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Date: Sat, 29 Mar 2014 14:41:17 -0400
From: Alan Reiner <etotheipi@gmail.com>
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To: Tamas Blummer <tamas@bitsofproof.com>
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	<19FE9882-7FC2-4518-BD50-8818B059271B@bitsofproof.com>
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Presenting a BIP for Shamir's Secret
 Sharing of Bitcoin private keys
Precedence: list

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Armory does exactly this: it defines the "Fragment ID" as the first few
bytes of the hash of the root pubKey + M-parameter, converted to
base58.  Then it explains to the user "All fragments with the same
fragment ID are compatible" (which only works if you use deterministic
coefficients).  Each fragment is then labeled with "[FragID]-#1",
"[FragID]-#2", etc.  It became quite useful for organizing the fragments
and documenting how I was distributing them, especially if I had printed
or saved the same fragment twice by accident.



On 03/29/2014 02:16 PM, Tamas Blummer wrote:
> I also think that we can add usability features if the underlying
> secret remains well protected.
> I do not think there is any reason to assume that the knowledge of the
> degree of the polynomial, would aid an attacker.
>
> Similarly a fingerprint of the secret if it is unrelated to the hash
> used in the polinomyal should leak no useful information,
>
> The length of such fingerpring (say 4 bytes) and the degree (1 byte)
> does not seem a big overhead for me.
>
> Remember that the biggest obstacle of Bitcoin is usability not security.
>
> Regards,
>
> Tamas Blummer
> http://bitsofproof.com
>
> On 29.03.2014, at 18:52, Alan Reiner <etotheipi@gmail.com
> <mailto:etotheipi@gmail.com>> wrote:
>
>> On 03/29/2014 01:19 PM, Matt Whitlock wrote:
>>> I intentionally omitted the parameter M (minimum subset size) from
>>> the shares because including it would give an adversary a vital
>>> piece of information. Likewise, including any kind of information
>>> that would allow a determination of whether the secret has been
>>> correctly reconstituted would give an adversary too much
>>> information. Failing silently when given incorrect shares or an
>>> insufficient number of shares is intentional.
>>
>> I do not believe this is a good tradeoff.  It's basically obfuscation of
>> something that is already considered secure at the expense of
>> usability.  It's much more important to me that the user understands
>> what is in their hands (or their family members after they get hit by a
>> bus), than to obfuscate the parameters of the secret sharing to provide
>> a tiny disadvantage to an adversary who gets ahold of one.
>>
>> The fact that it fails silently is really all downside, not a benefit.
>> If I have enough fragments, I can reconstruct the seed and see that it
>> produces addresses with money.  If not, I know I need more fragments.
>> I'm much more concerned about my family having all the info they need to
>> recover the money, than an attacker knowing that he needs two more
>> fragments instead of which are well-secured anyway.
>>
>>
>>
>> ------------------------------------------------------------------------------
>> _______________________________________________
>> Bitcoin-development mailing list
>> Bitcoin-development@lists.sourceforge.net
>> <mailto:Bitcoin-development@lists.sourceforge.net>
>> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
>>
>


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    Armory does exactly this: it defines the "Fragment ID" as the first
    few bytes of the hash of the root pubKey + M-parameter, converted to
    base58.&nbsp; Then it explains to the user "All fragments with the same
    fragment ID are compatible" (which only works if you use
    deterministic coefficients).&nbsp; Each fragment is then labeled with
    "[FragID]-#1", "[FragID]-#2", etc.&nbsp; It became quite useful for
    organizing the fragments and documenting how I was distributing
    them, especially if I had printed or saved the same fragment twice
    by accident.<br>
    <br>
    <br>
    <br>
    <div class="moz-cite-prefix">On 03/29/2014 02:16 PM, Tamas Blummer
      wrote:<br>
    </div>
    <blockquote
      cite="mid:19FE9882-7FC2-4518-BD50-8818B059271B@bitsofproof.com"
      type="cite">
      <meta http-equiv="Content-Type" content="text/html;
        charset=ISO-8859-1">
      I also think that we can add usability features if the underlying
      secret remains well protected.
      <div>I do not think there is any reason to assume that the
        knowledge of the degree of the polynomial, would aid an
        attacker.</div>
      <div><br>
      </div>
      <div>Similarly a fingerprint of the secret if it is unrelated to
        the hash used in the polinomyal should leak no useful
        information,</div>
      <div><br>
      </div>
      <div>The length of such fingerpring (say 4 bytes) and the degree
        (1 byte) does not seem a big overhead for me.</div>
      <div><br>
      </div>
      <div>Remember that the biggest obstacle of Bitcoin is usability
        not security.</div>
      <div>
        <div apple-content-edited="true"><br style="color: rgb(0, 0, 0);
            font-family: Helvetica; font-size: medium; font-style:
            normal; font-variant: normal; font-weight: normal;
            letter-spacing: normal; line-height: normal; orphans: 2;
            text-align: -webkit-auto; text-indent: 0px; text-transform:
            none; white-space: normal; widows: 2; word-spacing: 0px;
            -webkit-text-size-adjust: auto; -webkit-text-stroke-width:
            0px; ">
          <span style="color: rgb(0, 0, 0); font-family: Helvetica;
            font-size: medium; font-style: normal; font-variant: normal;
            font-weight: normal; letter-spacing: normal; line-height:
            normal; orphans: 2; text-align: -webkit-auto; text-indent:
            0px; text-transform: none; white-space: normal; widows: 2;
            word-spacing: 0px; -webkit-text-size-adjust: auto;
            -webkit-text-stroke-width: 0px; display: inline !important;
            float: none; ">Regards,</span><br style="color: rgb(0, 0,
            0); font-family: Helvetica; font-size: medium; font-style:
            normal; font-variant: normal; font-weight: normal;
            letter-spacing: normal; line-height: normal; orphans: 2;
            text-align: -webkit-auto; text-indent: 0px; text-transform:
            none; white-space: normal; widows: 2; word-spacing: 0px;
            -webkit-text-size-adjust: auto; -webkit-text-stroke-width:
            0px; ">
          <br style="color: rgb(0, 0, 0); font-family: Helvetica;
            font-size: medium; font-style: normal; font-variant: normal;
            font-weight: normal; letter-spacing: normal; line-height:
            normal; orphans: 2; text-align: -webkit-auto; text-indent:
            0px; text-transform: none; white-space: normal; widows: 2;
            word-spacing: 0px; -webkit-text-size-adjust: auto;
            -webkit-text-stroke-width: 0px; ">
          <span style="color: rgb(0, 0, 0); font-family: Helvetica;
            font-size: medium; font-style: normal; font-variant: normal;
            font-weight: normal; letter-spacing: normal; line-height:
            normal; orphans: 2; text-align: -webkit-auto; text-indent:
            0px; text-transform: none; white-space: normal; widows: 2;
            word-spacing: 0px; -webkit-text-size-adjust: auto;
            -webkit-text-stroke-width: 0px; display: inline !important;
            float: none; ">Tamas Blummer</span><br style="color: rgb(0,
            0, 0); font-family: Helvetica; font-size: medium;
            font-style: normal; font-variant: normal; font-weight:
            normal; letter-spacing: normal; line-height: normal;
            orphans: 2; text-align: -webkit-auto; text-indent: 0px;
            text-transform: none; white-space: normal; widows: 2;
            word-spacing: 0px; -webkit-text-size-adjust: auto;
            -webkit-text-stroke-width: 0px; ">
          <span style="color: rgb(0, 0, 0); font-family: Helvetica;
            font-size: medium; font-style: normal; font-variant: normal;
            font-weight: normal; letter-spacing: normal; line-height:
            normal; orphans: 2; text-align: -webkit-auto; text-indent:
            0px; text-transform: none; white-space: normal; widows: 2;
            word-spacing: 0px; -webkit-text-size-adjust: auto;
            -webkit-text-stroke-width: 0px; display: inline !important;
            float: none; "><a moz-do-not-send="true"
              href="http://bitsofproof.com">http://bitsofproof.com</a></span>
        </div>
        <br>
        <div>
          <div>On 29.03.2014, at 18:52, Alan Reiner &lt;<a
              moz-do-not-send="true" href="mailto:etotheipi@gmail.com">etotheipi@gmail.com</a>&gt;
            wrote:</div>
          <br class="Apple-interchange-newline">
          <blockquote type="cite">On 03/29/2014 01:19 PM, Matt Whitlock
            wrote:<br>
            <blockquote type="cite">I intentionally omitted the
              parameter M (minimum subset size) from the shares because
              including it would give an adversary a vital piece of
              information. Likewise, including any kind of information
              that would allow a determination of whether the secret has
              been correctly reconstituted would give an adversary too
              much information. Failing silently when given incorrect
              shares or an insufficient number of shares is intentional.<br>
            </blockquote>
            <br>
            I do not believe this is a good tradeoff. &nbsp;It's basically
            obfuscation of<br>
            something that is already considered secure at the expense
            of<br>
            usability. &nbsp;It's much more important to me that the user
            understands<br>
            what is in their hands (or their family members after they
            get hit by a<br>
            bus), than to obfuscate the parameters of the secret sharing
            to provide<br>
            a tiny disadvantage to an adversary who gets ahold of one. <br>
            <br>
            The fact that it fails silently is really all downside, not
            a benefit. <br>
            If I have enough fragments, I can reconstruct the seed and
            see that it<br>
            produces addresses with money. &nbsp;If not, I know I need more
            fragments. <br>
            I'm much more concerned about my family having all the info
            they need to<br>
            recover the money, than an attacker knowing that he needs
            two more<br>
            fragments instead of which are well-secured anyway.<br>
            <br>
            <br>
            <br>
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          </blockquote>
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        <br>
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