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Bitcoin ZK Bounty Series: Pay for Decryption Key


kram1

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Previously, we have used Zero-Knowledge Bounties (ZKB) to pay for a Sudoku solution on Bitcoin. In this series, we are going to use ZKB to solve a wide range of more practical problems.

In Part 1, we apply ZKB to pay for a decryption key. Suppose Alice encrypts her files with a symmetric key K. For safety, she contacts Bob, an online backup service, to keep a copy of K. She does not want Bob to know K, so Bob generates an elliptic curve private/public key pair (skpk) and gives pk to Alice. Alice encrypts K with pk with an asymmetric encryption algorithm, such as ElGamal or ECIES, and sends it to Bob.

In case Alice loses her local K, she asks Bob for the decryption key sk, and Bob only delivers sk if he gets paid. This is the classic fair exchange problem that ZKB solves, as it guarantees that payment will result in the disclosure of the right decryption key.

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