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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-31) exists of draft-ietf-dtn-bpbis-14 ** Obsolete normative reference: RFC 7049 (Obsoleted by RFC 8949) == Outdated reference: A later version (-02) exists of draft-ietf-dtn-bpsec-interop-sc-00 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Delay-Tolerant Networking E. Birrane 3 Internet-Draft K. McKeever 4 Intended status: Standards Track JHU/APL 5 Expires: March 21, 2020 September 18, 2019 7 Bundle Protocol Security Specification 8 draft-ietf-dtn-bpsec-12 10 Abstract 12 This document defines a security protocol providing end to end data 13 integrity and confidentiality services for the Bundle Protocol. 15 Status of This Memo 17 This Internet-Draft is submitted in full conformance with the 18 provisions of BCP 78 and BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF). Note that other groups may also distribute 22 working documents as Internet-Drafts. The list of current Internet- 23 Drafts is at https://datatracker.ietf.org/drafts/current/. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 This Internet-Draft will expire on March 21, 2020. 32 Copyright Notice 34 Copyright (c) 2019 IETF Trust and the persons identified as the 35 document authors. All rights reserved. 37 This document is subject to BCP 78 and the IETF Trust's Legal 38 Provisions Relating to IETF Documents 39 (https://trustee.ietf.org/license-info) in effect on the date of 40 publication of this document. Please review these documents 41 carefully, as they describe your rights and restrictions with respect 42 to this document. Code Components extracted from this document must 43 include Simplified BSD License text as described in Section 4.e of 44 the Trust Legal Provisions and are provided without warranty as 45 described in the Simplified BSD License. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 50 1.1. Supported Security Services . . . . . . . . . . . . . . . 3 51 1.2. Specification Scope . . . . . . . . . . . . . . . . . . . 4 52 1.3. Related Documents . . . . . . . . . . . . . . . . . . . . 5 53 1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 54 2. Design Decisions . . . . . . . . . . . . . . . . . . . . . . 7 55 2.1. Block-Level Granularity . . . . . . . . . . . . . . . . . 7 56 2.2. Multiple Security Sources . . . . . . . . . . . . . . . . 8 57 2.3. Mixed Security Policy . . . . . . . . . . . . . . . . . . 8 58 2.4. User-Defined Security Contexts . . . . . . . . . . . . . 9 59 2.5. Deterministic Processing . . . . . . . . . . . . . . . . 9 60 3. Security Blocks . . . . . . . . . . . . . . . . . . . . . . . 9 61 3.1. Block Definitions . . . . . . . . . . . . . . . . . . . . 9 62 3.2. Uniqueness . . . . . . . . . . . . . . . . . . . . . . . 10 63 3.3. Target Multiplicity . . . . . . . . . . . . . . . . . . . 11 64 3.4. Target Identification . . . . . . . . . . . . . . . . . . 11 65 3.5. Block Representation . . . . . . . . . . . . . . . . . . 12 66 3.6. Abstract Security Block . . . . . . . . . . . . . . . . . 12 67 3.7. Block Integrity Block . . . . . . . . . . . . . . . . . . 15 68 3.8. Block Confidentiality Block . . . . . . . . . . . . . . . 16 69 3.9. Block Interactions . . . . . . . . . . . . . . . . . . . 17 70 3.10. Parameter and Result Identification . . . . . . . . . . . 19 71 3.11. BSP Block Examples . . . . . . . . . . . . . . . . . . . 19 72 3.11.1. Example 1: Constructing a Bundle with Security . . . 19 73 3.11.2. Example 2: Adding More Security At A New Node . . . 20 74 4. Canonical Forms . . . . . . . . . . . . . . . . . . . . . . . 22 75 5. Security Processing . . . . . . . . . . . . . . . . . . . . . 23 76 5.1. Bundles Received from Other Nodes . . . . . . . . . . . . 23 77 5.1.1. Receiving BCBs . . . . . . . . . . . . . . . . . . . 23 78 5.1.2. Receiving BIBs . . . . . . . . . . . . . . . . . . . 24 79 5.2. Bundle Fragmentation and Reassembly . . . . . . . . . . . 25 80 6. Key Management . . . . . . . . . . . . . . . . . . . . . . . 25 81 7. Security Policy Considerations . . . . . . . . . . . . . . . 25 82 8. Security Considerations . . . . . . . . . . . . . . . . . . . 27 83 8.1. Attacker Capabilities and Objectives . . . . . . . . . . 27 84 8.2. Attacker Behaviors and BPSec Mitigations . . . . . . . . 28 85 8.2.1. Eavesdropping Attacks . . . . . . . . . . . . . . . . 28 86 8.2.2. Modification Attacks . . . . . . . . . . . . . . . . 29 87 8.2.3. Topology Attacks . . . . . . . . . . . . . . . . . . 30 88 8.2.4. Message Injection . . . . . . . . . . . . . . . . . . 30 89 9. Security Context Considerations . . . . . . . . . . . . . . . 31 90 9.1. Identification and Configuration . . . . . . . . . . . . 31 91 9.2. Authorship . . . . . . . . . . . . . . . . . . . . . . . 32 92 10. Defining Other Security Blocks . . . . . . . . . . . . . . . 33 93 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 94 11.1. Bundle Block Types . . . . . . . . . . . . . . . . . . . 34 95 11.2. Security Context Identifiers . . . . . . . . . . . . . . 34 96 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 97 12.1. Normative References . . . . . . . . . . . . . . . . . . 35 98 12.2. Informative References . . . . . . . . . . . . . . . . . 35 99 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 36 100 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 102 1. Introduction 104 This document defines security features for the Bundle Protocol (BP) 105 [I-D.ietf-dtn-bpbis] and is intended for use in Delay Tolerant 106 Networks (DTNs) to provide end-to-end security services. 108 The Bundle Protocol specification [I-D.ietf-dtn-bpbis] defines DTN as 109 referring to "a networking architecture providing communications in 110 and/or through highly stressed environments" where "BP may be viewed 111 as sitting at the application layer of some number of constituent 112 networks, forming a store-carry-forward overlay network". The term 113 "stressed" environment refers to multiple challenging conditions 114 including intermittent connectivity, large and/or variable delays, 115 asymmetric data rates, and high bit error rates. 117 The BP might be deployed such that portions of the network cannot be 118 trusted, posing the usual security challenges related to 119 confidentiality and integrity. However, the stressed nature of the 120 BP operating environment imposes unique conditions where usual 121 transport security mechanisms may not be sufficient. For example, 122 the store-carry-forward nature of the network may require protecting 123 data at rest, preventing unauthorized consumption of critical 124 resources such as storage space, and operating without regular 125 contact with a centralized security oracle (such as a certificate 126 authority). 128 An end-to-end security service is needed that operates in all of the 129 environments where the BP operates. 131 1.1. Supported Security Services 133 BPSec provides end-to-end integrity and confidentiality services for 134 BP bundles, as defined in this section. 136 Integrity services ensure that changes to target data within a 137 bundle, if any, can be discovered. Data changes may be caused by 138 processing errors, environmental conditions, or intentional 139 manipulation. In the context of BPSec, integrity services apply to 140 plain-text in the bundle. 142 Confidentiality services ensure that target data is unintelligible to 143 nodes in the DTN, except for authorized nodes possessing special 144 information. This generally means producing cipher-text from plain- 145 text and generating authentication information for that cipher-text. 146 Confidentiality, in this context, applies to the contents of target 147 data and does not extend to hiding the fact that confidentiality 148 exists in the bundle. 150 NOTE: Hop-by-hop authentication is NOT a supported security service 151 in this specification, for three reasons. 153 1. The term "hop-by-hop" is ambiguous in a BP overlay, as nodes that 154 are adjacent in the overlay may not be adjacent in physical 155 connectivity. This condition is difficult or impossible to 156 detect and therefore hop-by-hop authentication is difficult or 157 impossible to enforce. 159 2. Networks in which BPSec may be deployed may have a mixture of 160 security-aware and not-security-aware nodes. Hop-by-hop 161 authentication cannot be deployed in a network if adjacent nodes 162 in the network have different security capabilities. 164 3. Hop-by-hop authentication is a special case of data integrity and 165 can be achieved with the integrity mechanisms defined in this 166 specification. Therefore, a separate authentication service is 167 not necessary. 169 1.2. Specification Scope 171 This document defines the security services provided by the BPSec. 172 This includes the data specification for representing these services 173 as BP extension blocks, and the rules for adding, removing, and 174 processing these blocks at various points during the bundle's 175 traversal of the DTN. 177 BPSec applies only to those nodes that implement it, known as 178 "security-aware" nodes. There might be other nodes in the DTN that 179 do not implement BPSec. While all nodes in a BP overlay can exchange 180 bundles, BPSec security operations can only happen at BPSec security- 181 aware nodes. 183 BPSec addresses only the security of data traveling over the DTN, not 184 the underlying DTN itself. Furthermore, while the BPSec protocol can 185 provide security-at-rest in a store-carry-forward network, it does 186 not address threats which share computing resources with the DTN and/ 187 or BPSec software implementations. These threats may be malicious 188 software or compromised libraries which intend to intercept data or 189 recover cryptographic material. Here, it is the responsibility of 190 the BPSec implementer to ensure that any cryptographic material, 191 including shared secret or private keys, is protected against access 192 within both memory and storage devices. 194 This specification addresses neither the fitness of externally- 195 defined cryptographic methods nor the security of their 196 implementation. Different networking conditions and operational 197 considerations require varying strengths of security mechanism such 198 that mandating a cipher suite in this specification may result in too 199 much security for some networks and too little security in others. 200 It is expected that separate documents will be standardized to define 201 security contexts and cipher suites compatible with BPSec, to include 202 those that should be used to assess interoperability and those fit 203 for operational use in various network scenarios. A sample security 204 context has been defined ([I-D.ietf-dtn-bpsec-interop-sc]) to support 205 interoperability testing and serve as an exemplar for how security 206 contexts should be defined for this specification. 208 This specification does not address the implementation of security 209 policy and does not provide a security policy for the BPSec. Similar 210 to cipher suites, security policies are based on the nature and 211 capabilities of individual networks and network operational concepts. 212 This specification does provide policy considerations when building a 213 security policy. 215 With the exception of the Bundle Protocol, this specification does 216 not address how to combine the BPSec security blocks with other 217 protocols, other BP extension blocks, or other best practices to 218 achieve security in any particular network implementation. 220 1.3. Related Documents 222 This document is best read and understood within the context of the 223 following other DTN documents: 225 "Delay-Tolerant Networking Architecture" [RFC4838] defines the 226 architecture for DTNs and identifies certain security assumptions 227 made by existing Internet protocols that are not valid in a DTN. 229 The Bundle Protocol [I-D.ietf-dtn-bpbis] defines the format and 230 processing of bundles, defines the extension block format used to 231 represent BPSec security blocks, and defines the canonicalization 232 algorithms used by this specification. 234 The Concise Binary Object Representation (CBOR) format [RFC7049] 235 defines a data format that allows for small code size, fairly small 236 message size, and extensibility without version negotiation. The 237 block-specific data associated with BPSec security blocks are encoded 238 in this data format. 240 The Bundle Security Protocol [RFC6257] and Streamlined Bundle 241 Security Protocol [I-D.birrane-dtn-sbsp] documents introduced the 242 concepts of using BP extension blocks for security services in a DTN. 243 The BPSec is a continuation and refinement of these documents. 245 1.4. Terminology 247 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 248 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 249 "OPTIONAL" in this document are to be interpreted as described in 250 [RFC2119]. 252 This section defines terminology either unique to the BPSec or 253 otherwise necessary for understanding the concepts defined in this 254 specification. 256 o Bundle Destination - the node which receives a bundle and delivers 257 the payload of the bundle to an application. Also, the Node ID of 258 the BPA receiving the bundle. The bundle destination acts as the 259 security destination for every security target in every security 260 block in every bundle it receives. 262 o Bundle Source - the node which originates a bundle. Also, the 263 Node ID of the BPA originating the bundle. 265 o Cipher Suite - a set of one or more algorithms providing integrity 266 and confidentiality services. Cipher suites may define necessary 267 parameters but do not provide values for those parameters. 269 o Forwarder - any node that transmits a bundle in the DTN. Also, 270 the Node ID of the Bundle Protocol Agent (BPA) that sent the 271 bundle on its most recent hop. 273 o Intermediate Receiver, Waypoint, or Next Hop - any node that 274 receives a bundle from a Forwarder that is not the Destination. 275 Also, the Node ID of the BPA at any such node. 277 o Path - the ordered sequence of nodes through which a bundle passes 278 on its way from Source to Destination. The path is not 279 necessarily known in advance by the bundle or any BPAs in the DTN. 281 o Security Block - a BPSec extension block in a bundle. 283 o Security Context - the set of assumptions, algorithms, 284 configurations and policies used to implement security services. 286 o Security Destination - a bundle node that processes one or more 287 security blocks in a bundle. Also, the Node ID of that node. 289 o Security Operation - the application of a security service to a 290 security target, notated as OP(security service, security target). 291 For example, OP(confidentiality, payload). Every security 292 operation in a bundle MUST be unique, meaning that a security 293 service can only be applied to a security target once in a bundle. 294 A security operation is implemented by a security block. 296 o Security Service - the security features supported by this 297 specification: either integrity or confidentiality. 299 o Security Source - a bundle node that adds a security block to a 300 bundle. Also, the Node ID of that node. 302 o Security Target - the block within a bundle that receives a 303 security-service as part of a security-operation. 305 2. Design Decisions 307 The application of security services in a DTN is a complex endeavor 308 that must consider physical properties of the network, policies at 309 each node, and various application security requirements. This 310 section identifies those desirable properties that guide design 311 decisions for this specification and are necessary for understanding 312 the format and behavior of the BPSec protocol. 314 2.1. Block-Level Granularity 316 Security services within this specification must allow different 317 blocks within a bundle to have different security services applied to 318 them. 320 Blocks within a bundle represent different types of information. The 321 primary block contains identification and routing information. The 322 payload block carries application data. Extension blocks carry a 323 variety of data that may augment or annotate the payload, or 324 otherwise provide information necessary for the proper processing of 325 a bundle along a path. Therefore, applying a single level and type 326 of security across an entire bundle fails to recognize that blocks in 327 a bundle represent different types of information with different 328 security needs. 330 For example, a payload block might be encrypted to protect its 331 contents and an extension block containing summary information 332 related to the payload might be integrity signed but unencrypted to 333 provide waypoints access to payload-related data without providing 334 access to the payload. 336 2.2. Multiple Security Sources 338 A bundle can have multiple security blocks and these blocks can have 339 different security sources. BPSec implementations MUST NOT assume 340 that all blocks in a bundle have the same security operations and/or 341 security sources. 343 The Bundle Protocol allows extension blocks to be added to a bundle 344 at any time during its existence in the DTN. When a waypoint adds a 345 new extension block to a bundle, that extension block MAY have 346 security services applied to it by that waypoint. Similarly, a 347 waypoint MAY add a security service to an existing extension block, 348 consistent with its security policy. 350 When a waypoint adds a security service to the bundle, the waypoint 351 is the security source for that service. The security block(s) which 352 represent that service in the bundle may need to record this security 353 source as the bundle destination might need this information for 354 processing. 356 For example, a bundle source may choose to apply an integrity service 357 to its plain-text payload. Later a waypoint node, representing a 358 gateway to an insecure portion of the DTN, may receive the bundle and 359 choose to apply a confidentiality service. In this case, the 360 integrity security source is the bundle source and the 361 confidentiality security source is the waypoint node. 363 2.3. Mixed Security Policy 365 The security policy enforced by nodes in the DTN may differ. 367 Some waypoints might not be security aware and will not be able to 368 process security blocks. Therefore, security blocks must have their 369 processing flags set such that the block will be treated 370 appropriately by non-security-aware waypoints. 372 Some waypoints will have security policies that require evaluating 373 security services even if they are not the bundle destination or the 374 final intended destination of the service. For example, a waypoint 375 could choose to verify an integrity service even though the waypoint 376 is not the bundle destination and the integrity service will be 377 needed by other nodes along the bundle's path. 379 Some waypoints will determine, through policy, that they are the 380 intended recipient of the security service and terminate the security 381 service in the bundle. For example, a gateway node could determine 382 that, even though it is not the destination of the bundle, it should 383 verify and remove a particular integrity service or attempt to 384 decrypt a confidentiality service, before forwarding the bundle along 385 its path. 387 Some waypoints could understand security blocks but refuse to process 388 them unless they are the bundle destination. 390 2.4. User-Defined Security Contexts 392 A security context is the union of security algorithms (cipher 393 suites), policies associated with the use of those algorithms, and 394 configuration values. Different contexts may specify different 395 algorithms, different polices, or different configuration values used 396 in the implementation of their security services. BPSec must provide 397 a mechanism for users to define their own security contexts. 399 For example, some users might prefer a SHA2 hash function for 400 integrity whereas other users might prefer a SHA3 hash function. The 401 security services defined in this specification must provide a 402 mechanism for determining what cipher suite, policy, and 403 configuration has been used to populate a security block. 405 2.5. Deterministic Processing 407 Whenever a node determines that it must process more than one 408 security block in a received bundle (either because the policy at a 409 waypoint states that it should process security blocks or because the 410 node is the bundle destination) the order in which security blocks 411 are processed must be deterministic. All nodes must impose this same 412 deterministic processing order for all security blocks. This 413 specification provides determinism in the application and evaluation 414 of security services, even when doing so results in a loss of 415 flexibility. 417 3. Security Blocks 419 3.1. Block Definitions 421 This specification defines two types of security block: the Block 422 Integrity Block (BIB) and the Block Confidentiality Block (BCB). 424 The BIB is used to ensure the integrity of its plain-text security 425 target(s). The integrity information in the BIB MAY be verified 426 by any node along the bundle path from the BIB security source to 427 the bundle destination. Security-aware waypoints add or remove 428 BIBs from bundles in accordance with their security policy. BIBs 429 are never used to sign the cipher- text provided by a BCB. 431 The BCB indicates that the security target(s) have been encrypted 432 at the BCB security source in order to protect their content while 433 in transit. The BCB is decrypted by security- aware nodes in the 434 network, up to and including the bundle destination, as a matter 435 of security policy. BCBs additionally provide authentication 436 mechanisms for the cipher-text they generate. 438 3.2. Uniqueness 440 Security operations in a bundle MUST be unique; the same security 441 service MUST NOT be applied to a security target more than once in a 442 bundle. Since a security operation is represented as a security 443 block, this limits what security blocks may be added to a bundle: if 444 adding a security block to a bundle would cause some other security 445 block to no longer represent a unique security operation then the new 446 block MUST NOT be added. It is important to note that any cipher- 447 text integrity mechanism supplied by the BCB is considered part of 448 the confidentiality service and, therefore, unique from the plain- 449 text integrity service provided by the BIB. 451 If multiple security blocks representing the same security operation 452 were allowed in a bundle at the same time, there would exist 453 ambiguity regarding block processing order and the property of 454 deterministic processing blocks would be lost. 456 Using the notation OP(service, target), several examples illustrate 457 this uniqueness requirement. 459 o Signing the payload twice: The two operations OP(integrity, 460 payload) and OP(integrity, payload) are redundant and MUST NOT 461 both be present in the same bundle at the same time. 463 o Signing different blocks: The two operations OP(integrity, 464 payload) and OP(integrity, extension_block_1) are not redundant 465 and both may be present in the same bundle at the same time. 466 Similarly, the two operations OP(integrity, extension_block_1) and 467 OP(integrity,extension_block_2) are also not redundant and may 468 both be present in the bundle at the same time. 470 o Different Services on same block: The two operations OP(integrity, 471 payload) and OP(confidentiality, payload) are not inherently 472 redundant and may both be present in the bundle at the same time, 473 pursuant to other processing rules in this specification. 475 3.3. Target Multiplicity 477 Under special circumstances, a single security block MAY represent 478 multiple security operations as a way of reducing the overall number 479 of security blocks present in a bundle. In these circumstances, 480 reducing the number of security blocks in the bundle reduces the 481 amount of redundant information in the bundle. 483 A set of security operations can be represented by a single security 484 block when all of the following conditions are true. 486 o The security operations apply the same security service. For 487 example, they are all integrity operations or all confidentiality 488 operations. 490 o The security context parameters and key information for the 491 security operations are identical. 493 o The security source for the security operations is the same. 494 Meaning the set of operations are being added by the same node. 496 o No security operations have the same security target, as that 497 would violate the need for security operations to be unique. 499 o None of the security operations conflict with security operations 500 already present in the bundle. 502 When representing multiple security operations in a single security 503 block, the information that is common across all operations is 504 represented once in the security block, and the information which is 505 different (e.g., the security targets) are represented individually. 507 It is RECOMMENDED that if a node processes any security operation in 508 a security block that it process all security operations in the 509 security block. This allows security sources to assert that the set 510 of security operations in a security block are expected to be 511 processed by the same security destination. However, the 512 determination of whether a node actually is a security destination or 513 not is a matter of the policy of the node itself. In cases where a 514 receiving node determines that it is the security destination of only 515 a subset of the security operations in a security block, the node may 516 choose to only process that subset of security operations. 518 3.4. Target Identification 520 A security target is a block in the bundle to which a security 521 service applies. This target must be uniquely and unambiguously 522 identifiable when processing a security block. The definition of the 523 extension block header from [I-D.ietf-dtn-bpbis] provides a "Block 524 Number" field suitable for this purpose. Therefore, a security 525 target in a security block MUST be represented as the Block Number of 526 the target block. 528 3.5. Block Representation 530 Each security block uses the Canonical Bundle Block Format as defined 531 in [I-D.ietf-dtn-bpbis]. That is, each security block is comprised 532 of the following elements: 534 o Block Type Code 536 o Block Number 538 o Block Processing Control Flags 540 o CRC Type and CRC Field (if present) 542 o Block Data Length 544 o Block Type Specific Data Fields 546 Security-specific information for a security block is captured in the 547 "Block Type Specific Data Fields". 549 3.6. Abstract Security Block 551 The structure of the security-specific portions of a security block 552 is identical for both the BIB and BCB Block Types. Therefore, this 553 section defines an Abstract Security Block (ASB) data structure and 554 discusses the definition, processing, and other constraints for using 555 this structure. An ASB is never directly instantiated within a 556 bundle, it is only a mechanism for discussing the common aspects of 557 BIB and BCB security blocks. 559 The fields of the ASB SHALL be as follows, listed in the order in 560 which they must appear. 562 Security Targets: 563 This field identifies the block(s) targeted by the security 564 operation(s) represented by this security block. Each target 565 block is represented by its unique Block Number. This field 566 SHALL be represented by a CBOR array of data items. Each 567 target within this CBOR array SHALL be represented by a CBOR 568 unsigned integer. This array MUST have at least 1 entry and 569 each entry MUST represent the Block Number of a block that 570 exists in the bundle. There MUST NOT be duplicate entries in 571 this array. 573 Security Context Id: 574 This field identifies the security context used to implement 575 the security service represented by this block and applied to 576 each security target. This field SHALL be represented by a 577 CBOR unsigned integer. 579 Security Context Flags: 580 This field identifies which optional fields are present in the 581 security block. This field SHALL be represented as a CBOR 582 unsigned integer whose contents shall be interpreted as a bit 583 field. Each bit in this bit field indicates the presence (bit 584 set to 1) or absence (bit set to 0) of optional data in the 585 security block. The association of bits to security block data 586 is defined as follows. 588 Bit 1 (the least-significant bit, 0x01): Security Context 589 Parameters Present Flag. 591 Bit 2 (0x02): Security Source Present Flag. 593 Bit >2 Reserved 595 Implementations MUST set reserved bits to 0 when writing this 596 field and MUST ignore the values of reserved bits when reading 597 this field. For unreserved bits, a value of 1 indicates that 598 the associated security block field MUST be included in the 599 security block. A value of 0 indicates that the associated 600 security block field MUST NOT be in the security block. 602 Security Source (Optional): 603 This field identifies the Endpoint that inserted the security 604 block in the bundle. If the security source field is not 605 present then the source MUST be inferred from other 606 information, such as the bundle source, previous hop, or other 607 values defined by security policy. This field SHALL be 608 represented by a CBOR array in accordance with 609 [I-D.ietf-dtn-bpbis] rules for representing Endpoint 610 Identifiers (EIDs). 612 Security Context Parameters (Optional): 613 This field captures one or more security context parameters 614 that should be provided to security-aware nodes when processing 615 the security service described by this security block. This 616 field SHALL be represented by a CBOR array. Each entry in this 617 array is a single security context parameter. A single 618 parameter SHALL also be represented as a CBOR array comprising 619 a 2-tuple of the id and value of the parameter, as follows. 621 * Parameter Id. This field identifies which parameter is 622 being specified. This field SHALL be represented as a CBOR 623 unsigned integer. Parameter Ids are selected as described 624 in Section 3.10. 626 * Parameter Value. This field captures the value associated 627 with this parameter. This field SHALL be represented by the 628 applicable CBOR representation of the parameter, in 629 accordance with Section 3.10. 631 The logical layout of the parameters array is illustrated in 632 Figure 1. 634 +----------------+----------------+ +----------------+ 635 | Parameter 1 | Parameter 2 | ... | Parameter N | 636 +------+---------+------+---------+ +------+---------+ 637 | Id | Value | Id | Value | | Id | Value | 638 +------+---------+------+---------+ +------+---------+ 640 Figure 1: Security Context Parameters 642 Security Results: 643 This field captures the results of applying a security service 644 to the security targets of the security block. This field 645 SHALL be represented as a CBOR array of target results. Each 646 entry in this array represents the set of security results for 647 a specific security target. The target results MUST be ordered 648 identically to the Security Targets field of the security 649 block. This means that the first set of target results in this 650 array corresponds to the first entry in the Security Targets 651 field of the security block, and so on. There MUST be one 652 entry in this array for each entry in the Security Targets 653 field of the security block. 655 The set of security results for a target is also represented as 656 a CBOR array of individual results. An individual result is 657 represented as a 2-tuple of a result id and a result value, 658 defined as follows. 660 * Result Id. This field identifies which security result is 661 being specified. Some security results capture the primary 662 output of a cipher suite. Other security results contain 663 additional annotative information from cipher suite 664 processing. This field SHALL be represented as a CBOR 665 unsigned integer. Security result Ids will be as specified 666 in Section 3.10. 668 * Result Value. This field captures the value associated with 669 the result. This field SHALL be represented by the 670 applicable CBOR representation of the result value, in 671 accordance with Section 3.10. 673 The logical layout of the security results array is illustrated 674 in Figure 2. In this figure there are N security targets for 675 this security block. The first security target contains M 676 results and the Nth security target contains K results. 678 +------------------------------+ +------------------------------+ 679 | Target 1 | | Target N | 680 +------------+----+------------+ +------------------------------+ 681 | Result 1 | | Result M | ... | Result 1 | | Result K | 682 +----+-------+ .. +----+-------+ +----+-------+ .. +----+-------+ 683 | Id | Value | | Id | Value | | Id | Value | | Id | Value | 684 +----+-------+ +----+-------+ +----+-------+ +----+-------+ 686 Figure 2: Security Results 688 3.7. Block Integrity Block 690 A BIB is a bundle extension block with the following characteristics. 692 o The Block Type Code value is as specified in Section 11.1. 694 o The Block Type Specific Data Fields follow the structure of the 695 ASB. 697 o A security target listed in the Security Targets field MUST NOT 698 reference a security block defined in this specification (e.g., a 699 BIB or a BCB). 701 o The Security Context Id MUST utilize an end-to-end authentication 702 cipher or an end-to-end error detection cipher. 704 o An EID-reference to the security source MAY be present. If this 705 field is not present, then the security source of the block SHOULD 706 be inferred according to security policy and MAY default to the 707 bundle source. The security source MAY be specified as part of 708 key information described in Section 3.10. 710 Notes: 712 o It is RECOMMENDED that cipher suite designers carefully consider 713 the effect of setting flags that either discard the block or 714 delete the bundle in the event that this block cannot be 715 processed. 717 o Since OP(integrity, target) is allowed only once in a bundle per 718 target, it is RECOMMENDED that users wishing to support multiple 719 integrity signatures for the same target define a multi-signature 720 cipher suite. 722 o For some cipher suites, (e.g., those using asymmetric keying to 723 produce signatures or those using symmetric keying with a group 724 key), the security information MAY be checked at any hop on the 725 way to the destination that has access to the required keying 726 information, in accordance with Section 3.9. 728 o The use of a generally available key is RECOMMENDED if custodial 729 transfer is employed and all nodes SHOULD verify the bundle before 730 accepting custody. 732 3.8. Block Confidentiality Block 734 A BCB is a bundle extension block with the following characteristics. 736 The Block Type Code value is as specified in Section 11.1. 738 The Block Processing Control flags value can be set to whatever 739 values are required by local policy, except that this block MUST 740 have the "replicate in every fragment" flag set if the target of 741 the BCB is the Payload Block. Having that BCB in each fragment 742 indicates to a receiving node that the payload portion of each 743 fragment represents cipher-text. 745 The Block Type Specific Data Fields follow the structure of the 746 ASB. 748 A security target listed in the Security Targets field can 749 reference the payload block, a non-security extension block, or a 750 BIB. A BCB MUST NOT include another BCB as a security target. A 751 BCB MUST NOT target the primary block. 753 The Security Context Id MUST utilize a confidentiality cipher that 754 provides authenticated encryption with associated data (AEAD). 756 Additional information created by a cipher suite (such as 757 additional authenticated data) can be placed either in a security 758 result field or in the generated cipher-text. The determination 759 of where to place these data is a function of the cipher suite and 760 security context used. 762 An EID-reference to the security source MAY be present. If this 763 field is not present, then the security source of the block SHOULD 764 be inferred according to security policy and MAY default to the 765 bundle source. The security source MAY be specified as part of 766 the key information described in Section 3.10. 768 The BCB modifies the contents of its security target(s). When a BCB 769 is applied, the security target body data are encrypted "in-place". 770 Following encryption, the security target Block Type Specific Data 771 field contains cipher-text, not plain-text. Other block fields 772 remain unmodified, with the exception of the Block Data Length field, 773 which MUST be updated to reflect the new length of the Block Type 774 Specific Data field. 776 Notes: 778 o It is RECOMMENDED that cipher suite designers carefully consider 779 the effect of setting flags that either discard the block or 780 delete the bundle in the event that this block cannot be 781 processed. 783 o The BCB block processing control flags can be set independently 784 from the processing control flags of the security target(s). The 785 setting of such flags SHOULD be an implementation/policy decision 786 for the encrypting node. 788 3.9. Block Interactions 790 The security block types defined in this specification are designed 791 to be as independent as possible. However, there are some cases 792 where security blocks may share a security target creating processing 793 dependencies. 795 If a security target of a BCB is also a security target of a BIB, an 796 undesirable condition occurs where a security aware waypoint would be 797 unable to validate the BIB because one of its security target's 798 contents have been encrypted by a BCB. To address this situation the 799 following processing rules MUST be followed. 801 o When adding a BCB to a bundle, if some (or all) of the security 802 targets of the BCB also match all of the security targets of an 803 existing BIB, then the existing BIB MUST also be encrypted. This 804 can be accomplished by either adding a new BCB that targets the 805 existing BIB, or by adding the BIB to the list of security targets 806 for the BCB. Deciding which way to represent this situation is a 807 matter of security policy. 809 o When adding a BCB to a bundle, if some (or all) of the security 810 targets of the BCB match some (but not all) of the security 811 targets of a BIB then that BIB MUST be altered in the following 812 way. Any security results in the BIB associated with the BCB 813 security targets MUST be removed from the BIB and placed in a new 814 BIB. This newly created BIB MUST then be encrypted. The 815 encryption of the new BIB can be accomplished by either adding a 816 new BCB that targets the new BIB, or by adding the new BIB to the 817 list of security targets for the BCB. Deciding which way to 818 represent this situation is a matter of security policy. 820 o A BIB MUST NOT be added for a security target that is already the 821 security target of a BCB. In this instance, the BCB is already 822 providing authentication and integrity of the security target and 823 the BIB would be redundant, insecure, and cause ambiguity in block 824 processing order. 826 o A BIB integrity value MUST NOT be evaluated if the BIB is the 827 security target of an existing BCB. In this case, the BIB data is 828 encrypted. 830 o A BIB integrity value MUST NOT be evaluated if the security target 831 of the BIB is also the security target of a BCB. In such a case, 832 the security target data contains cipher-text as it has been 833 encrypted. 835 o As mentioned in Section 3.7, a BIB MUST NOT have a BCB as its 836 security target. 838 These restrictions on block interactions impose a necessary ordering 839 when applying security operations within a bundle. Specifically, for 840 a given security target, BIBs MUST be added before BCBs. This 841 ordering MUST be preserved in cases where the current BPA is adding 842 all of the security blocks for the bundle or whether the BPA is a 843 waypoint adding new security blocks to a bundle that already contains 844 security blocks. 846 NOTE: Since any cipher suite used with a BCB MUST be an AEAD cipher 847 suite, it is inefficient and possibly insecure for a single security 848 source to add both a BIB and a BCB for the same security target. In 849 cases where a security source wishes to calculate both a plain-text 850 integrity mechanism and encrypt a security target, a BCB with a 851 cipher suite that generates such signatures as additional security 852 results SHOULD be used instead. 854 3.10. Parameter and Result Identification 856 Each security context MUST define its own context parameters and 857 security results. Each defined parameter and result is represented 858 as the tuple of an identifier and a value. Identifiers are always 859 represented as a CBOR unsigned integer. The CBOR encoding of values 860 is as defined by the security context specification. 862 Identifiers MUST be unique for a given security context but do not 863 need to be unique amongst all security contexts. 865 3.11. BSP Block Examples 867 This section provides two examples of BPSec blocks applied to a 868 bundle. In the first example, a single node adds several security 869 operations to a bundle. In the second example, a waypoint node 870 received the bundle created in the first example and adds additional 871 security operations. In both examples, the first column represents 872 blocks within a bundle and the second column represents the Block 873 Number for the block, using the terminology B1...Bn for the purpose 874 of illustration. 876 3.11.1. Example 1: Constructing a Bundle with Security 878 In this example a bundle has four non-security-related blocks: the 879 primary block (B1), two extension blocks (B4,B5), and a payload block 880 (B6). The bundle source wishes to provide an integrity signature of 881 the plain-text associated with the primary block, one of the 882 extension blocks, and the payload. The resultant bundle is 883 illustrated in Figure 3 and the security actions are described below. 885 Block in Bundle ID 886 +======================================+====+ 887 | Primary Block | B1 | 888 +--------------------------------------+----+ 889 | BIB | B2 | 890 | OP(integrity, targets=B1, B5, B6) | | 891 +--------------------------------------+----+ 892 | BCB | B3 | 893 | OP(confidentiality, target=B4) | | 894 +--------------------------------------+----+ 895 | Extension Block (encrypted) | B4 | 896 +--------------------------------------+----+ 897 | Extension Block | B5 | 898 +--------------------------------------+----+ 899 | Payload Block | B6 | 900 +--------------------------------------+----+ 902 Figure 3: Security at Bundle Creation 904 The following security actions were applied to this bundle at its 905 time of creation. 907 o An integrity signature applied to the canonicalized primary block 908 (B1), the second extension block (B5) and the payload block (B6). 909 This is accomplished by a single BIB (B2) with multiple targets. 910 A single BIB is used in this case because all three targets share 911 a security source, security context, and security context 912 parameters. Had this not been the case, multiple BIBs could have 913 been added instead. 915 o Confidentiality for the first extension block (B4). This is 916 accomplished by a BCB (B3). Once applied, the contents of 917 extension block B4 are encrypted. The BCB MUST hold an 918 authentication signature for the cipher-text either in the cipher- 919 text that now populated the first extension block or as a security 920 result in the BCB itself, depending on which cipher suite is used 921 to form the BCB. A plain-text integrity signature may also exist 922 as a security result in the BCB if one is provided by the selected 923 confidentiality cipher suite. 925 3.11.2. Example 2: Adding More Security At A New Node 927 Consider that the bundle as it is illustrated in Figure 3 is now 928 received by a waypoint node that wishes to encrypt the first 929 extension block and the bundle payload. The waypoint security policy 930 is to allow existing BIBs for these blocks to persist, as they may be 931 required as part of the security policy at the bundle destination. 933 The resultant bundle is illustrated in Figure 4 and the security 934 actions are described below. Note that block IDs provided here are 935 ordered solely for the purpose of this example and not meant to 936 impose an ordering for block creation. The ordering of blocks added 937 to a bundle MUST always be in compliance with [I-D.ietf-dtn-bpbis]. 939 Block in Bundle ID 940 +======================================+====+ 941 | Primary Block | B1 | 942 +--------------------------------------+----+ 943 | BIB | B2 | 944 | OP(integrity, targets=B1) | | 945 +--------------------------------------+----+ 946 | BIB (encrypted) | B7 | 947 | OP(integrity, targets=B5, B6) | | 948 +--------------------------------------+----+ 949 | BCB | B8 | 950 | OP(confidentiality, target=B4,B6,B7) | | 951 +--------------------------------------+----+ 952 | BCB | B3 | 953 | OP(confidentiality, target=B4) | | 954 +--------------------------------------+----+ 955 | Extension Block (encrypted) | B4 | 956 +--------------------------------------+----+ 957 | Extension Block (encrypted) | B5 | 958 +--------------------------------------+----+ 959 | Payload Block (encrypted) | B6 | 960 +--------------------------------------+----+ 962 Figure 4: Security At Bundle Forwarding 964 The following security actions were applied to this bundle prior to 965 its forwarding from the waypoint node. 967 o Since the waypoint node wishes to encrypt blocks B5 and B6, it 968 MUST also encrypt the BIBs providing plain-text integrity over 969 those blocks. However, BIB B2 could not be encrypted in its 970 entirety because it also held a signature for the primary block 971 (B1). Therefore, a new BIB (B7) is created and security results 972 associated with B5 and B6 are moved out of BIB B2 and into BIB B7. 974 o Now that there is no longer confusion of which plain-text 975 integrity signatures must be encrypted, a BCB is added to the 976 bundle with the security targets being the second extension block 977 (B5) and the payload (B6) as well as the newly created BIB holding 978 their plain-text integrity signatures (B7). A single new BCB is 979 used in this case because all three targets share a security 980 source, security context, and security context parameters. Had 981 this not been the case, multiple BCBs could have been added 982 instead. 984 4. Canonical Forms 986 Security services require consistency and determinism in how 987 information is presented to cipher suites at the security source and 988 at a receiving node. For example, integrity services require that 989 the same target information (e.g., the same bits in the same order) 990 is provided to the cipher suite when generating an original signature 991 and when generating a comparison signature. Canonicalization 992 algorithms are used to construct a stable, end-to-end bit 993 representation of a target block. 995 Canonical forms are not transmitted, they are used to generate input 996 to a cipher suite for security processing at a security-aware node. 998 The canonicalization of the primary block is as specified in 999 [I-D.ietf-dtn-bpbis]. 1001 All non-primary blocks share the same block structure and are 1002 canonicalized as specified in [I-D.ietf-dtn-bpbis] with the following 1003 exceptions. 1005 o If the service being applied is a confidentiality service, then 1006 the Block Type Code, Block Number, Block Processing Control Flags, 1007 CRC Type and CRC Field (if present), and Block Data Length fields 1008 MUST NOT be included in the canonicalization. Confidentiality 1009 services are used solely to convert the Block Type Specific Data 1010 Fields from plain-text to cipher-text. 1012 o Reserved flags MUST NOT be included in any canonicalization as it 1013 is not known if those flags will change in transit. 1015 These canonicalization algorithms assume that Endpoint IDs do not 1016 change from the time at which a security source adds a security block 1017 to a bundle and the time at which a node processes that security 1018 block. 1020 Cipher suites and security contexts MAY define their own 1021 canonicalization algorithms and require the use of those algorithms 1022 over the ones provided in this specification. In the event of 1023 conflicting canonicalization algorithms, those algorithms take 1024 precedence over this specification. 1026 5. Security Processing 1028 This section describes the security aspects of bundle processing. 1030 5.1. Bundles Received from Other Nodes 1032 Security blocks must be processed in a specific order when received 1033 by a security-aware node. The processing order is as follows. 1035 o When BIBs and BCBs share a security target, BCBs MUST be evaluated 1036 first and BIBs second. 1038 5.1.1. Receiving BCBs 1040 If a received bundle contains a BCB, the receiving node MUST 1041 determine whether it is the security destination for any of the 1042 security operations in the BCB. If so, the node MUST process those 1043 operations and remove any operation-specific information from the BCB 1044 prior to delivering data to an application at the node or forwarding 1045 the bundle. If processing a security operation fails, the target 1046 SHALL be processed according to the security policy. A bundle status 1047 report indicating the failure MAY be generated. When all security 1048 operations for a BCB have been removed from the BCB, the BCB MUST be 1049 removed from the bundle. 1051 If the receiving node is the destination of the bundle, the node MUST 1052 decrypt any BCBs remaining in the bundle. If the receiving node is 1053 not the destination of the bundle, the node MUST process the BCB if 1054 directed to do so as a matter of security policy. 1056 If the security policy of a security-aware node specifies that a 1057 bundle should have applied confidentiality to a specific security 1058 target and no such BCB is present in the bundle, then the node MUST 1059 process this security target in accordance with the security policy. 1060 This may involve removing the security target from the bundle. If 1061 the removed security target is the payload block, the bundle MUST be 1062 discarded. 1064 If an encrypted payload block cannot be decrypted (i.e., the cipher- 1065 text cannot be authenticated), then the bundle MUST be discarded and 1066 processed no further. If an encrypted security target other than the 1067 payload block cannot be decrypted then the associated security target 1068 and all security blocks associated with that target MUST be discarded 1069 and processed no further. In both cases, requested status reports 1070 (see [I-D.ietf-dtn-bpbis]) MAY be generated to reflect bundle or 1071 block deletion. 1073 When a BCB is decrypted, the recovered plain-text MUST replace the 1074 cipher-text in the security target Block Type Specific Data Fields. 1075 If the Block Data Length field was modified at the time of encryption 1076 it MUST be updated to reflect the decrypted block length. 1078 If a BCB contains multiple security operations, each operation 1079 processed by the node MUST be be treated as if the security operation 1080 has been represented by a single BCB with a single security 1081 operation. 1083 5.1.2. Receiving BIBs 1085 If a received bundle contains a BIB, the receiving node MUST 1086 determine whether it is the security destination for any of the 1087 security operations in the BIB. If so, the node MUST process those 1088 operations and remove any operation-specific information from the BIB 1089 prior to delivering data to an application at the node or forwarding 1090 the bundle. If processing a security operation fails, the target 1091 SHALL be processed according to the security policy. A bundle status 1092 report indicating the failure MAY be generated. When all security 1093 operations for a BIB have been removed from the BIB, the BIB MUST be 1094 removed from the bundle. 1096 A BIB MUST NOT be processed if the security target of the BIB is also 1097 the security target of a BCB in the bundle. Given the order of 1098 operations mandated by this specification, when both a BIB and a BCB 1099 share a security target, it means that the security target must have 1100 been encrypted after it was integrity signed and, therefore, the BIB 1101 cannot be verified until the security target has been decrypted by 1102 processing the BCB. 1104 If the security policy of a security-aware node specifies that a 1105 bundle should have applied integrity to a specific security target 1106 and no such BIB is present in the bundle, then the node MUST process 1107 this security target in accordance with the security policy. This 1108 may involve removing the security target from the bundle. If the 1109 removed security target is the payload or primary block, the bundle 1110 MAY be discarded. This action can occur at any node that has the 1111 ability to verify an integrity signature, not just the bundle 1112 destination. 1114 If a receiving node is not the security destination of a security 1115 operation in a BIB it MAY attempt to verify the security operation 1116 anyway to prevent forwarding corrupt data. If the verification 1117 fails, the node SHALL process the security target in accordance to 1118 local security policy. It is RECOMMENDED that if a payload integrity 1119 check fails at a waypoint that it is processed in the same way as if 1120 the check fails at the bundle destination. If the check passes, the 1121 node MUST NOT remove the security operation from the BIB prior to 1122 forwarding. 1124 If a BIB contains multiple security operations, each operation 1125 processed by the node MUST be be treated as if the security operation 1126 has been represented by a single BIB with a single security 1127 operation. 1129 5.2. Bundle Fragmentation and Reassembly 1131 If it is necessary for a node to fragment a bundle payload, and 1132 security services have been applied to that bundle, the fragmentation 1133 rules described in [I-D.ietf-dtn-bpbis] MUST be followed. As defined 1134 there and summarized here for completeness, only the payload block 1135 can be fragmented; security blocks, like all extension blocks, can 1136 never be fragmented. 1138 Due to the complexity of payload block fragmentation, including the 1139 possibility of fragmenting payload block fragments, integrity and 1140 confidentiality operations are not to be applied to a bundle 1141 representing a fragment. Specifically, a BCB or BIB MUST NOT be 1142 added to a bundle if the "Bundle is a Fragment" flag is set in the 1143 Bundle Processing Control Flags field. 1145 Security processing in the presence of payload block fragmentation 1146 may be handled by other mechanisms outside of the BPSec protocol or 1147 by applying BPSec blocks in coordination with an encapsulation 1148 mechanism. 1150 6. Key Management 1152 There exist a myriad of ways to establish, communicate, and otherwise 1153 manage key information in a DTN. Certain DTN deployments might 1154 follow established protocols for key management whereas other DTN 1155 deployments might require new and novel approaches. BPSec assumes 1156 that key management is handled as a separate part of network 1157 management and this specification neither defines nor requires a 1158 specific key management strategy. 1160 7. Security Policy Considerations 1162 When implementing BPSec, several policy decisions must be considered. 1163 This section describes key policies that affect the generation, 1164 forwarding, and receipt of bundles that are secured using this 1165 specification. No single set of policy decisions is envisioned to 1166 work for all secure DTN deployments. 1168 o If a bundle is received that contains more than one security 1169 operation, in violation of BPSec, then the BPA must determine how 1170 to handle this bundle. The bundle may be discarded, the block 1171 affected by the security operation may be discarded, or one 1172 security operation may be favored over another. 1174 o BPAs in the network must understand what security operations they 1175 should apply to bundles. This decision may be based on the source 1176 of the bundle, the destination of the bundle, or some other 1177 information related to the bundle. 1179 o If a waypoint has been configured to add a security operation to a 1180 bundle, and the received bundle already has the security operation 1181 applied, then the receiver must understand what to do. The 1182 receiver may discard the bundle, discard the security target and 1183 associated BPSec blocks, replace the security operation, or some 1184 other action. 1186 o It is recommended that security operations only be applied to the 1187 blocks that absolutely need them. If a BPA were to apply security 1188 operations such as integrity or confidentiality to every block in 1189 the bundle, regardless of need, there could be downstream errors 1190 processing blocks whose contents must be inspected or changed at 1191 every hop along the path. 1193 o It is recommended that BCBs be allowed to alter the size of 1194 extension blocks and the payload block. However, care must be 1195 taken to ensure that changing the size of the payload block while 1196 the bundle is in transit do not negatively affect bundle 1197 processing (e.g., calculating storage needs, scheduling 1198 transmission times, caching block byte offsets). 1200 o Adding a BIB to a security target that has already been encrypted 1201 by a BCB is not allowed. If this condition is likely to be 1202 encountered, there are (at least) three possible policies that 1203 could handle this situation. 1205 1. At the time of encryption, a plain-text integrity signature 1206 may be generated and added to the BCB for the security target 1207 as additional information in the security result field. 1209 2. The encrypted block may be replicated as a new block and 1210 integrity signed. 1212 3. An encapsulation scheme may be applied to encapsulate the 1213 security target (or the entire bundle) such that the 1214 encapsulating structure is, itself, no longer the security 1215 target of a BCB and may therefore be the security target of a 1216 BIB. 1218 o It is recommended that security policy address whether cipher 1219 suites whose cipher-text is larger (or smaller) than the initial 1220 plain-text are permitted and, if so, for what types of blocks. 1221 Changing the size of a block may cause processing difficulties for 1222 networks that calculate block offsets into bundles or predict 1223 transmission times or storage availability as a function of bundle 1224 size. In other cases, changing the size of a payload as part of 1225 encryption has no significant impact. 1227 8. Security Considerations 1229 Given the nature of DTN applications, it is expected that bundles may 1230 traverse a variety of environments and devices which each pose unique 1231 security risks and requirements on the implementation of security 1232 within BPSec. For these reasons, it is important to introduce key 1233 threat models and describe the roles and responsibilities of the 1234 BPSec protocol in protecting the confidentiality and integrity of the 1235 data against those threats. This section provides additional 1236 discussion on security threats that BPSec will face and describes how 1237 BPSec security mechanisms operate to mitigate these threats. 1239 The threat model described here is assumed to have a set of 1240 capabilities identical to those described by the Internet Threat 1241 Model in [RFC3552], but the BPSec threat model is scoped to 1242 illustrate threats specific to BPSec operating within DTN 1243 environments and therefore focuses on man-in-the-middle (MITM) 1244 attackers. In doing so, it is assumed that the DTN (or significant 1245 portions of the DTN) are completely under the control of an attacker. 1247 8.1. Attacker Capabilities and Objectives 1249 BPSec was designed to protect against MITM threats which may have 1250 access to a bundle during transit from its source, Alice, to its 1251 destination, Bob. A MITM node, Mallory, is a non-cooperative node 1252 operating on the DTN between Alice and Bob that has the ability to 1253 receive bundles, examine bundles, modify bundles, forward bundles, 1254 and generate bundles at will in order to compromise the 1255 confidentiality or integrity of data within the DTN. For the 1256 purposes of this section, any MITM node is assumed to effectively be 1257 security-aware even if it does not implement the BPSec protocol. 1258 There are three classes of MITM nodes which are differentiated based 1259 on their access to cryptographic material: 1261 o Unprivileged Node: Mallory has not been provisioned within the 1262 secure environment and only has access to cryptographic material 1263 which has been publicly-shared. 1265 o Legitimate Node: Mallory is within the secure environment and 1266 therefore has access to cryptographic material which has been 1267 provisioned to Mallory (i.e., K_M) as well as material which has 1268 been publicly-shared. 1270 o Privileged Node: Mallory is a privileged node within the secure 1271 environment and therefore has access to cryptographic material 1272 which has been provisioned to Mallory, Alice and/or Bob (i.e. 1273 K_M, K_A, and/or K_B) as well as material which has been publicly- 1274 shared. 1276 If Mallory is operating as a privileged node, this is tantamount to 1277 compromise; BPSec does not provide mechanisms to detect or remove 1278 Mallory from the DTN or BPSec secure environment. It is up to the 1279 BPSec implementer or the underlying cryptographic mechanisms to 1280 provide appropriate capabilities if they are needed. It should also 1281 be noted that if the implementation of BPSec uses a single set of 1282 shared cryptographic material for all nodes, a legitimate node is 1283 equivalent to a privileged node because K_M == K_A == K_B. 1285 A special case of the legitimate node is when Mallory is either Alice 1286 or Bob (i.e., K_M == K_A or K_M == K_B). In this case, Mallory is 1287 able to impersonate traffic as either Alice or Bob, which means that 1288 traffic to and from that node can be decrypted and encrypted, 1289 respectively. Additionally, messages may be signed as originating 1290 from one of the endpoints. 1292 8.2. Attacker Behaviors and BPSec Mitigations 1294 8.2.1. Eavesdropping Attacks 1296 Once Mallory has received a bundle, she is able to examine the 1297 contents of that bundle and attempt to recover any protected data or 1298 cryptographic keying material from the blocks contained within. The 1299 protection mechanism that BPSec provides against this action is the 1300 BCB, which encrypts the contents of its security target, providing 1301 confidentiality of the data. Of course, it should be assumed that 1302 Mallory is able to attempt offline recovery of encrypted data, so the 1303 cryptographic mechanisms selected to protect the data should provide 1304 a suitable level of protection. 1306 When evaluating the risk of eavesdropping attacks, it is important to 1307 consider the lifetime of bundles on a DTN. Depending on the network, 1308 bundles may persist for days or even years. Long-lived bundles imply 1309 that the data exists in the network for a longer period of time and, 1310 thus, there may be more opportunities to capture those bundles. 1311 Additionally, bundles that are long-lived imply that the information 1312 stored within them may remain relevant and sensitive for long enough 1313 that, once captured, there is sufficient time to crack encryption 1314 associated with the bundle. If a bundle does persist on the network 1315 for years and the cipher suite used for a BCB provides inadequate 1316 protection, Mallory may be able to recover the protected data either 1317 before that bundle reaches its intended destination or before the 1318 information in the bundle is no longer considered sensitive. 1320 8.2.2. Modification Attacks 1322 As a node participating in the DTN between Alice and Bob, Mallory 1323 will also be able to modify the received bundle, including non-BPSec 1324 data such as the primary block, payload blocks, or block processing 1325 control flags as defined in [I-D.ietf-dtn-bpbis]. Mallory will be 1326 able to undertake activities which include modification of data 1327 within the blocks, replacement of blocks, addition of blocks, or 1328 removal of blocks. Within BPSec, both the BIB and BCB provide 1329 integrity protection mechanisms to detect or prevent data 1330 manipulation attempts by Mallory. 1332 The BIB provides that protection to another block which is its 1333 security target. The cryptographic mechanisms used to generate the 1334 BIB should be strong against collision attacks and Mallory should not 1335 have access to the cryptographic material used by the originating 1336 node to generate the BIB (e.g., K_A). If both of these conditions 1337 are true, Mallory will be unable to modify the security target or the 1338 BIB and lead Bob to validate the security target as originating from 1339 Alice. 1341 Since BPSec security operations are implemented by placing blocks in 1342 a bundle, there is no in-band mechanism for detecting or correcting 1343 certain cases where Mallory removes blocks from a bundle. If Mallory 1344 removes a BCB, but keeps the security target, the security target 1345 remains encrypted and there is a possibility that there may no longer 1346 be sufficient information to decrypt the block at its destination. 1347 If Mallory removes both a BCB (or BIB) and its security target there 1348 is no evidence left in the bundle of the security operation. 1349 Similarly, if Mallory removes the BIB but not the security target 1350 there is no evidence left in the bundle of the security operation. 1351 In each of these cases, the implementation of BPSec must be combined 1352 with policy configuration at endpoints in the network which describe 1353 the expected and required security operations that must be applied on 1354 transmission and are expected to be present on receipt. This or 1355 other similar out-of-band information is required to correct for 1356 removal of security information in the bundle. 1358 A limitation of the BIB may exist within the implementation of BIB 1359 validation at the destination node. If Mallory is a legitimate node 1360 within the DTN, the BIB generated by Alice with K_A can be replaced 1361 with a new BIB generated with K_M and forwarded to Bob. If Bob is 1362 only validating that the BIB was generated by a legitimate user, Bob 1363 will acknowledge the message as originating from Mallory instead of 1364 Alice. In order to provide verifiable integrity checks, both a BIB 1365 and BCB should be used and the BCB should require an IND-CCA2 1366 encryption scheme. Such an encryption scheme will guard against 1367 signature substitution attempts by Mallory. In this case, Alice 1368 creates a BIB with the protected data block as the security target 1369 and then creates a BCB with both the BIB and protected data block as 1370 its security targets. 1372 8.2.3. Topology Attacks 1374 If Mallory is in a MITM position within the DTN, she is able to 1375 influence how any bundles that come to her may pass through the 1376 network. Upon receiving and processing a bundle that must be routed 1377 elsewhere in the network, Mallory has three options as to how to 1378 proceed: not forward the bundle, forward the bundle as intended, or 1379 forward the bundle to one or more specific nodes within the network. 1381 Attacks that involve re-routing the packets throughout the network 1382 are essentially a special case of the modification attacks described 1383 in this section where the attacker is modifying fields within the 1384 primary block of the bundle. Given that BPSec cannot encrypt the 1385 contents of the primary block, alternate methods must be used to 1386 prevent this situation. These methods may include requiring BIBs for 1387 primary blocks, using encapsulation, or otherwise strategically 1388 manipulating primary block data. The specifics of any such 1389 mitigation technique are specific to the implementation of the 1390 deploying network and outside of the scope of this document. 1392 Furthermore, routing rules and policies may be useful in enforcing 1393 particular traffic flows to prevent topology attacks. While these 1394 rules and policies may utilize some features provided by BPSec, their 1395 definition is beyond the scope of this specification. 1397 8.2.4. Message Injection 1399 Mallory is also able to generate new bundles and transmit them into 1400 the DTN at will. These bundles may either be copies or slight 1401 modifications of previously-observed bundles (i.e., a replay attack) 1402 or entirely new bundles generated based on the Bundle Protocol, 1403 BPSec, or other bundle-related protocols. With these attacks 1404 Mallory's objectives may vary, but may be targeting either the bundle 1405 protocol or application-layer protocols conveyed by the bundle 1406 protocol. 1408 BPSec relies on cipher suite capabilities to prevent replay or forged 1409 message attacks. A BCB used with appropriate cryptographic 1410 mechanisms (e.g., a counter-based cipher mode) may provide replay 1411 protection under certain circumstances. Alternatively, application 1412 data itself may be augmented to include mechanisms to assert data 1413 uniqueness and then protected with a BIB, a BCB, or both along with 1414 other block data. In such a case, the receiving node would be able 1415 to validate the uniqueness of the data. 1417 9. Security Context Considerations 1419 9.1. Identification and Configuration 1421 Security blocks must uniquely define the security context for their 1422 services. This context MUST be uniquely identifiable and MAY use 1423 parameters for customization. Where policy and configuration 1424 decisions can be captured as parameters, the security context 1425 identifier may identify a cipher suite. In cases where the same 1426 cipher suites are used with differing predetermined configurations 1427 and policies, users can define multiple security contexts. 1429 Network operators must determine the number, type, and configuration 1430 of security contexts in a system. Networks with rapidly changing 1431 configurations may define relatively few security contexts with each 1432 context customized with multiple parameters. For networks with more 1433 stability, or an increased need for confidentiality, a larger number 1434 of contexts can be defined with each context supporting few, if any, 1435 parameters. 1437 Security Context Examples 1439 +---------+------------+--------------------------------------------+ 1440 | Context | Parameters | Definition | 1441 | Id | | | 1442 +---------+------------+--------------------------------------------+ 1443 | 1 | Key, IV | AES-GCM-256 cipher suite with provided | 1444 | | | ephemeral key and initialization vector. | 1445 | 2 | IV | AES-GCM-256 cipher suite with | 1446 | | | predetermined key and predetermined key | 1447 | | | rotation policy. | 1448 | 3 | Nil | AES-GCM-256 cipher suite with all info | 1449 | | | predetermined. | 1450 +---------+------------+--------------------------------------------+ 1452 Table 1 1454 9.2. Authorship 1456 Cipher suite developers or implementers should consider the diverse 1457 performance and conditions of networks on which the Bundle Protocol 1458 (and therefore BPSec) will operate. Specifically, the delay and 1459 capacity of delay-tolerant networks can vary substantially. Cipher 1460 suite developers should consider these conditions to better describe 1461 the conditions when those suites will operate or exhibit 1462 vulnerability, and selection of these suites for implementation 1463 should be made with consideration to the reality. There are key 1464 differences that may limit the opportunity to leverage existing 1465 cipher suites and technologies that have been developed for use in 1466 traditional, more reliable networks: 1468 o Data Lifetime: Depending on the application environment, bundles 1469 may persist on the network for extended periods of time, perhaps 1470 even years. Cryptographic algorithms should be selected to ensure 1471 protection of data against attacks for a length of time reasonable 1472 for the application. 1474 o One-Way Traffic: Depending on the application environment, it is 1475 possible that only a one-way connection may exist between two 1476 endpoints, or if a two-way connection does exist, the round- trip 1477 time may be extremely large. This may limit the utility of 1478 session key generation mechanisms, such as Diffie-Hellman, as a 1479 two-way handshake may not be feasible or reliable. 1481 o Opportunistic Access: Depending on the application environment, a 1482 given endpoint may not be guaranteed to be accessible within a 1483 certain amount of time. This may make asymmetric cryptographic 1484 architectures which rely on a key distribution center or other 1485 trust center impractical under certain conditions. 1487 When developing new security contexts for use with BPSec, the 1488 following information SHOULD be considered for inclusion in these 1489 specifications. 1491 o Security Context Parameters. Security contexts MUST define their 1492 parameter Ids, the data types of those parameters, and their CBOR 1493 encoding. 1495 o Security Results. Security contexts MUST define their security 1496 result Ids, the data types of those results, and their CBOR 1497 encoding. 1499 o New Canonicalizations. Security contexts may define new 1500 canonicalization algorithms as necessary. 1502 o Cipher-Text Size. Security contexts MUST state whether their 1503 associated cipher suites generate cipher-text (to include any 1504 authentication information) that is of a different size than the 1505 input plain-text. 1507 If a security context does not wish to alter the size of the 1508 plain-text, it should consider defining the following policy. 1510 * Place overflow bytes, authentication signatures, and any 1511 additional authenticated data in security result fields rather 1512 than in the cipher-text itself. 1514 * Pad the cipher-text in cases where the cipher-text is smaller 1515 than the plain-text. 1517 10. Defining Other Security Blocks 1519 Other security blocks (OSBs) may be defined and used in addition to 1520 the security blocks identified in this specification. Both the usage 1521 of BIB, BCB, and any future OSBs can co-exist within a bundle and can 1522 be considered in conformance with BPSec if each of the following 1523 requirements are met by any future identified security blocks. 1525 o Other security blocks (OSBs) MUST NOT reuse any enumerations 1526 identified in this specification, to include the block type codes 1527 for BIB and BCB. 1529 o An OSB definition MUST state whether it can be the target of a BIB 1530 or a BCB. The definition MUST also state whether the OSB can 1531 target a BIB or a BCB. 1533 o An OSB definition MUST provide a deterministic processing order in 1534 the event that a bundle is received containing BIBs, BCBs, and 1535 OSBs. This processing order MUST NOT alter the BIB and BCB 1536 processing orders identified in this specification. 1538 o An OSB definition MUST provide a canonicalization algorithm if the 1539 default non-primary-block canonicalization algorithm cannot be 1540 used to generate a deterministic input for a cipher suite. This 1541 requirement can be waived if the OSB is defined so as to never be 1542 the security target of a BIB or a BCB. 1544 o An OSB definition MUST NOT require any behavior of a BPSEC-BPA 1545 that is in conflict with the behavior identified in this 1546 specification. In particular, the security processing 1547 requirements imposed by this specification must be consistent 1548 across all BPSEC-BPAs in a network. 1550 o The behavior of an OSB when dealing with fragmentation must be 1551 specified and MUST NOT lead to ambiguous processing states. In 1552 particular, an OSB definition should address how to receive and 1553 process an OSB in a bundle fragment that may or may not also 1554 contain its security target. An OSB definition should also 1555 address whether an OSB may be added to a bundle marked as a 1556 fragment. 1558 Additionally, policy considerations for the management, monitoring, 1559 and configuration associated with blocks SHOULD be included in any 1560 OSB definition. 1562 NOTE: The burden of showing compliance with processing rules is 1563 placed upon the standards defining new security blocks and the 1564 identification of such blocks shall not, alone, require maintenance 1565 of this specification. 1567 11. IANA Considerations 1569 This specification includes fields requiring registries managed by 1570 IANA. 1572 11.1. Bundle Block Types 1574 This specification allocates two block types from the existing 1575 "Bundle Block Types" registry defined in [I-D.ietf-dtn-bpbis]. 1577 Additional Entries for the Bundle Block-Type Codes Registry: 1579 +-------+-----------------------------+---------------+ 1580 | Value | Description | Reference | 1581 +-------+-----------------------------+---------------+ 1582 | 2 | Block Integrity Block | This document | 1583 | 3 | Block Confidentiality Block | This document | 1584 +-------+-----------------------------+---------------+ 1586 Table 2 1588 11.2. Security Context Identifiers 1590 BPSec has a Security Context Identifier field for which IANA is 1591 requested to create and maintain a new registry named "BPSec Security 1592 Context Identifiers". Initial values for this registry are given 1593 below. 1595 The registration policy for this registry is: Specification Required. 1597 The value range is: unsigned 16-bit integer. 1599 BPSec Security Context Identifier Registry 1601 +-------+-------------+---------------+ 1602 | Value | Description | Reference | 1603 +-------+-------------+---------------+ 1604 | 0 | Reserved | This document | 1605 +-------+-------------+---------------+ 1607 Table 3 1609 12. References 1611 12.1. Normative References 1613 [I-D.ietf-dtn-bpbis] 1614 Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol 1615 Version 7", draft-ietf-dtn-bpbis-14 (work in progress), 1616 August 2019. 1618 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1619 Requirement Levels", BCP 14, RFC 2119, 1620 DOI 10.17487/RFC2119, March 1997, 1621 . 1623 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 1624 Text on Security Considerations", BCP 72, RFC 3552, 1625 DOI 10.17487/RFC3552, July 2003, 1626 . 1628 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object 1629 Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, 1630 October 2013, . 1632 12.2. Informative References 1634 [I-D.birrane-dtn-sbsp] 1635 Birrane, E., Pierce-Mayer, J., and D. Iannicca, 1636 "Streamlined Bundle Security Protocol Specification", 1637 draft-birrane-dtn-sbsp-01 (work in progress), October 1638 2015. 1640 [I-D.ietf-dtn-bpsec-interop-sc] 1641 Birrane, E., "BPSec Interoperability Security Contexts", 1642 draft-ietf-dtn-bpsec-interop-sc-00 (work in progress), 1643 March 2019. 1645 [RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, 1646 R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant 1647 Networking Architecture", RFC 4838, DOI 10.17487/RFC4838, 1648 April 2007, . 1650 [RFC6257] Symington, S., Farrell, S., Weiss, H., and P. Lovell, 1651 "Bundle Security Protocol Specification", RFC 6257, 1652 DOI 10.17487/RFC6257, May 2011, 1653 . 1655 Appendix A. Acknowledgements 1657 The following participants contributed technical material, use cases, 1658 and useful thoughts on the overall approach to this security 1659 specification: Scott Burleigh of the Jet Propulsion Laboratory, Amy 1660 Alford and Angela Hennessy of the Laboratory for Telecommunications 1661 Sciences, and Angela Dalton and Cherita Corbett of the Johns Hopkins 1662 University Applied Physics Laboratory. 1664 Authors' Addresses 1666 Edward J. Birrane, III 1667 The Johns Hopkins University Applied Physics Laboratory 1668 11100 Johns Hopkins Rd. 1669 Laurel, MD 20723 1670 US 1672 Phone: +1 443 778 7423 1673 Email: Edward.Birrane@jhuapl.edu 1675 Kenneth McKeever 1676 The Johns Hopkins University Applied Physics Laboratory 1677 11100 Johns Hopkins Rd. 1678 Laurel, MD 20723 1679 US 1681 Phone: +1 443 778 2237 1682 Email: Ken.McKeever@jhuapl.edu