Who signs

Chain 12

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The exchange holds 500 M USD and signs a withdrawal every 5 s at a fee of 0.4 USD. Yearly chance of compromise: 1 percent for an HSM, 2 percent for each online key or key share. Pick who signs: expected theft under 0.1 percent of funds a year and fees under 10,000 USD/day.

Goals

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99 of 100 requests finish faster than this.The share of requests that fail or get no answer.What the servers and services cost each month.
Choose a design
One HSM holds the whole key and signs each withdrawal
A 2-of-3 multisig contract: two of three keys sign on chain
2-of-3 MPC: two of three key shares make one signature