Offline Bitcoin signing isolates private keys from networked devices, using air-gapped hardware like Coldcard to create signatures. The process splits transaction creation (on a networked watch-only wallet) and signing (on an isolated device), with data transferred via microSD, QR codes, or NFC. BIP-174 standardized the Partially Signed Bitcoin Transaction (PSBT) format, enabling seamless offline workflows across wallets.

Step 1: A networked computer builds a transaction and saves it as a PSBT file. Step 2: The PSBT moves offline via physical media. Step 3: The signing device verifies details on its screen and adds a cryptographic signature using the private key. Step 4: The signed PSBT returns online for broadcast. This eliminates exposure to malware or keyloggers, critical for institutional funds holding large balances.

However, Dark Skippy (2024) revealed vulnerabilities: compromised firmware can leak seed data via transaction signatures. Mitigations like BitBox's anti-exfiltration protocol mix random data into nonces, but most hardware wallets remain at risk. Key at rest—private keys stored long-term—poses inherent security tradeoffs. Alternatives like Frozen Security's physical key derivation eliminate digital storage entirely, redefining custody models.

The evolution of offline signing reflects Bitcoin's security arms race. While PSBT remains the gold standard, innovations like anti-exfiltration protocols and keyless custody address emerging threats, ensuring Bitcoin's long-term viability as a secure digital asset.