wdiff rfc9172v4.txt rfc9172.txt

Internet Engineering Task Force (IETF) E. Birrane, III
Request for Comments: 9172 K. McKeever
Category: Standards Track JHU/APL
ISSN: 2070-1721 November 2021 January 2022

Bundle Protocol Security (BPSec)

Abstract

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

Status of This Memo

This is an Internet Standards Track document.

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

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9172.

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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Supported Security Services
1.2. Specification Scope
1.3. Related Documents
1.4. Terminology
2. Design Decisions
2.1. Block-Level Granularity
2.2. Multiple Security Sources
2.3. Mixed Security Policy
2.4. User-Defined Security Contexts
2.5. Deterministic Processing
3. Security Blocks
3.1. Block Definitions
3.2. Uniqueness
3.3. Target Multiplicity
3.4. Target Identification
3.5. Block Representation
3.6. Abstract Security Block
3.7. Block Integrity Block
3.8. Block Confidentiality Block
3.9. Block Interactions
3.10. Parameter and Result Identification
3.11. BPSec Block Examples
3.11.1. Example 1: Constructing a Bundle with Security
3.11.2. Example 2: Adding More Security at a New Node
4. Canonical Forms
5. Security Processing
5.1. Bundles Received from Other Nodes
5.1.1. Receiving BCBs
5.1.2. Receiving BIBs
5.2. Bundle Fragmentation and Reassembly
6. Key Management
7. Security Policy Considerations
7.1. Security Reason Codes
8. Security Considerations
8.1. Attacker Capabilities and Objectives
8.2. Attacker Behaviors and BPSec Mitigations
8.2.1. Eavesdropping Attacks
8.2.2. Modification Attacks
8.2.3. Topology Attacks
8.2.4. Message Injection
9. Security Context Considerations
9.1. Mandating Security Contexts
9.2. Identification and Configuration
9.3. Authorship
10. Defining Other Security Blocks
11. IANA Considerations
11.1. Bundle Block Types
11.2. Bundle Status Report Reason Codes
11.3. Security Context Identifiers
12. References
12.1. Normative References
12.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

This document defines security features for the Bundle Protocol (BP)
[RFC9171] and is intended for use in Delay-Tolerant Networking (DTN)
to provide security services between a security source and a security
acceptor. When the security source is the bundle source and the
security acceptor is the bundle destination, the security service
provides end-to-end protection.

The Bundle Protocol specification [RFC9171] defines DTN as referring
to "a network architecture providing communications in and/or through
highly stressed environments" where "BP may be viewed as sitting at
the application layer of some number of constituent networks, forming
a store-carry-forward overlay network". The phrase "stressed
environment" refers to multiple challenging conditions including
intermittent connectivity, large and/or variable delays, asymmetric
data rates, and high bit error rates.

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

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

1.1. Supported Security Services

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

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

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

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

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

2. Hop-by-hop authentication cannot be deployed in a network if
adjacent nodes in the network have incompatible security
capabilities.

1.2. Specification Scope

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

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

Completely trusted networks are extremely uncommon. Among untrusted
networks, different networking conditions and operational
considerations require security mechanisms of varying strengths.
Mandating a single security context, which is a set of assumptions,
algorithms, configurations, and policies used to implement security
services, may result in too much security for some networks and too
little security in others. Default security contexts are defined in
[RFC9173] to provide basic security services for interoperability
testing and for operational use on the terrestrial Internet. It is
expected that separate documents will define different security
contexts for use in different networks.

This specification addresses neither the fitness of externally
defined cryptographic methods nor the security of their
implementation.

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

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

1.3. Related Documents

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

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

* "Bundle Protocol Version 7" [RFC9171] defines the format and
processing of bundles, the extension block format used to
represent BPSec security blocks, and the canonical block structure
used by this specification.

* "Concise Binary Object Representation (CBOR)" [RFC8949] defines a
data format that allows for small code size, fairly small message
size, and extensibility without version negotiation. The block-
type-specific data associated with BPSec security blocks is
encoded in this data format.

* "Bundle Security Protocol Specification" [RFC6257] introduces the
concept of using BP extension blocks for security services in DTN.
BPSec is a continuation and refinement of this document.

1.4. Terminology

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

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

Bundle Destination: the Bundle Protocol Agent (BPA) that receives a
bundle and delivers the payload of the bundle to an Application
Agent. Also, an endpoint comprising the node(s) at which the
bundle is to be delivered. The bundle destination acts as the
security acceptor for every security target in every security
block in every bundle it receives.

Bundle Source: the BPA that originates a bundle. Also, any node ID
of the node of which the BPA is a component.

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

Forwarder: any BPA that transmits a bundle in DTN. Also, any node
ID of the node of which the BPA that sent the bundle on its most
recent hop is a component.

Intermediate Receiver, Waypoint, or Next Hop: any BPA that receives
a bundle from a forwarder that is not the bundle destination.
Also, any node ID of the node of which the BPA is a component.

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

Security Acceptor: a BPA that processes and dispositions one or more
security blocks in a bundle. Security acceptors act as the
endpoint of a security service represented in a security block.
They remove the security blocks they act upon as part of
processing and disposition. Also, any node ID of the node of
which the BPA is a component.

Security Block: a BPSec extension block in a bundle.

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

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

Security Service: a process that gives some protection to a security
target. For example, this specification defines security services
for plaintext integrity (bib-integrity) and authenticated
plaintext confidentiality with additional authenticated data (bcb-
confidentiality).

Security Source: a BPA that adds a security block to a bundle.
Also, any node ID of the node of which the BPA is a component.

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

Security Verifier: a BPA that verifies the data integrity of one or
more security blocks in a bundle. Unlike security acceptors,
security verifiers do not act as the endpoint of a security
service, and they do not remove verified security blocks. Also,
any node ID of the node of which the BPA is a component.

2. Design Decisions

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

2.1. Block-Level Granularity

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

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

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

2.2. Multiple Security Sources

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

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

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

For example, a bundle source may choose to apply an integrity service
to its plaintext payload. Later a waypoint node, representing a
gateway to another portion of the delay-tolerant network, may receive
the bundle and choose to apply a confidentiality service. In this
case, the integrity security source is the bundle source and the
confidentiality security source is the waypoint node.

In cases where the security source and security acceptor are not the
bundle source and bundle destination, respectively, it is possible
that the bundle will reach the bundle destination prior to reaching a
security acceptor. In cases where this may be a practical problem,
it is recommended that solutions such as bundle encapsulation be used
to ensure that a bundle be delivered to a security acceptor prior to
being delivered to the bundle destination. Generally, if a bundle
reaches a waypoint that has the appropriate configuration and policy
to act as a security acceptor for a security service in the bundle,
then the waypoint should act as that security acceptor.

2.3. Mixed Security Policy

The security policy enforced by nodes in the delay-tolerant network
may differ.

Some waypoints will have security policies that require the waypoint
to evaluate security services even if the waypoint is neither the
bundle destination nor the final intended acceptor of the service.
For example, a waypoint could choose to verify an integrity service
even though the waypoint is not the bundle destination and the
integrity service will be needed by other nodes along the bundle's
path.

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

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

2.4. User-Defined Security Contexts

A security context is the set of assumptions, algorithms,
configurations, and policies used to implement security services.
Different contexts may specify different algorithms, different
polices, or different configuration values used in the implementation
of their security services. BPSec provides a mechanism to define
security contexts. Users may select from registered security
contexts and customize those contexts through security context
parameters.

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

2.5. Deterministic Processing

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

3. Security Blocks

3.1. Block Definitions

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

* The BIB is used to ensure the integrity of its plaintext security
target(s). The integrity information in the BIB MAY be verified
by any node along the bundle path from the BIB security source to
the bundle destination. Waypoints add or remove BIBs from bundles
in accordance with their security policy. BIBs are never used for
integrity protection of the ciphertext provided by a BCB. Because
security policy at BPSec nodes may differ regarding integrity
verification, BIBs do not guarantee hop-by-hop authentication, as
discussed in Section 1.1.

* The BCB indicates that the security target or targets have been
encrypted at the BCB security source in order to protect their
content while in transit. As a matter of security policy, the BCB
is decrypted by security acceptor nodes in the network, up to and
including the bundle destination. BCBs additionally provide
integrity-protection mechanisms for the ciphertext they generate.

3.2. Uniqueness

Security operations in a bundle MUST be unique; the same security
service MUST NOT be applied to a security target more than once in a
bundle. Since a security operation is represented by a security
block, this means that multiple security blocks of the same type
cannot share the same security targets. A new security block MUST
NOT be added to a bundle if a preexisting security block of the same
type is already defined for the security target of the new security
block.

This uniqueness requirement ensures that there is no ambiguity
related to the order in which security blocks are processed or how
security policy can be specified to require certain security services
be present in a bundle.

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

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

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

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

Different services from different block types: The notation
OP(service, target) refers specifically to a security block, as
the security block is the embodiment of a security service applied
to a security target in a bundle. Were some Other Security Block
(OSB) to be defined providing an integrity service, then the
operations OP(bib-integrity, target) and OP(osb-integrity, target)
MAY both be present in the same bundle if so allowed by the
definition of the OSB, as discussed in Section 10.

NOTES:

* A security block may be removed from a bundle as part of security
processing at a waypoint node with a new security block being
added to the bundle by that node. In this case, conflicting
security blocks never coexist in the bundle at the same time and
the uniqueness requirement is not violated.

* A ciphertext integrity-protection mechanism (such as associated
authenticated data) calculated by a cipher suite and transported
in a BCB is considered part of the confidentiality service;
therefore, it is unique from the plaintext integrity service
provided by a BIB.

* The security blocks defined in this specification (BIB and BCB)
are designed with the intention that the BPA adding these blocks
is the authoritative source of the security service. If a BPA
adds a BIB on a security target, then the BIB is expected to be
the authoritative source of integrity for that security target.
If a BPA adds a BCB to a security target, then the BCB is expected
to be the authoritative source of confidentiality for that
security target. More complex scenarios, such as having multiple
nodes in a network sign the same security target, can be
accommodated using the definition of custom security contexts (see
Section 9) and/or the definition of OSBs (see Section 10).

3.3. Target Multiplicity

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

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

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

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

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

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

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

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

If a node processes any security operation in a security block, it is
RECOMMENDED that it process all security operations in the security
block. This allows security sources to assert that the set of
security operations in a security block are expected to be processed
by the same security acceptor. However, the determination of whether
a node actually is a security acceptor or not is a matter of the
policy of the node itself. In cases where a receiving node
determines that it is the security acceptor of only a subset of the
security operations in a security block, the node may choose to only
process that subset of security operations.

3.4. Target Identification

A security target is a block in the bundle to which a security
service applies. This target must be uniquely and unambiguously
identifiable when processing a security block. The definition of the
extension block header from [RFC9171] provides a "block number" field
suitable for this purpose. Therefore, a security target in a
security block MUST be represented as the block number of the target
block.

3.5. Block Representation

Each security block uses the Canonical Bundle Block Format as defined
in [RFC9171]. That is, each security block is comprised of the
following elements:

* block type code
* block number
* block processing control flags
* cyclic redundancy check (CRC) type
* block-type-specific data
* CRC field (if present)

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

3.6. Abstract Security Block

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

The fields of the ASB SHALL be as follows, listed in the order in
which they must appear. The encoding of these fields MUST be in
accordance with the canonical forms provided in Section 4.

Security Targets:
This field identifies the block(s) targeted by the security
operation(s) represented by this security block. Each target
block is represented by its unique block number. This field
SHALL be represented by a Concise Binary Object Representation
(CBOR) array of data items. Each target within this CBOR array
SHALL be represented by a CBOR unsigned integer. This array
MUST have at least one entry and each entry MUST represent the
block number of a block that exists in the bundle. There MUST
NOT be duplicate entries in this array. The order of elements
in this list has no semantic meaning outside of the context of
this block. Within the block, the ordering of targets must
match the ordering of results associated with these targets.

Security Context Id:
This field identifies the security context used to implement
the security service represented by this block and applied to
each security target. This field SHALL be represented by a
CBOR unsigned integer. The values for this Id should come from
the registry defined in Section 11.3.

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

Bit 0 (the least-significant bit, 0x01): "Security context
parameters present" flag.

Bit >0 Reserved

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

Security Source:
This field identifies the BPA that inserted the security block
in the bundle. Also, any node ID of the node of which the BPA
is a component. This field SHALL be represented by a CBOR
array in accordance with the rules in [RFC9171] for
representing endpoint IDs (EIDs).

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

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

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

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

+----------------+----------------+ +----------------+
| Parameter 1 | Parameter 2 | ... | Parameter N |
+------+---------+------+---------+ +------+---------+
| Id | Value | Id | Value | | Id | Value |
+------+---------+------+---------+ +------+---------+

Figure 1: Security Context Parameters

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

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

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

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

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

+--------------------------+ +---------------------------+
| Target 1 | | Target N |
+----------+----+----------+ +---------------------------+
| Result 1 | | Result M | ... | Result 1 | | Result K |
+----+-----+ .. +----+-----+ +---+------+ .. +----+------+
| Id |Value| | Id |Value| | Id |Value| | Id | Value|
+----+-----+ +----+-----+ +----+-----+ +----+------+

Figure 2: Security Results

3.7. Block Integrity Block

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

* The block type code value is as specified in Section 11.1.

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

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

* The security context MUST utilize an authentication mechanism or
an error detection mechanism.

Notes:

* Designers SHOULD carefully consider the effect of setting flags
that either discard the block or delete the bundle in the event
that this block cannot be processed.

* Since OP(bib-integrity, target) is allowed only once in a bundle
per target, it is RECOMMENDED that users wishing to support
multiple integrity-protection mechanisms for the same target
define a multi-result security context. Such a context could
generate multiple security results for the same security target
using different integrity-protection mechanisms or different
configurations for the same integrity-protection mechanism.

* A BIB is used to verify the plaintext integrity of its security
target. However, a single BIB MAY include security results for
blocks other than its security target when doing so establishes a
needed relationship between the BIB security target and other
blocks in the bundle (such as the primary block).

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

3.8. Block Confidentiality Block

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

* The block type code value is as specified in Section 11.1.

* The block processing control flags value can be set to whatever
values are required by local policy with the following exceptions:

- BCBs MUST have the "Block must be replicated in every fragment"
flag set if one of the targets is the payload block. Having
that BCB in each fragment indicates to a receiving node that
the payload portion of each fragment represents ciphertext.

- BCBs MUST NOT have the "Block must be removed from bundle if it
can't be processed" flag set. Removing a BCB from a bundle
without decrypting its security targets removes information
from the bundle necessary for their later decryption.

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

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

* Any security context used by a BCB MUST utilize a confidentiality
cipher that provides authenticated encryption with associated data
(AEAD).

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

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

Notes:

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

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

3.9. Block Interactions

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

If a BCB and a BIB share a security target, an undesirable condition
occurs: a waypoint would be unable to validate the BIB because the
shared security target has been encrypted by the BCB. To address
this situation, the following processing rules MUST be followed:

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

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

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

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

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

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

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

In cases where a security source wishes to calculate both a plaintext
integrity-protection mechanism and encrypt a security target, a BCB
with a security context that generates an integrity-protection
mechanism as one or more additional security results MUST be used
instead of adding both a BIB and then a BCB for the security target
at the security source.

3.10. Parameter and Result Identification

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

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

An example of a security context can be found in [RFC9173].

3.11. BPSec Block Examples

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

3.11.1. Example 1: Constructing a Bundle with Security

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

Block in Bundle ID
+==========================================+====+
| Primary Block | B1 |
+------------------------------------------+----+
| BIB | B2 |
| OP(bib-integrity, targets = B1, B5, B6)| |
+------------------------------------------+----+
| BCB | B3 |
| OP(bcb-confidentiality, target = B4) | |
+------------------------------------------+----+
| Extension Block (encrypted) | B4 |
+------------------------------------------+----+
| Extension Block | B5 |
+------------------------------------------+----+
| Payload Block | B6 |
+------------------------------------------+----+

Figure 3: Security at Bundle Creation

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

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

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

3.11.2. Example 2: Adding More Security at a New Node

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

The resultant bundle is illustrated in Figure 4 and the security
actions are described below. Note that block IDs provided here are
ordered solely for the purpose of this example and are not meant to
impose an ordering for block creation. The ordering of blocks added
to a bundle MUST always be in compliance with [RFC9171].

Block in Bundle ID
+==========================================+====+
| Primary Block | B1 |
+------------------------------------------+----+
| BIB | B2 |
| OP(bib-integrity, target = B1) | |
+------------------------------------------+----+
| BIB (encrypted) | B7 |
| OP(bib-integrity, targets = B5, B6) | |
+------------------------------------------+----+
| BCB | B8 |
|OP(bcb-confidentiality,targets = B5,B6,B7)| |
+------------------------------------------+----+
| BCB | B3 |
| OP(bcb-confidentiality, target = B4) | |
+------------------------------------------+----+
| Extension Block (encrypted) | B4 |
+------------------------------------------+----+
| Extension Block (encrypted) | B5 |
+------------------------------------------+----+
| Payload Block (encrypted) | B6 |
+------------------------------------------+----+

Figure 4: Security at Bundle Forwarding

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

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

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

4. Canonical Forms

Security services require consistency and determinism in how
information is presented to cipher suites at security sources,
verifiers, and acceptors. For example, integrity services require
that the same target information (e.g., the same bits in the same
order) is provided to the cipher suite when generating an original
signature and when validating a signature. Canonicalization
algorithms transcode the contents of a security target into a
canonical form.

Canonical forms are used to generate input to a security context for
security processing at a BP node. If the values of a security target
are unchanged, then the canonical form of that target will be the
same even if the encoding of those values for wire transmission is
different.

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

The canonical form of the primary block is as specified in [RFC9171]
with the following constraint.

* CBOR values from the primary block MUST be canonicalized using the
rules for Deterministically Encoded CBOR, as specified in
[RFC8949].

All non-primary blocks share the same block structure and are
canonicalized as specified in [RFC9171] with the following
constraints.

* CBOR values from the non-primary block MUST be canonicalized using
the rules for Deterministically Encoded CBOR, as specified in
[RFC8949].

* Only the block-type-specific data field may be provided to a
cipher suite for encryption as part of a confidentiality security
service. Other fields within a non-primary block MUST NOT be
encrypted or decrypted and MUST NOT be included in the canonical
form used by the cipher suite for encryption and decryption. An
integrity-protection mechanism MAY be applied to these other
fields as supported by the security context. For example, these
fields might be treated as associated authenticated data.

* Reserved and unassigned flags in the block processing control
flags field MUST be set to 0 in a canonical form as it is not
known if those flags will change in transit.

Security contexts MAY define their own canonicalization algorithms
and require the use of those algorithms over the ones provided in
this specification. In the event of conflicting canonicalization
algorithms, algorithms defined in a security context take precedence
over this specification when constructing canonical forms for that
security context.

5. Security Processing

This section describes the security aspects of bundle processing.

5.1. Bundles Received from Other Nodes

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

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

5.1.1. Receiving BCBs

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

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

If the security policy of a node specifies that a node should have
applied confidentiality to a specific security target and no such BCB
is present in the bundle, then the node MUST process this security
target in accordance with the security policy. It is RECOMMENDED
that the node remove the security target from the bundle because the
confidentiality (and possibly the integrity) of the security target
cannot be guaranteed. If the removed security target is the payload
block, the bundle MUST be discarded.

If an encrypted payload block cannot be decrypted (i.e., the
ciphertext cannot be authenticated), then the bundle MUST be
discarded and processed no further. If an encrypted security target
other than the payload block cannot be decrypted, then the associated
security target and all security blocks associated with that target
MUST be discarded and processed no further. In both cases, requested
status reports (see [RFC9171]) MAY be generated to reflect bundle or
block deletion.

When a BCB is decrypted, the recovered plaintext for each security
target MUST replace the ciphertext in each of the security targets'
block-type-specific data fields. If the plaintext is of a different
size than the ciphertext, the framing of the CBOR byte string of this
field must be updated to ensure this field remains a valid CBOR byte
string. The length of the recovered plaintext is known by the
decrypting security context.

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

5.1.2. Receiving BIBs

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

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

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

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

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

5.2. Bundle Fragmentation and Reassembly

If it is necessary for a node to fragment a bundle payload, and
security services have been applied to that bundle, the fragmentation
rules described in [RFC9171] MUST be followed. As defined there and
summarized here for completeness, only the payload block can be
fragmented; security blocks, like all extension blocks, can never be
fragmented.

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

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

6. Key Management

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

7. Security Policy Considerations

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

* If a bundle is received that contains combinations of security
operations that are disallowed by this specification, the BPA must
determine how to handle the bundle: the bundle may be discarded,
the block affected by the security operation may be discarded, or
one security operation may be favored over another.

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

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

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

* BCB security contexts can alter the size of extension blocks and
the payload block. Security policy SHOULD consider how changes to
the size of a block could negatively effect bundle processing
(e.g., calculating storage needs and scheduling transmission
times).

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

1. At the time of encryption, a security context can be selected
that computes a plaintext integrity-protection mechanism that
is included as a security context result field.

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

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

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

7.1. Security Reason Codes

BPAs must process blocks and bundles in accordance with both BP
policy and BPSec policy. The decision to receive, forward, deliver,
or delete a bundle may be communicated to the report-to address of
the bundle in the form of a status report, as a method of tracking
the progress of the bundle through the network. The status report
for a bundle may be augmented with a "reason code" explaining why the
particular action was taken on the bundle.

This section describes a set of reason codes associated with the
security processing of a bundle. The communication of security-
related status reports might reduce the security of a network if
these reports are intercepted by unintended recipients. BPSec policy
SHOULD specify the conditions in which sending security reason codes
are appropriate. Examples of appropriate conditions for the use of
security reason codes could include the following.

* When the report-to address is verified as unchanged from the
bundle source. This can occur by placing an appropriate BIB on
the bundle primary block.

* When the block containing a status report with a security reason
code is encrypted by a BCB.

* When a status report containing a security reason code is only
sent for security issues relating to bundles and/or blocks
associated with non-operational user data or test data.

* When a status report containing a security reason code is only
sent for security issues associated with non-operational security
contexts, or security contexts using non-operational
configurations, such as test keys.

Security reason codes are assigned in accordance with Section 11.2
and are as described below.

Missing Security Operation: security operation:
This reason code indicates that a bundle was missing one or
more required security operations. This reason code is
typically used by a security verifier or security acceptor.

Unknown Security Operation: security operation:
This reason code indicates that one or more security operations
present in a bundle cannot be understood by the security
verifier or security acceptor for the operation. For example,
this reason code may be used if a security block references an
unknown security context identifier or security context
parameter. This reason code should not be used for security
operations for which the node is not a security verifier or
security acceptor; there is no requirement that all nodes in a
network understand all security contexts, security context
parameters, and security services for every bundle in a
network.

Unexpected Security Operation: security operation:
This reason code indicates that a receiving node is neither a
security verifier nor a security acceptor for at least one
security operation in a bundle. This reason code should not be
seen as an error condition: not every node is a security
verifier or security acceptor for every security operation in
every bundle. In certain networks, this reason code may be
useful in identifying misconfigurations of security policy.

Failed Security Operation: security operation:
This reason code indicates that one or more security operations
in a bundle failed to process as expected for reasons other
than misconfiguration. This may occur when a security-source
is unable to add a security block to a bundle. This may occur
if the target of a security operation fails to verify using the
defined security context at a security verifier. This may also
occur if a security operation fails to be processed without
error at a security acceptor.

Conflicting Security Operations: security operation:
This reason code indicates that two or more security operations
in a bundle are not conformant with the BPSec specification and
that security processing was unable to proceed because of a
BPSec protocol violation.

8. Security Considerations

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

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

8.1. Attacker Capabilities and Objectives

BPSec was designed to protect against OPA threats that may have
access to a bundle during transit from its source, Alice, to its
destination, Bob. An OPA node, Olive, is a noncooperative node
operating on the delay-tolerant network between Alice and Bob that
has the ability to receive bundles, examine bundles, modify bundles,
forward bundles, and generate bundles at will in order to compromise
the confidentiality or integrity of data within the delay-tolerant
network. There are three classes of OPA nodes that are
differentiated based on their access to cryptographic material:

Unprivileged Node: Olive has not been provisioned within the secure
environment and only has access to cryptographic material that has
been publicly shared.

Legitimate Node: Olive is within the secure environment; therefore,
Olive has access to cryptographic material that has been
provisioned to Olive (i.e., K_M) as well as material that has been
publicly shared.

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

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

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

8.2. Attacker Behaviors and BPSec Mitigations

8.2.1. Eavesdropping Attacks

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

When evaluating the risk of eavesdropping attacks, it is important to
consider the lifetime of bundles on DTN. Depending on the network,
bundles may persist for days or even years. Long-lived bundles imply
that the data exists in the network for a longer period of time and,
thus, there may be more opportunities to capture those bundles.
Additionally, the implication is that long-lived bundles store
information within that remains relevant and sensitive for long
enough that, once captured, there is sufficient time to crack
encryption associated with the bundle. If a bundle does persist on
the network for years and the cipher suite used for a BCB provides
inadequate protection, Olive may be able to recover the protected
data either before that bundle reaches its intended destination or
before the information in the bundle is no longer considered
sensitive.

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

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

8.2.2. Modification Attacks

As a node participating in the delay-tolerant network between Alice
and Bob, Olive will also be able to modify the received bundle,
including non-BPSec data such as the primary block, payload blocks,
or block processing control flags as defined in [RFC9171]. Olive
will be able to undertake activities including modification of data
within the blocks, replacement of blocks, addition of blocks, or
removal of blocks. Within BPSec, both the BIB and BCB provide
integrity-protection mechanisms to detect or prevent data
manipulation attempts by Olive.

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

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

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

8.2.3. Topology Attacks

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

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

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

8.2.4. Message Injection

Olive is also able to generate new bundles and transmit them into the
delay-tolerant network at will. These bundles may be either 1)
copies or slight modifications of previously observed bundles (i.e.,
a replay attack) or 2) entirely new bundles generated based on the
Bundle Protocol, BPSec, or other bundle-related protocols. With
these attacks, Olive's objectives may vary, but may be targeting
either the Bundle Protocol or application-layer protocols conveyed by
the Bundle Protocol. The target could also be the storage and
computing capabilities of the nodes running the bundle or
application-layer protocols (e.g., a denial of service to flood on
the storage of the store-and-forward mechanism or a computation that
would process the bundles and perhaps prevent other activities).

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

For example, a BIB may be used to validate the integrity of a
bundle's primary block, which includes a timestamp and lifetime for
the bundle. If a bundle is replayed outside of its lifetime, then
the replay attack will fail as the bundle will be discarded.
Similarly, additional blocks, such as the Bundle Age, may be signed
and validated to identify replay attacks. Finally, security context
parameters within BIBs and BCBs may include anti-replay mechanisms
such as session identifiers, nonces, and dynamic passwords as
supported by network characteristics.

9. Security Context Considerations

9.1. Mandating Security Contexts

Because of the diversity of networking scenarios and node
capabilities that may utilize BPSec, there is a risk that a single
security context mandated for every possible BPSec implementation is
not feasible. For example, a security context appropriate for a
resource-constrained node with limited connectivity may be
inappropriate for use in a well-resourced, well-connected node.

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

To ensure interoperability among various implementations, all BPSec
implementations MUST support at least the current, mandatory security
context(s) defined in IETF Standards Track RFCs. As of this writing,
that BP mandatory security context is specified in [RFC9173], but the
mandatory security context(s) might change over time in accordance
with usual IETF processes. Such changes are likely to occur in the
future if/when flaws are discovered in the applicable cryptographic
algorithms, for example.

Additionally, BPSec implementations need to support the security
contexts that are required by the BP networks in which they are
deployed.

If a node serves as a gateway between two or more networks, the BPSec
implementation at that node needs to support the union of security
contexts mandated in those networks.

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

Implementers should monitor the state of security context
specifications to check for future updates and replacement.

9.2. Identification and Configuration

Security blocks uniquely identify the security context to be used in
the processing of their security services. The security context for
a security block MUST be uniquely identifiable and MAY use parameters
for customization.

To reduce the number of security contexts used in a network, security
context designers should make security contexts customizable through
the definition of security context parameters. For example, a single
security context could be associated with a single cipher suite and
security context parameters could be used to configure the use of
this security context with different key lengths and different key
management options without needing to define separate security
contexts for each possible option.

A single security context may be used in the application of more than
one security service. This means that a security context identifier
MAY be used with a BIB, with a BCB, or with any other BPSec-compliant
security block. The definition of a security context MUST identify
which security services may be used with the security context, how
security context parameters are interpreted as a function of the
security operation being supported, and which security results are
produced for each security service.

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

Table 1: Security Context Examples

9.3. Authorship

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

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

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

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

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

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

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

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

Ciphertext Size: Security contexts MUST state whether their
associated cipher suites generate ciphertext (to include any
authentication information) that is of a different size than the
input plaintext.

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

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

Handling CRC Fields: Security contexts may include algorithms that
alter the contexts of their security target block, such as the
case when encrypting the block-type-specific data of a target
block as part of a BCB confidentiality service. Security context
specifications SHOULD address how preexisting CRC type and CRC
value fields be handled. For example, a BCB security context
could remove the plaintext CRC value from its target upon
encryption and replace or recalculate the value upon decryption.

10. Defining Other Security Blocks

Other Security Blocks (OSBs) may be defined and used in addition to
the security blocks identified in this specification. BIB, BCB, and
any future OSBs can coexist within a bundle and can be considered in
conformance with BPSec if all of the following requirements are met
by any future identified security blocks.

* OSBs MUST NOT reuse any enumerations identified in this
specification, to include the block type codes for BIB and BCB.

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

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

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

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

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

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

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

11. IANA Considerations

This specification includes fields that require registries managed by
IANA.

11.1. Bundle Block Types

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

+=======+=======================+===============+
| Value | Description | Reference |
+=======+=======================+===============+
| 11 | Block Integrity | This document |
+-------+-----------------------+---------------+
| 12 | Block Confidentiality | This document |
+-------+-----------------------+---------------+

Table 2: Additional Entries for the "Bundle
Block Types" Registry

The "Bundle Block Types" registry notes whether a block type is meant
for use in BP version 6, BP version 7 (BPv7), or both. The two block
types defined in this specification are meant for use with BPv7.

11.2. Bundle Status Report Reason Codes

This specification allocates five reason codes from the existing
"Bundle Status Report Reason Codes" registry defined in [RFC6255].

+============+=======+============================+================+
| BP Version | Value | Description | Reference |
+============+=======+============================+================+
| 7 | 12 | Missing security operation | This document, |
| | | | Section 7.1 |
+------------+-------+----------------------------+----------------+
| 7 | 13 | Unknown security operation | This document, |
| | | | Section 7.1 |
+------------+-------+----------------------------+----------------+
| 7 | 14 | Unexpected security | This document, |
| | | operation | Section 7.1 |
+------------+-------+----------------------------+----------------+
| 7 | 15 | Failed security operation | This document, |
| | | | Section 7.1 |
+------------+-------+----------------------------+----------------+
| 7 | 16 | Conflicting security | This document, |
| | | operation | Section 7.1 |
+------------+-------+----------------------------+----------------+

Table 3: Additional Entries for the "Bundle Status Report Reason
Codes" Registry

11.3. Security Context Identifiers

BPSec has a Security Context Identifier field for which IANA has
created a new registry named "BPSec Security Context Identifiers".
Initial values for this registry are given below.

The registration policy for this registry is Specification Required
(see [RFC8126]).

The value range: signed 16-bit integer.

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

Table 4: "BPSec Security Context
Identifier" Registry

Negative security context identifiers are reserved for local/site-
specific uses. The use of 0 as a security context identifier is for
nonoperational testing purposes only.

12. References

12.1. Normative References

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

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

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

[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>.

[RFC9171] Burleigh, S., Fall, K., and E. Birrane, III, "Bundle
Protocol Version 7", RFC 9171, DOI 10.17487/RFC9171,
November 2021,
January 2022, <https://www.rfc-editor.org/info/rfc9171>.

[RFC9173] Birrane, III, E., "BPSec Default White, A., and S. Heiner, "Default
Security Contexts for Bundle Protocol Security Contexts", (BPSec)",
RFC 9173, DOI 10.17487/RFC9173, November 2021, January 2022,
<https://www.rfc-editor.org/info/rfc9173>.

12.2. Informative References

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

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

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

Acknowledgments

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

Additionally, Benjamin Kaduk of Akamai Technologies provided a
detailed technical review that resulted in a stronger and more
precise specification.

Authors' Addresses

Edward J. Birrane, III
The Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Rd.
Laurel, MD 20723
United States of America

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

Kenneth McKeever
The Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Rd.
Laurel, MD 20723
United States of America

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