An interesting new cryptocurrency has been developed solely for IoT called IOTA. In this case, the IOT devices themselves are the backbone of the trust network and the architecture is based on a Directed Acyclic Graph (DAG). Bitcoin has an associated fee per transaction. IOTA has no fees. This is very important in the world of IoT, which may be serving micro-transactions. For example, a sensor may provide a service reporting to many subscribers of MQTT. The service has value in aggregate terms but is measurably very small per single transaction, so small that a Bitcoin fee to provide that information may be larger than the value of the data.

The IOTA architecture has the following aspects:

• No centralization of monetary control; with blockchain, miners can form large groups in order to increase the number of blocks they can mine as well as increase their rewards. This can potentially lead to a concentration of power and possibly harm the network.
• No costly hardware equipment. In order to mine Bitcoins. cryptocurrency, powerful processors are necessary due to the complexity of the processing logic required.
• Micro and nano transactions at the IoT level of data.
• Proven security even against quantum computing brute force attacks.
• Data that can be transferred through IOTA just as currency can. The data is fully authenticated and tamper-proof.
• Since IOTA is agnostic to the payload of a transaction, a national tamper-proof voting system can be designed.
• Anything with a small SOC can become a service. If you own a device such as a drill, personal Wi-Fi, a microwave, or bicycle, the devices with a small SOC or microcontroller can join IOTA and become leasing sources of revenue.

The IOTA DAG is called a tangle and is used store transactions as a distributed ledger. Transactions are issued by nodes (IoT devices) and they constitute a set on the tangle DAG. If there is no directed edge between transaction A and transaction B, but there is a path of at least length two from A to B, it can be said A indirectly approves B. 

There is also the notion of a genesis transaction. Since there is no mining for graph edges (and no incentives or fees) to start the tangle, one node contains all tokens. The genesis event sends the tokens to other "founder" addresses. This is the static set of all tokens and no new tokens will ever be created. 

When a new transaction arrives, it must approve (or deny) two previous transactions; this is called direct approval. This forms a direct edge on the graph. Anything performing a transaction is required to produce a "work" product on behalf of the tangle. This work involves finding a nonce that works with the hash of a portion of the approved transaction. Thus by using IOTA, the network becomes more distributed and more secure. The transaction may have many approvers. As more transactions occur, the confidence that the transaction is legitimate increases. If a node attempts to approve a transaction that for all intents is not legitimate, it risks having its own transaction be continuously disproved and falling into oblivion. 

While still early in the rollout, it is a technology to watch. More information can be found at http://iota.org.