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Date: Sun, 14 Apr 2013 01:21:21 -0400
From: Alan Reiner <etotheipi@gmail.com>
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Subject: Re: [Bitcoin-development] RFC: extend signmessage/verifymessage to
 P2SH multisig
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If we're going to extend/expand message signing, can we please add a
proper ASCII-armored format for it?  Really, anything that encodes the
signed message next to the signature, so that there's no ambiguities
about what was signed.  You can keep the "bare signatures" as an option
for backwards compatiblity, but offer this as the primary one.

What we really want is to have the user copy an ASCII-armored block of
text into the client (or we could have a URI-extension for this), and
the app pops up with a window that says "The following message has a
valid signature from address 1XKjf32kJbf...:   <message>".  

I know people argue they'd like to get away from raw addresses and
copy-and-paste.  But it'll be a while before that happens, and there's a
lot of demand for Armory to become compatible with Bitcoin-Qt signing. 
People are obviously using it.

-Alan





On 04/14/2013 01:09 AM, Peter Todd wrote:
> Currently signmessage/verifymessage only supports messages signed by a
> single key. We should extend that to messages signed by n-of-m keys, or
> from the users point of view, P2SH multisig addresses.
>
> rpc.cpp:signmessage() returns the output of SignCompact(). That in turn
> starts with a header byte marking the signs of the various keys to allow
> for key recovery. The header byte can be one of 0x1B, 0x1C, 0x1D or 0x1E
>
>
> For multisig signmessage signatures this is extended:
>
>     <01> <varint n> <varint m> <sig or key> {<sig or key>, ...}
>
> Each signature or key can be one of the following forms:
>
>     sig: <1B/1C/1D/1E> <32 byte r> <32 byte s>
>     compress key: <02/03> <32 byte x>
>     uncompressed key: <04> <32 byte x> <32 byte y>
>
> Note that we have to provide all pubkeys, even if they aren't used for a
> given signature, to allow the P2SH address to be reconstructed.
>
> Decoding/encoding is a bit code-intensive due to the all the cases, but
> on the other hand this format keeps the size down to an absolute
> minimum. Alternatively I could use length bytes.
>
> The format is backwards compatible in the sense that older versions will
> fail safely on new signatures, even ones that have been truncated.
> Similarly new signatures are easily distinguished from old, and going
> forward if we for some reason need yet another signature format the
> leading byte can be incremented.
>
> Signing incomplete signatures on messages can be handled by converting
> pubkeys to signatures. Similarly the RPC signmessage command can be
> extended with an optional "existing signature" option.
>
>
>
> ------------------------------------------------------------------------------
> Precog is a next-generation analytics platform capable of advanced
> analytics on semi-structured data. The platform includes APIs for building
> apps and a phenomenal toolset for data science. Developers can use
> our toolset for easy data analysis & visualization. Get a free account!
> http://www2.precog.com/precogplatform/slashdotnewsletter
>
>
> _______________________________________________
> Bitcoin-development mailing list
> Bitcoin-development@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/bitcoin-development


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    <div class="moz-cite-prefix">If we're going to extend/expand message
      signing, can we please add a proper ASCII-armored format for it?&nbsp;
      Really, anything that encodes the signed message next to the
      signature, so that there's no ambiguities about what was signed.&nbsp;
      You can keep the "bare signatures" as an option for backwards
      compatiblity, but offer this as the primary one.<br>
      <br>
      What we really want is to have the user copy an ASCII-armored
      block of text into the client (or we could have a URI-extension
      for this), and the app pops up with a window that says "The
      following message has a valid signature from address
      1XKjf32kJbf...:&nbsp;&nbsp; &lt;message&gt;".&nbsp;&nbsp; <br>
      <br>
      I know people argue they'd like to get away from raw addresses and
      copy-and-paste.&nbsp; But it'll be a while before that happens, and
      there's a lot of demand for Armory to become compatible with
      Bitcoin-Qt signing.&nbsp; People are obviously using it.<br>
      <br>
      -Alan<br>
      <br>
      <br>
      <br>
      <br>
      <br>
      On 04/14/2013 01:09 AM, Peter Todd wrote:<br>
    </div>
    <blockquote cite="mid:20130414050958.GA11142@savin" type="cite">
      <pre wrap="">Currently signmessage/verifymessage only supports messages signed by a
single key. We should extend that to messages signed by n-of-m keys, or
from the users point of view, P2SH multisig addresses.

rpc.cpp:signmessage() returns the output of SignCompact(). That in turn
starts with a header byte marking the signs of the various keys to allow
for key recovery. The header byte can be one of 0x1B, 0x1C, 0x1D or 0x1E


For multisig signmessage signatures this is extended:

    &lt;01&gt; &lt;varint n&gt; &lt;varint m&gt; &lt;sig or key&gt; {&lt;sig or key&gt;, ...}

Each signature or key can be one of the following forms:

    sig: &lt;1B/1C/1D/1E&gt; &lt;32 byte r&gt; &lt;32 byte s&gt;
    compress key: &lt;02/03&gt; &lt;32 byte x&gt;
    uncompressed key: &lt;04&gt; &lt;32 byte x&gt; &lt;32 byte y&gt;

Note that we have to provide all pubkeys, even if they aren't used for a
given signature, to allow the P2SH address to be reconstructed.

Decoding/encoding is a bit code-intensive due to the all the cases, but
on the other hand this format keeps the size down to an absolute
minimum. Alternatively I could use length bytes.

The format is backwards compatible in the sense that older versions will
fail safely on new signatures, even ones that have been truncated.
Similarly new signatures are easily distinguished from old, and going
forward if we for some reason need yet another signature format the
leading byte can be incremented.

Signing incomplete signatures on messages can be handled by converting
pubkeys to signatures. Similarly the RPC signmessage command can be
extended with an optional "existing signature" option.

</pre>
      <br>
      <fieldset class="mimeAttachmentHeader"></fieldset>
      <br>
      <pre wrap="">------------------------------------------------------------------------------
Precog is a next-generation analytics platform capable of advanced
analytics on semi-structured data. The platform includes APIs for building
apps and a phenomenal toolset for data science. Developers can use
our toolset for easy data analysis &amp; visualization. Get a free account!
<a class="moz-txt-link-freetext" href="http://www2.precog.com/precogplatform/slashdotnewsletter">http://www2.precog.com/precogplatform/slashdotnewsletter</a></pre>
      <br>
      <fieldset class="mimeAttachmentHeader"></fieldset>
      <br>
      <pre wrap="">_______________________________________________
Bitcoin-development mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Bitcoin-development@lists.sourceforge.net">Bitcoin-development@lists.sourceforge.net</a>
<a class="moz-txt-link-freetext" href="https://lists.sourceforge.net/lists/listinfo/bitcoin-development">https://lists.sourceforge.net/lists/listinfo/bitcoin-development</a>
</pre>
    </blockquote>
    <br>
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