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Clarifying the encoding of ik #46

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Nov 7, 2023
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Clarifying the encoding of ik #46

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9 changes: 7 additions & 2 deletions zip-0227.html
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Original file line number Diff line number Diff line change
Expand Up @@ -184,7 +184,9 @@
</ul>
<p>where the
<span class="math">\(\textit{PubKey}\)</span>
algorithm is defined in BIP 340 <a id="footnote-reference-16" class="footnote_reference" href="#bip-0340">17</a>.</p>
algorithm is defined in BIP 340 <a id="footnote-reference-16" class="footnote_reference" href="#bip-0340">17</a>. Note that the byte representation of
<span class="math">\(\mathsf{ik}\)</span>
is in big-endian order as defined in BIP 340.</p>
<p>It is possible for the
<span class="math">\(\textit{PubKey}\)</span>
algorithm to fail with very low probability, which means that
Expand Down Expand Up @@ -276,8 +278,11 @@
, where</p>
<ul>
<li>
<span class="math">\(\mathsf{EncodeAssetId}(\mathsf{AssetId}) = \mathsf{EncodeAssetId}((\mathsf{ik}, \mathsf{asset\_desc})) := \mathsf{0x00} || \mathsf{repr}_{\mathbb{P}}(\mathsf{ik}) || \mathsf{asset\_desc}\!\)</span>
<span class="math">\(\mathsf{EncodeAssetId}(\mathsf{AssetId}) = \mathsf{EncodeAssetId}((\mathsf{ik}, \mathsf{asset\_desc})) := \mathsf{0x00} || \mathsf{ik} || \mathsf{asset\_desc}\!\)</span>
.</li>
<li>Note that the initial
<span class="math">\(\mathsf{0x00}\)</span>
byte is a version byte.</li>
</ul>
<p>Define
<span class="math">\(\mathsf{AssetBase_{\mathsf{AssetId}}} := \mathsf{ZSAValueBase}(\mathsf{AssetDigest}_{\mathsf{AssetId}})\)</span>
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4 changes: 3 additions & 1 deletion zip-0227.rst
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Expand Up @@ -147,6 +147,7 @@ Define :math:`\mathsf{IssueAuthSig.DerivePublic}\: : \: (\mathsf{isk}\: : \: \ma
* Return :math:`\bot` if the :math:`\textit{PubKey}` algorithm invocation fails, otherwise return :math:`\mathsf{ik}`.

where the :math:`\textit{PubKey}` algorithm is defined in BIP 340 [#bip-0340]_.
Note that the byte representation of :math:`\mathsf{ik}` is in big-endian order as defined in BIP 340.

It is possible for the :math:`\textit{PubKey}` algorithm to fail with very low probability, which means that :math:`\mathsf{IssueAuthSig.DerivePublic}` could return :math:`\bot` with very low probability.
If this happens, discard the keys and repeat with a different :math:`\mathsf{isk}`.
Expand Down Expand Up @@ -189,7 +190,8 @@ Let
Define :math:`\mathsf{AssetDigest_{\mathsf{AssetId}}} := \textsf{BLAKE2b-512}(\texttt{"ZSA-Asset-Digest"},\; \mathsf{EncodeAssetId}(\mathsf{AssetId}))`,
where

- :math:`\mathsf{EncodeAssetId}(\mathsf{AssetId}) = \mathsf{EncodeAssetId}((\mathsf{ik}, \mathsf{asset\_desc})) := \mathsf{0x00} || \mathsf{repr}_{\mathbb{P}}(\mathsf{ik}) || \mathsf{asset\_desc}\!`.
- :math:`\mathsf{EncodeAssetId}(\mathsf{AssetId}) = \mathsf{EncodeAssetId}((\mathsf{ik}, \mathsf{asset\_desc})) := \mathsf{0x00} || \mathsf{ik} || \mathsf{asset\_desc}\!`.
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Note that the specification defines ik to be a byte string. So there is no need to use the I2BEBSP function. I have instead clarified that the representation is big-endian above (line 150)

- Note that the initial :math:`\mathsf{0x00}` byte is a version byte.

Define :math:`\mathsf{AssetBase_{\mathsf{AssetId}}} := \mathsf{ZSAValueBase}(\mathsf{AssetDigest}_{\mathsf{AssetId}})`

Expand Down