95 lines
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95 lines
7.8 KiB
HTML
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<title>ZIP 302: Standardized Memo Field Format</title>
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<pre>ZIP: 302
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Title: Standardized Memo Field Format
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Owners: Jack Grigg <jack@electriccoin.co>
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Original-Author: Jay Graber
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Status: Draft
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Category: Standards / RPC / Wallet
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Created: 2017-02-08
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Discussions-To: <<a href="https://github.com/zcash/zips/issues/366">https://github.com/zcash/zips/issues/366</a>>
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Pull-Request: <<a href="https://github.com/zcash/zips/pull/105">https://github.com/zcash/zips/pull/105</a>></pre>
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<section id="abstract"><h2><span class="section-heading">Abstract</span><span class="section-anchor"> <a rel="bookmark" href="#abstract"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h2>
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<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>
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</section>
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<section id="motivation"><h2><span class="section-heading">Motivation</span><span class="section-anchor"> <a rel="bookmark" href="#motivation"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h2>
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<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>
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</section>
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<section id="specification"><h2><span class="section-heading">Specification</span><span class="section-anchor"> <a rel="bookmark" href="#specification"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h2>
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<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>
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<ul>
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<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>
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<li>the byte <code>0xF6</code> followed by 511 <code>0x00</code> bytes, indicating "no memo"; or</li>
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<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>
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</ul>
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<p>This ZIP refines the specification of the third case.</p>
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<p>The following specification constrains a party, called the "reader", that interprets the contents of a memo. It does not define consensus requirements.</p>
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<ul>
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<li>If the first byte (byte 0) has a value of 0xF4 or smaller, then the reader MUST:
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<ul>
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<li>strip any trailing zero bytes</li>
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<li>decode it as a UTF-8 string (if decoding fails then report an error).</li>
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</ul>
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</li>
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<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>
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<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:
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<ul>
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<li>The first byte has a value of 0xF5.</li>
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<li>The first byte has a value of 0xF6, and the remaining 511 bytes are not all 0x00.</li>
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<li>The first byte has a value between 0xF7 and 0xFE inclusive.</li>
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</ul>
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</li>
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<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>
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</ul>
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</section>
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<section id="rationale"><h2><span class="section-heading">Rationale</span><span class="section-anchor"> <a rel="bookmark" href="#rationale"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h2>
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<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>
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<blockquote>
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<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>
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<ul>
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<li>a UTF-8 human-readable string [Unicode], padded by appending zero bytes; or</li>
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<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>
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</ul>
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<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>
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<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>
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</blockquote>
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<p>See issue <a href="https://github.com/zcash/zcash/issues/1849">#1849</a> for further discussion.</p>
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</section>
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<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" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h2>
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<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>
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</section>
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<section id="references"><h2><span class="section-heading">References</span><span class="section-anchor"> <a rel="bookmark" href="#references"><img width="24" height="24" class="section-anchor" src="assets/images/section-anchor.png" alt=""></a></span></h2>
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<table id="protocol" class="footnote">
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<tbody>
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<tr>
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<th>1</th>
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<td><a href="protocol/protocol.pdf">Zcash Protocol Specification, Version 2021.1.19</a></td>
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</tr>
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</tbody>
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</table>
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<table id="utf-8" class="footnote">
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<tbody>
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<tr>
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<th>2</th>
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<td><a href="https://www.rfc-editor.org/rfc/rfc3629.html">UTF-8, a transformation format of ISO 10646</a></td>
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</tr>
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</tbody>
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</table>
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<table id="bitcoin-compactsize" class="footnote">
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<tbody>
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<tr>
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<th>3</th>
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<td><a href="https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer">Variable length integer. Bitcoin Wiki</a></td>
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</tr>
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</tbody>
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</table>
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</section>
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</section>
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