





Independent Submission                                        A. Santoni
Request for Comments: 5544                                Actalis S.p.A.
Category: Informational                                    February 2010
ISSN: 2070-1721


             Syntax for Binding Documents with Time-Stamps

Abstract

   This document describes an envelope that can be used to bind a file
   (not necessarily protected by means of cryptographic techniques) with
   one or more time-stamp tokens obtained for that file, where "time-
   stamp token" has the meaning defined in RFC 3161 or its successors.
   Additional types of temporal evidence are also allowed.

   The proposed envelope is based on the Cryptographic Message Syntax as
   defined in RFC 5652.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5544.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   standards track specification RFC 4998, Evidence Record Syntax (ERS),
   specifies an alternative mechanism.  Readers are encouraged to also
   review RFC 4998 when evaluating the suitability of this mechanism.










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Copyright Notice

   Copyright (c) 2010 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
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................3
   2. Syntax for TimeStampedData ......................................3
   3. Compliance Requirements .........................................6
   4. Recommended Processing ..........................................6
      4.1. Generating a New TimeStampedData Structure .................7
      4.2. Verifying an Existing TimeStampedData Structure ............8
      4.3. Extending the Validity of an Existing
           TimeStampedData Structure ..................................9
   5. Security Considerations .........................................9
   6. Normative References ...........................................10
   7. Informative References .........................................10
   Appendix A. ASN.1 Module ..........................................11
   Appendix B. Acknowledgments .......................................12

1.  Introduction

   Time-stamping has become the standard technique for proving the
   existence of a document before a certain point in time.  Several
   legislations around the world embrace the concept and provide for
   time-stamping services, mainly for the purpose of extending the
   validity of signed documents.  However, while time-stamping enhances
   digital signatures, its value does not depend on them.  It can
   clearly be useful to time-stamp a document even if it is not signed.
   And it can also be useful, or even mandatory in some cases, to time-
   stamp a signed document in its entirety, regardless of how many
   signatures it contains.

   When a time-stamp is related to a digital signature, there already
   exists a way to keep the two pieces together: RFC 3161 [TSP]
   describes how one or more TimeStampTokens can be included in a
   SignerInfo structure as unsigned attributes.  On the other hand,
   there is no standard way to keep together a time-stamped document,
   whether signed or not, and the related time-stamps.



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   In such cases, two approaches are typically being adopted:

   o  time-stamps are kept as separate files (keeping track of what
      time-stamps belong to what documents is up to the user);

   o  an ad hoc solution is adopted for specific applications, e.g., a
      ZIP archive or a proprietary "envelope" of some kind.

   Both solutions impede interoperability, which is the objective of
   this memo.

   This document describes a simple syntax for binding one document
   (actually, any kind of file) to the corresponding temporal evidence;
   the latter is typically represented by one or more RFC 3161
   TimeStampTokens.  Additional types of temporal evidence, e.g., an RFC
   4998 EvidenceRecord [ERS], are also supported via an "open" syntax.
   However, for the sake of interoperability, the emphasis in this
   document is on TimeStampTokens.

   The proposed syntax is broadly based on the Cryptographic Message
   Syntax (CMS) defined in RFC 5652 [CMS].

1.1.  Conventions Used in This Document

   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 [KWORDS].

   The terms "document" and "file" are used interchangeably.  The terms
   "TimeStampToken" and "time-stamp token" are used interchangeably,
   both referring to the data structure defined in RFC 3161.

2.  Syntax for TimeStampedData

   The proposed data structure is called TimeStampedData, and it is
   based on the ContentInfo envelope defined in [CMS]:

      ContentInfo ::= SEQUENCE {
         contentType ContentType,
         content [0] EXPLICIT ANY DEFINED BY contentType }

      ContentType ::= OBJECT IDENTIFIER

   While CMS defines six content types (data, signed-data, enveloped-
   data, digested-data, encrypted-data, and authenticated-data), this
   memo defines an additional content type, timestamped-data, identified
   by the following Object Identifier (OID):




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      id-ct-timestampedData OBJECT IDENTIFIER ::= {
               iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
               pkcs9(9) id-smime(16) id-ct(1) 31 }

   This particular OID signals that the content field of the ContentInfo
   has the following syntax:

      TimeStampedData ::= SEQUENCE {
         version              INTEGER { v1(1) },
         dataUri              IA5String OPTIONAL,
         metaData             MetaData OPTIONAL,
         content              OCTET STRING OPTIONAL,
         temporalEvidence     Evidence
      }

      MetaData ::= SEQUENCE {
         hashProtected        BOOLEAN,
         fileName             UTF8String OPTIONAL,
         mediaType            IA5String OPTIONAL,
         otherMetaData        Attributes OPTIONAL
      }

      Attributes ::=
         SET SIZE(1..MAX) OF Attribute -- according to RFC 5652

      Evidence ::= CHOICE {
         tstEvidence    [0] TimeStampTokenEvidence,   -- see RFC 3161
         ersEvidence    [1] EvidenceRecord,           -- see RFC 4998
         otherEvidence  [2] OtherEvidence
      }

      OtherEvidence ::= SEQUENCE {
         oeType               OBJECT IDENTIFIER,
         oeValue              ANY DEFINED BY oeType }

      TimeStampTokenEvidence ::=
         SEQUENCE SIZE(1..MAX) OF TimeStampAndCRL

      TimeStampAndCRL ::= SEQUENCE {
         timeStamp   TimeStampToken,          -- according to RFC 3161
         crl         CertificateList OPTIONAL -- according to RFC 5280
      }

   The version field contains the version number of the TimeStampedData
   syntax.  It SHALL be 1 for this version of the document.






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   The dataUri field contains a URI reference conforming to [URI].  When
   the content field is absent, dataUri MUST be present and contain a
   URI allowing retrieval of the document that was time-stamped (unless
   the document is later moved).  When the content field is present,
   this field MAY also be present.

   The metaData field contains metadata related to the document that was
   time-stamped, if applicable.  In particular:

      The hashProtected field indicates whether the metadata have been
      included in the computation of the digest within the first
      TimeStampToken (see further on).  This makes it possible to detect
      a subsequent alteration of the metadata.

      The fileName field contains the original filename of the document
      that was time-stamped.

      The mediaType field contains a media type/subtype and possible
      parameters for the time-stamped document, according to [MIME].
      This information may help decide how to "open" or deal with the
      time-stamped document.

      The otherMetaData field contains further attributes of the time-
      stamped document (e.g., a description, claimed author, etc.),
      where each attribute is specified by an object identifier and a
      corresponding set of values, as described in [CMS].  When this
      field is present, it MUST contain at least one Attribute.

   Within the metaData field (if present), at least one of the fileName,
   mediaType, and otherMetaData sub-fields MUST be present.

   The Attribute values within the otherMetaData field MUST be DER
   encoded, even if the rest of the structure is BER encoded.

   The content field, when present, carries the entire contents, in its
   original format and encoding, of the document that was time-stamped.
   This can actually be any kind of data, e.g., a text document, an
   executable, a movie, a message, etc.  The omission of the content
   field makes it possible to bind the temporal evidence to external
   data.  In such a case, the temporal evidence is computed as though
   the content field were present.

   The temporalEvidence field carries the evidence that the time-stamped
   document did exist before a certain point in time.  Several types of
   evidence are allowed, but compliant applications are only required to
   support the RFC 3161 type -- namely, the tstEvidence choice.





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   The TimeStampTokenEvidence sequence MUST contain at least one element
   of type TimeStampAndCRL.

   The elements of the TimeStampTokenEvidence sequence MUST conform to
   the following rule:

   o  if the metaData field is absent or the value of its hashProtected
      field is FALSE, then the TimeStampToken within the first element
      SHALL be computed over the value octets of the content field (if
      this field is absent, use the octets retrieved via the dataUri
      field);

   o  otherwise (the metaData field is present and the value of its
      hashProtected field is TRUE), the TimeStampToken within the first
      element SHALL be computed over the concatenation of the following
      fields:

      - the DER encoding of the metaData field;

      - the value octets of the content field (if this field is absent,
        use the octets retrieved via the dataUri field);

   o  the TimeStampToken within the second element SHALL be computed
      over the first element;

   o  the TimeStampToken within each subsequent element SHALL be
      computed over its preceding element in the sequence.

   Within the TimeStampAndCRL construct, the optional crl field carries
   a suitable CRL (Certificate Revocation List) demonstrating that the
   certificate of the TSA (Time-Stamping Authority) that issued the
   TimeStampToken was not revoked at the time when the subsequent
   element in the TimeStampTokenEvidence sequence was added.  See the
   Security Considerations section for further discussion on this topic.

3.  Compliance Requirements

   Compliant applications MUST support at least the RFC 3161-based type
   of evidence (i.e., the tstEvidence CHOICE).

4.  Recommended Processing

   This section is focused on the RFC 3161-based type of evidence.
   Processing of the structure for other types of evidence would be done
   in a similar manner.






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4.1.  Generating a New TimeStampedData Structure

   In this case, applications are supposed to behave as follows:

   o  populate the version field with the integer value v1(1);

   o  if a self-contained envelope is to be generated, always populate
      the content field with the content of the file in its original
      format and encoding; depending on the application, the dataUri
      field may also be added;

   o  otherwise (a detached envelope is to be generated), always
      populate the dataUri field with the URI of the time-stamped
      document (e.g., http://foo.example.com/Contract12345.pdf); using
      an absolute URI or a relative reference depends on the
      application;

   o  if the metaData field is being added, decide on the value of its
      hashProtected field; set its value to TRUE if the application
      needs the remaining fields of the metaData construct to be hash-
      protected as described in Section 2; otherwise, set it to FALSE;

   o  if the metaData field is being added, optionally populate the
      fileName field (e.g., "Contract12345.pdf"), the mediaType field
      with a suitable media type/subtype and possible parameters
      according to [MIME], and the otherMetaData field, depending on the
      application;

   o  select a suitable one-way hash function and compute a hash value
      using that function over the content, or the concatenation of the
      metadata and the content, as described in Section 2; this hash
      value will then be used for requesting the first TimeStampToken;

   o  obtain the first temporal evidence from a TSA and add it to the
      temporalEvidence field;

   o  insert the TimeStampedData into a ContentInfo structure, with the
      id-ct-timestampedData OID in the contentType field;

   o  BER-encode the ContentInfo structure (except for the fields that
      are required to be DER encoded) and save it with a reasonable file
      name (e.g., derived from the name of the time-stamped file).









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4.2.  Verifying an Existing TimeStampedData Structure

   In this case, applications are supposed to behave as follows:

   o  check that the contentType field of the ContentInfo structure has
      the expected value (id-ct-timestampedData) in its contentType
      field; then, extract the inner TimeStampedData structure and
      continue processing;

   o  check the version field (it should be v1);

   o  check that the temporalEvidence field is not empty;

   o  check whether the content is present; if it is not, use the
      dataUri field to retrieve the file;

   o  open the first element of the TimeStampTokenEvidence sequence,
      open the time-stamp token within it and use the hash function that
      was used to obtain it to re-compute the hash of the fields
      indicated in Section 2; if the re-computed hash value matches the
      one within the time-stamp token, continue processing; otherwise,
      the TimeStampedData structure has been modified;

   o  validate the temporalEvidence by checking that:

      - each TimeStampToken in the chain does contain the correct digest
        value (according to the rule described in Section 2) and it was
        signed by a trusted TSA,

      - the corresponding TSA signing certificate was not revoked at the
        time when the subsequent TimeStampToken was issued, based on the
        associated CRL;

   o  depending on the application, use the temporal evidence for
      whatever purpose the application was designed for;

   o  depending on the application, show the dataUri, the fileName, the
      mediaType, the otherMetaData, and the temporal evidence to the
      user;

   o  depending on the application, save the content to a separate file;

   o  depending on the application, store at a different place the
      content that has been retrieved using the dataUri field, then
      update the dataUri field accordingly;

   o  depending on the application, show the time-stamped file to the
      user, possibly by activating a suitable "viewer".



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4.3.  Extending the Validity of an Existing TimeStampedData Structure

   In this case, applications are supposed to behave as follows:

   o  validate the TimeStampedData structure as described above;

   o  select the time-stamp token from the last TimeStampAndCRL element
      in the chain and obtain the latest available CRL for the
      corresponding TSA certificate (if this CRL is not fresh enough,
      wait until the next one is available), then store it in the
      TimeStampAndCRL element;

   o  instantiate a new TimeStampAndCRL element and obtain a new time-
      stamp token computed over the previous one, according to the rule
      described in Section 2; insert the new time-stamp token into the
      new TimeStampAndCRL element, then append the latter to the end of
      the chain.

   See the Security Considerations section for further discussion on
   extending the validity of an existing TimeStampedData structure.

5.  Security Considerations

   When the metaData field is present and the hashProtected sub-field is
   set to TRUE, the metadata are also included in the computation of the
   digest within the first time-stamp token, so that any subsequent
   alteration of the metadata will be easily detected.  However, the
   integrity of hash-protected metadata does not imply that the metadata
   were correct at the time when the TimeStampedData object was created.
   That can only be inferred by other means (e.g., from context).  For
   instance, when TimeStampedData objects are created by an archival
   service provider, it may be reasonable to assume that the metadata
   are correct at creation time.  Instead, when a TimeStampedData object
   is received from an unknown party, the recipient cannot safely assume
   that the metadata are correct, lacking further information.

   In general, a time-stamp token should not be considered valid after
   the certificate of the issuing TSA is expired (also, this
   consideration depends on the legislation and the policy under which
   the TSA operates).  However, a time-stamp token can itself be time-
   stamped to extend the validity of the TSA's signature.  By repeatedly
   applying this technique, a whole chain of time-stamp tokens can be
   grown to extend the validity of the first one ad libitum.  Thus, this
   approach can be adopted to extend the validity of a TimeStampedData
   structure beyond the expiry date of the first TimeStampToken within
   it, by adding further elements to the TimeStampTokenEvidence sequence





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   according to the rule described in Section 2.  Of course, each
   additional TimeStampToken must be added in a timely manner (before
   the previous one is expired or has been revoked).

   The validity extension technique described above requires that the
   TSA signing certificates can still be verified long after they have
   expired, typically by checking a CRL.  The CRL must be captured at
   the suitable time, because expired certificates are typically removed
   from the CRL regardless of their being revoked.  The TimeStampAndCRL
   construct allows adding a CRL next to the related TimeStampToken, so
   that the TSA certificate will still be verifiable at any later time.
   The CRL must be captured at the time when another element is about to
   be added to the TimeStampTokenEvidence sequence, or even later -- to
   allow for a last-minute revocation request to be processed by the CA
   (see the discussion about "grace periods" in [CADES]).

6.  Normative References

   [CMS]     Housley, R., "Cryptographic Message Syntax (CMS)", RFC
             5652, September 2009.

   [ERS]     Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
             Record Syntax (ERS)", RFC 4998, August 2007.

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

   [MIME]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, November 1996.

   [PKIX1]   Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
             Housley, R., and W. Polk, "Internet X.509 Public Key
             Infrastructure Certificate and Certificate Revocation List
             (CRL) Profile", RFC 5280, May 2008.

   [TSP]     Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
             "Internet X.509 Public Key Infrastructure Time-Stamp
             Protocol (TSP)", RFC 3161, August 2001.

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

7.  Informative References

   [CADES]   Pinkas, D., Pope, N., and J. Ross, "CMS Advanced Electronic
             Signatures (CAdES)", RFC 5126, March 2008.



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Appendix A.  ASN.1 Module

   The ASN.1 module contained in this appendix defines the structures
   that are needed to implement this specification.  It is expected to
   be used in conjunction with the ASN.1 modules in [CMS], [TSP],
   [PKIX1], and [ERS].

   TimeStampedDataModule
      { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
        pkcs-9(9) smime(16) modules(0) 35 }

      DEFINITIONS IMPLICIT TAGS ::=
      BEGIN

      IMPORTS

         -- Imports from RFC 5652 [CMS]
         Attribute
            FROM CryptographicMessageSyntax2004
               { iso(1) member-body(2) us(840) rsadsi(113549)
                 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }

         -- Imports from RFC 3161 [TSP]
         TimeStampToken
            FROM PKIXTSP
               { iso(1) identified-organization(3) dod(6) internet(1)
                 security(5) mechanisms(5) pkix(7) id-mod(0)
                 id-mod-tsp(13)}

         -- Imports from RFC 5280 [PKIX1]
         CertificateList
            FROM PKIX1Explicit88
               { iso(1) identified-organization(3) dod(6) internet(1)
                 security(5) mechanisms(5) pkix(7) id-mod(0)
                 id-pkix1-explicit-88(18)}

         -- Imports from RFC 4998 [ERS]
         EvidenceRecord
            FROM ERS
               { iso(1) identified-organization(3) dod(6) internet(1)
                 security(5) mechanisms(5) ltans(11) id-mod(0)
                 id-mod-ers88(2) id-mod-ers88-v1(1) };

      -- TimeStampedData Content Type and Object Identifier

      id-ct-timestampedData OBJECT IDENTIFIER ::= {
         iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
         id-smime(16) id-ct(1) 31 }



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      TimeStampedData ::= SEQUENCE {
         version              INTEGER { v1(1) },
         dataUri              IA5String OPTIONAL,
         metaData             MetaData OPTIONAL,
         content              OCTET STRING OPTIONAL,
         temporalEvidence     Evidence
      }

      MetaData ::= SEQUENCE {
         hashProtected        BOOLEAN,
         fileName             UTF8String OPTIONAL,
         mediaType            IA5String OPTIONAL,
         otherMetaData        Attributes OPTIONAL
      }

      Attributes ::=
         SET SIZE(1..MAX) OF Attribute -- according to RFC 5652

      Evidence ::= CHOICE {
         tstEvidence    [0] TimeStampTokenEvidence,   -- see RFC 3161
         ersEvidence    [1] EvidenceRecord,           -- see RFC 4998
         otherEvidence  [2] OtherEvidence
      }

      OtherEvidence ::= SEQUENCE {
         oeType            OBJECT IDENTIFIER,
         oeValue           ANY DEFINED BY oeType }

      TimeStampTokenEvidence ::=
         SEQUENCE SIZE(1..MAX) OF TimeStampAndCRL

      TimeStampAndCRL ::= SEQUENCE {
         timeStamp   TimeStampToken,          -- according to RFC 3161
         crl         CertificateList OPTIONAL -- according to RFC 5280
      }

      END

Appendix B.  Acknowledgments

   Thanks to Stephen Kent for encouraging the author in the early stages
   of this work.

   Thanks to Russ Housley for reviewing this memo, suggesting useful
   amendments and assigning a value to the OIDs herein defined.

   Thanks are also due to other people who reviewed this memo and helped
   improving it, but prefer not to be mentioned.



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Author's Address

   Adriano Santoni
   Actalis S.p.A.
   Via Taramelli 26
   I-20124 Milano
   Italy

   EMail: adriano.santoni@actalis.it










































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