Структура данных для обеспечения безопасности криптографических алгоритмов (DSSC)
Абстрактные
Поскольку криптографические алгоритмы могут быть слабыми на протяжении многих лет, это
необходимых для оценки их пригодности для обеспечения безопасности. При подписании или
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структура, которая позволяет автоматизировать анализ безопасности
пригодность данного криптографического алгоритма в данной точке
время, которое может быть в прошлом, настоящем или будущем.
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=====================================
Network Working Group T. Kunz
Request for Comments: 5698 Fraunhofer SIT
Category: Standards Track S. Okunick
pawisda systems GmbH
U. Pordesch
Fraunhofer Gesellschaft
November 2009
Data Structure for the Security Suitability
of Cryptographic Algorithms (DSSC)
Abstract
Since cryptographic algorithms can become weak over the years, it is
necessary to evaluate their security suitability. When signing or
verifying data, or when encrypting or decrypting data, these
evaluations must be considered. This document specifies a data
structure that enables an automated analysis of the security
suitability of a given cryptographic algorithm at a given point of
time, which may be in the past, the present, or the future.
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) 2009 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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Kunz, et al. Standards Track [Page 1]
RFC 5698 DSSC November 2009
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Kunz, et al. Standards Track [Page 2]
RFC 5698 DSSC November 2009
Table of Contents
1. Introduction ....................................................4
1.1. Motivation .................................................4
1.2. Terminology ................................................5
1.2.1. Conventions Used in This Document ...................5
1.3. Use Cases ..................................................5
2. Requirements and Assumptions ....................................5
2.1. Requirements ...............................................6
2.2. Assumptions ................................................6
3. Data Structures .................................................7
3.1. SecuritySuitabilityPolicy ..................................7
3.2. PolicyName .................................................8
3.3. Publisher ..................................................9
3.4. PolicyIssueDate ............................................9
3.5. NextUpdate .................................................9
3.6. Usage ......................................................9
3.7. Algorithm ..................................................9
3.8. AlgorithmIdentifier .......................................10
3.9. Evaluation ................................................10
3.10. Parameter ................................................11
3.11. Validity .................................................12
3.12. Information ..............................................12
3.13. Signature ................................................12
4. DSSC Policies ..................................................13
5. Definition of Parameters .......................................13
6. Processing .....................................................14
6.1. Inputs ....................................................14
6.2. Verify Policy .............................................14
6.3. Algorithm Evaluation ......................................15
6.4. Evaluation of Parameters ..................................15
6.5. Output ....................................................16
7. Security Considerations ........................................16
8. IANA Considerations ............................................18
9. References .....................................................23
9.1. Normative References ......................................23
9.2. Informative References ....................................24
Appendix A. DSSC and ERS .........................................27
A.1. Verification of Evidence Records Using DSSC
(Informative) .............................................27
A.2. Storing DSSC Policies in Evidence Records (Normative) .....27
Appendix B. XML Schema (Normative) ...............................28
Appendix C. ASN.1 Module in 1988 Syntax (Informative) ............30
Appendix D. ASN.1 Module in 1997 Syntax (Normative) ..............32
Appendix E. Example ..............................................34
Kunz, et al. Standards Track [Page 3]
RFC 5698 DSSC November 2009
1. Introduction
1.1. Motivation
Digital signatures can provide data integrity and authentication.
They are based on cryptographic algorithms that are required to have
certain security properties. For example, hash algorithms must be
resistant to collisions, and in case of public key algorithms,
computation of the private key that corresponds to a given public key
must be infeasible. If algorithms lack the required properties,
signatures could be forged, unless they are protected by a strong
cryptographic algorithm.
Cryptographic algorithms that are used in signatures shall be
selected to resist such attacks during their period of use. For
signature keys included in public key certificates, this period of
use is the validity period of the certificate. Cryptographic
algorithms that are used for encryption shall resist such attacks
during the period it is planned to keep the information confidential.
Only very few algorithms satisfy the security requirements. Besides,
because of the increasing performance of computers and progresses in
cryptography, algorithms or their parameters become insecure over the
years. The hash algorithm MD5, for example, is unsuitable today for
many purposes. A digital signature using a "weak" algorithm has no
probative value, unless the "weak" algorithm has been protected by a
strong algorithm before the time it was considered to be weak. Many
kinds of digital signed data (including signed documents, timestamps,
certificates, and revocation lists) are affected, particularly in the
case of long-term archiving. Over long periods of time, it is
assumed that the algorithms used in signatures become insecure.
For this reason, it is important to periodically evaluate an
algorithm's fitness and to consider the results of these evaluations
when creating and verifying signatures, or when maintaining the
validity of signatures made in the past. One result is a projected
validity period for the algorithm, i.e., a prediction of the period
of time during which the algorithm is fit for use. This prediction
can help to detect whether a weak algorithm is used in a signature
and whether that signature has been properly protected in due time by
another signature made using an algorithm that is suitable at the
present point of time. Algorithm evaluations are made by expert
committees. In Germany, the Federal Network Agency annually
publishes evaluations of cryptographic algorithms [BNetzAg.2008].
Examples of other European and international evaluations are
[ETSI-TS102176-1-2005] and [NIST.800-57-Part1.2006].
Kunz, et al. Standards Track [Page 4]
RFC 5698 DSSC November 2009
These evaluations are published in documents intended to be read by
humans. Therefore, to enable automated processing, it is necessary
to define a data structure that expresses the content of the
evaluations. This standardized data structure can be used for
publication and can be interpreted by signature generation and
verification tools. Algorithm evaluations are pooled in a security
suitability policy. In this document, a data structure for a
security suitability policy is specified. Therefore, the document
provides a framework for expressing evaluations of cryptographic
algorithms. This document does not attempt to catalog the security
properties of cryptographic algorithms. Furthermore, no guidelines
are made about which kind of algorithms shall be evaluated, for
example, security suitability policies may be used to evaluate public
key and hash algorithms, signature schemes, and encryption schemes.
1.2. Terminology
Algorithm: A cryptographic algorithm, i.e., a public key or hash
algorithm. For public key algorithms, this is the algorithm with
its parameters, if any. Furthermore, the term "algorithm" is used
for cryptographic schemes and for actually padding functions.
Operator: Instance that uses and interprets a policy, e.g., a
signature-verification component.
Policy: An abbreviation for security suitability policy.
Publisher: Instance that publishes the policy containing the
evaluation of algorithms.
Security suitability policy: The evaluation of cryptographic
algorithms with regard to their security in a specific application
area, e.g., signing or verifying data. The evaluation is
published in an electronic format.
Suitable algorithm: An algorithm that is evaluated against a policy
and determined to be valid, i.e., resistant against attacks, at a
particular point of time.
1.2.1. 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 [RFC2119].
Kunz, et al. Standards Track [Page 5]
RFC 5698 DSSC November 2009
1.3. Use Cases
Some use cases for a security suitability policy are presented here.
Long-term archiving: The most important use case is long-term
archiving of signed data. Algorithms or their parameters become
insecure over long periods of time. Therefore, signatures of
archived data and timestamps have to be periodically renewed. A
policy provides information about suitable and threatened
algorithms. Additionally, the policy assists in verifying
archived as well as re-signed documents.
Services: Services may provide information about cryptographic
algorithms. On the basis of a policy, a service is able to
provide the date when an algorithm became insecure or presumably
will become insecure, as well as information regarding which
algorithms are presently valid. Verification tools or long-term
archiving systems can request such services and therefore do not
need to deal with the algorithm security by themselves.
Long-term Archive Services (LTA) as defined in [RFC4810] may use
the policy for signature renewal.
Signing and verifying: When signing documents or certificates, it
must be assured that the algorithms used for signing or verifying
are suitable. Accordingly, when verifying Cryptographic Message
Syntax (CMS) [RFC5652] or XML signatures ([RFC3275],
[ETSI-TS101903]), not only the validity of the certificates but
also the validity of all involved algorithms may be checked.
Re-encryption: A security suitability policy can also be used to
decide if encrypted documents must be re-encrypted because the
encryption algorithm is no longer secure.
2. Requirements and Assumptions
Section 2.1 describes general requirements for a data structure
containing the security suitability of algorithms. In Section 2.2,
assumptions are specified concerning both the design and the usage of
the data structure.
A policy contains a list of algorithms that have been evaluated by a
publisher. An algorithm evaluation is described by its identifier,
security constraints, and validity period. By these constraints, the
requirements for algorithm properties must be defined, e.g., a public
key algorithm is evaluated on the basis of its parameters.
Kunz, et al. Standards Track [Page 6]
RFC 5698 DSSC November 2009
2.1. Requirements
Automatic interpretation: The data structure of the policy must
allow automated evaluation of the security suitability of an
algorithm.
Flexibility: The data structure must be flexible enough to support
new algorithms. Future policy publications may include
evaluations of algorithms that are currently unknown. It must be
possible to add new algorithms with the corresponding security
constraints in the data structure. Additionally, the data
structure must be independent of the intended use, e.g.,
encryption, signing, verifying, and signature renewing. Thus, the
data structure is usable in every use case.
Source authentication: Policies may be published by different
institutions, e.g., on the national or European Union (EU) level,
whereas one policy needs not to be in agreement with the other
one. Furthermore, organizations may undertake their own
evaluations for internal purposes. For this reason a policy must
be attributable to its publisher.
Integrity and authenticity: It must be possible to assure the
integrity and authenticity of a published security suitability
policy. Additionally, the date of issue must be identifiable.
2.2. Assumptions
It is assumed that a policy contains the evaluations of all currently
known algorithms, including the expired ones.
An algorithm is suitable at a time of interest if it is contained in
the current policy and the time of interest is within the validity
period. Additionally, if the algorithm has any parameters, these
parameters must meet the requirements defined in the security
constraints.
If an algorithm appears in a policy for the first time, it may be
assumed that the algorithm has already been suitable in the past.
Generally, algorithms are used in practice prior to evaluation.
To avoid inconsistencies, multiple instances of the same algorithm
are prohibited. The publisher must take care to prevent conflicts
within a policy.
Assertions made in the policy are suitable at least until the next
policy is published.
Kunz, et al. Standards Track [Page 7]
RFC 5698 DSSC November 2009
Publishers may extend the lifetime of an algorithm prior to reaching
the end of the algorithm's validity period by publishing a revised
policy. Publishers should not resurrect algorithms that are expired
at the time a revised policy is published.
3. Data Structures
This section describes the syntax of a security suitability policy
defined as an XML schema [W3C.REC-xmlschema-1-20041028]. ASN.1
modules are defined in Appendix C and Appendix D. The schema uses
the following XML namespace [W3C.REC-xml-names-20060816]:
urn:ietf:params:xml:ns:dssc
Within this document, the prefix "dssc" is used for this namespace.
The schema starts with the following schema definition:
< xml version="1.0" encoding="UTF-8"?>
3.1. SecuritySuitabilityPolicy
The SecuritySuitabilityPolicy element is the root element of a
policy. It has an optional id attribute, which MUST be used as a
reference when signing the policy (Section 3.13). The optional lang
attribute defines the language according to [RFC5646]. The language
is applied to all human-readable text within the policy. If the lang
attribute is omitted, the default language is English ("en"). The
element is defined by the following schema:
Kunz, et al. Standards Track [Page 8]
RFC 5698 DSSC November 2009
3.2. PolicyName
The PolicyName element contains an arbitrary name for the policy.
The optional elements Object Identifier (OID) and Uniform Resource
Identifier (URI) MAY be used for the identification of the policy.
OIDs MUST be expressed in the dot notation.
Kunz, et al. Standards Track [Page 9]
RFC 5698 DSSC November 2009
3.3. Publisher
The Publisher element contains information about the publisher of the
policy. It is composed of the name (e.g., name of institution), an
optional address, and an optional URI. The Address element contains
arbitrary free-format text not intended for automatic processing.
3.4. PolicyIssueDate
The PolicyIssueDate element indicates the point of time when the
policy was issued.
3.5. NextUpdate
The optional NextUpdate element MAY be used to indicate when the next
policy will be issued.
3.6. Usage
The optional Usage element determines the intended use of the policy
(e.g., certificate validation, signing and verifying documents). The
element contains free-format text intended only for human
readability.
3.7. Algorithm
A security suitability policy MUST contain at least one Algorithm
element. An algorithm is identified by an AlgorithmIdentifier
element. Additionally, the Algorithm element contains all
evaluations of the specific cryptographic algorithm. More than one
evaluation may be necessary if the evaluation depends on the
parameter constraints. The optional Information element MAY be used
to provide additional information like references on algorithm
specifications. In order to give the option to extend the Algorithm
element, it additionally contains a wildcard. The Algorithm element
is defined by the following schema:
Kunz, et al. Standards Track [Page 10]
RFC 5698 DSSC November 2009
3.8. AlgorithmIdentifier
The AlgorithmIdentifier element is used to identify a cryptographic
algorithm. It consists of the algorithm name, at least one OID, and
optional URIs. The algorithm name is not intended to be parsed by
automatic processes. It is only intended to be read by humans. The
OID MUST be expressed in dot notation (e.g., "1.3.14.3.2.26"). The
element is defined as follows:
3.9. Evaluation
The Evaluation element contains the evaluation of one cryptographic
algorithm in dependence of its parameter constraints. For example,
the suitability of the RSA algorithm depends on the modulus length
(RSA with a modulus length of 1024 may have another suitability
period as RSA with a modulus length of 2048). Current hash
algorithms like SHA-1 or RIPEMD-160 do not have any parameters.
Therefore, the Parameter element is optional. The suitability of the
algorithm is expressed by a validity period, which is defined by the
Validity element. An optional wildcard MAY be used to extend the
Evaluation element.
Kunz, et al. Standards Track [Page 11]
RFC 5698 DSSC November 2009
3.10. Parameter
The Parameter element is used to express constraints on algorithm-
specific parameters.
The Parameter element has a name attribute, which holds the name of
the parameter (e.g., "moduluslength" for RSA [RFC3447]). Section 5
defines parameter names for currently known public key algorithms;
these parameter names SHOULD be used. For the actual parameter, a
range of values or an exact value may be defined. These constraints
are expressed by the following elements:
Min: The Min element defines the minimum value of the parameter.
That means values equal or greater than the given value meet the
requirements.
Max: The Max element defines the maximum value the parameter may
take.
At least one of both elements MUST be set to define a range of
values. A range MAY also be specified by a combination of both
elements, whereas the value of the Min element MUST be less than or
equal to the value of the Max element. The parameter may have any
value within the defined range, including the minimum and maximum
values. An exact value is expressed by using the same value in both
the Min and the Max element.
These constraints are sufficient for all current algorithms. If
future algorithms need constraints that cannot be expressed by the
elements above, an arbitrary XML structure MAY be inserted that meets
the new constraints. For this reason, the Parameter element contains
a wildcard. A parameter MUST contain at least one constraint. The
schema for the Parameter element is as follows:
Kunz, et al. Standards Track [Page 12]
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3.11. Validity
The Validity element is used to define the period of the (predicted)
suitability of the algorithm. It is composed of an optional start
date and an optional end date. Defining no end date means the
algorithm has an open-end validity. Of course, this may be
restricted by a future policy that sets an end date for the
algorithm. If the end of the validity period is in the past, the
algorithm was suitable until that end date. The element is defined
by the following schema:
3.12. Information
The Information element MAY be used to give additional textual
information about the algorithm or the evaluation, e.g., references
on algorithm specifications. The element is defined as follows:
Kunz, et al. Standards Track [Page 13]
RFC 5698 DSSC November 2009
3.13. Signature
The optional Signature element MAY be used to guarantee the integrity
and authenticity of the policy. It is an XML signature specified in
[RFC3275]. The signature MUST relate to the
SecuritySuitabilityPolicy element. If the Signature element is set,
the SecuritySuitabilityPolicy element MUST have the optional id
attribute. This attribute MUST be used to reference the
SecuritySuitabilityPolicy element within the Signature element.
Since it is an enveloped signature, the signature MUST use the
transformation algorithm identified by the following URI:
http://www.w3.org/2000/09/xmldsig#enveloped-signature
4. DSSC Policies
DSSC policies MUST be expressed either in XML or ASN.1. However, in
order to reach interoperability, DSSC policies SHOULD be published in
both XML and ASN.1.
In the case of XML, a DSSC policy is an XML document that MUST be
well-formed and SHOULD be valid. XML-encoded DSSC policies MUST be
based on XML 1.0 [W3C.REC-xml-20081126] and MUST be encoded using
UTF-8 [RFC3629]. This specification makes use of XML namespaces
[W3C.REC-xml-names-20060816] for identifying DSSC policies. The
namespace URI for elements defined by this specification is a URN
[RFC2141] using the namespace prefix "dssc". This URN is:
urn:ietf:params:xml:ns:dssc
XML-encoded DSSC policies are identified with the MIME type
"application/dssc+xml" and are instances of the XML schema
[W3C.REC-xmlschema-1-20041028] defined in Appendix B.
A file containing a DSSC policy in ASN.1 representation (for
specification of ASN.1 refer to [CCITT.x208.1988], [CCITT.x209.1988],
[CCITT.x680.2002] and [CCITT.x690.2002]) MUST contain only the DER
encoding of one DSSC policy, i.e., there MUST NOT be extraneous
header or trailer information in the file. ASN.1-based DSSC policies
are identified with the MIME type "application/dssc+der".
Appropriate ASN.1 modules are defined in Appendices C (1988-ASN.1
syntax) and D (1997-ASN.1 syntax).
Kunz, et al. Standards Track [Page 14]
RFC 5698 DSSC November 2009
5. Definition of Parameters
This section defines the parameter names for the currently known
public key algorithms. The following parameters also refer to
cryptographic schemes based on these public key algorithms (e.g., the
PKCS#1 v1.5 signature scheme SHA-256 with RSA [RFC3447]).
The parameter of RSA [RFC3447] SHOULD be named "moduluslength".
The parameters for the Digital Signature Algorithm (DSA)
[FIPS186-2] SHOULD be "plength" and "qlength".
These parameter names have been registered by IANA (see Section 8).
It may be necessary to register further algorithms not given in this
section (in particular, future algorithms). The process for
registering parameter names of further algorithms is described in
Section 8. Publishers of policies SHOULD use these parameter names
so that the correct interpretation is guaranteed.
6. Processing
Evaluation of an algorithm's security suitability is described in
three parts: verification of the policy, determination of algorithm
validity, and evaluation of algorithm parameters, if any.
In the following sections, a process is described
o to determine if an algorithm was suitable at a particular point of
time, and
o to determine until what time an algorithm was or will be suitable.
6.1. Inputs
To determine the security suitability of an algorithm, the following
information is required:
o Policy
o Current time
o Algorithm identifier and parameter constraints (if associated)
o Time of interest (optional). Providing no time of interest means
determination of the validity end date of the algorithm.
Kunz, et al. Standards Track [Page 15]
RFC 5698 DSSC November 2009
6.2. Verify Policy
The signature on the policy SHOULD be verified and a certification
path from the policy signer's certificate to a current trust anchor
SHOULD be constructed and validated [RFC5280]. The algorithms used
to verify the digital signature and validate the certification path
MUST be suitable per the contents of the policy being verified. If
signature verification fails, certification path validation fails or
an unsuitable algorithm is required to perform these checks, then the
policy MUST be rejected.
The nextUpdate time in the policy MUST be either greater than the
current time or absent. If the nextUpdate time is less than the
current time, the policy MUST be rejected.
6.3. Algorithm Evaluation
To determine the validity period of an algorithm, locate the
Algorithm element in the policy that corresponds to the algorithm
identifier provided as input. The Algorithm element is located by
comparing the OID in the element to the OID included in the algorithm
identifier provided as input.
If no matching Algorithm element is found, then the algorithm is
unknown.
If the time of interest was provided as input, the validity of each
Evaluation element MUST be checked in order to determine if the
algorithm was suitable at the time of interest. For each Evaluation
element:
o Confirm the Start time is either less than the time of interest or
absent. Discard the entry if the Start time is present and
greater than the time of interest.
o Confirm the End time is either greater than the time of interest
or absent. Discard the entry if the End time is present and less
than the time of interest.
If all Evaluation elements were rejected, the algorithm is not
suitable according to the policy.
Any entries not rejected will be used for the evaluation of the
parameters, if any.
Kunz, et al. Standards Track [Page 16]
RFC 5698 DSSC November 2009
6.4. Evaluation of Parameters
Any necessary parameters of the entries not rejected MUST be
evaluated within the context of the type and usage of the algorithm.
Details of parameter evaluation are defined on a per-algorithm basis.
To evaluate the parameters, the Parameter elements of each Evaluation
element that has not been rejected in the process described in
Section 6.3 MUST be checked. For each Parameter element:
o Confirm that the parameter was provided as input. Discard the
Evaluation element if the parameter does not match to any of the
parameters provided as input.
o If the Parameter element has a Min element, confirm that the
parameter value is less than or equal to the corresponding
parameter provided as input. Discard the Evaluation element if
the parameter value does not meet the constraint.
o If the Parameter element has a Max element, confirm that the
parameter value is greater than or equal to the corresponding
parameter provided as input. Discard the Evaluation element if
the parameter value does not meet the constraint.
o If the Parameter has another constraint, confirm that the value of
the corresponding parameter provided as input meets this
constraint. If it does not or if the constraint is unrecognized,
discard the Evaluation element.
If all Evaluation elements were rejected, the algorithm is not
suitable according to the policy.
Any entries not rejected will be provided as output.
6.5. Output
If the algorithm is not in the policy, return an error "algorithm
unknown".
If no time of interest was provided as input, return the maximum End
time of the Evaluation elements that were not discarded. If at least
one End time of these Evaluation elements is absent, return
"algorithm has an indefinite End time".
Otherwise, if the algorithm is not suitable relative to the time of
interest, return an error "algorithm unsuitable".
Kunz, et al. Standards Track [Page 17]
RFC 5698 DSSC November 2009
If the algorithm is suitable relative to the time of interest, return
the Evaluation elements that were not discarded.
7. Security Considerations
The policy for an algorithm's security suitability has a great impact
on the quality of the results of signature generation and
verification operations. If an algorithm is incorrectly evaluated
against a policy, signatures with a low probative force could be
created or verification results could be incorrect. The following
security considerations have been identified:
1. Publishers MUST ensure unauthorized manipulation of any security
suitability is not possible prior to a policy being signed and
published. There is no mechanism provided to revoke a policy
after publication. Since the algorithm evaluations change
infrequently, the lifespan of a policy should be carefully
considered prior to publication.
2. Operators SHOULD only accept policies issued by a trusted
publisher. Furthermore, the validity of the certificate used to
sign the policy SHOULD be verifiable by Certificate Revocation
List (CRL) [RFC5280] or Online Certificate Status Protocol (OCSP)
[RFC2560]. The certificate used to sign the policy SHOULD be
revoked if the algorithms used in this certificate are no longer
suitable. It MUST NOT be possible to alter or replace a policy
once accepted by an operator.
3. Operators SHOULD periodically check to see if a new policy has
been published to avoid using obsolete policy information. For
publishers, it is suggested not to omit the NextUpdate element in
order to give operators a hint regarding when the next policy
will be published.
4. When signing a policy, algorithms that are suitable according to
this policy SHOULD be used.
5. The processing rule described in Section 6 is about one
cryptographic algorithm independent of the use case. Depending
upon the use case, an algorithm that is no longer suitable at the
time of interest, does not necessarily mean that the data
structure where it is used is no longer secure. For example, a
signature has been made with an RSA signer's key of 1024 bits.
This signature is timestamped with a timestamp token that uses an
RSA key of 2048 bits, before an RSA key size of 1024 bits will be
broken. The fact that the signature key of 1024 bits is no
longer suitable at the time of interest does not mean that the
Kunz, et al. Standards Track [Page 18]
RFC 5698 DSSC November 2009
whole data structure is no longer secure, if an RSA key size of
2048 bits is still suitable at the time of interest.
6. In addition to the key size considerations, other considerations
must be applied, like whether a timestamp token has been provided
by a trusted authority. This means that the simple use of a
suitability policy is not the single element to consider when
evaluating the security of a complex data structure that uses
several cryptographic algorithms.
7. The policies described in this document are suitable to evaluate
basic cryptographic algorithms, like public key or hash
algorithms, as well as cryptographic schemes (e.g., the PKCS#1
v1.5 signature schemes [RFC3447]). But it MUST be kept in mind
that a basic cryptographic algorithm that is suitable according
to the policy does not necessarily mean that any cryptographic
schemes based on this algorithm are also secure. For example, a
signature scheme based on RSA must not necessarily be secure if
RSA is suitable. In case of a complete signature verification,
including validation of the certificate path, various algorithms
have to be checked against the policy (i.e., signature schemes of
signed data objects and revocation information, public key
algorithms of the involved certificates, etc.). Thus, a policy
SHOULD contain evaluations of public key and hash algorithms as
well as of signature schemes.
8. Re-encrypting documents that were originally encrypted using an
algorithm that is no longer suitable will not protect the
semantics of the document if the document has been intercepted.
However, for documents stored in an encrypted form, re-encryption
must be considered, unless the document has lost its original
value.
8. IANA Considerations
This document defines the XML namespace "urn:ietf:params:xml:ns:dssc"
according to the guidelines in [RFC3688]. This namespace has been
registered in the IANA XML Registry.
This document defines an XML schema (see Appendix B) according to the
guidelines in [RFC3688]. This XML schema has been registered in the
IANA XML Registry and can be identified with the URN
"urn:ietf:params:xml:schema:dssc".
This document defines the MIME type "application/dssc+xml". This
MIME type has been registered by IANA under "MIME Media Types"
according to the procedures of [RFC4288].
Kunz, et al. Standards Track [Page 19]
RFC 5698 DSSC November 2009
Type name: application
Subtype name: dssc+xml
Required parameters: none
Optional parameters: "charset" as specified for "application/xml"
in [RFC3023].
Encoding considerations: Same as specified for "application/xml"
in [RFC3023].
Security considerations: Same as specified for "application/xml"
in Section 10 of [RFC3023]. For further security considerations,
see Section 7 of this document.
Interoperability considerations: Same as specified for
"application/xml" in [RFC3023].
Published specification: This document.
Applications that use this media: Applications for long-term
archiving of signed data, applications for signing data /
verifying signed data, and applications for encrypting /
decrypting data.
Additional information:
Magic number(s): none
File extension(s): .xdssc
Macintosh file type code: "TEXT"
Object Identifiers: none
Person to contact for further information: Thomas Kunz
(thomas.kunz@sit.fraunhofer.de)
Intended usage: COMMON
Restrictions on usage: none
Author/Change controller: IETF
This document defines the MIME type "application/dssc+der". This
MIME type has been registered by IANA under "MIME Media Types"
according to the procedures of [RFC4288].
Kunz, et al. Standards Track [Page 20]
RFC 5698 DSSC November 2009
Type name: application
Subtype name: dssc+der
Required parameters: none
Optional parameters: none
Encoding considerations: binary
Security considerations: See Section 7 of this document.
Interoperability considerations: none
Published specification: This document.
Applications that use this media: Applications for long-term
archiving of signed data, applications for signing data /
verifying signed data, and applications for encrypting /
decrypting data.
Additional information:
Magic number(s): none
File extension(s): .dssc
Macintosh file type code: none
Object Identifiers: none
Person to contact for further information: Thomas Kunz
(thomas.kunz@sit.fraunhofer.de)
Intended usage: COMMON
Restrictions on usage: none
Author/Change controller: IETF
This specification creates a new IANA registry entitled "Data
Structure for the Security Suitability of Cryptographic Algorithms
(DSSC)". This registry contains two sub-registries entitled
"Parameter Definitions" and "Cryptographic Algorithms". The policy
for future assignments to the sub-registry "Parameter Definitions" is
"RFC Required".
Kunz, et al. Standards Track [Page 21]
RFC 5698 DSSC November 2009
The initial values for the "Parameter Definitions" sub-registry are:
Value Description Reference
-------------- ------------------------------- ------------------
moduluslength Parameter for RSA RFC 5698
(integer value)
plength Parameter for DSA RFC 5698
(integer value, used together
with parameter "qlength")
qlength Parameter for DSA RFC 5698
(integer value, used together
with parameter "plength")
The sub-registry "Cryptographic Algorithms" contains textual names as
well as Object Identifiers (OIDs) and Uniform Resource Identifiers
(URIs) of cryptographic algorithms. It serves as assistance when
creating a new policy. The policy for future assignments is "First
Come First Served". When registering a new algorithm, the following
information MUST be provided:
o The textual name of the algorithm.
o The OID of the algorithm.
o A reference to a publicly available specification that defines the
algorithm and its identifiers.
Optionally, a URI MAY be provided if possible.
The initial values for the "Cryptographic Algorithms" sub-registry
are:
Kunz, et al. Standards Track [Page 22]
RFC 5698 DSSC November 2009
Name OID / URI Reference
----------------------- --------------------------------- ----------
rsaEncryption 1.2.840.113549.1.1.1 [RFC3447]
dsa 1.2.840.10040.4.1 [RFC3279]
md2 1.2.840.113549.2.2 [RFC3279]
md5 1.2.840.113549.2.5 [RFC3279]
http://www.w3.org/2001/04/xmldsig-more#md5 [RFC4051]
sha-1 1.3.14.3.2.26 [RFC3279]
http://www.w3.org/2000/09/xmldsig#sha1 [RFC3275]
sha-224 2.16.840.1.101.3.4.2.4 [RFC4055]
http://www.w3.org/2001/04/xmldsig-more#sha224 [RFC4051]
sha-256 2.16.840.1.101.3.4.2.1 [RFC4055]
sha-384 2.16.840.1.101.3.4.2.2 [RFC4055]
http://www.w3.org/2001/04/xmldsig-more#sha384 [RFC4051]
sha-512 2.16.840.1.101.3.4.2.3 [RFC4055]
md2WithRSAEncryption 1.2.840.113549.1.1.2 [RFC3443]
md5WithRSAEncryption 1.2.840.113549.1.1.4 [RFC3443]
http://www.w3.org/2001/04/xmldsig-more#rsa-md5 [RFC4051]
sha1WithRSAEncryption 1.2.840.113549.1.1.5 [RFC3443]
http://www.w3.org/2000/09/xmldsig#rsa-sha1 [RFC3275]
sha256WithRSAEncryption 1.2.840.113549.1.1.11 [RFC3443]
http://www.w3.org/2001/04/xmldsig-more#rsa-sha256 [RFC4051]
sha384WithRSAEncryption 1.2.840.113549.1.1.12 [RFC3443]
http://www.w3.org/2001/04/xmldsig-more#rsa-sha384 [RFC4051]
sha512WithRSAEncryption 1.2.840.113549.1.1.13 [RFC3443]
http://www.w3.org/2001/04/xmldsig-more#rsa-sha512 [RFC4051]
sha1WithDSA 1.2.840.10040.4.3 [RFC3279]
http://www.w3.org/2000/09/xmldsig#dsa-sha1 [RFC3275]
Kunz, et al. Standards Track [Page 23]
RFC 5698 DSSC November 2009
9. References
9.1. Normative References
[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, "AASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3275] Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
Language) XML-Signature Syntax and Processing", RFC 3275,
March 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4998] Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
Record Syntax (ERS)", RFC 4998, August 2007.
Kunz, et al. Standards Track [Page 24]
RFC 5698 DSSC November 2009
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 5652, September 2009.
[W3C.REC-xml-20081126]
Yergeau, F., Maler, E., Paoli, J., Sperberg-McQueen, C.,
and T. Bray, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
.
[W3C.REC-xml-names-20060816]
Layman, A., Hollander, D., Tobin, R., and T. Bray,
"Namespaces in XML 1.0 (Second Edition)", World Wide Web
Consortium Recommendation REC-xml-names-20060816,
August 2006,
.
[W3C.REC-xmlschema-1-20041028]
Thompson, H., Beech, D., Mendelsohn, N., and M. Maloney,
"XML Schema Part 1: Structures Second Edition", World Wide
Web Consortium Recommendation REC-xmlschema-1-20041028,
October 2004,
.
9.2. Informative References
[BNetzAg.2008]
Federal Network Agency for Electricity, Gas,
Telecommunications, Post and Railway, "Bekanntmachung zur
elektronischen Signatur nach dem Signaturgesetz und der
Signaturverordnung (Uebersicht ueber geeignete
Algorithmen)", December 2007,
.
[CCITT.x208.1988]
International Telephone and Telegraph Consultative
Committee, "Specification of Abstract Syntax Notation One
(ASN.1)", CCITT Recommendation X.208, November 1988.
Kunz, et al. Standards Track [Page 25]
RFC 5698 DSSC November 2009
[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, November 1988.
[ETSI-TS101903]
European Telecommunication Standards Institute (ETSI),
"XML Advanced Electronic Signatures (XAdES)", ETSI TS 101
903 V1.3.2, March 2006.
[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 V2.0.0, November 2007.
[FIPS186-2]
National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", FIPS PUB 186-2 with Change
Notice, January 2000.
[NIST.800-57-Part1.2006]
National Institute of Standards and Technology,
"Recommendation for Key Management - Part 1: General
(Revised)", NIST 800-57 Part 1, May 2006.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks", RFC
3443, January 2003.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4051] Eastlake, D., "Additional XML Security Uniform Resource
Identifiers (URIs)", RFC 4051, April 2005.
Kunz, et al. Standards Track [Page 26]
RFC 5698 DSSC November 2009
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
June 2005.
[RFC4810] Wallace, C., Pordesch, U., and R. Brandner, "Long-Term
Archive Service Requirements", RFC 4810, March 2007.
Kunz, et al. Standards Track [Page 27]
RFC 5698 DSSC November 2009
Appendix A. DSSC and ERS
A.1. Verification of Evidence Records Using DSSC (Informative)
This section describes the verification of an Evidence Record
according to the Evidence Record Syntax (ERS, [RFC4998]), using the
presented data structure.
An Evidence Record contains a sequence of ArchiveTimeStampChains,
which consist of ArchiveTimeStamps. For each ArchiveTimeStamp the
hash algorithm used for the hash tree (digestAlgorithm) as well as
the public key algorithm and hash algorithm in the timestamp
signature have to be examined. The relevant date is the time
information in the timestamp (date of issue). Starting with the
first ArchiveTimeStamp, it has to be assured that:
1. The timestamp uses public key and hash algorithms that were
suitable at the date of issue.
2. The hashtree was built with a hash algorithm that was suitable at
the date of issue as well.
3. Algorithms for timestamp and hashtree in the preceding
ArchiveTimeStamp must have been suitable at the issuing date of
considered ArchiveTimeStamp.
4. Algorithms in the last ArchiveTimeStamp have to be suitable now.
If the check of one of these items fails, this will lead to a failure
of the verification.
A.2. Storing DSSC Policies in Evidence Records (Normative)
This section describes how to store a policy in an Evidence Record.
ERS provides the field cryptoInfos for the storage of additional
verification data. For the integration of a security suitability
policy in an Evidence Record, the following content types are defined
for both ASN.1 and XML representation:
DSSC_ASN1 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-ct(1) id-ct-dssc-asn1(2) }
DSSC_XML {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-ct(1) id-ct-dssc-xml(3) }
Kunz, et al. Standards Track [Page 28]
RFC 5698 DSSC November 2009
Appendix B. XML Schema (Normative)
< xml version="1.0" encoding="UTF-8"?>
Kunz, et al. Standards Track [Page 29]
RFC 5698 DSSC November 2009
Kunz, et al. Standards Track [Page 30]
RFC 5698 DSSC November 2009
Appendix C. ASN.1 Module in 1988 Syntax (Informative)
ASN.1-Module
DSSC {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-mod(0) id-mod-dssc88(6) id-mod-dssc88-v1(1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORT ALL --
IMPORTS
-- Import from RFC 5280 [RFC5280]
-- Delete following import statement
-- if "new" types are supported
UTF8String FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Import from RFC 5652 [RFC5652]
ContentInfo FROM CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms-2004(24)}
;
SecuritySuitabilityPolicy ::= ContentInfo
-- contentType is id-signedData as defined in [RFC5652]
-- content is SignedData as defined in [RFC5652]
-- eContentType within SignedData is id-ct-dssc
-- eContent within SignedData is TBSPolicy
Kunz, et al. Standards Track [Page 31]
RFC 5698 DSSC November 2009
id-ct-dssc OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-ct(1) id-ct-dssc-tbsPolicy(6) }
TBSPolicy ::= SEQUENCE {
version INTEGER DEFAULT {v1(1)},
language UTF8String DEFAULT "en",
policyName PolicyName,
publisher Publisher,
policyIssueDate GeneralizedTime,
nextUpdate GeneralizedTime OPTIONAL,
usage UTF8String OPTIONAL,
algorithms SEQUENCE OF Algorithm
}
PolicyName ::= SEQUENCE {
name UTF8String,
oid OBJECT IDENTIFIER OPTIONAL,
uri IA5String OPTIONAL
}
Publisher ::= SEQUENCE {
name UTF8String,
address [0] UTF8String OPTIONAL,
uri [1] IA5String OPTIONAL
}
Algorithm ::= SEQUENCE {
algorithmIdentifier AlgID,
evaluations SEQUENCE OF Evaluation,
information [0] SEQUENCE OF UTF8String OPTIONAL,
other [1] Extension OPTIONAL
}
Extension ::= SEQUENCE {
extensionType OBJECT IDENTIFIER,
extension ANY DEFINED BY extensionType
}
AlgID ::= SEQUENCE {
name UTF8String,
oid [0] SEQUENCE OF OBJECT IDENTIFIER,
uri [1] SEQUENCE OF IA5String OPTIONAL
}
Evaluation ::= SEQUENCE {
parameters [0] SEQUENCE OF Parameter OPTIONAL,
Kunz, et al. Standards Track [Page 32]
RFC 5698 DSSC November 2009
validity [1] Validity,
other [2] Extension OPTIONAL
}
Parameter ::= SEQUENCE {
name UTF8String,
min [0] INTEGER OPTIONAL,
max [1] INTEGER OPTIONAL,
other [2] Extension OPTIONAL
}
Validity ::= SEQUENCE {
start [0] GeneralizedTime OPTIONAL,
end [1] GeneralizedTime OPTIONAL
}
END
Appendix D. ASN.1 Module in 1997 Syntax (Normative)
ASN.1-Module
DSSC {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-mod(0) id-mod-dssc(7) id-mod-dssc-v1(1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORT ALL --
IMPORTS
-- Import from RFC 5280 [RFC5280]
-- Delete following import statement
-- if "new" types are supported
UTF8String FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Import from RFC 5652 [RFC5652]
ContentInfo FROM CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms-2004(24)}
Kunz, et al. Standards Track [Page 33]
RFC 5698 DSSC November 2009
;
SecuritySuitabilityPolicy ::= ContentInfo
-- contentType is id-signedData as defined in [RFC5652]
-- content is SignedData as defined in [RFC5652]
-- eContentType within SignedData is id-ct-dssc
-- eContent within SignedData is TBSPolicy
id-ct-dssc OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
ltans(11) id-ct(1) id-ct-dssc-tbsPolicy(6) }
TBSPolicy ::= SEQUENCE {
version INTEGER DEFAULT {v1(1)},
language UTF8String DEFAULT "en",
policyName PolicyName,
publisher Publisher,
policyIssueDate GeneralizedTime,
nextUpdate GeneralizedTime OPTIONAL,
usage UTF8String OPTIONAL,
algorithms SEQUENCE OF Algorithm
}
PolicyName ::= SEQUENCE {
name UTF8String,
oid OBJECT IDENTIFIER OPTIONAL,
uri IA5String OPTIONAL
}
Publisher ::= SEQUENCE {
name UTF8String,
address [0] UTF8String OPTIONAL,
uri [1] IA5String OPTIONAL
}
Algorithm ::= SEQUENCE {
algorithmIdentifier AlgID,
evaluations SEQUENCE OF Evaluation,
information [0] SEQUENCE OF UTF8String OPTIONAL,
other [1] Extension OPTIONAL
}
Extension ::= SEQUENCE {
extensionType EXTENSION-TYPE.&id ({SupportedExtensions}),
extension EXTENSION-TYPE.&Type
({SupportedExtensions}{@extensionType})
Kunz, et al. Standards Track [Page 34]
RFC 5698 DSSC November 2009
}
EXTENSION-TYPE ::= TYPE-IDENTIFIER
SupportedExtensions EXTENSION-TYPE ::= {...}
AlgID ::= SEQUENCE {
name UTF8String,
oid [0] SEQUENCE OF OBJECT IDENTIFIER,
uri [1] SEQUENCE OF IA5String OPTIONAL
}
Evaluation ::= SEQUENCE {
parameters [0] SEQUENCE OF Parameter OPTIONAL,
validity [1] Validity,
other [2] Extension OPTIONAL
}
Parameter ::= SEQUENCE {
name UTF8String,
min [0] INTEGER OPTIONAL,
max [1] INTEGER OPTIONAL,
other [2] Extension OPTIONAL
}
Validity ::= SEQUENCE {
start [0] GeneralizedTime OPTIONAL,
end [1] GeneralizedTime OPTIONAL
}
END
Appendix E. Example
The following example shows a policy that may be used for signature
verification. It contains hash algorithms, public key algorithms,
and signature schemes. SHA-1 as well as RSA with modulus length of
1024 are examples for expired algorithms.
Evaluation of cryptographic algorithmsSome Evaluation Authority2009-01-01T00:00:00
Kunz, et al. Standards Track [Page 35]
RFC 5698 DSSC November 2009
Digital signature verificationSHA-11.3.14.3.2.262008-06-30SHA-2562.16.840.1.101.3.4.2.12014-12-31SHA-5122.16.840.1.101.3.4.2.32014-12-31RSA1.2.840.113549.1.1.110242008-03-31
Kunz, et al. Standards Track [Page 36]
RFC 5698 DSSC November 2009
20482014-12-31DSA1.2.840.10040.4.110241602007-12-3120482242014-12-31PKCS#1 v1.5 SHA-1 with RSA1.2.840.113549.1.1.510242008-03-31
Kunz, et al. Standards Track [Page 37]
RFC 5698 DSSC November 2009
20482008-06-30PKCS#1 v1.5 SHA-256 with RSA1.2.840.113549.1.1.1110242008-03-3120482014-12-31PKCS#1 v1.5 SHA-512 with RSA1.2.840.113549.1.1.1310242008-03-312048
Kunz, et al. Standards Track [Page 38]
RFC 5698 DSSC November 2009
2014-12-31SHA-1 with DSA1.2.840.10040.4.310241602007-12-3120482242008-06-30
Kunz, et al. Standards Track [Page 39]
RFC 5698 DSSC November 2009
Authors' Addresses
Thomas Kunz
Fraunhofer Institute for Secure Information Technology
Rheinstrasse 75
Darmstadt D-64295
Germany
EMail: thomas.kunz@sit.fraunhofer.de
Susanne Okunick
pawisda systems GmbH
Robert-Koch-Strasse 9
Weiterstadt D-64331
Germany
EMail: susanne.okunick@pawisda.de
Ulrich Pordesch
Fraunhofer Gesellschaft
Rheinstrasse 75
Darmstadt D-64295
Germany
EMail: ulrich.pordesch@zv.fraunhofer.de
Kunz, et al. Standards Track [Page 40]
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