In March 2019 we released the fourth TAST Technical White Paper and the first TAST research prototype. Since then we explored further ways of allowing efficient cross-blockchain token transfers. While we made good progress in evaluating the current state, the efficient interoperability between blockchains still remains an open problem.

In the fifth TAST Technical White Paper we discuss concrete requirements and define open research questions. To overcome this challenge, we present two approaches to make cross-blockchain transfers as efficient as possible, which we will further explore in upcoming research.

You can find the full version of the fifth TAST Technical White Paper on TU Wien’s TAST homepage or you can download it directly here.

Specifying requirements for cross-blockchain token transfers

One of the main goals of our current research is to connect the fragmented field of research and development by investigating possible means of interconnecting various blockchain-related projects. The TAST protocol has to overcome a couple of challenges, in order to enable a token like PAN to exist on multiple blockchains at the same time. The goal is to let holders of the token freely choose on which blockchain they want to keep their assets.

The general requirements for a cross- blockchain token transfers are as follows:

It should not be possible to create tokens on the destination blockchain, without first burning the same amount of tokens on the source blockchain. When transferring some amount of tokens from some source blockchain A to some destination blockchain B, the specified amount should only be created on B, if it can be proven that the same amount was actually burnt on A. It should not be possible to fake the burning of tokens. Every token that was burnt on one blockchain, can only be (re-)created once on another blockchain. It should not be possible to destroy tokens on one blockchain without recreating them on another.

Progress and challenges of TAST

The TAST research project has resulted in numerous contributions throughout its progress to date. The first research prototype demonstrated the use of the concepts discussed in previous publications using Solidity. This prototype already provides the means for realising cross-blockchain token transfers. The designed protocol so far suffers from multiple limitations, e.g., very high synchronisation costs since token balances are synchronised across all participating blockchains. Ideally, a cross-blockchain token transfer only requires the interaction of the two blockchains directly involved in the transfer.

To solve the existing challenges and limitations, we identified two approaches as potential solutions: First SPV-based cross-chain token transfers. And second an approach relying on incentivised majority voting.

Approach 1: SPV-Based Cross-Chain Token Transfer

The basic idea of this approach is to enable users to construct a cryptographic proof certifying that a particular amount of assets has been burnt on the source blockchain.

By submitting this proof to a destination blockchain, users can prove to the destination blockchain that the specified amount of assets has been destroyed on the source blockchain. After a successful verification of this proof, the claimed tokens are (re-)created on the destination blockchain.

A major advantage of this combination is the ability to verify the inclusion of a transaction in a blockchain without the need to download the full blockchain. In order to be able to verify that a particular transaction is included in a certain block it is sufficient to have access to the block headers (e.g., in case of Bitcoin and Ethereum to a copy of the block headers of the longest proof-of-work chain). Storing only these headers requires less space than storing the complete blocks. This lightweight verification is also known as Simplified Payment Verification (SPV).

In summary, this approach aims to leverage SPV to verify the inclusion of a transaction burning tokens on the source blockchain before creating the same amount of tokens on the destination blockchain. To make SPV-based token transfers work in practice, solutions are being developed that minimize the storage and computational requirements for verifying such proofs.

Approach 2: Incentivised Majority Voting

Instead of relying on cryptographic proofs as in approach 1, approach 2 relies on observing parties (witnesses) to determine whether or not tokens can be created on the destination blockchain. Witnesses are participants which do not partake directly in the token transfer, but rather observe transfers and vote on whether the correct amount of tokens has actually been destroyed on the source blockchain before new tokens are created on the destination blockchain.

Summarising, this approach declines or accepts token transfers between blockchains based on the outcome of a validity vote by witnesses. The main challenge consists of building an incentive structure that (a) motivates as many witnesses as possible to participate in the vote, and that (b) distributes rewards and penalties in such a way that witnesses are always inclined to vote honestly rather than dishonestly.

Conclusion and outlook

In the fifth TAST White Paper, we revisited our original vision and defined requirements for efficient cross-blockchain token transfers. We summarised our previous work and explained the limitations of the developed prototypes. We then introduced two approaches that are currently being explored for overcoming the described limitations. One approach uses cryptographic proofs while the other one relies on reward-incentivised voting contests to determine the validity of transfers. While both approaches come with their own set of problems and challenges, both ultimately aim to enable cross-blockchain token transfers which only require the interaction of the two blockchains directly involved in a transfer. We are currently working on prototypes for both approaches in order to determine the most promising solution.

We are looking for talented developers

We are constantly looking for talented people to join our amazing team. If you are interested, take a look at our career page if you want to work with us as for example Pantos Developer (f/m/d) or Pantos Researcher (f/m/d) and shape the future of the cryptocurrency industry.

About Pantos

As the first multi-blockchain token system, Pantos aims to bring blockchain projects closer together, improve communication between developers, researchers and users, and set innovative standards for cross-chain token transfers.

The goal is to serve as a lighthouse project in an increasingly fragmented blockchain space. With multiple blockchains serving all kinds of different purposes, Pantos is seeking to allow these projects to talk to one another in a standardised way. This will speed up innovation by creating a link between blockchains which then can scale together.

To get the latest news on the progress of the Pantos project you can follow our official channels: