Information Technology Reference
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action), Bob (the acceptor who buys the financial transaction) and the CA. Be-
fore any transaction occurs, Alice generates her public key/private key pair and
submits a certificate issuance request to the CA. If this request satisfies the CA's
certificate issuance policy, CA replies to Alice with a certificate
Cert
that binds
Alice's name and her public key. Let the validity period of
Cert
be from
t
1
to
t
2
.
We assume that
t
1
is the certificate issuance time and
t
2
denotes the expiration
time. When Alice wants to send a message
M
to Bob, she signs
M
and sends
the message with the signature value
S
and the certificate
Cert
. After receiving
M
, Bob decides whether he accepts this message or not according
to his acceptance policy.
We define the acceptor, Bob's recency period
,
S
and
Cert
t
. If the signer, Alice's certifi-
cate
t
1
, Bob trusts this
certificate as valid without checking its revocation status. In case Alice's certifi-
cate
Cert
is within time
t
from the certificate issuance time
Cert
is not within time
t
from the certificate issuance time
t
1
, Bob queries
whether
Cert
is revoked or not to the CA. Hence, Bob's recency period
t
is a
kind of recency requirement. If Bob sets
as a very short time, he will enjoy
almost real-time certificate revocation status. If Bob sets
t
t
as a very long time,
he will skip revocation status checking processes for many certificates to shorten
the overall processing time. Since every acceptor may have his own recency pe-
riod
t
, and change his recency period
t
arbitrarily, ACSP can be used in a very
flexible manner.
To answer Bob's query, CA generates an ACSP response
1
. If Alice's certificate
Cert
Cert
is not revoked, CA issues a new certificate
as the ACSP response
Cert
to Bob.
contains the same information as
Cert
except that the validity
Cert
is from
period of
t
q
is Bob's query time. This can be seen
as a certificate renewal process of Alice's certificate. After receiving the ACSP
response
t
q
to
t
2
, where
Cert
, Bob forwards
Cert
to Alice, who can replace
Cert
.
Cert
with
This certificate re-issuance technique together with recency period
t
will improve
the system performance. If Alice's
Cert
is already revoked at
t
R
, where
t
R
<t
q
,
Cert
, whose validity period of
Cert
is from
the CA issues a new certificate
t
1
to
t
R
. Bob can conclude that Alice's certificate
Cert
is already revoked, for the
Cert
is an expired certificate. Additionally, Bob knows the fact
ACSP response
that
t
R
. Note that the generation of an ACSP response does
not take more risks than that of an OCSP response, since the expiration time
of an ACSP response is the same as that of queried certificate and an OCSP
response is just another certificate [2].
The ACSP response
Cert
was revoked at
Cert
can be generated by use of exiting certificate
issuance mechanisms and infrastructure.
At this point, we will present our new online certificate status checking pro-
tocol, ACSP. There are two types of ACSP and their performances are not much
different.
1
The ACSP request/response may include other information such as a protocol ver-
sion and optional extensions. However, we do not consider these implementation
details.
