Anais - Engenharia de Redes de Comunicação - UnB

From all those possibilities, one realizes that it should be possible to let the author
specify which signatures are required for the electronic document he creates. The process
would then become similar to the way it is done with paper documents. This would allow
applications performing digital signature validation to gather identity and authorization
requirements directly from the document. Those requirements would then be enforced
against the PKCs and ACs present in the signatures.
In order to address this necessity, we propose to bind identity and authorization
requirements to a document through a creator signature. For this, we introduce a new
signed signature attribute.
The structure of the paper is as follows. In Section 2 we briefly describe Attribute
Certificates and the support offered by digital signature standards CAdES and XAdES
to the inclusion of these certificates. Section 3 describes different alternatives for the inclusion
of authorization constraints in a document. Section 4 proposes the concept of
a creator signature and introduces a new signed signature attribute. Section 5 discusses
advantages and limitations of the proposed solution in comparison to the existing alternatives.
Section 6 concludes the paper and describes future work.
2. Attribute Certificates and Digital Signature Standards
The digital signature standards CAdES[ETSI 2011] and XAdES[ETSI 2010] currently
support the use of X.509 Attribute Certificates [Farrell et al. 2010] to carry the signatories’
authorization credentials within the signature.
X.509 Attribute Certificates(ACs) are certificates that can provide authorization
information about a given entity. They are issued by an Authorization Authority(AA) and
they reference a single Public Key Certificate(PKC) [Cooper et al. 2008]. These certificates
are widely used in access control schemes. A well know example is the Permis
Project[Chadwick and Otenko 2002].
The CAdES and XAdES digital signature standards are respectively evolutions
of the Cryptographic Message Syntax(CMS) [Housley 2009] and XML Signature Syntax
and Processing(XMLDSIG)[Eastlake et al. 2002] formats. They define the attributes that
can be present in a digital signature and how those attributes shall be interpreted. Those
attributes are classified as signed or unsigned attributes. Signed attributes are included in
the signature container before the actual signature value is calculated, therefore becoming
part of the signed content along with the document’s content itself. Thus, these attributes
cannot be altered after the signature is completed. An example of a signed attribute is the
Signing Certificate attribute, which holds a reference of the signatory’s PKC. Unsigned
attributes, in the other hand, are included in the signature container after the signature
value calculation. These attributes can be altered at any time. They are used mainly to
carry validation data, as certificates and certificate revocation data, and artifacts to extend
the lifetime os the signature, such as timestamps.
ACs can be included in a CAdES signature with a signed attribute called signer-
Attributes. The equivalent in XAdES is the signed property signerRoles.
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