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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-31) exists of draft-ietf-dtn-bpbis-11 ** Downref: Normative reference to an Informational RFC: RFC 6255 -- Obsolete informational reference (is this intentional?): RFC 8152 (Obsoleted by RFC 9052, RFC 9053) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). 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: January 2, 2019 July 1, 2018 7 Bundle Protocol Security Specification 8 draft-ietf-dtn-bpsec-07 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 January 2, 2019. 32 Copyright Notice 34 Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . . . . . 5 54 2. Design Decisions . . . . . . . . . . . . . . . . . . . . . . 6 55 2.1. Block-Level Granularity . . . . . . . . . . . . . . . . . 7 56 2.2. Multiple Security Sources . . . . . . . . . . . . . . . . 7 57 2.3. Mixed Security Policy . . . . . . . . . . . . . . . . . . 8 58 2.4. User-Selected Cipher Suites . . . . . . . . . . . . . . . 8 59 2.5. Deterministic Processing . . . . . . . . . . . . . . . . 8 60 3. Security Blocks . . . . . . . . . . . . . . . . . . . . . . . 9 61 3.1. Block Definitions . . . . . . . . . . . . . . . . . . . . 9 62 3.2. Uniqueness . . . . . . . . . . . . . . . . . . . . . . . 9 63 3.3. Target Multiplicity . . . . . . . . . . . . . . . . . . . 10 64 3.4. Target Identification . . . . . . . . . . . . . . . . . . 11 65 3.5. Block Representation . . . . . . . . . . . . . . . . . . 11 66 3.6. Abstract Security Block . . . . . . . . . . . . . . . . . 11 67 3.7. Block Integrity Block . . . . . . . . . . . . . . . . . . 14 68 3.8. Block Confidentiality Block . . . . . . . . . . . . . . . 15 69 3.9. Block Interactions . . . . . . . . . . . . . . . . . . . 16 70 3.10. Cipher Suite Parameter and Result Identification . . . . 18 71 3.11. BSP Block Examples . . . . . . . . . . . . . . . . . . . 18 72 3.11.1. Example 1: Constructing a Bundle with Security . . . 18 73 3.11.2. Example 2: Adding More Security At A New Node . . . 19 74 4. Canonical Forms . . . . . . . . . . . . . . . . . . . . . . . 21 75 5. Security Processing . . . . . . . . . . . . . . . . . . . . . 22 76 5.1. Bundles Received from Other Nodes . . . . . . . . . . . . 22 77 5.1.1. Receiving BCBs . . . . . . . . . . . . . . . . . . . 22 78 5.1.2. Receiving BIBs . . . . . . . . . . . . . . . . . . . 23 79 5.2. Bundle Fragmentation and Reassembly . . . . . . . . . . . 24 80 6. Key Management . . . . . . . . . . . . . . . . . . . . . . . 24 81 7. Security Policy Considerations . . . . . . . . . . . . . . . 24 82 8. Security Considerations . . . . . . . . . . . . . . . . . . . 26 83 8.1. Attacker Capabilities and Objectives . . . . . . . . . . 26 84 8.2. Attacker Behaviors and BPSec Mitigations . . . . . . . . 27 85 8.2.1. Eavesdropping Attacks . . . . . . . . . . . . . . . . 27 86 8.2.2. Modification Attacks . . . . . . . . . . . . . . . . 28 87 8.2.3. Topology Attacks . . . . . . . . . . . . . . . . . . 29 88 8.2.4. Message Injection . . . . . . . . . . . . . . . . . . 29 89 9. Cipher Suite Authorship Considerations . . . . . . . . . . . 30 90 10. Defining Other Security Blocks . . . . . . . . . . . . . . . 31 91 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 92 11.1. Bundle Block Types . . . . . . . . . . . . . . . . . . . 32 93 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 94 12.1. Normative References . . . . . . . . . . . . . . . . . . 33 95 12.2. Informative References . . . . . . . . . . . . . . . . . 33 96 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 34 97 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 99 1. Introduction 101 This document defines security features for the Bundle Protocol (BP) 102 [I-D.ietf-dtn-bpbis] and is intended for use in Delay Tolerant 103 Networks (DTNs) to provide end-to-end security services. 105 The Bundle Protocol specification [I-D.ietf-dtn-bpbis] defines DTN as 106 referring to "a networking architecture providing communications in 107 and/or through highly stressed environments" where "BP may be viewed 108 as sitting at the application layer of some number of constituent 109 networks, forming a store-carry-forward overlay network". The term 110 "stressed" environment refers to multiple challenging conditions 111 including intermittent connectivity, large and/or variable delays, 112 asymmetric data rates, and high bit error rates. 114 The BP might be deployed such that portions of the network cannot be 115 trusted, posing the usual security challenges related to 116 confidentiality and integrity. However, the stressed nature of the 117 BP operating environment imposes unique conditions where usual 118 transport security mechanisms may not be sufficient. For example, 119 the store-carry-forward nature of the network may require protecting 120 data at rest, preventing unauthorized consumption of critical 121 resources such as storage space, and operating without regular 122 contact with a centralized security oracle (such as a certificate 123 authority). 125 An end-to-end security service is needed that operates in all of the 126 environments where the BP operates. 128 1.1. Supported Security Services 130 BPSec provides end-to-end integrity and confidentiality services for 131 BP bundles, as defined in this section. 133 Integrity services ensure that target data within a bundle are not 134 changed from the time they are provided to the network to the time 135 they are delivered at their destination. Data changes may be caused 136 by processing errors, environmental conditions, or intentional 137 manipulation. In the context of BPSec, integrity services apply to 138 plain-text in the bundle. 140 Confidentiality services ensure that target data is unintelligible to 141 nodes in the DTN, except for authorized nodes possessing special 142 information. This generally means producing cipher-text from plain- 143 text and generating authentication information for that cipher-text. 144 Confidentiality, in this context, applies to the contents of target 145 data and does not extend to hiding the fact that confidentiality 146 exists in the bundle. 148 NOTE: Hop-by-hop authentication is NOT a supported security service 149 in this specification, for three reasons. 151 1. The term "hop-by-hop" is ambiguous in a BP overlay, as nodes that 152 are adjacent in the overlay may not be adjacent in physical 153 connectivity. This condition is difficult or impossible to 154 detect and therefore hop-by-hop authentication is difficult or 155 impossible to enforce. 157 2. Networks in which BPSec may be deployed may have a mixture of 158 security-aware and not-security-aware nodes. Hop-by-hop 159 authentication cannot be deployed in a network if adjacent nodes 160 in the network have different security capabilities. 162 3. Hop-by-hop authentication is a special case of data integrity and 163 can be achieved with the integrity mechanisms defined in this 164 specification. Therefore, a separate authentication service is 165 not necessary. 167 1.2. Specification Scope 169 This document defines the security services provided by the BPSec. 170 This includes the data specification for representing these services 171 as BP extension blocks, and the rules for adding, removing, and 172 processing these blocks at various points during the bundle's 173 traversal of the DTN. 175 BPSec applies only to those nodes that implement it, known as 176 "security-aware" nodes. There might be other nodes in the DTN that 177 do not implement BPSec. While all nodes in a BP overlay can exchange 178 bundles, BPSec security operations can only happen at BPSec security- 179 aware nodes. 181 BPSec addresses only the security of data traveling over the DTN, not 182 the underlying DTN itself. Furthermore, while the BPSec protocol can 183 provide security-at-rest in a store-carry-forward network, it does 184 not address threats which share computing resources with the DTN and/ 185 or BPSec software implementations. These threats may be malicious 186 software or compromised libraries which intend to intercept data or 187 recover cryptographic material. Here, it is the responsibility of 188 the BPSec implementer to ensure that any cryptographic material, 189 including shared secret or private keys, is protected against access 190 within both memory and storage devices. 192 This specification addresses neither the fitness of externally- 193 defined cryptographic methods nor the security of their 194 implementation. Different networking conditions and operational 195 considerations require varying strengths of security mechanism such 196 that mandating a cipher suite in this specification may result in too 197 much security for some networks and too little security in others. 198 It is expected that separate documents will be standardized to define 199 cipher suites compatible with BPSec, to include operational cipher 200 suites and interoperability cipher suites. 202 This specification does not address the implementation of security 203 policy and does not provide a security policy for the BPSec. Similar 204 to cipher suites, security policies are based on the nature and 205 capabilities of individual networks and network operational concepts. 206 This specification does provide policy considerations when building a 207 security policy. 209 With the exception of the Bundle Protocol, this specification does 210 not address how to combine the BPSec security blocks with other 211 protocols, other BP extension blocks, or other best practices to 212 achieve security in any particular network implementation. 214 1.3. Related Documents 216 This document is best read and understood within the context of the 217 following other DTN documents: 219 "Delay-Tolerant Networking Architecture" [RFC4838] defines the 220 architecture for DTNs and identifies certain security assumptions 221 made by existing Internet protocols that are not valid in a DTN. 223 The Bundle Protocol [I-D.ietf-dtn-bpbis] defines the format and 224 processing of bundles, defines the extension block format used to 225 represent BPSec security blocks, and defines the canonicalization 226 algorithms used by this specification. 228 The Bundle Security Protocol [RFC6257] and Streamlined Bundle 229 Security Protocol [I-D.birrane-dtn-sbsp] documents introduced the 230 concepts of using BP extension blocks for security services in a DTN. 231 The BPSec is a continuation and refinement of these documents. 233 1.4. Terminology 235 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 236 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 237 "OPTIONAL" in this document are to be interpreted as described in 238 [RFC2119]. 240 This section defines terminology either unique to the BPSec or 241 otherwise necessary for understanding the concepts defined in this 242 specification. 244 o Bundle Source - the node which originates a bundle. The Node ID 245 of the BPA originating the bundle. 247 o Forwarder - any node that transmits a bundle in the DTN. The Node 248 ID of the Bundle Protocol Agent (BPA) that sent the bundle on its 249 most recent hop. 251 o Intermediate Receiver, Waypoint, or "Next Hop" - any node that 252 receives a bundle from a Forwarder that is not the Destination. 253 The Node ID of the BPA at any such node. 255 o Path - the ordered sequence of nodes through which a bundle passes 256 on its way from Source to Destination. The path is not 257 necessarily known in advance by the bundle or any BPAs in the DTN. 259 o Security Block - a BPSec extension block in a bundle. 261 o Security Operation - the application of a security service to a 262 security target, notated as OP(security service, security target). 263 For example, OP(confidentiality, payload). Every security 264 operation in a bundle MUST be unique, meaning that a security 265 service can only be applied to a security target once in a bundle. 266 A security operation is implemented by a security block. 268 o Security Service - the security features supported by this 269 specification: integrity and confidentiality. 271 o Security Source - a bundle node that adds a security block to a 272 bundle. The Node ID of that node. 274 o Security Target - the block within a bundle that receives a 275 security-service as part of a security-operation. 277 2. Design Decisions 279 The application of security services in a DTN is a complex endeavor 280 that must consider physical properties of the network, policies at 281 each node, and various application security requirements. This 282 section identifies those desirable properties that guide design 283 decisions for this specification and are necessary for understanding 284 the format and behavior of the BPSec protocol. 286 2.1. Block-Level Granularity 288 Security services within this specification must allow different 289 blocks within a bundle to have different security services applied to 290 them. 292 Blocks within a bundle represent different types of information. The 293 primary block contains identification and routing information. The 294 payload block carries application data. Extension blocks carry a 295 variety of data that may augment or annotate the payload, or 296 otherwise provide information necessary for the proper processing of 297 a bundle along a path. Therefore, applying a single level and type 298 of security across an entire bundle fails to recognize that blocks in 299 a bundle represent different types of information with different 300 security needs. 302 For example, a payload block might be encrypted to protect its 303 contents and an extension block containing summary information 304 related to the payload might be integrity signed but unencrypted to 305 provide waypoints access to payload-related data without providing 306 access to the payload. 308 2.2. Multiple Security Sources 310 A bundle can have multiple security blocks and these blocks can have 311 different security sources. BPSec implementations MUST NOT assume 312 that all blocks in a bundle have the same security operations and/or 313 security sources. 315 The Bundle Protocol allows extension blocks to be added to a bundle 316 at any time during its existence in the DTN. When a waypoint adds a 317 new extension block to a bundle, that extension block MAY have 318 security services applied to it by that waypoint. Similarly, a 319 waypoint MAY add a security service to an existing extension block, 320 consistent with its security policy. 322 When a waypoint adds a security service to the bundle, the waypoint 323 is the security source for that service. The security block(s) which 324 represent that service in the bundle may need to record this security 325 source as the bundle destination might need this information for 326 processing. For example, a destination node might interpret policy 327 as it related to security blocks as a function of the security source 328 for that block. 330 For example, a bundle source may choose to apply an integrity service 331 to its plain-text payload. Later a waypoint node, representing a 332 gateway to an insecure portion of the DTN, may receive the bundle and 333 choose to apply a confidentiality service. In this case, the 334 integrity security source is the bundle source and the 335 confidentiality security source is the waypoint node. 337 2.3. Mixed Security Policy 339 The security policy enforced by nodes in the DTN may differ. 341 Some waypoints might not be security aware and will not be able to 342 process security blocks. Therefore, security blocks must have their 343 processing flags set such that the block will be treated 344 appropriately by non-security-aware waypoints. 346 Some waypoints will have security policies that require evaluating 347 security services even if they are not the bundle destination or the 348 final intended destination of the service. For example, a waypoint 349 could choose to verify an integrity service even though the waypoint 350 is not the bundle destination and the integrity service will be 351 needed by other nodes along the bundle's path. 353 Some waypoints will determine, through policy, that they are the 354 intended recipient of the security service and terminate the security 355 service in the bundle. For example, a gateway node could determine 356 that, even though it is not the destination of the bundle, it should 357 verify and remove a particular integrity service or attempt to 358 decrypt a confidentiality service, before forwarding the bundle along 359 its path. 361 Some waypoints could understand security blocks but refuse to process 362 them unless they are the bundle destination. 364 2.4. User-Selected Cipher Suites 366 The security services defined in this specification rely on a variety 367 of cipher suites providing integrity signatures, cipher-text, and 368 other information necessary to populate security blocks. Users may 369 select different cipher suites to implement security services. For 370 example, some users might prefer a SHA2 hash function for integrity 371 whereas other users might prefer a SHA3 hash function instead. The 372 security services defined in this specification must provide a 373 mechanism for identifying what cipher suite has been used to populate 374 a security block. 376 2.5. Deterministic Processing 378 Whenever a node determines that it must process more than one 379 security block in a received bundle (either because the policy at a 380 waypoint states that it should process security blocks or because the 381 node is the bundle destination) the order in which security blocks 382 are processed must be deterministic. All nodes must impose this same 383 deterministic processing order for all security blocks. This 384 specification provides determinism in the application and evaluation 385 of security services, even when doing so results in a loss of 386 flexibility. 388 3. Security Blocks 390 3.1. Block Definitions 392 This specification defines two types of security block: the Block 393 Integrity Block (BIB) and the Block Confidentiality Block (BCB). 395 The BIB is used to ensure the integrity of its plain-text security 396 target(s). The integrity information in the BIB MAY be verified 397 by any node along the bundle path from the BIB security source to 398 the bundle destination. Security-aware waypoints add or remove 399 BIBs from bundles in accordance with their security policy. BIBs 400 are never used to sign the cipher-text provided by a BCB. 402 The BCB indicates that the security target(s) have been encrypted 403 at the BCB security source in order to protect their content while 404 in transit. The BCB is decrypted by security-aware nodes in the 405 network, up to and including the bundle destination, as a matter 406 of security policy. BCBs additionally provide authentication 407 mechanisms for the cipher-text they generate. 409 3.2. Uniqueness 411 Security operations in a bundle MUST be unique; the same security 412 service MUST NOT be applied to a security target more than once in a 413 bundle. Since a security operation is represented as a security 414 block, this limits what security blocks may be added to a bundle: if 415 adding a security block to a bundle would cause some other security 416 block to no longer represent a unique security operation then the new 417 block MUST NOT be added. It is important to note that any cipher- 418 text integrity mechanism supplied by the BCB is considered part of 419 the confidentiality service and, therefore, unique from the plain- 420 text integrity service provided by the BIB. 422 If multiple security blocks representing the same security operation 423 were allowed in a bundle at the same time, there would exist 424 ambiguity regarding block processing order and the property of 425 deterministic processing blocks would be lost. 427 Using the notation OP(service,target), several examples illustrate 428 this uniqueness requirement. 430 o Signing the payload twice: The two operations OP(integrity, 431 payload) and OP(integrity, payload) are redundant and MUST NOT 432 both be present in the same bundle at the same time. 434 o Signing different blocks: The two operations OP(integrity, 435 payload) and OP(integrity, extension_block_1) are not redundant 436 and both may be present in the same bundle at the same time. 437 Similarly, the two operations OP(integrity, extension_block_1) and 438 OP(integrity,extension_block_2) are also not redundant and may 439 both be present in the bundle at the same time. 441 o Different Services on same block: The two operations 442 OP(integrity,payload) and OP(confidentiality, payload) are not 443 inherently redundant and may both be present in the bundle at the 444 same time, pursuant to other processing rules in this 445 specification. 447 3.3. Target Multiplicity 449 Under special circumstances, a single security block MAY represent 450 multiple security operations as a way of reducing the overall number 451 of security blocks present in a bundle. In these circumstances, 452 reducing the number of security blocks in the bundle reduces the 453 amount of redundant information in the bundle. 455 A set of security operations can be represented by a single security 456 block when all of the following conditions are true. 458 o The security operations apply the same security service. For 459 example, they are all integrity operations or all confidentiality 460 operations. 462 o The cipher suite parameters and key information for the security 463 operations are identical. 465 o The security source for the security operations is the same. 466 Meaning the set of operations are being added/removed by the same 467 node. 469 o No security operations have the same security target, as that 470 would violate the need for security operations to be unique. 472 o None of the security operations conflict with security operations 473 already present in the bundle. 475 When representing multiple security operations in a single security 476 block, the information that is common across all operations is 477 represented once in the security block, and the information which is 478 different (e.g., the security targets) are represented individually. 479 When the security block is processed all security operations 480 represented by the security block MUST be applied/evaluated at that 481 time. 483 3.4. Target Identification 485 A security target is a block in the bundle to which a security 486 service applies. This target must be uniquely and unambiguously 487 identifiable when processing a security block. The definition of the 488 extension block header from [I-D.ietf-dtn-bpbis] provides a "Block 489 Number" field suitable for this purpose. Therefore, a security 490 target in a security block MUST be represented as the Block Number of 491 the target block. 493 3.5. Block Representation 495 Each security block uses the Canonical Bundle Block Format as defined 496 in [I-D.ietf-dtn-bpbis]. That is, each security block is comprised 497 of the following elements: 499 o Block Type Code 501 o Block Number 503 o Block Processing Control Flags 505 o CRC Type and CRC Field (if present) 507 o Block Data Length 509 o Block Type Specific Data Fields 511 Security-specific information for a security block is captured in the 512 "Block Type Specific Data Fields". 514 3.6. Abstract Security Block 516 The structure of the security-specific portions of a security block 517 is identical for both the BIB and BCB Block Types. Therefore, this 518 section defines an Abstract Security Block (ASB) data structure and 519 discusses the definition, processing, and other constraints for using 520 this structure. An ASB is never directly instantiated within a 521 bundle, it is only a mechanism for discussing the common aspects of 522 BIB and BCB security blocks. 524 The fields of the ASB SHALL be as follows, listed in the order in 525 which they must appear. 527 Security Targets: 528 This field identifies the block(s) targeted by the security 529 operation(s) represented by this security block. Each target 530 block is represented by its unique Block Number. This field 531 SHALL be represented by a CBOR array of data items. Each 532 target within this CBOR array SHALL be represented by a CBOR 533 unsigned integer. This array MUST have at least 1 entry and 534 each entry MUST represent the Block Number of a block that 535 exists in the bundle. There MUST NOT be duplicate entries in 536 this array. 538 Cipher Suite Id: 539 This field identifies the cipher suite used to implement the 540 security service represented by this block and applied to each 541 security target. This field SHALL be represented by a CBOR 542 unsigned integer. 544 Cipher Suite Flags: 545 This field identifies which optional fields are present in the 546 security block. This field SHALL be represented as a CBOR 547 unsigned integer containing a bit field of 5 bits indicating 548 the presence or absence of other security block fields, as 549 follows. 551 Bit 1 (the most-significant bit, 0x10): reserved. 553 Bit 2 (0x08): reserved. 555 Bit 3 (0x04): reserved. 557 Bit 4 (0x02): Security Source Present Flag. 559 Bit 5 (the least-significant bit, 0x01): Cipher Suite 560 Parameters Present Flag. 562 In this field, a value of 1 indicates that the associated 563 security block field MUST be included in the security block. A 564 value of 0 indicates that the associated security block field 565 MUST NOT be in the security block. 567 Security Source (Optional Field): 568 This field identifies the Endpoint that inserted the security 569 block in the bundle. If the security source field is not 570 present then the source MUST be inferred from other 571 information, such as the bundle source, previous hop, or other 572 values defined by security policy. This field SHALL be 573 represented by a CBOR array in accordance with 575 [I-D.ietf-dtn-bpbis] rules for representing Endpoint 576 Identifiers (EIDs). 578 Cipher Suite Parameters (Optional Field): 579 This field captures one or more cipher suite parameters that 580 should be provided to security-aware nodes when processing the 581 security service described by this security block. This field 582 SHALL be represented by a CBOR array. Each entry in this array 583 is a single cipher suite parameter. A single cipher suite 584 parameter SHALL also be represented as a CBOR array comprising 585 a 2-tuple of the id and value of the parameter, as follows. 587 * Parameter Id. This field identifies which cipher suite 588 parameter is being specified. This field SHALL be 589 represented as a CBOR unsigned integer. Parameter ids are 590 selected as described in Section 3.10. 592 * Parameter Value. This field captures the value associated 593 with this parameter. This field SHALL be represented by the 594 applicable CBOR representation of the parameter, in 595 accordance with Section 3.10. 597 The logical layout of the cipher suite parameters array is 598 illustrated in Figure 1. 600 +----------------+----------------+ +----------------+ 601 | Parameter 1 | Parameter 2 | ... | Parameter N | 602 +------+---------+------+---------+ +------+---------+ 603 | Id | Value | Id | Value | | Id | Value | 604 +------+---------+------+---------+ +------+---------+ 606 Figure 1: Cipher Suite Parameters 608 Security Results: 609 This field captures the results of applying a security service 610 to the security targets of the security block. This field 611 SHALL be represented as a CBOR array of target results. Each 612 entry in this array represents the set of security results for 613 a specific security target. The target results MUST be ordered 614 identically to the Security Targets field of the security 615 block. This means that the first set of target results in this 616 array corresponds to the first entry in the Security Targets 617 field of the security block, and so on. There MUST be one 618 entry in this array for each entry in the Security Targets 619 field of the security block. 621 The set of security results for a target is also represented as 622 a CBOR array of individual results. An individual result is 623 represented as a 2-tuple of a result id and a result value, 624 defined as follows. 626 * Result Id. This field identifies which security result is 627 being specified. Some security results capture the primary 628 output of a cipher suite. Other security results contain 629 additional annotative information from cipher suite 630 processing. This field SHALL be represented as a CBOR 631 unsigned integer. Security result ids will be as specified 632 in Section 3.10. 634 * Result Value. This field captures the value associated with 635 the result. This field SHALL be represented by the 636 applicable CBOR representation of the result value, in 637 accordance with Section 3.10. 639 The logical layout of the security results array is illustrated 640 in Figure 2. In this figure there are N security targets for 641 this security block. The first security target contains M 642 results and the Nth security target contains K results. 644 +------------------------------+ +------------------------------+ 645 | Target 1 | | Target N | 646 +------------+----+------------+ +------------------------------+ 647 | Result 1 | | Result M | ... | Result 1 | | Result K | 648 +----+-------+ .. +----+-------+ +----+-------+ .. +----+-------+ 649 | Id | Value | | Id | Value | | Id | Value | | Id | Value | 650 +----+-------+ +----+-------+ +----+-------+ +----+-------+ 652 Figure 2: Security Results 654 3.7. Block Integrity Block 656 A BIB is a bundle extension block with the following characteristics. 658 o The Block Type Code value is as specified in Section 11.1. 660 o The Block Type Specific Data Fields follow the structure of the 661 ASB. 663 o A security target listed in the Security Targets field MUST NOT 664 reference a security block defined in this specification (e.g., a 665 BIB or a BCB). 667 o The Cipher Suite Id MUST be documented as an end-to-end 668 authentication-cipher suite or as an end-to-end error-detection- 669 cipher suite. 671 o An EID-reference to the security source MAY be present. If this 672 field is not present, then the security source of the block SHOULD 673 be inferred according to security policy and MAY default to the 674 bundle source. The security source MAY be specified as part of 675 key information described in Section 3.10. 677 Notes: 679 o It is RECOMMENDED that cipher suite designers carefully consider 680 the effect of setting flags that either discard the block or 681 delete the bundle in the event that this block cannot be 682 processed. 684 o Since OP(integrity, target) is allowed only once in a bundle per 685 target, it is RECOMMENDED that users wishing to support multiple 686 integrity signatures for the same target define a multi-signature 687 cipher suite. 689 o For some cipher suites, (e.g., those using asymmetric keying to 690 produce signatures or those using symmetric keying with a group 691 key), the security information MAY be checked at any hop on the 692 way to the destination that has access to the required keying 693 information, in accordance with Section 3.9. 695 o The use of a generally available key is RECOMMENDED if custodial 696 transfer is employed and all nodes SHOULD verify the bundle before 697 accepting custody. 699 3.8. Block Confidentiality Block 701 A BCB is a bundle extension block with the following characteristics. 703 The Block Type Code value is as specified in Section 11.1. 705 The Block Processing Control flags value can be set to whatever 706 values are required by local policy, except that this block MUST 707 have the "replicate in every fragment" flag set if the target of 708 the BCB is the Payload Block. Having that BCB in each fragment 709 indicates to a receiving node that the payload portion of each 710 fragment represents cipher-text. 712 The Block Type Specific Data Fields follow the structure of the 713 ASB. 715 A security target listed in the Security Targets field can 716 reference the payload block, a non-security extension block, or a 717 BIB. A BCB MUST NOT include another BCB as a security target. A 718 BCB MUST NOT target the primary block. 720 The Cipher Suite Id MUST be documented as a confidentiality cipher 721 suite that supports authenticated encryption with associated data 722 (AEAD). 724 Additional information created by a cipher suite (such as 725 additional authenticated data) can be placed either in a security 726 result field or in the generated cipher-text. The determination 727 of where to place these data is a function of the cipher suite 728 used. 730 An EID-reference to the security source MAY be present. If this 731 field is not present, then the security source of the block SHOULD 732 be inferred according to security policy and MAY default to the 733 bundle source. The security source MAY be specified as part of 734 key information described in Section 3.10. 736 The BCB modifies the contents of its security target(s). When a BCB 737 is applied, the security target body data are encrypted "in-place". 738 Following encryption, the security target Block Type Specific Data 739 field contains cipher-text, not plain-text. Other block fields 740 remain unmodified, with the exception of the Block Data Length field, 741 which MUST be updated to reflect the new length of the Block Type 742 Specific Data field. 744 Notes: 746 o It is RECOMMENDED that cipher suite designers carefully consider 747 the effect of setting flags that either discard the block or 748 delete the bundle in the event that this block cannot be 749 processed. 751 o The BCB block processing control flags can be set independently 752 from the processing control flags of the security target(s). The 753 setting of such flags SHOULD be an implementation/policy decision 754 for the encrypting node. 756 3.9. Block Interactions 758 The security block types defined in this specification are designed 759 to be as independent as possible. However, there are some cases 760 where security blocks may share a security target creating processing 761 dependencies. 763 If a security target of a BCB is also a security target of a BIB, an 764 undesirable condition occurs where a security aware waypoint would be 765 unable to validate the BIB because one of its security target's 766 contents have been encrypted by a BCB. To address this situation the 767 following processing rules MUST be followed. 769 o When adding a BCB to a bundle, if some (or all) of the security 770 targets of the BCB also match all of the security targets of an 771 existing BIB, then the existing BIB MUST also be encrypted. This 772 can be accomplished by either adding a new BCB that targets the 773 existing BIB, or by adding the BIB to the list of security targets 774 for the BCB. Deciding which way to represent this situation is a 775 matter of security policy. 777 o When adding a BCB to a bundle, if some (or all) of the security 778 targets of the BCB match some (but not all) of the security 779 targets of a BIB, then a new BIB MUST be created and all entries 780 relating to those BCB security targets MUST be moved from the 781 original BIB to the newly created BIB. The newly created BIB MUST 782 then be encrypted. This can be accomplished by either adding a 783 new BCB that targets the new BIB, or by adding the new BIB to the 784 list of security targets for the BCB. Deciding which way to 785 represent this situation is a matter of security policy. 787 o A BIB MUST NOT be added for a security target that is already the 788 security target of a BCB. In this instance, the BCB is already 789 providing authentication and integrity of the security target and 790 the BIB would be redundant, insecure, and cause ambiguity in block 791 processing order. 793 o A BIB integrity value MUST NOT be evaluated if the BIB is the 794 security target of an existing BCB. In this case, the BIB data is 795 encrypted. 797 o A BIB integrity value MUST NOT be evaluated if the security target 798 of the BIB is also the security target of a BCB. In such a case, 799 the security target data contains cipher-text as it has been 800 encrypted. 802 o As mentioned in Section 3.7, a BIB MUST NOT have a BCB as its 803 security target. 805 These restrictions on block interactions impose a necessary ordering 806 when applying security operations within a bundle. Specifically, for 807 a given security target, BIBs MUST be added before BCBs. This 808 ordering MUST be preserved in cases where the current BPA is adding 809 all of the security blocks for the bundle or whether the BPA is a 810 waypoint adding new security blocks to a bundle that already contains 811 security blocks. 813 NOTE: Since any cipher suite used with a BCB MUST be an AEAD cipher 814 suite, it is inefficient and possible insecure for a single security 815 source to add both a BIB and a BCB for the same security target. In 816 cases where a security source wishes to calculate both a plain-text 817 integrity mechanism and encrypt a security target, a BCB with a 818 cipher suite that generates such signatures as additional security 819 results SHOULD be used instead. 821 3.10. Cipher Suite Parameter and Result Identification 823 Cipher suite parameters and security results each represent multiple 824 distinct pieces of information in a security block. Each piece of 825 information is assigned an identifier and a CBOR encoding. 826 Identifiers MUST be unique for a given cipher suite but do not need 827 to be unique across all cipher suites. Therefore, parameter ids and 828 security result ids are specified in the context of a cipher suite 829 definition. 831 Individual BPSec cipher suites SHOULD use existing registries of 832 identifiers and CBOR encodings, such as those defined in [RFC8152], 833 whenever possible. Cipher suites SHOULD define their own identifiers 834 and CBOR encodings when necessary. 836 A cipher suite can include multiple instances of the same identifier 837 for a parameter or result in a security block. Parameters and 838 results are represented using CBOR, and any identification of a new 839 parameter or result must include how the value will be represented 840 using the CBOR specification. Ids themselves are always represented 841 as a CBOR unsigned integer. 843 3.11. BSP Block Examples 845 This section provides two examples of BPSec blocks applied to a 846 bundle. In the first example, a single node adds several security 847 operations to a bundle. In the second example, a waypoint node 848 received the bundle created in the first example and adds additional 849 security operations. In both examples, the first column represents 850 blocks within a bundle and the second column represents the Block 851 Number for the block, using the terminology B1...Bn for the purpose 852 of illustration. 854 3.11.1. Example 1: Constructing a Bundle with Security 856 In this example a bundle has four non-security-related blocks: the 857 primary block (B1), two extension blocks (B4,B5), and a payload block 858 (B6). The bundle source wishes to provide an integrity signature of 859 the plain-text associated with the primary block, one of the 860 extension blocks, and the payload. The resultant bundle is 861 illustrated in Figure 3 and the security actions are described below. 863 Block in Bundle ID 864 +======================================+====+ 865 | Primary Block | B1 | 866 +--------------------------------------+----+ 867 | BIB | B2 | 868 | OP(integrity, targets=B1, B5, B6) | | 869 +--------------------------------------+----+ 870 | BCB | B3 | 871 | OP(confidentiality, target=B4) | | 872 +--------------------------------------+----+ 873 | Extension Block (encrypted) | B4 | 874 +--------------------------------------+----+ 875 | Extension Block | B5 | 876 +--------------------------------------+----+ 877 | Payload Block | B6 | 878 +--------------------------------------+----+ 880 Figure 3: Security at Bundle Creation 882 The following security actions were applied to this bundle at its 883 time of creation. 885 o An integrity signature applied to the canonicalized primary block 886 (B1), the second extension block (B5) and the payload block (B6). 887 This is accomplished by a single BIB (B2) with multiple targets. 888 A single BIB is used in this case because all three targets share 889 a security source and policy has them share the same cipher suite, 890 key, and cipher suite parameters. Had this not been the case, 891 multiple BIBs could have been added instead. 893 o Confidentiality for the first extension block (B4). This is 894 accomplished by a BCB (B3). Once applied, the contents of 895 extension block B4 are encrypted. The BCB MUST hold an 896 authentication signature for the cipher-text either in the cipher- 897 text that now populated the first extension block or as a security 898 result in the BCB itself, depending on which cipher suite is used 899 to form the BCB. A plain-text integrity signature may also exist 900 as a security result in the BCB if one is provided by the selected 901 confidentiality cipher suite. 903 3.11.2. Example 2: Adding More Security At A New Node 905 Consider that the bundle as it is illustrated in Figure 3 is now 906 received by a waypoint node that wishes to encrypt the first 907 extension block and the bundle payload. The waypoint security policy 908 is to allow existing BIBs for these blocks to persist, as they may be 909 required as part of the security policy at the bundle destination. 911 The resultant bundle is illustrated in Figure 4 and the security 912 actions are described below. Note that block IDs provided here are 913 ordered solely for the purpose of this example and not meant to 914 impose an ordering for block creation. The ordering of blocks added 915 to a bundle MUST always be in compliance with [I-D.ietf-dtn-bpbis]. 917 Block in Bundle ID 918 +======================================+====+ 919 | Primary Block | B1 | 920 +--------------------------------------+----+ 921 | BIB | B2 | 922 | OP(integrity, targets=B1) | | 923 +--------------------------------------+----+ 924 | BIB (encrypted) | B7 | 925 | OP(integrity, targets=B5, B6) | | 926 +--------------------------------------+----+ 927 | BCB | B8 | 928 | OP(confidentiality, target=B4,B6,B7) | | 929 +--------------------------------------+----+ 930 | BCB | B3 | 931 | OP(confidentiality, target=B4) | | 932 +--------------------------------------+----+ 933 | Extension Block (encrypted) | B4 | 934 +--------------------------------------+----+ 935 | Extension Block (encrypted) | B5 | 936 +--------------------------------------+----+ 937 | Payload Block (encrypted) | B6 | 938 +--------------------------------------+----+ 940 Figure 4: Security At Bundle Forwarding 942 The following security actions were applied to this bundle prior to 943 its forwarding from the waypoint node. 945 o Since the waypoint node wishes to encrypt blocks B5 and B6, it 946 MUST also encrypt the BIBs providing plain-text integrity over 947 those blocks. However, BIB B2 could not be encrypted in its 948 entirety because it also held a signature for the primary block 949 (B1). Therefore, a new BIB (B7) is created and security results 950 associated with B5 and B6 are moved out of BIB B2 and into BIB B7. 952 o Now that there is no longer confusion of which plain-text 953 integrity signatures must be encrypted, a BCB is added to the 954 bundle with the security targets being the second extension block 955 (B5) and the payload (B6) as well as the newly created BIB holding 956 their plain-text integrity signatures (B7). A single new BCB is 957 used in this case because all three targets share a security 958 source and policy has them share the same cipher suite, key, and 959 cipher suite parameters. Had this not been the case, multiple 960 BCBs could have been added instead. 962 4. Canonical Forms 964 Security services require consistency and determinism in how 965 information is presented to cipher suites at the security source and 966 at a receiving node. For example, integrity services require that 967 the same target information (e.g., the same bits in the same order) 968 is provided to the cipher suite when generating an original signature 969 and when generating a comparison signature. Canonicalization 970 algorithms are used to construct a stable, end-to-end bit 971 representation of a target block. 973 Canonical forms are not transmitted, they are used to generate input 974 to a cipher suite for security processing at a security-aware node. 976 The canonicalization of the primary block is as specified in 977 [I-D.ietf-dtn-bpbis]. 979 All non-primary blocks share the same block structure and are 980 canonicalized as specified in [I-D.ietf-dtn-bpbis] with the following 981 exceptions. 983 o If the service being applied is a confidentiality service, then 984 the Block Type Code, Block Number, Block Processing Control Flags, 985 CRC Type and CRC Field (if present), and Block Data Length fields 986 MUST NOT be included in the canonicalization. Confidentiality 987 services are used solely to convert the Block Type Specific Data 988 Fields from plain-text to cipher-text. 990 o Reserved flags MUST NOT be included in any canonicalization as it 991 is not known if those flags will change in transit. 993 These canonicalization algorithms assume that Endpoint IDs do not 994 change from the time at which a security source adds a security block 995 to a bundle and the time at which a node processes that security 996 block. 998 Cipher suites MAY define their own canonicalization algorithms and 999 require the use of those algorithms over the ones provided in this 1000 specification. In the event of conflicting canonicalization 1001 algorithms, cipher suite algorithms take precedence over this 1002 specification. 1004 5. Security Processing 1006 This section describes the security aspects of bundle processing. 1008 5.1. Bundles Received from Other Nodes 1010 Security blocks must be processed in a specific order when received 1011 by a security-aware node. The processing order is as follows. 1013 o When BIBs and BCBs share a security target, BCBs MUST be evaluated 1014 first and BIBs second. 1016 5.1.1. Receiving BCBs 1018 If a received bundle contains a BCB, the receiving node MUST 1019 determine whether it has the responsibility of decrypting the BCB 1020 security target and removing the BCB prior to delivering data to an 1021 application at the node or forwarding the bundle. 1023 If the receiving node is the destination of the bundle, the node MUST 1024 decrypt any BCBs remaining in the bundle. If the receiving node is 1025 not the destination of the bundle, the node MUST decrypt the BCB if 1026 directed to do so as a matter of security policy. 1028 If the security policy of a security-aware node specifies that a 1029 bundle should have applied confidentiality to a specific security 1030 target and no such BCB is present in the bundle, then the node MUST 1031 process this security target in accordance with the security policy. 1032 This may involve removing the security target from the bundle. If 1033 the removed security target is the payload block, the bundle MUST be 1034 discarded. 1036 If an encrypted payload block cannot be decrypted (i.e., the cipher- 1037 text cannot be authenticated), then the bundle MUST be discarded and 1038 processed no further. If an encrypted security target other than the 1039 payload block cannot be decrypted then the associated security target 1040 and all security blocks associated with that target MUST be discarded 1041 and processed no further. In both cases, requested status reports 1042 (see [I-D.ietf-dtn-bpbis]) MAY be generated to reflect bundle or 1043 block deletion. 1045 When a BCB is decrypted, the recovered plain-text MUST replace the 1046 cipher-text in the security target Block Type Specific Data Fields. 1047 If the Block Data Length field was modified at the time of encryption 1048 it MUST be updated to reflect the decrypted block length. 1050 If a BCB contains multiple security targets, all security targets 1051 MUST be processed when the BCB is processed. Errors and other 1052 processing steps SHALL be made as if each security target had been 1053 represented by an individual BCB with a single security target. 1055 5.1.2. Receiving BIBs 1057 If a received bundle contains a BIB, the receiving node MUST 1058 determine whether it has the final responsibility of verifying the 1059 BIB security target and removing it prior to delivering data to an 1060 application at the node or forwarding the bundle. If a BIB check 1061 fails, the security target has failed to authenticate and the 1062 security target SHALL be processed according to the security policy. 1063 A bundle status report indicating the failure MAY be generated. 1064 Otherwise, if the BIB verifies, the security target is ready to be 1065 processed for delivery. 1067 A BIB MUST NOT be processed if the security target of the BIB is also 1068 the security target of a BCB in the bundle. Given the order of 1069 operations mandated by this specification, when both a BIB and a BCB 1070 share a security target, it means that the security target must have 1071 been encrypted after it was integrity signed and, therefore, the BIB 1072 cannot be verified until the security target has been decrypted by 1073 processing the BCB. 1075 If the security policy of a security-aware node specifies that a 1076 bundle should have applied integrity to a specific security target 1077 and no such BIB is present in the bundle, then the node MUST process 1078 this security target in accordance with the security policy. This 1079 may involve removing the security target from the bundle. If the 1080 removed security target is the payload or primary block, the bundle 1081 MAY be discarded. This action can occur at any node that has the 1082 ability to verify an integrity signature, not just the bundle 1083 destination. 1085 If a receiving node does not have the final responsibility of 1086 verifying the BIB it MAY attempt to verify the BIB to prevent the 1087 needless forwarding of corrupt data. If the check fails, the node 1088 SHALL process the security target in accordance to local security 1089 policy. It is RECOMMENDED that if a payload integrity check fails at 1090 a waypoint that it is processed in the same way as if the check fails 1091 at the destination. If the check passes, the node MUST NOT remove 1092 the BIB prior to forwarding. 1094 If a BIB contains multiple security targets, all security targets 1095 MUST be processed if the BIB is processed by the Node. Errors and 1096 other processing steps SHALL be made as if each security target had 1097 been represented by an individual BIB with a single security target. 1099 5.2. Bundle Fragmentation and Reassembly 1101 If it is necessary for a node to fragment a bundle payload, and 1102 security services have been applied to that bundle, the fragmentation 1103 rules described in [I-D.ietf-dtn-bpbis] MUST be followed. As defined 1104 there and summarized here for completeness, only the payload block 1105 can be fragmented; security blocks, like all extension blocks, can 1106 never be fragmented. 1108 Due to the complexity of payload block fragmentation, including the 1109 possibility of fragmenting payload block fragments, integrity and 1110 confidentiality operations are not to be applied to a bundle 1111 representing a fragment. Specifically, a BCB or BIB MUST NOT be 1112 added to a bundle if the "Bundle is a Fragment" flag is set in the 1113 Bundle Processing Control Flags field. 1115 Security processing in the presence of payload block fragmentation 1116 may be handled by other mechanisms outside of the BPSec protocol or 1117 by applying BPSec blocks in coordination with an encapsulation 1118 mechanism. 1120 6. Key Management 1122 There exist a myriad of ways to establish, communicate, and otherwise 1123 manage key information in a DTN. Certain DTN deployments might 1124 follow established protocols for key management whereas other DTN 1125 deployments might require new and novel approaches. BPSec assumes 1126 that key management is handled as a separate part of network 1127 management and this specification neither defines nor requires a 1128 specific key management strategy. 1130 7. Security Policy Considerations 1132 When implementing BPSec, several policy decisions must be considered. 1133 This section describes key policies that affect the generation, 1134 forwarding, and receipt of bundles that are secured using this 1135 specification. No single set of policy decisions is envisioned to 1136 work for all secure DTN deployments. 1138 o If a bundle is received that contains more than one security 1139 operation, in violation of BPSec, then the BPA must determine how 1140 to handle this bundle. The bundle may be discarded, the block 1141 affected by the security operation may be discarded, or one 1142 security operation may be favored over another. 1144 o BPAs in the network must understand what security operations they 1145 should apply to bundles. This decision may be based on the source 1146 of the bundle, the destination of the bundle, or some other 1147 information related to the bundle. 1149 o If a waypoint has been configured to add a security operation to a 1150 bundle, and the received bundle already has the security operation 1151 applied, then the receiver must understand what to do. The 1152 receiver may discard the bundle, discard the security target and 1153 associated BPSec blocks, replace the security operation, or some 1154 other action. 1156 o It is recommended that security operations only be applied to the 1157 blocks that absolutely need them. If a BPA were to apply security 1158 operations such as integrity or confidentiality to every block in 1159 the bundle, regardless of need, there could be downstream errors 1160 processing blocks whose contents must be inspected or changed at 1161 every hop along the path. 1163 o It is recommended that BCBs be allowed to alter the size of 1164 extension blocks and the payload block. However, care must be 1165 taken to ensure that changing the size of the payload block while 1166 the bundle is in transit do not negatively affect bundle 1167 processing (e.g., calculating storage needs, scheduling 1168 transmission times, caching block byte offsets). 1170 o Adding a BIB to a security target that has already been encrypted 1171 by a BCB is not allowed. If this condition is likely to be 1172 encountered, there are (at least) three possible policies that 1173 could handle this situation. 1175 1. At the time of encryption, a plain-text integrity signature 1176 may be generated and added to the BCB for the security target 1177 as additional information in the security result field. 1179 2. The encrypted block may be replicated as a new block and 1180 integrity signed. 1182 3. An encapsulation scheme may be applied to encapsulate the 1183 security target (or the entire bundle) such that the 1184 encapsulating structure is, itself, no longer the security 1185 target of a BCB and may therefore be the security target of a 1186 BIB. 1188 o It is recommended that security policy address whether cipher 1189 suites whose cipher-text is larger (or smaller) than the initial 1190 plain-text are permitted and, if so, for what types of blocks. 1191 Changing the size of a block may cause processing difficulties for 1192 networks that calculate block offsets into bundles or predict 1193 transmission times or storage availability as a function of bundle 1194 size. In other cases, changing the size of a payload as part of 1195 encryption has no significant impact. 1197 8. Security Considerations 1199 Given the nature of DTN applications, it is expected that bundles may 1200 traverse a variety of environments and devices which each pose unique 1201 security risks and requirements on the implementation of security 1202 within BPSec. For these reasons, it is important to introduce key 1203 threat models and describe the roles and responsibilities of the 1204 BPSec protocol in protecting the confidentiality and integrity of the 1205 data against those threats. This section provides additional 1206 discussion on security threats that BPSec will face and describes how 1207 BPSec security mechanisms operate to mitigate these threats. 1209 The threat model described here is assumed to have a set of 1210 capabilities identical to those described by the Internet Threat 1211 Model in [RFC3552], but the BPSec threat model is scoped to 1212 illustrate threats specific to BPSec operating within DTN 1213 environments and therefore focuses on man-in-the-middle (MITM) 1214 attackers. In doing so, it is assumed that the DTN (or significant 1215 portions of the DTN) are completely under the control of an attacker. 1217 8.1. Attacker Capabilities and Objectives 1219 BPSec was designed to protect against MITM threats which may have 1220 access to a bundle during transit from its source, Alice, to its 1221 destination, Bob. A MITM node, Mallory, is a non-cooperative node 1222 operating on the DTN between Alice and Bob that has the ability to 1223 receive bundles, examine bundles, modify bundles, forward bundles, 1224 and generate bundles at will in order to compromise the 1225 confidentiality or integrity of data within the DTN. For the 1226 purposes of this section, any MITM node is assumed to effectively be 1227 security-aware even if it does not implement the BPSec protocol. 1228 There are three classes of MITM nodes which are differentiated based 1229 on their access to cryptographic material: 1231 o Unprivileged Node: Mallory has not been provisioned within the 1232 secure environment and only has access to cryptographic material 1233 which has been publicly-shared. 1235 o Legitimate Node: Mallory is within the secure environment and 1236 therefore has access to cryptographic material which has been 1237 provisioned to Mallory (i.e., K_M) as well as material which has 1238 been publicly-shared. 1240 o Privileged Node: Mallory is a privileged node within the secure 1241 environment and therefore has access to cryptographic material 1242 which has been provisioned to Mallory, Alice and/or Bob (i.e. 1243 K_M, K_A, and/or K_B) as well as material which has been publicly- 1244 shared. 1246 If Mallory is operating as a privileged node, this is tantamount to 1247 compromise; BPSec does not provide mechanisms to detect or remove 1248 Mallory from the DTN or BPSec secure environment. It is up to the 1249 BPSec implementer or the underlying cryptographic mechanisms to 1250 provide appropriate capabilities if they are needed. It should also 1251 be noted that if the implementation of BPSec uses a single set of 1252 shared cryptographic material for all nodes, a legitimate node is 1253 equivalent to a privileged node because K_M == K_A == K_B. 1255 A special case of the legitimate node is when Mallory is either Alice 1256 or Bob (i.e., K_M == K_A or K_M == K_B). In this case, Mallory is 1257 able to impersonate traffic as either Alice or Bob, which means that 1258 traffic to and from that node can be decrypted and encrypted, 1259 respectively. Additionally, messages may be signed as originating 1260 from one of the endpoints. 1262 8.2. Attacker Behaviors and BPSec Mitigations 1264 8.2.1. Eavesdropping Attacks 1266 Once Mallory has received a bundle, she is able to examine the 1267 contents of that bundle and attempt to recover any protected data or 1268 cryptographic keying material from the blocks contained within. The 1269 protection mechanism that BPSec provides against this action is the 1270 BCB, which encrypts the contents of its security target, providing 1271 confidentiality of the data. Of course, it should be assumed that 1272 Mallory is able to attempt offline recovery of encrypted data, so the 1273 cryptographic mechanisms selected to protect the data should provide 1274 a suitable level of protection. 1276 When evaluating the risk of eavesdropping attacks, it is important to 1277 consider the lifetime of bundles on a DTN. Depending on the network, 1278 bundles may persist for days or even years. Long-lived bundles imply 1279 that the data exists in the network for a longer period of time and, 1280 thus, there may be more opportunities to capture those bundles. 1281 Additionally, bundles that are long-lived imply that the information 1282 stored within them may remain relevant and sensitive for long enough 1283 that, once captured, there is sufficient time to crack encryption 1284 associated with the bundle. If a bundle does persist on the network 1285 for years and the cipher suite used for a BCB provides inadequate 1286 protection, Mallory may be able to recover the protected data either 1287 before that bundle reaches its intended destination or before the 1288 information in the bundle is no longer considered sensitive. 1290 8.2.2. Modification Attacks 1292 As a node participating in the DTN between Alice and Bob, Mallory 1293 will also be able to modify the received bundle, including non-BPSec 1294 data such as the primary block, payload blocks, or block processing 1295 control flags as defined in [I-D.ietf-dtn-bpbis]. Mallory will be 1296 able to undertake activities which include modification of data 1297 within the blocks, replacement of blocks, addition of blocks, or 1298 removal of blocks. Within BPSec, both the BIB and BCB provide 1299 integrity protection mechanisms to detect or prevent data 1300 manipulation attempts by Mallory. 1302 The BIB provides that protection to another block which is its 1303 security target. The cryptographic mechanisms used to generate the 1304 BIB should be strong against collision attacks and Mallory should not 1305 have access to the cryptographic material used by the originating 1306 node to generate the BIB (e.g., K_A). If both of these conditions 1307 are true, Mallory will be unable to modify the security target or the 1308 BIB and lead Bob to validate the security target as originating from 1309 Alice. 1311 Since BPSec security operations are implemented by placing blocks in 1312 a bundle, there is no in-band mechanism for detecting or correcting 1313 certain cases where Mallory removes blocks from a bundle. If Mallory 1314 removes a BCB, but keeps the security target, the security target 1315 remains encrypted and there is a possibility that there may no longer 1316 be sufficient information to decrypt the block at its destination. 1317 If Mallory removes both a BCB (or BIB) and its security target there 1318 is no evidence left in the bundle of the security operation. 1319 Similarly, if Mallory removes the BIB but not the security target 1320 there is no evidence left in the bundle of the security operation. 1321 In each of these cases, the implementation of BPSec must be combined 1322 with policy configuration at endpoints in the network which describe 1323 the expected and required security operations that must be applied on 1324 transmission and are expected to be present on receipt. This or 1325 other similar out-of-band information is required to correct for 1326 removal of security information in the bundle. 1328 A limitation of the BIB may exist within the implementation of BIB 1329 validation at the destination node. If Mallory is a legitimate node 1330 within the DTN, the BIB generated by Alice with K_A can be replaced 1331 with a new BIB generated with K_M and forwarded to Bob. If Bob is 1332 only validating that the BIB was generated by a legitimate user, Bob 1333 will acknowledge the message as originating from Mallory instead of 1334 Alice. In order to provide verifiable integrity checks, both a BIB 1335 and BCB should be used and the BCB should require an IND-CCA2 1336 encryption scheme. Such an encryption scheme will guard against 1337 signature substitution attempts by Mallory. In this case, Alice 1338 creates a BIB with the protected data block as the security target 1339 and then creates a BCB with both the BIB and protected data block as 1340 its security targets. 1342 8.2.3. Topology Attacks 1344 If Mallory is in a MITM position within the DTN, she is able to 1345 influence how any bundles that come to her may pass through the 1346 network. Upon receiving and processing a bundle that must be routed 1347 elsewhere in the network, Mallory has three options as to how to 1348 proceed: not forward the bundle, forward the bundle as intended, or 1349 forward the bundle to one or more specific nodes within the network. 1351 Attacks that involve re-routing the packets throughout the network 1352 are essentially a special case of the modification attacks described 1353 in this section where the attacker is modifying fields within the 1354 primary block of the bundle. Given that BPSec cannot encrypt the 1355 contents of the primary block, alternate methods must be used to 1356 prevent this situation. These methods may include requiring BIBs for 1357 primary blocks, using encapsulation, or otherwise strategically 1358 manipulating primary block data. The specifics of any such 1359 mitigation technique are specific to the implementation of the 1360 deploying network and outside of the scope of this document. 1362 Furthermore, routing rules and policies may be useful in enforcing 1363 particular traffic flows to prevent topology attacks. While these 1364 rules and policies may utilize some features provided by BPSec, their 1365 definition is beyond the scope of this specification. 1367 8.2.4. Message Injection 1369 Mallory is also able to generate new bundles and transmit them into 1370 the DTN at will. These bundles may either be copies or slight 1371 modifications of previously-observed bundles (i.e., a replay attack) 1372 or entirely new bundles generated based on the Bundle Protocol, 1373 BPSec, or other bundle-related protocols. With these attacks 1374 Mallory's objectives may vary, but may be targeting either the bundle 1375 protocol or application-layer protocols conveyed by the bundle 1376 protocol. 1378 BPSec relies on cipher suite capabilities to prevent replay or forged 1379 message attacks. A BCB used with appropriate cryptographic 1380 mechanisms (e.g., a counter-based cipher mode) may provide replay 1381 protection under certain circumstances. Alternatively, application 1382 data itself may be augmented to include mechanisms to assert data 1383 uniqueness and then protected with a BIB, a BCB, or both along with 1384 other block data. In such a case, the receiving node would be able 1385 to validate the uniqueness of the data. 1387 9. Cipher Suite Authorship Considerations 1389 Cipher suite developers or implementers should consider the diverse 1390 performance and conditions of networks on which the Bundle Protocol 1391 (and therefore BPSec) will operate. Specifically, the delay and 1392 capacity of delay-tolerant networks can vary substantially. Cipher 1393 suite developers should consider these conditions to better describe 1394 the conditions when those suites will operate or exhibit 1395 vulnerability, and selection of these suites for implementation 1396 should be made with consideration to the reality. There are key 1397 differences that may limit the opportunity to leverage existing 1398 cipher suites and technologies that have been developed for use in 1399 traditional, more reliable networks: 1401 o Data Lifetime: Depending on the application environment, bundles 1402 may persist on the network for extended periods of time, perhaps 1403 even years. Cryptographic algorithms should be selected to ensure 1404 protection of data against attacks for a length of time reasonable 1405 for the application. 1407 o One-Way Traffic: Depending on the application environment, it is 1408 possible that only a one-way connection may exist between two 1409 endpoints, or if a two-way connection does exist, the round-trip 1410 time may be extremely large. This may limit the utility of 1411 session key generation mechanisms, such as Diffie-Hellman, as a 1412 two-way handshake may not be feasible or reliable. 1414 o Opportunistic Access: Depending on the application environment, a 1415 given endpoint may not be guaranteed to be accessible within a 1416 certain amount of time. This may make asymmetric cryptographic 1417 architectures which rely on a key distribution center or other 1418 trust center impractical under certain conditions. 1420 When developing new cipher suites for use with BPSec, the following 1421 information SHOULD be considered for inclusion in these 1422 specifications. 1424 o Cipher Suite Parameters. Cipher suites MUST define their 1425 parameter ids, the data types of those parameters, and their CBOR 1426 encoding. 1428 o Security Results. Cipher suites MUST define their security result 1429 ids, the data types of those results, and their CBOR encoding. 1431 o New Canonicalizations. Cipher suites may define new 1432 canonicalization algorithms as necessary. 1434 o Cipher-Text Size. Cipher suites MUST state whether they generate 1435 cipher-text (to include any included authentication information) 1436 that is of a different size than the input plain-text. 1438 If a cipher suite does not wish to alter the size of the plain- 1439 text, it should consider the following. 1441 * Place overflow bytes, authentication signatures, and any 1442 additional authenticated data in security result fields rather 1443 than in the cipher-text itself. 1445 * Pad the cipher-text in cases where the cipher-text is smaller 1446 than the plain-text. 1448 o If a BCB cannot alter the size of the security target then 1449 differences in the size of the cipher-text and plain-text MUST be 1450 handled in the following way. If the cipher-text is shorter in 1451 length than the plain-text, padding MUST be used in accordance 1452 with the cipher suite policy. If the cipher-text is larger than 1453 the plain-text, overflow bytes MUST be placed in overflow 1454 parameters in the Security Result field. Any additional 1455 authentication information can be treated either as overflow 1456 cipher-text or represented separately in the BCB in a security 1457 result field, in accordance with cipher suite documentation and 1458 security policy. 1460 10. Defining Other Security Blocks 1462 Other security blocks (OSBs) may be defined and used in addition to 1463 the security blocks identified in this specification. Both the usage 1464 of BIB, BCB, and any future OSBs can co-exist within a bundle and can 1465 be considered in conformance with BPSec if each of the following 1466 requirements are met by any future identified security blocks. 1468 o Other security blocks (OSBs) MUST NOT reuse any enumerations 1469 identified in this specification, to include the block type codes 1470 for BIB and BCB. 1472 o An OSB definition MUST state whether it can be the target of a BIB 1473 or a BCB. The definition MUST also state whether the OSB can 1474 target a BIB or a BCB. 1476 o An OSB definition MUST provide a deterministic processing order in 1477 the event that a bundle is received containing BIBs, BCBs, and 1478 OSBs. This processing order MUST NOT alter the BIB and BCB 1479 processing orders identified in this specification. 1481 o An OSB definition MUST provide a canonicalization algorithm if the 1482 default non-primary-block canonicalization algorithm cannot be 1483 used to generate a deterministic input for a cipher suite. This 1484 requirement can be waived if the OSB is defined so as to never be 1485 the security target of a BIB or a BCB. 1487 o An OSB definition MUST NOT require any behavior of a BPSEC-BPA 1488 that is in conflict with the behavior identified in this 1489 specification. In particular, the security processing 1490 requirements imposed by this specification must be consistent 1491 across all BPSEC-BPAs in a network. 1493 o The behavior of an OSB when dealing with fragmentation must be 1494 specified and MUST NOT lead to ambiguous processing states. In 1495 particular, an OSB definition should address how to receive and 1496 process an OSB in a bundle fragment that may or may not also 1497 contain its security target. An OSB definition should also 1498 address whether an OSB may be added to a bundle marked as a 1499 fragment. 1501 Additionally, policy considerations for the management, monitoring, 1502 and configuration associated with blocks SHOULD be included in any 1503 OSB definition. 1505 NOTE: The burden of showing compliance with processing rules is 1506 placed upon the standards defining new security blocks and the 1507 identification of such blocks shall not, alone, require maintenance 1508 of this specification. 1510 11. IANA Considerations 1512 A registry of cipher suite identifiers will be required. 1514 11.1. Bundle Block Types 1516 This specification allocates two block types from the existing 1517 "Bundle Block Types" registry defined in [RFC6255]. 1519 Additional Entries for the Bundle Block-Type Codes Registry: 1521 +-------+-----------------------------+---------------+ 1522 | Value | Description | Reference | 1523 +-------+-----------------------------+---------------+ 1524 | TBD | Block Integrity Block | This document | 1525 | TBD | Block Confidentiality Block | This document | 1526 +-------+-----------------------------+---------------+ 1528 Table 1 1530 12. References 1532 12.1. Normative References 1534 [I-D.ietf-dtn-bpbis] 1535 Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol 1536 Version 7", draft-ietf-dtn-bpbis-11 (work in progress), 1537 May 2018. 1539 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1540 Requirement Levels", BCP 14, RFC 2119, 1541 DOI 10.17487/RFC2119, March 1997, 1542 . 1544 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 1545 Text on Security Considerations", BCP 72, RFC 3552, 1546 DOI 10.17487/RFC3552, July 2003, 1547 . 1549 [RFC6255] Blanchet, M., "Delay-Tolerant Networking Bundle Protocol 1550 IANA Registries", RFC 6255, DOI 10.17487/RFC6255, May 1551 2011, . 1553 12.2. Informative References 1555 [I-D.birrane-dtn-sbsp] 1556 Birrane, E., Pierce-Mayer, J., and D. Iannicca, 1557 "Streamlined Bundle Security Protocol Specification", 1558 draft-birrane-dtn-sbsp-01 (work in progress), October 1559 2015. 1561 [RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, 1562 R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant 1563 Networking Architecture", RFC 4838, DOI 10.17487/RFC4838, 1564 April 2007, . 1566 [RFC6257] Symington, S., Farrell, S., Weiss, H., and P. Lovell, 1567 "Bundle Security Protocol Specification", RFC 6257, 1568 DOI 10.17487/RFC6257, May 2011, 1569 . 1571 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 1572 RFC 8152, DOI 10.17487/RFC8152, July 2017, 1573 . 1575 Appendix A. Acknowledgements 1577 The following participants contributed technical material, use cases, 1578 and useful thoughts on the overall approach to this security 1579 specification: Scott Burleigh of the Jet Propulsion Laboratory, Amy 1580 Alford and Angela Hennessy of the Laboratory for Telecommunications 1581 Sciences, and Angela Dalton and Cherita Corbett of the Johns Hopkins 1582 University Applied Physics Laboratory. 1584 Authors' Addresses 1586 Edward J. Birrane, III 1587 The Johns Hopkins University Applied Physics Laboratory 1588 11100 Johns Hopkins Rd. 1589 Laurel, MD 20723 1590 US 1592 Phone: +1 443 778 7423 1593 Email: Edward.Birrane@jhuapl.edu 1595 Kenneth McKeever 1596 The Johns Hopkins University Applied Physics Laboratory 1597 11100 Johns Hopkins Rd. 1598 Laurel, MD 20723 1599 US 1601 Phone: +1 443 778 2237 1602 Email: Ken.McKeever@jhuapl.edu