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Network Working Group                                       P. Thiemann
Internet-Draft                                      Freiburg University
Category: Informational                                4 September 2003
Expires: March 4, 2004


     A URN Namespace For Identifiers Based on Cryptographic Hashes
                     draft-thiemann-hash-urn-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of section 10 of RFC2026.

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   This Internet-Draft will expire on March 4, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes a URN namespace to identify immutable, typed
   resources using content-based unique identifiers.  The naming scheme
   relies on an algorithm that computes identifiers from media types and
   cryptographic hashes without a central authority.

1. Conventions used in this document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [RFC2119].



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2. Introduction

   A URN serves as a unique name for a resource [RFC1630].  Most URN
   namespaces involve a central authority to ensure uniqueness of
   assigned names.  This approach has its merits but it requires
   organizational structures for processing requests for naming and for
   bookkeeping about used names.  Thus, acquiring a URN becomes an
   involved task not to be undertaken on a day-to-day basis.

   A URN namespace based on cryptographic hashes enables using and
   creating URNs on a day-to-day basis for storing and retrieving
   immutable resources.  It relies on a decentralized, algorithmic
   assignment of identifiers by exploiting the uniqueness guarantees of
   (cryptographic) hashes.  This document contains the assignment
   algorithm so that everyone can generate identifiers in this
   namespace.

   The namespace provides identifiers for typed resources with
   application/octet-stream as a default type.

   This namespace specification is for a formal namespace.  The
   specification adheres to the guidelines given in "Uniform Resource
   Names (URN) Namespace Definition Mechanisms" [RFC3406].

3. Specification Template

   Namespace ID:

         "hash" requested.

   Registration Information:

         Registration Version Number: 1

         Registration Date: 2003-09-??

   Declared registrant of the namespace:

         The CBUID Project
         Institut fuer Informatik
         Universitaet Freiburg
         Georges-Koehler-Allee 079
         D-79110 Freiburg
         Germany

      Contact:
         Peter Thiemann
         info@cbuid.org



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   Declaration of syntactic structure:

         The Namespace Specific Strings (NSS) of all URNs assigned by
         the schema described in this document will conform to the
         syntax defined in section 2.2 of RFC2141 [RFC2141].  The formal
         syntax of the NSS is defined by the following normative ABNF
         [RFC2234] rules for <hash-nss>:

     hash-nss    = [media-type] ":" [hash-scheme] ":" hash-value
     hash-scheme = "md5" / "sha1" / "sha256" / "sha384" / "sha512"
     hash-value  = 1*(ALPHA / DIGIT / ".")

         The following are comments and restrictions not captured by the
         above grammar.

         A <media-type> is any MIME media type [RFC2046] which is
         registered in the appropriate IANA registry [IANA-MT].  There
         is no default for the <media-type> specification.  If omitted,
         then the media type is unspecified, thus leaving the
         application complete freedom to interpret the resource.

         If the <hash-scheme> specification is omitted, then the length
         of the <hash-value> unambiguously selects one of "sha1",
         "sha256", "sha384", or "sha512" according to the following
         table.

                length of <hash-value>    |    implied <hash-scheme>
            ------------------------------+-----------------------------
                       32                 |          "sha1"
                       56                 |          "sha256"
                       80                 |          "sha384"
                      104                 |          "sha512"

         A <hash-value> is a non-empty sequence of characters encoding a
         sequence of bits which must be a valid hash for the specified
         hash-scheme.  The encoding depends on the <hash-scheme>.  If
         <hash-scheme> is "md5", then <hash-value> is the base16
         encoding [RFC3548] of the 16 octets of the MD5 hash value of
         the resource (most significant octet first) so that the <hash-
         value> consists of 32 HEXDIG.  If <hash-scheme> is "sha1", then
         <hash-value> is the base32 encoding [RFC3548] of the 20 octets
         of the SHA1 hash value of the resource (most significant octet
         first) so that the <hash-value> consists of 32 BASE32DIG.  The
         other "sha" <hash-value>s are handled analogously according to
         the above table.

         In any case, the <hash-value> MUST provide the correct number
         of bits for the chosen <hash-scheme>, 128 for "sha1", 256 for



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         "sha256", 384 for "sha384", and 512 for "sha512".

         Examples:

     urn:hash::md5:5307d294b6ccd9854f2deed8c1628b72

     urn:hash::sha1:LBPI666ED2QSWVD3VSO5BG5R54TE22QL

     urn:hash:::JRBFASJWGY3EKRBSKFJVOVSEGNLFGTZVIJDTKURVGRKEKMRSKFGA====

         The implied <hash-scheme> for this identifier is "sha256" since
         the <hash-value> consists of 56 BASE32DIG and specifies 256
         bits.

     urn:hash:text/plain::LBPI666ED2QSWVD3VSO5BG5R54TE22QL

         The implied <hash-scheme> for this identifier is "sha1" since
         the <hash-value> consists of 32 BASE32DIG and specifies 160
         bits.

     urn:hash:message/rfc822:md5:5307d294b6ccd9854f2deed8c1628b72

   Relevant ancillary documentation:

         None as yet.

   Identifier uniqueness considerations:

         Each identifier contains a cryptographic hash value for the
         referenced resource.  The probability that two different
         resources have the same hash value depends on the hash
         function.  For the MD5 hash where the hash value has 128 bits,
         it is conjectured [RFC1321] that the probability of a collision
         is in the order of 1/2^64 by reasoning with the birthday
         attack.  For the sha1 hash where the hash value has 160 bits,
         the same attach yields a probability of 1/2^80 for a collision.

   Identifier persistence considerations:

         The binding between the identifier and the referenced resource
         is permanently established by the assignment algorithm that
         computes the identifier from the resource.

         The persistence of an identifier for some resource A might be
         compromised by coming up with a different resource B with the
         same identifier.  However, this corresponds to solving the
         "second preimage problem" for either the MD5 algorithm or an
         algorithm of the SHA family.  This problem turns out to be much



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         harder than just producing a collision.  In fact, the handbook
         of applied cryptography [HAC] estimates that computing a second
         preimage takes on the order of 2^128 steps for MD5 and 2^160
         steps for SHA1.

   Process of identifier assignment:

         Assignment is completely open, following the algorithm below.

         The inputs of the algorithm are
            - the name <hash-scheme> of a hash function
            - a media type for <media-type>
            - a resource (a sequence of octets)

         The algorithm applies the hash function to the resource,
         converts the resulting bit sequence into a valid <hash-value>
         according to the <hash-scheme>, and constructs the URN by
         concatenating the <media-type>, the <hash-scheme>, and the
         <hash-value> using the syntax described above.  Algorithms for
         computing the hash functions mentioned in this document are
         defined in the following references:

            md5      [RFC1321]
            sha1     [RFC3174]
            sha256   [FIPS180-2]
            sha384   [FIPS180-2]
            sha512   [FIPS180-2]

         The conversion of a <hash-value> to a string in base16
         enconding proceeds as follows.  The bits in the <hash-value>
         are converted from most significant to least significant bit,
         four bits at a time to their ASCII presentation.  Each sequence
         of four bits is represented by its hexadecimal digit from
         "0123456789abcdef".  That is, binary 0000 gets represented by
         the character '0', 0001, by '1', and so on up to the
         representation of 1111 as 'f'.

         The conversion of a <hash-value> to a string in base32
         enconding proceeds as follows.  The bits in the <hash-value>
         are converted from most significant to least significant bit,
         five bits at a time to their ASCII presentation.  Each sequence
         of five bits is represented by its base32 digit from
         "abcdefghijklmnopqrstuvwxyz234567" as defined in [RFC3548].
         That is, binary 00000 gets represented by the character 'a',
         00001, by 'b', and so on up to the representation of 11111 as
         '7'. A value that does not consist of a number of bits which is
         divisible by five is padded with zero bits to the next multiple
         of five. The length of a base32 encoded bit string is always



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         divisible by eight. Padding of an incomplete 8 character group
         is done using the character '='.

   Process of identifier resolution:

         Not specified.

   Rules for Lexical Equivalence:

         Lexical equivalence is identity after normalization.  An
         identifier in the cbuid URN namespace is normalized by
         converting all characters to lower case

   Conformance with URN Syntax:

         There are no additional characters reserved.

   Validation mechanism:

         Each identifier in the namespace MUST conform with the syntax
         specified above.

   Scope:

         The namespace is global and public.

4. IANA Considerations

   This document includes a URN namespace registration that is to be
   entered into the IANA registry for URN NIDs.

5. Namespace Considerations

   Many URN namespaces are assigned to organizations and rely on a
   centralized registry to achieve uniqueness and persistency.  In
   contrast, the hash namespace is not tied to any organization.
   Assignment of identifiers can be performed and verified individually,
   while uniqueness is still preserved (with a probability close to 1).

   The hard coding of the hashing schemes into the namespace definition
   is intentional.  This is because a valid identifier should be able to
   act as a proxy for the the named resource.  That way, metainformation
   of descriptive or authoritative nature (such as endorsements,
   signatures, etc) can be attached to the identifier and need not be
   bundled with the actual resource.  Such a proxy functionality is only
   guaranteed as long as the underlying hashing scheme is not
   compromised, that is, as long as no collisions are found.




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   The encoding of the hash value is also hard coded into the
   definition.  We have chosen not to make the encoding an additional
   parameter of the URN scheme for two reasons

      1. it would make identifier normalization non-trivial;

      2. each hashing scheme has a standard encoding, which should be
         reflected in the identifier.

   One problem is the phasing out of compromised hash schemes. For
   instance, many believe that MD5 is "not sufficiently secure" on the
   grounds that it only provides 128 bit hashes and that colliding
   inputs have been constructed.  However, the only known approach for
   solving the second preimage problem, which appears to be more
   relevant for the application as an identifier, is brute force search
   through on the order of 2^128 inputs.

   If a procedure for computing a second preimage in significantly fewer
   operations is ever published, then resolvers SHOULD refuse to resolve
   the compromised hash scheme.  This is in line with the semantics of
   URNs which need to identify a resource uniquely but the resource need
   not be available forever (cf. the discussion in BCP 66 [RFC3406]).

6. Community Considerations

   Similar URNs are in use in peer-to-peer file transfer systems.  Most
   of them do not include a mediatype, although this practice can
   provide extra guarantees.  For example, a provider of metainformation
   can state that mediatype of the resource has been verified by
   including the mediatype in the published URN.  For many formats, the
   mediatype provides an additional self-verifiable attribute.

   Some URI schemes in common use may be easily derived from the hash
   scheme.

         1. The sha1 scheme

            urn:sha1:<sha1-hash-value>

         is equivalent to

            urn:hash::sha1:<sha1-hash>

         and even to

            urn:hash:::<sha1-hash>

         2. Another proposed scheme is based on the data URL



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            urn:data-hash:text/plain;sha1,<sha1-hash>

         which is equivalent to

            urn:hash:text/plain:sha1:<sha1-hash>

         In this case, the identifier from the hash namespace has a
         simpler, more regular structure.

7. Security Considerations

   The use of the namespace per se does have security implications.
   However, it should be kept in mind that the uniqueness guarantee
   given by cryptographic hashes is only probabilistic and that no known
   procedure (save bitwise comparision) can provide a 100% guarantee of
   the identify of the hashed resource.

Normative References

   [FIPS180-2] National Institute of Standards and Technology,
   "Specifications for the SECURE HASH STANDARD", August 2002.
   http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf

   [RFC1321] Rivest, R. L., "The MD5 Message-Digest Algorithm", RFC
   1321, April 1992.

   [RFC2046] Freed, N., and Borenstein, N., "Multipurpose Internet Mail
   Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996.

   [RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
   Requirement Levels", RFC 2119, March 1997.

   [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2234] Crocker, D., Editor, and P. Overell, "Augmented BNF for
   Syntax Specifications: ABNF", RFC 2234, November 1997.

   [RFC3174] Eastlake, E., and Jones, P., "US Secure Hash Algorithm 1
   (SHA1)", RFC 3174, September 2001.

   [RFC3548] Josefsson, S. (Ed.), "The Base16, Base32, and Base64 Data
   Encodings", RFC 3548, July 2003.

Informational References

   [HAC]  Menezes, Alfred J., van Oorschot, Paul C., and Vanstone, Scott
   A., Handbook of Applied Cryptography, CRC Press, 5th printing, August
   2001.



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   [IANA-MT] IANA Registry of Media Types: ftp://ftp.isi.edu/in-
   notes/iana/assignments/media-types/

   [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW,"
   RFC 1630, June 1994.

   [RFC3406] Daigle, L., van Gulik, D.W., Iannella, R., and Faltstrom,
   P., "Uniform Resource Names (URN) Namespace Definition Mechanisms",
   RFC 3406, October 2002.

Contributors

         Stephanie Kollenz

         Matthias Neubauer

Author's Address

         Peter Thiemann
         Institut fuer Informatik
         Universitaet Freiburg
         Georges-Koehler-Allee 079
         D-79110 Freiburg
         Germany

         Phone: +49 761 203 8051
         EMail: thiemann@acm.org
         URL: http://www.informatik.uni-freiburg.de/~thiemann

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   this standard.  Please address the information to the IETF Executive
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