A. Santoni
Internet Draft Actalis S.p.A.
Intended status: Informational June 10, 2008
Expires: December 2008
Syntax for binding documents with time stamps
draft-santoni-timestampeddata-04.txt
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document describes a syntax which can be used to bind a generic
document (or any set of data, not necessarily protected by means of
cryptographic techniques) to one or more time-stamp tokens obtained
for that document, where "time-stamp token" has the meaning defined
in RFC 3161. Additional types of temporal evidence are also
supported.
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This document proposes a simple syntax based on the Cryptographic
Message Syntax (RFC 3852).
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].
Table of Contents
1. Introduction...................................................2
2. Syntax for TimeStampedData.....................................3
3. Compliance requirements........................................5
4. Recommended processing.........................................5
5. Recommended file extensions....................................6
6. Security Considerations........................................6
7. IANA Considerations............................................6
8. References.....................................................7
Author's Addresses................................................7
Intellectual Property Statement...................................7
Disclaimer of Validity............................................8
1. Introduction
Digital time stamping has become the standard technique for proving
the existence of a document before a certain point in time. Several
digital signature legislations around the world embrace the concept
and provide for time-stamping services as an approved means for
attesting the signing time and/or for extending the validity of
signed documents beyond the expiry date of the signer's certificate.
However, while digital time stamping enhances digital signature, its
value does not depend on this latter. It can obviously be useful to
time-stamp a document even if this is not signed. And it can also be
useful, or even mandatory in some cases, to time-stamp a document in
its entirety, regardless of how many signatures it contains.
When a time-stamp is related to a digital signature, there already
exist a way to keep the two pieces together: RFC 3161 describes how
one or more TimeStampTokens can be included in a SignerInfo structure
as unsigned attributes. On the other hand, when time-stamps are not
related to a digital signature, there is no standard way to keep
together the time-stamped document and the related time-stamps.
In such cases two approaches are typically being adopted:
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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, like e.g.
a ZIP archive or a proprietary "envelope" of some kind.
Both solutions impede interoperability, the objective of this memo.
This document proposes a simple syntax for bundling one document
(actually, any kind of file) to the corresponding temporal evidence,
this latter being typically represented by one or more RFC 3161
TimeStampTokens. Additional types of temporal evidence, like e.g. an
RFC 4998 EvidenceRecord, are also supported via an "open" syntax.
However, for the sake of interoperability, the emphasis is given to
TimeStampTokens.
The proposed syntax is broadly based on the Cryptographic Message
Syntax (CMS) defined in RFC 3852 [CMS].
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 specific contentType OID:
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:
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TimeStampedData ::= SEQUENCE {
version INTEGER { v1(1) },
fileName UTF8String,
mimeType PrintableString,
content OCTET STRING,
evidence Evidence
}
Evidence ::= CHOICE {
timeStamps [0] SET (SIZE(1..MAX)) OF TimeStampToken,
evidenceRecord [1] EvidenceRecord
-- additional evidence types to be registered with the IETF
}
The version field contains the version number of the TimeStampedData
syntax. The initial version number is 1.
The fileName field contains the original filename of the document
which was time-stamped and whose content was inserted into the
TimeStampedData structure.
The mimeType field contains a MIME media type and subtype for the
bundled file (e.g. "text/rtf"), according to RFC 2045 [MIME]. It is
an advisory information which may help decide how to open or deal
with the file after having "detached" it from the TimeStampedData
structure, regardless of the filename extension (which could be
missing or unknown).
The content field carries the entire content, in its original format,
of the file which was time-stamped. The file need not be a document
in the strict sense; it can be any kind of file (e.g. an executable,
a database, etc).
The evidence field carries the evidence that the content data existed
before a certain point in time. The TimeStampedData syntax allows for
different types of evidence (like e.g. an EvidenceRecord according to
RFC 4998). However, this document mandates support for one type only:
a non-empty set of RFC 3161 TimeStampToken's [TSP].
Additional types of evidence may be used after having registered them
(and having had a distinguishing tag assigned to them) with the IETF.
A suitable registration procedure should be defined for that purpose.
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3. Compliance requirements
Compliant applications MUST support the RFC 3161-based type of
evidence (i.e. the timeStamps CHOICE).
Compliant applications MUST always populate the mimeType field of
TimeStampedData structure with a valid MIME type/subtype string
according to RFC 2045 [MIME]. A valid example is "application/pdf".
An invalid example is "whatever". An empty string is not allowed.
4. Recommended processing
When generating the TimeStampedData structure, applications are
supposed to behave like follows:
o populate the version field with the integer value v1(1);
o populate the fileName field with the real name of the file;
o populate the mimeType field with an appropriate MIME type/subtype
string, preferably, or at least with "application/octet-stream";
o populate the content field with the entire contents of the file in
its original format and encoding;
o add the necessary evidence (e.g. one or more TimeStampTokens);
o insert the TimeStampedData into a ContentInfo structure, with the
id-timestamped-data OID in the contentType field;
o BER-encode the ContentInfo structure and save it with the same
name of the time-stamped file, but with the file extension
recommended in section 5.
When parsing an existing TimeStampedData structure, applications are
supposed to behave like follows:
o check that the contentType field of the ContentInfo structure has
the expected value (id-timestamped-data) in its contentType field;
then, extract the inner TimeStampedData structure and continue
processing;
o check the version field (it should be v1);
o check the fileName field and keep it for later use;
o check the mimeType field and keep it for later use;
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o read the content field and prepare to save it in a separate file
and/or show it to the user (or otherwise deal with it);
o check that the evidence field not be empty; extract the inner data
and prepare to show them to the user and/or save them to separate
files;
o validate the evidence data (e.g. in case of timeStamps: check that
each TimeStampToken does indeed contain the hash of the document
and it was signed by a trusted TSA);
o depending on the application, show the evidence data to the user;
o depending on the application, show the time-stamped document to
the user, possibly by activating a suitable external "viewer"
based on the fileName extension and the mimeType field;
o depending on the application, save the content field into a
separate file with the name specified by the fileName field (see
Security Considerations) or let the user specify the desired
filename.
5. Recommended file extensions
A file containing a TimeStampedData structure SHOULD bear the .tsd
extension. Example: "patent123.tsd"
6. Security Considerations
Any consumer of TimeStampedData should validate the entire filename
(carried in the filename field of the TimeStampedData structure)
according the rules of its local filesystem and its intended usage
before using some or all of the name to store the data.
7. IANA Considerations
This document defines one object identifier under the pkcs9 arc:
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-ct OBJECT IDENTIFIER ::= { id-smime 1 } -- content types
id-ct-timestampedData OBJECT IDENTIFIER ::= { id-ct 31 }
(OID assignment courtesy of Russ Housley)
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8. References
[KWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
http://www.ietf.org/rfc/rfc2119.txt
[TSP] Adams, C., Cain, P., Pinkas, D. and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
http://www.ietf.org/rfc/rfc3161.txt
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
http://www.ietf.org/rfc/rfc3852.txt
[MIME] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
http://www.ietf.org/rfc/rfc2045.txt
[ERS] Gondrom, T., Brandner, R., and Pordesch, U., "Evidence
Record Syntax (ERS)", RFC 4998, August 2007.
http://www.ietf.org/rfc/rfc4998.txt
Author's Addresses
Adriano Santoni
Actalis S.p.A.
Via Taramelli 26
I-20124 Milano
Phone: +39-02-68825.1
Email: adriano.santoni@actalis.it
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Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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