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Network Working Group T. Gondrom
Request for Comments: 4998 Open Text Corporation
Category: Standards Track R. Brandner
InterComponentWare AG
U. Pordesch
Fraunhofer Gesellschaft
August 2007
Evidence Record Syntax (ERS)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
In many scenarios, users must be able prove the existence and
integrity of data, including digitally signed data, in a common and
reproducible way over a long and possibly undetermined period of
time. This document specifies the syntax and processing of an
Evidence Record, a structure designed to support long-term non-
repudiation of existence of data.
Gondrom, et al. Standards Track [Page 1]
RFC 4998 ERS August 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. General Overview and Requirements . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Conventions Used in This Document . . . . . . . . . . . . 6
2. Identification and References . . . . . . . . . . . . . . . . 7
2.1. ASN.1 Module Definition . . . . . . . . . . . . . . . . . 7
2.1.1. ASN.1 Module Definition for 1988 ASN.1 Syntax . . . . 7
2.1.2. ASN.1 Module Definition for 1997-ASN.1 Syntax . . . . 7
2.2. ASN.1 Imports and Exports . . . . . . . . . . . . . . . . 7
2.2.1. Imports and Exports Conform with 1988 ASN.1 . . . . . 8
2.2.2. Imports and Exports Conform with 1997-ASN.1 . . . . . 8
2.3. LTANS Identification . . . . . . . . . . . . . . . . . . . 9
3. Evidence Record . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Generation . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Verification . . . . . . . . . . . . . . . . . . . . . . . 11
4. Archive Timestamp . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Generation . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. Verification . . . . . . . . . . . . . . . . . . . . . . . 15
5. Archive Timestamp Chain and Archive Timestamp Sequence . . . . 16
5.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Generation . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3. Verification . . . . . . . . . . . . . . . . . . . . . . . 19
6. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1.1. EncryptionInfo in 1988 ASN.1 . . . . . . . . . . . . . 21
6.1.2. EncryptionInfo in 1997-ASN.1 . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Evidence Record Using CMS . . . . . . . . . . . . . . 26
Appendix B. ASN.1-Module with 1988 Syntax . . . . . . . . . . . . 27
Appendix C. ASN.1-Module with 1997 Syntax . . . . . . . . . . . . 29
Gondrom, et al. Standards Track [Page 2]
RFC 4998 ERS August 2007
1. Introduction
1.1. Motivation
In many application areas of electronic data exchange, a non-
repudiable proof of the existence of digital data must be possible.
In some cases, this proof must survive the passage of long periods of
time. An important example is digitally signed data. Digital
signatures can be used to demonstrate data integrity and to perform
source authentication. In some cases, digitally signed data must be
archived for 30 years or more. However, the reliability of digital
signatures over long periods is not absolute. During the archival
period, hash algorithms and public key algorithms can become weak or
certificates can become invalid. These events complicate the
reliance on digitally signed data after many years by increasing the
likelihood that forgeries can be created. To avoid losing the
desired security properties derived from digital signatures, it is
necessary to prove that the digitally signed data already existed
before such a critical event. This can be accomplished using a
timestamp. However, some timestamps rely upon mechanisms that will
be subject to the same problems. To counter this problem, timestamps
are renewed by simply obtaining a new timestamp that covers the
original data and its timestamps prior to the compromise of
mechanisms used to generate the timestamps. This document provides a
syntax to support the periodic renewal of timestamps.
It is necessary to standardize the data formats and processing
procedures for such timestamps in order to be able to verify and
communicate preservation evidence. A first approach was made by IETF
within [RFC3126], where an optional Archive Timestamp Attribute was
specified for integration in signatures according to the
Cryptographic Messages Syntax (CMS) [RFC3852].
Evidence Record Syntax (ERS) broadens and generalizes this approach
for data of any format and takes long-term archive service
requirements [RFC4810] into account -- in particular, the handling of
large sets of data objects. ERS specifies a syntax for an
EvidenceRecord, which contains a set of Archive Timestamps and some
additional data. This Evidence Record can be stored separately from
the archived data, as a file, or integrated into the archived data,
i.e., as an attribute. ERS also specifies processes for generation
and verification of Evidence Records. Appendix A describes the
integration and use of an EvidenceRecord in context of signed and
enveloped messages according to the Cryptographic Message Syntax
(CMS). ERS does not specify a protocol for interacting with a long-
term archive system. The Long-term Archive Protocol specification
being developed by the IETF LTANS WG addresses this interface.
Gondrom, et al. Standards Track [Page 3]
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1.2. General Overview and Requirements
ERS is designed to meet the requirements for data structures set
forth in [RFC4810].
The basis of the ERS are Archive Timestamps, which can cover a single
data object (as an RFC3161 compliant timestamp does) or can cover a
group of data objects. Groups of data objects are addressed using
hash trees, first described by Merkle [MER1980], combined with a
timestamp. The leaves of the hash tree are hash values of the data
objects in a group. A timestamp is requested only for the root hash
of the hash tree. The deletion of a data object in the tree does not
influence the provability of others. For any particular data object,
the hash tree can be reduced to a few sets of hash values, which are
sufficient to prove the existence of a single data object.
Similarly, the hash tree can be reduced to prove existence of a data
group, provided all members of the data group have the same parent
node in the hash tree. Archive Timestamps are comprised of an
optional reduced hash tree and a timestamp.
An EvidenceRecord may contain many Archive Timestamps. For the
generation of the initial Archive Timestamp, the data objects to be
timestamped have to be determined. Depending on the context, this
could be a file or a data object group consisting of multiple files,
such as a document and its associated digital signature.
Before the cryptographic algorithms used within the Archive Timestamp
become weak or timestamp certificates become invalid, Archive
Timestamps have to be renewed by generating a new Archive Timestamp.
(Note: Information about the weakening of the security properties of
public key and hash algorithms, as well as the risk of compromise of
private keys of Time Stamping Units, has to be closely watched by the
Long-Term Archive provider or the owner of the data objects himself.
This information should be gathered by "out-of-band" means and is out
of scope of this document.) ERS distinguishes two ways for renewal
of an Archive Timestamp: Timestamp Renewal and Hash-Tree Renewal.
Depending on the conditions, the respective type of renewal is
required: The timestamp renewal is necessary if the private key of a
Timestamping Unit has been compromised, or if an asymmetric algorithm
or a hash algorithm used for the generation of the timestamps is no
longer secure for the given key size. If the hash algorithm used to
build the hash trees in the Archive Timestamp loses its security
properties, the Hash-Tree Renewal is required.
In the case of Timestamp Renewal, the timestamp of an Archive
Timestamp has to be hashed and timestamped by a new Archive
Timestamp. This mode of renewal can only be used when it is not
Gondrom, et al. Standards Track [Page 4]
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necessary to access the archived data objects covered by the
timestamp. For example, this simple form of renewal is sufficient if
the public key algorithm of the timestamp is going to lose its
security or the timestamp authority certificate is about to expire.
This is very efficient, in particular, if Archive Timestamping is
done by an archiving system or service, which implements a central
management of Archive Timestamps.
Timestamp renewal is not sufficient if the hash algorithm used to
build the hash tree of an Archive Timestamp becomes insecure. In the
case of Hash-Tree Renewal, all evidence data must be accessed and
timestamped. This includes not only the timestamps but also the
complete Archive Timestamps and the archived data objects covered by
the timestamps, which must be hashed and timestamped again by a new
Archive Timestamp.
1.3. Terminology
Archived data object: A data unit that is archived and has to be
preserved for a long time by the Long-term Archive Service.
Archived data object group: A set of two or more of data objects,
which for some reason belong together. For example, a document file
and a signature file could be an archived data object group, which
represent signed data.
Archive Timestamp: A timestamp and typically lists of hash values,
which allow the verification of the existence of several data objects
at a certain time. (In its most simple variant, when it covers only
one object, it may only consist of the timestamp.)
Archive Timestamp Chain: Part of an Archive Timestamp Sequence, it is
a time-ordered sequence of Archive Timestamps, where each Archive
Timestamp preserves non-repudiation of the previous Archive
Timestamp, even after the previous Archive Timestamp becomes invalid.
Overall non-repudiation is maintained until the new Archive Timestamp
itself becomes invalid. The process of generating such an Archive
Timestamp Chain is called Timestamp Renewal.
Archive Timestamp Sequence: Part of the Evidence Record, it is a
sequence of Archive Timestamp Chains, where each Archive Timestamp
Chain preserves non-repudiation of the previous Archive Timestamp
Chains, even after the hash algorithm used within the previous
Archive Timestamp's hash tree became weak. Non-repudiation is
preserved until the last Archive Timestamp of the last chain becomes
invalid. The process of generating such an Archive Timestamp
Sequence is called Hash-Tree Renewal.
Gondrom, et al. Standards Track [Page 5]
RFC 4998 ERS August 2007
Evidence: Information that may be used to resolve a dispute about
various aspects of authenticity of archived data objects.
Evidence record: Collection of evidence compiled for one or more
given archived data objects over time. An evidence record includes
all Archive Timestamps (within structures of Archive Timestamp Chains
and Archive Timestamp Sequences) and additional verification data,
like certificates, revocation information, trust anchors, policy
details, role information, etc.
Long-term Archive (LTA) Service: A service responsible for preserving
data for long periods of time, including generation and collection of
evidence, storage of archived data objects and evidence, etc.
Reduced hash tree: The process of reducing a Merkle hash tree
[MER1980] to a list of lists of hash values. This is the basis of
storing the evidence for a single data object.
Timestamp: A cryptographically secure confirmation generated by a
Time Stamping Authority (TSA). [RFC3161] specifies a structure for
timestamps and a protocol for communicating with a TSA. Besides
this, other data structures and protocols may also be appropriate,
e.g., such as defined in [ISO-18014-1.2002], [ISO-18014-2.2002],
[ISO-18014-3.2004], and [ANSI.X9-95.2005].
An Archive Timestamp relates to a data object, if the hash value of
this data object is part of the first hash value list of the Archive
Timestamp. An Archive Timestamp relates to a data object group, if
it relates to every data object of the group and no other data
objects. An Archive Timestamp Chain relates to a data object / data
object group, if its first Archive Timestamp relates to this data
object/data object group. An Archive Timestamp Sequence relates to a
data object / data object group, if its first Archive Timestamp Chain
relates to this data object/data object group.
1.4. 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 [RFC2119].
Gondrom, et al. Standards Track [Page 6]
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2. Identification and References
2.1. ASN.1 Module Definition
As many open ASN.1 compilers still support the 1988 syntax, this
standard offers to support two versions of ASN.1 1997-ASN.1 and 1988-
ASN.1. (For specification of ASN.1 refer to [CCITT.X208.1988],
[CCITT.X209.1988], [CCITT.X680.2002] and [CCITT.X690.2002].) This
specification defines the two ASN.1 modules, one for 1988 conform
ASN.1 and another in 1997-ASN.1 syntax. Depending on the syntax
version of your compiler implementation, you can use the imports for
the 1988 conformant ASN.1 syntax or the imports for the 1997-ASN.1
syntax. The appendix of this document lists the two complete
alternative ASN.1 modules. If there is a conflict between both
modules, the 1988-ASN.1 module precedes.
2.1.1. ASN.1 Module Definition for 1988 ASN.1 Syntax
1988 ASN.1 Module start
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) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
2.1.2. ASN.1 Module Definition for 1997-ASN.1 Syntax
ASN.1 Module start
ERS {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-mod(0) id-mod-ers(1) id-mod-ers-v1(1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
2.2. ASN.1 Imports and Exports
The specification exports all definitions and imports various
definitions. Depending on the ASN.1 syntax version of your
implementation, you can use the imports for the 1988 conform ASN.1
syntax below or the imports for the 1997-ASN.1 syntax in
Section 2.2.2.
Gondrom, et al. Standards Track [Page 7]
RFC 4998 ERS August 2007
2.2.1. Imports and Exports Conform with 1988 ASN.1
-- EXPORTS ALL --
IMPORTS
-- Imports from RFC 3852 Cryptographic Message Syntax
ContentInfo, Attribute
FROM CryptographicMessageSyntax2004 -- FROM [RFC3852]
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }
-- Imports from RFC 3280 [RFC3280], Appendix A.1
AlgorithmIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
;
2.2.2. Imports and Exports Conform with 1997-ASN.1
-- EXPORTS ALL --
IMPORTS
-- Imports from PKCS-7
ContentInfo
FROM PKCS7
{iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-7(7) modules(0)}
-- Imports from AuthenticationFramework
AlgorithmIdentifier
FROM AuthenticationFramework
{joint-iso-itu-t ds(5) module(1)
authenticationFramework(7) 4}
-- Imports from InformationFramework
Attribute
FROM InformationFramework
{joint-iso-itu-t ds(5) module(1)
informationFramework(1) 4}
;
Gondrom, et al. Standards Track [Page 8]
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2.3. LTANS Identification
This document defines the LTANS object identifier tree root.
LTANS Object Identifier tree root
ltans OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) ltans(11) }
3. Evidence Record
An Evidence Record is a unit of data, which can be used to prove the
existence of an archived data object or an archived data object group
at a certain time. The Evidence Record contains Archive Timestamps,
generated during a long archival period and possibly useful data for
validation. It is possible to store this Evidence Record separately
from the archived data objects or to integrate it into the data
itself. For data types, signed data and enveloped data of the CMS
integration are specified in Appendix A.
3.1. Syntax
Evidence Record has the following ASN.1 Syntax:
ASN.1 Evidence Record
EvidenceRecord ::= SEQUENCE {
version INTEGER { v1(1) } ,
digestAlgorithms SEQUENCE OF AlgorithmIdentifier,
cryptoInfos [0] CryptoInfos OPTIONAL,
encryptionInfo [1] EncryptionInfo OPTIONAL,
archiveTimeStampSequence ArchiveTimeStampSequence
}
CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF Attribute
The fields have the following meanings:
The 'version' field indicates the syntax version, for compatibility
with future revisions of this specification and to distinguish it
from earlier non-conformant or proprietary versions of the ERS. The
value 1 indicates this specification. Lower values indicate an
earlier version of the ERS has been used. An implementation
conforming to this specification SHOULD reject a version value below
1.
Gondrom, et al. Standards Track [Page 9]
RFC 4998 ERS August 2007
digestAlgorithms is a sequence of all the hash algorithms used to
hash the data object over the archival period. It is the union of
all digestAlgorithm values from the ArchiveTimestamps contained in
the EvidenceRecord. The ordering of the values is not relevant.
cryptoInfos allows the storage of data useful in the validation of
the archiveTimeStampSequence. This could include possible Trust
Anchors, certificates, revocation information, or the current
definition of the suitability of cryptographic algorithms, past and
present (e.g., RSA 768-bit valid until 1998, RSA 1024-bit valid until
2008, SHA1 valid until 2010). These items may be added based on the
policy used. Since this data is not protected within any timestamp,
the data should be verifiable through other mechanisms. Such
verification is out of scope of this document.
encryptionInfo contains the necessary information to support
encrypted content to be handled. For discussion of syntax, please
refer to Section 6.1.
ArchiveTimeStampSequence is a sequence of ArchiveTimeStampChain,
described in Section 5.
If the archive data objects were encrypted before generating Archive
Timestamps but a non-repudiation proof is needed for unencrypted data
objects, the optional encryptionInfos field contains data necessary
to unambiguously re-encrypt data objects. If omitted, it means that
data objects are not encrypted or that a non-repudiation proof for
the unencrypted data is not required. For further details, see
Section 6.
3.2. Generation
The generation of an EvidenceRecord can be described as follows:
1. Select a data object or group of data objects to archive.
2. Create the initial Archive Timestamp (see Section 4, "Archive
Timestamp").
3. Refresh the Archive Timestamp when necessary, by Timestamp
Renewal or Hash-Tree Renewal (see Section 5).
The process of generation depends on whether the Archive Timestamps
are generated, stored, and managed by a centralized instance. In the
case of central management, it is possible to collect many data
objects, build hash trees, store them, and reduce them later. In
case of local generation, it might be easier to generate a simple
Archive Timestamp without building hash trees. This can be
Gondrom, et al. Standards Track [Page 10]
RFC 4998 ERS August 2007
accomplished by omitting the reducedHashtree field from the
ArchiveTimestamp. In this case, the ArchiveTimestamp covers a single
data object. Using this approach, it is possible to "convert"
existing timestamps into ArchiveTimestamps for renewal.
3.3. Verification
The Verification of an EvidenceRecord overall can be described as
follows:
1. Select an archived data object or group of data objects
2. Re-encrypt data object/data object group, if encryption field is
used (for details, see Section 6).
3. Verify Archive Timestamp Sequence (details in Section 4 and
Section 5).
4. Archive Timestamp
An Archive Timestamp is a timestamp and a set of lists of hash
values. The lists of hash values are generated by reduction of an
ordered Merkle hash tree [MER1980]. The leaves of this hash tree are
the hash values of the data objects to be timestamped. Every inner
node of the tree contains one hash value, which is generated by
hashing the concatenation of the children nodes. The root hash
value, which represents unambiguously all data objects, is
timestamped.
4.1. Syntax
An Archive Timestamp has the following ASN.1 Syntax:
ASN.1 Archive Timestamp
ArchiveTimeStamp ::= SEQUENCE {
digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
attributes [1] Attributes OPTIONAL,
reducedHashtree [2] SEQUENCE OF PartialHashtree OPTIONAL,
timeStamp ContentInfo}
PartialHashtree ::= SEQUENCE OF OCTET STRING
Attributes ::= SET SIZE (1..MAX) OF Attribute
The fields of type ArchiveTimeStamp have the following meanings:
Gondrom, et al. Standards Track [Page 11]
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digestAlgorithm identifies the digest algorithm and any associated
parameters used within the reduced hash tree. If the optional field
digestAlgorithm is not present, the digest algorithm of the timestamp
MUST be used. Which means, if timestamps according to [RFC3161] are
used in this case, the content of this field is identical to
hashAlgorithm of messageImprint field of TSTInfo.
attributes contains information an LTA might want to provide to
document individual renewal steps and the creation of the individual
ArchiveTimeStamps, e.g., applied policies. As the structure of the
ArchiveTimeStamp may be protected by hash and timestamps, the
ordering is relevant, which is why a SET is used instead of a
SEQUENCE.
reducedHashtree contains lists of hash values, organized in
PartialHashtrees for easier understanding. They can be derived by
reducing a hash tree to the nodes necessary to verify a single data
object. Hash values are represented as octet strings. If the
optional field reducedHashtree is not present, the ArchiveTimestamp
simply contains an ordinary timestamp.
timeStamp should contain the timestamp as defined in Section 1.3.
(e.g., as defined with TimeStampToken in [RFC3161]). Other types of
timestamp MAY be used, if they contain time data, timestamped data,
and a cryptographically secure confirmation from the TSA of these
data.
4.2. Generation
The lists of hash values of an Archive Timestamp can be generated by
building and reducing a Merkle hash tree [MER1980].
Such a hash tree can be built as follows:
1. Collect data objects to be timestamped.
2. Choose a secure hash algorithm H and generate hash values for the
data objects. These values will be the leaves of the hash tree.
3. For each data group containing more than one document, its
respective document hashes are binary sorted in ascending order,
concatenated, and hashed. The hash values are the complete
output from the hash algorithm, i.e., leading zeros are not
removed, with the most significant bit first.
4. If there is more than one hash value, place them in groups and
sort each group in binary ascending order. Concatenate these
values and generate new hash values, which are inner nodes of
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this tree. (If additional hash values are needed, e.g., so that
all nodes have the same number of children, any data may be
hashed using H and used.) Repeat this step until there is only
one hash value, which is the root node of the hash tree.
5. Obtain a timestamp for this root hash value. The hash algorithm
in the timestamp request MUST be the same as the hash algorithm
of the hash tree, or the digestAlgorithm field of the
ArchiveTimeStamp MUST be present and specify the hash algorithm
of the hash tree.
An example of a constructed hash tree for 3 data groups, where data
groups 1 and 3 only contain one document, and data group 2 contains 3
documents:
+------+
| h123 |
+------+
/ \
/ \
+----+ +----+
| h12| | h3 |
+----+ +----+
/ \
/ \
+----+ +-------+
| h1 | | h2abc |
+----+ +-------+
/ | \
/ | \
/ | \
/ | \
+----+ +----+ +----+
| h2a| | h2b| | h2c|
+----+ +----+ +----+
Figure 1: Hash tree
h1 = H(d1) where d1 is the only data object in data group 1
h3 = H(d3) where d3 is the only data object in data group 3
h12 = H( binary sorted and concatenated (h1, h2abc))
h123 = H( binary sorted and concatenated (h12, h3))
h2a = H(first data object of data object group 2)
h2b = H(second data object of data object group 2)
h2c = H(third data object of data object group 2)
h2abc = H( binary sorted and concatenated (h2a, h2b, h2c))
Gondrom, et al. Standards Track [Page 13]
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The hash tree can be reduced to lists of hash values, necessary to
have a proof of existence for a single data object:
1. Generate hash value h of the data object, using hash algorithm H
of the hash tree.
2. Select all hash values, which have the same father node as h.
Generate the first list of hash values by arranging these hashes,
in binary ascending order. This will be stored in the structure
of the PartialHashtree. Repeat this step for the father node of
all hashes until the root hash is reached. The father nodes
themselves are not saved in the hash lists -- they are
computable.
3. The list of all partialHashtrees finally is the reducedHashtree.
(All of the specified hash values under the same father node,
except the father node of the nodes below, are grouped in a
PartialHashtree. The sequence list of all Partialhashtrees is
the reducedHashtree.)
4. Finally, add the timestamp and the info about the hash algorithm
to get an Archive Timestamp.
Assuming that the sorted binary ordering of the hashes in Figure 1
is: h2abc < h1, then the reduced hash tree for data group 1 (d1) is:
+--------------------------------+
| +-----------------+ +--------+ |
| | +------+ +----+ | | +----+ | |
| | | h2abc| | h1 | | | | h3 | | |
| | +------+ +----+ | | +----+ | |
| +-----------------+ +--------+ |
+--------------------------------+
Figure 2: Reduced hash tree for data group 1
The pseudo ASN1 for this reduced hash tree rht1 would look like:
rht1 = SEQ(pht1, pht2)
with the PartialHashtrees pht1 and pht2
pht1 = SEQ (h2abc, h1)
pht2 = SEQ (h3)
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RFC 4998 ERS August 2007
Assuming the same hash tree as in Figure 1, the reduced hash tree for
all data objects in data group 2 is identical.
+-------------------------------------------------+
| +----------------------+ +--------+ +--------+ |
| | +----+ +----+ +----+ | | +----+ | | +----+ | |
| | | h2b| | h2c| | h2a| | | | h1 | | | | h3 | | |
| | +----+ +----+ +----+ | | +----+ | | +----+ | |
| +----------------------+ +--------+ +--------+ |
+-------------------------------------------------+
Figure 3: Reduced hash tree for data object group 2
The pseudo ASN1 for this reduced hash tree would look like:
rht2 = SEQ(pht3, pht4, pht5)
with the PartialHashtrees pht3, pht4, and pht5
pht3 = SEQ (h2b, h2c, h2a)
pht4 = SEQ (h1)
pht5 = SEQ (h3)
Note there are no restrictions on the quantity or length of hash-
value lists. Also note that it is profitable but not required to
build hash trees and reduce them. An Archive Timestamp may consist
only of one list of hash-values and a timestamp or only a timestamp
with no hash value lists.
The data (e.g. certificates, Certificate Revocation Lists (CRLs), or
Online Certificate Status Protocol (OCSP) responses) needed to verify
the timestamp MUST be preserved, and SHOULD be stored in the
timestamp itself unless this causes unnecessary duplication. A
timestamp according to [RFC3161] is a CMS object in which
certificates can be stored in the certificates field and CRLs can be
stored in the crls field of signed data. OCSP responses can be
stored as unsigned attributes [RFC3126]. Note [ANSI.X9-95.2005],
[ISO-18014-2.2002], and [ISO-18014-3.2004], which specify verifiable
timestamps that do not depend on certificates, CRLs, or OCSP
responses.
4.3. Verification
An Archive Timestamp shall prove that a data object existed at a
certain time, given by timestamp. This can be verified as follows:
1. Calculate hash value h of the data object with hash algorithm H
given in field digestAlgorithm of the Archive Timestamp.
Gondrom, et al. Standards Track [Page 15]
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2. Search for hash value h in the first list (partialHashtree) of
reducedHashtree. If not present, terminate verification process
with negative result.
3. Concatenate the hash values of the actual list (partialHashtree)
of hash values in binary ascending order and calculate the hash
value h' with algorithm H. This hash value h' MUST become a
member of the next higher list of hash values (from the next
partialHashtree). Continue step 3 until a root hash value is
calculated.
4. Check timestamp. In case of a timestamp according to [RFC3161],
the root hash value must correspond to hashedMessage, and
digestAlgorithm must correspond to hashAlgorithm field, both in
messageImprint field of timeStampToken. In case of other
timestamp formats, the hash value and digestAlgorithm must also
correspond to their equivalent fields if they exist.
If a proof is necessary for more than one data object, steps 1 and 2
have to be done for all data objects to be proved. If an additional
proof is necessary that the Archive Timestamp relates to a data
object group (e.g., a document and all its signatures), it can be
verified additionally, that only the hash values of the given data
objects are in the first hash-value list.
5. Archive Timestamp Chain and Archive Timestamp Sequence
An Archive Timestamp proves the existence of single data objects or
data object group at a certain time. However, this first Archive
Timestamp in the first ArchiveTimeStampChain can become invalid, if
hash algorithms or public key algorithms used in its hash tree or
timestamp become weak or if the validity period of the timestamp
authority certificate expires or is revoked.
Prior to such an event, the existence of the Archive Timestamp or
archive timestamped data has to be reassured. This can be done by
creating a new Archive Timestamp. Depending on whether the timestamp
becomes invalid or the hash algorithm of the hash tree becomes weak,
two kinds of Archive Timestamp renewal are possible:
o Timestamp Renewal: A new Archive Timestamp is generated, which
covers the timestamp of the old one. One or more Archive
Timestamps generated by Timestamp Renewal yield an Archive
Timestamp Chain for a data object or data object group.
Gondrom, et al. Standards Track [Page 16]
RFC 4998 ERS August 2007
o Hash-Tree Renewal: A new Archive Timestamp is generated, which
covers all the old Archive Timestamps as well as the data objects.
A new Archive Timestamp Chain is started. One or more Archive
Timestamp Chains for a data object or data object group yield an
Archive Timestamp Sequence.
After the renewal, always only the last (i.e., most recent)
ArchiveTimeStamp and the algorithms and timestamps used by it must be
watched regarding expiration and loss of security.
5.1. Syntax
ArchiveTimeStampChain and ArchiveTimeStampSequence have the following
ASN.1 Syntax:
ASN.1 ArchiveTimeStampChain and ArchiveTimeStampSequence
ArchiveTimeStampChain ::= SEQUENCE OF ArchiveTimeStamp
ArchiveTimeStampSequence ::= SEQUENCE OF
ArchiveTimeStampChain
ArchiveTimeStampChain and ArchiveTimeStampSequence MUST be ordered
ascending by time of timestamp. Within an ArchiveTimeStampChain, all
reducedHashtrees of the contained ArchiveTimeStamps MUST use the same
Hash-Algorithm.
5.2. Generation
The initial Archive Timestamp relates to a data object or a data
object group. Before cryptographic algorithms that are used within
the most recent Archive Timestamp (which is, at the beginning, the
initial one) become weak or their timestamp certificates become
invalid, Archive Timestamps have to be renewed by generating a new
Archive Timestamp.
In the case of Timestamp Renewal, the content of the timeStamp field
of the old Archive Timestamp has to be hashed and timestamped by a
new Archive Timestamp. The new Archive Timestamp MAY not contain a
reducedHashtree field, if the timestamp only simply covers the
previous timestamp. However, generally one can collect a number of
old Archive Timestamps and build the new hash tree with the hash
values of the content of their timeStamp fields.
The new Archive Timestamp MUST be added to the ArchiveTimestampChain.
This hash tree of the new Archive Timestamp MUST use the same hash
algorithm as the old one, which is specified in the digestAlgorithm
Gondrom, et al. Standards Track [Page 17]
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field of the Archive Timestamp or, if this value is not set (as it is
optional), within the timestamp itself.
In the case of Hash-Tree Renewal, the Archive Timestamp and the
archived data objects covered by the Archive Timestamp must be hashed
and timestamped again, as described below:
1. Select a secure hash algorithm H.
2. Select data objects d(i) referred to by initial Archive Timestamp
(objects that are still present and not deleted). Generate hash
values h(i) = H((d(i)). If data groups with more than one
document are present, then one will have more than one hash for a
group, i.e., h(i_a), h(i_b).., h(i_n)
3. atsc(i) is the encoded ArchiveTimeStampSequence, the
concatenation of all previous Archive Timestamp Chains (in
chronological order) related to data object d(i). Generate hash
value ha(i) = H(atsc(i)).
Note: The ArchiveTimeStampChains used are DER encoded, i.e., they
contain sequence and length tags.
4. Concatenate each h(i) with ha(i) and generate hash values
h(i)' = H (h(i)+ ha(i)). For multi-document groups, this is:
h(i_a)' = H (h(i_a)+ ha(i))
h(i_b)' = H (h(i_b)+ ha(i)), etc.
5. Build a new Archive Time Stamp for each h(i)'. (Hash-tree
generation and reduction is defined in Section 4.2; note that
each h(i)' will be treated in Section 4.2 as the document hash.
The first hash value list in the reduced hash tree should only
contain h(i)'. For a multi-document group, the first hash value
list will contain the new hashes for all the documents in this
group, i.e., h(i_a)', h(i_b)'.., h(i_n)')
6. Create new ArchiveTimeStampChain containing the new Archive
Timestamp and append this ArchiveTimeStampChain to the
ArchiveTimeStampSequence.
Gondrom, et al. Standards Track [Page 18]
RFC 4998 ERS August 2007
+-------+
| h123' |
+-------+
/ \
/ \
+-----+ +----+
| h12'| | h3'|
+-----+ +----+
/ \
/ \
+----+ +--------+
| h1'| | h2abc' |
+----+ +--------+
/ | \
/ | \
/ | \
/ | \
+----+ +----+ +----+
|h2a'| |h2b'| |h2c'|
+----+ +----+ +----+
Figure 4: Hash tree from Hash-Tree Renewal
Let H be the new secure hash algorithm
ha(1), ha(2), ha(3) are as defined in step 4 above
h1' = H( binary sorted and concatenated (H(d1), ha(1)))
d1 is the original document from data group 1
h3' = H( binary sorted and concatenated (H(d3), ha(3)))
d3 is the original document from data group 3
h2a = H(first data object of data object group 2)
...
h2c = H(third data object of data object group 2)
h2a' = H( binary sorted and concatenated (h2a, ha(2)))
...
h2c' = H( binary sorted and concatenated (h2c, ha(2)))
h2abc = H( binary sorted and concatenated (h2a', h2b', h2c'))
ArchiveTimeStamps that are not necessary for verification should not
be added to an ArchiveTimeStampChain or ArchiveTimeStampSequence.
5.3. Verification
To get a non-repudiation proof that a data object existed at a
certain time, the Archive Timestamp Chains and their relations to
each other and to the data objects have to be proved:
Gondrom, et al. Standards Track [Page 19]
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1. Verify that the first Archive Timestamp of the first
ArchiveTimestampChain (the initial Archive Timestamp) contains
the hash value of the data object.
2. Verify each ArchiveTimestampChain. The first hash value list of
each ArchiveTimeStamp MUST contain the hash value of the
timestamp of the Archive Timestamp before. Each Archive
Timestamp MUST be valid relative to the time of the following
Archive Timestamp. All Archive Timestamps within a chain MUST
use the same hash algorithm and this algorithm MUST be secure at
the time of the first Archive Timestamp of the following
ArchiveTimeStampChain.
3. Verify that the first hash value list (partialHashtree) of the
first Archive Timestamp of all other ArchiveTimeStampChains
contains a hash value of the concatenation of the data object
hash and the hash value of all older ArchiveTimeStampChain.
Verify that this Archive Timestamp was generated before the last
Archive Timestamp of the ArchiveTimeStampChain became invalid.
In order to complete the non-repudiation proof for the data objects,
the last Archive Timestamp has to be valid at the time the
verification is performed.
If the proof is necessary for more than one data object, steps 1 and
3 have to be done for all these data objects. To prove the Archive
Timestamp Sequence relates to a data object group, verify that each
first Archive Timestamp of the first ArchiveTimeStampChain of the
ArchiveTimeStampSequence of each data object does not contain other
hash values in its first hash value list (than the hash values of the
other data objects).
6. Encryption
If service providers are used to archive data and generate Archive
Timestamps, it might be desirable or required that clients only send
encrypted data to be archived. However, this means that evidence
records refer to encrypted data objects. ERS directly protects the
integrity of the bit-stream and this freezes the bit structure at the
time of archiving. This precludes changing of the encryption scheme
during the archival period, e.g., if the encryption scheme is no
longer secure, a change is not possible without losing the integrity
proof of the EvidenceRecord. In such cases, the services of a data
transformation (and by this also possible re-encryption) done by a
notary service might be a possible solution. To avoid problems when
using the evidence records in the future, additional special
precautions have to be taken:
Gondrom, et al. Standards Track [Page 20]
RFC 4998 ERS August 2007
o Evidence generated to prove the existence of encrypted data cannot
always be relied upon to prove the existence of unencrypted data.
It may be possible to choose an algorithm or a key for decryption
that is not the algorithm or key used for encryption. In this
case, the evidence record would not be a non-repudiation proof for
the unencrypted data. Therefore, only encryption methods should
be used that make it possible to prove that archive-timestamped
encrypted data objects unambiguously represent unencrypted data
objects. All data necessary to prove unambiguous representation
should be included in the archived data objects. (Note: In
addition, the long-term security of the encryption schemes should
be analyzed to determine if it could be used to create collision
attacks.)
o When a relying party uses an evidence record to prove the
existence of encrypted data objects, it may be desirable for
clients to only store the unencrypted data objects and to delete
the encrypted copy. In order to use the evidence record, it must
then be possible to unambiguously re-encrypt the unencrypted data
to get exactly the data that was originally archived. Therefore,
additional data necessary to re-encrypt data objects should be
inserted into the evidence record by the client, i.e., the LTA
never sees these values.
An extensible structure is defined to store the necessary parameters
of the encryption methods. The use of the specified
encryptionInfoType and encryptionInfoValue may be heavily dependent
on the mechanisms and has to be defined in other specifications.
6.1. Syntax
The EncryptionInfo field in EvidenceRecord has the following syntax
depending on the version of ASN.1.
6.1.1. EncryptionInfo in 1988 ASN.1
1988 ASN.1 EncryptionInfo
EncryptionInfo ::= SEQUENCE {
encryptionInfoType OBJECT IDENTIFIER,
encryptionInfoValue ANY DEFINED BY encryptionInfoType
}
Gondrom, et al. Standards Track [Page 21]
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6.1.2. EncryptionInfo in 1997-ASN.1
1997-ASN.1 EncryptionInfo
EncryptionInfo ::= SEQUENCE {
encryptionInfoType ENCINFO-TYPE.&id
({SupportedEncryptionAlgorithms}),
encryptionInfoValue ENCINFO-TYPE.&Type
({SupportedEncryptionAlgorithms}{@encryptionInfoType})
}
ENCINFO-TYPE ::= TYPE-IDENTIFIER
SupportedEncryptionAlgorithms ENCINFO-TYPE ::= {...}
encryptionInfo contains information necessary for the unambiguous
re-encryption of unencrypted content so that it exactly matches with
the encrypted data objects protected by the EvidenceRecord.
7. Security Considerations
Secure Algorithms
Cryptographic algorithms and parameters that are used within Archive
Timestamps must be secure at the time of generation. This concerns
the hash algorithm used in the hash lists of Archive Timestamp as
well as hash algorithms and public key algorithms of the timestamps.
Publications regarding security suitability of cryptographic
algorithms ([NIST.800-57-Part1.2006] and [ETSI-TS102176-1-2005]) have
to be considered by verifying components. A generic solution for
automatic interpretation of security suitability policies in
electronic form is desirable but not the subject of this
specification.
Redundancy
Retrospectively, certain parts of an Archive Timestamp may turn out
to have lost their security suitability before this has been publicly
known. For example, retrospectively, it may turn out that algorithms
have lost their security suitability, and that even TSAs are
untrustworthy. This can result in Archive Timestamps using those
losing their probative force. Many TSAs are using the same signature
algorithms. While the compromise of a private key will only affect
the security of one specific TSA, the retrospective loss of security
of a signature algorithm will have impact on a potentially large
number of TSAs at once. To counter such risks, it is recommended to
Gondrom, et al. Standards Track [Page 22]
RFC 4998 ERS August 2007
generate and manage at least two redundant Evidence Records with
ArchiveTimeStampSequences using different hash algorithms and
different TSAs using different signature algorithms.
To best achieve and manage this redundancy, it is recommended to
manage the Archive Timestamps in a central LTA.
Secure Timestamps
Archive Timestamping depends upon the security of normal time
stamping. Security requirements for Time Stamping Authorities stated
in security policies have to be met. Renewed Archive Timestamps
should have the same or higher quality as the initial Archive
Timestamp. Archive Timestamps used for signature renewal of signed
data, should have the same or higher quality than the maximum quality
of the signatures.
Secure Encryption
For non-repudiation proof, it does not matter whether encryption has
been broken or not. Nevertheless, users should keep secret their
private keys and randoms used for encryption and disclose them only
if needed, e.g., in a lawsuit to a judge or expert. They should use
encryption algorithms and parameters that are prospected to be
unbreakable as long as confidentiality of the archived data is
important.
8. References
8.1. Normative References
[CCITT.X208.1988]
International Telephone and Telegraph Consultative
Committee, "Specification of Abstract Syntax Notation One
(ASN.1)", CCITT Recommendation X.208, November 1988.
[CCITT.X209.1988]
International Telephone and Telegraph Consultative
Committee, "Specification of Basic Encoding Rules for
Abstract Syntax Notation One (ASN.1)",
CCITT Recommendation X.209, 1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
Gondrom, et al. Standards Track [Page 23]
RFC 4998 ERS August 2007
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
8.2. Informative References
[ANSI.X9-95.2005]
American National Standard for Financial Services,
"Trusted Timestamp Management and Security", ANSI X9.95,
June 2005.
[CCITT.X680.2002]
International Telephone and Telegraph Consultative
Committee, "Abstract Syntax Notation One (ASN.1):
Specification of basic notation", CCITT Recommendation
X.680, July 2002.
[CCITT.X690.2002]
International Telephone and Telegraph Consultative
Committee, "ASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.
[ETSI-TS102176-1-2005]
European Telecommunication Standards Institute (ETSI),
Electronic Signatures and Infrastructures (ESI);,
"Algorithms and Parameters for Secure Electronic
Signatures; Part 1: Hash functions and asymmetric
algorithms", ETSI TS 102 176-1 V1.2.1, July 2005.
[ISO-18014-1.2002]
ISO/IEC JTC 1/SC 27, "Time stamping services - Part 1:
Framework", ISO ISO-18014-1, February 2002.
[ISO-18014-2.2002]
ISO/IEC JTC 1/SC 27, "Time stamping services - Part 2:
Mechanisms producing independent tokens", ISO ISO-18014-2,
December 2002.
[ISO-18014-3.2004]
ISO/IEC JTC 1/SC 27, "Time stamping services - Part 3:
Mechanisms producing linked tokens", ISO ISO-18014-3,
February 2004.
Gondrom, et al. Standards Track [Page 24]
RFC 4998 ERS August 2007
[MER1980] Merkle, R., "Protocols for Public Key Cryptosystems,
Proceedings of the 1980 IEEE Symposium on Security and
Privacy (Oakland, CA, USA)", pages 122-134, April 1980.
[NIST.800-57-Part1.2006]
National Institute of Standards and Technology,
"Recommendation for Key Management - Part 1: General
(Revised)", NIST 800-57 Part1, May 2006.
[RFC3126] Pinkas, D., Ross, J., and N. Pope, "Electronic Signature
Formats for long term electronic signatures", RFC 3126,
September 2001.
[RFC4810] Wallace, C., Pordesch, U., and R. Brandner, "Long-Term
Archive Service Requirements", RFC 4810, March 2007.
Gondrom, et al. Standards Track [Page 25]
RFC 4998 ERS August 2007
Appendix A. Evidence Record Using CMS
An Evidence Record can be added to signed data or enveloped data in
order to transfer them in a conclusive way. For CMS, a sensible
place to store such an Evidence Record is an unsigned attribute
(signed message) or an unprotected attribute (enveloped message).
One advantage of storing the Evidence Record within the CMS structure
is that all data can be transferred in one conclusive file and is
directly connected. The documents, the signatures, and their
Evidence Records can be bundled and managed together. The
description in the appendix contains the normative specification of
how to integrate ERS in CMS structures.
The Evidence Record also contains information about the selection
method that was used for the generation of the data objects to be
timestamped. In the case of CMS, two selection methods can be
distinguished:
1. The CMS Object as a whole including contentInfo is selected as
data object and archive timestamped. This means that a hash
value of the CMS object MUST be located in the first list of hash
values of Archive Timestamps.
2. The CMS Object and the signed or encrypted content are included
in the Archive Timestamp as separated objects. In this case, the
hash value of the CMS Object as well as the hash value of the
content have to be stored in the first list of hash values as a
group of data objects.
However, other selection methods could also be applied, for instance,
as in [RFC3126].
In the case of the two selection methods defined above, the Evidence
Record has to be added to the first signature of the CMS Object of
signed data. Depending on the selection method, the following Object
Identifiers are defined for the Evidence Record:
ASN.1 Internal EvidenceRecord Attribute
id-aa-er-internal OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 49 }
ASN.1 External EvidenceRecord Attribute
id-aa-er-external OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 50 }
Gondrom, et al. Standards Track [Page 26]
RFC 4998 ERS August 2007
The attributes SHOULD only occur once. If they appear several times,
they have to be stored within the first signature in chronological
order.
If the CMS object doesn't have the EvidenceRecord Attributes -- which
indicates that the EvidenceRecord has been provided externally -- the
archive timestamped data object has to be generated over the complete
CMS object within the existing coding.
In case of verification, if only one EvidenceRecord is contained in
the CMS object, the hash value must be generated over the CMS object
without the one EvidenceRecord. This means that the attribute has to
be removed before verification. The length of fields containing tags
has to be adapted. Apart from that, the existing coding must not be
modified.
If several Archive Timestamps occur, the data object has to be
generated as follows:
o During verification of the first (in chronological order)
EvidenceRecord, all EvidenceRecord have to be removed in order to
generate the data object.
o During verification of the nth one EvidenceRecord, the first n-1
attributes should remain within the CMS object.
o The verification of the nth one EvidenceRecord must result in a
point of time when the document must have existed with the first n
attributes. The verification of the n+1th attribute must prove
that this requirement has been met.
Appendix B. ASN.1-Module with 1988 Syntax
ASN.1-Module
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) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
-- Imports from RFC 3852 Cryptographic Message Syntax
ContentInfo, Attribute
Gondrom, et al. Standards Track [Page 27]
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FROM CryptographicMessageSyntax2004 -- FROM [RFC3852]
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }
-- Imports from RFC 3280 [RFC3280], Appendix A.1
AlgorithmIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
;
ltans OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) ltans(11) }
EvidenceRecord ::= SEQUENCE {
version INTEGER { v1(1) } ,
digestAlgorithms SEQUENCE OF AlgorithmIdentifier,
cryptoInfos [0] CryptoInfos OPTIONAL,
encryptionInfo [1] EncryptionInfo OPTIONAL,
archiveTimeStampSequence ArchiveTimeStampSequence
}
CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF Attribute
ArchiveTimeStamp ::= SEQUENCE {
digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
attributes [1] Attributes OPTIONAL,
reducedHashtree [2] SEQUENCE OF PartialHashtree OPTIONAL,
timeStamp ContentInfo}
PartialHashtree ::= SEQUENCE OF OCTET STRING
Attributes ::= SET SIZE (1..MAX) OF Attribute
ArchiveTimeStampChain ::= SEQUENCE OF ArchiveTimeStamp
ArchiveTimeStampSequence ::= SEQUENCE OF
ArchiveTimeStampChain
EncryptionInfo ::= SEQUENCE {
Gondrom, et al. Standards Track [Page 28]
RFC 4998 ERS August 2007
encryptionInfoType OBJECT IDENTIFIER,
encryptionInfoValue ANY DEFINED BY encryptionInfoType}
id-aa-er-internal OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 49 }
id-aa-er-external OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 50 }
END
Appendix C. ASN.1-Module with 1997 Syntax
ASN.1-Module
ERS {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-mod(0) id-mod-ers(1) id-mod-ers-v1(1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
-- Imports from PKCS-7
ContentInfo
FROM PKCS7
{iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-7(7) modules(0)}
-- Imports from AuthenticationFramework
AlgorithmIdentifier
FROM AuthenticationFramework
{joint-iso-itu-t ds(5) module(1)
authenticationFramework(7) 4}
-- Imports from InformationFramework
Attribute
FROM InformationFramework
{joint-iso-itu-t ds(5) module(1)
informationFramework(1) 4}
;
ltans OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) ltans(11) }
Gondrom, et al. Standards Track [Page 29]
RFC 4998 ERS August 2007
EvidenceRecord ::= SEQUENCE {
version INTEGER { v1(1) } ,
digestAlgorithms SEQUENCE OF AlgorithmIdentifier,
cryptoInfos [0] CryptoInfos OPTIONAL,
encryptionInfo [1] EncryptionInfo OPTIONAL,
archiveTimeStampSequence ArchiveTimeStampSequence
}
CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF Attribute
(WITH COMPONENTS {
...,
valuesWithContext ABSENT
})
ArchiveTimeStamp ::= SEQUENCE {
digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
attributes [1] Attributes OPTIONAL,
reducedHashtree [2] SEQUENCE OF PartialHashtree OPTIONAL,
timeStamp ContentInfo}
PartialHashtree ::= SEQUENCE OF OCTET STRING
Attributes ::= SET SIZE (1..MAX) OF Attribute
(WITH COMPONENTS {
...,
valuesWithContext ABSENT
})
ArchiveTimeStampChain ::= SEQUENCE OF ArchiveTimeStamp
ArchiveTimeStampSequence ::= SEQUENCE OF
ArchiveTimeStampChain
EncryptionInfo ::= SEQUENCE {
encryptionInfoType ENCINFO-TYPE.&id
({SupportedEncryptionAlgorithms}),
encryptionInfoValue ENCINFO-TYPE.&Type
({SupportedEncryptionAlgorithms}{@encryptionInfoType})
}
ENCINFO-TYPE ::= TYPE-IDENTIFIER
SupportedEncryptionAlgorithms ENCINFO-TYPE ::= {...}
id-aa-er-internal OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 49 }
id-aa-er-external OBJECT IDENTIFIER ::= { iso(1) member-body(2)
Gondrom, et al. Standards Track [Page 30]
RFC 4998 ERS August 2007
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 50 }
END
Authors' Addresses
Tobias Gondrom
Open Text Corporation
Werner-von-Siemens-Ring 20
Grasbrunn, Munich D-85630
Germany
Phone: +49 (0) 89 4629-1816
Fax: +49 (0) 89 4629-33-1816
EMail: tobias.gondrom@opentext.com
Ralf Brandner
InterComponentWare AG
Industriestra?e 41
Walldorf D-69119
Germany
EMail: ralf.brandner@intercomponentware.com
Ulrich Pordesch
Fraunhofer Gesellschaft
Rheinstra?e 75
Darmstadt D-64295
Germany
EMail: ulrich.pordesch@zv.fraunhofer.de
Gondrom, et al. Standards Track [Page 31]
RFC 4998 ERS August 2007
Full Copyright Statement
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Gondrom, et al. Standards Track [Page 32]