Elektronika 2009-11.pdf - Instytut Systemów Elektronicznych

In the context of CL-PKC, the idea of appending validity
periods (for example, year, date or time) to identifiers ID U can
be used ensuring that any partial private key, and hence any
private key, has a limited lifetime. Then the revocation in CL-
PKC systems can be handled in the same way as in ID-PKC
systems using standard certificate-based revocation techniques.
For example, this can be done by deploying an analogue
of an online certificate status protocol (e.g. OCSP) or
variants of certificate revocation lists (CRLs).
3. Keeping the probative value
of electronic documents
Ensuring a long-term validity of stored electronic documents
depends on both organizational and technical measures (see
e.g. CWA 15579 [14]). When technical measures are used to
keep the probative value of electronic documents then first of
all there is necessary to apply the strong cryptographic algorithms
and to provide additional and sufficient information -
a certificate set (all certificates needed to generate a path that
leads to a point the verifier trusts) and the relevant revocation
information (2) (e.g. CRLs) related to this certificate set and to
all other involved certificates. Trusted third party (TTP) providing
a service of keeping the probative value of an electronic
document over long period has to issue the evidence records
attached to the electronic attestations (Section 5) and associated
with a given document or a group of documents. All these
electronic attestations and time stamp tokens cyclically created
for all stored evidences are needed later for making a
long-term verification, i.e. verifications when the signing certificate
has expired. In this case only the verifier can independently
perform the complete verification of a given
document authenticity and the validity of all signatures and
other evidences (e.g. time stamp tokens) related to it.
The storage mechanisms used by a trusted third party
(TTP) should be based on a long-term electronic signature
scheme (LT-CESS) like this:
The LT-CESS scheme in comparison to the CESS (see Section
2) contains two additional elements:
• (IV,SV) - a pair of interactive deterministic algorithms forming
the verification protocol (e.g. CWA 14171 [11]); IV - an
initial (or a short term) verification algorithm performed by
a verifier after an electronic signature is generated in order
to consolidate the signature by capturing additional information
that will support its subsequent verification; SV -
a subsequent (or a long-term) verification algorithm performed
by a verifier some time after the initial application of
a signature, to assess a signature’s validity, based on the
data collected at initial verification time;
• Preserve (P): a deterministic algorithm for long-term preservation
of the probative value of electronic signatures/documents;
this algorithm is based on the initial or subsequent
verification algorithm and makes the additional activities
needed to protect electronic signatures/documents from effects
of their deterioration, technology obsolescence, and
fading human recall.
Preserve and (IV,SV) algorithms generate the evidence
records attached to the electronic attestations. These elements
are subsequently linked to each preserved electronic document
and jointly form Virtual Signed Electronic Document (V-SED).
V-SED document should fulfil the following requirements:
(4)
• each electronic document D is stored where produced
(local repository) or in a deposit (regional repository);
• V-SED contains the references to the electronic documents,
the signatures associated with this document, the
time stamp tokens and other evidences references allowing
the creation of a real (complete) SED on demand.
Storing the references allows the values of V-SED elements
to be kept elsewhere, reducing the entire size of V-SED
document. A virtual document with above properties is a tuple:
where: ID R - the V-SED identifier in a repository, D R - the references
to the documents and their attachments, S R - the references
to the electronic signatures, E R - the references to the
evidences related to an electronic signature and a document
(including the references to the electronic attestations and the
evidence records linked to them). For V-SED the following definition
can be formulated:
Definition. The electronic document is a long-term provable
authentic, if exist a polynomial time Verify(V) algorithm
that takes as an input the certification scheme CS (i.e. PKC or
CL-PKC), the services related to a electronic signature TSI
(Trusted Services Infrastructure), V-SED, the verification policy
VP (Verification Policy), and outputs a value of each electronic
signature {valid, invalid}:
where: V is one from two algorithms being a member of the pair
(IV,SV), while the notation array[1…n] means the verification of
each electronic signature referenced in V-SED structure.
Note that CS parameter in Equation (6) includes the parameters
params of an electronic signature scheme, while V-
SED structure - (D, Σ, pk IDU ,cert) parameters; compare the
Equation (3).
4. Timestamp chain and sequence
The basic elements of V-SED structure, i.e. the electronic attestations
and the evidence records linked to them, can be
built in different ways. The most often used approach is based
on the appropriate mechanisms of the time stamp chains
building. These mechanisms have to ensure the requirements
counted in the definition of the provable authentic document
(see Section 3).
The starting point for the timestamp chains creation is an
archival electronic signature form CAdES-A [7], XAdES-A [8]
or other equivalent form (e.g. PAdES [9]). Each of these forms
should contain a time stamp of a system for preserving electronic
document for long periods of time (an archive mark for
shortly). This mark has the same meaning as the archive timestamp
that are included into the archival signature forms
CAdES-A or XAdES-A.
The archive timestamp is added to the CAdES or XadES
form as an unsigned attribute. The hash value related to this
archive timestamp is the result of using the hash algorithm to
the CAdES-X or XadES-X form. The first archive timestamp of
the obtained archival form CAdES or XAdES includes all necessary
information required for every signature verification and
so called an initial archive timestamp. This timestamp can become
invalid when a public key algorithm, or a hash function
used respectively to creation of the hash value or the timestamp:
(5)
(6)
32 ELEKTRONIKA 11/2009