Thanks for the interesting suggestion. I’d consider the blockchain for succinct, long-term or permanent information. However, I would not back up my forum credentials on the blockchain. It’s the world’s most expensive database, and it’s append-only. What do I do if I want to change my forum password next year, or maybe next month?
Forum credentials are only good to log in to the forum; if the forum ever goes kaput, the credentials are useless. Therefore, I think the forum is the logical storage place.
(for example, if you need to leave a country with strict border controls and censorship, and you cannot carry any data with you in physical form - or even a phone).
For
this threat model, I recommend splitting a symmetric key
m/
n with Shamir’s method, and exfiltrating the key shares by a variety of different covert channels, on both electronic and paper media, with steganography and other information-hiding methods, such that:
- Security: The risk is low of any adversary ever recovering at least m shares—or of today’s adversary detecting while you’re in their jurisdiction that you exfiltrate data; and,
- Availability: The risk is low that you fail to keep and reconstruct m shares, without which your precious data are lost forever.
Use that one key to encrypt other data—including PGP private keys, Bitcoin seeds, etc., and everything else. Use a symmetric cipher in an AEAD mode (AES-256-GCM or Chacha20-Poly1305). This is post-quantum, simple, and reliable
iff you are competent about avoiding the
nonce reuse footgun; I’d have other recommendations to use public-key cryptography for this, but it can get complicated. Simplicity is good, when you may need to rewrite custom software twice (on both sides of the border) first to encrypt, then to recover your data. Exfiltrate the encrypted bulk data by whatever insecure channels you may have available for this purpose; if someone is able to reach the Bitcoin Forum privately, I guess they can also simply use a storage VPS or other untrusted server.
Risks are never zero in these difficult situations. How low can the risks go? I must ask myself, how good am I at hiding things in plain sight?
In the Lauda memorial thread, I wasn’t trying to make
the puzzle impossible for humans—to the contrary; yet almost six years later, with nontrivial prize money accumulated, no one has solved it. I assure you, the answer is so much in plain sight that people would smack their foreheads if they saw it! If, say, I wanted to cross the Russian border with my coins and with all of my encrypted political writings, would I bet that I can hide information well enough to exfiltrate a 256-bit key? It is a grave question, not suitable for bragging.
Beyond that, I do not like to discuss such ideas publicly. But if any dissident in the Russian-language boards has a practical need to bypass border controls somewhere that the guards speak Russian, I’d be happy to discuss this privately.
ProtonMail includes PGP (likely the simplest method), so, in all likelihood, all you need to remember is your username and password, plus a 9-digit recovery key for 2FA ... not too difficult, really.
Proton Mail is not post-quantum for long-term security. For other reasons also, I don’t recommend them generally.
My current PGP key is also not post-quantum. I intend to upgrade it soon. However, it’s fine for storing my current forum login credentials.
