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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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