Long-term Archive And Notary T. Kunz
Services (LTANS) Fraunhofer Institute for Secure
Internet-Draft Information Technology
Intended status: Standards Track S. Okunick
Expires: May 7, 2009 pawisda systems GmbH
U. Pordesch
Fraunhofer Gesellschaft
November 3, 2008
Data Structure for the Security Suitability of Cryptographic Algorithms
(DSSC)
draft-ietf-ltans-dssc-05.txt
Status of this Memo
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Abstract
In many application areas it must be possible to prove the existence
and integrity of digital signed data. This proof depends on the
security suitability of the cryptographic algorithms used to generate
or verify the digital signature. Because algorithms can become weak
over the years, it is necessary to periodically evaluate their
security suitability. When signing or verifying data, these
evaluations must be considered. This document specifies a data
structure that enables automated analysis of the security suitability
of cryptographic algorithms.
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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].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Requirements and Assumptions . . . . . . . . . . . . . . . . . 8
2.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 8
3. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. SecuritySuitabilityPolicy . . . . . . . . . . . . . . . . 10
3.2. PolicyName . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Publisher . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4. Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5. PolicyIssueDate . . . . . . . . . . . . . . . . . . . . . 12
3.6. NextUpdate . . . . . . . . . . . . . . . . . . . . . . . . 12
3.7. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.8. Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 12
3.9. AlgorithmIdentifier . . . . . . . . . . . . . . . . . . . 13
3.10. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 13
3.11. Parameter . . . . . . . . . . . . . . . . . . . . . . . . 13
3.12. Validity . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.13. Information . . . . . . . . . . . . . . . . . . . . . . . 15
3.14. Signature . . . . . . . . . . . . . . . . . . . . . . . . 16
4. Definition of Parameters . . . . . . . . . . . . . . . . . . . 17
5. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1. Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Verify policy . . . . . . . . . . . . . . . . . . . . . . 18
5.3. Algorithm evaluation . . . . . . . . . . . . . . . . . . . 18
5.4. Evaluation of parameters . . . . . . . . . . . . . . . . . 19
5.5. Output . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. DSSC and ERS . . . . . . . . . . . . . . . . . . . . 25
A.1. Verification of Evidence Records using DSSC . . . . . . . 25
A.2. Storing DSSC Policies in Evidence Records . . . . . . . . 25
Appendix B. XML schema (normative) . . . . . . . . . . . . . . . 26
Appendix C. ASN.1 Module in 1988 Syntax (informative) . . . . . . 29
Appendix D. ASN.1 Module in 1997 Syntax (normative) . . . . . . . 32
Appendix E. Example . . . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
Intellectual Property and Copyright Statements . . . . . . . . . . 41
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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.
Very few algorithms satisfy the security requirements and are
suitable for usage in signatures. 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.
Many kinds of digital signed data, including signed documents, time
stamps, certificates, and revocation lists, are affected, in
particular 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, verifying or renewing signatures. 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 an insecure algorithm is used
in a signature or whether a signature has been properly preserved.
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 [NIST.800-57-Part1.2006] and
[ETSI-TS102176-1-2005].
These evaluations are published in documents intended to be read by
humans. Therefore it is necessary to define a data structure that
expresses the content of the evaluations to enable automated
processing. 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. This document does not attempt to
catalog the security properties of cryptographic algorithms.
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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.
Operator: Instance which 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 which is evaluated against a policy
and determined to be valid, i.e. resistant against attacks, at a
particular point of time.
1.3. Use Cases
In the following some use cases for a security suitability policy are
presented.
Long-term archiving: The most important use case is long-term
archiving of signed data. Algorithms or their parameters become
insecure over long time periods. 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 or to provide all algorithms which 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.
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Signing and verifying: When signing documents, certificates or
attestations, e.g. within an LTAP transaction
[I-D.ietf-ltans-ltap], it must be assured that the algorithms used
for signing or verifying are suitable. Accordingly, when
verifying CMS [RFC3852] or XML signatures [RFC3275]
[ETSI-TS101903], not only the validity of the certificates may be
checked but also the validity of the algorithms.
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.
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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.
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 designed independent of the intended use, e.g.,
encryption, signing, verifying, and signature renewing. Thus, the
interpretion of the data structure is same for every use case.
Source authentication: Policies may be published by different
institutions, e.g. on national or 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 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.
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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 about preventing
conflicts within a policy.
Assertions made in the policy are suitable at least until the next
policy is published.
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.
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3. Data Structures
This section describes the syntax of a security suitability policy
defined as an XML schema. ASN.1 modules are defined in Appendix C
and Appendix D. The schema uses the following namespace:
http://www.sit.fraunhofer.de/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"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:dssc="http://www.sit.fraunhofer.de/dssc"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
targetNamespace="http://www.sit.fraunhofer.de/dssc"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd"/>
<xs:import namespace="http://www.w3.org/2000/09/xmldsig#"
schemaLocation="xmldsig-core-schema.xsd"/>
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 3.14). The element is
defined by the following schema:
<xs:element name="SecuritySuitabilityPolicy"
type="dssc:SecuritySuitabilityPolicyType"/>
<xs:complexType name="SecuritySuitabilityPolicyType">
<xs:sequence>
<xs:element ref="dssc:PolicyName"/>
<xs:element ref="dssc:Publisher"/>
<xs:element name="PolicyIssueDate" type="xs:dateTime"/>
<xs:element name="NextUpdate" type="xs:dateTime" minOccurs="0"/>
<xs:element name="Usage" type="xs:string" minOccurs="0"/>
<xs:element ref="dssc:Algorithm" maxOccurs="unbounded"/>
<xs:element ref="ds:Signature" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="version" type="xs:string" default="1"/>
<xs:attribute name="id" type="xs:ID"/>
</xs:complexType>
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3.2. PolicyName
The PolicyName element consists of an arbitrary name of the policy
and an optional Uniform Resource Identifier (URI).
<xs:element name="PolicyName" type="dssc:PolicyNameType"/>
<xs:complexType name="PolicyNameType">
<xs:sequence>
<xs:element ref="dssc:Name"/>
<xs:element ref="dssc:URI" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Name" type="xs:string"/>
<xs:element name="URI" type="xs:anyURI"/>
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.
<xs:element name="Publisher" type="dssc:PublisherType"/>
<xs:complexType name="PublisherType">
<xs:sequence>
<xs:element ref="dssc:Name"/>
<xs:element ref="dssc:Address" minOccurs="0"/>
<xs:element ref="dssc:URI" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
3.4. Address
The Address element consists of the street, the locality, the
optional state or province, the postal code, and the country.
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<xs:element name="Address" type="dssc:AddressType"/>
<xs:complexType name="AddressType">
<xs:sequence>
<xs:element name="Street" type="xs:string"/>
<xs:element name="Locality" type="xs:string"/>
<xs:element name="StateOrProvince" type="xs:string" minOccurs="0"/>
<xs:element name="PostalCode" type="xs:string"/>
<xs:element name="Country" type="xs:string"/>
</xs:sequence>
</xs:complexType>
3.5. PolicyIssueDate
The PolicyIssueDate element indicates the point of time when the
policy was issued.
3.6. NextUpdate
The optional NextUpdate element may be used to indicate when the next
policy will be issued.
3.7. Usage
The optional Usage element determines the intended use of the policy
(e.g. certificate validation, signing and verifying documents).
3.8. 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 Algorithm element is defined by the following
schema:
<xs:element name="Algorithm" type="dssc:AlgorithmType"/>
<xs:complexType name="AlgorithmType">
<xs:sequence>
<xs:element ref="dssc:AlgorithmIdentifier"/>
<xs:element ref="dssc:Evaluation" maxOccurs="unbounded"/>
<xs:element ref="dssc:Information" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
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3.9. AlgorithmIdentifier
The AlgorithmIdentifier element is used to identify a cryptographic
algorithm. It consists of the algorithm name, at least one object
identifer, and optional URIs. The element is defined as follows:
<xs:element name="AlgorithmIdentifier"
type="dssc:AlgorithmIdentifierType"/>
<xs:complexType name="AlgorithmIdentifierType">
<xs:sequence>
<xs:element ref="dssc:Name"/>
<xs:element name="ObjectIdentifier" type="xs:string"
maxOccurs="unbounded"/>
<xs:element ref="dssc:URI" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
3.10. Evaluation
The evaluation element contains the evaluation of one cryptographic
algorithm in dependence of its parameter contraints. E.g. 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.
<xs:element name="Evaluation" type="dssc:EvaluationType"/>
<xs:complexType name="EvaluationType">
<xs:sequence>
<xs:element ref="dssc:Parameter" minOccurs="0"
maxOccurs="unbounded"/>
<xs:element ref="dssc:Validity"/>
</xs:sequence>
</xs:complexType>
3.11. Parameter
The Parameter element is used to express constraints on algorithm
specific parameters like the "moduluslength" parameter in case of
RSA.
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The Parameter element has a name attribute which holds the name of
the parameter (e.g. "moduluslength" for RSA [RFC3447]). Besides a
better readability of the policy, the attribute may be used by
implementations for output messages. In Section 4 the parameter
names of currently known signature algorithms are defined. For the
actual parameter, an exact value or a range of values may be defined.
These constraints are expressed by the following elements:
Exact: The Exact element specifies the exact value of the parameter.
Min: The Min element defines the minimum value of the parameter.
That means, also all other values greater than the given one meet
the requirements.
Max: The Max element defines the maximum value the parameter may
take.
Range: The Range element is used to define a range of values,
consisting of a minimum and a maximum value. The parameter may
have any value within the defined range, including the minimum and
maximum values.
For one algorithm it is recommended not to mix these elements in
order to avoid inconsistencies.
These constraints are sufficient for all current algorithms. If
future algorithms will need constraints which cannot be expressed by
the elements above, an arbitrary XML structure may be inserted which
meets the new constraints. For this reason, the Parameter element
contains an "any" element. The schema for the Parameter element is
as follows:
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<xs:element name="Parameter" type="dssc:ParameterType"/>
<xs:complexType name="ParameterType">
<xs:choice>
<xs:element name="Exact" type="xs:string"/>
<xs:element ref="dssc:Min"/>
<xs:element ref="dssc:Max"/>
<xs:element name="Range">
<xs:complexType>
<xs:sequence>
<xs:element ref="dssc:Min"/>
<xs:element ref="dssc:Max"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:any namespace="##other"/>
</xs:choice>
<xs:attribute name="name" type="xs:string" use="required"/>
</xs:complexType>
<xs:element name="Min" type="xs:string"/>
<xs:element name="Max" type="xs:string"/>
3.12. 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 which 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:
<xs:element name="Validity" type="dssc:ValidityType"/>
<xs:complexType name="ValidityType">
<xs:sequence>
<xs:element name="Start" type="xs:date" minOccurs="0"/>
<xs:element name="End" type="xs:date" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
3.13. 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:
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<xs:element name="Information" type="dssc:InformationType"/>
<xs:complexType name="InformationType">
<xs:sequence>
<xs:element name="Text" maxOccurs="unbounded">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="lang"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
3.14. 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
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4. Definition of Parameters
This section defines the parameter names for the currently known
public key algorithms. The signature algorithms RSA [RFC3447] and
DSA [FIPS.186-1.1998] are always used in conjunction with a one-way
hash algorithm. RSA with RIPEMD-160 is such a combined algorithm
with its own object identifier. RSA and DSA may be combined with the
suitable hash algorithms SHA-1, SHA-224, SHA-256, SHA-384, SHA-512,
and RIPEMD-160. The following parameters refer to the appropriate
combined algorithms as well.
The parameter of RSA should be named "moduluslength".
The parameters for DSA should be "plength" and "qlength".
Publishers of policies must use the same parameter names, so that the
correct interpretation is guaranteed.
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5. Processing
Evaluation of an algorithm's security suitability is described in
three parts: verification of the policy, determination of algorithm
validity at time of interest, and evaluation of algorithm parameters,
if any.
In the following, a process is described that can be used to
determine if an algorithm was suitable at a particular point of time.
5.1. 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
5.2. 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 greater than the current
time or absent. If the nextUpdate time is less than the current
time, the policy MUST be rejected.
5.3. Algorithm evaluation
To determine the validity period of an algorithm relative to the time
of interest, locate the Algorithm element in the policy that
corresponds to the algorithm identifier provided as input. The
Algorithm element is located by comparing the object identifier in
the element to the object identifier included in the algorithm
identifier provided as input.
If no matching Algorithm element is found, then the algorithm is not
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suitable according the policy.
If an Algorithm element is found, the validity of each Evaluation
element MUST be checked. For each Evaluation element,
o Confirm the Start time is 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 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 the policy.
Any entries not rejected will be used for the evaluation of the
parameters, if any.
5.4. Evaluation of parameters
After confirming an algorithm is suitable relative to the time of
interest, any necessary parameters 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 5.3 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 an Exact element, confirm that the
parameter value exactly complies with the according parameter
provided as input. Discard the Evaluation element if the
parameter value does not comply.
o If the Parameter element has a Min element, confirm that the
parameter value is less than or equal to the according 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 according
parameter provided as input. Discard the Evaluation element if
the parameter value does not meet the constraint.
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o If the Parameter element has a Range element, confirm that the
value of the according parameter provided as input is within the
range. 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 according 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 the policy.
Any entries not rejected will be provided as output.
5.5. Output
If the algorithm is not suitable, return an error.
If the algorithm is suitable, return the Evaluation elements that
were not discarded.
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6. Security Considerations
The policy for algorithm's security suitability has 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 is 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. It must not be possible to alter or replace a
security suitability once accepted by the client.
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, when the next policy will be
published.
4. When signing a policy, algorithms should be used which are
suitable according this policy.
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7. IANA Considerations
This document has no actions for IANA. Section can be removed prior
to publication as an RFC.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3275] Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
Language) XML-Signature Syntax and Processing", RFC 3275,
March 2002.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
[RFC4998] Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
Record Syntax (ERS)", RFC 4998, August 2007.
[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.
8.2. 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,
<http://www.bundesnetzagentur.de/media/archive/12198.pdf>.
[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.
[FIPS.186-1.1998]
National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-1, December 1998,
<http://csrc.nist.gov/fips/fips1861.pdf>.
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[I-D.ietf-ltans-ltap]
Jerman-Blazic, A., Sylvester, P., and C. Wallace, "Long-
term Archive Protocol (LTAP)", draft-ietf-ltans-ltap-07
(work in progress), November 2008.
[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.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4810] Wallace, C., Pordesch, U., and R. Brandner, "Long-Term
Archive Service Requirements", RFC 4810, March 2007.
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Appendix A. DSSC and ERS
A.1. Verification of Evidence Records using DSSC
This section gives an informative description of 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) and 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 which have been
suitable at the date of issue.
2. The hashtree was build with an hash algorithm that has been
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 ArchiveTimstamp 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
This normative 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) }
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Appendix B. XML schema (normative)
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:dssc="http://www.sit.fraunhofer.de/dssc"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
targetNamespace="http://www.sit.fraunhofer.de/dssc"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd"/>
<xs:import namespace="http://www.w3.org/2000/09/xmldsig#"
schemaLocation="xmldsig-core-schema.xsd"/>
<xs:element name="SecuritySuitabilityPolicy"
type="dssc:SecuritySuitabilityPolicyType"/>
<xs:complexType name="SecuritySuitabilityPolicyType">
<xs:sequence>
<xs:element ref="dssc:PolicyName"/>
<xs:element ref="dssc:Publisher"/>
<xs:element name="PolicyIssueDate" type="xs:dateTime"/>
<xs:element name="NextUpdate" type="xs:dateTime" minOccurs="0"/>
<xs:element name="Usage" type="xs:string" minOccurs="0"/>
<xs:element ref="dssc:Algorithm" maxOccurs="unbounded"/>
<xs:element ref="ds:Signature" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="version" type="xs:string" default="1"/>
<xs:attribute name="id" type="xs:ID"/>
</xs:complexType>
<xs:element name="PolicyName" type="dssc:PolicyNameType"/>
<xs:complexType name="PolicyNameType">
<xs:sequence>
<xs:element ref="dssc:Name"/>
<xs:element ref="dssc:URI" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Publisher" type="dssc:PublisherType"/>
<xs:complexType name="PublisherType">
<xs:sequence>
<xs:element ref="dssc:Name"/>
<xs:element ref="dssc:Address" minOccurs="0"/>
<xs:element ref="dssc:URI" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Name" type="xs:string"/>
<xs:element name="URI" type="xs:anyURI"/>
<xs:element name="Address" type="dssc:AddressType"/>
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<xs:complexType name="AddressType">
<xs:sequence>
<xs:element name="Street" type="xs:string"/>
<xs:element name="Locality" type="xs:string"/>
<xs:element name="StateOrProvince" type="xs:string"
minOccurs="0"/>
<xs:element name="PostalCode" type="xs:string"/>
<xs:element name="Country" type="xs:string"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Algorithm" type="dssc:AlgorithmType"/>
<xs:complexType name="AlgorithmType">
<xs:sequence>
<xs:element ref="dssc:AlgorithmIdentifier"/>
<xs:element ref="dssc:Evaluation" maxOccurs="unbounded"/>
<xs:element ref="dssc:Information" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:element name="AlgorithmIdentifier"
type="dssc:AlgorithmIdentifierType"/>
<xs:complexType name="AlgorithmIdentifierType">
<xs:sequence>
<xs:element ref="dssc:Name"/>
<xs:element name="ObjectIdentifier" type="xs:string"
maxOccurs="unbounded"/>
<xs:element ref="dssc:URI" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Validity" type="dssc:ValidityType"/>
<xs:complexType name="ValidityType">
<xs:sequence>
<xs:element name="Start" type="xs:date" minOccurs="0"/>
<xs:element name="End" type="xs:date" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Information" type="dssc:InformationType"/>
<xs:complexType name="InformationType">
<xs:sequence>
<xs:element name="Text" maxOccurs="unbounded">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="lang"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:sequence>
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</xs:complexType>
<xs:element name="Evaluation" type="dssc:EvaluationType"/>
<xs:complexType name="EvaluationType">
<xs:sequence>
<xs:element ref="dssc:Parameter" minOccurs="0"
maxOccurs="unbounded"/>
<xs:element ref="dssc:Validity"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Parameter" type="dssc:ParameterType"/>
<xs:complexType name="ParameterType">
<xs:choice>
<xs:element name="Exact" type="xs:string"/>
<xs:element ref="dssc:Min"/>
<xs:element ref="dssc:Max"/>
<xs:element name="Range">
<xs:complexType>
<xs:sequence>
<xs:element ref="dssc:Min"/>
<xs:element ref="dssc:Max"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:any namespace="##other"/>
</xs:choice>
<xs:attribute name="name" type="xs:string" use="required"/>
</xs:complexType>
<xs:element name="Min" type="xs:string"/>
<xs:element name="Max" type="xs:string"/>
</xs:schema>
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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
-- Imports from RFC 5280 [RFC5280]
-- and RFC 3852 [RFC3852], Section 7.1
UTF8String FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
ContentInfo FROM CryptographicMessageSyntax
{ iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1)}
;
SecuritySuitabilityPolicy ::= ContentInfo
-- contentType is id-signedData as defined in [RFC3852]
-- content is SignedData as defined in [RFC3852]
-- 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 { v1(1) } OPTIONAL,
policyName PolicyName,
publisher Publisher,
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policyIssueDate GeneralizedTime,
nextUpdate GeneralizedTime OPTIONAL,
usage UTF8String OPTIONAL,
algorithms SEQUENCE OF Algorithm
}
PolicyName ::= SEQUENCE {
name UTF8String,
oid OBJECT IDENTIFIER OPTIONAL
}
Publisher ::= SEQUENCE {
name UTF8String,
address [0] Address OPTIONAL,
uri [1] IA5String OPTIONAL
}
Address ::= SEQUENCE {
street [0] UTF8String,
locality [1] UTF8String,
stateOrProvince [2] UTF8String OPTIONAL,
postalCode [3] UTF8String,
country [4] UTF8String
}
Algorithm ::= SEQUENCE {
algorithmIdentifier AlgID,
evaluations SEQUENCE OF Evaluation,
information [0] SEQUENCE OF UTF8String OPTIONAL
}
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
}
Parameter ::= SEQUENCE {
name UTF8String,
constraint CHOICE {
exact [0] OCTET STRING,
min [1] OCTET STRING,
max [2] OCTET STRING,
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range [3] Range,
other [4] OtherConstraints
}
}
OtherConstraints ::= SEQUENCE {
otherConstraintType OBJECT IDENTIFIER,
otherConstraint ANY DEFINED BY otherConstraintType
}
Range ::= SEQUENCE {
min [0] OCTET STRING,
max [1] OCTET STRING
}
Validity ::= SEQUENCE {
start [0] GeneralizedTime OPTIONAL,
end [1] GeneralizedTime OPTIONAL
}
END
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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
-- Imports from RFC 5280 [RFC5280]
-- and RFC 3852 [RFC3852], Section 7.1
UTF8String FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
ContentInfo FROM CryptographicMessageSyntax
{ iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1)}
;
SecuritySuitabilityPolicy ::= ContentInfo
-- contentType is id-signedData as defined in [RFC3852]
-- content is SignedData as defined in [RFC3852]
-- 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 { v1(1) } OPTIONAL,
policyName PolicyName,
publisher Publisher,
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policyIssueDate GeneralizedTime,
nextUpdate GeneralizedTime OPTIONAL,
usage UTF8String OPTIONAL,
algorithms SEQUENCE OF Algorithm
}
PolicyName ::= SEQUENCE {
name UTF8String,
oid OBJECT IDENTIFIER OPTIONAL
}
Publisher ::= SEQUENCE {
name UTF8String,
address [0] Address OPTIONAL,
uri [1] IA5String OPTIONAL
}
Address ::= SEQUENCE {
street [0] UTF8String,
locality [1] UTF8String,
stateOrProvince [2] UTF8String OPTIONAL,
postalCode [3] UTF8String,
country [4] UTF8String
}
Algorithm ::= SEQUENCE {
algorithmIdentifier AlgID,
evaluations SEQUENCE OF Evaluation,
information [0] SEQUENCE OF UTF8String OPTIONAL
}
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
}
Parameter ::= SEQUENCE {
name UTF8String,
constraint CHOICE {
exact [0] OCTET STRING,
min [1] OCTET STRING,
max [2] OCTET STRING,
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range [3] Range,
other [4] OtherConstraints
}
}
OtherConstraints ::= SEQUENCE {
otherConstraintType CONSTRAINT-TYPE.&id ({SupportedConstraints}),
otherConstraint CONSTRAINT-TYPE.&Type
({SupportedConstraints}{@otherConstraintType})
}
CONSTRAINT-TYPE ::= TYPE-IDENTIFIER
SupportedConstraints CONSTRAINT-TYPE ::= {...}
Range ::= SEQUENCE {
min [0] OCTET STRING,
max [1] OCTET STRING
}
Validity ::= SEQUENCE {
start [0] GeneralizedTime OPTIONAL,
end [1] GeneralizedTime OPTIONAL
}
END
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Appendix E. Example
In the following an example of a policy is presented. It is
generated on the basis of the last evaluation of the German Federal
Network Agency ([BNetzAg.2008]). The policy consists on hash
algorithms as well as public key algorithms. RSA with modulus length
of 768 is an example for an expired algorithm.
<SecuritySuitabilityPolicy xmlns="http://www.sit.fraunhofer.de/dssc"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<PolicyName>
<name>Evaluation of suitable signature algorithms 2008</Name>
</PolicyName>
<Publisher>
<Name>Federal Network Agency</Name>
</Publisher>
<PolicyIssueDate>2007-12-17T00:00:00</PolicyIssueDate>
<Usage>Qualified electronic signatures</Usage>
<Algorithm>
<AlgorithmIdentifier>
<Name>SHA-1</Name>
<ObjectIdentifier>1.3.14.3.2.26</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Validity>
<End>2008-06-31</End>
</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>RIPEMD-160</Name>
<ObjectIdentifier>1.3.36.3.2.1</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Validity>
<End>2010-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>SHA-224</Name>
<ObjectIdentifier>2.16.840.1.101.3.4.2.4</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Validity>
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<End>2014-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>SHA-256</Name>
<ObjectIdentifier>2.16.840.1.101.3.4.2.1</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Validity>
<End>2014-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>SHA-384</Name>
<ObjectIdentifier>2.16.840.1.101.3.4.2.2</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Validity>
<End>2014-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>SHA-512</Name>
<ObjectIdentifier>2.16.840.1.101.3.4.2.3</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Validity>
<End>2014-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>RSA</Name>
<ObjectIdentifier>1.2.840.113549.1.1.1</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Parameter name="moduluslength">
<Min>768</Min>
</Parameter>
<Validity>
<End>2000-12-31</End>
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</Validity>
</Evaluation>
<Evaluation>
<Parameter name="moduluslength">
<Min>1024</Min>
</Parameter>
<Validity>
<End>2008-03-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="moduluslength">
<Min>1280</Min>
</Parameter>
<Validity>
<End>2008-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="moduluslength">
<Min>1536</Min>
</Parameter>
<Validity>
<End>2009-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="moduluslength">
<Min>1728</Min>
</Parameter>
<Validity>
<End>2010-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="moduluslength">
<Min>1976</Min>
</Parameter>
<Validity>
<End>2014-12-31</End>
</Validity>'
</Evaluation>
<Evaluation>
<Parameter name="moduluslength">
<Min>2048</Min>
</Parameter>
<Validity>
<End>2014-12-31</End>
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</Validity>
</Evaluation>
</Algorithm>
<Algorithm>
<AlgorithmIdentifier>
<Name>DSA</Name>
<ObjectIdentifier>1.2.840.10040.4.1</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Parameter name="plength">
<Min>1024</Min>
</Parameter>
<Parameter name="qlength">
<Min>160</Min>
</Parameter>
<Validity>
<End>2007-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="plength">
<Min>1280</Min>
</Parameter>
<Parameter name="qlength">
<Min>160</Min>
</Parameter>
<Validity>
<End>2008-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="plength">
<Min>1536</Min>
</Parameter>
<Parameter name="qlength">
<Min>160</Min>
</Parameter>
<Validity>
<End>2009-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="plength">
<Min>2048</Min>
</Parameter>
<Parameter name="qlength">
<Min>160</Min>
</Parameter>
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<Validity>
<End>2009-12-31</End>
</Validity>
</Evaluation>
<Evaluation>
<Parameter name="plength">
<Min>2048</Min>
</Parameter>
<Parameter name="qlength">
<Min>224</Min>
</Parameter>
<Validity>
<End>2014-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
</SecuritySuitabilityPolicy>
Combined algorithms should also be part of the policy since some
programs know the object identifiers of combined algorithms instead
of the general public key algorithm. The following excerpt describes
a combined algorithm. The validity end date is given by the end
dates of RSA and RIPEMD-160, in particular it is the former one.
Combined algorithms could replace the public key algorithms in the
policy example. They could also be listed together with public key
algorithms.
<Algorithm>
<AlgorithmIdentifier>
<Name>RIPEMD-160 with RSA 2048</Name>
<ObjectIdentifier>1.3.36.3.3.1.2</ObjectIdentifier>
</AlgorithmIdentifier>
<Evaluation>
<Parameter name="moduluslength">
<Min>2048</Min>
</Parameter>
<Validity>
<End>2010-12-31</End>
</Validity>
</Evaluation>
</Algorithm>
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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
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Internet-Draft DSSC November 2008
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