Network Working Group W. Eddy Internet-Draft Verizon Intended status: Experimental L. Wood Expires: March 20, 2008 Cisco Systems W. Ivancic NASA September 17, 2007 Checksum Ciphersuites for the Bundle Protocol draft-irtf-dtnrg-bundle-checksum-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 20, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Eddy, et al. Expires March 20, 2008 [Page 1] Internet-Draft Bundle Checksum Ciphersuites September 2007 Abstract The Delay-Tolerant Networking Bundle Protocol includes a custody transfer mechanism to provide acknowledgements of receipt for particular bundles. No checksum is included in the basic DTN Bundle Protocol, however, so it is not possible to verify that bundles have been either forwarded or passed through various convergence layers without errors having been introduced. This document remedies the situation by defining ciphersuites for use within the Bundle Security Protocol's Payload Security Block to provide error-detection functions regardless of an implementation's support for other, more heavy-weight, security-providing ciphersuites. This creates the checksum service needed for error-free reliability, but does so at the expense of divorcing security concerns from the few new ciphersuite definitions. This document lengthily discusses the pros and cons of this approach and the discussion points that combined to drive this design. Eddy, et al. Expires March 20, 2008 [Page 2] Internet-Draft Bundle Checksum Ciphersuites September 2007 Table of Contents 1. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Use of the Payload Security Block . . . . . . . . . . . . . . 6 2.1. Differences from Intended Use of the Payload Security Block . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. INSECURE Ciphersuites . . . . . . . . . . . . . . . . . . . . 10 3.1. Generation and Processing Rules . . . . . . . . . . . . . 12 4. Security Considerations . . . . . . . . . . . . . . . . . . . 14 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.1. Normative References . . . . . . . . . . . . . . . . . . . 18 7.2. Informative References . . . . . . . . . . . . . . . . . . 18 Appendix A. Mandatory BSP Elements Needed to Implement Error-Detection . . . . . . . . . . . . . . . . . . . 20 A.1. Mutable Canonicalization . . . . . . . . . . . . . . . . . 20 A.2. PSB Format . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 24 Eddy, et al. Expires March 20, 2008 [Page 3] Internet-Draft Bundle Checksum Ciphersuites September 2007 1. Motivations Reliable transmission of information is a well-known problem for all protocol layers. Error-detection and correction capabilities are frequently found in lower layers, but are also present in many higher-layer protocols in order to detect residual bit errors and bugs that introduce errors. For example, IPv4 verifies a simple header checksum before processing inbound packets, even when running over a data link like Ethernet that already performs a stronger CRC. TCP and UDP segments further include a checksum covering their contents as well as some IP header fields. What may seem like paranoia is actually not unfounded, as errors in received data or packet corruption are known to creep into networking systems from many causes other than channel noise [SP00]. Although coding of data on the channel can reduce the impact of channel noise, end-to-end checksums are understood to be necessary for applications requiring certainty that the data received is error-free [SGHP98]. The Delay/Disruption-Tolerant Networking (DTN) architecture [RFC4838] is founded on an overlay of Bundle Agents (BAs). These Bundle Agents forward data units called bundles via the Bundle Protocol [I-D.irtf-dtnrg-bundle-spec]. Bundles may be lost or errored both during transmission between BAs, or within a BA itself. Bundles belonging to applications that are not tolerant of lost data have a "custody transfer" flag that requests reliable transmission between bundle agents. The notion of reliability used in the basic custody transfer mechanism does not take the integrity of bundles into account, but acknowledges a bundle's receipt and transfers its custody without verifying its internal data integrity. It is assumed that the "convergence layer adapters" that connect BAs to each other will detect and correct errors before presenting bundle data to the BAs themselves. This may be adequate in many cases, but is not always sufficient, and the insufficiency is encapsulated in the well- known end-to-end principle [SRC84]. It is possible (and even statistically likely) that either transmission errors will go unnoticed, or unchecked errors will be introduced within a BA's memory, storage, or forwarding systems. For example, the UDP convergence-layer adapter that has been popularly implemented in DTN stacks uses UDP's usual 16-bit one's- complement checksum (the same algorithm used for IPv4 headers and TCP segments) to validate incoming datagrams. This checksum is computed by summing 16-bit values within a packet. If two strings are swapped in position within the packet, the checksum can remain unchanged even though the datagram is now different from the original, and clearly errored. The proposed TCP-based convergence layer [I-D.irtf-dtnrg-tcp-clayer] relies on the same checksum algorithm. This checksum algorithm is considered weak, and in more recent Eddy, et al. Expires March 20, 2008 [Page 4] Internet-Draft Bundle Checksum Ciphersuites September 2007 transport protocols has been reconsidered; for instance, SCTP uses the CRC-32c algorithm instead [RFC3309]. Within the context of DTN, even stronger convergence-layer adapter error detection is not sufficient. Errors within a BA's device drivers, errors due to memory issues within the BA's host, e.g. radiation-induced soft errors, or errors introduced from file-system corruption cannot be detected by convergence layer adapters, as these errors occur in gaps between successive phases of forwarding and convergence-layer processing. End-to-end computation and verification of checksums is required to ensure integrity of DTN bundles forwarded across a system composed of BAs and convergence layer adapters [SRC84]. Within this document, we describe a use of the Bundle Security Protocol (BSP) [I-D.irtf-dtnrg-bundle-security] in order to provide the desired error-detection service by defining suitable BSP ciphersuites. The design decisions for doing this are painstakingly explained in Section 2. It should be clearly understood by readers, implementers, and users that we are not using the BSP in a way that provides any level of security, which we explain fully in Section 2.1. The guarantee that we attempt to provide is that specific blocks within a received bundle are highly likely to have been propagated across the overlay without errors, under the assumptions of no malicious activity within or between Bundle Agents and no capability to inject forged bundles. The actual format and use of this error-detection mechanism based on the BSP and requirements for support are described in Section 3. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. [RFC2119] Eddy, et al. Expires March 20, 2008 [Page 5] Internet-Draft Bundle Checksum Ciphersuites September 2007 2. Use of the Payload Security Block The BSP defines three types of blocks: o the Bundle Authentication Block (BAB), o the Payload Security Block (PSB), o the Confidentiality Block (CB). Algorithms applied within each of these blocks could be pressed into use to detect errors introduced in bundle contents. However, based on the different design goals motivating these three block types, the PSB is the only candidate that is truly suitable for supporting the type of checksum fields needed to yield end-to-end reliability of received bundles. The BAB is intended to operate along a single hop within a DTN overlay network, and due to the issues discussed in the previous section, even an end-to-end chain of hops using the BAB over each hop is not sufficient for ensuring reliability. The CB is primarily concerned with the operation of privacy transforms over a bundle's contents, which are intended to significantly alter and disguise the protected data while in transit, rather than simply performing consistency checks over untransformed data. The PSB is intended to be used end-to-end; that is, by two endpoints, rather than hop-by-hop at each intermediate point. The PSB is intended to be used with sets of cryptographic algorithms (ciphersuites) that provide Message Authentication Codes (MACs) or signatures over bundle or block contents. MAC and signature algorithms are security constructions that may allow verification of a legitimate sender (authentication), detection of in-transit tampering (integrity), and proof of a particular sender (non- repudiation). As a consequence of the integrity goal, which is based on the assumption of an adversary that can alter messages in-flight, MACs and signatures can also be effective at detecting errors that occur without the presence of an attacker and in the absence of any malicious intent (e.g. due to bit errors within transmission media, file system corruption, etc.). Since the PSB uses the BSP's mutable canonicalization and covers the Primary Bundle Block, the EIDs and other data influencing forwarding and delivery of payloads are also protected by the MACs or signatures in addition to the payload data. The error-detecting and rejecting capabilities of a MAC or signature are similar to those of more-simple checksum algorithms that are Eddy, et al. Expires March 20, 2008 [Page 6] Internet-Draft Bundle Checksum Ciphersuites September 2007 intended only for error-detection. In fact, several popular MAC and signature constructions use checksums as primary components. For instance, the MD5 hash (checksum) algorithm [RFC1321] is used within the HMAC-MD5 keyed-hash MAC construction [RFC2104]. Computationally, for large messages, the efficiency of a security construction providing integrity is similar to that of a simple checksum, although for short messages, it may be much worse. For instance, HMAC requires multiple applications of the underlying hash function, with the final one being over a very short input, but if the message itself fits within a single block, this results in twice the overhead compared to a simple checksum. Thus, assuming large bundles in relation to the block size of typical hash functions, the PSB can provide end-to-end error-detection capability for bundles from the standpoints of both reasonable effectiveness and reasonable computational cost. 2.1. Differences from Intended Use of the Payload Security Block The main difference between any simple error-detecting checksum and a security construction designed for integrity is that the security construction requires keying material. Key management is recognized as an outstanding problem within the DTNRG [I-D.irtf-dtnrg-sec-overview] and thought to be quite difficult [I-D.farrell-dtnrg-km]. Key management in well-connected systems like the Internet is difficult itself, without the additional complications of a DTN networking environment. However, if using a keyed security construction for simple error-detection, the secrecy of the key is unimportant, and a feasible approach is to specify a hard-coded key that all nodes use in the error-detection mechanism (e.g. the NULL ciphersuite in LTP's authentication extension is one example [I-D.irtf-dtnrg-ltp-extensions]). Using this approach, existing keyed ciphersuites defined for the PSB could be used with NULL keys to provide an error-detection mechanism, without requiring a key management mechanism. However, this key-based method reuses a security mechanism for error detection, which it is not designed for. As there the Bundle Protocol has no separate outer error detection wrapping this security payload, if a private key is used, then third- party nodes that do not possess that key cannot determine the reliability of the content, and would be unable to prevent further forwarding of errored bundles. The only PSB ciphersuite included in the BSP to date is PSB-RSA- SHA256, which creates and verifies signatures of bundles using RSA asymmetric-key operations over a SHA-256 hash of the bundle. The length of the SHA-256 output is excessive for providing simple error- detection on all but extremely large bundles and the overhead of SHA- 256 and RSA computations are also detrimental. It would also require some small change in either security block metadata bits or the block Eddy, et al. Expires March 20, 2008 [Page 7] Internet-Draft Bundle Checksum Ciphersuites September 2007 type field to signal that the special (unsecure) NULL key pair for error-detection only was to be used with PSB-RSA-SHA256. But the biggest problem with PSB-RSA-SHA256 is the bulk of code needed to support the RSA operations, which include bignum math and modular arithmetic libraries. Since error-detection and rejection is a vital and absolutely essential component of reliable networking protocols, and much of the purpose of the DTN architecture is to enable internetworking of devices with limited resources, e.g. motes), it would be burdensome to require all Bundle Protocol implementations to include RSA code. Furthermore, RSA was historically patent- encumbered, and this is very typical of modern asymmetric-key cryptography techniques in general, so although the RSA patents have expired, we are hesitant to require implementation of asymmetric-key techniques unless needed for security purposes. The BAB-HMAC ciphersuite that uses SHA1 [RFC3174] within the HMAC construction (HMAC-SHA1) has been specified as mandatory for BSP support. Even though the BAB is not appropriate for end-to-end error-detection, it is certain that BSP implementations will include HMAC-SHA1 routines, and that creating another ciphersuite for PSB- HMAC (which does not exist in the base BSP specification) would impose very little additional code. Partial support for the BSP's elements (at least the PSB's format and mutable canonicalization) could be made mandatory in the Bundle Protocol along with support for the PSB-HMAC ciphersuite with NULL keys while retaining as optional all of the other components of the BSP (BAB, CB, and other ciphersuites). This path seems to be desirable in that it allows re- use of existing code along with re-use of existing specifications, but does not significantly burden lightweight implementations or deployments unconcerned with overlay-layer security. This approach is followed in Section 3 within this document. Our previous proposal to add checksumming to bundles defined a new block type for carrying error-detecting checksums computed over the bundle payload. This was motivated by the fact that some deployments on limited hardware within closed networks will not desire to run heavyweight security protocols nor include the full BSP and its mandatory ciphersuites within their code footprints. Another motivation was the desire to avoid creating a false sense of security by using a mechanism labelled the Bundle _Security_ Protocol with either a ciphersuite or NULL key that provides absolutely no security services. For instance, if an implementation allowed this to be configured using the same mechanisms or policy directive configuration files, formats, etc. that are normally used to configure BSP mechanisms providing real security, then a misconfiguration or misunderstanding could have a negative security impact to an operational system. Eddy, et al. Expires March 20, 2008 [Page 8] Internet-Draft Bundle Checksum Ciphersuites September 2007 In order to allay these concerns, it was decided to define simple error-detection ciphersuites with the string "INSECURE" in their mnemonics and draw a line as to which portions of the BSP security framework become mandatory and which remain optional. This allows implementation of error-detection capabilities either with or without the majority of the BSP, and with reduced potential for misleading users with regards to security. Implementations that provide both the full BSP and simple error- detection ciphersuites MUST subjectively ensure that their configuration by users is sufficiently dissimilar from the normal BSP configuration. Implementations MUST NOT implement the "INSECURE" ciphersuites in such a way that leads to their being construed as security mechanisms in logging output or configuration directives. Eddy, et al. Expires March 20, 2008 [Page 9] Internet-Draft Bundle Checksum Ciphersuites September 2007 3. INSECURE Ciphersuites Any PSB ciphersuite providing only integrity checking for error- detection and is unkeyed MUST contain the string "INSECURE" in its mnemonic. PSBs that use these ciphersuites are otherwise indistinguishable from PSBs used to implement security services. PSB-HMAC is keyed, and so does not use "INSECURE" in its name. When used with a secret key, PSB-HMAC is useful for security, although this is not the case when it is used with a NULL key, we assume that the presence of a NULL key in the configuration significantly alerts users to the fact that it is not providing security. To provide the desired functionality, three new ciphersuites are defined in this document (PSB-HMAC used with either real or NULL keys, PSB-INSECURE-MD5, and PSB-INSECURE-CRC32). The motivations behind defining all three of these ciphersuites are outlined below in the more detailed description of each ciphersuite. All of the ciphersuites defined here use the mutable canonicalization algorithm that is defined in the BSP and compute their checksums over the canonical forms of bundles. Since error-detection support is mandatory, this means that the minimal BSP elements that all Bundle Protocol implementations MUST support include: 1. Mutable Canonicalization - fully described in Appendix A.1 of this document. 2. PSB wire-format - fully described in Appendix A.2 of this document. 3. PSB-HMAC ciphersuite with NULL key definition, PSB-INSECURE-MD5 and PSB-INSECURE-CRC32 ciphersuite The new ciphersuites are identified by the following ciphersuite IDs within the abstract security block: o 0x04 - PSB-HMAC o 0x05 - PSB-INSECURE-MD5 o 0x06 - PSB-INSECURE-CRC32 PSB-HMAC is defined to use the same HMAC-SHA1 construction as the BSP's BAB-HMAC ciphersuite, and can thus leverage existing code. Three ciphersuite parameters are needed, all of which are SDNVs. The first SDNV is a key identifier. A zero value in the key identifier field of a PSB using the PSB-HMAC ciphersuite indicates that the algorithm is keyed with the special NULL key. The NULL key used here is defined to be 0xc37b 7e64 9258 4340 bed1 2207 8089 4115 5068 f738, Eddy, et al. Expires March 20, 2008 [Page 10] Internet-Draft Bundle Checksum Ciphersuites September 2007 the same fixed NULL key used with LTP's NULL ciphersuite. The later two EIDs are offsets describing the protected bits of the bundle, identical to the offset and length parameters describe in the BSP's PSB-RSA-SHA256 ciphersuite. The first identifies the first covered octet and the second identifies the last covered octet. PSB-HMAC creates a 10-octet security result and should provide adequate error-detection capabilities for large bundles of at least several gigabytes in size. Its advantage lies in that the NULL-keyed version can be implemented very nearly for free within existing BSP codebases, and support for HMAC-SHA1 is known to only require around 200 lines of portable C code for implementations that do not already contain BSP support. The existence of the PSB-INSECURE-MD5 ciphersuite is motivated by the fact that an MD5 checksum can be computed on the order of twice as fast as a SHA1 checksum over the same data, as demonstrated by benchmarking activities, yet still yields robust error-detection over fairly large inputs. This may be desirable in environments that have only limited computational resources to expend on bundle generation or processing. For instance, the authors have implemented generation of bundles of up to several hundred megabytes in size, onboard an imaging satellite solid-state data recorder using only a 200 MHz processor. The PSB-INSECURE-MD5 parameters consist of two SDNVs, an offset and length, that convey the covered portion of the bundle, in an identical way to the corresponding PSB-HMAC and PSB-RSA-SHA256 parameters. The security result included with the PSB-INSECURE-MD5 ciphersuite is a full 16-octet MD5 output. The longer security result than PSB-HMAC may provide better error-detection for very large bundles, in addition to being faster to compute. For small bundles, the lack of the HMAC construction's second application of the hash function also improves efficiency in PSB-INSECURE-MD5 compared to PSB-HMAC. Implementations of MD5 are known to require only around 200 lines of portable C code and are widely available as open-source and within the MD5 RFC [RFC1321]. The PSB-INSECURE-CRC32 ciphersuite is intended for small bundles, and MUST only be used on bundles whose payload length is less than 65535 bytes. The parameters included with this ciphersuite are identical to those used with PSB-INSECURE-MD5. The security result is computed using the CRC-32c algorithm, identically to that defined for use with SCTP [RFC3309]. The security result is always a 4-octet quantity when PSB-INSECURE-CRC32 is used. The advantage of this checksum is that it implies a lower header overhead to on-wire or in-memory bundles with small payloads in comparison to the length of the PSB- HMAC and PSB-INSECURE-MD5 security results. This may be highly Eddy, et al. Expires March 20, 2008 [Page 11] Internet-Draft Bundle Checksum Ciphersuites September 2007 desirable in environments where small messaging bundles are normal and only bandwidth-limited links are available. The CRC-32c algorithm is known to be implementable in only several dozen lines of portable C code. 3.1. Generation and Processing Rules Since the INSECURE ciphersuites and NULL-keyed PSB-HMAC use the same block type code and format as the more secure uses of PSB, they inherit the existing generation and processing rules of the PSB. This is good from a security standpoint in two respects: 1. The existing PSB processing rules consider interaction with any BABs that might be added to a bundle and prevent interactions that would cause wrongful failure of MACs, signatures, and checksums. 2. The PSB and CB processing rules remove the possibility of a MAC, signature, or checksum revealing information about the private contents of the CB via the ordering of the applied security/ error-detection transforms. This is discussed within the CB-RSA- AES128-PAYLOAD-PSB ciphersuite definition in the BSP specification. Although the BSP was intended as an optional suite of extensions to the Bundle Protocol, and only needed in cases where certain security services are desired at the bundle layer, a subset of its components is always needed to implement the PSB-based error-detection mechanism. These MUST always be supported in any implementation of the Bundle Protocol, since error-detection must be an essential property of the Bundle Protocol. To be very clear. error-free delivery does not require mandatory implementation of the full BSP and accompanying security ciphersuites, only the PSB block format, and the mutable canonicalization rules. These two portions of the BSP are fully described in teh BSP specification [I-D.irtf-dtnrg-bundle-security] with some commentary regarding their use for error-detection in Appendix A of this document. When applications request custody transfer, the generated bundles MUST use an unkeyed or NULL-keyed PSB ciphersuite defined in this document between the source and destination bundle agents. For other bundles, not requiring custody transfer, an unkeyed or NULL-keyed PSB ciphersuite SHOULD be used. Checking of unkeyed or NULL-keyed PSBs at intermediate bundle agents SHOULD be performed, when possible, and an agent which fails to match the PSB security result within a bundle SHOULD immediately discard the bundle. This limits the wasted resources involved in propagating Eddy, et al. Expires March 20, 2008 [Page 12] Internet-Draft Bundle Checksum Ciphersuites September 2007 known-errored data. A future version of the Bundle Protocol might include a bundle processing flag that signals that errored-delivery is acceptable to a receiving application. However, the current version does not define such a flag. A future version of the Bundle Protocol specification might also define an administrative record that signals when a bundle has been dropped due to a corruption event detected via an unkeyed or NULL-keyed PSB check; this has not been defined in the current Bundle Protocol. Eddy, et al. Expires March 20, 2008 [Page 13] Internet-Draft Bundle Checksum Ciphersuites September 2007 4. Security Considerations This document has attempted to assuage any security concerns that would result from applying non-security-providing algorithms within a mechanism intended for security. This is accomplished through semantic overloading of the PSB, reusing its structure to hold a simple checksum when it is not intended to provide security services. The potential leakage of information if checksums are not covered by some BSP confidentiality transform that is applied later in the transmission path is eliminated by the fact that the existing PSB block type code is used, and the BSP itself already contains rules for ensuring that confidentiality transforms applied by the CB protect the security result fields within PSB instances. This design decision to reuse a security block for error-detection may seem bizarre to both security and networking experts unfamiliar with the travails of the IRTF DTN research group. However, this decision was necessitated by the late addition of checksum support to the Bundle Protocol. By the time this work was recognized as needed by the group, the Bundle Protocol itself was in final review phases and had been implemented multiple times. The Bundle Security Protocol was also no longer malleable enough to ensure compatibility with checksum support, as it had obtained a level of relative stability in its specification and there were existing implementation efforts based on these which could have required modification in order to not pass checksums carried in a non-BSP block as unprotected after performing a confidentiality transform of the payload. Ideally, the error-detection functions would be implemented within the basic networking portions of the Bundle Protocol, and not as a subset of the security framework, however the horse was too far out of the gate for the current definition of the Bundle Protocol to be affected. Since at this time, the Bundle Protocol has only been proposed as Experimental with some disclaimers, this situation is temporarily palatable. It is well-understood that future revisions of the Bundle Protocol need to include appropriate checksums within the basic format of each block. In order to limit overhead, shorter checksums, e.g. CRC-16, could be used for small blocks, with longer checksums, e.g. MD5, reserved for large payload blocks. This would allow checksums to cover and provide confident processing of even blocks with mutable fields, and retain efficient updating in-network as a mutable field changes, without the recomputation also covering large unchanging payload blocks. This might also constrain the damage caused by errors to the functions provided by an individual block, rather than affecting the whole bundle and causing the whole bundle to be discarded, although the overall value of this is currently unknown. Eddy, et al. Expires March 20, 2008 [Page 14] Internet-Draft Bundle Checksum Ciphersuites September 2007 The need to conserve limited network resources by detecting and avoiding further propagating errored bundles in-transit means that Bundle Agents SHOULD always validate checksums of in-flight bundles, even if the Agents are not the ultimate destination. This opens the door for a potential denial-of-service attack on DTN Bundle Agents by forcing them to expend computational cycles on bundles with large payloads. In this case, the attacker would also have to send these bundles over some link towards the target Bundle Agent, which will often be more constrained in bandwidth or availability than the Bundle Agent is in compuational cycles, so this threat may be unrealistic, or better combatted through access-control on links. If this threat does turn out to be realistic in some set of circumstances, intermediary validation of PSBs was intentionally left as a SHOULD-level activity rather than a MUST, and could be dynamically disabled at some threshold of CPU use. Using the same protocol mechanism to provide (1) error-detection without security claims, (2) error-detection using a security protocol insecurely-keyed with a known NULL key, and (3) actual security protection using the same protocol but with secret keys, any of which can defined and used in the same "Payload Security Block", is confusing at best, and not a good clean-sheet approach to helping ensure secure configurations, interoperable implementations, or efficient handling of errored bundles. The normal method for handling error detection with security is to cover the encrypted payload with an outer error-detecting checksum. Use of a private key without a separate outer error-detecting checksum prevents determination of the bundle's fidelity by any in- path forwarding nodes lacking the key. This discourages interoperability between parties that do not share keys. Use of a NULL key is inferior to separately handling the security concerns of sender-authentication and integrity-protection from that of error- checking as it opens the door to private keys that prevent standalone error-detection, and should be discouraged. Also, the NULL key cannot provide error-detection needed for the mutable parts of the bundle. Providing any error detection for the mutable parts of the bundle has not been done here, and reliance on the fidelity of mutable payloads should be avoided for this reason. Eddy, et al. Expires March 20, 2008 [Page 15] Internet-Draft Bundle Checksum Ciphersuites September 2007 5. IANA Considerations This document has no considerations for IANA. Eddy, et al. Expires March 20, 2008 [Page 16] Internet-Draft Bundle Checksum Ciphersuites September 2007 6. Acknowledgements Some of the work on this document was performed at NASA's Glenn Research Center under funding from the Earth Science Technology Office (ESTO) and the Space Communications Architecture Working Group (SCAWG). Discussion in the DTNRG and particular suggestions from (alphabetically) Mike Demmer, Stephen Farrell, Darren Long, Peter Lovell, and Susan Symington guided the genesis of this document and were crucial to adding error-detection capabilities to the Bundle Protocol within the limitations of the existing pre-established security framework. Eddy, et al. Expires March 20, 2008 [Page 17] Internet-Draft Bundle Checksum Ciphersuites September 2007 7. References 7.1. Normative References [I-D.irtf-dtnrg-bundle-security] Symington, S., "Bundle Security Protocol Specification", draft-irtf-dtnrg-bundle-security-03 (work in progress), April 2007. [I-D.irtf-dtnrg-bundle-spec] Scott, K. and S. Burleigh, "Bundle Protocol Specification", draft-irtf-dtnrg-bundle-spec-10 (work in progress), July 2007. [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. [RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control Transmission Protocol (SCTP) Checksum Change", RFC 3309, September 2002. 7.2. Informative References [I-D.farrell-dtnrg-km] Farrell, S., "DTN Key Management Requirements", draft-farrell-dtnrg-km-00 (work in progress), June 2007. [I-D.irtf-dtnrg-ltp-extensions] Farrell, S., "Licklider Transmission Protocol - Extensions", draft-irtf-dtnrg-ltp-extensions-05 (work in progress), April 2007. [I-D.irtf-dtnrg-sec-overview] Farrell, S., "Delay-Tolerant Networking Security Overview", draft-irtf-dtnrg-sec-overview-03 (work in progress), July 2007. [I-D.irtf-dtnrg-tcp-clayer] Eddy, et al. Expires March 20, 2008 [Page 18] Internet-Draft Bundle Checksum Ciphersuites September 2007 Demmer, M. and J. Ott, "Delay Tolerant Networking TCP Convergence Layer Protocol", draft-irtf-dtnrg-tcp-clayer-00 (work in progress), July 2007. [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, July 1994. [RFC1810] Touch, J., "Report on MD5 Performance", RFC 1810, June 1995. [RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant Networking Architecture", RFC 4838, April 2007. [SGHP98] Stone, J., Greenwald, M., Hughes, J., and C. Partridge, "Performance of checksums and CRCs over real data", IEEE Transactions on Networks vol. 6 issue 5, pp. 529-543. [SP00] Stone, J. and C. Partridge, "When the CRC and TCP Checksum Disagree", Proceedings of ACM SIGCOMM 2000, September 2000. [SRC84] Saltzer, J., Reed, D., and D. Clark, "End-to-end Arguments in System Design", ACM Transactions on Computer Systems 2 (4), November 1984. Eddy, et al. Expires March 20, 2008 [Page 19] Internet-Draft Bundle Checksum Ciphersuites September 2007 Appendix A. Mandatory BSP Elements Needed to Implement Error-Detection This document makes some BSP components mandatory to Bundle Protocol implementations, while the BSP was previously entirely optional. The appendix discusses these elements in greater detail, and highlights some further drawbacks of basing error detection and protocol reliability upon them. A.1. Mutable Canonicalization While impressively named, the mutable canonicalization proceedure is actually quite simple to understand. The requirement for mutable canonicalization stems from the Bundle Protocol's forwarding design that allows several "mutable" fields (e.g. the "dictionary", custodian, and some flags and length fields), to change in-transit at intermediate nodes. In order for the checksum, MAC, or signature computed and placed in the security result of a sent PSB to match the result computed over the received bundle, the sender and receiver need to leave mutable fields out of these computations. The format of a bundle that is input to the PSB algorithms thus differs from its wire-format, and is called its "mutable canonicalization". Using mutable canonicalization implies either using an incrementally- updatable checksum algorithm and feeding many small pieces of data to it, or entirely rewriting a bundle block-by-block based on mutable canonicalization rules before feeding it to the checksum function. (The mutable fields still require protection against errors; a hop- by-hop checksum over only the mutable fields could be used to provide this. Hop-by-hop checksum coverage could be provided by a convergence layer, but this would likely cover the entire bundle or fragment. Some proposed convergence layers do not provide any form of checksum to compensate for this oversight in bundle design.) Several problems are known to plague mutable canonicalization: 1. The Bundle Protocol specification describes the bundle processing control flags as a single variable-length SDNV whose bits are sub-divided in-order by function into "SRR" (Status Report Request), "COS" (Class of Service), and "General". The BSP's mutable canonicalization description shows three separate fields, with only slightly differing names, but in totally opposite order: "Proc. Flags", "COS Flags", and "SRR Flags", instead of one single SDNV, yet has text that describes operating on these as a single 64-bit value and applying a fixed-length mask to them. This is unclear at best, and feared to be uninteroperable in implementations. Eddy, et al. Expires March 20, 2008 [Page 20] Internet-Draft Bundle Checksum Ciphersuites September 2007 2. Many bits within the bundle processing control flags are masked out (i.e. forced to zero within) the mutable canonicalization format. This includes all of the reserved class of service (COS) bits that are highly likely to be needed to overcome the limitation of having only three defined priority levels in the Bundle Specification (compare to DiffServ, CLNP's priority field, Aeronautical Mobile Radio Service message priorities, or mechanisms in other networking stacks that provide many more bits). This means that these bits, and any other bundle processing control bits, will be unprotected by the end-to-end checksum and may change in-transit, potentially causing mis- treatment or mal-delivery of bundles. 3. The existing "bundle is a fragment" bit is unprotected in mutable canonicalization. Errors in this bit itself can probably be caught through other means, such as careful length and bound checking in processing the rest of the bundle. 4. The entire mutable canonicalization procedure of parsing and re- formatting bundles in order to perform a checksum validation is significantly more complex than is typical in most existing protocols that are designed to be capable of simply computing a validation over a frame either without modifications [RFC1662], with only a small fixed-length and position field masked [RFC0791], or with only a simple fixed-size pseudoheader [RFC0793]. The significant additional complexity of mutable canonicalization prevents high performance in forwarding nodes that follow the guideline of verifying unkeyed or NULL-keyed PSBs. A.2. PSB Format PSBs follow the format of the abstract security block, defined in the BSP specification [I-D.irtf-dtnrg-bundle-security], with a block type code that identifies them as PSBs. Some of the processing rules for PSBs that make the PSB less than ideal for error-detection purposes include: 1. If a CB is placed into a bundle that already has a PSB, then another CB is created that hides the PSB. This means that for end-to-end error-detection PSBs, any in-network security proxies that add CB blocks also prevent the checksum in the PSB from being verifiable before the CB's security destination recovers the cleartext PSB. If the CB security destination is never reached, the bundle cannot be checked for errors. Errored bundles will consume resources between these two security gateways, since the errors cannot be detected and the bundles discarded en route. The RSA signature of such an errored bundle Eddy, et al. Expires March 20, 2008 [Page 21] Internet-Draft Bundle Checksum Ciphersuites September 2007 will only fail at the security destination, and the bundle will only be discarded at that end point, but there may be significant resources expended in delivering the useless bundle to that point. 2. A previously-generated PSB's security result cannot be retained outside a CB in the clear, because an observer could correlate the value to some known probable plaintext payload value. It might be better to reverse the order of operations and always generate rewritten PSB ciphersuite checksums after generating CBs that encrypt the payload, so that the PSB security result covers the CB's encrypted form of the payload rather than the unencrypted form, and uses the same security destination as the CB. Upon reaching this security destination, another PSB destined for the receiver, covering the payload revealed at the security destination, could be generated. Requiring this would allow detection of errored bundles between CB security source and CB security destination, but would involve adding another instruction to the CB generation process within the BSP. This assumes no errors are introduced during the decryption process of the CB, as such errors would go undetected. If bundles pass through nested security domains, this could compound the error rate. There seems to be both benefits and drawbacks to any approach to PSB and CB interaction that doesn't involve layering multiple PSBs that can be pushed and popped off of a bundle at various security sources and destinations. By some reasonable metrics, the BSP-prescribed interaction that we have attempted to build on and fix here may be among the least desirable of known methods for error-detection. Eddy, et al. Expires March 20, 2008 [Page 22] Internet-Draft Bundle Checksum Ciphersuites September 2007 Authors' Addresses Wesley M. Eddy Verizon Federal Network Systems NASA Glenn Research Center 21000 Brookpark Rd, MS 54-5 Cleveland, OH 44135 United States of America Phone: +1-216-433-6682 Email: weddy@grc.nasa.gov Lloyd Wood Cisco Systems 11 New Square Park, Bedfont Lakes Feltham, Middlesex TW14 8HA United Kingdom Phone: +44-20-8824-4236 Email: lwood@cisco.com Will Ivancic NASA Glenn Research Center 21000 Brookpark Road Cleveland, Ohio 44135 USA Phone: +1-216-433-3494 Fax: +1-216-433-8705 Email: William.D.Ivancic@nasa.gov Eddy, et al. Expires March 20, 2008 [Page 23] Internet-Draft Bundle Checksum Ciphersuites September 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Eddy, et al. Expires March 20, 2008 [Page 24]