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I am working on a PGP scheme where a private key is created offline (and in volatile memory) and must be reused on very rare occasions to sign a piece of exportable data. The private key is locked using a random passphrase with ~256 bits of entropy, and the passphrase and resultant PGP secret key are written on paper and locked up for later use. The key needs to remain usable for years in the future.

To simplify the future import procedure by reducing the amount of manually entered data, would it be safe to publicly export the secret key file provided the passphrase is kept locked up? That way, the secret key can be imported off of a flash drive or similar, while only the passphrase needs to be keyed in.

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    $\begingroup$ Why not use a pc, not connected to the internet, as a sign machine, and carry the data by air gap, sign with this pc, and take back? $\endgroup$ – kelalaka Nov 3 '18 at 20:00
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    $\begingroup$ Linux on a stick plugged into a TV would do the trick. There are also PGP smart cards. $\endgroup$ – Maarten Bodewes♦ Nov 4 '18 at 4:15
  • $\begingroup$ I'd be concerned about the devices dying or becoming otherwise inaccessible. I suppose redundant Pi Zeroes could be a good option. $\endgroup$ – cjgriscom Nov 5 '18 at 0:28
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What I understand from the question, as if you have a Certificate Authority key around.

Using an air-gapped pc, raspberry pi zero, etc. is a good choice. The important points;

  1. Sign the document in the offline solution, do not carry the key.
  2. You can have more than one air-gapped solution. Extra hardware against failures.
  3. You can keep your old paper around.
  4. Have an extra security against the easy copying from the device

Extream care

Off-line encryption: In the military, when a message is top secret, the message, firstly, encrypted with an off-line system that has no connection to the network. After the off-line encryption, the message is sent by on-line crypto. The problem is not finished here.

Tempest Attack: The signal from a pc can be carried by air or the power lines. This requires an expert in this area to filter the machine. So the off-line system and the power lines are filtered.

Covert channel: If an adversary can install a software to your air gapped solution that he can use the heat to transfer the data. Have a mechanism to prevent access to the solution.

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  • $\begingroup$ I'd be surprised if he could use heat to transfer data for a Raspberry Pi Zero, considering the original heat side-channel is done with bulky hot servers in a server room raising the entire ambient temperature to exfiltrate data. A Raspi might only be able to do that if it's being received by a sensitive IR camera. $\endgroup$ – forest Nov 8 '18 at 3:28
  • $\begingroup$ @forest It is just a generic answer to all extra hardware. You are right that Raspberry Pi Zero cannot be realistically used to data transfer by heat. $\endgroup$ – kelalaka Nov 8 '18 at 10:26

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