MIME-Version: 1.0
From: Tobias Kaupat <Tobias@kaupat-hh.de>
Date: Wed, 5 May 2021 19:32:05 +0200
Message-ID: <CAPyCnfvqVT00C2TZ86GXf856jNJqPXY0duRa1CfdCqC0ecC6xA@mail.gmail.com>
To: bitcoin-dev@lists.linuxfoundation.org
Content-Type: multipart/alternative; boundary="0000000000007965ff05c198933f"
Subject: [bitcoin-dev] Encryption of an existing BIP39 mnemonic without
	changing the seed
Precedence: list

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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

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<div dir=3D"ltr">Hi all,<div>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.</div><div><br><div><b>Use case:</b></div><div>An existi=
ng mnemonic (e.g. for a hardware wallet) should be saved on a paper backup =
in a password encrypted=C2=A0form. The encrypted=C2=A0form should be a mnem=
onic itself to keep all backup properties like error correction.</div><div>=
<br></div><div><b>Suggested=C2=A0solution:</b></div><div>1) Take the existi=
ng mnemonic and extract=C2=A0the related entropy</div><div>2) Create a SHA5=
26 hash (key) from a user defined password</div><div>3) Use the key as inpu=
t for an AES CTR (empty IV) to encrypt the entropy</div><div>4) Derive a ne=
w mnemonic from the encrypted entropy to be stored on a paper backup</div><=
div><br></div><div>We can add some hints to the paper backp that the mnemon=
ic=C2=A0is encrypted, or prefix it with &quot;*&quot; to make clear it&#39;=
s not usable without applying the password via the algorithm above.</div><d=
iv><br></div><div>To restore the original mnemonic, one must know the passw=
ord and need to follow the process above again.</div><div><br></div><div>An=
 example implementation=C2=A0in GoLang can be found here:</div><div><a href=
=3D"https://github.com/Niondir/go-bip39/blob/master/encyrption_test.go" tar=
get=3D"_blank">https://github.com/Niondir/go-bip39/blob/master/encyrption_t=
est.go</a><br></div><div><br></div><div><b>Why not use the existing BIP-39 =
Passphrase?</b></div><div>When generating a mnemonic=C2=A0with passphrase, =
the entropy is derived from the passphrase. When you have an existing mnemo=
nic 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 t=
o encrypt the entropy.</div><div><br></div><div>I&#39;m open for your feedb=
ack. All encryption parameters are up to discussion and the whole proposal =
needs a security review. It&#39;s just the first draft.</div><div><br></div=
><div><b>Existing solutions</b></div><div>One solution I found is &quot;See=
dshift&quot; which can be found here: <a href=3D"https://github.com/mifunet=
oshiro/Seedshift">https://github.com/mifunetoshiro/Seedshift</a></div><div>=
<br></div><div>But I consider it less secure and I would like to suggest a =
solution based on provably secure algorithms rather than a &quot;rot23 deri=
vation&quot;. Also using a date as password seems not very clever to me.</d=
iv><div><br></div><div>Kind regards<br>Tobias</div></div></div>

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