  The blockchain space is changing constantly. New chains are being implemented
frequently with different use cases in mind. As more and more types of crypto
assets are getting real world value there is an increasing need for blockchain
interoperability. Exchange services today are still dominated by central
parties which require custody of funds. This trust imposes costs and security
risks as frequent breaches testify. Atomic cross-chain swaps (ACCS) allow
mutual distrusting parties to securely exchange crypto assets in a peer-to-peer
manner while preserving self-custody. Fundamental ACCS protocols leveraged the
scripting capabilities of blockchains to conditionalize the transfer of funds
between trading parties. Recent work showed that such protocols can be realized
in a scriptless setting. This has many benefits to blockchains throughput,
efficiency of swap protocols and also to fungibility and privacy. The proposed
protocols are limited to assets transferable by either Schnorr signatures or
ECDSA that are assuming the same elliptic curve parameters. In this work we
present JugglingSwap, a scriptless atomic cross-chain swap protocol with a
higher degree of interoperability. We weaken the assumptions about blockchains
that can be included in the ACCS protocol, and only require that (1) a
threshold variant exists to the underlying digital signature scheme and (2) it
is based on the elliptic curve discrete logarithm problem (ECDLP). The fair
exchange is achieved by a gradual release of secrets. To achieve this we use a
new building block we call Juggling: a public key verifiable encryption scheme
to transfer segments of secret shares between parties, which can also be of
separate interest. Juggling is then tailored to a specific private key
management system design with threshold signatures security.
