Many prominent smart-contract applications such as payment channels,
auctions, and voting systems often involve a mechanism in which some party must
respond to a challenge or appeal some action within a fixed time limit. This
pattern of challenge-response mechanisms poses great risks if during periods of
high transaction volume, the network becomes congested. In this case fee market
competition can prevent the inclusion of the response in blocks, causing great
harm. As a result, responders are allowed long periods to submit their response
and overpay in fees. To overcome these problems and improve challenge-response
protocols, we suggest a secure mechanism that detects congestion in blocks and
adjusts the deadline of the response accordingly. The responder is thus
guaranteed a deadline extension should congestion arise. We lay theoretical
foundations for congestion signals in blockchains and then proceed to analyze
and discuss possible attacks on the mechanism and evaluate its robustness. Our
results show that in Ethereum, using short response deadlines as low as 3
hours, the protocol has >99% defense rate from attacks even by miners with up
to 33% of the computational power. Using shorter deadlines such as one hour is
also possible with a similar defense rate for attackers with up to 27% of the

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