BPSec Default Security Contexts
draft-ietf-dtn-bpsec-default-sc-04

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Document Type Active Internet-Draft (dtn WG)
Author Edward Birrane 
Last updated 2021-04-11
Replaces draft-ietf-dtn-bpsec-interop-sc
Stream Internet Engineering Task Force (IETF)
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Delay-Tolerant Networking                                     E. Birrane
Internet-Draft                                                   JHU/APL
Intended status: Standards Track                          April 11, 2021
Expires: October 13, 2021

                    BPSec Default Security Contexts
                   draft-ietf-dtn-bpsec-default-sc-04

Abstract

   This document defines default integrity and confidentiality security
   contexts that may be used with the Bundle Protocol Security Protocol
   (BPSec) implementations.  These security contexts are intended to be
   used for both testing the interoperability of BPSec implementations
   and for providing basic security operations when no other security
   contexts are defined or otherwise required for a network.

Status of This Memo

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

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

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

   This Internet-Draft will expire on October 13, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Integrity Security Context BIB-HMAC-SHA2  . . . . . . . . . .   3
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  SHA Variant . . . . . . . . . . . . . . . . . . . . .   6
       3.3.2.  Wrapped Key . . . . . . . . . . . . . . . . . . . . .   6
       3.3.3.  Integrity Scope Flags . . . . . . . . . . . . . . . .   7
       3.3.4.  Enumerations  . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Results . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Key Considerations  . . . . . . . . . . . . . . . . . . .   8
     3.6.  Canonicalization Algorithms . . . . . . . . . . . . . . .   8
     3.7.  Processing  . . . . . . . . . . . . . . . . . . . . . . .   9
       3.7.1.  Keyed Hash Generation . . . . . . . . . . . . . . . .   9
       3.7.2.  Keyed Hash Verification . . . . . . . . . . . . . . .  10
   4.  Security Context BCB-AES-GCM  . . . . . . . . . . . . . . . .  11
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .  14
       4.3.1.  Initialization Vector (IV)  . . . . . . . . . . . . .  14
       4.3.2.  AES Variant . . . . . . . . . . . . . . . . . . . . .  14
       4.3.3.  Wrapped Key . . . . . . . . . . . . . . . . . . . . .  15
       4.3.4.  AAD Scope Flags . . . . . . . . . . . . . . . . . . .  15
       4.3.5.  Enumerations  . . . . . . . . . . . . . . . . . . . .  16
     4.4.  Results . . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.4.1.  Authentication Tag  . . . . . . . . . . . . . . . . .  16
       4.4.2.  Enumerations  . . . . . . . . . . . . . . . . . . . .  17
     4.5.  Key Considerations  . . . . . . . . . . . . . . . . . . .  17
     4.6.  GCM Considerations  . . . . . . . . . . . . . . . . . . .  18
     4.7.  Canonicalization Algorithms . . . . . . . . . . . . . . .  19
       4.7.1.  Cipher text related calculations  . . . . . . . . . .  19
       4.7.2.  Additional Authenticated Data . . . . . . . . . . . .  20
     4.8.  Processing  . . . . . . . . . . . . . . . . . . . . . . .  20
       4.8.1.  Encryption  . . . . . . . . . . . . . . . . . . . . .  20
       4.8.2.  Decryption  . . . . . . . . . . . . . . . . . . . . .  22
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     5.1.  Security Context Identifiers  . . . . . . . . . . . . . .  23
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
     6.1.  Key Handling  . . . . . . . . . . . . . . . . . . . . . .  24
     6.2.  AES GCM . . . . . . . . . . . . . . . . . . . . . . . . .  24
     6.3.  Bundle Fragmentation  . . . . . . . . . . . . . . . . . .  24
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  25

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   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  26
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   The Bundle Protocol Security Protocol (BPSec) [I-D.ietf-dtn-bpsec]
   specification provides inter-bundle integrity and confidentiality
   operations for networks deploying the Bundle Protocol (BP)
   [I-D.ietf-dtn-bpbis].  BPSec defines BP extension blocks to carry
   security information produced under the auspices of some security
   context.

   This document defines two security contexts (one for an integrity
   service and one for a confidentiality service) for populating BPSec
   Block Integrity Blocks (BIBs) and Block Confidentiality Blocks
   (BCBs).

   These contexts generate information that MUST be encoded using the
   CBOR specification documented in [RFC8949].

2.  Requirements Language

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

3.  Integrity Security Context BIB-HMAC-SHA2

3.1.  Overview

   The BIB-HMAC-SHA2 security context provides a keyed hash over a set
   of plain text information.  This context uses the Secure Hash
   Algorithm 2 (SHA-2) discussed in [SHS] combined with the HMAC keyed
   hash discussed in [HMAC].  The combination of HMAC and SHA-2 as the
   integrity mechanism for this security context was selected for two
   reasons:

   1.  The use of symmetric keys allows this security context to be used
       in places where an asymmetric-key infrastructure (such as a
       public key infrastructure) may be impractical.

   2.  The combination HMAC-SHA2 represents a well-supported and well-
       understood integrity mechanism with multiple implementations
       available.

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   BIB-HMAC-SHA2 supports three variants of HMAC-SHA, based on the
   supported length of the SHA-2 hash value.  These variants correspond
   to "HMAC 256/256", "HMAC 384/384", and "HMAC 512/512" as defined in
   [RFC8152] Table 7: HMAC Algorithm Values.  The selection of which
   variant is used by this context is provided as a security context
   parameter.

   The output of the HMAC MUST be equal to the size of the SHA2 hashing
   function: 256 bits for SHA-256, 384 bits for SHA-384, and 512 bits
   for SHA-512.

   The BIB-HMAC-SHA2 security context MUST have the security context
   identifier specified in Section 5.1.

3.2.  Scope

   The scope of BIB-HMAC-SHA2 is the set of information used to produce
   the plain text over which a keyed hash is calculated.  This plain
   text is termed the "Integrity Protected Plain Text" (IPPT).  The
   content of the IPPT is constructed as the concatenation of
   information whose integrity is being preserved from the BIB-HMAC-SHA2
   security source to its security acceptor.  There are four types of
   information that can be used in the generation of the IPPT, based on
   how broadly the concept of integrity is being applied.  These four
   types of information, whether they are required, and why they are
   important for integrity, are discussed as follows.

   Security target contents
       The contents of the block-type-specific data field of the
       security target MUST be included in the IPPT.  Including this
       information protects the security target data and is considered
       the minimal, required set of information for an integrity service
       on the security target.

   Primary block
       The primary block identifies a bundle and, once created, the
       contents of this block are immutable.  Changes to the primary
       block associated with the security target indicate that the
       security target (and BIB) may no longer be in the correct bundle.

       For example, if a security target and associated BIB are copied
       from one bundle to another bundle, the BIB may still contain a
       verifiable signature for the security target unless information
       associated with the bundle primary block is included in the keyed
       hash carried by the BIB.

       Including this information in the IPPT protects the integrity of
       the association of the security target with a specific bundle.

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   Security target other fields
       The other fields of the security target include block
       identification and processing information.  Changing this
       information changes how the security target is treated by nodes
       in the network even when the "user data" of the security target
       are otherwise unchanged.

       For example, if the block processing control flags of a security
       target are different at a security verifier than they were
       originally set at the security source then the policy for
       handling the security target has been modified.

       Including this information in the IPPT protects the integrity of
       the policy and identification of the security target data.

   BIB other fields
       The other fields of the BIB include block identification and
       processing information.  Changing this information changes how
       the BIB is treated by nodes in the network, even when other
       aspects of the BIB are unchanged.

       For example, if the block processing control flags of the BIB are
       different at a security verifier than they were originally set at
       the security source, then the policy for handling the BIB has
       been modified.

       Including this information in the IPPT protects the integrity of
       the policy and identification of the security service in the
       bundle.

       NOTE: The security context identifier and security context
       parameters of the security block are not included in the IPPT
       because these parameters, by definition, are required to verify
       or accept the security service.  Successful verification at
       security verifiers and security acceptors implies that these
       parameters were unchanged since being specified at the security
       source.

   The scope of the BIB-HMAC-SHA2 security context is configured using
   an optional security context parameter.

3.3.  Parameters

   BIB-HMAC-SHA2 can be parameterized to select SHA-2 variants,
   communicate key information, and define the scope of the IPPT.

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3.3.1.  SHA Variant

   This optional parameter identifies which variant of the SHA-2
   algorithm is to be used in the generation of the authentication code.

   This value MUST be encoded as a CBOR unsigned integer.

   Valid values for this parameter are as follows.

                       SHA Variant Parameter Values

   +-------+-----------------------------------------------------------+
   | Value |                        Description                        |
   +-------+-----------------------------------------------------------+
   |   5   |     HMAC 256/256 as defined in [RFC8152] Table 7: HMAC    |
   |       |                      Algorithm Values                     |
   |   6   |     HMAC 384/384 as defined in [RFC8152] Table 7: HMAC    |
   |       |                      Algorithm Values                     |
   |   7   |     HMAC 512/512 as defined in [RFC8152] Table 7: HMAC    |
   |       |                      Algorithm Values                     |
   +-------+-----------------------------------------------------------+

                                  Table 1

   When not provided, implementations SHOULD assume a value of 6
   (indicating use of HMAC 384/384), unless an alternate default is
   established by local security policy at the security source,
   verifiers, or acceptor of this integrity service.

3.3.2.  Wrapped Key

   This optional parameter contains the output of the AES key wrap
   authenticated encryption function (KW-AE) as defined in [AES-KW].
   Specifically, this parameter holds the cipher text produced when
   running the KW-AE algorithm with the input string being the symmetric
   HMAC key used to generate the security results present in the
   security block.  The value of this parameter is used as input to the
   AES key wrap authenticated decryption function (KW-AD) at security
   verifiers and security acceptors to determine the symmetric HMAC key
   needed for the proper validation of the security results in the
   security block.

   This value MUST be encoded as a CBOR byte string.

   If this parameter is not present then security verifiers and
   acceptors MUST determine the proper key as a function of their local
   BPSec policy and configuration.

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3.3.3.  Integrity Scope Flags

   This optional parameter contains a series of flags that describe what
   information is to be included with the block-type-specific data when
   constructing the IPPT value.

   This value MUST be represented as a CBOR unsigned integer, the value
   of which MUST be processed as a bit field containing no more than 8
   bits.

   Bits in this field represent additional information to be included
   when generating an integrity signature over the security target.
   These bits are defined as follows.

      - Bit 0 (the low-order bit, 0x1): Primary Block Flag.

      - Bit 1 (0x02): Target Header Flag.

      - Bit 2 (0x03): Security Header Flag.

      - Bits 3-7 are reserved.

3.3.4.  Enumerations

   BIB-HMAC-SHA2 defines the following security context parameters.

                     BIB-HMAC-SHA2 Security Parameters

    +----+-----------------------+--------------------+---------------+
    | Id |          Name         | CBOR Encoding Type | Default Value |
    +----+-----------------------+--------------------+---------------+
    | 1  |      SHA Variant      |        UINT        |       6       |
    | 2  |      Wrapped Key      |    Byte String     |      NONE     |
    | 4  | Integrity Scope Flags |        UINT        |      0x7      |
    +----+-----------------------+--------------------+---------------+

                                  Table 2

3.4.  Results

   BIB-HMAC-SHA2 defines the following security results.

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                      BIB-HMAC-SHA2 Security Results

   +--------+----------+-------------+---------------------------------+
   | Result |  Result  |     CBOR    |           Description           |
   |   Id   |   Name   |   Encoding  |                                 |
   |        |          |     Type    |                                 |
   +--------+----------+-------------+---------------------------------+
   |   1    | Expected | byte string |      The output of the HMAC     |
   |        |   HMAC   |             |   calculation at the security   |
   |        |          |             |             source.             |
   +--------+----------+-------------+---------------------------------+

                                  Table 3

3.5.  Key Considerations

   HMAC keys used with this context MUST be symmetric and MUST have a
   key length equal to the output of the HMAC.  For this reason, HMAC
   keys will be integer divisible by 8 bytes and special padding-aware
   AES key wrap algorithms are not needed.

   It is assumed that any security verifier or security acceptor
   performing an integrity verification can determine the proper HMAC
   key to be used.  Potential sources of the HMAC key include (but are
   not limited to) the following:

      Pre-placed keys selected based on local policy.

      Keys extracted from material carried in the BIB.

      Session keys negotiated via a mechanism external to the BIB.

   When an AES-KW wrapped key is present in a security block, it is
   assumed that security verifiers and security acceptors can
   independently determine the key encryption key (KEK) used in the
   wrapping of the symmetric HMAC key.

   As discussed in Section 6 and emphasized here, it is strongly
   recommended that keys be protected once generated, both when they are
   stored and when they are transmitted.

3.6.  Canonicalization Algorithms

   This section defines the canonicalization algorithm used to prepare
   the IPPT input to the BIB-HMAC-SHA2 integrity mechanism.  The
   construction of the IPPT depends on the settings of the integrity
   scope flags that may be provided as part of customizing the behavior
   of this security context.

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   In all cases, the canonical form of any portion of an extension block
   MUST be performed as described in [I-D.ietf-dtn-bpsec].  The
   canonicalization algorithms defined in [I-D.ietf-dtn-bpsec] adhere to
   the canonical forms for extension blocks defined in
   [I-D.ietf-dtn-bpbis] but resolve ambiguities related to how values
   are represented in CBOR.

   The IPPT is constructed using the following process.

   1.  The canonical form of the IPPT starts as the empty set with
       length 0.

   2.  If the integrity scope parameter is present and the primary block
       flag is set to 1, then a canonical form of the bundle's primary
       block MUST be calculated and the result appended to the IPPT.

   3.  If the integrity scope parameter is present and the target header
       flag is set to 1, then the canonical form of the block type code,
       block number, and block processing control flags associated with
       the security target MUST be calculated and, in that order,
       appended to the IPPT.

   4.  If the integrity scope parameter is present and the security
       header flag is set to 1, then the canonical form of the block
       type code, block number, and block processing control flags
       associated with the BIB MUST be calculated and, in that order,
       appended to the IPPT.

   5.  The canonical form of the security target block-type-specific
       data MUST be calculated and appended to the IPPT.

3.7.  Processing

3.7.1.  Keyed Hash Generation

   During keyed hash generation, two inputs are prepared for the the
   appropriate HMAC/SHA2 algorithm: the HMAC key and the IPPT.  These
   data items MUST be generated as follows.

      The HMAC key MUST have the appropriate length as required by local
      security policy.  The key can be generated specifically for this
      integrity service, given as part of local security policy, or
      through some other key management mechanism as discussed in
      Section 3.5.

      Prior to the generation of the IPPT, if a CRC value is present for
      the target block of the BIB, then that CRC value MUST be removed
      from the target block.  This involves both removing the CRC value

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      from the target block and setting the CRC Type field of the target
      block to "no CRC is present."

      Once CRC information is removed, the IPPT MUST be generated as
      discussed in Section 3.6.

   Upon successful hash generation the following actions MUST occur.

      The keyed hash produced by the HMAC/SHA2 variant MUST be added as
      a security result for the BIB representing the security operation
      on this security target, as discussed in Section 3.4).

   Finally, the BIB containing information about this security operation
   MUST be updated as follows.  These operations may occur in any order.

      The security context identifier for the BIB MUST be set to the
      context identifier for BIB-HMAC-SHA2.

      Any local flags used to generate the IPPT SHOULD be placed in the
      integrity scope flags security parameter for the BIB unless these
      flags are expected to be correctly configured at security
      verifiers and acceptors in the network.

      The HMAC key MAY be wrapped using the NIST AES-KW algorithm and
      the results of the wrapping added as the wrapped key security
      parameter for the BIB.

      The SHA variant used by this security context SHOULD be added as
      the SHA variant security parameter for the BIB if it differs from
      the default key length.  Otherwise, this parameter MAY be omitted
      if doing so provides a useful reduction in message sizes.

   Problems encountered in the keyed hash generation MUST be processed
   in accordance with local BPSec security policy.

3.7.2.  Keyed Hash Verification

   During keyed hash verification, the input of the security target and
   a HMAC key are provided to the appropriate HMAC/SHA2 algorithm.

   During keyed hash verification, two inputs are prepared for the
   appropriate HMAC/SHA2 algorithm: the HMAC key and the IPPT.  These
   data items MUST be generated as follows.

      The HMAC key MUST be derived using the wrapped key security
      parameter if such a parameter is included in the security context
      parameters of the BIB.  Otherwise, this key MUST be derived in

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      accordance with security policy at the verifying node as discussed
      in Section 3.5.

      The IPPT MUST be generated as discussed in Section 3.6 with the
      value of integrity scope flags being taken from the integrity
      scope flags security context parameter.  If the integrity scope
      flags parameter is not included in the security context parameters
      then these flags MAY be derived from local security policy.

   The calculated HMAC output MUST be compared to the expected HMAC
   output encoded in the security results of the BIB for the security
   target.  If the calculated HMAC and expected HMAC are identical, the
   verification MUST be considered a success.  Otherwise, the
   verification MUST be considered a failure.

   If the verification fails or otherwise experiences an error, or if
   any needed parameters are missing, then the verification MUST be
   treated as failed and processed in accordance with local security
   policy.

   This security service is removed from the bundle at the security
   acceptor as required by the BPSec specification.  If the security
   acceptor is not the bundle destination and if no other integrity
   service is being applied to the target block, then a CRC MUST be
   included for the target block.  The CRC type, as determined by
   policy, is set in the target block's CRC type field and the
   corresponding CRC value is added as the CRC field for that block.

4.  Security Context BCB-AES-GCM

4.1.  Overview

   The BCB-AES-GCM security context replaces the block-type-specific
   data field of its security target with cipher text generated using
   the Advanced Encryption Standard (AES) cipher operating in Galois/
   Counter Mode (GCM) [AES-GCM].  The use of AES-GCM was selected as the
   cipher suite for this confidentiality mechanism for several reasons:

   1.  The selection of a symmetric-key cipher suite allows for
       relatively smaller keys than asymmetric-key cipher suites.

   2.  The selection of a symmetric-key cipher suite allows this
       security context to be used in places where an asymmetric-key
       infrastructure (such as a public key infrastructure) may be
       impractical.

   3.  The use of the Galois/Counter Mode produces cipher-text with the
       same size as the plain text making the replacement of target

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       block information easier as length fields do not need to be
       changed.

   4.  The AES-GCM cipher suite provides authenticated encryption, as
       required by the BPSec protocol.

   Additionally, the BCB-AES-GCM security context generates an
   authentication tag based on the plain text value of the block-type-
   specific data and other additional authenticated data that may be
   specified via parameters to this security context.

   This security context supports two variants of AES-GCM, based on the
   supported length of the symmetric key.  These variants correspond to
   A128GCM and A256GCM as defined in [RFC8152] Table 9: Algorithm Value
   for AES-GCM.

   The BCB-AES-GCM security context MUST have the security context
   identifier specified in Section 5.1.

4.2.  Scope

   There are two scopes associated with BCB-AES-GCM: the scope of the
   confidentiality service and the scope of the authentication service.
   The first defines the set of information provided to the AES-GCM
   cipher for the purpose of producing cipher text.  The second defines
   the set of information used to generate an authentication tag.

   The scope of the confidentiality service defines the set of
   information provided to the AES-GCM cipher for the purpose of
   producing cipher text.  This MUST be the full set of plain text
   contained in the block-type-specific data field of the security
   target.

   The scope of the authentication service defines the set of
   information used to generate an authentication tag carried with the
   security block.  This information includes the data included in the
   confidentiality service and MAY include other information (additional
   authenticated data), as follows.

   Primary block
       The primary block identifies a bundle and, once created, the
       contents of this block are immutable.  Changes to the primary
       block associated with the security target indicate that the
       security target (and BCB) may no longer be in the correct bundle.

       For example, if a security target and associated BCB are copied
       from one bundle to another bundle, the BCB may still be able to

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       decrypt the security target even though these blocks were never
       intended to exist in the copied-to bundle.

       Including this information as part of additional authenticated
       data ensures that security target (and security block) appear in
       the same bundle at the time of decryption as at the time of
       encryption.

   Security target other fields
       The other fields of the security target include block
       identification and processing information.  Changing this
       information changes how the security target is treated by nodes
       in the network even when the "user data" of the security target
       are otherwise unchanged.

       For example, if the block processing control flags of a security
       target are different at a security verifier than they were
       originally set at the security source then the policy for
       handling the security target has been modified.

       Including this information as part of additional authenticated
       data ensures that the cipher text in the security target will not
       be used with a different set of block policy than originally set
       at the time of encryption.

   BCB other fields
       The other fields of the BCB include block identification and
       processing information.  Changing this information changes how
       the BCB is treated by nodes in the network, even when other
       aspects of the BCB are unchanged.

       For example, if the block processing control flags of the BCB are
       different at a security acceptor than they were originally set at
       the security source then the policy for handling the BCB has been
       modified.

       Including this information as part of additional authenticated
       data ensures that the policy and identification of the security
       service in the bundle has not changed.

       NOTE: The security context identifier and security context
       parameters of the security block are not included as additional
       authenticated data because these parameters, by definition, are
       those needed to verify or accept the security service.
       Therefore, it is expected that changes to these values would
       result in failures at security verifiers and security acceptors.

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   The scope of the BCB-AES-GCM security context is configured using an
   optional security context parameter.

4.3.  Parameters

   BCB-AES-GCM can be parameterized to specify the AES variant,
   initialization vector, key information, and identify additional
   authenticated data.

4.3.1.  Initialization Vector (IV)

   This optional parameter identifies the initialization vector (IV)
   used to initialize the AES-GCM cipher.

   The length of the initialization vector, prior to any CBOR encoding,
   MUST be between 8-16 bytes.  A value of 12 bytes SHOULD be used
   unless local security policy requires a different length.

   This value MUST be encoded as a CBOR byte string.

   The initialization vector may have any value with the caveat that a
   value MUST NOT be re-used for multiple encryptions using the same
   encryption key.  This value MAY be re-used when encrypting with
   different keys.  For example, if each encryption operation using BCB-
   AES-GCM uses a newly generated key, then the same IV may be reused.

4.3.2.  AES Variant

   This optional parameter identifies the AES variant being used for the
   AES-GCM encryption, where the variant is identified by the length of
   key used.

   This value MUST be encoded as a CBOR unsigned integer.

   Valid values for this parameter are as follows.

                       AES Variant Parameter Values

   +-------+-----------------------------------------------------------+
   | Value |                        Description                        |
   +-------+-----------------------------------------------------------+
   |   1   | A128GCM as defined in [RFC8152] Table 9: Algorithm Values |
   |       |                        for AES-GCM                        |
   |   3   | A256GCM as defined in [RFC8152] Table 9: Algorithm Values |
   |       |                        for AES-GCM                        |
   +-------+-----------------------------------------------------------+

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   When not provided, implementations SHOULD assume a value of 3
   (indicating use of A256GCM), unless an alternate default is
   established by local security policy at the security source,
   verifier, or acceptor of this integrity service.

   Regardless of the variant, the generated authentication tag MUST
   always be 128 bits.

4.3.3.  Wrapped Key

   This optional parameter contains the output of the AES key wrap
   authenticated encryption function (KW-AE) as defined in [AES-KW].
   Specifically, this parameter holds the cipher text produced when
   running the KW-AE algorithm with the input string being the symmetric
   AES key used to generate the security results present in the security
   block.  The value of this parameter is used as input to the AES key
   wrap authenticated decryption function (KW-AD) at security verifiers
   and security acceptors to determine the symmetric AES key needed for
   the proper decryption of the security results in the security block.

   This value MUST be encoded as a CBOR byte string.

   If this parameter is not present then security verifiers and
   acceptors MUST determine the proper key as a function of their local
   BPSec policy and configuration.

4.3.4.  AAD Scope Flags

   This optional parameter contains a series of flags that describe what
   information is to be included with the block-type-specific data of
   the security target as part of additional authenticated data (AAD).

   This value MUST be represented as a CBOR unsigned integer, the value
   of which MUST be processed as a bit field containing no more than 8
   bits.

   Bits in this field represent additional information to be included
   when generating an integrity signature over the security target.
   These bits are defined as follows.

      - Bit 0 (the low-order bit, 0x1): Primary Block Flag.

      - Bit 1 (0x02): Target Header Flag.

      - Bit 2 (0x03): Security Header Flag.

      - Bits 3-7 are reserved.

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4.3.5.  Enumerations

   BCB-AES-GCM defines the following security context parameters.

                      BCB-AES-GCM Security Parameters

    +----+-----------------------+--------------------+---------------+
    | Id |          Name         | CBOR Encoding Type | Default Value |
    +----+-----------------------+--------------------+---------------+
    | 1  | Initialization Vector |    Byte String     |      NONE     |
    | 2  |      AES Variant      |        UINT        |       3       |
    | 3  |      Wrapped Key      |    Byte String     |      NONE     |
    | 4  |    AAD Scope Flags    |        UINT        |      0x7      |
    +----+-----------------------+--------------------+---------------+

                                  Table 4

4.4.  Results

   The BCB-AES-GCM security context produces a single security result
   carried in the security block: the authentication tag.

   NOTES:

      The cipher text generated by the cipher suite is not considered a
      security result as it is stored in the block-type-specific data
      field of the security target block.  When operating in GCM mode,
      AES produces cipher text of the same size as its plain text and,
      therefore, no additional logic is required to handle padding or
      overflow caused by the encryption in most cases (see below).

      If the generated cipher text contains the authentication tag and
      the tag can be separated from the cipher text then the tag MUST be
      separated and stored in the authentication tag security result
      field.

      If the generated cipher text contains the authentication tag and
      the tag cannot be separated from the cipher text then the tag MUST
      NOT be included in the authentication tag security result field.
      Instead the security target block MUST be resized to accommodate
      the additional 128 bits of authentication tag included in the
      generated cipher text.

4.4.1.  Authentication Tag

   The authentication tag is generated by the cipher suite over the
   security target plain text input to the cipher suite as combined with
   any optional additional authenticated data.  This tag is used to

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   ensure that the plain text (and important information associated with
   the plain text) is authenticated prior to decryption.

   If the authentication tag is included in the cipher text placed in
   the security target block-type-specific data field, then this
   security result MUST NOT be included in the BCB for that security
   target.

   The length of the authentication tag, prior to any CBOR encoding,
   MUST be 128 bits.

   This value MUST be encoded as a CBOR byte string.

4.4.2.  Enumerations

   BCB-AES-GCM defines the following security context parameters.

                       BCB-AES-GCM Security Results

          +-----------+--------------------+--------------------+
          | Result Id |    Result Name     | CBOR Encoding Type |
          +-----------+--------------------+--------------------+
          |     1     | Authentication Tag |    Byte String     |
          +-----------+--------------------+--------------------+

                                  Table 5

4.5.  Key Considerations

   Keys used with this context MUST be symmetric and MUST have a key
   length equal to the key length defined in the security context
   parameters or as defined by local security policy at security
   verifiers and acceptors.  For this reason, content-encrypting keys
   will be integer divisible by 8 bytes and special padding-aware AES
   key wrap algorithms are not needed.

   It is assumed that any security verifier or security acceptor can
   determine the proper key to be used.  Potential sources of the key
   include (but are not limited to) the following.

      Pre-placed keys selected based on local policy.

      Keys extracted from material carried in the BCB.

      Session keys negotiated via a mechanism external to the BCB.

   When an AES-KW wrapped key is present in a security block, it is
   assumed that security verifiers and security acceptors can

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   independently determine the key encryption key (KEK) used in the
   wrapping of the symmetric AES content-encrypting key.

   The security provided by block ciphers is reduced as more data is
   processed with the same key.  The total number of bytes processed
   with a single key for AES-GCM is recommended to be less than 2^64, as
   described in Appendix B of [AES-GCM].

   As discussed in Section 6 and emphasized here, it is strongly
   recommended that keys be protected once generated, both when they are
   stored and when they are transmitted.

4.6.  GCM Considerations

   The GCM cryptographic mode of AES has specific requirements that MUST
   be followed by implementers for the secure function of the BCB-AES-
   GCM security context.  While these requirements are well documented
   in [AES-GCM], some of them are repeated here for emphasis.

      The pairing of an IV and a security key MUST be unique.  An IV
      MUST NOT be used with a security key more than one time.  If an IV
      and key pair are repeated then the GCM implementation may be
      vulnerable to forgery attacks.  More information regarding the
      importance of the uniqueness of the IV value can be found in
      Appendix A of [AES-GCM].

      While any tag-based authentication mechanism has some likelihood
      of being forged, this probability is increased when using AES-GCM.
      In particular, short tag lengths combined with very long messages
      SHOULD be avoided when using this mode.  The BCB-AES-GCM security
      context requires the use of 128-bit authentication tags at all
      times.  Concerns relating to the size of authentication tags is
      discussed in Appendices B and C of [AES-GCM].

      As discussed in Appendix B of [AES-GCM], implementations SHOULD
      limit the number of unsuccessful verification attempts for each
      key to reduce the likelihood of guessing tag values.

      As discussed in the Security Considerations section of
      [I-D.ietf-dtn-bpsec], delay-tolerant networks may have a higher
      occurrence of replay attacks due to the store-and-forward nature
      of the network.  Because GCM has no inherent replay attack
      protection, implementors SHOULD attempt to detect replay attacks
      by using mechanisms such as those described in Appendix D of
      [AES-GCM].

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4.7.  Canonicalization Algorithms

   This section defines the canonicalization algorithms used to prepare
   the inputs used to generate both the cipher text and the
   authentication tag.

   In all cases, the canonical form of any portion of an extension block
   MUST be performed as described in [I-D.ietf-dtn-bpsec].  The
   canonicalization algorithms defined in [I-D.ietf-dtn-bpsec] adhere to
   the canonical forms for extension blocks defined in
   [I-D.ietf-dtn-bpbis] but resolve ambiguities related to how values
   are represented in CBOR.

4.7.1.  Cipher text related calculations

   The plain text used during encryption MUST be calculated as the
   single, definite-length CBOR byte string representing the block-type-
   specific data field of the security target excluding the CBOR byte
   string identifying byte and optional CBOR byte string length field.

   For example, consider the following two CBOR byte strings and the
   plain text that would be extracted from them.

                         CBOR Byte String Examples

   +------------------------------+---------+--------------------------+
   |    CBOR Byte String (Hex)    |   CBOR  |  Plain Text Part (Hex)   |
   |                              |   Part  |                          |
   |                              |  (Hex)  |                          |
   +------------------------------+---------+--------------------------+
   |             18ED             |    18   |            ED            |
   +------------------------------+---------+--------------------------+
   | C24CDEADBEEFDEADBEEFDEADBEEF |   C24C  | DEADBEEFDEADBEEFDEADBEEF |
   +------------------------------+---------+--------------------------+

                                  Table 6

   Similarly, the cipher text used during decryption MUST be calculated
   as the single, definite-length CBOR byte string representing the
   block-type-specific data field excluding the CBOR byte string
   identifying byte and optional CBOR byte string length field.

   All other fields of the security target (such as the block type code,
   block number, block processing control flags, or any CRC information)
   MUST NOT be considered as part of encryption or decryption.

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4.7.2.  Additional Authenticated Data

   The construction of additional authenticated data depends on the AAD
   scope flags that may be provided as part of customizing the behavior
   of this security context.

   The canonical form of the AAD input to the BCB-AES-GCM mechanism is
   constructed using the following process.  This process MUST be
   followed when generating AAD for either encryption or decryption.

   1.  The canonical form of the AAD starts as the empty set with length
       0.

   2.  If the AAD scope parameter is present and the primary block flag
       is set to 1, then a canonical form of the bundle's primary block
       MUST be calculated and the result appended to the AAD.

   3.  If the AAD scope parameter is present and the target header flag
       is set to 1, then the canonical form of the block type code,
       block number, and block processing control flags associated with
       the security target MUST be calculated and, in that order,
       appended to the AAD.

   4.  If the AAD scope parameter is present and the security header
       flag is set to 1, then the canonical form of the block type code,
       block number, and block processing control flags associated with
       the BIB MUST be calculated and, in that order, appended to the
       AAD.

   If, after this process, the AAD remains at length 0, then no AAD
   exists to be input to the cipher suite.

4.8.  Processing

4.8.1.  Encryption

   During encryption, four inputs are prepared for input to the AES/GCM
   cipher: the encryption key, the IV, the security target plain text to
   be encrypted, and any additional authenticated data.  These data
   items MUST be generated as follows.

   Prior to encryption, if a CRC value is present for the target block,
   then that CRC value MUST be removed.  This requires removing the CRC
   field from the target block and setting the CRC type field of the
   target block to "no CRC is present."

      The encryption key MUST have the appropriate length as required by
      local security policy.  The key may be generated specifically for

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      this encryption, given as part of local security policy, or
      through some other key management mechanism as discussed in
      Section 4.5.

      The IV selected MUST be of the appropriate length.  Because
      replaying an IV in counter mode voids the confidentiality of all
      messages encrypted with said IV, this context also requires a
      unique IV for every encryption performed with the same key.  This
      means the same key and IV combination MUST NOT be used more than
      once.

      The security target plain text for encryption MUST be generated as
      discussed in Section 4.7.1.

      Additional authenticated data, if present, MUST be generated as
      discussed in Section 4.7.2 with the value of AAD scope flags being
      taken from local security policy.

   Upon successful encryption the following actions MUST occur.

      The cipher text produced by AES/GCM MUST replace the bytes used to
      define the plain text in the security target block's block-type-
      specific data field.  The block length of the security target MUST
      be updated if the generated cipher text is larger than the plain
      text (which can occur when the authentication tag is included in
      the cipher text calculation, as discussed in Section 4.4).

      The authentication tag calculated by the AES/GCM cipher MUST be
      added as a security result for the security target in the BCB
      holding results for this security operation.

      Cases where the authentication tag is generated as part of the
      cipher text MUST be processed as described in Section 4.4.

   Finally, the BCB containing information about this security operation
   MUST be updated as follows.  These operations may occur in any order.

      The security context identifier for the BCB MUST be set to the
      context identifier for BCB-AES-GCM.

      The IV input to the cipher MUST be added as the IV security
      parameter for the BCB.

      Any local flags used to generated AAD for this cipher MUST be
      added as the AAD scope flags security parameter for the BCB.

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      The encryption key MAY be wrapped using the NIST AES-KW algorithm
      and the results of the wrapping added as the wrapped key security
      parameter for the BCB.

      The key length used by this security context MUST be considered
      when setting the AES variant security parameter for the BCB if it
      differs from the default AES variant.  Otherwise, the AES variant
      MAY be omitted if doing so provides a useful reduction in message
      sizes.

   Problems encountered in the encryption MUST be processed in
   accordance with local security policy.  This MAY include restoring a
   CRC value removed from the target block prior to encryption, if the
   target block is allowed to be transmitted after an encryption error.

4.8.2.  Decryption

   During encryption, five inputs are prepared for input to the AES/GCM
   cipher: the decryption key, the IV, the security target cipher text
   to be decrypted, any additional authenticated data, and the
   authentication tag generated from the original encryption.  These
   data items MUST be generated as follows.

      The decryption key MUST be derived using the wrapped key security
      parameter if such a parameter is included in the security context
      parameters of the BCB.  Otherwise this key MUST be derived in
      accordance with local security policy at the decrypting node as
      discussed in Section 4.5.

      The IV MUST be set to the value of the IV security parameter
      included in the BCB.  If the IV parameter is not included as a
      security parameter, an IV MAY be derived as a function of local
      security policy and other BCB contents or a lack of an IV security
      parameter in the BCB MAY be treated as an error by the decrypting
      node.

      The security target cipher text for decryption MUST be generated
      as discussed in Section 4.7.1.

      Additional authenticated data, if present, MUST be generated as
      discussed in Section 4.7.2 with the value of AAD scope flags being
      taken from the AAD scope flags security context parameter.  If the
      AAD scope flags parameter is not included in the security context
      parameters then these flags MAY be derived from local security
      policy in cases where the set of such flags is determinable in the
      network.

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      The authentication tag MUST be present in the BCB security context
      parameters field if additional authenticated data are defined for
      the BCB (either in the AAD scope flags parameter or as specified
      by local policy).  This tag MUST be 128 bits in length.

   Upon successful decryption the following actions MUST occur.

      The plain text produced by AES/GCM MUST replace the bytes used to
      define the cipher text in the security target block's block-type-
      specific data field.  Any changes to the security target block
      length field MUST be corrected in cases where the plain text has a
      different length than the replaced cipher text.

   If the security acceptor is not the bundle destination and if no
   other integrity or confidentiality service is being applied to the
   target block, then a CRC MUST be included for the target block.  The
   CRC type, as determined by policy, is set in the target block's CRC
   type field and the corresponding CRC value is added as the CRC field
   for that block.

   If the cipher text fails to authenticate, if any needed parameters
   are missing, or if there are other problems in the decryption then
   the decryption MUST be treated as failed and processed in accordance
   with local security policy.

5.  IANA Considerations

5.1.  Security Context Identifiers

   This specification allocates two security context identifiers from
   the "BPSec Security Context Identifier" registry defined in
   [I-D.ietf-dtn-bpsec].

       Additional Entries for the BPSec Security Context Identifiers
                                 Registry:

                 +-------+---------------+---------------+
                 | Value |  Description  |   Reference   |
                 +-------+---------------+---------------+
                 |  TBA  | BIB-HMAC-SHA2 | This document |
                 |  TBA  |  BCB-AES-GCM  | This document |
                 +-------+---------------+---------------+

                                  Table 7

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6.  Security Considerations

   Security considerations specific to a single security context are
   provided in the description of that context.  This section discusses
   security considerations that should be evaluated by implementers of
   any security context described in this document.  Considerations may
   also be found in documents listed as normative references and they
   should also be reviewed by security context implementors.

6.1.  Key Handling

   In addition to the key considerations listed in each security
   context, the following also apply to the generation, transmission,
   and use of keys associated with all of the security contexts defined
   in this document.

      It is strongly RECOMMENDED that implementations protect keys both
      when they are stored and when they are transmitted.

      In the event that a key is compromised, any security operations
      using a security context associated with that key SHOULD also be
      considered compromised.  This means that the BIB-HMAC-SHA2
      security context SHOULD NOT provide integrity when used with a
      compromised key and BCB-AES-GCM SHOULD NOT provide confidentiality
      when used with a compromised key.

      The same key SHOULD NOT be used for different algorithms as doing
      so may leak information about the key.

      Unless otherwise specified, the security contexts provided in this
      document do not mandate any specific method for key exchange,
      encryption, or encapsulation.  The derivation of an appropriate
      key is considered separate from the application of the
      authenticated confidentiality service provided by this context.

6.2.  AES GCM

   There are a significant number of considerations related to the use
   of the GCM mode of AES to provide a confidentiality service.  These
   considerations are provided in Section 4.6 as part of the
   documentation of the BCB-AES-GCM security context.

6.3.  Bundle Fragmentation

   Bundle fragmentation may prevent security services in a bundle from
   being verified after a bundle is fragmented and before the bundle is
   re-assembled.  Examples of potential issues include the following.

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      If a security block and its security target do not exist in the
      same fragment, then the security block cannot be processed until
      the bundle is re-assembled.  If a fragment includes an encrypted
      target block, but not its BCB, then a receiving bundle processing
      agent (BPA) will not know that the target block has been
      encrypted.

      If a security block is cryptographically bound to a bundle, it
      cannot be processed even if the security block and target both
      coexist in the fragment.  This is because fragments have different
      primary blocks than the original bundle.

      If security blocks and their target blocks are repeated in
      multiple fragments, policy must determine how to deal with issues
      where a security operation verifies in one fragment but fails in
      another fragment.  This may happen, for example, if a BIB block
      becomes corrupted in one fragment but not in another fragment.

   Implementors should consider how security blocks are processed when a
   BPA fragments a received bundle.  For example, security blocks and
   their targets could be placed in the same fragment if the security
   block is not otherwise cryptographically bound to the bundle being
   fragmented.  Alternatively, if security blocks are cryptographically
   bound to a bundle, then a fragmenting BPA should consider
   encapsulating the bundle first and then fragmenting the encapsulating
   bundle.

7.  Normative References

   [AES-GCM]  Dworkin, M., "NIST Special Publication 800-38D:
              Recommendation for Block Cipher Modes of Operation:
              Galois/Counter Mode (GCM) and GMAC.", November 2007.

   [AES-KW]   Dworkin, M., "NIST Special Publication 800-38F:
              Recommendation for Block Cipher Modes of Operation:
              Methods for Key Wrapping.", December 2012.

   [HMAC]     US NIST, "The Keyed-Hash Message Authentication Code
              (HMAC).", FIPS-198-1, Gaithersburg, MD, USA, July 2008.

              https://csrc.nist.gov/publications/detail/fips/198/1/final

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
              Version 7", draft-ietf-dtn-bpbis-31 (work in progress),
              January 2021.

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   [I-D.ietf-dtn-bpsec]
              Birrane, E. and K. McKeever, "Bundle Protocol Security
              Specification", draft-ietf-dtn-bpsec-27 (work in
              progress), February 2021.

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

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

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

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [SHS]      US NIST, "Secure Hash Standard (SHS).", FIPS-
              180-4, Gaithersburg, MD, USA, August 2015.

              https://csrc.nist.gov/publications/detail/fips/180/4/final

Appendix A.  Acknowledgements

   The following participants contributed useful review and analysis of
   these security contexts: Amy Alford and Sarah Heiner of the Johns
   Hopkins University Applied Physics Laboratory.

Author's Address

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

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

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