2nd USENIX Conference on Web Application Development ...

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34 WebApps ’11: <strong>2nd</strong> <strong>USENIX</strong> <strong>Conference</strong> on Web Application Development <strong>USENIX</strong> Association
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Figure 3: Invocation costs: (left) measurement preserver; latency vs. payload size, (middle) stock preserver; latency vs. number of
ticker events, (right) targeted ads preservers; latency vs number of ads.
Stock Preserver:To reflect a typical trading scenario with
hundreds of ticker events per second, we plotted latency
versus numbers of back-to-back invocations (stock ticker
events) in Figure 3(middle). As expected, the TTP case
requiring network access is substantially slower as compared
to the Xen case and the base case. Comparing the
Xen case and the base case for a sequence of 100 backto-back
invocations, the latencies are 43.4ms and 9.05ms
respectively; though the overhead due to Xen is substantial,
it is still a significant improvement over the TTP
case, which requires 12 seconds. Regarding the traffic,
in the TTP case, the traffic generated for a sequence of
5, 10, 50, 100, 500 and 1000 invocations are respectively
2.8, 4.8, 20.2, 39.4, 194, 387 KB respectively. Thus, the
bandwidth required for say, 500 events per second, is
about 1.6 MB/s per user per stock symbol (and a single
user typically trades multiple symbols). These results reflect
that server site colocation can offer significant benefit
in terms of network bandwidth.
Targeted Ads Preserver: We present two results for the
targeted ads scenario. Figure 3(right) shows the latency
per invocation for the preserver to serve targeted ads using
the ChooseAd interface (we do not show results for
the GetInterestVector interface due to lack of space). This
graph shows that the Xen preserver has nearly the same
overhead as the Base case preserver once the number
of ads out of which the preserver selects one exceeds
5. This is mainly because the payload size dominates
the transfer time; the context switch overhead is minimal.
In the TTP case, the latency is clearly impacted by
the wide-area network delay. The traffic generated in the
TTP case was measured to be 2.6, 8.3, 15.8, 29.2 KB for
10, 50, 100, 200 ads respectively. These reflect the bandwidth
savings of colocation; since this bandwidth is incurred
for every website the user visits, this could be significant.
Our technical report [16] has performance results
on two other operations: allowing a user to update her web
history and the data aggregation transformation.
6.3 Security Analysis
We now discuss the desirable security properties of our
preserver, determine the TCB for these to hold, and argue
that our TCB is a significant improvement over the status
quo. Our security goal is that any data access or preserver
transfer obeys the user-sanctioned interface and policies.
Depending on the deployment, the adversary either has
control of the service software stack (above the VMM
layer) or has physical access to the machine hosting the
preserver; the first applies to VMMs, TPMs, and the second
to TTPs, client-side preservers, secure co-processors.
In the TTP/client case, we rely on physical isolation and
the preserver implementation’s correctness; the rest of this
section is concerned with the security goal for colocation.
Colocation has one basic caveat: the service can launch
an availability attack by refusing to service requests using
its host hub or by preventing a user from updating her
preserver. We assume that the trusted module provides the
four security properties detailed in Section 3: isolation,
anti-replay protection, random number generation, and remote
attestation. We also assume that any side-channels
(such as memory page caching, CPU usage) have limited
bandwidth (a subject of ongoing research [30]). We first
analyze the installation protocol and then examine the invocation
protocol.
Installation Protocol: The installation protocol is
somewhat complex since it involves a Diffie-Hellman key
exchange along with remote attestation. In order to argue
its correctness, we now present an informal argument (our
technical report [16] has a formal argument in LS2 [10],
a formal specification language). Our informal argument
is based on three salient features of the installation protocol
(Section 4.1). First, the attestation guarantees that
the public key is generated by the base layer. Second, the
Diffie-Hellman-based installation protocol ensures confidentiality;
only the party that generated the public key itself
can decrypt the preserver. From the first observation,
this party can only be the base layer. This relies on the
random number generation ability of the trusted module
to ensure that the generated public key is truly random.
Third, since attestation verifies the purported input and
output to the code which generated the public key, manin-the-middle
attacks are ruled out; an adversary cannot
tamper with the attestation without rendering it invalid.
These three properties together ensure that the preserver