Delay-Tolerant Networking E. Birrane
Internet-Draft K. McKeever
Intended status: Experimental JHU/APL
Expires: January 9, 2017 July 8, 2016

Bundle Protocol Security Specification
draft-ietf-dtn-bpsec-02

Abstract

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.

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

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This Internet-Draft will expire on January 9, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

This document defines security features for the Bundle Protocol [BPBIS] intended for use in delay-tolerant networks, in order to provide Delay-Tolerant Networking (DTN) security services.

1.1. Motivation

The Bundle Protocol is used in DTNs that overlay multiple networks, some of which may be challenged by limitations such as intermittent and possibly unpredictable loss of connectivity, long or variable delay, asymmetric data rates, and high error rates. The purpose of the Bundle Protocol is to support interoperability across such stressed networks.

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

1.2. Supported Security Services

This specification supports end-to-end integrity and confidentiality services associated with BP bundles.

Integrity services ensure data within a bundle are not changed. Data changes may be caused by processing errors, environmental conditions, or intentional manipulation. An integrity service is one that provides sufficient confidence to a data receiver that data has not changed since its value was last asserted.

Confidentiality services ensure that the values of some data within a bundle can only be determined by authorized receivers of the data. When a bundle traverses a DTN, many nodes in the network other than the destination node MAY see the contents of a bundle. A confidentiality service allows a destination node to generate data values from otherwise encrypted contents of a 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 predict in the overlay and therefore makes the concept of hop-by-hop authentication difficult or impossible to enforce at the overlay.
  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 can be viewed as a special case of data integrity. As such, it is possible to develop policy that provides a version of authentication using the integrity mechanisms defined in this specification.

1.3. Specification Scope

This document describes the Bundle Protocol Security Specification (BPSec), which provides security services for blocks within a bundle. This includes the data specification for individual BP extension blocks and the processing instructions for those blocks.

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

This specification does not address individual cipher suite implementations. The definition and enumeration of cipher suites should be undertaken in separate specification documents.

This specification does not address the implementation of security policy and does not provide a security policy for the BPSec. Security policies are typically based on the nature and capabilities of individual networks and network operational concepts. However, this specification does recommend 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.

1.4. 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 delay-tolerant networks, but does not discuss security at any length.

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

The Bundle Security Protocol [RFC6257] and Streamlind Bundle Security Protocol [SBSP] introduce the concepts of security blocks for security services. BPSec is based off of these documents.

1.5. 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 those terms whose definition is important to the understanding of concepts within this specification.

The application of these terms applied to a sample network topology is shown in Figure 1. This figure shows four bundle nodes (BN1, BN2, BN3, BN4) residing above some transport layer(s). Three distinct transport and network protocols (T1/N1, T2/N2, and T3/N3) are also shown.

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

Figure 1: Bundle Nodes Sitting Above the Transport Layer.

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

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

The following security-specific terminology is also defined to clarify security operations in this specifiation.

2. Key Properties

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. Rather than enumerate all potential security implementations in all potential DTN topologies, this specification defines a set of key properties of a security system. The security primitives outlined in this document MUST enable the realization of these properties in a DTN deploying the Bundle Protocol.

2.1. Block-Level Granularity

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.

Security services within this specification MUST provide block level granularity where applicable such that different blocks within a bundle may have different security services applied to them.

For example, within a bundle, a payload might be encrypted to protect its contents, whereas an extension block containing summary information related to the payload might be integrity signed but otherwise unencrypted to provide certain nodes access to payload-related data without providing access to the payload.

Each security block in a bundle will be associated with a specific security-operation.

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 node adds a new extension block to a bundle, that extension block may have security services applied to it by that waypoint. Similarly, a waypoint node 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.

In each case, a node other than the bundle originator may add a security service to the bundle and, as such, the source for the security service will be different than the source of the bundle itself. Security services MUST track their orginating node so as to properly apply policy and key selection associated with processing the security service at the bundle destination.

Referring to Figure 1, if the bundle that originates at BN1 is given security blocks by BN1, then BN1 is the security-source for those blocks as well as being the source of the bundle. If the bundle that originates at BN1 is then given a security block by BN2, then BN2 is the security-source for that block even though BN1 remains the bundle source.

2.3. Mixed Security Policy

Different nodes in a DTN may have different security-related capabilities. Some nodes may not be security-aware and will not understand any security-related extension blocks. Other nodes may have security policies that require evaluation of security services at places other than the bundle destination (such as verifying integrity signatures at certain waypoint nodes). Other nodes may ignore any security processing if they are not the destination of the bundle. The security services described in this specification must allow each of these scenarios.

Extension blocks representing security services MUST have their block processing flags set such that the block will be treated appropriately by non-security-aware nodes.

Extension blocks providing integrity services within a bundle MUST support options to allow waypoint nodes to evaluate these signatures if such nodes have the proper configuraton to do so.

2.4. User-Selected Ciphersuites

The security services defined in this specification rely on a variety of cipher suites providing integrity signatures, ciphertext, and other information necessary to populate security blocks. Users may wish to select different cipher suites to implement different security services. For example, some users may wish to use a SHA-256 based hash for integrity whereas other users may require a SHA-384 hash 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

In all cases, the processing order of security services within a bundle must avoid ambiguity when evaluating security at the bundle destination. This specification MUST provide determinism in the application and evaluation of security services, even when doing so results in a loss of flexibility.

3. Security Block Definitions

There are two types of security blocks that may be included in a bundle. These are the Block Integrity Block (BIB) and the Block Confidentiality Block (BCB).

A security-operation MUST NOT be applied more than once in a bundle. For example, the two security-operations: OP(integrity, payload) and OP(integrity, payload) are considered redundant and MUST NOT appear together in a bundle. However, the two security operations OP(integrity, payload) and OP(integrity, extension_block_1) MAY both be present in the bundle. Also, the two security operations OP(integrity, extension_block_1) and OP(integrity, extension_block_2) are unique and may both appear in the same bundle.

If the same security-service is to be applied to multiple security-targets, and cipher suite parameters for each security service are identical, then the set of security-operations can be represented as a single security-block with multiple security-targets. In such a case, all security-operations represented in the security-block MUST be applied/evaluated together.

3.1. Block Identification

This specification requires that every target block of a security operation be uniquely identifiable. The definition of the extension block header from [BPBIS] provides such a mechanism in the "Block Number" field, which provides a unique identifier for a block within a bundle. Within this specification, a security-target will be identified by its unique Block Number.

3.2. 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:

The structure of the BIB and BCB Block Type Specific Data fields are identifcal and illustrated in Figure 2. In this figure, field names prefaced with an '*' are optional and their inclusion in the block is indicated by the Cipher Suite Flags field.


+=================================================
|    Field Name       |     Field Data Type      |    
+=================================================
| # Security Targets  | Unsigned Integer         |
+---------------------+--------------------------+
| Security Targets    | Array (Unsigned Integer) |
+---------------------+--------------------------+
| Cipher Suite ID     | Unsigned Integer         |
+---------------------+--------------------------+
| Cipher Suite Flags  | Unsigned Integer         |
+---------------------+--------------------------+
| Security Source     | URI - OPTIONAL           |
+---------------------+--------------------------+
| Cipher Parameters   | Byte Array - OPTIONAL    |
+---------------------+--------------------------+
| Security Result     | Byte Array               |
+---------------------+--------------------------+
                
               

Figure 2: BIB and BCB Block Structure

  Bit   Bit   Bit   Bit   Bit   Bit   Bit   Bit
   7     6     5     4     3     2     1     0
+-----------------------------------+-----+-----+
|    reserved                       | src |parm |
+-----------------------------------+-----+-----+
  MSB                                       LSB                          
                      

Figure 3: Cipher Suite Flags

Where the block fields are identified as follows.

  • # Security Targets - The number of security targets for this security block. This value MUST be at least 1.
  • Security-Targets - This array contains the unique identifier of the blocks targetted by this security operation. Each security-target MUST represent a block present in the bundle. A security-target MUST NOT be repeated in this array.
  • Cipher suite ID - Identifies the cipher suite used to implement the security service represented by this block and applied to each security-target.
  • Cipher suite flags - Identifies which optional security block fields are present in the block. The structure of the Cipher Suite Flags field is shown in Figure 3. The presence of an optional field is indicated by setting the value of the corresponding flag to one. A value of zero indicates the corresponding optional field is not present. The BPSEC Cipher Suite Flags are defined as follows.
    • bits 7-2 are reserved for future use.
    • src - bit 1 indicates whether the Security Source EID is present in the block.
    • parm - bit 0 indicates whether or not the Cipher Suite Parameters field is present in the block.

  • (OPTIONAL) Security Source (URI) - This identifies the EID that inserted the security service in the bundle. If the security source is not present then the souce of the block MAY be taken to be the bundle source, the previous hop, or some other EID as defined by security policy.
  • (OPTIONAL) Parameters (Byte Array) - Compound field of the following two items.
    • Length (Unsigned Integer) - specifies the length of the next field, which captures the parameters data.
    • Data (Byte Array) - A byte array encoding one or more cipher suite parameters, with each parameter represented as a Type-Length-Value (TLV) triplet, defined as follows.
      • Type (Byte) - The parameter type.
      • Length (Unsigned Integer) - The length of the parameter.
      • Value (Byte Array) - The parameter value.

      See

      Section 3.7 for a list of parameter types that MUST be supported by BPSEC implementations. BPSEC cipher suite specifications MAY define their own parameters to be represented in this byte array.

  • Security Result (Byte Array) - Compound field of the next two items.
    • Length (Unsigned Integer) - specifies the length of the next field, which is the security-result data.
    • Data (Byte Array) - A byte array encoding a security result for each security-target covered by the security-block, with each entry represented as a TLV and optionally prepended with information on which security-target is referenced by the result, as follows.
      • Target (Optional Unsigned Integer) - If the security-block has multiple security-targets, the target field is the Block Number of the security-target to which this result field applies. If the security-block only has a single security-target, this field is omitted.
      • Type (Unsigned Integer)(Byte) - The type of security result field.
      • Length (Unsigned Integer) - The length of the result field.
      • Value (Byte Array) - The results of the appropriate cipher suite specific calculation (e.g., a signature, Message Authentication Code (MAC), or cipher-text block key).

3.3. Block Integrity Block

A BIB is an ASB with the following characteristics:

  • The Block Type Code value MUST be 0x02.
  • The Block Processing Control flags value can be set to whatever values are required by local policy. Cipher suite 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.
  • A security-target for a BIB MUST NOT reference a security-block defined in this specification (e.g., a BIB or a BCB).
  • The cipher suite ID MUST be documented as an end-to-end authentication-cipher suite or as an end-to-end error-detection-cipher suite.
  • An EID-reference to the security-source MAY be present. If this field is not present, then the security-source of the block SHOULD be inferred according to security policy and MAY default to the bundle source. The security-source may also be specified as part of key-information described in Section 3.7.
  • The security-result captures the result of applying the cipher suite calculation (e.g., the MAC or signature) to the relevant parts of the security-target, as specified in the cipher suite definition. This field MUST be present.
  • The cipher suite MAY process less than the entire security-target. If the cipher suite processes less than the complete, original security-target, the cipher suite parameters MUST specify which bytes of the security-target are protected.

Notes:

  • Since OP(integrity, target) is allowed only once in a bundle per target, it is RECOMMENDED that users wishing to support multiple integrity signatures for the same target define a multi-signature cipher suite.
  • For some cipher suites, (e.g., those using asymmetric keying to produce signatures or those using symmetric keying with a group key), the security information MAY be checked at any hop on the way to the destination that has access to the required keying information, in accordance with Section 3.5.
  • The use of a generally available key is RECOMMENDED if custodial transfer is employed and all nodes SHOULD verify the bundle before accepting custody.

3.4. Block Confidentiality Block

A BCB is an ASB with the following characteristics:

  • The Block Type Code value MUST be 0x03.
  • The Block Processing Control flags value can be set to whatever values are required by local policy, except that this block MUST have the "replicate in every fragment" flag set if the target of the BCB is the Payload Block. Having that BCB in each fragment indicates to a receiving node that the payload portion of each fragment represents cipher-text. Cipher suite 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.
  • A security-target for a BCB MAY reference the payload block, a non-security extension block, or a BIB block. A security-target in a BCB MUST NOT be another BCB.
  • The cipher suite ID MUST be documented as a confidentiality cipher suite.
  • Any additional bytes generated as a result of encryption and/or authentication processing of the security-target SHOULD be placed in an "integrity check value" field (see Section 3.7) or other such appropriate area in the security-result of the BCB.
  • An EID-reference to the security-source MAY be present. If this field is not present, then the security-source of the block SHOULD be inferred according to security policy and MAY default to the bundle source. The security-source may also be specified as part of key-information described in Section 3.7.
  • The security-result MUST be present in the BCB. This compound field normally contains fields such as an encrypted bundle encryption key and/or authentication tag.

The BCB modifies the contents of its security-target. When a BCB is applied, the security-target body data are encrypted "in-place". Following encryption, the security-target body data contains cipher-text, not plain-text. Other security-target block fields (such as type, processing control flags, and length) remain unmodified.

Fragmentation, reassembly, and custody transfer are adversely affected by a change in size of the payload 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. Cipher suites SHOULD place any block expansion, such as authentication tags (integrity check values) and any padding generated by a block-mode cipher, into an integrity check value item in the security-result field (see Section 3.7) of the BCB. This "in-place" encryption allows fragmentation, reassembly, and custody transfer to operate without knowledge of whether or not encryption has occurred.

Notes:

  • The cipher suite MAY process less than the entire original security-target body data. If the cipher suite processes less than the complete, original security-target body data, the BCB for that security-target MUST specify, as part of the cipher suite parameters, which bytes of the body data are protected.
  • The BCB's "discard" flag may be set independently from its security-target's "discard" flag. Whether or not the BCB's "discard" flag is set is an implementation/policy decision for the encrypting node. (The "discard" flag is more properly called the "Discard if block cannot be processed" flag.)
  • A BCB MAY include information as part of additional authenticated data to address parts of the target block, such as EID references, that are not converted to cipher-text.

3.5. 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 intermediate node 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.

  • If confidentiality is to be applied to a target, it MUST also be applied to any integrity operation already defined for that target. This means that if a BCB is added to encrypt a block, another BCB MUST also be added to encrypt a BIB also targeting that block.
  • An integrity operation MUST NOT be applied to a security-target if a BCB in the bundle shares the same security-target. This prevents ambiguity in the order of evaluation when receiving a BIB and a BCB for a given security-target.
  • An integrity value MUST NOT be evaluated if the BIB providing the integrity value is the security target of an existing BCB block in the bundle. In such a case, the BIB data contains cipher-text as it has been encrypted.
  • An integrity value MUST NOT be evaluated if the security-target of the BIB is also the security-target of a BCB in the bundle. In such a case, the security-target data contains cipher-text as it has been encrypted.
  • As mentioned in Section 3.3, a BIB MUST NOT have a BCB as its security target. BCBs may embed integrity results as part of cipher suite parameters.

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.

3.6. Multi-Target Block Definitions

A security-block MAY target multiple security-targets if and only if all cipher suite parameters, security source, and key information are common for each security operation. The following processing directives apply for these multi-target blocks.

  • If a security-block has more than one security-target, then each type identifier in the security result TLV MUST be interpretted as a tuple with the first entry being the security-target for which the security result applies and the second entry being the type value enumeration of the security result value.
  • If the security-block has a single security-target, the type field of every entry in the security result array MUST simply be the type field and MUST NOT be a tuple as described above.

3.7. Parameters and Result Fields

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

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

Item types, name, and descriptions are defined as follows.

Cipher suite parameters and result fields.

Type Name Description Field
0 Reserved
1 Initialization Vector (IV) A random value, typically eight to sixteen bytes. Cipher Suite Parameters
2 Reserved
3 Key Information Material encoded or protected by the key management system and used to transport an ephemeral key protected by a long-term key. Cipher Suite Parameters
4 Content Range Pair of Unsigned Integers (offset,length) specifying the range of payload bytes to which an operation applies. The offset MUST be the offset within the original bundle, even if the current bundle is a fragment. Cipher Suite Parameters
5 Integrity Signatures Result of BAB or BIB digest or other signing operation. Security Results
6 Unassigned
7 Salt An IV-like value used by certain confidentiality suites. Cipher Suite Parameters
8 BCB Integrity Check Value (ICV) / Authentication Tag Output from certain confidentiality cipher suite operations to be used at the destination to verify that the protected data has not been modified. This value MAY contain padding if required by the cipher suite. Security Results
9-255 Reserved

3.8. BSP Block Example

An example of BPSec blocks applied to a bundle is illustrated in Figure 4. In this figure the first column represents blocks within a bundle and the second column represents a unique identifier for each block, suitable for use as the security-target of a BPSec security-block. Since the mechanism and format of a security-target is not specified in this document, the terminology B1...Bn is used to identify blocks in the bundle for the purposes 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,target=B8,B9) |    |
+-----------------------------------+----+                
|   BIB  (encrypted by B7)          | B8 |
|  OP(integrity, target=B9)         |    |
+-----------------------------------+----|                                                        
|         Payload Block             | B9 |
+-----------------------------------+----+
               

Figure 4: Sample Use of BSP Blocks

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

  • An integrity signature applied to the canonicalized primary block. This is accomplished by a single BIB (B2).
  • Confidentiality for the first extension block (B4). This is accomplished by a BCB block (B3).
  • Integrity for the second extension block (B6). This is accomplished by a BIB block (B5). NOTE: If the extension block B6 contains a representation of the serialized bundle (such as a hash over all blocks in the bundle at the time of its last transmission) then the BIB block is also providing an authentication service from the prior BPSEC-BPA to this BPSEC-BPA.
  • An integrity signature on the payload (B10). This is accomplished by a BIB block (B8).
  • Confidentiality for the payload block and it's integrity signature. This is accomplished by a BCB block, B7, encrypting B8 and B9.

4. Canonical Forms

By definition, an integrity service determines whether any aspect of a block was changed from the moment the security service was applied at the security source until the point of current evaluation. To successfully verify the integrity of a block, the data passed to the verifying cipher suite MUST be the same bits, in the same order, as those passed to the signature-generating cipher suite at the security source.

However, [BPBIS] does not specify a single on-the-wire encoding of bundles. In cases where a security source generates a different encoding than that used at a receiving node, care MUST be taken to ensure that the inputs to cipher suites at the receiving node is a bitwise match to inputs provided at the security source.

This section provides guidance on how to create a canonical form for each type of block in a bundle. This form MUST be used when generating inputs to cipher suites for use by BPSec blocks.

This specification does not define any security operation over the entire bundle and, therefore, provides no canonical form for a serialized bundle.

4.1. Technical Notes

The following technical considerations hold for all canonicalizations in this section.

  • Any numeric fields defined as variable-length MUST be expanded to their "unpacked" form. For example, a 32-bit integer value MUST be unpacked to a four-byte representation.
  • Each block encoding MUST follow the CBOR encodings provided in [BPBISCBOR].
  • Canonical forms are not transmitted, they are used to generate input to a cipher suite for secuity processing at a security-aware node.
  • Reserved flags MUST NOT be included in any canonicalization as it is not known if those flags will chaneg in transit.
  • These canonicalization algorithms assume that endpoint IDs themselves are immutable and they are unsuitable for use in environments where that assumption might be violated.
  • 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.

4.2. Primary Block Canonicalization

The primary block canonical form is the same as the CBOR encoding of the block, with certain modifications to account for allowed block changes as the bundle traverses the DTN. The fields that compromise the primary block, and any special considerations for their representation in a canonical form, are as follows.

  • The Version field is included, without modification.
  • The Bundle Processing Flags field is used, with modification. Certain bundle processing flags MAY change as a bundle transits the DTN without indicating an integrity error. These flags, which are identified below, MUST NOT be represented in the canonicalized form of the bundle processing flags and, instead, be represented by the bit 0.
    • Reserved flags.
    • Bundle is a Fragment flag.

  • The CRC Type, Destination EID, Source Node ID, Report-To EID, Creation Timestamp, and Lifetime fields are included, without modification.
  • The fragment ID field MAY change if the bundle is fragmented in transit and, as such, this field MUST NOT be included in the canonicalization.
  • The CRC field MAY change at each hop - for example, if a bundle becomes fragmented, each fragment will have a different CRC value from the original signed primary block. As such, this field MUST NOT be included in the canonicalization.

4.3. Non-Primary-Block Canonicalization

All non-primary blocks (NPBs) in [BPBIS] share the same block structure and should be canonicalized in the same way.

Canonicalization for NPBs is dependent on whether the security operation being performed is integrity or confidentiality. Integrity operations consider every field in the block, whereas confidentiality operations only consider the block-type-specific data. Since confidentiality is applied to hide information (replacing plaintext with ciphertext) it provides no benefit to include in the confidentiality calculation information that MUST remain readable, such as block fields other than the block-type-specific data.

The fields that comprise a NPB, and any special considerations for their representation in a canonical form, are as follows.

  • The Block Type Code field is included, without modification, for integrity operations and omitted for confidentiality operations.
  • The Block Number field is included, without modification, for integrity operations and omitted for confidentiality operations.
  • The Block Processing Control Flags field is included, without modification, for integrity operations and omitted for confidentiality operations, with the exception of reserved flags which are treated as 0 in both cases.
  • The CRC type and CRC fields are included, without modification, for integrity operations and omitted for confidentiality operations.
  • The Block Type Specific Data field is included, without modification, for both integrity and confidentiality operations, with the exception that in some cases only a portion of the payload data is to be processed. In such a case, only those bytes are included in the canonical form and additional cipher suite parameters are required to specify which part of the field is included.

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.

  • All BCB blocks in the bundle MUST be evaluated prior to evaluating any BIBs in the bundle. When BIBs and BCBs share a security-target, BCBs MUST be evaluated first and BIBs second.

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 relevant parts of 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 body data

If a BCB contains multiple security-targets, all security-targets MUST be processed if the BCB is processed by the Node. The effect of this is to be the same 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 responsibility of verifying the BIB security target and whether to remove the BIB prior to delivering data to an application at the node or forwarding 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 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 the bundle has a BIB and the receiving node is the destination for the bundle, the node MUST verify the security-target in accordance with the cipher suite specification. 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.

If the bundle has a BIB and the receiving node is not the bundle destination, the receiving node MAY attempt to verify the value in the security-result field. 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 a BIB contains multiple security-targets, all security-targets MUST be processed if the BIB is processed by the Node. The effect of this is to be the same 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 and security services have been applied to that bundle, the fragmentation rules described in [BPBIS] MUST be followed. As defined there and repeated here for completeness, only the payload may be fragmented; security blocks, like all extension blocks, can never be fragmented.

Due to the complexity of bundle fragmentation, including the possibility of fragmenting bundle fragments, integrity and confidentiality operations are not to be applied to a bundle representing a fragment (i.e., a bundle whose "bundle is a Fragment" flag is set in the Bundle Processing Control Flags field). Specifically, a BCB or BIB MUST NOT be added to a bundle fragment, even if the security-target of the security block is not the payload. When integrity and confidentiality must be applied to a fragment, we RECOMMEND that encapsulation be used instead.

6. Key Management

Key management in delay-tolerant networks is recognized as a difficult topic and is one that this specification does not attempt to solve.

7. 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.

  • If a bundle is received that contains more than one security-operation, in violation of BPSec, then the BPA must determine how to handle this 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 an intermediate receiver 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 some other action.
  • It is recommended that security operations only be applied to the payload block, the primary block, and any block-types specifically identified in the security policy. If a BPA were to apply security operations such as integrity or confidentiality to every block in the bundle, regardless of the block type, there could be downstream errors processing blocks whose contents must be inspected at every hop in the network path.
  • Adding a BIB to a security-target that has already been encrypted by a BCB is not allowed. Therefore, we recommend three methods to add an integrity signature to an encrypted security-target.
    1. At the time of encryption, an integrity signature may be generated and added to the BCB for the security-target as additional information in the security-result field.
    2. The encrypted block may be replicated as a new block and integrity signed.
    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.

8. Security Considerations

Given the nature of delay-tolerant networking 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 throughout the DTN. This section provides additional discussion on security threats that BPSEC will face and describe in additional detail 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:

  • Unprivileged Node: Mallory has not been provisioned within the secure environment and only has access to cryptographic material which has been publicly-shared.
  • Legitimate Node: Mallory is within the secure environment and therefore has access to cryptographic material which has been provisioned to Mallory (i.e., K_M) as well as material which has been publicly-shared.
  • Privileged Node: Mallory is a privileged node within the secure environment and therefore has access to cryptographic material which has been provisioned to Mallory, Alice and/or Bob (i.e. K_M, K_A, and/or K_B) as well as material which has been publicly-shared.

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 mechansims 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. Ciphersuite 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:

  • 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 which rely on a key distribution center or other trust center impractical under certain conditions.

10. Conformance

All implementations are strongly RECOMMENDED to provide some method of hop-by-hop verification by generating a hash to some canonical form of the bundle and placing an integrity signature on that form using a BIB.

11. IANA Considerations

This protocol has fields that have been registered by IANA.

11.1. Bundle Block Types

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

Additional Entries for the Bundle Block-Type Codes Registry:

Value Description Reference
2 Block Integrity Block This document
3 Block Confidentiality Block This document

11.2. Cipher Suite Flags

This protocol has a cipher suite flags field and certain flags are defined. An IANA registry has been set up as follows.

The registration policy for this registry is: Specification Required

The Value range is: Variable Length

Cipher Suite Flag Registry:

Bit Position (right to left) Description Reference
0 Block contains result This document
1 Block Contains parameters This document
2 Source EID ref present This document
>3 Reserved This document

11.3. Parameters and Results

This protocol has fields for cipher suite parameters and results. The field is a type-length-value triple and a registry is required for the "type" sub-field. The values for "type" apply to both the cipher suite parameters and the cipher suite results fields. Certain values are defined. An IANA registry has been set up as follows.

The registration policy for this registry is: Specification Required

The Value range is: 8-bit unsigned integer.

Cipher Suite Parameters and Results Type Registry:

Value Description Reference
0 reserved Section 3.7
1 initialization vector (IV) Section 3.7
2 reserved Section 3.7
3 key-information Section 3.7
4 content-range (pair of Unsigned Integers) Section 3.7
5 integrity signature Section 3.7
6 unassigned Section 3.7
7 salt Section 3.7
8 BCB integrity check value (ICV) Section 3.7
9-191 reserved Section 3.7
192-250 private use Section 3.7
251-255 reserved Section 3.7

12. References

12.1. Normative References

[BPBIS] Burleigh, S., Fall, K. and E. Birrane, "Bundle Protocol", Internet-Draft draft-ietf-dtn-bpbis-04, 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

[BPBISCBOR] Burleigh, S., "Bundle Protocol CBOR Representation Specification", Internet-Draft draft-burleigh-dtn-rs-cbor-01, April 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: Edward.Birrane@jhuapl.edu
Kenneth McKeever The Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Rd. Laurel, MD 20723 US Phone: +1 443 778 2237 EMail: Ken.McKeever@jhuapl.edu