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From: Erik Aronesty <erik@q32.com>
To: Jonas Nick <jonasdnick@gmail.com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Blinded 2-party Musig2
Date: Mon, 24 Jul 2023 10:40:10 -0400	[thread overview]
Message-ID: <CAJowKgKM-8kz2b7W5qihYfNvdzGkUxF4T4puXK95MKMuu83xkA@mail.gmail.com> (raw)
In-Reply-To: <ca674cee-6fe9-f325-7e09-f3efda082b6b@gmail.com>

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You can't choose R if you provide posk

On Mon, Jul 24, 2023 at 10:31 AM Jonas Nick via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi Tom,
>
> I'm not convinced that this works. As far as I know blind musig is still
> an open
> research problem. What the scheme you propose appears to try to prevent is
> that
> the server signs K times, but the client ends up with K+1 Schnorr
> signatures for
> the aggregate of the server's and the clients key. I think it's possible to
> apply a variant of the attack that makes MuSig1 insecure if the nonce
> commitment
> round was skipped or if the message isn't determined before sending the
> nonce.
> Here's how a malicious client would do that:
>
> - Obtain K R-values R1[0], ..., R1[K-1] from the server
> - Let
>      R[i] := R1[i] + R2[i] for all i <= K-1
>      R[K] := R1[0] + ... + R1[K-1]
>      c[i] := H(X, R[i], m[i]) for all i <= K.
>    Using Wagner's algorithm, choose R2[0], ..., R2[K-1] such that
>      c[0] + ... + c[K-1] = c[K].
> - Send c[0], ..., c[K-1] to the server to obtain s[0], ..., s[K-1].
> - Let
>      s[K] = s[0] + ... + s[K-1].
>    Then (s[K], R[K]) is a valid signature from the server, since
>      s[K]*G = R[K] + c[K]*a1*X1,
>    which the client can complete to a signature for public key X.
>
> What may work in your case is the following scheme:
> - Client sends commitment to the public key X2, nonce R2 and message m to
> the
>    server.
> - Server replies with nonce R1 = k1*G
> - Client sends c to the server and proves in zero knowledge that c =
>    SHA256(X1 + X2, R1 + R2, m).
> - Server replies with s1 = k1 + c*x1
>
> However, this is just some quick intuition and I'm not sure if this
> actually
> works, but maybe worth exploring.
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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  reply	other threads:[~2023-07-24 14:40 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-24  7:46 [bitcoin-dev] Blinded 2-party Musig2 Tom Trevethan
2023-07-24 10:50 ` ZmnSCPxj
2023-07-24 14:25   ` Erik Aronesty
2023-07-24 16:08     ` Tom Trevethan
2023-07-24 15:57   ` Tom Trevethan
2023-07-24 14:12 ` Jonas Nick
2023-07-24 14:40   ` Erik Aronesty [this message]
2023-07-24 15:40     ` Jonas Nick
2023-07-24 16:51   ` AdamISZ
2023-07-25 14:12     ` Erik Aronesty
2023-07-25 16:05       ` Tom Trevethan
2023-07-26  4:09         ` Erik Aronesty
2023-07-26 17:40           ` Andrew Poelstra
2023-07-26 19:59           ` Jonas Nick
2023-07-26 20:35             ` Tom Trevethan
2023-07-26 22:06               ` Erik Aronesty
2023-07-27  2:54                 ` Lloyd Fournier
2023-07-27  8:07               ` Jonas Nick
     [not found]                 ` <CAJvkSsfa8rzbwXiatZBpwQ6d4d94yLQifK8gyq3k-rq_1SH4OQ@mail.gmail.com>
2023-07-27 13:25                   ` [bitcoin-dev] Fwd: " Tom Trevethan
2023-08-07  0:55                     ` [bitcoin-dev] " Tom Trevethan
2023-08-08 17:44                       ` moonsettler
2023-08-09 15:14                         ` Tom Trevethan
2023-08-10  3:30                           ` Lloyd Fournier
2023-08-10 11:59                             ` Tom Trevethan
2023-08-14  6:31                               ` Lloyd Fournier
2023-08-30 10:52                       ` Tom Trevethan
2023-07-24 15:39 ` Jonas Nick
2023-07-24 16:22   ` Tom Trevethan
2023-07-26  9:44   ` moonsettler
2023-07-26 14:59     ` Jonas Nick
2023-07-26 19:19     ` AdamISZ
2023-07-26 19:28       ` moonsettler
2023-07-27  5:51         ` AdamISZ
     [not found] <mailman.125690.1690381971.956.bitcoin-dev@lists.linuxfoundation.org>
2023-07-26 16:32 ` Tom Trevethan

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