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Delay Tolerant Networking Research Group                      S. Farrell
Internet Draft                                    Trinity College Dublin
Intended Status: Experimental                                 M. Ramadas
<draft-irtf-dtnrg-ltp-extensions-08.txt>                 Ohio University
June 24 2008                                                 S. Burleigh
Expires December 24 2008                  NASA/Jet Propulsion Laboratory



         Licklider Transmission Protocol - Security Extensions


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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   The Licklider Transmission Protocol (LTP), is intended to serve as a
   reliable convergence layer over single hop deep-space RF links. LTP
   does Automatic Repeat reQuest (ARQ) of data transmissions by



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   soliciting selective-acknowledgment reception reports.  It is
   stateful and has no negotiation or handshakes.

   LTP is designed to provide retransmission-based reliability over
   links characterized by extremely long message round-trip times (RTTs)
   and/or frequent interruptions in connectivity.  Since communication
   across interplanetary space is the most prominent example of this
   sort of environment, LTP is principally aimed at supporting "long-
   haul" reliable transmission in interplanetary space, but has
   applications in other environments as well.

   This document describes security extensions to LTP, and is part of a
   series of related documents describing LTP. Other documents in this
   series cover the motivation for LTP and the main protocol
   specification. We recommend reading all the documents in the series
   before writing code based on this document.

   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.


Table of Contents

   1. Introduction..................................................  2
   2. Security Extensions...........................................  3
       2.1 LTP Authentication ......................................  3
       2.2 Cookie Mechanism.........................................  6
   3. Security Considerations ......................................  7
   4. IANA Considerations ..........................................  7
   5. Acknowledgments ..............................................  7
   6. References ...................................................  8
      6.1 Normative References .....................................  8
      6.2 Informative References ...................................  8
   7. Author's Addresses ...........................................  9


1. Introduction

   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 [B97].

   This document describes extensions to the base LTP protocol
   [LTPSPEC]. The background to LTP is described in the "motivation"
   document [LTPMOTIVE].  All the extensions defined in this document



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   provide additional security features for LTP.

2. Security Extensions

   The syntactical layout of the extensions are defined in Section 3.1.4
   of the base protocol specification [LTPSPEC].

   Implementers should note that the LTP extension mechanism allows for
   multiple occurrences of any extension tag, in both (or either) the
   header or trailer. For example, the LTP authentication mechanism
   defined below requires both header and trailer extensions, which both
   use the same tag.

   This document defines new security extensions for LTP but does not
   address key management since key management in DTNs remains an open
   research question.

   If LTP were deployed layered on top of UDP, it might be possible to
   use IPsec or other existing security mechanisms. In a general DTN
   however IPsec's key exchange (IKE) cannot work (e.g. where link
   delays are measured in minutes).

2.1 LTP Authentication

   The LTP Authentication mechanism provides cryptographic
   authentication of the segment.

   Implementations MAY support this extension field. If they do not
   support this header then they MUST ignore it.

   The LTP authentication extension field has the extension tag value
   0x00.

   LTP authentication requires three new fields, the first two of which
   are carried as the value of the extensions field of the LTP segment
   header, and the third of which is carried in the segment trailer.

   The fields which are carried in the header extensions field are
   catenated together to form the extension value (with the leftmost
   octet representing the ciphersuite and the remaining octets the
   KeyID). The KeyID field is optional, and is determined to be absent
   if the extension value consists of a single octet.

      Ciphersuite: an eight bit integer value with values defined below.

      KeyID: An optional key identifier, the interpretation of which is



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      out of scope for this specification (that is, implementers MUST
      treat these KeyID fields as raw octets, even if they contained an
      ASN.1 DER encoding of an X.509 IssuerSerial construct [PKIXPROF],
      for example).

   The LTP-auth header extension MUST be present in the first segment
   from any LTP session which uses LTP authentication, but MAY be
   omitted from subsequent segments in that session. To guard against
   additional problems arising from lost segments, implementations
   SHOULD, where bandwidth allows, include these fields in a number of
   segments in the LTP session. If the first segment (or any part
   thereof) is re-transmitted, then the LTP-auth header extension MUST
   be included in the re-transmission.

   The field carried as a trailer extension is the AuthVal field. It
   contains the authentication value, which is either a message
   authentication code (MAC) or a digital signature. This is itself a
   structured field whose length and formatting depends on the
   ciphersuite.

   If for some reason the sender includes two instances of LTP auth
   headers then there is a potential problem for the receiver in that
   presumably at least one of the AuthVal fields will not verify. There
   are very few situations where it would make sense to include more
   than one LTP auth extension in a single segment, since LTP is a peer-
   to-peer protocol. If however, keys are being upgraded then the sender
   might protect the segment with both the new and old keys. In such
   cases the receiver MUST search and can consider the LTP
   authentication valid so long as one AuthVal is correct.

   For all ciphersuites, the input to the calculation is the entire
   encoded segment including the AuthVal extension tag and length, but
   not of course, including the AuthVal value.

   We define three ciphersuites in this specification. Our approach is
   to follow the precedent set by TLS [TLS], and to "hardcode" all
   algorithm options in a single ciphersuite number. This means that
   there are 256 potential ciphersuites supported by this version of
   LTP-auth. Since this is a limited space, we request IANA to establish
   a registry for LTP Ciphersuites as described in the IANA
   considerations section below. Current ciphersuite assignments are:

         Ciphersuite                        Value
         -----------                        -----
         HMAC-SHA1-80                          0
         RSA-SHA256                            1



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         Unassigned                          2-127
         Reserved                           128-191
         Private/Experimental use           192-254
         NULL                                 255

   1. HMAC-SHA1-80 Ciphersuite

      The HMAC-SHA1-80 ciphersuite involves generating a MAC over the
      LTP segment and appending the resulting AuthVal field to the end
      of the segment.  There is only one MACing algorithm defined for
      this which is HMAC-SHA1-80 [HMAC]. The AuthVal field in this case
      contains just the output of the HMAC-SHA1-80 algorithm which is a
      fixed width field (10 octets).


   2. RSA-SHA256 Ciphersuite

      The RSA-SHA256 ciphersuite involves generating a digital signature
      of the LTP segment and appending the resulting AuthVal field to
      the end of the segment.  There is only one signature algorithm
      currently defined for this which is RSA with SHA256 as defined in
      [RSA], section 8.2.  The AuthVal field in this case is simply the
      signature value, where the signature value occupies the minimum
      number of octets, e.g. 128 octets for a 1024 bit signature).


   3. NULL Ciphersuite

      The NULL ciphersuite is basically the same as the HMAC-SHA1-80
      ciphersuite, but with a hardcoded key. This ciphersuite
      effectively provides only a strong checksum without
      authentication, and thus is subject to active attacks and is the
      equivalent of providing a CRC.

      The hardcoded key to be used with this ciphersuite is the
      following:

         HMAC_KEY     :  c37b7e64 92584340
                      :  bed12207 80894115
                      :  5068f738
      (The above is the test vector from RFC 3537 [WRAP].)

      In each case the bytes which are input to the cryptographic
      algorithm consist of the entire LTP segment except the AuthVal. In
      particular the header extensions field which may contain the
      ciphersuite number and the KeyID field are part of the input.



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      The output bytes of the cryptographic operation are the payload of
      the AuthVal field.

   The following shows an example LTP-auth header, starting from and
   including the extensions field

       ext  tag  sdnv  c-s  k-id
      +----+----+----+----+----+
      |0x11|0x00|0x02|0x00|0x24|
      +----+----+----+----+----+

   The Extensions field has the value 0x11 with the most-significant and
   least-significant nibble value 1, indicating the presence of one
   header and one trailer extension respectively. The next octet is the
   extension tag (0x00 for LTP-auth), followed by the SDNV encoded
   length of the ensuing data : a one-octet ciphersuite (0x00 meaning
   HMAC-SHA1-80) and the KeyID (in this case with a short value of
   0x24). The trailer extension (not shown above) should contain the
   AuthVal).

2.2 A Cookie mechanism

   The use of cookies is a well known way to make denial-of-service
   attacks harder to mount.  We define the cookie extension for use in
   environments where an LTP implementation is liable to such attacks.

   The cookie is placed in a header extension field, and has no related
   trailer extension field. It has the extension tag value 0x01.

   The cookie value can essentially be viewed as a sufficiently long
   random number, where the length can be determined by the
   implementation (longer cookies are harder to guess and therefore
   better, though using more bandwidth).  Note that cookie values can be
   derived using lots of different schemes so long as they produce
   random-looking and hard-to-predict values.

   The first cookie inserted into a segment for this session is called
   the initial cookie.

   Note that cookies do not outlast an LTP session.

   The basic mode of operation is that an LTP engine can include a
   cookie in a segment at any time. After that time all segments
   corresponding to that LTP session MUST contain a good cookie value -
   that is, all segments both to and from the engine MUST contain a good
   cookie. Clearly, there will be some delay before the cookie is seen



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   in incoming segments - implementations MUST determine an acceptable
   delay for these cases, and MUST only accept segments without a cookie
   until that time.

   The cookie value can be extended at any time by catenating more
   random bits.  This allows both LTP engines to contribute to the
   randomness of the cookie, where that is useful. It also allows a node
   which considers the cookie value too short (say due to changing
   circumstances) to add additional security.  In this case, the
   extended cookie value becomes the "to-be-checked-against" cookie
   value for all future segments (modulo the communications delay as
   above).

   It can happen that both sides emit segments containing an initial
   cookie before their peer has a chance to see any cookie. In that case
   two cookie extension fields MUST be included in all segments
   subsequently (once the traffic has caught up). That is, the sender
   and recipient cookies are handled independently. In such cases, both
   cookie values MUST be "good" at all relevant times (i.e. modulo the
   delay). In this case, the peer's initial cookie MUST arrive before
   the calculated delay for receipt of segments containing this engine's
   cookie - there is only a finite window during which a second cookie
   can be inserted into the session.

   A "good" cookie is therefore one which starts with the currently
   stored cookie value, or else a new cookie where none has been seen in
   that session so far.  Once a cookie value is seen and treated as
   "good" (e.g. an extended value), the previous value is no longer
   "good".

   Modulo the communications delay, segments with an incorrect or
   missing cookie value MUST be silently discarded.

   If a segment is to be re-transmitted, (e.g. as a result of a timer
   expiring) then it needs to contain the correct cookie value at the
   time of (re-)transmission.  Note that this may differ from what was
   the correct cookie value at the time of the original transmission.

3.  Security Considerations

   The extensions specified above are generally intended to help thwart
   DoS attacks. For environments where lower layers provide neither
   integrity nor freshness, it makes sense to use both extensions
   together. For example, in the case where a node extends an existing
   cookie, the lack of origin authentication would allow a man in the
   middle to lock out the session.



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   While there are currently some concerns about using the SHA-1
   algorithm, these appear to only make it easier to find collisions. In
   that case, the use of HMAC with SHA-1 can still be considered safe.
   However, we have changed to use SHA-256 for the signature
   ciphersuite.

4.  IANA Considerations

   <<Note to IANA - to be removed on publication - please create a
   registry for LTP ciphersuites with the "Specification Required"
   update rule as specified below. Since this is not an IETF RFC,
   appointing an expert requires a bit of new thought - in this case, I
   guess that the IRTF DTN RG is the appropriate group, and that the
   IRTF chair (or IRSG) should appoint some other experts whenever the
   DTNRG no longer exists.>>

   Section 2.1 above calls for a new registry to be created by the IANA,
   maintaining the set of known LTP ciphersuites.  The registry will be
   initially populated using the values given in Section 2.1 above. IANA
   may assign LTP Extension Tag values from the range 2..127 (decimal,
   inclusive) using the Specifiction Required rule. [GUIDE] The
   Specification concerned can be an RFC (whether Standards Track,
   Experimental or Informational), or a specification from any other
   standards development organisation recognised by IANA or with a
   liaison with the IESG, specifically including CCSDS.
   (http://www.ccsds.org/)

5.  Acknowledgments

   Many thanks to Tim Ray, Vint Cerf, Bob Durst, Kevin Fall, Adrian
   Hooke, Keith Scott, Leigh Torgerson, Eric Travis, and Howie Weiss for
   their thoughts on this protocol and its role in Delay-Tolerant
   Networking architecture.

   Part of the research described in this document was carried out at
   the Jet Propulsion laboratory, California Institute of Technology,
   under a contract with the National Aeronautics and Space
   Administration. This work was performed under DOD Contract DAA-B07-
   00-CC201, DARPA AO H912; JPL Task Plan No. 80-5045, DARPA AO H870;
   and NASA Contract NAS7-1407.

   Thanks are also due to Shawn Ostermann, Hans Kruse, and Dovel Myers
   at Ohio University for their suggestions and advice in making various
   design decisions.

   Part of this work was carried out at Trinity College Dublin as part



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   of the Dev-SeNDT contract funded by Enterprise Ireland's technology
   development programme.

6.  References

6.1 Normative References

   [B97] S. Bradner, "Key words for use in RFCs to Indicate Requirement
   Levels", BCP 14, RFC 2119, March 1997.

   [GUIDE] Narten, T. & Alvestrand, H. "Guidelines for Writing IANA
   Considerations Sections in RFCs," BCP 26, RFC 5226, May 2008.

   [HMAC] Krawczyk, H. et al, "HMAC: Keyed-Hashing for Message
   Authentication", RFC 2104, February 1997.

   [LTPSPEC] Ramadas, M., Burleigh, S., and Farrell, S., "Licklider
   Transmission Protocol - Specification", draft-irtf-dtnrg-ltp-10.txt
   (Work in Progress), June 2008.

   [RSA] Kaliski, B, Staddon J, "PKCS1: RSA Cryptography Specifications
   Version 2.1", RFC 3447, February 2003.

6.2 Informative References

   [LTPMOTIVE] Burleigh, S., Ramadas, M., and Farrell, S., "Licklider
   Transmission Protocol - Motivation", draft-irtf-dtnrg-ltp-
   motivation-07.txt (Work in Progress), June 2008.

   [PKIXPROF] Cooper, D., et al, "Internet X.509 Public Key
   Infrastructure Certificate and Certificate Revocation List (CRL)
   Profile", RFC 5280, May 2008.

   [TLS] Dierks, T., Allen, C. "The TLS Protocol - Version 1.1", RFC
   4346, April 2006.

   [WRAP] Schaad, J. Housley, R. "Wrapping a Hashed Message
   Authentication Code (HMAC) key with a Triple-Data Encryption Standard
   (DES) Key or an Advanced Encryption Standard (AES) Key", RFC 3537,
   May 2003.

7.  Author's Addresses

      Stephen Farrell
      Computer Science Department
      Trinity College Dublin



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      Ireland
      Telephone +353-1-896-1761
      Email stephen.farrell@cs.tcd.ie

      Manikantan Ramadas
      Internetworking Research Group
      301 Stocker Center
      Ohio University
      Athens, OH 45701
      Telephone +1 (740) 593-1562
      Email mramadas@irg.cs.ohiou.edu

      Scott C. Burleigh
      Jet Propulsion Laboratory
      4800 Oak Grove Drive
      M/S: 301-485B
      Pasadena, CA 91109-8099
      Telephone +1 (818) 393-3353
      FAX +1 (818) 354-1075
      Email Scott.Burleigh@jpl.nasa.gov

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