Internet Research Task Force (IRTF) S. Symington Request for Comments: 6257 The MITRE Corporation Category: Experimental S. Farrell ISSN: 2070-1721 Trinity College Dublin H. Weiss P. Lovell SPARTA, Inc. May 2011 Bundle Security Protocol Specification
AbstractThis document defines the bundle security protocol, which provides data integrity and confidentiality services for the Bundle Protocol. Separate capabilities are provided to protect the bundle payload and additional data that may be included within the bundle. We also describe various security considerations including some policy options. This document is a product of the Delay-Tolerant Networking Research Group and has been reviewed by that group. No objections to its publication as an RFC were raised. Status of This Memo This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internet-related research and development activities. These results might not be suitable for deployment. This RFC represents the consensus of the Delay-Tolerant Networking Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6257.
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1. Introduction ....................................................4 1.1. Related Documents ..........................................4 1.2. Terminology ................................................5 2. Security Blocks .................................................8 2.1. Abstract Security Block ....................................9 2.2. Bundle Authentication Block ...............................13 2.3. Payload Integrity Block ...................................15 2.4. Payload Confidentiality Block .............................16 2.5. Extension Security Block ..................................20 2.6. Parameters and Result Fields ..............................21 2.7. Key Transport .............................................23 2.8. PIB and PCB Combinations ..................................24 3. Security Processing ............................................25 3.1. Nodes as Policy Enforcement Points ........................26 3.2. Processing Order of Security Blocks .......................26 3.3. Security Regions ..........................................29 3.4. Canonicalization of Bundles ...............................31 3.5. Endpoint ID Confidentiality ...............................37 3.6. Bundles Received from Other Nodes .........................38 3.7. The At-Most-Once-Delivery Option ..........................39 3.8. Bundle Fragmentation and Reassembly .......................40 3.9. Reactive Fragmentation ....................................41 3.10. Attack Model .............................................42 4. Mandatory Ciphersuites .........................................42 4.1. BAB-HMAC ..................................................42 4.2. PIB-RSA-SHA256 ............................................43 4.3. PCB-RSA-AES128-PAYLOAD-PIB-PCB ............................44 4.4. ESB-RSA-AES128-EXT ........................................48 5. Key Management .................................................51 6. Default Security Policy ........................................51 7. Security Considerations ........................................53 8. Conformance ....................................................55 9. IANA Considerations ............................................56 9.1. Bundle Block Types ........................................56 9.2. Ciphersuite Numbers .......................................56 9.3. Ciphersuite Flags .........................................56 9.4. Parameters and Results ....................................57 10. References ....................................................58 10.1. Normative References .....................................58 10.2. Informative References ...................................59
DTNBP] intended for use in delay-tolerant networks, in order to provide Delay-Tolerant Networking (DTN) security services. The Bundle Protocol is used in DTNs that overlay multiple networks, some of which may be challenged by limitations such as intermittent and possibly unpredictable loss of connectivity, long or variable delay, asymmetric data rates, and high error rates. The purpose of the Bundle Protocol is to support interoperability across such stressed networks. The Bundle Protocol is layered on top of underlay-network-specific convergence layers, on top of network- specific lower layers, to enable an application in one network to communicate with an application in another network, both of which are spanned by the DTN. Security will be important for the Bundle Protocol. The stressed environment of the underlying networks over which the Bundle Protocol will operate makes it important for the DTN to be protected from unauthorized use, and this stressed environment poses unique challenges for the mechanisms needed to secure the Bundle Protocol. Furthermore, DTNs may very likely be deployed in environments where a portion of the network might become compromised, posing the usual security challenges related to confidentiality, integrity, and availability. Different security processing applies to the payload and extension blocks that may accompany it in a bundle, and different rules apply to various extension blocks. This document describes both the base Bundle Security Protocol (BSP) and a set of mandatory ciphersuites. A ciphersuite is a specific collection of various cryptographic algorithms and implementation rules that are used together to provide certain security services. The Bundle Security Protocol applies, by definition, only to those nodes that implement it, known as "security-aware" nodes. There MAY be other nodes in the DTN that do not implement BSP. All nodes can interoperate with the exception that BSP security operations can only happen at security-aware nodes.
"Delay-Tolerant Networking Architecture" [DTNarch] defines the architecture for delay-tolerant networks, but does not discuss security at any length. The DTN Bundle Protocol [DTNBP] defines the format and processing of the blocks used to implement the Bundle Protocol, excluding the security-specific blocks defined here. RFC2119]. We introduce the following terminology for purposes of clarity: source - the bundle node from which a bundle originates destination - the bundle node to which a bundle is ultimately destined forwarder - the bundle node that forwarded the bundle on its most recent hop intermediate receiver or "next hop" - the neighboring bundle node to which a forwarder forwards a bundle. path - the ordered sequence of nodes through which a bundle passes on its way from source to destination In the figure below, which is adapted from figure 1 in the Bundle Protocol Specification [DTNBP], four bundle nodes (denoted BN1, BN2, BN3, and BN4) reside above some transport layer(s). Three distinct transport and network protocols (denoted T1/N1, T2/N2, and T3/N3) are also shown.
+---------v-| +->>>>>>>>>>v-+ +->>>>>>>>>>v-+ +-^---------+ | BN1 v | | ^ BN2 v | | ^ BN3 v | | ^ BN4 | +---------v-+ +-^---------v-+ +-^---------v-+ +-^---------+ | T1 v | + ^ T1/T2 v | + ^ T2/T3 v | | ^ T3 | +---------v-+ +-^---------v-+ +-^---------v + +-^---------+ | N1 v | | ^ N1/N2 v | | ^ N2/N3 v | | ^ N3 | +---------v-+ +-^---------v + +-^---------v-+ +-^---------+ | >>>>>>>>^ >>>>>>>>>>^ >>>>>>>>^ | +-----------+ +------------+ +-------------+ +-----------+ | | | | |<-- An Internet --->| |<--- An Internet --->| | | | | BN = "Bundle Node" as defined in the Bundle Protocol Specification Figure 1: Bundle Nodes Sit at the Application Layer of the Internet Model Bundle node BN1 originates a bundle that it forwards to BN2. BN2 forwards the bundle to BN3, and BN3 forwards the bundle to BN4. BN1 is the source of the bundle and BN4 is the destination of the bundle. BN1 is the first forwarder, and BN2 is the first intermediate receiver; BN2 then becomes the forwarder, and BN3 the intermediate receiver; BN3 then becomes the last forwarder, and BN4 the last intermediate receiver, as well as the destination. If node BN2 originates a bundle (for example, a bundle status report or a custodial signal), which is then forwarded on to BN3, and then to BN4, then BN2 is the source of the bundle (as well as being the first forwarder of the bundle) and BN4 is the destination of the bundle (as well as being the final intermediate receiver). We introduce the following security-specific DTN terminology: security-source - a bundle node that adds a security block to a bundle security-destination - a bundle node that processes a security block of a bundle security path - the ordered sequence of security-aware nodes through which a bundle passes on its way from the security-source to the security-destination
Referring to Figure 1 again: If the bundle that originates at BN1 is given a security block by BN1, then BN1 is the security-source of this bundle with respect to that security block, as well as being the source of the bundle. If the bundle that originates at BN1 is given a security block by BN2, then BN2 is the security-source of this bundle with respect to that security block, even though BN1 is the source. If the bundle that originates at BN1 is given a security block by BN1 that is intended to be processed by BN3, then BN1 is the security- source and BN3 is the security-destination with respect to this security block. The security path for this block is BN1 to BN3. A bundle MAY have multiple security blocks. The security-source of a bundle, with respect to a given security block in the bundle, MAY be the same as or different from the security-source of the bundle with respect to a different security block in the bundle. Similarly, the security-destination of a bundle, with respect to each of that bundle's security blocks, MAY be the same or different. Therefore, the security paths for various blocks MAY be, and often will be, different. If the bundle that originates at BN1 is given a security block by BN1 that is intended to be processed by BN3, and BN2 adds a security block with security-destination BN4, the security paths for the two blocks overlap but not completely. This problem is discussed further in Section 3.3. As required in [DTNBP], forwarding nodes MUST transmit blocks in a bundle in the same order in which they were received. This requirement applies to all DTN nodes, not just ones that implement security processing. Blocks in a bundle MAY be added or deleted according to the applicable specification, but those blocks that are both received and transmitted MUST be transmitted in the same order that they were received. If a node is not security-aware, then it forwards the security blocks in the bundle unchanged unless the bundle's block processing flags specify otherwise. If a network has some nodes that are not security-aware, then the block processing flags SHOULD be set such that security blocks are not discarded at those nodes solely because they cannot be processed there. Except for this, the non-security- aware nodes are transparent relay points and are invisible as far as security processing is concerned.
The block sequence also indicates the order in which certain significant actions have affected the bundle, and therefore the sequence in which actions MUST occur in order to produce the bundle at its destination. Section 2.2. The PIB is used to ensure the authenticity and integrity of the payload from the PIB security-source, which creates the PIB, to the PIB security-destination, which verifies the PIB authenticator. The authentication information in the PIB MAY (if the ciphersuite allows) be verified by any node in between the PIB security-source and the PIB security-destination that has access to the cryptographic keys and revocation status information required to do so. Since a BAB protects a bundle on a "hop-by-hop" basis and other security blocks MAY be protecting over several hops or end-to-end, whenever both are present, the BAB MUST form the "outer" layer of protection -- that is, the BAB MUST always be calculated and added to the bundle after all other security blocks have been calculated and added to the bundle. The PCB indicates that the payload has been encrypted, in whole or in part, at the PCB security-source in order to protect the bundle content while in transit to the PCB security-destination. PIB and PCB protect the payload and are regarded as "payload- related" for purposes of the security discussion in this document. Other blocks are regarded as "non-payload" blocks. Of course, the primary block is unique and has separate rules. The ESB provides security for non-payload blocks in a bundle. Therefore, ESB is not applied to PIBs or PCBs and, of course, is not appropriate for either the payload block or primary block.
Each of the security blocks uses the Canonical Bundle Block Format as defined in the Bundle Protocol Specification. That is, each security block is comprised of the following elements: o Block-type code o Block processing control flags o Block EID-reference list (OPTIONAL) o Block data length o Block-type-specific data fields Since the four security blocks have most fields in common, we can shorten the description of the Block-type-specific data fields of each security block if we first define an abstract security block (ASB) and then specify each of the real blocks in terms of the fields that are present/absent in an ASB. Note that no bundle ever contains an actual ASB, which is simply a specification artifact. DTNBP]. SDNV stands for Self-Delimiting Numeric Value. An ASB consists of the following mandatory and optional fields: o Block-type code (one byte) - as in all bundle protocol blocks except the primary bundle block. The block-type codes for the security blocks are: BundleAuthenticationBlock - BAB: 0x02 PayloadIntegrityBlock - PIB: 0x03 PayloadConfidentialityBlock - PCB: 0x04 ExtensionSecurityBlock - ESB: 0x09 o Block processing control flags (SDNV) - defined as in all bundle protocol blocks except the primary bundle block (as described in the Bundle Protocol Specification [DTNBP]). SDNV encoding is described in the Bundle Protocol. There are no general constraints on the use of the block processing control flags, and some specific requirements are discussed later.
o EID-references - composite field defined in [DTNBP] containing references to one or two endpoint identifiers (EIDs). Presence of the EID-reference field is indicated by the setting of the "Block contains an EID-reference field" (EID_REF) bit of the block processing control flags. If one or more references are present, flags in the ciphersuite ID field, described below, specify which. If no EID fields are present, then the composite field itself MUST be omitted entirely and the EID_REF bit MUST be unset. A count field of zero is not permitted. o The possible EIDs are: * (OPTIONAL) Security-source - specifies the security-source for the block. If this is omitted, then the source of the bundle is assumed to be the security-source unless otherwise indicated. * (OPTIONAL) Security-destination - specifies the security- destination for the block. If this is omitted, then the destination of the bundle is assumed to be the security- destination unless otherwise indicated. If two EIDs are present, security-source is first and security- destination comes second. o Block data length (SDNV) - as in all bundle protocol blocks except the primary bundle block. SDNV encoding is described in the Bundle Protocol. o Block-type-specific data fields as follows: * Ciphersuite ID (SDNV) * Ciphersuite flags (SDNV) * (OPTIONAL) Correlator - when more than one related block is inserted, then this field MUST have the same value in each related block instance. This is encoded as an SDNV. See the note in Section 3.8 with regard to correlator values in bundle fragments. * (OPTIONAL) Ciphersuite-parameters - compound field of the next two items + Ciphersuite-parameters length - specifies the length of the following Ciphersuite-parameters data field and is encoded as an SDNV.
+ Ciphersuite-parameters data - parameters to be used with the ciphersuite in use, e.g., a key identifier or initialization vector (IV). See Section 2.6 for a list of potential parameters and their encoding rules. The particular set of parameters that is included in this field is defined as part of the ciphersuite specification. * (OPTIONAL) Security-result - compound field of the next two items + Security-result length - contains the length of the next field and is encoded as an SDNV. + Security-result data - contains the results of the appropriate ciphersuite-specific calculation (e.g., a signature, Message Authentication Code (MAC), or ciphertext block key). Although the diagram hints at a 32-bit layout, this is purely for the purpose of exposition. Except for the "type" field, all fields are variable in length. +----------------+----------------+----------------+----------------+ | type | flags (SDNV) | EID-ref list(comp) | +----------------+----------------+----------------+----------------+ | length (SDNV) | ciphersuite (SDNV) | +----------------+----------------+----------------+----------------+ | ciphersuite flags (SDNV) | correlator (SDNV) | +----------------+----------------+----------------+----------------+ |params len(SDNV)| ciphersuite params data | +----------------+----------------+----------------+----------------+ |res-len (SDNV) | security-result data | +----------------+----------------+----------------+----------------+ Figure 2: Abstract Security Block Structure Some ciphersuites are specified in Section 4, which also specifies the rules that MUST be satisfied by ciphersuite specifications. Additional ciphersuites MAY be defined in separate specifications. Ciphersuite IDs not specified are reserved. Implementations of the Bundle Security Protocol decide which ciphersuites to support, subject to the requirements of Section 4. It is RECOMMENDED that implementations that allow additional ciphersuites permit ciphersuite ID values at least up to and including 127, and they MAY decline to allow larger ID values.
The structure of the ciphersuite flags field is shown in Figure 3. In each case, the presence of an optional field is indicated by setting the value of the corresponding flag to one. A value of zero indicates the corresponding optional field is missing. Presently, there are five flags defined for the field; for convenience, these are shown as they would be extracted from a single-byte SDNV. Future additions may cause the field to grow to the left so, as with the flags fields defined in [DTNBP], the description below numbers the bit positions from the right rather than the standard RFC definition, which numbers bits from the left. src - bit 4 indicates whether the EID-reference field of the ASB contains the optional reference to the security-source. dest - bit 3 indicates whether the EID-reference field of the ASB contains the optional reference to the security-destination. parm - bit 2 indicates whether or not the ciphersuite-parameters length and ciphersuite-parameters data fields are present. corr - bit 1 indicates whether or not the ASB contains an optional correlator. res - bit 0 indicates whether or not the ASB contains the security-result length and security-result data fields. bits 5-6 are reserved for future use. Bit Bit Bit Bit Bit Bit Bit 6 5 4 3 2 1 0 +-----+-----+-----+-----+-----+-----+-----+ | reserved | src |dest |parm |corr |res | +-----+-----+-----+-----+-----+-----+-----+ Figure 3: Ciphersuite Flags A little bit more terminology: if the block is a PIB, when we refer to the PIB-source, we mean the security-source for the PIB as represented by the EID-reference in the EID-reference field. Similarly, we may refer to the "PCB-dest", meaning the security- destination of the PCB, again as represented by an EID reference. For example, referring to Figure 1 again, if the bundle that originates at BN1 is given a Payload Confidentiality Block (PCB) by BN1 that is protected using a key held by BN3, and it is given a Payload Integrity Block (PIB) by BN1, then BN1 is both the PCB-source and the PIB-source of the bundle, and BN3 is the PCB-destination of the bundle.
The correlator field is used to associate several related instances of a security block. This can be used to place a BAB that contains the ciphersuite information at the "front" of a (probably large) bundle, and another correlated BAB that contains the security-result at the "end" of the bundle. This allows even very memory-constrained nodes to be able to process the bundle and verify the BAB. There are similar use cases for multiple related instances of PIB and PCB as will be seen below. The ciphersuite specification MUST make it clear whether or not multiple block instances are allowed, and if so, under what conditions. Some ciphersuites can, of course, leave flexibility to the implementation, whereas others might mandate a fixed number of instances. For convenience, we use the term "first block" to refer to the initial block in a group of correlated blocks or to the single block if there are no others in the set. Obviously, there can be several unrelated groups in a bundle, each containing only one block or more than one, and each having its own "first block".
An EID-reference to the security-source MAY be present. The security-source can also be specified as part of key-information described in Section 2.6 or another block such as the Previous-Hop Insertion Block [PHIB]. The security-source might also be inferred from some implementation-specific means such as the convergence layer. An EID-reference to the security-destination MAY be present and is useful to ensure that the bundle has been forwarded to the correct next-hop node. The security-result MUST be present as it is effectively the "output" from the ciphersuite calculation (e.g., the MAC or signature) applied to the (relevant parts of the) bundle (as specified in the ciphersuite definition). For the case using two related BAB instances, the first instance is as defined above, except the ciphersuite ID MUST be documented as a hop-by-hop authentication ciphersuite that requires two instances of the BAB. In addition, the correlator MUST be present and the security-result length and security-result fields MUST be absent. The second instance of the BAB MUST have the same correlator value present and MUST contain security-result length and security-result data fields. The other optional fields MUST NOT be present. Typically, this second instance of a BAB will be the last block of the bundle. The details of key transport for BAB are specified by the particular ciphersuite. In the absence of conflicting requirements, the following should be noted by implementors: o the key-information item in Section 2.6 is OPTIONAL, and if not provided, then the key SHOULD be inferred from the source- destination tuple, being the previous key used, a key created from a key-derivation function, or a pre-shared key. o if all the nodes are security-aware, the capabilities of the underlying convergence layer might be useful for identifying the security-source. o depending upon the key mechanism used, bundles can be signed by the sender, or authenticated for one or more recipients, or both.
Section 2.6. An EID-reference to the security-destination MAY be present. The security-result is effectively the "output" from the ciphersuite calculation (e.g., the MAC or signature) applied to the (relevant parts of the) bundle. As in the case of the BAB, this field MUST be present if the correlator is absent. If more than one related instance of the PIB is required, then this is handled in the same way as described for the BAB above. The ciphersuite MAY process less than the entire original bundle payload. This might be because it is defined to process some subset of the bundle, or perhaps because the current payload is a fragment of an original bundle. For whatever reason, if the ciphersuite processes less than the complete, original bundle payload, the ciphersuite-parameters of this block MUST specify which bytes of the bundle payload are protected.
For some ciphersuites, (e.g., those using asymmetric keying to produce signatures or those using symmetric keying with a group key), the security information can be checked at any hop on the way to the security-destination that has access to the required keying information. This possibility is further discussed in Section 3.6. The use of a generally available key is RECOMMENDED if custodial transfer is employed and all nodes SHOULD verify the bundle before accepting custody. Most asymmetric PIB ciphersuites will use the PIB-source to indicate who the signer is and will not require the PIB-dest field because the key needed to verify the PIB authenticator will be a public key associated with the PIB-source.
Fragmentation, reassembly, and custody transfer are adversely affected by a change in size of the payload due to ambiguity about what byte range of the original payload is actually in any particular fragment. Ciphersuites SHOULD place any payload expansion, such as authentication tags (integrity check values) and any padding generated by a block-mode cipher, into an integrity check value item in the security-result field (see Section 2.6) of the confidentiality block. Payload super-encryption is allowed, that is, encrypting a payload that has already been encrypted, perhaps more than once. Ciphersuites SHOULD define super-encryption such that, as well as re- encrypting the payload, it also protects the parameters of earlier encryption. Failure to do so may represent a vulnerability in some circumstances. Confidentiality is normally applied to the payload, and possibly to additional blocks. It is RECOMMENDED to apply a Payload Confidentiality ciphersuite to non-payload blocks only if these SHOULD be super-encrypted with the payload. If super-encryption of the block is not desired, then protection of the block SHOULD be done using the Extension Security Block mechanism rather than PCB. Multiple related PCB instances are required if both the payload and PIBs and PCBs in the bundle are to be encrypted. These multiple PCB instances require correlators to associate them with each other since the key-information is provided only in the first PCB. There are situations where more than one PCB instance is required but the instances are not "related" in the sense that requires correlators. One example is where a payload is encrypted for more than one security-destination so as to be robust in the face of routing uncertainties. In this scenario, the payload is encrypted using a BEK. Several PCBs contain the BEK encrypted using different KEKs, one for each destination. These multiple PCB instances are not "related" and SHOULD NOT contain correlators. The ciphersuite MAY apply different rules to confidentiality for non- payload blocks. A PCB is an ASB with the following additional restrictions: The block-type code value MUST be 0x04. The block processing control flags value can be set to whatever values are required by local policy, except that a PCB "first block" MUST have the "replicate in every fragment" flag set. This flag SHOULD NOT be set otherwise. Ciphersuite designers should
carefully consider the effect of setting flags that either discard the block or delete the bundle in the event that this block cannot be processed. The ciphersuite ID MUST be documented as a confidentiality ciphersuite. The correlator MUST be present if there is more than one related PCB instance. The correlator MUST NOT be present if there are no related PCB instances. If a correlator is present, the key-information MUST be placed in the PCB "first block". Any additional bytes generated as a result of encryption and/or authentication processing of the payload SHOULD be placed in an "integrity check value" field (see Section 2.6) in the security- result of the first PCB. The ciphersuite-parameters field MAY be present. An EID-reference to the security-source MAY be present. The security-source can also be specified as part of key-information described in Section 2.6. An EID-reference to the security-destination MAY be present. The security-result MAY be present and normally contains fields such as an encrypted bundle encryption key, authentication tag, or the encrypted versions of bundle blocks other than the payload block. The ciphersuite MAY process less than the entire original bundle payload, either because the current payload is a fragment of the original bundle or just because it is defined to process some subset. For whatever reason, if the ciphersuite processes less than the complete, original bundle payload, the "first" PCB MUST specify, as part of the ciphersuite-parameters, which bytes of the bundle payload are protected. PCB ciphersuites MUST specify which blocks are to be encrypted. The specification MAY be flexible and be dependent upon block type, security policy, various data values, and other inputs, but it MUST be deterministic. The determination of whether or not a block is to be encrypted MUST NOT be ambiguous.
As was the case for the BAB and PIB, if the ciphersuite requires more than one instance of the PCB, then the "first block" MUST contain any optional fields (e.g., security-destination, etc.) that apply to all instances with this correlator. These MUST be contained in the first instance and MUST NOT be repeated in other correlated blocks. Fields that are specific to a particular instance of the PCB MAY appear in that PCB. For example, security-result fields MAY (and probably will) be included in multiple related PCB instances, with each result being specific to that particular block. Similarly, several PCBs might each contain a ciphersuite-parameters field with an IV specific to that PCB instance. Put another way: when confidentiality will generate multiple blocks, it MUST create a "first" PCB with the required ciphersuite ID, parameters, etc., as specified above. Typically, this PCB will appear early in the bundle. This "first" PCB contains the parameters that apply to the payload and also to the other correlated PCBs. The correlated PCBs follow the "first" PCB and MUST NOT repeat the ciphersuite-parameters, security-source, or security-destination fields from the first PCB. These correlated PCBs need not follow immediately after the "first" PCB, and probably will not do so. Each correlated block, encapsulating an encrypted PIB or PCB, is at the same place in the bundle as the original PIB or PCB. A ciphersuite MUST NOT mix payload data and a non-payload block in a single PCB. Even if a to-be-encrypted block has the "discard" flag set, whether or not the PCB's "discard" flag is set is an implementation/policy decision for the encrypting node. (The "discard" flag is more properly called the "Discard if block can't be processed" flag.) Any existing EID-list in the to-be-encapsulated original block remains exactly as-is, and is copied to become the EID-list for the replacing block. The encapsulation process MUST NOT replace or remove the existing EID-list entries. This is critically important for correct updating of entries at the security-destination. At the security-destination, either the specific destination or the bundle-destination, the processes described above are reversed. The payload is decrypted "in-place" using the salt, IV, and key values in the first PCB, including verification using the ICV. These values are described in Section 2.6. Each correlated PCB is also processed at the same destination, using the salt and key values from the first PCB and the block-specific IV item. The encapsulated block item in the security-result is decrypted and validated, using also the tag that SHOULD have been appended to the ciphertext of the original block data. Assuming the validation succeeds, the resultant
plaintext, which is the entire content of the original block, replaces the PCB at the same place in the bundle. The block type reverts to that of the original block prior to encapsulation, and the other block-specific data fields also return to their original values. Implementors are cautioned that this "replacement" process requires delicate stitchery, as the EID-list contents in the decapsulated block are invalid. As noted above, the EID-list references in the original block were preserved in the "replacing" PCB, and will have been updated as necessary as the bundle has toured the DTN. The references from the PCB MUST replace the references within the EID-list of the newly decapsulated block. Caveat implementor. DTNMD]. ESBs are typically used to apply confidentiality protection. While it is possible to create an integrity-only ciphersuite, the block protection is not transparent and makes access to the data more difficult. For simplicity, this discussion describes the use of a confidentiality ciphersuite. The protection mechanisms in ESBs are similar to other security blocks with two important differences: o different key values are used (using the same key as that for payload would defeat the purpose) o the block is not encrypted or super-encrypted with the payload A typical ESB ciphersuite will encrypt the extension block using a randomly generated ephemeral key and will use the key-information item in the security-parameters field to carry the key encrypted with some long-term key encryption key (KEK) or well-known public key. If neither the destination nor security-destination resolves the key to use for decryption, the key-information item in the ciphersuite- parameters field can be used also to indicate the decryption key with which the BEK can be recovered.
It is strongly RECOMMENDED that a data integrity mechanism be used in conjunction with confidentiality, and that encryption-only ciphersuites NOT be used. AES-GCM satisfies this requirement. The ESB is placed in the bundle in the same position as the block being protected. That is, the entire original block is processed (encrypted, etc.) and encapsulated in a "replacing" ESB-type block, and this appears in the bundle at the same sequential position as the original block. The processed data is placed in the security-result field. The process is reversed at the security-destination with the recovered plaintext block replacing the ESB that had encapsulated it. Processing of EID-list entries, if any, is described in Section 2.4, and this MUST be followed in order to correctly recover EIDs. An ESB is an ASB with the following additional restrictions: The block type is 0x09. Ciphersuite flags indicate which fields are present in this block. Ciphersuite designers should carefully consider the effect of setting flags that either discard the block or delete the bundle in the event that this block cannot be processed. EID-references MUST be stored in the EID-reference list. The security-source MAY be present. The security-source can also be specified as part of key-information described in Section 2.6. If neither is present, then the bundle-source is used as the security-source. The security-destination MAY be present. If not present, then the bundle-destination is used as the security-destination. The security-parameters MAY optionally contain a block-type code field to indicate the type of the encapsulated block. Since this replicates a field in the encrypted portion of the block, it is a slight security risk, and its use is therefore OPTIONAL.
Each item is represented as a type-length-value. Type is a single byte indicating which item this is. Length is the count of data bytes to follow, and is an SDNV-encoded integer. Value is the data content of the item. Item types are 0: reserved 1: initialization vector (IV) 2: reserved 3: key-information 4: fragment-range (offset and length as a pair of SDNVs) 5: integrity signature 6: unassigned 7: salt 8: PCB integrity check value (ICV) 9: reserved 10: encapsulated block 11: block type of encapsulated block 12 - 191: reserved 192 - 250: private use 251 - 255: reserved The following descriptions apply to the usage of these items for all ciphersuites. Additional characteristics are noted in the discussion for specific suites. o initialization vector (IV): random value, typically eight to sixteen bytes. o key-information: key material encoded or protected by the key management system and used to transport an ephemeral key protected by a long-term key. This item is discussed further in Section 2.7.
o fragment-range: pair of SDNV values (offset then length) specifying the range of payload bytes to which a particular operation applies. This is termed "fragment-range" since that is its typical use, even though sometimes it describes a subset range that is not a fragment. The offset value MUST be the offset within the original bundle, which might not be the offset within the current bundle if the current bundle is already a fragment. o integrity signature: result of BAB or PIB digest or signing operation. This item is discussed further in Section 2.7. o salt: an IV-like value used by certain confidentiality suites. o PCB integrity check value (ICV): output from certain confidentiality ciphersuite operations to be used at the destination to verify that the protected data has not been modified. o encapsulated block: result of confidentiality operation on certain blocks, contains the ciphertext of the block and MAY also contain an integrity check value appended to the ciphertext; MAY also contain padding if required by the encryption mode; used for non- payload blocks only. o block type of encapsulated block: block-type code for a block that has been encapsulated in ESB. Section 2.6: key-information (item type 3) and integrity signature (item type 5). The ciphersuite MUST define the details and format for these items. To facilitate
interoperability, it is strongly RECOMMENDED that the implementations use the appropriate definitions from the Cryptographic Message Syntax (CMS) [RFC5652] and related RFCs. Many situations will require several pieces of key-information. Again, ciphersuites MUST define whether they accept these packed into a single key-information item and/or separated into multiple instances of key-information. For interoperability, it is RECOMMENDED that ciphersuites accept these packed into a single key- information item, and that they MAY additionally choose to accept them sent as separate items. Section 2.4 to ensure that parameters are protected in the case of super-encryption.
Multiple parallel authenticators - a single security-source might wish to protect the integrity of a bundle in multiple ways. This could be required if the bundle's path is unpredictable and if various nodes might be involved as security-destinations. Similarly, if the security-source cannot determine in advance which algorithms to use, then using all might be reasonable. This would result in uses of PIB that, presumably, all protect the payload, and which cannot in general protect one another. Note that this logic can also apply to a BAB, if the unpredictable routing happens in the convergence layer, so we also envisage support for multiple parallel uses of BAB. Multiple sequential authenticators - if some security-destination requires assurance about the route that bundles have taken, then it might insist that each forwarding node add its own PIB. More likely, however, would be that outbound "bastion" nodes would be configured to sign bundles as a way of allowing the sending "domain" to take accountability for the bundle. In this case, the various PIBs will likely be layered, so that each protects the earlier applications of PIB. Authenticated and encrypted bundles - a single bundle MAY require both authentication and confidentiality. Some specifications first apply the authenticator and follow this by encrypting the payload and authenticator. As noted previously in the case where the authenticator is a signature, there are security reasons for this ordering. (See the PCB-RSA-AES128-PAYLOAD-PIB-PCB ciphersuite defined in Section 4.3.) Others apply the authenticator after encryption, that is, to the ciphertext. This ordering is generally RECOMMENDED and minimizes attacks that, in some cases, can lead to recovery of the encryption key. There are, no doubt, other valid ways to combine PIB and PCB instances, but these are the "core" set supported in this specification. Having said that, as will be seen, the mandatory ciphersuites defined here are quite specific and restrictive in terms of limiting the flexibility offered by the correlator mechanism. This is primarily designed to keep this specification as simple as possible, while at the same time supporting the above scenarios.