Information Technology Reference
In-Depth Information
well too. Due to that, it is necessary to reduce the number of time-consuming
calculations like generation and verification of digital signatures and to minimize
the amount of data transmitted.
3 Related Work
In this Section we present in short the main approaches to the certificate sta-
tus checking. The simplest approach is traditional
Certificate Revocation List
(CRL) [5]. CRL is the most mature revocation system and it is part of X.509
since its version 1. A CRL is a digitally signed collection of serial numbers with
additional information about version, CRL serial number, issuer, algorithm used
to sign, signature, issuing date, expiration date and some optional fields called
extensions. The CA that issued the certificate acts as RDI and repositories can
be used to distribute the CRL. The
Indirect CRL
(I-CRL) enables a RDI to pick
up revocation records from multiple CAs to be issued within a single CRL. In
1994
Delta CRL
(D-CRL) was introduced; a Delta-CRL is a small CRL that
provides information about the certificates whose status have changed since the
issuance of a complete list called Base-CRL.
CRL Distribution Points
(CRL-DP)
was introduced in version 3 of X.509 [5]. In CRL Distribution Points, each CRL
contains the status information of a certain subgroup of certificates. The criteria
to create these subgroups can be geographic, level of importance, scope of use,
revocation reason, etc.
Micali proposed the
Certificate Revocation System
(CRS) [8], recently re-
named as
Novomodo
[9]. In [2] an improvement over CRS called
Hierarchical
Certificate Revocation Scheme
is suggested
.
The
Certificate Revocation Tree
(CRT) [6] and the
Authenticated Dictionary
(AD) [11] are both based on the Merkle Hash Tree (MHT) [7]. The AD is further
discussed in Section 4. The PKIX workgroup of the IETF has proposed the
On-
line Certificate Status Protocol
(OCSP) [10], where the
SD
is available through
a responder that signs each response that it produces.
4 The Authenticated Dictionary
The AD is based on two data structures: the Merkle Hash Tree (MHT) and the
2-3 tree.
The Merkle Hash Tree (MHT).
The MHT [7] relies on the properties of the OWHF (One Way Hash Functions).
It exploits the fact that a OWHF is at least 10,000 times faster to compute than
a digital signature, so the majority of the cryptographic operations performed in
the revocation system are hash functions instead of digital signatures. A sample
MHT is represented in Fig. 2.
We denote by
N
i,j
the nodes within the MHT where
i
and
j
represent re-
spectively the
i
-th level and the
j
-th node. We denote by
H
i,j
the cryptographic
