Abstract
The loosely defined terms hard fork and soft fork have established themselves as descriptors of different classes of upgrade mechanisms for the underlying consensus rules of (proof-of-work) blockchains. Recently, a novel approach termed velvet fork, which expands upon the concept of a soft fork, was outlined in [22]. Specifically, velvet forks intend to avoid the possibility of disagreement by a change of rules through rendering modifications to the protocol backward compatible and inclusive to legacy blocks. We present an overview and definitions of these different upgrade mechanisms and outline their relationships. Hereby, we expose examples where velvet forks or similar constructions are already actively employed in Bitcoin and other cryptocurrencies. Furthermore, we expand upon the concept of velvet forks by proposing possible applications and discuss potentially arising security implications.
A. Zamyatin and N. Stifter—Contributed equally to this work.
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Notes
- 1.
For example, repurposing anyone-can-spend outputs as is the case with SegWit (BIP 141).
- 2.
The target \(d_{share}\) is adjusted such that the sharechain maintains an average block interval of 30 s.
- 3.
We point out that the agreement problem on the overlay protocol rules themselves is hereby of course not solved, and an upgrade \( \mathcal {O} \rightarrow \mathcal {O'} \) may cause a logical fork with similar problems to those of the underlying consensus protocol, discussed previously.
- 4.
And do not exceed Bitcoin’s block size limitations.
- 5.
In the data used as input to the proof-of-work of the block.
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Acknowledgments
This research was funded by Blockchain (GB) Ltd., FFG-Austrian Research Promotion Agency Bridge Early Stage 846573 A2Bit, Bridge 1 858561 SESC, and COMET K1.
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Zamyatin, A., Stifter, N., Judmayer, A., Schindler, P., Weippl, E., Knottenbelt, W.J. (2019). A Wild Velvet Fork Appears! Inclusive Blockchain Protocol Changes in Practice. In: Zohar, A., et al. Financial Cryptography and Data Security. FC 2018. Lecture Notes in Computer Science(), vol 10958. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-58820-8_3
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