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Wiki page [Experiment: Chainsignature] by martin_vahi on 2017-05-26 18:06:58.
D 2017-05-26T18:06:58.094
L Experiment:\sChainsignature
P 14ff92efb518a00258bf09a491ecc10f51778ef6
U martin_vahi
W 2031
<p><b>The purpose of a signature is to authenticate the creator of the signed
object.</b>&nbsp;</p>

<p>In the case of blind dates, internet shopping <b>there is nothing to
authenticate at the first exchange of information</b>, because all merchants
and blind date partners are total strangers anyway. What matters is that the
correspondence that follows the first exchange of information is with the same
stranger, not with some other stranger.&nbsp;</p>

<p><br>
</p>

<h1>The Scheme</h1>

<p>The scheme is that message M_n signature is a secure hash of a concatenation
of all messages since the first message, message M_n included. If M_1 is a file
with the very first message that the signer sent and M_2 is the 2. message that
the signer sent, then a way to calculate the signature is:</p>
<pre>    cat ./M_1.blob  &gt; ./x.blob   # The "&gt;" is required for overwriting the old x.blob .</pre>
<pre>    cat ./M_2.blob &gt;&gt; ./x.blob   # The "&gt;&gt;" in stead of the "&gt;" is </pre>
<pre>                                 # required to add in stead of overwriting.</pre>
<pre>    cat ./M_3.blob &gt;&gt; ./x.blob</pre>
<pre>    ...</pre>
<pre>    cat ./M_n.blob &gt;&gt; ./x.blob</pre>
<pre>    sha256sum ./x.blob &gt; ./the_signature.txt</pre>

<p><br>
</p>

<p>The M_n must be included to the signature calculation x.blob to make sure
that an eavesdropper can not reuse the signature for signing its own message.
To make eavesdropping more difficult, different messages might be sent through
different channels.&nbsp;</p>

<p><br>
</p>

<h1>Optional Bootstrapping</h1>

<p>The message M_1 might be accompanied by M_0, which might be some bitstream
that is distributed as a small piece of paper with a QR-code. Given that NSA
and alike do not intercept paper-mail/snail-mail without specifically targeting
someone, because the cost of doing physical processing of physical letters is
too high, some initial symmetric cryptography keys and the M_0 might be sent by
using snail-mail.&nbsp;</p>

<p><br>
</p>

<p><br>
</p>

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