RFC 5126
CMS Advanced Electronic Signatures (CAdES)
4. Overview
The present document defines a number of Electronic Signature (ES) formats that build on CMS (RFC 3852 [4]) by adding signed and unsigned attributes. This section: - provides an introduction to the major parties involved (Section 4.1),
- introduces the concept of signature policies (Section 4.2),
- provides an overview of the various ES formats (Section 4.3),
- introduces the concept of validation data, and provides an
overview of formats that incorporate validation data
(Section 4.4), and
- presents relevant considerations on arbitration
(Section 4.5) and for the validation process (Section 4.6).
The formal specifications of the attributes are specified in Sections
5 and 6; Annexes C and D provide rationale for the definitions of the
different ES forms.
4.1. Major Parties
The major parties involved in a business transaction supported by
electronic signatures, as defined in the present document, are:
- the signer;
- the verifier;
- Trusted Service Providers (TSP); and
- the arbitrator.
The signer is the entity that creates the electronic signature. When
the signer digitally signs over data using the prescribed format,
this represents a commitment on behalf of the signing entity to the
data being signed.
The verifier is the entity that validates the electronic signature;
it may be a single entity or multiple entities.
The Trusted Service Providers (TSPs) are one or more entities that
help to build trust relationships between the signer and verifier.
They support the signer and verifier by means of supporting services
including: user certificates, cross-certificates, time-stamp tokens,
CRLs, ARLs, and OCSP responses. The following TSPs are used to
support the functions defined in the present document:
- Certification Authorities;
- Registration Authorities;
- CRL Issuers;
- OCSP Responders;
- Repository Authorities (e.g., a Directory);
- Time-Stamping Authorities;
- Time-Marking Authorities; and
- Signature Policy Issuers.
Certification Authorities provide users with public key certificates
and a revocation service.
Registration Authorities allow the identification and registration of
entities before a CA generates certificates.
Repository Authorities publish CRLs issued by CAs, signature policies
issued by Signature Policy Issuers, and optionally public key
certificates.
Time-Stamping Authorities attest that some data was formed before a
given trusted time.
Time-Marking Authorities record that some data was formed before a
given trusted time.
Signature Policy Issuers define the signature policies to be used by
signers and verifiers.
In some cases, the following additional TSPs are needed:
- Attribute Authorities.
Attributes Authorities provide users with attributes linked to public
key certificates.
An Arbitrator is an entity that arbitrates in disputes between a
signer and a verifier.
4.2. Signature Policies
The present document includes the concept of signature policies that
can be used to establish technical consistency when validating
electronic signatures.
When a comprehensive signature policy used by the verifier is either
explicitly indicated by the signer or implied by the data being
signed, then a consistent result can be obtained when validating an
electronic signature.
When the signature policy being used by the verifier is neither
indicated by the signer nor can be derived from other data, or the
signature policy is incomplete, then verifiers, including
arbitrators, may obtain different results when validating an
electronic signature. Therefore, comprehensive signature policies
that ensure consistency of signature validation are recommended from
both the signer's and verifier's point of view.
Further information on signature policies is provided in:
- TR 102 038 [TR102038];
- Sections 5.8.1, C.1, and C.3.1 of the present document;
- RFC 3125 [RFC3125]; and
- TR 102 272 [TR102272].
4.3. Electronic Signature Formats
The current section provides an overview for two forms of CMS
advanced electronic signature specified in the present document,
namely, the CAdES Basic Electronic Signature (CAdES-BES) and the
CAdES Explicit Policy-based Electronic Signature (CAdES-EPES).
Conformance to the present document mandates that the signer create
one of these formats.
4.3.1. CAdES Basic Electronic Signature (CAdES-BES)
A CAdES Basic Electronic Signature (CAdES-BES), in accordance with
the present document, contains:
- The signed user data (e.g., the signer's document), as defined
in CMS (RFC 3852 [4]);
- A collection of mandatory signed attributes, as defined in CMS
(RFC 3852 [4]) and in ESS (RFC 2634 [5]);
- Additional mandatory signed attributes, defined in the present
document; and
- The digital signature value computed on the user data and, when
present, on the signed attributes, as defined in CMS (RFC 3852
[4]).
A CAdES Basic Electronic Signature (CAdES-BES), in accordance with
the present document, may contain:
- a collection of additional signed attributes; and
- a collection of optional unsigned attributes.
The mandatory signed attributes are:
- Content-type. It is defined in RFC 3852 [4] and specifies the
type of the EncapsulatedContentInfo value being signed. Details
are provided in Section 5.7.1 of the present document.
Rationale for its inclusion is provided in Annex C.3.7;
- Message-digest. It is defined in RFC 3852 [4] and specifies the
message digest of the eContent OCTET STRING within
encapContentInfo being signed. Details are provided in Section
5.7.2;
- ESS signing-certificate OR ESS signing-certificate-v2. The ESS
signing-certificate attribute is defined in Enhanced Security
Services (ESS), RFC 2634 [5], and only allows for the use of
SHA-1 as a digest algorithm. The ESS signing-certificate-v2
attribute is defined in "ESS Update: Adding CertID Algorithm
Agility", RFC 5035 [15], and allows for the use of any digest
algorithm. A CAdES-BES claiming compliance with the present
document must include one of them. Section 5.7.3 provides the
details of these attributes. Rationale for its inclusion is
provided in Annex C.3.3.
Optional signed attributes may be added to the CAdES-BES, including
optional signed attributes defined in CMS (RFC 3852 [4]), ESS (RFC
2634 [5]), and the present document. Listed below are optional
attributes that are defined in Section 5 and have a rationale
provided in Annex C:
- Signing-time: as defined in CMS (RFC 3852 [4]), indicates the
time of the signature, as claimed by the signer. Details and
short rationale are provided in Section 5.9.1. Annex C.3.6
provides the rationale.
- content-hints: as defined in ESS (RFC 2634 [5]), provides
information that describes the innermost signed content of a
multi-layer message where one content is encapsulated in
another. Section 5.10.1 provides the specification details.
Annex C.3.8 provides the rationale.
- content-reference: as defined in ESS (RFC 2634 [5]), can be
incorporated as a way to link request and reply messages in an
exchange between two parties. Section 5.10.1 provides the
specification details. Annex C.3.9 provides the rationale.
- content-identifier: as defined in ESS (RFC 2634 [5]), contains
an identifier that may be used later on in the previous
content-reference attribute. Section 5.10.2 provides the
specification details.
- commitment-type-indication: this attribute is defined by the
present document as a way to indicate the commitment endorsed by
the signer when producing the signature. Section 5.11.1
provides the specification details. Annex C.3.2 provides the
rationale.
- signer-location: this attribute is defined by the present
document. It allows the signer to indicate the place where the
signer purportedly produced the signature. Section 5.11.2
provides the specification details. Annex C.3.5 provides the
rationale.
- signer-attributes: this attribute is defined by the present
document. It allows a claimed or certified role to be
incorporated into the signed information. Section 5.11.3
provides the specification details. Annex C.3.4 provides the
rationale.
- content-time-stamp: this attribute is defined by the present
document. It allows a time-stamp token of the data to be signed
to be incorporated into the signed information. It provides
proof of the existence of the data before the signature was
created. Section 5.11.4 provides the specification details.
Annex C.3.6 provides the rationale.
A CAdES-BES form can also incorporate instances of unsigned
attributes, as defined in CMS (RFC 3852 [4]) and ESS (RFC 2634 [5]).
- CounterSignature, as defined in CMS (RFC 3852 [4]); it can be
incorporated wherever embedded signatures (i.e., a signature on
a previous signature) are needed. Section 5.9.2 provides the
specification details. Annex C.5 in Annex C provides the
rationale.
The structure of the CAdES-BES is illustrated in Figure 1.
+------Elect.Signature (CAdES-BES)------+
|+----------------------------------- + |
||+---------+ +----------+ | |
|||Signer's | | Signed | Digital | |
|||Document | |Attributes| Signature | |
||| | | | | |
||+---------+ +----------+ | |
|+------------------------------------+ |
+---------------------------------------+
Figure 1: Illustration of a CAdES-BES
The signer's conformance requirements of a CAdES-BES are defined in
Section 8.1.
NOTE: The CAdES-BES is the minimum format for an electronic
signature to be generated by the signer. On its own, it does not
provide enough information for it to be verified in the longer
term. For example, revocation information issued by the relevant
certificate status information issuer needs to be available for
long-term validation (see Section 4.4.2).
The CAdES-BES satisfies the legal requirements for electronic
signatures, as defined in the European Directive on Electronic
Signatures, (see Annex C for further discussion on the relationship
of the present document to the Directive). It provides basic
authentication and integrity protection.
The semantics of the signed data of a CAdES-BES or its context may
implicitly indicate a signature policy to the verifier.
Specification of the contents of signature policies is outside the
scope of the present document. However, further information on
signature policies is provided in TR 102 038 [TR102038], RFC 3125
[RFC3125], and Sections 5.8.1, C.1, and C.3.1 of the present
document.
4.3.2. CAdES Explicit Policy-based Electronic Signatures (CAdES-EPES)
A CAdES Explicit Policy-based Electronic Signature (CAdES-EPES), in
accordance with the present document, extends the definition of an
electronic signature to conform to the identified signature policy.
A CAdES Explicit Policy-based Electronic Signature (CAdES-EPES)
incorporates a signed attribute (sigPolicyID attribute) indicating
the signature policy that shall be used to validate the electronic
signature. This signed attribute is protected by the signature. The
signature may also have other signed attributes required to conform
to the mandated signature policy.
Section 5.7.3 provides the details on the specification of
signature-policy-identifier attribute. Annex C.1 provides a short
rationale. Specification of the contents of signature policies is
outside the scope of the present document.
Further information on signature policies is provided in TR 102 038
[TR102038] and Sections 5.8.1, C.1, and C.3.1 of the present
document.
The structure of the CAdES-EPES is illustrated in Figure 2. +------------- Elect.Signature (CAdES-EPES) ---------------+ | | |+-------------------------------------------------------+ | || +-----------+ | | || | | +---------------------------+ | | || | | | +----------+ | | | || | Signer's | | |Signature | Signed | Digital | | || | Document | | |Policy ID | Attributes |Signature| | || | | | +----------+ | | | || | | +---------------------------+ | | || +-----------+ | | |+-------------------------------------------------------+ | | | +----------------------------------------------------------+ Figure 2: Illustration of a CAdES-EPES The signer's conformance requirements of CAdES-EPES are defined in Section 8.2.4.4. Electronic Signature Formats with Validation Data
Validation of an electronic signature, in accordance with the present document, requires additional data needed to validate the electronic signature. This additional data is called validation data, and includes: - Public Key Certificates (PKCs); - revocation status information for each PKC; - trusted time-stamps applied to the digital signature, otherwise a time-mark shall be available in an audit log. - when appropriate, the details of a signature policy to be used to verify the electronic signature. The validation data may be collected by the signer and/or the verifier. When the signature-policy-identifier signed attribute is present, it shall meet the requirements of the signature policy.
Validation data includes CA certificates as well as revocation status information in the form of Certificate Revocation Lists (CRLs) or certificate status information (OCSP) provided by an online service. Validation data also includes evidence that the signature was created before a particular point in time; this may be either a time-stamp token or time-mark. The present document defines unsigned attributes able to contain validation data that can be added to CAdES-BES and CAdES-EPES, leading to electronic signature formats that include validation data. The sections below summarize these formats and their most relevant characteristics.4.4.1. Electronic Signature with Time (CAdES-T)
An electronic signature with time (CAdES-T), in accordance with the present document, is when there exits trusted time associated with the ES. The trusted time may be provided by: - a time-stamp attribute as an unsigned attribute added to the ES; and - a time-mark of the ES provided by a Trusted Service Provider. The time-stamp attribute contains a time-stamp token of the electronic signature value. Section 6.1.1 provides the specification details. Annex C.4.3 provides the rationale. A time-mark provided by a Trusted Service would have a similar effect to the signature-time-stamp attribute, but in this case, no attribute is added to the ES, as it is the responsibility of the TSP to provide evidence of a time-mark when required to do so. The management of time marks is outside the scope of the present document. Trusted time provides the initial steps towards providing long-term validity. Electronic signatures with the time-stamp attribute or a time-marked BES/EPES, forming the CAdES-T are illustrated in Figure 3.
+-------------------------------------------------CAdES-T ---------+ |+------ CAdES-BES or CAdES-EPES -------+ | ||+-----------------------------------+ | +----------------------+ | |||+---------+ +----------+ | | | | | ||||Signer's | | Signed | Digital | | | Signature-time-stamp | | ||||Document | |Attributes| Signature | | | attribute required | | |||| | | | | | | when using time | | |||+---------+ +----------+ | | | stamps. | | ||+-----------------------------------+ | | | | |+--------------------------------------+ | or the BES/EPES | | | | shall be time-marked | | | | | | | | Management and | | | | provision of time | | | | mark is the | | | | responsibility of | | | | the TSP. | | | +----------------------+ | +------------------------------------------------------------------+ Figure 3: Illustration of CAdES-T formats NOTE 1: A time-stamp token is added to the CAdES-BES or CAdES-EPES as an unsigned attribute. NOTE 2: Time-stamp tokens that may themselves include unsigned attributes required to validate the time-stamp token, such as the complete-certificate-references and complete-revocation-references attributes, as defined by the present document.4.4.2. ES with Complete Validation Data References (CAdES-C)
Electronic Signature with Complete validation data references (CAdES-C), in accordance with the present document, adds to the CAdES-T the complete-certificate-references and complete-revocation-references attributes, as defined by the present document. The complete-certificate-references attribute contains references to all the certificates present in the certification path used for verifying the signature. The complete-revocation-references attribute contains references to the CRLs and/or OCSPs responses used for verifying the signature. Section 6.2 provides the specification details. Storing the references allows the values of the certification path and the CRLs or OCSPs responses to be stored elsewhere, reducing the size of a stored electronic signature format.
Sections C.4.1 to C.4.2 provide rationale on the usage of validation data and when it is suitable to generate the CAdES-C form. Electronic signatures, with the additional validation data forming the CAdES-C, are illustrated in Figure 4. +------------------------- CAdES-C --------------------------------+ |+----------------------------- CAdES-T ---------+ | || +----------+ | +-------------+ | || |Timestamp | | | | | || |attribute | | | | | ||+- CAdES-BES or CAdES-EPES ------+|over | | | | | ||| ||digital | | | Complete | | |||+---------++----------+ ||signature | | | certificate | | ||||Signer's || Signed | Digital ||is | | | and | | ||||Document ||Attributes|Signature||mandatory | | | revocation | | |||| || | ||if is not | | | references | | |||+---------++----------+ ||timemarked| | | | | ||+--------------------------------++----------+ | | | | |+-----------------------------------------------+ +-------------+ | +------------------------------------------------------------------+ Figure 4: Illustration of CAdES-C format NOTE 1: The complete certificate and revocation references are added to the CAdES-T as an unsigned attribute. NOTE 2: As a minimum, the signer will provide the CAdES-BES or, when indicating that the signature conforms to an explicit signing policy, the CAdES-EPES. NOTE 3: To reduce the risk of repudiating signature creation, the trusted time indication needs to be as close as possible to the time the signature was created. The signer or a TSP could provide the CAdES-T; if not, the verifier should create the CAdES-T on first receipt of an electronic signature because the CAdES-T provides independent evidence of the existence of the signature prior to the trusted time indication. NOTE 4: A CAdES-T trusted time indication must be created before a certificate has been revoked or expired. NOTE 5: The signer and TSP could provide the CAdES-C to minimize this risk, and when the signer does not provide the CAdES-C, the verifier should create the CAdES-C when the required component of revocation and validation data become available; this may require a grace period.
NOTE 6: A grace period permits certificate revocation information
to propagate through the revocation processes. This period could
extend from the time an authorized entity requests certificate
revocation to when the information is available for the relying
party to use. In order to make sure that the certificate was not
revoked at the time the signature was time-marked or time-stamped,
verifiers should wait until the end of the grace period. A
signature policy may define specific values for grace periods.
An illustration of a grace period is provided in Figure 5.
+<--------------Grace Period --------->+
----+-------+-------+--------+---------------------+----------+
^ ^ ^ ^ ^ ^
| | | | | |
| | | | | |
Signature | First | Second |
creation | revocation | revocation |
time | status | status |
| checking | checking |
| | |
Time-stamp Certification Build
or path CAdES-C
time-mark construction
over & verification
signature
Figure 5: Illustration of a grace period
NOTE 7: CWA 14171 [CWA14171] specifies a signature validation
process using CAdES-T, CAdES-C, and a grace period. Annex B
provides example validation processes. Annex C.4 provides
additional information about applying grace periods during the
validation process.
The verifier's conformance requirements are defined in Section 8.3
for time-stamped CAdES-C, and Section 8.4 for time-marked CAdES-C.
The present document only defines conformance requirements for the
verifier up to an ES with Complete validation data (CAdES-C). This
means that none of the extended and archive forms of electronic
signatures, as defined in Sections 4.4.3 to 4.4.4, need to be
implemented to achieve conformance to the present document.
4.4.3. Extended Electronic Signature Formats
CAdES-C can be extended by adding unsigned attributes to the
electronic signature. The present document defines various unsigned
attributes that are applicable for very long-term verification, and
for preventing some disaster situations that are discussed in Annex C. Annex B provides the details of the various extended formats, all the required unsigned attributes for each type, and how they can be used within the electronic signature validation process. The sections below give an overview of the various forms of extended signature formats in the present document.4.4.3.1. EXtended Long Electronic Signature (CAdES-X Long)
Extended Long format (CAdES-X Long), in accordance with the present document, adds the certificate-values and revocation-values attributes to the CAdES-C format. The first one contains the whole certificate path required for verifying the signature; the second one contains the CRLs and/OCSP responses required for the validation of the signature. This provides a known repository of certificate and revocation information required to validate a CAdES-C and prevents such information from getting lost. Sections 6.3.3 and 6.3.4 give specification details. Annex B.1.1 gives details on the production of the format. Annexes C4.1 to C.4.2 provide the rationale. The structure of the CAdES-X Long format is illustrated in Figure 6. +----------------------- CAdES-X-Long -----------------------------+ |+------------------------------------ CadES-C --+ | || +----------+ | +-------------+ | ||+------ CAdES -------------------+|Timestamp | | | | | ||| || over | | | Complete | | |||+---------++----------+ ||digital | | | certificate | | ||||Signer's || Signed | Digital ||signature | | | and | | ||||Document ||Attributes|Signature|| | | | revocation | | |||| || | ||Optional | | | data | | |||+---------++----------+ ||when | | | | | ||+--------------------------------+|timemarked| | | | | || +----------+ | | | | || +-------------+ | +-------------+ | || | Complete | | | || | certificate | | | || | and | | | || | revocation | | | || | references | | | || +-------------+ | | |+-----------------------------------------------+ | | | +------------------------------------------------------------------+ Figure 6: Illustration of CAdES-X-Long
4.4.3.2. EXtended Electronic Signature with Time Type 1 (CAdES-X Type 1)
Extended format with time type 1 (CAdES-X Type 1), in accordance with the present document, adds the CAdES-C-time-stamp attribute, whose content is a time-stamp token on the CAdES-C itself, to the CAdES-C format. This provides an integrity and trusted time protection over all the elements and references. It may protect the certificates, CRLs, and OCSP responses in case of a later compromise of a CA key, CRL key, or OCSP issuer key. Section 6.3.5 provides the specification details. Annex B.1.2 gives details on the production of the time-stamping process. Annex C.4.4.1 provides the rationale. The structure of the CAdES-X Type 1 format is illustrated in Figure 7. +----------------------- CAdES-X-Type 1 ------------------------------+ |+-------------------------------------- CAdES-C -----+ | || +-------------+ | +-----------+ | ||+--------- CAdES ------------------+| Timestamp | | | | | ||| || over | | | | | |||+---------++----------+ || digital | | | | | ||||Signer's || Signed | Digital || signature | | | Timestamp | | ||||Document ||Attributes| Signature || | | | over | | |||| || | || Optional | | | CAdES-C | | |||+---------++----------+ || when | | | | | ||+----------------------------------+| time-marked | | | | | || +-------------+ | | | | || +-------------+ | +-----------+ | || | Complete | | | || | certificate | | | || | and | | | || | revocation | | | || | references | | | || +-------------+ | | |+----------------------------------------------------+ | +---------------------------------------------------------------------+ Figure 7: Illustration of CAdES-X Type 1
4.4.3.3. EXtended Electronic Signature with Time Type 2 (CAdES-X Type 2)
Extended format with time type 2 (CAdES-X Type 2), in accordance with the present document, adds to the CAdES-C format the CAdES-C-time-stamped-certs-crls-references attribute, whose content is a time-stamp token on the certification path and revocation information references. This provides an integrity and trusted time protection over all the references. It may protect the certificates, CRLs and OCSP responses in case of a later compromise of a CA key, CRL key or OCSP issuer key. Both CAdES-X Type 1 and CAdES-X Type 2 counter the same threats, and the usage of one or the other depends on the environment. Section 6.3.5 provides the specification details. Annex B.1.3 gives details on the production of the time-stamping process. Annex C.4.4.2 provides the rationale. The structure of the CAdES-X Type 2 format is illustrated in Figure 8. +------------------------- CAdES-X-Type 2 ----------------------------+ |+----------------------------------------CAdES-C ---+ | || +------------+| | ||+----- CAdES -----------------------+| Timestamp || | ||| || over || | |||+---------+ +----------+ || digital || +-------------+| ||||Signer's | | Signed | Digital || signature || | Time-stamp || ||||Document | |Attributes| signature || || | only over || |||| | | | || optional || | complete || |||+---------+ +----------+ || when || | certificate || ||+-----------------------------------+| timemarked || | and || || +------------+| | revocation || || +-------------+ | | references || || | Complete | | +-------------+| || | certificate | | | || | and | | | || | revocation | | | || | references | | | || +-------------+ | | |+---------------------------------------------------+ | +---------------------------------------------------------------------+ Figure 8: Illustration of CAdES-X Type 2
4.4.3.4. EXtended Long Electronic Signature with Time (CAdES-X Long Type 1 or 2)
Extended Long with Time (CAdES-X Long Type 1 or 2), in accordance with the present document, is a combination of CAdES-X Long and one of the two former types (CAdES-X Type 1 and CAdES-X Type 2). Annex B.1.4 gives details on the production of the time-stamping process. Annex C.4.8 in Annex C provides the rationale. The structure of the CAdES-X Long Type 1 and CAdES-X Long Type 2 format is illustrated in Figure 9. +------------------ CAdES-X Long Type 1 or 2 -----------------------+ | +--------------+| |+-------------------------------------- CAdES-C --+|+------------+|| || ||| Timestamp ||| ||+------- CAdES --------------------++----------+ ||| over ||| ||| ||Timestamp | ||| CAdES-C ||| ||| ||over | ||+------------+|| |||+---------++----------+ ||digital | || OR || ||||Signer's || Signed | Digital ||signature | ||+------------+|| ||||Document ||Attributes| signature || | ||| Timestamp ||| |||| || | ||Optional | ||| only over ||| |||+---------++----------+ ||when | ||| complete ||| ||+----------------------------------+|timemarked| ||| certificate||| || +----------+ ||| and ||| || ||| Revocation ||| || +-------------+ ||| References ||| || | Complete | ||+------------+|| || | certificate | |+--------------+| || | and | | +------------+ | || | revocation | | | Complete | | || | references | | |certificate | | || +-------------+ | | and | | |+-------------------------------------------------+ |revocation | | | | value | | | +------------+ | +-------------------------------------------------------------------+ Figure 9: Illustration of CAdES-X Long Type 1 and CAdES Long Type 24.4.4. Archival Electronic Signature (CAdES-A)
Archival Form (CAdES-A), in accordance with the present document, builds on a CAdES-X Long or a CAdES-X Long Type 1 or 2 by adding one or more archive-time-stamp attributes. This form is used for archival of long-term signatures. Successive time-stamps protect the whole material against vulnerable hashing algorithms or the breaking
of the cryptographic material or algorithms. Section 6.4 contains the specification details. Sections C.4.5 and C.4.8 provide the rationale. The structure of the CAdES-A form is illustrated in Figure 10. +---------------------------CAdES-A ---------------------------------+ |+----------------------------------------------------+ | || +--------------+| +----------+ | ||+----------------------CAdES-C ----+|+------------+|| | | | ||| +----------+ ||| Timestamp ||| | | | |||+---- CAdES-BES ----+|Timestamp | ||| over ||| | | | |||| or CAdeS-EPES || over | ||| CAdES-C ||| | Archive | | |||| ||digital | ||+------------+|| | | | |||| ||signature | || or || |Timestamp | | |||| || | ||+------------+|| | | | |||| ||Optional | ||| Timestamp ||| | | | |||| ||when | ||| only over ||| | | | |||| ||Timemarked| ||| complete ||| | | | |||+-------------------+| | ||| certificate||| +----------+ | ||| +----------+ ||| and ||| | ||| +-------------+ ||| revocation ||| | ||| | Complete | ||| references ||| | ||| | certificate | ||+------------+|| | ||| | and | |+--------------+| | ||| | revocation | | +------------+ | | ||| | references | | | Complete | | | ||| +-------------+ | |certificate | | | ||| | | and | | | ||+----------------------------------+ |revocation | | | || | values | | | || +------------+ | | |+----------------------------------------------------+ | +--------------------------------------------------------------------+ Figure 10: Illustration of CAdES-A4.5. Arbitration
The CAdES-C may be used for arbitration should there be a dispute between the signer and verifier, provided that: - the arbitrator knows where to retrieve the signer's certificate (if not already present), all the cross-certificates and the required CRLs, ACRLs, or OCSP responses referenced in the CAdES-C;
- when time-stamping in the CAdES-T is being used, the certificate
from the TSU that has issued the time-stamp token in the CAdES-T
format is still within its validity period;
- when time-stamping in the CAdES-T is being used, the certificate
from the TSU that has issued the time-stamp token in the CAdES-T
format is not revoked at the time of arbitration;
- when time-marking in the CAdES-T is being used, a reliable audit
trail from the Time-Marking Authority is available for
examination regarding the time;
- none of the private keys corresponding to the certificates used
to verify the signature chain have ever been compromised;
- the cryptography used at the time the CAdES-C was built has not
been broken at the time the arbitration is performed; and
- if the signature policy can be explicitly or implicitly
identified, then an arbitrator is able to determine the rules
required to validate the electronic signature.
4.6. Validation Process
The validation process validates an electronic signature; the output
status of the validation process can be:
- invalid;
- incomplete validation; or
- valid.
An invalid response indicates that either the signature format is
incorrect or that the digital signature value fails verification
(e.g., the integrity check on the digital signature value fails, or
any of the certificates on which the digital signature verification
depends is known to be invalid or revoked).
An incomplete validation response indicates that the signature
validation status is currently unknown. In the case of incomplete
validation, additional information may be made available to the
application or user, thus allowing them to decide what to do with the
electronic signature. In the case of incomplete validation, the
electronic signature may be checked again at some later time when
additional information becomes available.
NOTE: For example, an incomplete validation may be because all the
required certificates are not available or the grace period is not
completed.
A valid response indicates that the signature has passed
verification, and it complies with the signature validation policy.
Example validation sequences are illustrated in Annex B.
5. Electronic Signature Attributes
This section builds upon the existing Cryptographic Message Syntax
(CMS), as defined in RFC 3852 [4], and Enhanced Security Services
(ESS), as defined in RFC 2634 [5]. The overall structure of an
Electronic Signature is as defined in CMS. The Electronic Signature
(ES) uses attributes defined in CMS, ESS, and the present document.
The present document defines ES attributes that it uses and that are
not defined elsewhere.
The mandated set of attributes and the digital signature value is
defined as the minimum Electronic Signature (ES) required by the
present document. A signature policy may mandate that other signed
attributes be present.
5.1. General Syntax
The general syntax of the ES is as defined in CMS (RFC 3852 [4]).
NOTE: CMS defines content types for id-data, id-signedData,
id-envelopedData, id-digestedData, id-encryptedData, and
id-authenticatedData. Although CMS permits other documents to
define other content types, the ASN.1 type defined should not be a
CHOICE type. The present document does not define other content
types.
5.2. Data Content Type
The data content type of the ES is as defined in CMS (RFC 3852 [4]).
NOTE: If the content type is id-data, it is recommended that the
content be encoded using MIME, and that the MIME type is used to
identify the presentation format of the data. See Annex F.1 for
an example of using MIME to identify the encoding type.
5.3. Signed-data Content Type
The Signed-data content type of the ES is as defined in CMS (RFC 3852
[4]).
5.4. SignedData Type
The syntax of the SignedData of the ES is as defined in CMS (RFC 3852 [4]). The fields of type SignedData are as defined in CMS (RFC 3852 [4]). The identification of a signer's certificate used to create the signature is always signed (see Section 5.7.3). The validation policy may specify requirements for the presence of certain certificates. The degenerate case, where there are no signers, is not valid in the present document.5.5. EncapsulatedContentInfo Type
The syntax of the EncapsulatedContentInfo type ES is as defined in CMS (RFC 3852 [4]). For the purpose of long-term validation, as defined by the present document, it is advisable that either the eContent is present, or the data that is signed is archived in such as way as to preserve any data encoding. It is important that the OCTET STRING used to generate the signature remains the same every time either the verifier or an arbitrator validates the signature. NOTE: The eContent is optional in CMS : - When it is present, this allows the signed data to be encapsulated in the SignedData structure, which then contains both the signed data and the signature. However, the signed data may only be accessed by a verifier able to decode the ASN.1 encoded SignedData structure. - When it is missing, this allows the signed data to be sent or stored separately from the signature, and the SignedData structure only contains the signature. It is, in the case of the signature, only the data that is signed that needs to be stored and distributed in such as way as to preserve any data encoding. The degenerate case where there are no signers is not valid in the present document.5.6. SignerInfo Type
The syntax of the SignerInfo type ES is as defined in CMS (RFC 3852 [4]).
Per-signer information is represented in the type SignerInfo. In the case of multiple independent signatures (see Annex B.5), there is an instance of this field for each signer. The fields of type SignerInfo have the meanings defined in CMS (RFC 3852 [4]), but the signedAttrs field shall contain the following attributes: - content-type, as defined in Section 5.7.1; and - message-digest, as defined in Section 5.7.2; - signing-certificate, as defined in Section 5.7.3.5.6.1. Message Digest Calculation Process
The message digest calculation process is as defined in CMS (RFC 3852 [4]).5.6.2. Message Signature Generation Process
The input to the message signature generation process is as defined in CMS (RFC 3852 [4]).5.6.3. Message Signature Verification Process
The procedures for message signature verification are defined in CMS (RFC 3852 [4]) and enhanced in the present document: the input to the signature verification process must be the signer's public key, which shall be verified as correct using the signing certificate reference attribute containing a reference to the signing certificate, i.e., when SigningCertificateV2 from RFC 5035 [16] or SigningCertificate from ESS [5] is used, the public key from the first certificate identified in the sequence of certificate identifiers from SigningCertificate must be the key used to verify the digital signature.5.7. Basic ES Mandatory Present Attributes
The following attributes shall be present with the signed-data defined by the present document. The attributes are defined in CMS (RFC 3852 [4]).5.7.1. content-type
The content-type attribute indicates the type of the signed content. The syntax of the content-type attribute type is as defined in CMS (RFC 3852 [4]) Section 11.1.
NOTE 1: As stated in RFC 3852 [4] , the content-type attribute
must have its value (i.e., ContentType) equal to the eContentType
of the EncapsulatedContentInfo value being signed.
NOTE 2: For implementations supporting signature generation, if
the content-type attribute is id-data, then it is recommended that
the eContent be encoded using MIME. For implementations
supporting signature verification, if the signed data (i.e.,
eContent) is MIME-encoded, then the OID of the content-type
attribute must be id-data. In both cases, the MIME
content-type(s) must be used to identify the presentation format
of the data. See Annex F for further details about the use of
MIME.
5.7.2. Message Digest
The syntax of the message-digest attribute type of the ES is as
defined in CMS (RFC 3852 [4]).
5.7.3. Signing Certificate Reference Attributes
The Signing certificate reference attributes are supported by using
either the ESS signing-certificate attribute or the
ESS-signing-certificate-v2 attribute.
These attributes shall contain a reference to the signer's
certificate; they are designed to prevent simple substitution and
reissue attacks and to allow for a restricted set of certificates to
be used in verifying a signature. They have a compact form (much
shorter than the full certificate) that allows for a certificate to
be unambiguously identified.
One, and only one, of the following alternative attributes shall be
present with the signedData, defined by the present document:
- The ESS signing-certificate attribute, defined in ESS [5], must
be used if the SHA-1 hashing algorithm is used.
- The ESS signing-certificate-v2 attribute, defined in "ESS
Update: Adding CertID Algorithm Agility", RFC 5035 [15], which
shall be used when other hashing algorithms are to be used.
The certificate to be used to verify the signature shall be
identified in the sequence (i.e., the certificate from the signer),
and the sequence shall not be empty. The signature validation policy
may mandate other certificates be present that may include all the
certificates up to the trust anchor.
5.7.3.1. ESS signing-certificate Attribute Definition
The syntax of the signing-certificate attribute type of the ES is as defined in Enhanced Security Services (ESS), RFC 2634 [5], and further qualified in the present document. The sequence of the policy information field is not used in the present document. The ESS signing-certificate attribute shall be a signed attribute. The encoding of the ESSCertID for this certificate shall include the issuerSerial field. If present, the issuerAndSerialNumber in SignerIdentifier field of the SignerInfo shall match the issuerSerial field present in ESSCertID. In addition, the certHash from ESSCertID shall match the SHA-1 hash of the certificate. The certificate identified shall be used during the signature verification process. If the hash of the certificate does not match the certificate used to verify the signature, the signature shall be considered invalid. NOTE: Where an attribute certificate is used by the signer to associate a role, or other attributes of the signer, with the electronic signature; this is placed in the signer-attributes attribute as defined in Section 5.8.3.5.7.3.2. ESS signing-certificate-v2 Attribute Definition
The ESS signing-certificate-v2 attribute is similar to the ESS signing-certificate defined above, except that this attribute can be used with hashing algorithms other than SHA-1. The syntax of the signing-certificate-v2 attribute type of the ES is as defined in "ESS Update: Adding CertID Algorithm Agility", RFC 5035 [15], and further qualified in the present document. The sequence of the policy information field is not used in the present document. This attribute shall be used in the same manner as defined above for the ESS signing-certificate attribute. The object identifier for this attribute is: id-aa-signingCertificateV2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 47 }
If present, the issuerAndSerialNumber in SignerIdentifier field of
the SignerInfo shall match the issuerSerial field present in
ESSCertIDv2. In addition, the certHash from ESSCertIDv2 shall match
the hash of the certificate computed using the hash function
specified in the hashAlgorithm field. The certificate identified
shall be used during the signature verification process. If the hash
of the certificate does not match the certificate used to verify the
signature, the signature shall be considered invalid.
NOTE 1: Where an attribute certificate is used by the signer to
associate a role, or other attributes of the signer, with the
electronic signature; this is placed in the signer-attributes
attribute as defined in Section 5.8.3.
NOTE 2: RFC 3126 was using the other signing-certificate attribute
(see Section 5.7.3.3) for the same purpose. Its use is now
deprecated, since this structure is simpler.
5.7.3.3. Other signing-certificate Attribute Definition
RFC 3126 was using the other signing-certificate attribute as an
alternative to the ESS signing-certificate when hashing algorithms
other than SHA-1 were being used. Its use is now deprecated, since
the structure of the signing-certificate-v2 attribute is simpler.
Its description is however still present in this version for
backwards compatibility.
id-aa-ets-otherSigCert OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 19 }
The other-signing-certificate attribute value has the ASN.1 syntax
OtherSigningCertificate:
OtherSigningCertificate ::= SEQUENCE {
certs SEQUENCE OF OtherCertID,
policies SEQUENCE OF PolicyInformation OPTIONAL
-- NOT USED IN THE PRESENT DOCUMENT }
OtherCertID ::= SEQUENCE {
otherCertHash OtherHash,
issuerSerial IssuerSerial OPTIONAL }
OtherHash ::= CHOICE {
sha1Hash OtherHashValue, -- This contains a SHA-1 hash
otherHash OtherHashAlgAndValue}
OtherHashValue ::= OCTET STRING
OtherHashAlgAndValue ::= SEQUENCE {
hashAlgorithm AlgorithmIdentifier,
hashValue OtherHashValue }
5.8. Additional Mandatory Attributes for Explicit Policy-based
Electronic Signatures
5.8.1. signature-policy-identifier
The present document mandates that for CAdES-EPES, a reference to the
signature policy is included in the signedData. This reference is
explicitly identified. A signature policy defines the rules for
creation and validation of an electronic signature, and is included
as a signed attribute with every Explicit Policy-based Electronic
Signature. The signature-policy-identifier shall be a signed
attribute.
The following object identifier identifies the
signature-policy-identifier attribute:
id-aa-ets-sigPolicyId OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 15 }
signature-policy-identifier attribute values have ASN.1 type
SignaturePolicyIdentifier:
SignaturePolicyIdentifier ::= CHOICE {
signaturePolicyId SignaturePolicyId,
signaturePolicyImplied SignaturePolicyImplied
-- not used in this version
}
SignaturePolicyId ::= SEQUENCE {
sigPolicyId SigPolicyId,
sigPolicyHash SigPolicyHash,
sigPolicyQualifiers SEQUENCE SIZE (1..MAX) OF
SigPolicyQualifierInfo OPTIONAL}
SignaturePolicyImplied ::= NULL
The sigPolicyId field contains an object-identifier that uniquely
identifies a specific version of the signature policy. The syntax of
this field is as follows:
SigPolicyId ::= OBJECT IDENTIFIER
The sigPolicyHash field optionally contains the identifier of the
hash algorithm and the hash of the value of the signature policy.
The hashValue within the sigPolicyHash may be set to zero to indicate
that the policy hash value is not known.
NOTE: The use of a zero sigPolicyHash value is to ensure backwards
compatibility with earlier versions of the current document. If
sigPolicyHash is zero, then the hash value should not be checked
against the calculated hash value of the signature policy.
If the signature policy is defined using ASN.1, then the hash is
calculated on the value without the outer type and length fields, and
the hashing algorithm shall be as specified in the field
sigPolicyHash.
If the signature policy is defined using another structure, the type
of structure and the hashing algorithm shall be either specified as
part of the signature policy, or indicated using a signature policy
qualifier.
SigPolicyHash ::= OtherHashAlgAndValue
OtherHashAlgAndValue ::= SEQUENCE {
hashAlgorithm AlgorithmIdentifier,
hashValue OtherHashValue }
OtherHashValue ::= OCTET STRING
A Signature Policy Identifier may be qualified with other information
about the qualifier. The semantics and syntax of the qualifier is as
associated with the object-identifier in the sigPolicyQualifierId
field. The general syntax of this qualifier is as follows:
SigPolicyQualifierInfo ::= SEQUENCE {
sigPolicyQualifierId SigPolicyQualifierId,
sigQualifier ANY DEFINED BY sigPolicyQualifierId }
The present document specifies the following qualifiers:
- spuri: this contains the web URI or URL reference to the
signature policy, and
- sp-user-notice: this contains a user notice that should be
displayed whenever the signature is validated.
sigpolicyQualifierIds defined in the present document:
SigPolicyQualifierId ::= OBJECT IDENTIFIER
id-spq-ets-uri OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-spq(5) 1 }
SPuri ::= IA5String
id-spq-ets-unotice OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-spq(5) 2 }
SPUserNotice ::= SEQUENCE {
noticeRef NoticeReference OPTIONAL,
explicitText DisplayText OPTIONAL}
NoticeReference ::= SEQUENCE {
organization DisplayText,
noticeNumbers SEQUENCE OF INTEGER }
DisplayText ::= CHOICE {
visibleString VisibleString (SIZE (1..200)),
bmpString BMPString (SIZE (1..200)),
utf8String UTF8String (SIZE (1..200)) }
(page 38 continued on part 3)
