What do you mean by "physical key", what are you trying to achieve? If you are looking for a hardware device to verify signed messages look into hardware wallets such as Trezor or Ledger:
https://trezor.io/
https://www.ledgerwallet.com/products/ledger-nano-s
Topic: Physical oracle services? (Read 150 times)
IS there somewhere a physical oracle service for verifying the authenticity of a signature in a transaction? like a physical key...
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