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Proofs of Work (POWs) and Bread Pudding Protocols

This article discusses the concept of Proofs of Work (POWs) and Bread Pudding Protocols, their applications in cryptography, and their effectiveness in spam deterrence and defense against denial-of-service attacks. It also explores the idea of reusing computational work in POWs to perform useful tasks, using the example of hash inversion in the context of distributed MicroMint.

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Proofs of Work (POWs) and Bread Pudding Protocols

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  1. POW Proofs of Work (POWs) and Bread Pudding Protocols with Markus Jakobsson Bell Laboratories Ari Juels RSA Laboratories

  2. Prover Verifier w = ge c s = cx +e Cryptography: About proofs of mathematical relations gs=ycw?

  3. Some proofs Proof of Identity = Cryptographic Authentication Protocol

  4. Some proofs Proof of Authorization (Signed Document) = Digital signature

  5. 1 ounce sweat = 1 hour of work Proof of work? We can make precise in cryptographic world

  6. Prover Verifier POW Query Response Proof of work (POW) Prover did at least 106 cycles of work

  7. Prover Verifier t = h(s) [k bits] s Example of a POW (Hash inversion) random secret s Prover computed an expected 2k-1hashes

  8. POW POW Service Request What are POWs good for? • Spam deterrent (DN94), “Hash cash” • Defense against denial-of-service attacks (JB99)

  9. POW Query Response What are POWs good for? • Benchmarking Server Client

  10. Formal notion of POW

  11. Breadpudding • Idea: Re-use the ``stale’’ computation in a POW to perform useful task • Achieve privacy in useful task • Example: Hash inversion POW for distributed MicroMint

  12. MicroMint Want a scheme that mimics economics of physical mint • Verifying validity of a coin is easy • Base minting cost is high so... • Forgery is expensive

  13. The minting process • .Throw balls into bins using “random” function h • . Any bin with two balls is a coin

  14. Minting in MicroMint h Collision = Coin Bin 1 Bin 3 Bin 2 Bin 4 Bin 5 Bin 6 Bin 9 Bin 7 Bin 8

  15. Checking a coin h Valid coin? Bin 2

  16. Features • Many bins, so need to throw many balls to mint successfully • Minting requires very intensive computation

  17. Minting requires special, e.g., $250,000 computer “Deep Crack”

  18. Another characteristic: Most balls are invalid h Bin 1 Bin 3 Bin 2 Bin 4 Bin 5 Bin 6 Bin 9 Bin 7 Bin 8 In fact, >99% of work goes to missed balls!

  19. Idea: Make three stage process • . Create “valid” balls, i.e., balls that won’t miss (>99% of work) • . Throw balls into bins using “random” function h (<1% of work) • . Any bin with two balls is a coin

  20. Have many other (untrusted) people do Step 1

  21. Now... • 99%+ of work is done for minter • No participant will get enough balls to do minting himself/herself (or else participants know “validity”h but not “throwing”h) • Minting is cheap for minter!

  22. Minter can use ordinary server

  23. + Questions? ?

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