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zeekay
abf99d05fc
Restructured from flat .tex/.pdf files into per-paper subdirectories. Added Antje Worring as co-author on all papers. Synced PDFs to pdfs/ for download site.
28 lines
2.0 KiB
TeX
28 lines
2.0 KiB
TeX
\section{Introduction}
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Private computation on blockchain networks faces a fundamental dilemma. Two mature cryptographic techniques exist---Fully Homomorphic Encryption and Multi-Party Computation---but each has a critical limitation when deployed in isolation.
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\textbf{FHE alone.} TFHE~\cite{cggi2020} enables arbitrary boolean circuit evaluation on encrypted data. The computational model is clean: encrypt inputs, evaluate the circuit homomorphically, decrypt the output. The problem is the decryption key. Whoever holds it can decrypt \emph{any} intermediate ciphertext, not just the final output. In a blockchain context, this means a single operator (or a compromised key) can observe all encrypted state.
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\textbf{MPC alone.} Protocols such as SPDZ~\cite{spdz2012} and GMW distribute computation across $N$ parties such that no coalition smaller than a threshold $t$ can learn any party's private input. The problem is that inputs must be secret-shared among the participants. For $N$ parties each holding private data, this requires $O(N^2)$ communication per operation, and every party must remain online for the entire computation.
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\textbf{The hybrid.} FHE handles computation. MPC handles decryption. The FHE secret key is split via Shamir secret sharing across $n$ MPC nodes. Computation proceeds homomorphically without any interaction. Only when a result must be revealed do $t$-of-$n$ MPC nodes participate in threshold decryption.
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\begin{center}
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\begin{tabular}{lccc}
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\toprule
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\textbf{Property} & \textbf{FHE only} & \textbf{MPC only} & \textbf{FHE+MPC} \\
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\midrule
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No single point of trust & $\times$ & \checkmark & \checkmark \\
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No data sharing required & \checkmark & $\times$ & \checkmark \\
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Non-interactive compute & \checkmark & $\times$ & \checkmark \\
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Threshold decryption & $\times$ & N/A & \checkmark \\
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Offline tolerance & \checkmark & $\times$ & \checkmark$^*$ \\
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\bottomrule
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\end{tabular}
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\end{center}
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$^*$Parties must be online only during threshold decryption, not during computation.
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