Subject: Re: [bitcoindev] CTV + CSFS: a letter

From: Owen Kemeys <[email-masked]>
Date: Sun, 15 Jun 2025 12:43:50 -0700 (PDT)

On Sunday, 15 June 2025 at 10:10:59 UTC-6 Jameson Lopp wrote:

It's the same problem as securely generating and storing keys. In order for 
presigned transaction vaults to actually be trustworthy then ephemeral key 
usage needs to occur on a hardened offline device that is highly unlikely 
to be compromised. I'm not aware of any of the hardware manufacturers 
offering functionality for generating and signing with ephemeral keys.

I'm talking my employer's book, but you can approximate this function for 
sure on Foundation Passport by generating a child seed then loading it as a 
temporary signing key (forgotten on power off). I'm sure Coldcard offers 
something similar and perhaps others. Of course, you'd have to remember to 
delete the seed before putting the device away, and it's derived, not 
generated from scratch, so undermining some of the security. But it's 
close, and the desired functionality could be added if there was demand, 
all the pieces are there.

The upcoming Passport Prime device would be perfectly placed to serve a 
workflow in a secure environment that generates an ephemeral key, signs, 
discards, and passes the PSBTs back to the online device. This is niche 
enough that we're unlikely to write the applet ourselves, but that's why 
it's an open source platform - hopefully some vault project will come along 
and assemble the building blocks in the right way; it shouldn't be hard.

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