Offline Submission with RSA Time-Lock Puzzles

We introduce a non-interactive RSA time-lock puzzle scheme whose level of difficulty can be arbitrarily chosen by artificially enlarging the public exponent. Solving a puzzle for a message m means for Bob to encrypt m with Alice's public puzzle key by repeated modular squaring. The number of squarings to perform determines the puzzle complexity. This puzzle is non-parallelizable. Thus, the solution time cannot be shortened significantly by employing many machines and it varies only slightly across modern CPUs. Alice can quickly verify the puzzle solution by decrypting the ciphertext with a regular private key operation. Our main contribution is an offline submission protocol which enables an author being currently offline to commit to his document before the deadline by continuously solving an RSA puzzle based on that document. When regaining Internet connectivity, he submits his document along with the puzzle solution which is a proof for the timely completion of the document. We have implemented a platform-independent tool performing all parts of our offline submission protocol: puzzle benchmark, issuing a time-lock RSA certificate, solving a puzzle and finally verifying the solution for a submitted document. Two other applications we propose for RSA time-lock puzzles are trial certificates from a well-known CA and a CEO disclosing the signing private key to his deputy.

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