Bitcoin is a distributed online payment system that organises transactions into blocks. The size of blocks is limited to 1 megabyte, which also limits the number of transactions per second that can be confirmed. This year several attempts have been made to create a fork or a split that removes this restriction. One such alternative is Bitcoin Unlimited (BTU). Proponents of BTU have suggested to use a type of majority attack to force other Bitcoin miners to adopt BTU. In this paper we model this attack in Uppaal, and analyse how long it will take for an attack to succeed, depending on the share the attacker has of the total network, and the so-called confirmation depth. The analysis shows that with a share of 20% an attack will be successful within a few days. This paper also looks at the effect of increasing the confirmation depth as a countermeasure.
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