  IOTA opened recently a new line of research in distributed ledgers area by
targeting algorithms that ensure a high throughput for the transactions
generated in IoT systems. Transactions are continuously appended to an acyclic
structure called tangle and each new transaction selects as parents two
existing transactions (called tips) that it approves. G-IOTA, a very recent
improvement of IOTA, targets to protect tips left behind offering hence a good
confidence level. However, this improvement had a cost: the use of an
additional tip selection mechanism which may be critical in IoT systems since
it needs additional energy consumption. In this paper we propose a new
metamorphic algorithm for tip selection that offers the best guaranties of both
IOTA and G-IOTA. Our contribution is two fold. First, we propose a
parameterized algorithm, E-IOTA, for tip selection which targets to reduce the
number of random walks executed in previous versions (IOTA and G-IOTA) while
maintaining the same security guaranties as IOTA and the same confidence level
and fairness with respect to tips selection as G-IOTA. Then we propose a formal
analysis of the security guaranties offered by E-IOTA against various attacks
mentioned in the original IOTA proposal (e.g. large weight attack, parasite
chain attack and splitting attack). Interestingly, to the best of our knowledge
this is the first formal analysis of the security guaranties of IOTA and its
derivatives.
