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Makes some clarifications, Sonja's comments
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dbosk committed Jul 1, 2015
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27 changes: 17 additions & 10 deletions otpkx-content.tex
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Secondly, we present a scheme which provides the desirable properties of
previous schemes, but with stronger deniability under the new adversary
model.
Our scheme uses physical meetings for exchanges of large amounts of random
key-material via near-field communication and later uses this random data to
key a one-time pad for text-messaging.
Our scheme requires physical meetings for exchanges of large amounts of
random key-material via near-field communication and later uses this random
data to key a one-time pad for text-messaging.
Finally we evaluate the practical feasibility of the suggested scheme.

\keywords{%
Expand Down Expand Up @@ -612,7 +612,7 @@ \subsection{The Amount of Randomness Needed}
mean_msg_size = mean_msg_size.quantize( precdata )
stddev_msg_size = stddev_msg_size.quantize( precdata )

mean_msg_freq, stddev_msg_freq = ( decimal.Decimal(20), decimal.Decimal(5) )
mean_msg_freq, stddev_msg_freq = ( decimal.Decimal(40), decimal.Decimal(5) )
# mailstat.mean_message_frequency( metadata )
mean_msg_freq = mean_msg_freq.quantize( precdata )
stddev_msg_freq = stddev_msg_freq.quantize( precdata )
Expand Down Expand Up @@ -670,8 +670,8 @@ \subsection{The Amount of Randomness Needed}
\(\unit{\py[random]{(data_per_day/1024).quantize( 10 )}}{\kibi\byte}\)
per day.
This means that we need approximately
\(\unit{\py[random]{(data_per_day*365*(mean_contacts+stddev_contacts)/1024/1024).quantize(
precdata )}}{\mebi\byte}\) to store one year's key-material.
\(\unit{\py[random]{(data_per_day*365/1024/1024).quantize( precdata
)}}{\mebi\byte}\) to store one year's key-material.


We use Android's <SecureRandom> to generate our randomness.
Expand Down Expand Up @@ -779,14 +779,21 @@ \section{Conclusions}
\item authenticated and
\item deniable encryption.
\end{itemize}
However, to achieve this scheme and these properties, we require physical
meetings to exchange the randomness.
If Alice and Bob run out of randomness they can fall back to e.g.~\ac{OTR}, but
then they lose deniability against Eve.
In either case, they are never worse of than using \ac{OTR} or TextSecure.

We also showed that our scheme is usable.
We found that a typical exchange of key material requires less than
\unit{10}{\second} to complete.
\unit{10}{\second} daily to complete.
If you exchange the key-material on a weekly basis, then it is still less than
a minute, while monthly and bimonthly require up to five minutes.
Thus the transmission rates are not a usability concern.
The effects on battery life under the considered use is not a limiting factor
in neither the generation of the key-material nor the transmission of the
key-material.
Also, the effects on battery life under the considered use is not a limiting
factor in neither the generation of the key-material nor the transmission of
the key-material.

The method for estimating the needed amount of data can be improved.
This estimate depends on the type of communication, e.g.~corporate emails
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