Return-Path: Received: from whitealder.osuosl.org (smtp1.osuosl.org [140.211.166.138]) by lists.linuxfoundation.org (Postfix) with ESMTP id 50772C013A for ; Fri, 12 Feb 2021 18:01:23 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by whitealder.osuosl.org (Postfix) with ESMTP id 3DF8286F75 for ; Fri, 12 Feb 2021 18:01:23 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from whitealder.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ej+JtCC2BmbU for ; Fri, 12 Feb 2021 18:01:22 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from mail-vs1-f46.google.com (mail-vs1-f46.google.com [209.85.217.46]) by whitealder.osuosl.org (Postfix) with ESMTPS id D8ECC86EC8 for ; Fri, 12 Feb 2021 18:01:21 +0000 (UTC) Received: by mail-vs1-f46.google.com with SMTP id u7so16899vsp.12 for ; Fri, 12 Feb 2021 10:01:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nunchuk-io.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=NwI2mqILTAR+G7QSJjr9zD2obeF5KytGpxFnMbcvIHc=; b=lR+YAKQH1PALqUp/R9z6vN1nMUoX4+j/GNp6ItGdWKDhPrAHj4uQ+laXD51QS0Q2z+ T3hCcrLk0k5rX9e01qfQEpoyA1H05B+Lk+rcgVGtsH5OF8YahB6s1vbgIp4IrIpIB8RZ tKVX9yFGPNr4FGHaCIWLGMaU/R5kbxx+YefKfC0VzddSLXrJAHTWG01qntd8FZ69ti89 269ct/C/8wQ4FhP3FsU9AQs3nT4a5kUtbjGTfgxeg11WT/yVqaeO/4aGBqOiJ/ZW1eVm bnsj6wZvy3aHAuc3X2tTiO9TVAjam5dzTwyF+rQU6SUglo+FfZJKJS395Y8ALWlEJ1SF 6i5A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=NwI2mqILTAR+G7QSJjr9zD2obeF5KytGpxFnMbcvIHc=; b=RH4SM1rtprAFbhjAFodk53lATzKbLht+VlcHGG4IpjF2dX1maEeUqtl6BJ71vkDqU3 oigmGazVKreUzQNPatAN9pidQ/2TzCMJqq1P61xiPAAtTYTQMgufuZQ4daI4iPiMPwDA KfAcVNKiAyPQgCrw//jw1kOuay+BUlueVlWU7vhRqkgLIptYL34SpkDba3ioj00mOXju BrT9169fW4EFgrgQO0xaizqmZlmbV7NMGPE+e3yZsYJcp80KeybGX7BDq1er4xGbGnMo 2xL/J32wcmjSVj1NEp9QAUei62VjSFXaQcsn4zsjchXzM+hYmJnaGYUJ2E4jrsJ/KRzw vtmw== X-Gm-Message-State: AOAM530v0NcL4yNz5UjY8MjZFz7SwgYG7HqZjNCCHJz4+2jTamZftyPQ cWZY8dvNd6iCTm1NGJOG4UZjAc+KJ74k8Bpi8f9dT0i2JeaSJcRAsuAArQ== X-Google-Smtp-Source: ABdhPJzuGXdxjFuW6VEpt49k8FNoTeuI2nGW9KnhOBOInyYJiZ2p7dUjAz49pRS2L+OOH9tq6/RaBvcK43odJp6QUeg= X-Received: by 2002:a1f:5606:: with SMTP id k6mr2573099vkb.6.1613152509023; Fri, 12 Feb 2021 09:55:09 -0800 (PST) MIME-Version: 1.0 References: <20210211172910.3b550706@simplexum.com> <20210212184231.22b517aa@simplexum.com> In-Reply-To: <20210212184231.22b517aa@simplexum.com> From: Hugo Nguyen Date: Fri, 12 Feb 2021 09:54:57 -0800 Message-ID: To: Dmitry Petukhov Content-Type: multipart/alternative; boundary="00000000000044e9c005bb27564f" X-Mailman-Approved-At: Fri, 12 Feb 2021 18:29:59 +0000 Cc: Bitcoin Protocol Discussion Subject: Re: [bitcoin-dev] Proposal: Bitcoin Secure Multisig Setup X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 12 Feb 2021 18:01:23 -0000 --00000000000044e9c005bb27564f Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Feb 12, 2021 at 9:36 AM Dmitry Petukhov wrote: > If HUMAN_READABLE_TITLE is the additional secret, the user would need > to enter it on the device in addition to the nonce, wouldn't it defeat > the advantage in UX that was gained by using (relatively) short nonce ? > > Is 64 bit nonce not enough ? > > Good question. If we don't need the extra entropy, we can fix the HUMAN_READABLE_TITLE string. Something like "No SPOF". (No Single Point Of Failure). > It seems that to crack this with fixed Pwd and 64 bit nonce, the > attacker will need to be about 10^15 more powerful than 80Mhz MCU: > (2^64)/(0.3*10^15)/3600 =3D 17 hours. I don't know if 10^15 is realistic > scale. Average desktop cpu seems to be about 10^3 more powerful than > the mentioned MCU for this task. > > Maybe for the UX it would be better to choose the number of rounds to > use in PBKDF2, instead of using variable Pwd. Number of rounds will be > easier to enter on the device (or just choose it from a set of > pre-defined values). The more money is at stake, the higher number of > rounds could the coordinator choose (taking into account the > characteristics of the participant devices) > > Or simply allow bigger entropy (more than 6 mnemonic words), if > the coordinator feels that 64 bit of entropy is not enough. That could work. Allowing variable iteration count is probably better UX-wise. Best, Hugo > > =D0=92 Fri, 12 Feb 2021 08:55:55 -0800 > Hugo Nguyen wrote: > > > Thanks everyone who has provided inputs so far! > > > > This is the new proposal for the encryption aspect of the scheme, > > based on all the feedback. > > > > The key derivation function would be PBKDF2, with PRF =3D SHA512. This > > should be readily available on today's hardware already, as they are > > used for BIP39. > > > > DK =3D PBKDF2(PRF, Password, Salt, c, dkLen) > > PRF =3D SHA512 > > Pwd =3D HUMAN_READABLE_TITLE > > Salt =3D NONCE > > c =3D 2048 > > dkLen =3D 256 > > > > HUMAN_READABLE_TITLE is in ASCII format, minimum length =3D 8, maximum > > length =3D 20. > > NONCE is a 64-bit number. > > > > Reason for going with SHA512 is due to legacy support on some > > hardware. c=3D2048 also mimics BIP39. It takes about ~3 seconds to > > derive the encryption key on a 80Mhz MCU. We feel like this is a good > > enough tradeoff for this use case. The assumption here is that the > > secure session is only needed temporarily for a few hours, maybe up > > to one day. > > > > The Coordinator and Signers agree and exchange these 2 secrets prior > > to the setup. The NONCE can be converted to either: > > (a) a 6-word phrase using BIP39 wordlist > > (b) a 20-digit decimal number > > (c) a QR code > > > > Depending on the vendor. This flexibility in the data format allows > > each vendor to customize the UX based on their respective device > > capabilities. > > > > Best, > > Hugo > > > > On Thu, Feb 11, 2021 at 8:25 AM Dmitry Petukhov via bitcoin-dev < > > bitcoin-dev@lists.linuxfoundation.org> wrote: > > > > > =D0=92 Thu, 11 Feb 2021 05:45:33 -0800 > > > Hugo Nguyen via bitcoin-dev > > > wrote: > > > > > > > > > ENCRYPTION_KEY =3D SHA256(SHA256(TOKEN)) > > > > > > > > > > This scheme might be vulnerable to rainbow table attack. > > > > > > > > > > > > > Thank you for pointing this out! Incidentally, Dmitry Petukhov > > > > also told me the same privately. > > > > > > My thought was that if TOKEN has the characteristics of a password > > > (short ASCII string), then it would be better to use key derivation > > > function designed for passwords, like PBKDF2. > > > > > > The counter-argument to this is that this adds another code > > > dependency for vendors, if the device firmware does not already > > > have the required key derivation function. > > > > > > Maybe this could be solved by going into opposite direction - make > > > the "token" even longer, use the mnemoic. > > > > > > The issue is that entering long data of the shared key into the > > > device manually is difficult UX-wise. > > > > > > Hww vendors that allow to enter custom keys into their device > > > already have to face this issue, and those who allow to enter > > > custom keys via mnemonic probably tackled this somehow. > > > > > > Maybe the shared key for multisig setup can be entered in the same > > > way ? (with maybe additional visual check via some fingerprint). > > > > > > Although we would then have another issue of potential confusion > > > between two procedures (entering the main key and entering the > > > shared key for multisig setup), and the measures has to be taken to > > > prevent such confusion. > > > > > > The approaches can be combined - specify a key derivation function > > > suitable for passwords; via secure channel, share a password and/or > > > the derived key. If hww supports derivation function, it can derive > > > the key from password. If hww supports only keys, the key can be > > > entered raw or via mnemonic. > > > _______________________________________________ > > > bitcoin-dev mailing list > > > bitcoin-dev@lists.linuxfoundation.org > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > > > > > --00000000000044e9c005bb27564f Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


=
On Fri, Feb 12, 2021 at 9:36 AM Dmitr= y Petukhov <dp@simplexum.com>= wrote:
If HUMAN= _READABLE_TITLE is the additional secret, the user would need
to enter it on the device in addition to the nonce, wouldn't it defeat<= br> the advantage in UX that was gained by using (relatively) short nonce ?

Is 64 bit nonce not enough ?


Good question. If we don't need=C2= =A0the extra entropy, we can fix the=C2=A0HUMAN_READABLE_TITLE string.
<= br>Something like "No SPOF". (No Single Point Of Failure).
<= div>
=C2=A0
It seems that to crack this with fixed Pwd and 64 bit nonce, the
attacker will need to be about 10^15 more powerful than 80Mhz MCU:
(2^64)/(0.3*10^15)/3600 =3D 17 hours. I don't know if 10^15 is realisti= c
scale. Average desktop cpu seems to be about 10^3 more powerful than
the mentioned MCU for this task.

Maybe for the UX it would be better to choose the number of rounds to
use in PBKDF2, instead of using variable Pwd. Number of rounds will be
easier to enter on the device (or just choose it from a set of
pre-defined values). The more money is at stake, the higher number of
rounds could the coordinator choose (taking into account the
characteristics of the participant devices)

Or simply allow bigger entropy (more= than 6 mnemonic words), if
the coordinator feels that 64 bit of entropy= is not enough.

That could work. Allowing variable ite= ration count is probably better UX-wise.

Best,
Hugo
=C2=A0

=D0=92 Fri, 12 Feb 2021 08:55:55 -0800
Hugo Nguyen <hugo@n= unchuk.io> wrote:

> Thanks everyone who has provided inputs so far!
>
> This is the new proposal for the encryption aspect of the scheme,
> based on all the feedback.
>
> The key derivation function would be PBKDF2, with PRF =3D SHA512. This=
> should be readily available on today's hardware already, as they a= re
> used for BIP39.
>
> DK =3D PBKDF2(PRF, Password, Salt, c, dkLen)
> PRF =3D SHA512
> Pwd =3D HUMAN_READABLE_TITLE
> Salt =3D NONCE
> c =3D 2048
> dkLen =3D 256
>
> HUMAN_READABLE_TITLE is in ASCII format, minimum length =3D 8, maximum=
> length =3D 20.
> NONCE is a 64-bit number.
>
> Reason for going with SHA512 is due to legacy support on some
> hardware. c=3D2048 also mimics BIP39. It takes about ~3 seconds to
> derive the encryption key on a 80Mhz MCU. We feel like this is a good<= br> > enough tradeoff for this use case. The assumption here is that the
> secure session is only needed temporarily for a few hours, maybe up > to one day.
>
> The Coordinator and Signers agree and exchange these 2 secrets prior > to the setup. The NONCE can be converted to either:
> (a) a 6-word phrase using BIP39 wordlist
> (b) a 20-digit decimal number
> (c) a QR code
>
> Depending on the vendor. This flexibility in the data format allows > each vendor to customize the UX based on their respective device
> capabilities.
>
> Best,
> Hugo
>
> On Thu, Feb 11, 2021 at 8:25 AM Dmitry Petukhov via bitcoin-dev < > bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> > =D0=92 Thu, 11 Feb 2021 05:45:33 -0800
> > Hugo Nguyen via bitcoin-dev <bitcoin-dev@lists.linuxfoundation= .org>
> > wrote:
> >=C2=A0
> > > > > ENCRYPTION_KEY =3D SHA256(SHA256(TOKEN))=C2=A0 > > > >
> > > > This scheme might be vulnerable to rainbow table attack= .
> > > >=C2=A0
> > >
> > > Thank you for pointing this out! Incidentally, Dmitry Petukh= ov
> > > also told me the same privately.=C2=A0
> >
> > My thought was that if TOKEN has the characteristics of a passwor= d
> > (short ASCII string), then it would be better to use key derivati= on
> > function designed for passwords, like PBKDF2.
> >
> > The counter-argument to this is that this adds another code
> > dependency for vendors, if the device firmware does not already > > have the required key derivation function.
> >
> > Maybe this could be solved by going into opposite direction - mak= e
> > the "token" even longer, use the mnemoic.
> >
> > The issue is that entering long data of the shared key into the > > device manually is difficult UX-wise.
> >
> > Hww vendors that allow to enter custom keys into their device
> > already have to face this issue, and those who allow to enter
> > custom keys via mnemonic probably tackled this somehow.
> >
> > Maybe the shared key for multisig setup can be entered in the sam= e
> > way ? (with maybe additional visual check via some fingerprint).<= br> > >
> > Although we would then have another issue of potential confusion<= br> > > between two procedures (entering the main key and entering the > > shared key for multisig setup), and the measures has to be taken = to
> > prevent such confusion.
> >
> > The approaches can be combined - specify a key derivation functio= n
> > suitable for passwords; via secure channel, share a password and/= or
> > the derived key. If hww supports derivation function, it can deri= ve
> > the key from password. If hww supports only keys, the key can be<= br> > > entered raw or via mnemonic.
> > _______________________________________________
> > bitcoin-dev mailing list
> > bitcoin-dev@lists.linuxfoundation.org
> > https://lists.linuxfoundatio= n.org/mailman/listinfo/bitcoin-dev
> >=C2=A0

--00000000000044e9c005bb27564f--