the primary bundle header.  There are no
      major constraints on the use of the header processing flags,
      however, we would expect that the BAH would not have the "transmit
      status report" bit set since it is only present on a hop-by-hop
      basis.

      For a PSH, or CH, we would not expect the "replicate in every
      fragment" bit to be set.[Comment.2]

      - Header data length (SDNV) - as in all bundle protocol headers
      except the primary bundle header.  SDNV encoding is described in
      the bundle protocol.

      - Header-type-specific data fields as follows:

         - Ciphersuite ID - identifies the ciphersuite in use.  This is
         two bytes long, though the top five bits are used to indicate
         the presence or absence of the optional fields below.

         - (optional) Correlator - when more than one related header is
         inserted then this field must have the same value in each
         related header instance.  This is encoded as an SDNV.

         - (optional) Ciphersuite parameters - parameters to be used
         with the ciphersuite in use, e.g. a key identifier or
         initialization vector (IV).  This is encoded as an SDNV.  The
         encoding rules for this are defined as part of the ciphersuite
         specification.

         - (optional) Security-source-length - specifies the length of
         the following security source field.





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         - (optional) Security-source - specifies the security source
         for the service.  If this is omitted, then the source of the
         bundle is assumed to be the security-source.  The length of
         this field in octets is the value of the security-source-length
         field.

         - (optional) Security-destination-length - specifies the length
         of the following security destination field.

         - (optional) Security-destination - specifies the security
         destination for the service.  If this is omitted, then the
         destination of the bundle is assumed to be the security-
         destination.  The length of this field in octets is the value
         of the security-destination-length field.

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

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


   +--------+--------+--------+--------+---------+------------+-------+
   |type    |flags   |len     | ciphersuite      |correlator  | params|
   +--------+--------+--------+--------+---------+------------+-------+
   | src-len         | security source                                |
   +-----------------+------------------------------------------------+
   | dest-len        | security destination                           |
   +--------+--------+------------------------------------------------+
   | res-len         | security result                                |
   +--------+--------+------------------------------------------------+

   The structure of an abstract security header

   Figure 2

   The ciphersuite ID is a 16-bit value with the top five bits
   indicating which of the optional fields are present (value = "1") or
   absent (value="0").  The remaining 11 bits indicate the ciphersuite.

   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.

   The structure of the ciphersuite ID bytes 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



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   indicates the corresponding optional field is missing.

      src - the most significant bit (bit 15) indicates whether the ASH
      contains the optional security-source-length and security-source
      fields.

      dest - bit 14 indicates whether the security-destination-length
      and security-destination fields are present or not.

      parm - bit 13 indicates whether the ASH contains optional
      ciphersuite parameters or not.

      corr - bit 12 indicates whether or not the ASH contains an
      optional correlator.

      res - bit 11 indicates whether or not the ASH contains security
      result length and security result fields.

      bits 10-0 represent the ciphersuite number, giving a maxium of
      2048 different ciphersuites.



   Ciphersuite ID
   Bit    Bit   Bit   Bit   Bit   Bit   Bit   Bit
   15    14    13    12    11    10     ...          1     0
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |src  |dest |parm |corr |res  | ciphersuite ID              |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

   Figure 3

   A little bit more terminology: when the header is a PSH then we refer
   to the PSH-source when we mean the security source field in the PSH.
   Similarly we may refer to the CH-dest, meaning the security-
   destination field of the CH.  For example, referring to Figure 1
   again, if the bundle that originates at BN1 as source is given a
   Confidentiality Header (CH) by BN1 that is protected using a key held
   by BN3 and it is given a Payload Security Header (PSH) by BN1, then
   BN1 is both the CH-source and the PSH-source of the bundle, and BN3
   is the CH-dest of the bundle.

   The correlator field is used to associate several related instances
   of a security header.  This can be used to place a BAH that contains
   the ciphersuite information at the "front" of a (probably large)
   bundle , and another correlated BAH 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 BAH.  There are



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   similar use cases for multiple related instances of PSH and CH as
   will be seen below.

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

2.2.  Bundle Authentication Header

   In this section we describe typical BAH field values for two
   scenarios - where a single instance of the BAH 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 BAH is used:

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

      The header processing control flags value can be set to whatever
      values are required by local policy.

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

      The correlator field MUST NOT be present.

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

      The security-source field MUST be present and MUST identify the
      forwarder of the bundle.

      The security-destination field SHOULD NOT be present unless the
      ciphersuite requires this information (since the first node
      receiving the bundle ought be the one to validate the BAH).

      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 BAH instances, the first instance is
   as defined above, except the correlator MUST be present and the
   security result MUST be absent.  The second instance of the BAH MUST
   have the same correlator value present and MUST contain a security



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   result.  The other optional fields MUST NOT be present.  Typically,
   this second instance of a BAH will be the last header of the bundle.

2.3.  Payload Security Header

   A PSH is an ASH with the following additional restrictions:

      The header type code value MUST be 0x03.

      The header processing control flags value can be set to whatever
      values are required by local policy.

      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 PSH be present in the bundle.  The
      correlator MUST NOT be present if the ciphersuite only requires
      one instance of the PSH in the bundle.

      The ciphersuite parameters field MAY be present.

      The security-source field MAY be present.

      The security-destination field 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 BAH, this
      field MUST be present if the correlator is absent.  If more than
      one related instance of the PSH is required then this is handled
      in the same way as described for the BAH above.

   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
   destination that has access to the required keying information.

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

2.4.  Confidentiality Header

   A typical confidentiality ciphersuite will encrypt the payload using
   a randomly generated bundle encrypting key (BEK) and will use a CH



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   security result to carry the BEK encrypted with some long term key
   encryption key (KEK). or well-known public key.  If neither the
   destination or security-destination resolves the key to use for
   decryption, the ciphersuite parameters field can be used to indicate
   the decryption key with which the BEK can be recovered.  Subsequent
   CH security results will contain headers encrypted using the BEK if
   non-payload headers are to be encrypted.

   The payload is encrypted "in-place", that is, following encryption,
   the payload header payload field contains ciphertext, not plaintext.
   The payload header processing flags are unmodified.[Comment.3]

   Payload super-encryption is allowed.  If a CH ciphersuite supports
   such super-encryption, then the ciphersuite MUST provide an
   unambiguous way to do the decryption operations in the correct order
   (e.g. by encoding the "layer" information as a ciphersuite
   parameter).  This "in-place" encryption of payload bytes is so as to
   allow bundle payload fragmentation and re-assembly to operate without
   knowledge of whether encryption has occurred.  A side-effect of this
   "in-place" encryption is that the payload will typically be expanded
   by up-to a ciphertext blocksize if the bulk cipher is a block cipher.
   Another is that the 2nd application of confidentiality does not
   generally protect the parameters of the first which represent a
   vulnerability in some circumstances.

   A CH is an ASH with the following additional restrictions:

      The header type code value MUST be 0x04.

      The header processing control flags value can be set to whatever
      values are required by local policy.

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

      The correlator MUST be present if more than one related CH
      instance is required.  More than one CH instance might be required
      if the payload is to be encrypted for more than one security-
      destination so as to be robust in the face of routing
      uncertainties.  These multiple CH instances, however, would not be
      related and would therefore not require correlators.  On the other
      hand, multiple related CH instances would be required if both the
      payload and the PSH headers in the bundle were to be encrypted.
      These multiple CH instances would require correlators to associate
      them with each other.

      The ciphersuite parameters field MAY be present




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      The security-source field MAY be present.

      The security-destination field MAY be present (and typically will
      be).

      The security result MAY be present and normally represents an
      encrypted bundle encryption key or encrypted versions of bundle
      headers other than the payload header.

   As was the case for the BAH and PSH, if the ciphersuite requires more
   than one instance of the CH, then the first occurrence MUST contain
   most of the optional fields, (e.g. security destination etc.) and
   subsequent instances MUST NOT contain those same fields.  Unlike the
   case for the BAH and PSH, however, the security result field MAY (and
   probably will) be included in multiple related CH instances.

   Put another way: when a node is encrypting some (non-payload)
   headers, it MUST first create a CH with the required ciphersuite ID,
   parameters etc. as specified above.  Typically, this CH will appear
   "early" in the bundle.  If this "first" CH doesn't contain all of the
   ciphertext, then it may be followed by other (correlated) CH, which
   MUST NOT repeat the ciphersuite or other information from the first
   CH.

   A CH ciphersuite may, or may not, specify which headers are to be
   encrypted.  If the ciphersuite doesn't specify this, then the node is
   free to encrypt whichever headers it wishes.  If a CH ciphersuite
   does specify which headers are to be encrypted, then doing otherwise
   is an error.

   Since a single CH security result can contain the ciphertext for
   multiple (non-payload) plaintext headers, the node simply catenates
   these plaintext headers prior to encryption.  After decryption the
   recovered plaintext should then replace the CH in the bundle for
   futher processing (e.g.  PSH verification).  This recovered plaintext
   MUST contain all the appropriate header type, processing flags and
   length information.  In other words delete the CH in question and
   place the recovered plaintext, which consists of additional (non-
   payload) headers, in the bundle at the location from which the CH was
   deleted.

   Even if a to-be-encrypted header has the "discard" flag set, whether
   or not the CH's "discard" flag is set is an implementation/policy
   decision for the encrypting node.  (The "discard" flag is more
   properly called the "discard if header cannot be processed"
   flag.)[Comment.4]





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 2.5.   PSH and CH combinations

   Given the above definitions, nodes are free to combine applications
   of PSH and CH in any way they wish - the correlator value allows for
   multiple applications of security services to be handled separately.

   However, there are some clear security problems that could arise when
   applying multiple services, for example, if we encrypted a payload
   but left a PSH security result containing a signature in clear, this
   would allow payload guesses to 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 PSH or CH, must be applied to all instances with the
   same correlator.

   We now list the PSH and CH 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 must 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 CH, but with the payload
      encrypted multiple times, "in-place".

      Multiple parallel authenticators - a single security source might
      wish to integrity protect a bundle in multiple ways, in the hope
      that one of them will be useful.  This could be required if the
      path the bundle will take 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 PSH which presumably all protect the payload, and which
      cannot in general protect one another.  Note that this logic can
      also apply to a BAH, if the unpredictable routing happens in the
      convergence layer, so we also envisage support for multiple
      parallel uses of BAH.

      Multiple sequential authenticators - if some security destination
      requires assurance about the route that bundles have taken, then
      it might insist on each forwarding node adding its own PSH.  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 PSH's will likely be layered, so that each protects the



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      earlier applications of PSH.

      Authenticated and encrypted bundles - a single bundle may require
      both authenticity and confidentiality.  In this case, most
      specifications 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.[Comment.5]

   There are no doubt other valid ways to combine PSH and CH instances,
   but these are the "core" set we wish to support.  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 in order 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.  If a particular transmission
   request satisfies the node's policy 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.[Comment.6]

   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 PSH.
   Similarly it is also valid to implement a node which always rejects
   all requests which imply the use of a PSH.

   Nodes MUST consult their security policy to determine the criteria
   that a received bundle ought to meet before it will be forwarded.
   These criteria MAY include a determination of whether or not the
   received bundle must include valid BAH, PSH or CH.  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, destined for the
   forwarding node's own endpoint.[Comment.7]

   The node's policy MAY call for the node to add or subtract some
   security headers, for example, requiring the node attempt to encrypt
   (parts of) the bundle for some security-destination, or requiring
   that the node add a PSH.  If the node's policy requires a BAH 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 headers in the bundle.

3.2.  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.  Because bundles may be modified while in



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   transit (either correctly or due to implementation errors), a
   canonical form of any given bundle (that contains a BAH or PSH) must
   be defined.

   This section defines two bundle canonicalisation algorithms which can
   be used by various ciphersuites.  [Comment.8]

 3.2.1.   Strict canonicalisation

   The first algorithm which can be used basically permits no changes at
   all to the bundle between when it is forwarded at the security-source
   and when it is received at the security-destination and is mainly
   intended for use in BAH ciphersuites.  This algorithm simply involves
   catenating all headers in the order presented, but omits all security
   result fields which are present in headers of the ciphersuite type in
   question - that is, when a BAH ciphersuite specifies this algorithm
   then we omit all BAH security results, when a PSH ciphersuite
   specifies this algorithm then we omit all PSH security results.

   Notes:

      - In the above we call for security results to be omitted, this
      means that no bytes at all are input - we do not set the security
      result length to zero or any equivalent.

      - The 'res' bit of the ciphersuite ID which indicates that a
      security value will be present is part of the canonical form.

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

3.2.2.  Mutable canonicalisation

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

   The basic approach is to define a canonical form for the primary
   header, and catenate that with the security and payload headers in
   the order that they will be transmitted.  This algorithm ignores all
   other headers on the basis that we cannot tell whether or not they
   are liable to change as the bundle transits the network.

   The canoncial form of the primary header is shown below.



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   Essentially, it de-references the dictionary header, adjusts lengths
   where necessary and ignores flags that may change in transit.


   +----------------+----------------+----------------+----------------+
   |    Version     |  Proc. Flags   |   COS Flags    |   SRR Flags    |
   +----------------+----------------+---------------------------------+
   |                Canonical primary header 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 (8 bytes)                   +
   |                                                                   |
   +---------------------------------+---------------------------------+
   |                             Lifetime                              |
   +----------------+----------------+----------------+----------------+
   |                      Fragment offset (optional)                   |
   +----------------+----------------+---------------------------------+
   |            Total application data unit length (optional)          |
   +----------------+----------------+---------------------------------+

   The canonical form of the primary bundle header.

   Figure 4

   The fields shown are:

      Version, Processing Flags, COS, SRR - are all copied from the
      first four bytes of the primary header and will contain the
      version and the immutable flag values from the primary header.
      Formed by copying the processing, COS and SRR flag fields from the



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      primary header and then subequently setting all of the mutable
      bits to zero.  This requires ANDing with the (four byte) value
      0xFF1E031F so that the mutable and reserved bits are set to zero.
      The only currently mutable bit masked out here is the "bundle is a
      fragment" bit - all others are reserved bits.

      There is an issue here which PSH ciphersuites MUST tackle.  If a
      bundle is fragmented before the PSH is applied then the PSH
      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 PSH.  For this reason, PSH ciphersuites which support
      applying a PSH to fragments MUST specify which bytes of the bundle
      payload are protected as part of the security parameters.  When
      verifying such a fragment only the bytes from the fragment are
      input to the PSH verification.  Of course, if is also valid for a
      ciphersuite to be specified so as to only apply to entire bundles
      and not to fragments.

      Length - a four-byte value containing the length (in bytes) of
      this structure.[Comment.9]

      Destination endpoint ID length and value - are the length (as a
      four byte value) and value of the destination endpoint ID from the
      primary bundle header.  The URI is simply copied from the relevant
      part(s) of the dictionary header and is not itself canonicalised.

      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 and Lifetime are simply copied from the primary
      header.

      Fragment offset and Total application data unit length are copied
      from the primary header if they are present there (which is
      controlled by one of the flags).

   Payload headers are generally copied as-is, with the exception that
   the processing flags value in the canonical version MUST be ANDed
   with 0x17 to ensure that currently "reserved" bits are clear, but
   that the "last header" flag is ignored.  The reason to ignore the
   "last header" flag is that if I create a bundle with both PSH and
   BAH, then the next hop will remove BAH instances, and if one of those
   was the last header, it may set the "last header" flag of e.g. a PSH
   instance.  The "last header" is therefore a mutable bit and should be



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   omitted from the canonical form.

   Another exception occurs in cases where only a fragment of the
   payload was protected, when only those bytes of the payload header
   payload field are considered part of the canonical form.

   Security headers are handled likewise, with the exception that the
   ciphersuite will likely specify that the "current" security header
   security result field not be considered part of the canonical form.
   This differs from the case in strict canonicalisation since we might
   use the mutable canonicalisation algorithm to handle sequential
   signatures, where later signatures should cover earlier ones.

   Notes:

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

      - We omit the reserved flags on the basis that we cannot determine
      whether or not they will change in transit.  This means that this
      algorithm may have to be revised if those flags are given a
      definition and if we want to protect them.

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

      - Note that the URI encoding above will be a cause for errors if
      any node rewrites the dictionary for example changing the DNS part
      of some HTTP URL from lower-case to upper case.  Since there are
      no exact general rules for canonicalising URIs (or even worse
      IRIs), we simply have to live with this problem.

      - All length fields here are four byte values in network byte
      order.  We do not need to optimise the encoding since the values
      are never sent over the network.

3.3.  Source confidentiality

   Since every bundle MUST contain a primary header that cannot be
   encrypted, and which contains the source endpoint ID (and others), if
   we want to provide source confidentiality, then we have to invent a
   fake primary header with false values for these fields and then a new
   header type to contain the actual values.

   Similarly, there may be confidentiality requirements applying to
   other parts of the primary header (e.g. the current-custodian) and we
   support these in the same way.



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   Since we don't know if we'll do this...details are TBD:-)[Comment.10]

 3.4.   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 BAH.  If the bundle is not required to have a
   BAH, then BAH processing on the received bundle is complete and the
   bundle is ready to be further processed for CH/PSH handling or
   delivery or forwarding.

   If the bundle is required to have a BAH but it does not, then the
   bundle MUST be discarded and processed no further; in this case a
   bundle status report indicating the authentication failure MAY be
   generated, destined for the receiving node's own endpoint.

   Otherwise, if the bundle does have a BAH, then the value in the
   security result field of the BAH of the received bundle MUST be
   verified according to the ciphersuite specification.  If the check
   fails for all BAHs in the bundle, the bundle has failed to
   authenticate and the bundle SHALL be discarded and processed no
   further; in this case, a bundle status report indicating the
   authentication failure MAY be generated, destined for the
   intermediate receiver's own endpoint.  Otherwise, if any of the BAHs
   present verify, the bundle is ready to have its CH processed (if it
   includes one).

   When forwarding a bundle that included some BAHs when it was
   received, these BAHs MUST be stripped from the bundle.  New BAHs MAY
   be added as required by policy.  This might require correcting the
   "last header" field of the to-be-forwarded bundle.

   If the bundle has a CH and the receiving node is the CH destination
   for the bundle (either because the node is listed in the bundle's CH-
   dest field or because the node is listed as the bundle's destination
   and there is no CH-dest field), the node MUST decrypt the relevant
   parts of the bundle according to the ciphersuite specification and
   delete the CH in question.  If the CH security result included the
   ciphertext of anything other than an encrypted BEK that was used to
   encrypt the bundle payload, the recovered plaintext headers MUST be
   placed in the bundle at the location from which the CH was deleted.

   If the bundle has a PSH and the receiving node is the PSH destination
   for the bundle (either because the node is listed in the bundle's
   PSH-dest field or because the node is listed as the bundle's
   destination and there is no PSH-dest field), the node MUST verify the
   value in the security result field of the PSH according to the
   ciphersuite specification.  If the check fails, the bundle has failed



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   to authenticate and the bundle SHALL be discarded and processed no
   further; a bundle status report indicating the failure MAY be
   generated, destined for the receiving node's own endpoint.
   Otherwise, if the PSH verifies, the bundle is ready to be processed
   for either delivery or forwarding.  Before forwarding the bundle, the
   node SHOULD remove the PSH from the bundle, unless there is the
   likelihood that some downstream node will also be able to verify the
   PSH.

   If the bundle has a PSH and the receiving node is not the PSH-dest
   for the bundle but the ciphersuite allows, the receiving node MAY, if
   it is able, verify the value in the security result field.  If the
   check fails, the node SHALL discard the bundle and it MAY send a
   bundle status report indicating the failure to the receiving node's
   own endpoint.

3.5.  The At-Most-Once-Delivery Option

   An application may request (in some 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.

   We define a new option for use in such cases, known as "at-most-once-
   delivery".  If this option is chosen, then the application is
   indicating that the node node SHALL check for replayed bundles,
   discard duplicates, and deliver at most one copy of each received
   bundle to the application.  If this option is not chosen, the
   application is indicating that node SHALL deliver all received bundle
   copies to the application.  If this option is not chosen, the node
   MAY (subject to policy) deliver all bundles or else behave as if the
   option had been chosen.

   To enforce this the node MUST look at the (source, timestamp) pair
   value of each complete (reassembled, if necessary) bundle received
   and determine if this pair, which should uniquely identify a bundle,
   has been received before.  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.

   The duration for which a node maintains entries on such a list is an
   implementation matter.

   If any application has indicated that it wants a node to use the "at
   most once" delivery option for a particular destination endpoint ID
   that is in a bundle, then the node MUST compare the (source,
   timestamp) pair values of the bundle with the local list of such



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   values of already-received bundles.

3.6.  Bundle Fragmentation and Reassembly

   If it is necessary for a node to fragment a bundle and security is
   being used on that bundle, the following security-specific processing
   is REQUIRED:

   Firstly, a BAH, PSH or CH MUST NOT be fragmented.  At this time, only
   the payload field of the payload header MAY be fragmented.

   If the bundle is required by the security policy to have a BAH before
   being forwarded, all fragments resulting from that bundle MUST
   contain individual BAH values.

   If the original bundle had a PSH, then each of the PSH instances MUST
   be included in some fragment.  A single PSH instance MUST NOT be sent
   more than once.

   If the original bundle had a CH, then the each of the CH instances
   MUST be included in some fragment.  A single CH instance MUST NOT be
   sent more than once.

3.7.  Reactive fragmentation

   When original bundle transmission is terminated before the entire
   bundle has been transmitted, the receiving node SHALL consult its
   security policy to determine whether it is permitted to transform the
   received portion of the bundle into a bundle fragment for further
   forwarding.  Whether or not such reactive fragmentation is permitted
   SHALL be dependent on the security policy in combination with the
   ciphersuite used to calculate the BAH authentication information if
   required.

   In such cases, if the original bundle is fragmented by the
   intermediate receiver (reactively), and the BAH-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 an original bundle transmission is terminated before the entire
   bundle has been transmitted, if the truncated bundle arriving at the
   intermediate receiver is reactively fragmented and forwarded, only
   the part of the bundle that was not received MUST be retransmitted.
   Before retransmitting this portion of the bundle, it SHALL be changed
   into a fragment and, if the original bundle included a BAH, the
   fragmented bundle MUST also, and its BAH SHALL be recalculated.



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

















































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4.  Mandatory Ciphersuites

   This section defines the mandatory ciphersuites for this
   specification.  There is currently one mandatory ciphersuite for each
   of BAH, PSH and CH.  The BAH ciphersuite is based on shared secrets
   using HMAC.  The PSH ciphersuite is based on digital signatures using
   RSA with SHA256.  The CH ciphersuite is based on using RSA for key
   transport and AES for bulk encryption.

4.1.  BAH-HMAC

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

   Security parameters are optional with this scheme, but if used then
   the value of the security parameter MUST be used as a key identifier.
   In the absence of a key identifier the intermediate receiver is
   expected to be able to find the correct key based on the sending
   identity (from the security-source and/or convergence layer).

   BAH-HMAC uses the strict canonicalisation algorithm in Section 3.2.1.

   The variant of HMAC to be used is HMAC-SHA1 as defined in RFC 2104
   [4].[Comment.11]

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

   This means that normally, the BAH will be the second and last headers
   of the bundle.  If a forwarder wishes to apply more than one
   correlated BAH 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" BAHs, and their "at back" "partners"
   (without any security result), before canonicalising.

   Inserting more than one correlated BAH 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
   input being the result of running the entire bundle through the
   strict canonicalisation algorithm.  Both required BAH instances MUST
   be included in the bundle before canonicalisation.





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4.2.  PSH-RSA-SHA256

   The PSH-RSA-SHA256 ciphersuite has ciphersuite ID value 0x002.

   If the bundle being signed has been fragemented before signing, then
   we have to specify which bytes were signed, in case the signed bundle
   is subequently fragmented for a second time.  So, if the bundle is a
   fragment, then the security parameters MUST include two SDNV encoded
   numbers, representing the offset and length of the signed fragment.
   If the entire bundle is signed then these numbers MUST be omitted.

   The security parameters field MAY also contain a key identifier.  The
   exact type of key identifier to be used is an implementation issue.
   In the absence of a key identifier the verifier of the PSH is
   expected to be able to use the security source (if supplied) or else
   the bundle source (if no security source is present) in order to
   determine the correct public key to use for PSH verification.

   PSH-RSA-SHA256 uses the mutable canonicalisation algorithm
   Section 3.2.2.  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 PSH.

   RSA is used with SHA256 as specified for the sha256WithRSAEncryption
   PKCSv1.5 signature scheme in RFC 4055 [5].  The output of the signing
   process is the security result for the PSH.

   "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.[Comment.12]

 4.3.   CH-RSA-AES-PAYLOAD-PSH
   [Comment.13]

   The CH-RSA-AES-PAYLOAD-PSH ciphersuite has cipersuite ID value 0x003.

   This scheme only allows for payload and PSH encryption and involves
   encrypting every instance of a PSH as well as the payload.

   This ciphersuite requires the use of a single CH instance if the
   bundle does not contain a PSH, and multiple CH instances if the
   bundle includes one or more PSHs.  A first CH is created which



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   contains the encrypted bundle encryption key (BEK).  Thereafter each
   PSH and the payload bytes are encrypted using a key derived from the
   BEK.

   For the first CH, there MUST be a security parameter which contains a
   16 byte IV, optionally followed by a key identifier (whose format is
   again out of scope here).  The security result contains the BEK
   encrypted using PKCSv1.5 rsaEncryption as specified in RFC 3370 [6].

   For each subsequent PSH, the entire header is replaced by a CH that
   is correlated with the first CH and whose security result is the
   ciphertext form of the PSH, including the header type, etc.

   For the payload, only the bytes of the bundle payload field are
   affected, being replaced by ciphertext.

   Since we are separately encrypting the payload and all of the PSH
   headers present, we would like to use different keys for each
   encryption.  We do this using a key derivation function (KDF) which
   takes a different input for each header (or part of the header, as in
   the case of payload encryption) that is to be encrypted.  The input
   to the KDF is the BEK catenated with the following values:

      - The IV from the security parameter. [16 bytes]

      - The "header number" (counting the primary header as 1, the next
      as 2 etc regardless of the header type). [ 1 octet]

      - The header type. [1 octet]

      - If the header is a PSH (i.e., if it is not the Bundle Payload
      Header), then the correlator field value.  [SDNV]

   Note: Identifying the right KDF here is stil TBD.  Plan is to see
   what happens with the recent NIST KDF paper (which is in conflict
   with KDF's used in various Internet protocols).  Expect change.

   The output from the KDF is called the result encryption key (REK) and
   is used to encrypt the payload or PSH bytes.

   The REK uses the AES algorithm in CBC mode as specified by the id-
   aes-cbc object identifier in RFC 3565 [7] [Comment.14][Comment.15]









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 5.   Key Management

   Since key management in delay tolerant networks is still a research
   topic we cannot provide much in the way of useful key management
   here.  However, solely in order to support implementation and
   testing, implementations SHOULD support:

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

      - The use of well-known RSA public keys for PSH-RSA-SHA256 and CH-
      RSA-AES-PAYLOAD-PSH ciphersuites.

   Since endpoint IDs are URIs and URIs can be placed in X.509 [8]
   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.

   Other than that, key management is for future study.






























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6.  Default Security Policy

   Every node serves as a Policy Enforcement Point (PEP) 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 BAHs.

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

      Under what conditions received bundles SHALL be required to have
      valid PSHs and/or CHs.

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

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

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

      Under what conditions a CH SHALL be added to 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.  [Comment.16]

   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:

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





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      The mandatory ciphersuites MUST be used.

      All bundles received MUST have a BAH which MUST be verified to
      contain a valid security result.  If the bundle does not have a
      BAH, then the bundle MUST be discarded and processed no further; a
      bundle status report indicating the authentication failure MAY be
      generated, destined for the receiving node's own endpoint.

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

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

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

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

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

      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 status report indicating the
      failure SHOULD be generated, destined for the receiving node's own
      endpoint.


















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7.  Security Considerations
   [Comment.17]

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

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

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














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8.  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 header types, and in particular for this specification a
   separate register of ciphersuite specifications.













































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9.  References

9.1.  Normative References

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

   [2]  Scott, K., "Bundle Protocol Specification",
        draft-irtf-dtnrg-bundle-spec-04.txt , December 2005.

   [3]  Cerf, V., "Delay-Tolerant Network Architecture",
        draft-irtf-dtnrg-arch-04.txt , December 2005,
        <draft-irtf-dtnrg-arch-02.txt>.

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

   [5]  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.

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

   [7]  Schaad, J., "Use of the Advanced Encryption Standard (AES)
        Encryption Algorithm in Cryptographic Message Syntax (CMS)",
        RFC 3565, July 2003.

   [8]  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.

9.2.  Informative References

   [9]  Farrell, S., Symington, S., and H. Weiss, "Delay-Tolerant
        Network Security Overview",
        draft-irtf-dtnrg-sec-overview-01.txt , March 2006.













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Editorial Comments

   [Comment.1]   Editors: There may be some (valid) question marks over
                 this terminology. We weren't entirely sure ourselves.

   [Comment.2]   Stephen: I guess there could be some weird corner case
                 where a  CH ciphersuite using counter-mode would allow
                 fragments to be  individually decrypted, and in that
                 case, we might want to set replication for each
                 fragment. So we can't fully rule out setting that flag
                 for all PSH/CH.

   [Comment.3]   Stephen: This to be revisited!

   [Comment.4]   Howie: It may be nice to be able to signal both
                 confidentiality and authentication using only one
                 header instead of using two separate headers (PSH and
                 CH).

   [Comment.5]   Add approppriate forward references to ciphersuites
                 defined later.

   [Comment.6]   Stephen: Do we need to specify error handling for the
                 case where a node drops a bundle for policy reasons?
                 Does/can it signal back to the source that its done so?
                 I dunno.

   [Comment.7]   Howie: The security policy database will need to be
                 discussed somewhere. Does it belong in this document,
                 the bundle protocol spec., both, some other document?

   [Comment.8]   Stephen: This is a fairly big new section and so
                 probably contains errors!

   [Comment.9]   Editors: Check that mask value at the very last moment
                 (incl. during auth-48) to be sure its (still) correct.

   [Comment.10]  Stephen: Should we support source confidentiality?
                 Might complicate PSH which is the downside IMO.

   [Comment.11]  Editors: At the moment there appears to be no security
                 reason to move away from HMAC-SHA1 since the HMAC
                 construct is not as far as we know affected by
                 collisions in the underlying digest algorithm (which
                 are nearly practically computable for SHA-1).
                 Nevertheless, since we use SHA-256 in the signature
                 ciphersuite (since collisions do matter there), it may
                 be desirable to move to HMAC-SHA-256 as specified in



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                 RFC 4321. So if you're writing code based on this...be
                 warned!

   [Comment.12]  Editors: There are currently unresolved "issues" with
                 digest  algorithms which might cause a change here
                 prior to, but more likely, after, an RFC has issued. So
                 expect change!

   [Comment.13]  Editors: This entire section is to be treated as a
                 strawman for the present.

   [Comment.14]  Speculation: There would be an interesting possibility
                 opened up were we to use a stream cipher with the REK.
                 That is that we could then encrypt and decrypt
                 independently - Alice could encrypt for Bob, then
                 Charlie could encrypt for Dessie, then Bob could
                 decrypt, then Dessie. Better in terms of not adding
                 padding but worse in that it trivially allows known
                 plaintext manipulation if there's no PSH.

   [Comment.15]  Editors: Another option might also be to switch to
                 using counter mode rather than CBC which would have the
                 benefit of allowing some fragments to be decrypted even
                 if not all fragments arrive. While that seems nice
                 enough to do, it would of course require us to think
                 more about fragments and so is for the next version if
                 at all.

   [Comment.16]  Howie: Eventually we will need to state where the
                 security policy information/DB does get discussed/
                 specified.

   [Comment.17]  Editors: Much more text is needed here no doubt.


















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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.
   7075 Samuel Morse Drive
   Columbia, MD  21046
   US

   Phone: +1-410-872-1515 x201
   Email: hsw@sparta.com


















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