[bitcoin-dev] Encryption of an existing BIP39 mnemonic without changing the seed

[Tobias Kaupat]

Hello Erik,
Thanks for your reply.
After a little research I came to the same conclusion. PDKDF2 makes sense,
since it is already used in BIP39.
I will update my code.



Regarding SeedXOR:
That's at least a similar solution, but than I have to store 2 phrases, I
really like to keep one part in my head, which is only possible with a
password.
Plus for anyone who want to use two seeds my proposal also works - it just
needs software to be applied.

Kind regards
Tobias Kaupat



Erik Aronesty <erik at q32.com> schrieb am Do., 6. Mai 2021, 15:19:

> i would stretch the password, with pbkdf2 or argon2 with like 30k
> rounds or something first, rather than "just hashing it".  remember,
> it's pretty easy to validate these seeds - not like you lock someone
> out after 9 guesses!
>
> On Wed, May 5, 2021 at 3:38 PM Tobias Kaupat via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org> wrote:
> >
> > Hi all,
> > I want to start a discussion about a use case I have and a possible
> solution. I have not found any satisfying solution to this use case yet.
> >
> > Use case:
> > An existing mnemonic (e.g. for a hardware wallet) should be saved on a
> paper backup in a password encrypted form. The encrypted form should be a
> mnemonic itself to keep all backup properties like error correction.
> >
> > Suggested solution:
> > 1) Take the existing mnemonic and extract the related entropy
> > 2) Create a SHA526 hash (key) from a user defined password
> > 3) Use the key as input for an AES CTR (empty IV) to encrypt the entropy
> > 4) Derive a new mnemonic from the encrypted entropy to be stored on a
> paper backup
> >
> > We can add some hints to the paper backp that the mnemonic is encrypted,
> or prefix it with "*" to make clear it's not usable without applying the
> password via the algorithm above.
> >
> > To restore the original mnemonic, one must know the password and need to
> follow the process above again.
> >
> > An example implementation in GoLang can be found here:
> > https://github.com/Niondir/go-bip39/blob/master/encyrption_test.go
> >
> > Why not use the existing BIP-39 Passphrase?
> > When generating a mnemonic with passphrase, the entropy is derived from
> the passphrase. When you have an existing mnemonic without a passphrase,
> any attempt to add a passphrase will end up in a different seed and thus a
> different private key. What we actually need is to encrypt the entropy.
> >
> > I'm open for your feedback. All encryption parameters are up to
> discussion and the whole proposal needs a security review. It's just the
> first draft.
> >
> > Existing solutions
> > One solution I found is "Seedshift" which can be found here:
> https://github.com/mifunetoshiro/Seedshift
> >
> > But I consider it less secure and I would like to suggest a solution
> based on provably secure algorithms rather than a "rot23 derivation". Also
> using a date as password seems not very clever to me.
> >
> > Kind regards
> > Tobias
> > _______________________________________________
> > bitcoin-dev mailing list
> > bitcoin-dev at lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
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