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Merge pull request #105 from zcash/memo-field-specification 

[ZIP 302] Standardized Memo Field Format
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README.rst
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@ -108,7 +108,7 @@ Index of ZIPs
<tr> <td>252</td> <td class="left"><a href="zip-0252.rst">Deployment of the NU5 Network Upgrade</a></td> <td>Proposed</td>
<tr> <td>300</td> <td class="left"><a href="zip-0300.rst">Cross-chain Atomic Transactions</a></td> <td>Proposed</td>
<tr> <td>301</td> <td class="left"><a href="zip-0301.rst">Zcash Stratum Protocol</a></td> <td>Final</td>
<tr> <td><span class="reserved">302</span></td> <td class="left"><a class="reserved" href="zip-0302.rst">Standardized Memo Field Format</a></td> <td>Reserved</td>
<tr> <td>302</td> <td class="left"><a href="zip-0302.rst">Standardized Memo Field Format</a></td> <td>Draft</td>
<tr> <td><span class="reserved">303</span></td> <td class="left"><a class="reserved" href="zip-0303.rst">Sprout Payment Disclosure</a></td> <td>Reserved</td>
<tr> <td>304</td> <td class="left"><a href="zip-0304.rst">Sapling Address Signatures</a></td> <td>Draft</td>
<tr> <td><span class="reserved">305</span></td> <td class="left"><a class="reserved" href="zip-0305.rst">Best Practices for Hardware Wallets supporting Sapling</a></td> <td>Reserved</td>

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index.html
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@ -81,7 +81,7 @@
<tr> <td>252</td> <td class="left"><a href="zip-0252">Deployment of the NU5 Network Upgrade</a></td> <td>Proposed</td>
<tr> <td>300</td> <td class="left"><a href="zip-0300">Cross-chain Atomic Transactions</a></td> <td>Proposed</td>
<tr> <td>301</td> <td class="left"><a href="zip-0301">Zcash Stratum Protocol</a></td> <td>Final</td>
<tr> <td><span class="reserved">302</span></td> <td class="left"><a class="reserved" href="zip-0302">Standardized Memo Field Format</a></td> <td>Reserved</td>
<tr> <td>302</td> <td class="left"><a href="zip-0302">Standardized Memo Field Format</a></td> <td>Draft</td>
<tr> <td><span class="reserved">303</span></td> <td class="left"><a class="reserved" href="zip-0303">Sprout Payment Disclosure</a></td> <td>Reserved</td>
<tr> <td>304</td> <td class="left"><a href="zip-0304">Sapling Address Signatures</a></td> <td>Draft</td>
<tr> <td><span class="reserved">305</span></td> <td class="left"><a class="reserved" href="zip-0305">Best Practices for Hardware Wallets supporting Sapling</a></td> <td>Reserved</td>

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zip-0302.html
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<pre>ZIP: 302
Title: Standardized Memo Field Format
Owners: Jack Grigg &lt;jack@electriccoin.co&gt;
Original-Authors: Jay Graber
Status: Reserved
Original-Author: Jay Graber
Status: Draft
Category: Standards / RPC / Wallet
Created: 2017-02-08
Discussions-To: &lt;<a href="https://github.com/zcash/zips/issues/366">https://github.com/zcash/zips/issues/366</a>&gt;
Pull-Request: &lt;<a href="https://github.com/zcash/zips/pull/105">https://github.com/zcash/zips/pull/105</a>&gt;</pre>
<section id="abstract"><h2><span class="section-heading">Abstract</span><span class="section-anchor"> <a rel="bookmark" href="#abstract"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>This ZIP describes a proposed specification for a standardized format for clients who wish to transmit or receive content within the encrypted memo field of shielded transactions.</p>
</section>
<section id="motivation"><h2><span class="section-heading">Motivation</span><span class="section-anchor"> <a rel="bookmark" href="#motivation"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>A well-defined standard for formatting content within the encrypted memo field will help expand its use within the Zcash ecosystem by providing a commonly recognized format for memo values carrying different types of data. Users and third-party services benefit from standardized formatting rules that define the type and length of the data contained within.</p>
</section>
<section id="specification"><h2><span class="section-heading">Specification</span><span class="section-anchor"> <a rel="bookmark" href="#specification"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>Section 5.5 of the Zcash protocol specification <a id="id1" class="footnote_reference" href="#protocol">1</a> defines three cases for the encoding of a memo field:</p>
<ul>
<li>a UTF-8 human-readable string <a id="id2" class="footnote_reference" href="#utf-8">2</a>, padded by appending zero bytes; or</li>
<li>the byte <code>0xF6</code> followed by 511 <code>0x00</code> bytes, indicating "no memo"; or</li>
<li>any other sequence of 512 bytes starting with a byte value <code>0xF5</code> or greater (which is therefore not a valid UTF-8 string), as specified in ZIP 302.</li>
</ul>
<p>This ZIP refines the specification of the third case.</p>
<p>The following specification constrains a party, called the "reader", that interprets the contents of a memo. It does not define consensus requirements.</p>
<ul>
<li>
<dl>
<dt>If the first byte (byte 0) has a value of 0xF4 or smaller, then the reader MUST:</dt>
<dd>
<ul>
<li>strip any trailing zero bytes</li>
<li>decode it as a UTF-8 string (if decoding fails then report an error).</li>
</ul>
</dd>
</dl>
</li>
<li>If the first byte has a value of 0xF6, and the remaining 511 bytes are 0x00, then the user supplied no memo, and the encrypted memo field is to be treated as empty.</li>
<li>If the memo matches any of these patterns, then this memo is from the future, because these ranges are reserved for future updates to this specification: - The first byte has a value of 0xF5. - The first byte has a value of 0xF6, and the remaining 511 bytes are not all 0x00. - The first byte has a value between 0xF7 and 0xFE inclusive.</li>
<li>If the first byte has a value of 0xFF then the reader should not make any other assumption about the memo. In order to put arbitrary data into a memo field (that might have some private non-standard structure), the value of the first byte SHOULD be set to 0xFF; the remaining 511 bytes are then unconstrained.</li>
</ul>
</section>
<section id="rationale"><h2><span class="section-heading">Rationale</span><span class="section-anchor"> <a rel="bookmark" href="#rationale"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>The new protocol specification is an improvement over the current memo field content specification that was in the protocol spec up to version 2020.1.0, which stated:</p>
<blockquote>
<p>The usage of the memo field is by agreement between the sender and recipient of the note. The memo field SHOULD be encoded either as:</p>
<ul>
<li>a UTF-8 human-readable string [Unicode], padded by appending zero bytes; or</li>
<li>an arbitrary sequence of 512 bytes starting with a byte value of 0xF5 or greater, which is therefore not a valid UTF-8 string.</li>
</ul>
<p>In the former case, wallet software is expected to strip any trailing zero bytes and then display the resulting UTF-8 string to the recipient user, where applicable. Incorrect UTF-8-encoded byte sequences should be displayed as replacement characters (U+FFFD).</p>
<p>In the latter case, the contents of the memo field SHOULD NOT be displayed. A start byte of 0xF5 is reserved for use by automated software by private agreement. A start byte of 0xF6 or greater is reserved for use in future Zcash protocol extensions.</p>
</blockquote>
<p>See issue <a href="https://github.com/zcash/zcash/issues/1849">#1849</a> for further discussion.</p>
</section>
<section id="backwards-compatibility"><h2><span class="section-heading">Backwards Compatibility</span><span class="section-anchor"> <a rel="bookmark" href="#backwards-compatibility"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<p>Encrypted memo field contents sent without the standardized format proposed here will be interpreted according to the specification set out in older versions of the protocol spec.</p>
</section>
<section id="references"><h2><span class="section-heading">References</span><span class="section-anchor"> <a rel="bookmark" href="#references"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<table id="protocol" class="footnote">
<tbody>
<tr>
<th>1</th>
<td><a href="protocol/protocol.pdf">Zcash Protocol Specification, Version 2021.1.19</a></td>
</tr>
</tbody>
</table>
<table id="utf-8" class="footnote">
<tbody>
<tr>
<th>2</th>
<td><a href="https://tools.ietf.org/html/rfc3629">UTF-8, a transformation format of ISO 10646</a></td>
</tr>
</tbody>
</table>
<table id="bitcoin-compactsize" class="footnote">
<tbody>
<tr>
<th>3</th>
<td><a href="https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer">Variable length integer. Bitcoin Wiki</a></td>
</tr>
</tbody>
</table>
</section>
</section>
</body>
</html>

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zip-0302.rst
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ZIP: 302
Title: Standardized Memo Field Format
Owners: Jack Grigg <jack@electriccoin.co>
Original-Authors: Jay Graber
Status: Reserved
Original-Author: Jay Graber
Status: Draft
Category: Standards / RPC / Wallet
Created: 2017-02-08
Discussions-To: <https://github.com/zcash/zips/issues/366>
Pull-Request: <https://github.com/zcash/zips/pull/105>
Abstract
========
This ZIP describes a proposed specification for a standardized format for clients who wish
to transmit or receive content within the encrypted memo field of shielded transactions.
Motivation
==========
A well-defined standard for formatting content within the encrypted memo field will help
expand its use within the Zcash ecosystem by providing a commonly recognized format for
memo values carrying different types of data. Users and third-party services benefit from
standardized formatting rules that define the type and length of the data contained within.
Specification
===============
Section 5.5 of the Zcash protocol specification [#protocol]_ defines three cases
for the encoding of a memo field:
* a UTF-8 human-readable string [#UTF-8]_, padded by appending zero bytes; or
* the byte ``0xF6`` followed by 511 ``0x00`` bytes, indicating "no memo"; or
* any other sequence of 512 bytes starting with a byte value ``0xF5`` or greater (which is
therefore not a valid UTF-8 string), as specified in ZIP 302.
This ZIP refines the specification of the third case.
The following specification constrains a party, called the "reader", that interprets the
contents of a memo. It does not define consensus requirements.
+ If the first byte (byte 0) has a value of 0xF4 or smaller, then the reader MUST:
+ strip any trailing zero bytes
+ decode it as a UTF-8 string (if decoding fails then report an error).
+ If the first byte has a value of 0xF6, and the remaining 511 bytes are 0x00, then the user
supplied no memo, and the encrypted memo field is to be treated as empty.
+ If the memo matches any of these patterns, then this memo is from the future, because
these ranges are reserved for future updates to this specification:
- The first byte has a value of 0xF5.
- The first byte has a value of 0xF6, and the remaining 511 bytes are not all 0x00.
- The first byte has a value between 0xF7 and 0xFE inclusive.
+ If the first byte has a value of 0xFF then the reader should not make any other
assumption about the memo. In order to put arbitrary data into a memo field (that
might have some private non-standard structure), the value of the first byte SHOULD
be set to 0xFF; the remaining 511 bytes are then unconstrained.
Rationale
===========
The new protocol specification is an improvement over the current memo field content
specification that was in the protocol spec up to version 2020.1.0, which stated:
The usage of the memo field is by agreement between the sender and recipient of the
note. The memo field SHOULD be encoded either as:
+ a UTF-8 human-readable string [Unicode], padded by appending zero bytes; or
+ an arbitrary sequence of 512 bytes starting with a byte value of 0xF5 or greater,
which is therefore not a valid UTF-8 string.
In the former case, wallet software is expected to strip any trailing zero bytes and
then display the resulting UTF-8 string to the recipient user, where applicable.
Incorrect UTF-8-encoded byte sequences should be displayed as replacement characters
(U+FFFD).
In the latter case, the contents of the memo field SHOULD NOT be displayed. A start
byte of 0xF5 is reserved for use by automated software by private agreement. A start
byte of 0xF6 or greater is reserved for use in future Zcash protocol extensions.
See issue `#1849`_ for further discussion.
.. _`#1849`: https://github.com/zcash/zcash/issues/1849
Backwards Compatibility
===========================
Encrypted memo field contents sent without the standardized format proposed here will be
interpreted according to the specification set out in older versions of the protocol spec.
References
==========
.. [#protocol] `Zcash Protocol Specification, Version 2021.1.19 <protocol/protocol.pdf>`_
.. [#UTF-8] `UTF-8, a transformation format of ISO 10646 <https://tools.ietf.org/html/rfc3629>`_
.. [#Bitcoin-CompactSize] `Variable length integer. Bitcoin Wiki <https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer>`_