SeedSigner is a Bitcoin wallet that allows you to build a device to sign your transactions in an air-gapped manner.
In this article, we will explore what SeedSigner is and how it works: 

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SeedSigner: A Great DIY & Air-Gapped Bitcoin Wallet
SeedSigner is a Bitcoin hardware wallet that allows you to sign transactions in an air-gapped manner, without connecting to Wi-Fi or Bluetooth.