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[ZSA] Issuance Key Derivation update to match with ZIP 32 (#72)
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This makes the changes to align with the change in notation for ZIP
32 done in zcash#908.
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@vivek-arte
vivek-arte authored Oct 8, 2024
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34 changes: 25 additions & 9 deletions rendered/zip-0227.html
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</section>
<section id="issuance-key-derivation"><h3><span class="section-heading">Issuance Key Derivation</span><span class="section-anchor"> <a rel="bookmark" href="#issuance-key-derivation"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<section id="issuance-authorizing-key-generation-for-hierarchical-deterministic-wallets"><h4><span class="section-heading">Issuance authorizing key generation for hierarchical deterministic wallets</span><span class="section-anchor"> <a rel="bookmark" href="#issuance-authorizing-key-generation-for-hierarchical-deterministic-wallets"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h4>
<p>The issuance authorizing key is generated using the Orchard master key derivation procedure defined in ZIP 32 <a id="footnote-reference-12" class="footnote_reference" href="#zip-0032-orchard-master">3</a>. We reuse the functions defined there in what follows in this section.</p>
<p>The issuance authorizing key is generated using the Hardened-only key derivation process defined in ZIP 32 <a id="footnote-reference-12" class="footnote_reference" href="#zip-0032-hardened-only-key-derivation">3</a>. For the
<span class="math">\(\mathsf{Issuance}\)</span>
context, we define the following constants:</p>
<ul>
<li>
<span class="math">\(\mathsf{Issuance.\!MKGDomain} := \texttt{"ZIP32ZSAIssue_V1"}\)</span>
</li>
<li>
<span class="math">\(\mathsf{Issuance.\!CKDDomain} := \mathtt{0x81}\!\)</span>
</li>
</ul>
<p>Let
<span class="math">\(S\)</span>
be a seed byte sequence of a chosen length, which MUST be at least 32 and at most 252 bytes. We define the master extended issuance key
<span class="math">\(m_{\mathsf{Issuance}} := \mathsf{MasterKeyGen}(\texttt{"ZIP32ZSAIssue_V1"}, S)\!\)</span>
<span class="math">\(m_{\mathsf{Issuance}} := \mathsf{MKGh}^{\mathsf{Issuance}}(S)\!\)</span>
.</p>
<p>As in ZIP 32 for Orchard <a id="footnote-reference-13" class="footnote_reference" href="#zip-0032-orchard-child-key-derivation">4</a>, we only use hardened child key derivation for the issuance authorizing key. We reuse the
<span class="math">\(\mathsf{CDKsk}\)</span>
function for Orchard child key derivation from ZIP 32.</p>
<p>We use hardened-only child key derivation as defined in ZIP 32 <a id="footnote-reference-13" class="footnote_reference" href="#zip-0032-hardened-only-child-key-derivation">4</a> for the issuance authorizing key.</p>
<p>
<span class="math">\(\mathsf{CDKsk}((\mathsf{sk}_{par},\mathsf{c}_{par}), i) \rightarrow (\mathsf{sk}_{i}, \mathsf{c}_{i})\)</span>
</p>
<ul>
<li>Return
<span class="math">\(\mathsf{CKDh}^{\mathsf{Issuance}}((\mathsf{sk}_{par},\mathsf{c}_{par}), i)\!\)</span>
</li>
</ul>
<p>We use the notation of ZIP 32 <a id="footnote-reference-14" class="footnote_reference" href="#zip-0032-orchard-key-path">6</a> for shielded HD paths, and define the issuance authorizing key path as
<span class="math">\(m_{\mathsf{Issuance}} / \mathit{purpose}' / \mathit{coin\_type}' / \mathit{account}'\!\)</span>
. We fix the path levels as follows:</p>
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</tr>
</tbody>
</table>
<table id="zip-0032-orchard-master" class="footnote">
<table id="zip-0032-hardened-only-key-derivation" class="footnote">
<tbody>
<tr>
<th>3</th>
<td><a href="zip-0032.html#orchard-master-key-generation">ZIP 32: Shielded Hierarchical Deterministic Wallets - Orchard master key generation</a></td>
<td><a href="zip-0032.html#specification-hardened-only-key-derivation">ZIP 32: Shielded Hierarchical Deterministic Wallets - Specification: Hardened-only key derivation</a></td>
</tr>
</tbody>
</table>
<table id="zip-0032-orchard-child-key-derivation" class="footnote">
<table id="zip-0032-hardened-only-child-key-derivation" class="footnote">
<tbody>
<tr>
<th>4</th>
<td><a href="zip-0032.html#orchard-child-key-derivation">ZIP 32: Shielded Hierarchical Deterministic Wallets - Orchard child key derivation</a></td>
<td><a href="zip-0032.html#hardened-only-child-key-derivation">ZIP 32: Shielded Hierarchical Deterministic Wallets - Hardened-only child key derivation</a></td>
</tr>
</tbody>
</table>
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19 changes: 13 additions & 6 deletions zips/zip-0227.rst
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Expand Up @@ -123,13 +123,20 @@ Issuance Key Derivation
Issuance authorizing key generation for hierarchical deterministic wallets
``````````````````````````````````````````````````````````````````````````

The issuance authorizing key is generated using the Orchard master key derivation procedure defined in ZIP 32 [#zip-0032-orchard-master]_. We reuse the functions defined there in what follows in this section.
The issuance authorizing key is generated using the Hardened-only key derivation process defined in ZIP 32 [#zip-0032-hardened-only-key-derivation]_.
For the :math:`\mathsf{Issuance}` context, we define the following constants:

- :math:`\mathsf{Issuance.\!MKGDomain} := \texttt{"ZIP32ZSAIssue_V1"}`
- :math:`\mathsf{Issuance.\!CKDDomain} := \mathtt{0x81}\!`

Let :math:`S` be a seed byte sequence of a chosen length, which MUST be at least 32 and at most 252 bytes.
We define the master extended issuance key :math:`m_{\mathsf{Issuance}} := \mathsf{MasterKeyGen}(\texttt{"ZIP32ZSAIssue_V1"}, S)\!`.
We define the master extended issuance key :math:`m_{\mathsf{Issuance}} := \mathsf{MKGh}^{\mathsf{Issuance}}(S)\!`.

We use hardened-only child key derivation as defined in ZIP 32 [#zip-0032-hardened-only-child-key-derivation]_ for the issuance authorizing key.

:math:`\mathsf{CDKsk}((\mathsf{sk}_{par},\mathsf{c}_{par}), i) \rightarrow (\mathsf{sk}_{i}, \mathsf{c}_{i})`

As in ZIP 32 for Orchard [#zip-0032-orchard-child-key-derivation]_, we only use hardened child key derivation for the issuance authorizing key.
We reuse the :math:`\mathsf{CDKsk}` function for Orchard child key derivation from ZIP 32.
- Return :math:`\mathsf{CKDh}^{\mathsf{Issuance}}((\mathsf{sk}_{par},\mathsf{c}_{par}), i)\!`

We use the notation of ZIP 32 [#zip-0032-orchard-key-path]_ for shielded HD paths, and define the issuance authorizing key path as :math:`m_{\mathsf{Issuance}} / \mathit{purpose}' / \mathit{coin\_type}' / \mathit{account}'\!`. We fix the path levels as follows:

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.. [#BCP14] `Information on BCP 14 — "RFC 2119: Key words for use in RFCs to Indicate Requirement Levels" and "RFC 8174: Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words" <https://www.rfc-editor.org/info/bcp14>`_
.. [#zip-0032] `ZIP 32: Shielded Hierarchical Deterministic Wallets <zip-0032.html>`_
.. [#zip-0032-orchard-master] `ZIP 32: Shielded Hierarchical Deterministic Wallets - Orchard master key generation <zip-0032.html#orchard-master-key-generation>`_
.. [#zip-0032-orchard-child-key-derivation] `ZIP 32: Shielded Hierarchical Deterministic Wallets - Orchard child key derivation <zip-0032.html#orchard-child-key-derivation>`_
.. [#zip-0032-hardened-only-key-derivation] `ZIP 32: Shielded Hierarchical Deterministic Wallets - Specification: Hardened-only key derivation <zip-0032.html#specification-hardened-only-key-derivation>`_
.. [#zip-0032-hardened-only-child-key-derivation] `ZIP 32: Shielded Hierarchical Deterministic Wallets - Hardened-only child key derivation <zip-0032.html#hardened-only-child-key-derivation>`_
.. [#zip-0032-key-path-levels] `ZIP 32: Shielded Hierarchical Deterministic Wallets - Key path levels <zip-0032.html#key-path-levels>`_
.. [#zip-0032-orchard-key-path] `ZIP 32: Shielded Hierarchical Deterministic Wallets - Orchard key path <zip-0032.html#orchard-key-path>`_
.. [#zip-0200] `ZIP 200: Network Upgrade Mechanism <zip-0200.html>`_
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