r/cryptography • u/martinkrafft • 5d ago

Designing your digital estate with PQC

Hello,

I am in the process of designing my digital estate, i.e. preparing for the case when I am dead or incapacitated, and close friends will have to take over my affairs. I'd like to make this as easy as possible for them.

With this post, I would like to sketch my current idea (the result of a couple of weeks of research), and ideally get your input from a cryptographic perspective.

Note that this is a lot more conceptual than it is technical. I hope this is okay for this sub, if not — I am sorry, maybe you can recommend a better place?

No AI is being used in the composition of this post.

Up front, here is what I am trying to protect: passwords and the like, from loss, and also from unauthorized use by a third party. My approach does involve a handful of family members and close friends, and I generally trust them, though my solution also does protect against one or two of them going rogue.

It's actually not rocket science:

  1. on an isolated, secure system, create a PQC-enabled GnuPG key pair (GPG 5, ky1024_cv448), secured with a 256bit passphrase;

  2. use SLIP39 to encode this passphrase into a m-of-n multi-share, i.e. 4 of a total of 7 shares together can yield the passphrase;

  3. copy each SLIP39 mnemonic share to individual paper, along with instructions;

  4. put the key onto n USB sticks or microSD cards each, and wipe any other copy. Do keep the public key, obviously.

  5. slip the USB keys/microSD cards and the paper menmonics into n envelopes each, and seal them. I won't go into details here, let's just assume the information therein is now no longer accessible without permanent tamper of the seal;

  6. distribute the envelopes among your n trusted peers, along with instructions such as "keep this secret until my will executor contacts you, or you find out about my death, in which case please inform those that survive me that you hold a piece of my digital estate".

  7. create (and regularly update) a collection of files, i.e. a tarball with the secrets and documents that make up the digital estate;

  8. encrypt this tarball with the public key of the aforementioned GnuPG key pair to create a so-called vault;

  9. mail the resulting vault to the trusted parties at regular intervals, asking them to confirm receipt, posession of the envelopes, and possibly randomly to prove that the envelopes are still untampered.

That's it. I've thought long and hard about alternative, redundant storages for the vault. Keeping them on USB sticks in bank safes just means that they'll go stale with time, so they have to be updateable.

And even though I've thought long and hard about possibilities to keep them redundantly around in a way that it's extremely unlikely that copies will have been made by the time post-post-quantum breaks Kyber, this remains I think an insoluble problem. Risk reduction here comes with a lot of extra complexity.¹

The solution with mailing the updated vaults out at regular intervals to trusted parties seems appropriate, as it creates redundancy, and no single one of those trusted people is in the threat model.

If post-quantum happens, and PQC turns out to be inadequate, then either I am still around (and we will have a whole slew of other problems) and could change the important passwords, or my digital estate needs to be accessed and handled anyway — not that I'd care, having tried to do my best to make it easy for those who survive me.

What do you think? Am I grossly overlooking attack vectors, or misunderstanding certain aspects of the cryptography?

Thanks for your time, m

Footnote:

¹) one could, for instance, include in the envelopes links to redundant storage of a "proxy archive", containing the GPG key, as well as two additional links to (1) the vault, and (2) a one-time-pad for XOR encryption. Each of those two files by themselves would be useless, and the likelihood of both of them being found together and the proxy archive also saved for a later point in time in a post-PQC world, is infinitesimally small, but the complexity is multi-fold.

• Upvotes

33% Upvoted

18 comments sorted by

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u/cmd-t 5d ago

What is your threat model? What are you afraid of?

Do you really have such interesting data that’s not stored elsewhere on other people’s servers?

Passwords to larger/reputable services really aren’t needed as executors get access to these things by other means (death certificates etc)

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u/martinkrafft 5d ago

As I wrote, my threat model is unauthorized use and loss, and a core motivation is to make it easiest for those who survive me.

Executors and heirs can eventually get access to banks, and even social media, etc., but there are two problems:

  1. there are no standardised processes, and some providers make you go through bureaucracy hell. Been there, done that…

  2. getting the certificates required by banks and other reputable services over here in Germany regularly takes 6–12 months, during which nobody can do anything.

And yes, most of my data is actually self-hosted, encrypted, decentralised, etc.…

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u/cmd-t 4d ago

By whom? The North Korean government or your in-laws?

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u/martinkrafft 4d ago

By whom what?

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u/No-Yogurtcloset-755 4d ago

This is riddiculous.

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u/martinkrafft 3d ago

how so?

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u/Dingbatdingbat 5d ago

Way too complicated for practical use and too many failure points.  Sounds like the start of a bad movie.

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u/martinkrafft 5d ago

I've played this through with normal people and it's not hard. Tools like GPG and SLIP39 have come a long way, are standardized and open-source, so readily available and future-proof.

Where do you see failure points?

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u/Dingbatdingbat 4d ago

If any one of those envelopes goes missing, or if any one of those people is unable or unwilling to cooperate, the whole thing fails.

And good luck updating it every time you change your password for any of those sites.

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u/martinkrafft 4d ago

The whole point of using SLIP39 is that only a subset of the people need to cooperate. I could make it that I have 9 envelopes, and any 4 of them together would be able to access the estate.

Changing the password for any site means updating my password database. Then, at regular intervals, I encrypt the vault again (for which I do not need to know the password) and send it around. Part of the design…

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u/Dingbatdingbat 4d ago

Are you a movie villain?

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u/martinkrafft 4d ago

No. I actually think I have a point and you seem to not have read or understood my design.

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u/Dingbatdingbat 4d ago

I read it, I understood it, and the best part of it is the entertainment and sense of satisfaction you got coming up with it.

It will work, unless it doesn’t, and all involved will have thoughts about it and the person who came up with it.

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u/martinkrafft 4d ago

just please disappear from my thread. Your comment yesterday clearly shows that you do not understand two core principles...

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u/kryptos- 4d ago

My recommendation would actually be the use of a dead man's switch. Establish a PQ secure communications channel with the individuals.

Connect a button, timer and alert/ beeper to a well-known circuit board like the raspberry pi. If you don't press the button within a week, the message gets sent. Use the timer and beeper to alert you.

Harden the device, set up a very strict firewall (to only the approved comms channels / destinations). These rules should require your device to egress first.

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u/tenoun 1d ago

sounds like AI crap !

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u/martinkrafft 1d ago

i did not use any AI