  We derive a new upper bound on the string oblivious transfer capacity of
discrete memoryless channels. The main tool we use is the tension region of a
pair of random variables introduced in Prabhakaran and Prabhakaran (2014) where
it was used to derive upper bounds on rates of secure sampling in the source
model. In this paper, we consider secure computation of string oblivious
transfer in the channel model. Our bound is based on a monotonicity property of
the tension region in the channel model. We show that our bound strictly
improves upon the upper bound of Ahlswede and Csisz\'ar (2013).
