Network Working Group P. Thiemann
Internet-Draft Freiburg University
Category: Informational 11 August 2003
Expires: February 11, 2004
A URN Namespace For Identifiers Based on Cryptographic Hashes
draft-thiemann-hash-urn-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC2026.
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 February 11, 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].
Thiemann [Page 1]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
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: 2003-08-??
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
Thiemann [Page 2]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
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"
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]. The
<media-type> specification defaults to "application/octet-
stream".
The <hash-scheme> specification defaults to "sha1".
A <hash-value> is a non-empty sequence 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 32 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 50 octet
of the SHA1 hash value of the resource (most significant octet
first) so that the <hash-value> consists of 20 BASE32DIG.
Examples:
urn:hash::md5:5307d294b6ccd9854f2deed8c1628b72
urn:hash::sha1:7660c8efbe7f656ce7612636c83a138c085bad3f
urn:hash:message/rfc822:md5:5307d294b6ccd9854f2deed8c1628b72
Relevant ancillary documentation:
None as yet.
Identifier uniqueness considerations:
Each identifier contains a cryptographic hash value for the
Thiemann [Page 3]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
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 the
SHA1 algorithm. This problem turns out to be much harder than
just producing a collision. In fact, the handbook of applied
cryptography [HAC] states 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]
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
Thiemann [Page 4]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
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
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
Thiemann [Page 5]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
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.
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
Thiemann [Page 6]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
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
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
[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.
Thiemann [Page 7]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
[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.
[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
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
Thiemann [Page 8]
Internet-Draft URNs Based on Cryptographic Hashes 11 August 2003
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication 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 implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Thiemann [Page 9]