  This work presents a new framework for Privacy-Preserving Investment systems
in a distributed model. In this model, independent investors can transfer funds
to independent projects, in the same way as it works on crowdfunding platforms.
The framework protects the investors' single payments from being detected (by
any other party), only the sums of each investor's payments are revealed.
Likewise, the projects' single incoming payments are concealed and only the
final sums of the incoming payments for every project are revealed. In this
way, no other party than the investor (not even the system administration) can
detect how much she paid to any single project. Though it is still possible to
confidentially exchange any part of an investment between any pair of
investors, such that market liquidity is unaffected by the system. On top, our
framework allows a privacy-preserving return of a multiple of all the held
investments (e.g. interest payments or dividends) to the indivdual investors
while still revealing nothing else than the sum of all returns for every
investor. We provide reasonable security guarantees for this framework that are
based on common notions from the Secure Multi-Party Computation literature. As
instantiation for this framework we present Investcoin. It is a proper
combination of three cryptographic protocols, namely a Private Stream
Aggregation scheme, a Commitment scheme and a Range test and it is usable in
connection with any existing currency. The security of these protocols is based
on the DDH assumption. By a composition theorem from the SMPC literature, the
security of the resulting Investcoin protocol is also based on the DDH
assumption. Furthermore, we provide a simple decentralised key generation
protocol for Investcoin supporting dynamic join/leave and fault-tolarance of
investors and moreover achieves some security guarantees against malicious
investors.
