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Versions: 00 01 02

Internet Engineering Task Force                          P. Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended status: Standards Track                            R. Stradling
Expires: April 5, 2012                                    Comodo CA Ltd.
                                                         October 3, 2011


                       The di (DIGEST) URI Scheme
                     draft-hallambaker-digesturi-02

Abstract

   A URI scheme for referencing static data abjects by means of a
   cryptographic digest mechanism is specified.  The format is designed
   to resist content type substitution attacks and supports a choice of
   digest algorithms.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 5, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
     1.2.  Defined Terms  . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Examples of Use  . . . . . . . . . . . . . . . . . . . . .  3
       2.1.1.  Simple Digest  . . . . . . . . . . . . . . . . . . . .  3
       2.1.2.  Truncated Digest . . . . . . . . . . . . . . . . . . .  3
       2.1.3.  Digest with Meta-Data  . . . . . . . . . . . . . . . .  4
       2.1.4.  Digest with Locator  . . . . . . . . . . . . . . . . .  4
       2.1.5.  Digest with Decryption Key . . . . . . . . . . . . . .  4
   3.  The di (DIGEST) URI TYPE . . . . . . . . . . . . . . . . . . .  5
     3.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Encoding Considerations  . . . . . . . . . . . . . . .  5
         3.1.1.1.  Use of base64url Encoding  . . . . . . . . . . . .  5
         3.1.1.2.  Query Parameter Encoding . . . . . . . . . . . . .  5
     3.2.  Semantics  . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  Digest Algorithms  . . . . . . . . . . . . . . . . . .  5
       3.2.2.  Parameters . . . . . . . . . . . . . . . . . . . . . .  6
         3.2.2.1.  Content Type (ct)  . . . . . . . . . . . . . . . .  6
         3.2.2.2.  HTTP Locator (http & https)  . . . . . . . . . . .  6
         3.2.2.3.  Encryption and MIME Encryption Specifiers (enc
                   & menc)  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
     4.1.  Integrity  . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Confidentiality  . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Weak Digest Algorithm  . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Assignment of URI Scheme di  . . . . . . . . . . . . . . .  8
     5.2.  Assignment of Well Known URI prefix di . . . . . . . . . .  8
     5.3.  Specification of Additional Cryptographic Algorithms . . .  8
     5.4.  Creation of di parameter registry  . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Non Normative References . . . . . . . . . . . . . . . . . 10
   Appendix A.  Test Vectors  . . . . . . . . . . . . . . . . . . . . 10
     A.1.  Example: Hello World ! . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11












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1.  Definitions

1.1.  Requirements Language

   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].

1.2.  Defined Terms

   The following terms are used in this document:

   URI


2.  Requirements

   Provides a strong reference to a static data object.

   Does not provide a means of resolution.

   Allows an authenticated data source to provide an authenticated
   reference to a static data object.

   Intended applications include creating references from

      Web pages delivered over HTTP/TLS

      DNS resource records signed using DNSSEC

      Data values embedded in certificates, CRLs, OCSP tokens and other
      signed data objects.

2.1.  Examples of Use

2.1.1.  Simple Digest

   For example, the following digest URI specifies a reference to the
   text "Hello World !" using the SHA-2 algorithm with 256 bit output:

   di:sha-256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc

2.1.2.  Truncated Digest

   Message Digest algorithms are designed to provide protection against
   a collision attack.  Due to the birthday paradox, this requires that
   the digest length be twice the length of a encryption or
   authentication key to achieve the same work factor.



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   The digest portion MAY be truncated at any 32 bit boundary.  If a
   truncated digest is used the query separator '?'  MUST be specified.

   di:sha-256;B_K97zTtFuOhug27fke4_Q?

2.1.3.  Digest with Meta-Data

   The semantics of a digest being used to establish a secure reference
   from an authenticated source to an external source may be a function
   of associated meta data such as the content type.  This data MAY be
   specified by means of a parameter:

   di:sha-256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc?ct=text/plain

2.1.4.  Digest with Locator

   A digest identifier MAY provide a location from which the referenced
   content MAY be available.  Note however that since it is
   statistically unlikely that a given identifier will correspond to
   more than one content sequence, the actual location from which the
   data is retrieved is immaterial.

   di:sha-
   256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc?http=di.example.com

   The corresponding content MAY be retrieved from the URL:

   http://di.example.com/.well-known/di/sha-256/
   B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc

   A digest identifier MAY specify multiple locations from which the
   content MAY be obtained:

   di:sha-256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc?
   http=one.example.com&http=two.example.com

   Asserts that the content may be retrieved from either of the
   following URIs:

   http://one.example.com/.well-known/di/sha-256/
   B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc http://two.example.com/
   .well-known/di/sha-256/B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc

2.1.5.  Digest with Decryption Key

   A digest identifier MAY provide a key for decrypting the referenced
   data.




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   di:sha-256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc? enc=aes-
   cbc:Fw3x20nEKfq6FDGzq7ttIQ


3.  The di (DIGEST) URI TYPE

3.1.  Syntax

   The DIGEST URI Type has the following format:

   "di:" algorithm ";" digest [ "?" tag "=" value [ "&" tag "=" value ]
   * ]

3.1.1.  Encoding Considerations

3.1.1.1.  Use of base64url Encoding

   Section 2.2 of [RFC4395] states [URI schemes that are not intended
   for use with relative URIs SHOULD avoid use of the forward slash "/"
   character, which is used for hierarchical delimiters, and the
   complete path segments "." and ".." (dot-segments).]

   Consequently the encoding of the digest value is effected using the
   base64url encoding specified in Section 5 of [RFC4648] which avoids
   the use of the forward slash '/' character.

3.1.1.2.  Query Parameter Encoding

   The query segment of a URI is NOT hierarchical.  Thus escape encoding
   of slash '/' characters is NOT required.

   Section 3.4 of [RFC3986] states [The characters slash ("/") and
   question mark ("?") may represent data within the query component.]

   Consequently no special escaping mechanism is required for the query
   parameter portion of the URI.  URI escaping is however frequently
   imposed automatically by scripting environments.  Thus to ensure
   interoperability, implementations SHOULD NOT generate URIs that
   employ URI character escaping and implementations MUST accept and
   URIs that employ URI character escaping.

3.2.  Semantics

3.2.1.  Digest Algorithms

   Implementations MUST support the sha-256 algorithm as specified in
   [RFC4055].




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   Implementations MAY support other algorithms specified in the Data
   Structure for the Security Suitability of Cryptographic Algorithms
   registry 'Cryptographic Algorithms' [RFC5698].

3.2.2.  Parameters

3.2.2.1.  Content Type (ct)

   The Content Type "ct" parameter specifies the MIME Content Type of
   the associated data as defined in [RFC4288]

3.2.2.2.  HTTP Locator (http & https)

   The http and https parameters are used to specify a possible means of
   resolving the referenced content.  Mulltiple locator parameters MAY
   be used to specify alternative sources for accessing the content.

   The http and https parameters take a single argument, the domain name
   to be used for resolution.  To permit the use of digest URIs in
   ASCII-only environments, the ASCII encoding (aka punycode) of the
   domain name MUST be used.

   To resolve such a location reference, a client first transforms the
   digest URI to obtain a http or https url as follows:

   URL = prefix + domain + "/.well-known/di/" + algorithm + digest

   Where:

   prefix  Is the string "http://" for the http parameter type and is
      the string "https://" for the https parameter type.

   domain  Is the value associated with the parameter.

   algorithm  Is the algorithm portion of the digest URI.

   digest  Is the digest portion of the digest URI.

   Implementations MUST NOT disclose any other data.  In particular
   implementations MUST NOT disclose the query parameter portion of the
   URI.

3.2.2.3.  Encryption and MIME Encryption Specifiers (enc & menc)

   The encryption and MIME encryption specifiers are used to provide a
   means of obtaining the plaintext of a reference to encrypted content.

   The enc specifier is used when the encrypted object consists of the



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   encrypted content alone.  The menc spcifier is used when the
   encrypted object consists of a MIME header containing metadata
   followed by the binary object encoding.

   The encryption specifiers both take an agrument of the form:

   algorithm ":" base64url (key) [":" base64url (iv)]

   Where

   algorithm  Is the algorithm used to encrypt the associated content

   key  Is the value of the cryptographic key

   iv (optional)  Is the value of the cryptographic Initialization
      Vector.

      If the IV is not spcified for a block cipher mode that requires
      one, the IV MUST be prepended to the encrypted content.

      [Actually the IV does not provide any additional security for this
      application but explaining the reason would be more effort than it
      is worth and what I really care about is saving bytes in the
      identifier, not the resulting data package.]


4.  Security Considerations

4.1.  Integrity

   No secret information is required to generate a DIGEST URI.
   Therefore a DIGEST URI only provides a proof of integrity for the
   referenced object and the proof of integrity provided is only as good
   as the proof of integrity for the DIGEST URI value.

4.2.  Confidentiality

   Disclosure of a DIGEST URI value does not necessarily entail
   disclosure of the referenced object but may enable an attacker to
   determine the contents of the referenced object by reference to a
   search engine or other data repository.

4.3.  Weak Digest Algorithm

   [The digest algorithm MUST be strong]

   [For most use cases collision resistance is a requirement]




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5.  IANA Considerations

5.1.  Assignment of URI Scheme di

   The procedures for registration of a URI scheme are specified in RFC
   4395 [RFC4395].  The following is the proposed assignment template.

   URI scheme name: di

   Status: Permanent

   URI scheme syntax.  See Section 3.1

   URI scheme semantics.  See Section 3.2

   Encoding considerations.  See Section 3.1.1

   Applications/protocols that use this URI scheme name: General
   applicability with initial use cases provided by WEBSEC and DECADE

   Interoperability considerations: TBS

   Security considerations: See Section 4

   Contact: IETF / Phillip Hallam-Baker

   Author/Change controller: IETF / Phillip Hallam-Baker

   References: As specified in this document

5.2.  Assignment of Well Known URI prefix di

   The procedures for registration of a Well Known URI entry are
   specified in RFC 5785 [RFC5785].  The following is the proposed
   assignment template.

   URI suffix: di

   Change controller: IETF

   Specification document(s): This document

   Related information: None

5.3.  Specification of Additional Cryptographic Algorithms

   [Added in case it is decided to specify truncated forms of the
   cryptographic digests]



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   The procedures for registration of a Cryptographic Algorithm
   identifier are specified in RFC 5698 [RFC5698].  The following is the
   proposed assignment template.

   Textual name of the algorithm: SHA-128

   OID of the algorithm: [TBS]

   Reference: This document.

5.4.  Creation of di parameter registry

   This specification creates a new IANA registry entitled "DI URI
   Parameter Definitions".

   The policy for future assignments to the registry is "RFC Required".


6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509 Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              June 2005.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List



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              (CRL) Profile", RFC 5280, May 2008.

   [RFC5698]  Kunz, T., Okunick, S., and U. Pordesch, "Data Structure
              for the Security Suitability of Cryptographic Algorithms
              (DSSC)", RFC 5698, November 2009.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

6.2.  Non Normative References

   [NIST-ALGS]
              National Institute of Standards and Technology,
              "Cryptographic Algorithm Registration", March 2009.

   [RFC3642]  Legg, S., "Common Elements of Generic String Encoding
              Rules (GSER) Encodings", RFC 3642, October 2003.


Appendix A.  Test Vectors

A.1.  Example: Hello World !

   The Digest URI of the text file "Hello World !" is computed as
   follows:

      Scheme "di"

      Algorithm Identifier: "sha-256"

      sha-256 ("Hello World !") " 07 f2 bd ef 34 ed 16 e3 a1 ba 0d bb 7e
      47 b8 fd 98 1c e0 cc b3 e1 bf e5 64 d8 2c 42 3c ba 7e 47 "

      BASE64URI (sha-256 ("Hello World !")): "
      B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc "

      Content Type Parameter 'text/plain' "ct=text/plain"

   Depending on the context, the digest URI MAY be specified using the
   digest value alone or the digest value plus content type parameter:

   di:sha-256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc

   di:sha-256;B_K97zTtFuOhug27fke4_Zgc4Myz4b_lZNgsQjy6fkc?ct=text/plain






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Authors' Addresses

   Phillip Hallam-Baker
   Comodo Group Inc.

   Email: philliph@comodo.com


   Rob Stradling
   Comodo CA Ltd.

   Email: rob.stradling@comodo.com







































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