Delay-Tolerant Networking E. Birrane
Internet-Draft K. McKeever
Intended status: Standards Track JHU/APL
Expires: January 3, 2018 July 2, 2017

Bundle Protocol Security Specification


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

Status of This Memo

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

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This Internet-Draft will expire on January 3, 2018.

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Table of Contents

1. Introduction

This document defines security features for the Bundle Protocol (BP) [BPBIS] and is intended for use in Delay Tolerant Networks (DTNs) to provide end-to-end security services.

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

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

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

1.1. Supported Security Services

BPSec provides end-to-end integrity and confidentiality services for BP bundles.

Integrity services ensure that protected data within a bundle are not changed from the time they are provided to the network to the time they are delivered at their destination. Data changes may be caused by processing errors, environmental conditions, or intentional manipulation.

Confidentiality services ensure that protected data is unintelligible to nodes in the DTN, except for authorized nodes possessing special information. Confidentiality, in this context, applies to the contents of protected data and does not extend to hiding the fact that protected data exist in the bundle.

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

  1. The term "hop-by-hop" is ambiguous in a BP overlay, as nodes that are adjacent in the overlay may not be adjacent in physical connectivity. This condition is difficult or impossible to detect and therefore hop-by-hop authentication is difficult or impossible to enforce.
  2. Networks in which BPSec may be deployed may have a mixture of security-aware and not-security-aware nodes. Hop-by-hop authentication cannot be deployed in a network if adjacent nodes in the network have different security capabilities.
  3. Hop-by-hop authentication is a special case of data integrity and can be achieved with the integrity mechanisms defined in this specification. Therefore, a separate authentication service is not necessary.

1.2. Specification Scope

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

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

This specification does not address individual cipher suite implementations. Different networking conditions and operational considerations require varying strengths of security mechanism such that mandating a cipher suite in this specification may result in too much security for some networks and too little security in others. It is expected that separate documents will be standardized to define cipher suites compatible with BPSec, to include operational cipher suites and interoperability cipher suites.

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

This specification does not address how to combine the BPSec security blocks with other protocols, other BP extension blocks, or other best practices to achieve security in any particular network implementation.

1.3. Related Documents

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

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

The Bundle Protocol [BPBIS] defines the format and processing of bundles, defines the extension block format used to represent BPSec security blocks, and defines the canonicalization algorithms used by this specification.

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

1.4. Terminology

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

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

2. Design Decisions

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

2.1. Block-Level Granularity

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

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

The Bundle Protocol allows extension blocks to be added to a bundle at any time during its existence in the DTN. When a waypoint adds a new extension block to a bundle, that extension block may have security services applied to it by that waypoint. Similarly, a waypoint may add a security service to an existing extension block, consistent with its security policy. For example, a node representing a boundary between a trusted part of the network and an untrusted part of the network may wish to apply payload encryption for bundles leaving the trusted portion of the network.

When a waypoint adds a security service to the bundle, the waypoint is the security source for that service. The security block(s) which represent that service in the bundle may need to record this security source as the bundle destination might need this information for processing. For example, a destination node might interpret policy as it related to security blocks as a function of the security source for that block.

2.3. Mixed Security Policy

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

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

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

Some waypoints will determine, through policy, that they are the intended recipient of the security service and terminate the security service in the bundle. For example, a gateway node may 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 may understand security blocks but refuse to process them unless they are the bundle destination.

2.4. User-Selected Cipher Suites

The security services defined in this specification rely on a variety of cipher suites providing integrity signatures, cipher-text, and other information necessary to populate security blocks. Users MAY select different cipher suites to implement security services. For example, some users might prefer a SHA2 hash function for integrity whereas other users may prefer a SHA3 hash function instead. The security services defined in this specification MUST provide a mechanism for identifying what cipher suite has been used to populate 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).

3.2. Uniqueness

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

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

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

3.3. Target Multiplicity

Under special circumstances, 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 may be represented by a single security block if and only if the following conditions are true.

When representing multiple security operations in a single security block, the information that is common across all operations is represented once in the security block, and the information which is different (e.g., the security targets) are represented individually. When the security block is processed all security operations represented by the security block MUST be applied/evaluated at that time.

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 [BPBIS] provides a "Block Number" field suitable for this purpose. Therefore, a security target in a security block MUST be represented as the Block Number of the target block.

3.5. Block Representation

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

Security-specific information for a security block is captured in the "Block Type Specific Data Fields".

3.6. Abstract Security Block

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

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

Figure 1: Cipher Suite Parameters

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

Figure 2: Security Results

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

Security Targets:

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

This field identifies the cipher suite 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.
Cipher Suite Flags:

This field identifies which optional fields are present in the security block. This field SHALL be represented as a CBOR unsigned integer containing a bit field of 5 bits indicating the presence or absence of other security block fields, as follows.
Bit 1
(the most-significant bit, 0x10): reserved.
Bit 2
(0x08): reserved.
Bit 3
(0x04): reserved.
Bit 4
(0x02): Security Source Present Flag.
Bit 5
(the least-significant bit, 0x01): Cipher Suite Parameters Present Flag.

In this field, a value of 1 indicates that the associated security block field MUST be included in the security block. A value of 0 indicates that the associated security block field MUST NOT be in the security block.

Security Source (Optional Field):

This field identifies the Endpoint that inserted the security block in the bundle. If the security source field is not present then the source MAY be inferred from other information, such as the bundle source or the previous hop, as defined by security policy. This field SHALL be represented by a CBOR array in accordance with [BPBIS] rules for representing Endpoint Identifiers (EIDs).
Cipher Suite Parameters (Optional Field):

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

The logical layout of the cipher suite parameters array is illustrated in Figure 1.
Security Results:

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

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

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.

3.7. Block Integrity Block

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


3.8. Block Confidentiality Block

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

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 Fields contains cipher-text, not plain-text. Other block fields remain unmodified, with the exception of the Block Data Length field, which may be changed if the BCB is allowed to change the length of the block (see below).

Fragmentation, reassembly, and custody transfer are adversely affected by a change in size of the payload block due to ambiguity about what byte range of the block is actually in any particular fragment. Therefore, when the security target of a BCB is the bundle payload, the BCB MUST NOT alter the size of the payload block body data. This "in-place" encryption allows fragmentation, reassembly, and custody transfer to operate without knowledge of whether or not encryption has occurred.

If a BCB cannot alter the size of the security target (e.g., the security target is the payload block or block length modifications are disallowed by policy) then differences in the size of the cipher-text and plain-text MUST be handled in the following way. If the cipher-text is shorter in length than the plain-text, padding must be used in accordance with the cipher suite policy. If the cipher-text is larger than the plain-text, overflow bytes MUST be placed in overflow parameters in the Security Result field.


3.9. Block Interactions

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

If confidentiality is being applied to a target that already has integrity applied to it, then an undesirable condition occurs where a security aware waypoint would be unable to check the integrity result of a block because the block contents have been encrypted after the integrity signature was generated. To address this concern, the following processing rules MUST be followed.

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

3.10. Cipher Suite Parameter and Result Identification

Cipher suite parameters and security results each represent multiple distinct pieces of information in a security block. Each piece of information is assigned an identifier and a CBOR encoding. Identifiers MUST be unique for a given cipher suite but do not need to be unique across all cipher suites. Therefore, parameter ids and security result ids are specified in the context of a cipher suite definition.

Individual BPSec cipher suites SHOULD use existing registries of identifiers and CBOR encodings, such as those defined in [COSE], whenever possible. Cipher suites MAY define their own identifiers and CBOR encodings when necessary.

A cipher suite MAY include multiple instances of the same identifier for a parameter or result in a security block. Parameters and results are represented using CBOR, and any identification of a new parameter or result MUST include how the value will be represented using the CBOR specification. Ids themselves are always represented as a CBOR unsigned integer.

3.11. BSP Block Example

An example of BPSec blocks applied to a bundle is illustrated in Figure 3. In this figure 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.

         Block in Bundle            ID    
|         Primary Block             | B1 |
|             BIB                   | B2 |
|  OP(integrity, target=B1)         |    |
|             BCB                   | B3 |
|  OP(confidentiality, target=B4)   |    |
|      Extension Block              | B4 |
|             BIB                   | B5 |
|  OP(integrity, target=B6)         |    |
|      Extension Block              | B6 |
|             BCB                   | B7 |
| OP(confidentiality,targets=B8,B9) |    |
|   BIB  (encrypted by B7)          | B8 |
|  OP(integrity, target=B9)         |    |
|         Payload Block             | B9 |

Figure 3: Sample Use of BPSec Blocks

In this example a bundle has four non-security-related blocks: the primary block (B1), two extension blocks (B4,B6), and a payload block (B9). The following security applications are applied to this bundle.

4. Canonical Forms

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

Canonical forms are not transmitted, they are used to generate input to a cipher suite for security processing at a security-aware node.

The canonicalization of the primary block is as specified in [BPBIS].

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

These canonicalization algorithms assume that Endpoint IDs do not change from the time at which a security source adds a security block to a bundle and the time at which a node processes that security block.

Cipher suites 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, cipher suite algorithms take precedence over this specification.

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 security-aware node. The processing order is as follows.

5.1.1. Receiving BCB Blocks

If a received bundle contains a BCB, the receiving node MUST determine whether it has the responsibility of decrypting the BCB security target and removing the BCB prior to delivering data to an application at the node or forwarding 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 MAY decrypt the BCB if directed to do so as a matter of security policy.

If the security policy of a security-aware node specifies that a bundle 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. This MAY involve removing the security target from the bundle. If the removed security target is the payload block, the bundle MAY be discarded.

If an encrypted payload block cannot be decrypted (i.e., the decryption key cannot be deduced or decryption fails), 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 [BPBIS]) MAY be generated to reflect bundle or block deletion.

When a BCB is decrypted, the recovered plain-text MUST replace the cipher-text in the security target Block Type Specific Data Fields. If the Block Data Length field was modified at the time of encryption it MUST be updated to reflect the decrypted block length.

If a BCB contains multiple security targets, all security targets MUST be processed when the BCB is processed. Errors and other processing steps SHALL be made as if each security target had been represented by an individual BCB with a single security target.

5.1.2. Receiving BIB Blocks

If a received bundle contains a BIB, the receiving node MUST determine whether it has the final responsibility of verifying the BIB security target and removing it prior to delivering data to an application at the node or forwarding the bundle. If a BIB check fails, the security target has failed to authenticate and the security target SHALL be processed according to the security policy. A bundle status report indicating the failure MAY be generated. Otherwise, if the BIB verifies, the security target is ready to be processed for delivery.

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

If the security policy of a security-aware node specifies that a bundle 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. This MAY involve removing the security target from the bundle. If the removed security target is the payload or primary block, the bundle MAY be discarded. This action may occur at any node that has the ability to verify an integrity signature, not just the bundle destination.

If a receiving node does not have the final responsibility of verifying the BIB it MAY still attempt to verify the BIB to prevent the needless forwarding of corrupt data. If the check fails, the node SHALL process the security target in accordance to local security policy. It is RECOMMENDED that if a payload integrity check fails at a waypoint that it is processed in the same way as if the check fails at the destination. If the check passes, the node MUST NOT remove the BIB prior to forwarding.

If a BIB contains multiple security targets, all security targets MUST be processed if the BIB is processed by the Node. Errors and other processing steps SHALL be made as if each security target had been represented by an individual BIB with a single security target.

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 [BPBIS] MUST be followed. As defined there and summarized here for completeness, only the payload block may 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.

6. Key Management

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

7. Security Policy Considerations

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

8. Security Considerations

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

It should be noted that BPSEC addresses only the security of data traveling over the DTN, not the underlying DTN itself. Additionally, BPSec addresses neither the fitness of externally-defined cryptographic methods nor the security of their implementation. It is the responsibility of the BPSec implementer that appropriate algorithms and methods are chosen. Furthermore, the BPSec protocol does not address threats which share computing resources with the DTN and/or BPSec software implementations. These threats may be malicious software or compromised libraries which intend to intercept data or recover cryptographic material. Here, it is the responsibility of the BPSec implementer to ensure that any cryptographic material, including shared secret or private keys, is protected against access within both memory and storage devices.

The threat model described here is assumed to have a set of capabilities identical to those described by the Internet Threat Model in [RFC3552], but the BPSec threat model is scoped to illustrate threats specific to BPSec operating within DTN environments and therefore focuses on man-in-the-middle (MITM) attackers.

8.1. Attacker Capabilities and Objectives

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

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

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

8.2. Attacker Behaviors and BPSec Mitigations

8.2.1. Eavesdropping Attacks

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

When evaluating the risk of eavesdropping attacks, it is important to consider the lifetime of bundles on a DTN. Depending on the network, bundles may persist for days or even years. If a bundle does persist on the network for years and the cipher suite used for a BCB provides inadequate protection, Mallory may be able to recover the protected data before that bundle reaches its intended destination.

8.2.2. Modification Attacks

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

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

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

A limitation of the BIB may exist within the implementation of BIB validation at the destination node. If Mallory is a legitimate node within the DTN, the BIB generated by Alice with K_A can be replaced with a new BIB generated with K_M and forwarded to Bob. If Bob is only validating that the BIB was generated by a legitimate user, Bob will acknowledge the message as originating from Mallory instead of Alice. In order to provide verifiable integrity checks, both a BIB and BCB should be used. 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. In this configuration, since Mallory is only a legitimate node and does not have access to Alice's key K_A, Mallory is unable to decrypt the BCB and replace the BIB.

8.2.3. Topology Attacks

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

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

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

8.2.4. Message Injection

Mallory is also able to generate new bundles and transmit them into the DTN at will. These bundles may either be copies or slight modifications of previously-observed bundles (i.e., a replay attack) or entirely new bundles generated based on the Bundle Protocol, BPSec, or other bundle-related protocols. With these attacks Mallory's objectives may vary, but may be targeting either the bundle protocol or application-layer protocols conveyed by the bundle protocol.

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

9. Cipher Suite Authorship Considerations

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

When developing new cipher suites for use with BPSec, the following information SHOULD be considered for inclusion in these specifications.

10. Defining Other Security Blocks

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

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 standards defining new security blocks and the identification of such blocks shall not, alone, require maintenance of this specification.

11. IANA Considerations

A registry of cipher suite identifiers will be required.

11.1. Bundle Block Types

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

Additional Entries for the Bundle Block-Type Codes Registry:

Value Description Reference
TBD Block Integrity Block This document
TBD Block Confidentiality Block This document

12. References

12.1. Normative References

[BPBIS] Burleigh, S., Fall, K. and E. Birrane, "Bundle Protocol", Internet-Draft draft-ietf-dtn-bpbis-06, July 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003.
[RFC6255] Blanchet, M., "Delay-Tolerant Networking Bundle Protocol IANA Registries", RFC 6255, May 2011.

12.2. Informative References

[COSE] Schaad, J., "CBOR Object Signing and Encryption (COSE)", Internet-Draft draft-ietf-cose-msg-24, November 2016.
[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, R., Scott, K., Fall, K. and H. Weiss, "Delay-Tolerant Networking Architecture", RFC 4838, April 2007.
[RFC6257] Symington, S., Farrell, S., Weiss, H. and P. Lovell, "Bundle Security Protocol Specification", RFC 6257, May 2011.
[SBSP] Birrane, E., "Streamlined Bundle Security Protocol", Internet-Draft draft-birrane-dtn-sbsp-01, October 2015.

Appendix A. Acknowledgements

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

Authors' Addresses

Edward J. Birrane, III The Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Rd. Laurel, MD 20723 US Phone: +1 443 778 7423 EMail:
Kenneth McKeever The Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Rd. Laurel, MD 20723 US Phone: +1 443 778 2237 EMail: