One of the problems with the transparency, a property usually provided by current blockchain techniques, is privacy since everyone
can look at the data inserted in the transactions compiled in the different block of a blockchain. In this way, the trust
obtained on the basis of the content of the transactions comes at the price of lack of privacy. From a commercial perspective, if it may
be needed to avoid sharing publicly the amounts and contents of transactions, however the payment history may be of interest for
credits for example or to prove that deliveries were realized successfully. Two main techniques are used to improve the privacy of
blockchains: zero-knowledge protocols and homomorphic Encryption.

Another research topic related to the subject of this proposed project is the design of fair exchange protocols. These
cryptographic protocols make it possible to implement, in a fair way, an exchange of goods via a network (a good against a payment, a
payment against the commitment to send a physical good...) while guaranteeing that either the different participants in the protocols
receive what they expect, or no participant receives anything that can be valuable (this property is called "fairness").
In the framework of this project we envisage to study how such protocols can be improved on the basis of a blockchain technology.

Also, blockchain technologies currently suffer from a range of limitations. For instance, Bitcoin, the main blockchain used for
payment today, is radically impractical for the type of use cases targeted by the SPE because each Bitcoin transaction can take
upto several minutes to get accepted and validated. This delay is impractical in a shop where the merchant could have to wait a long
time to be sure it is paid. Besides, the Bitcoin blockchain can accept only 3 transactions per second. Some blockchains try to
address Bitcoin’s limitations and currently promise to handle 15 000 to 20 000 transactions per second at the price of a lighter
transaction verification procedure. In the frame of this research project, we aim at working on an enhancement of blockchain
technologies to reach performance levels that are close to the load and transaction rate that centralized payment platforms can handle:
A SPE transaction should be validated in less than a second, and the blockchain sustaining the SPE should be able to process 30
000 transactions per second.