our node checks if the two transactions are not conflicting. Next, the node must do some proof of work by solving a cryptographic puzzle (hashcash). Hashcash works by repeatedly hashing the same data with a tiny variation until a hash is found with a certain number of leading zero bits. This PoW is to prevent spam and Sybil attacks. A Sybil attack is based on the assumption, that half of all hash power is coming from malicious nodes. Once you’ve done that, your transaction will be broadcast to the network. Someone else will come along, choose your transaction in the tip selection process and validate it. And just like that, transaction is confirmed. • It’s quite easy to determine the confirmation level of your transaction: it executes the MCMC algorithm N times, the probability of transaction being accepted is therefore M of N. • As a merchant, in IOTA you have complete freedom to decide with what probability you will start accepting transactions. If you are happy with 51% transaction you can increase the threshold to 99 or 100. How does IOTA prevent double spending • In the tangle, transactions are atomic. During processing though, bundles are. • When a full node is asked to provide tips to a light node to create a transaction, the full node will walk backwards along the edges of the DAG to the general transaction and check if there are any conflicting transactions along the way. If there is then that tip is discarded. If there isn’t then the tip is considered valid. • An attacker would have to outpace the input flow of new transactions. The tangle is network bound – requiring an attacker to be everywhere at the same time. • So full nodes are constantly being asked to provide branch and trunk tips to the light nodes for bundling purposes and will only select tips free of conflict. The attacker will try and do the same with the double spend and has to find a way to overwhelm the entire network’s influx.>