During military or peacekeeping operations it is often the case that coalitions are formed between nations or instances to achieve common objectives by joint decision making and resource sharing. Often a coalition partner participates for a certain period of time and other nations or instances might want to join the coalition in a later stage, resulting in a dynamic environment. In practice the members in these coalitions are dissimilar with regards to their disposition, requiring the coalition to be set up in a decentralised manner. In order to facilitate such an environment, a secure decentralised network should be in place that enables decision making, access control to shared information and the ability to join and leave in an appropriate way. To enable the coalition to put its signature on important decisions and to be able to provide certified public keys to enable access control, the network should include a signature scheme. A challenge for such a signature scheme is that it should not be able for previous compositions of the coalition to create valid signatures. In other words, Backlogging should be prevented in the system. Van der Lubbe et al. introduced a distributed (n,n) signature scheme for dynamic coalitions based on the distributed El Gamal signature scheme of Park et al., which can be expanded to a (n+1,n+1) signature scheme. The scheme prevents backlogging by the use of two Oblivious Third Parties. Van Elsas et al. then improved this signature scheme by enabling the signature scheme to be reduced to a (n-1,n-1) signature scheme and by replacing the Oblivious Third Parties by One-Way Accumulators, dismissing the need for Third Parties in general. Additionally, the currently popular Blockchain model has the capability to perform as a ledger. The ledger characteristics, specifically the chain of blocks, the Merkle Tree model and a consensus mechanism, can replace the functionality that prevents backlogging.

In this thesis we present a Blockchain-based Signature Scheme for Dynamic Coalitions. The signature scheme is a distributed (n,n)-signature scheme, that is able to expand to a distributed (n+1,n+1)-signature scheme and is able to reduce to a distributed (n-1,n-1)-signature scheme. The signature scheme uses the Blockchain model to tackle Backlogging Problem. In addition, we provide an analysis of aspects of the signature scheme that could introduce problems when implementing it.