<p>The key words "MUST", "MUST NOT", and "SHOULD" in this document are to be interpreted as described in RFC 2119. <aid="id1"class="footnote_reference"href="#rfc2119">1</a></p>
<p>The terms below are to be interpreted as follows:</p>
<dl>
<dt>Recipient</dt>
<dd>A wallet or other software that can receive transfers of assets (such as ZEC) or in the future potentially other transaction-based state changes.</dd>
<dt>Sender</dt>
<dd>A wallet or other software that can send transfers of assets, or other future consensus state side-effects.</dd>
<dt>Receiver</dt>
<dd>The necessary information to transfer an asset to a Recipient that generated that Receiver using a specific Transfer Protocol. Each Receiver is associated unambiguously with a specific Receiver Type, identified by an integer Typecode.</dd>
<dt>Legacy Address (or LA)</dt>
<dd>A transparent, Sprout, or Sapling Address.</dd>
<dd>Either a Legacy Address or a Unified Address.</dd>
<dt>Transfer Protocol</dt>
<dd>A specification of how a Sender can transfer assets to a Recipient. For example, the Transfer Protocol for a Sapling Receiver is the subset of the Zcash protocol required to successfully transfer ZEC using Sapling Spend/Output Transfers as specified in the Zcash Protocol Specification. (A single Zcash transaction can contain transfers of multiple Transfer Protocols. For example a t→z transaction that shields to the Sapling pool requires both Transparent and Sapling Transfer Protocols.)</dd>
<dt>Address Encoding</dt>
<dd>The externally visible encoding of an Address (e.g. as a string of characters or a QR code).</dd>
<p>This proposal defines Unified Addresses, which bundle together Zcash addresses of different types in a way that can be presented as a single Address Encoding. It also defines Unified Viewing Keys, which perform a similar function for Zcash viewing keys.</p>
<p>Up to and including the Canopy network upgrade, Zcash supported the following payment address types:</p>
<ul>
<li>Transparent Addresses (P2PKH and P2SH)</li>
<li>Sprout Addresses</li>
<li>Sapling Addresses</li>
</ul>
<p>Each of these has its own Address Encodings, as a string and as a QR code. (Since the QR code is derivable from the string encoding, for many purposes it suffices to consider the string encoding.)</p>
<p>The Orchard proposal <aid="id2"class="footnote_reference"href="#zip-0224">6</a> adds a new address type, Orchard Addresses.</p>
<p>The difficulty with defining new Address Encodings for each address type, is that end-users are forced to be aware of the various types, and in particular which types are supported by a given Sender or Recipient. In order to make sure that transfers are completed successfully, users may be forced to explicitly generate addresses of different types and re-distribute encodings of them, which adds significant friction and cognitive overhead to understanding and using Zcash.</p>
<p>The goals for a Unified Address standard are as follows:</p>
<ul>
<li>Simplify coordination between Recipients and Senders by removing complexity from negotiating address types.</li>
<li>Provide a “bridging mechanism” to allow shielded wallets to successfully interact with conformant Transparent-Only wallets.</li>
<li>Allow older conformant wallets to interact seamlessly with newer wallets.</li>
<li>Enable users of newer wallets to upgrade to newer transaction technologies and/or pools while maintaining seamless interactions with counterparties using older wallets.</li>
<li>Facilitate wallets to assume more sophisticated responsibilities for shielding and/or migrating user funds.</li>
<li>Allow wallets to potentially develop new transfer mechanisms without underlying protocol changes.</li>
<li>Provide forward compatibility that is standard for all wallets across a range of potential future features. Some examples might include Layer 2 features, cross-chain interoperability and bridging, and decentralized exchange.</li>
<li>The standard should work well for Zcash today and upcoming potential upgrades, and also anticipate even broader use cases down the road such as cross-chain functionality.</li>
<p>Unified Addresses specify multiple methods for payment to a Recipient's Wallet. The Sender's Wallet can then non-interactively select the method of payment.</p>
<p>Importantly, any wallet can support Unified Addresses, even when that wallet only supports a subset of payment methods for receiving and/or sending.</p>
<p>Despite having some similar characteristics, the Unified Address standard is orthogonal to Payment Request URIs <aid="id3"class="footnote_reference"href="#zip-0321">7</a> and similar schemes, and the Unified Address format is likely to be incorporated into such schemes as a new address type.</p>
<p>Wallets follow a model <em>Interaction Flow</em> as follows:</p>
<oltype="1">
<li>A Recipient <em>generates</em> an Address.</li>
<li>The Recipient wallet <em>encodes</em> the Address.</li>
<li>The Recipient wallet or human user <em>distributes</em> this Address Encoding, This ZIP leaves distribution mechanisms out of scope.</li>
<li>A Sender wallet or user <em>imports</em> the Address Encoding through any of a variety of mechanisms (QR Code scanning, Payment URIs, cut-and-paste, or “in-band” protocols like <code>Reply-To</code> memos).</li>
<li>A Sender wallet <em>decodes</em> the Address Encoding and performs validity checks.</li>
<li>(Perhaps later in time) the Sender wallet executes a transfer of ZEC (or other assets or future protocol state changes) to the Address.</li>
</ol>
<p>Encodings of the same Address may be distributed zero or more times through different means. Zero or more Senders may import addresses. Zero or more of those may execute a Transfer. A single Sender may execute multiple Transfers over time from a single import.</p>
<p>A Unified Address (or UA for short) combines one or more Receivers.</p>
<p>When new Transport Protocols are introduced to the Zcash protocol after Unified Addresses are standardized, those should introduce new Receiver Types but <em>not</em> different address types outside of the UA standard. There needs to be a compelling reason to deviate from the standard, since the benefits of UA come precisely from their applicability across all new protocol upgrades.</p>
<p>Every Wallet must properly <em>parse</em> a Unified Address containing unrecognized Receiver Types.</p>
<p>A Wallet may process unrecognized Receiver Types by indicating to the user their presence or similar information for usability or diagnostic purposes.</p>
<p>The string encoding is “opaque” to human readers: it does <em>not</em> allow visual identification of which Receivers or Receiver Types are present.</p>
<p>The string encoding is resilient against typos, transcription errors, cut-and-paste errors, unanticipated truncation, or other anticipated UX hazards.</p>
<p>There is a well-defined encoding of a Unified Address as a QR Code, which produces QR codes that are reasonably compact and robust.</p>
<p>There is a well-defined transformation between the QR Code and string encodings in either direction.</p>
<p>The string encoding fits into ZIP-321 Payment URIs <aid="id4"class="footnote_reference"href="#zip-0321">7</a> and general URIs without introducing parse ambiguities.</p>
<p>The encoding must support sufficiently many Recipient Types to allow for reasonable future expansion.</p>
<p>The encoding must allow all wallets to safely and correctly parse out unrecognized Receiver Types well enough to ignore them.</p>
<p>When executing a Transfer the Sender selects a transfer method via a Selection process.</p>
<p>Given a valid UA, Selection must treat any unrecognized Receiver as though it were absent.</p>
<ul>
<li>This property is crucial for forward compatibility to ensure users who upgrade to newer protocols / UAs don't lose the ability to smoothly interact with older wallets.</li>
<li>This property is crucial for allowing Transparent-Only UA-Conformant wallets to interact with newer shielded wallets, removing a disincentive for adopting newer shielded wallets.</li>
<li>This property also allows Transparent-Only wallets to upgrade to shielded support without re-acquiring counterparty UAs. If they are re-acquired, the user flow and usability will be minimally disrupted.</li>
</ul>
</section>
<sectionid="open-issues-and-known-concerns"><h3><spanclass="section-heading">Open Issues and Known Concerns</span><spanclass="section-anchor"><arel="bookmark"href="#open-issues-and-known-concerns"><imgwidth="24"height="24"src="assets/images/section-anchor.png"alt=""></a></span></h3>
<p>FIXME: We have a few of these I [Nathan] will add in future edits. This is especially true of privacy impacts of transparent or cross-pool transactions and the associated UX issues.</p>
<sectionid="encoding-of-unified-payment-addresses"><h3><spanclass="section-heading">Encoding of Unified Payment Addresses</span><spanclass="section-anchor"><arel="bookmark"href="#encoding-of-unified-payment-addresses"><imgwidth="24"height="24"src="assets/images/section-anchor.png"alt=""></a></span></h3>
<p>Rather than defining a Bech32 string encoding of Orchard shielded payment addresses, we instead define a Unified Address format that is able to encode a set of Receivers of different types. This enables the consumer of a Unified Address (i.e. the Sender) to choose the Receiver of the best type it supports, providing a better user experience as new Receiver Types are added in the future.</p>
<p>Assume that we are given a set of one or more raw encodings of Receivers of distinct types. That is, the set may optionally contain one Receiver of each of the Receiver Types in the following fixed Priority List:</p>
<ul>
<li>Typecode
<spanclass="math">\(\mathtt{0x03}\)</span>
— an Orchard raw address as defined in <aid="id5"class="footnote_reference"href="#protocol-orchardpaymentaddrencoding">4</a>;</li>
<li>Typecode
<spanclass="math">\(\mathtt{0x02}\)</span>
— a Sapling raw address as defined in <aid="id6"class="footnote_reference"href="#protocol-saplingpaymentaddrencoding">3</a>;</li>
<li>Typecode
<spanclass="math">\(\mathtt{0x01}\)</span>
— a transparent P2SH address, <em>or</em> Typecode
<spanclass="math">\(\mathtt{0x00}\)</span>
— a transparent P2PKH address.</li>
</ul>
<p>We say that a Receiver Type is “better” than another when it appears earlier in this Priority List.</p>
<p>A Unified Address MUST contain at least one shielded Receiver (Typecodes
<spanclass="math">\(\geq \mathtt{0x02}\)</span>
).</p>
<p>The Sender of a payment to a Unified Address MUST use the Receiver of the best Receiver Type that it supports from the set.</p>
<p>For example, consider a wallet that supports sending funds to Orchard Receivers and does not support sending to any better Receiver Type. If that wallet is given a UA that includes an Orchard Receiver and possibly other Receivers, it MUST send to the Orchard Receiver.</p>
<p>The raw encoding of a Unified Address is a concatenation of
— the raw encoding of a shielded payment address, or the
<spanclass="math">\(160\)</span>
-bit script hash of a P2SH address <aid="id7"class="footnote_reference"href="#p2sh">10</a>, or the
<spanclass="math">\(160\)</span>
-bit validating key hash of a P2PKH address <aid="id8"class="footnote_reference"href="#p2pkh">9</a>.</li>
</ul>
<p>The result of the concatenation is then encoded with Bech32m <aid="id9"class="footnote_reference"href="#bip-0350">8</a>, ignoring any length restrictions. This is chosen over Bech32 in order to better handle variable-length inputs.</p>
<p>For unified payment addresses on Mainnet, the Human-Readable Part (as defined in <aid="id10"class="footnote_reference"href="#bip-0350">8</a>) is “<code>u</code>”. For unified payment addresses on Testnet, the Human-Readable Part is “<code>utest</code>”.</p>
<p>Notes:</p>
<ul>
<li>The
<spanclass="math">\(\mathtt{length}\)</span>
field is always encoded as a single byte, <em>not</em> as a
<spanclass="math">\(\mathtt{compactSize}\)</span>
.</li>
<li>For transparent addresses, the
<spanclass="math">\(\mathtt{addr}\)</span>
field does not include the first two bytes of a raw encoding.</li>
<li>There is intentionally no Typecode defined for a Sprout shielded payment address. Since it is no longer possible (since activation of ZIP 211 in the Canopy network upgrade <aid="id11"class="footnote_reference"href="#zip-0211">5</a>) to send funds into the Sprout chain value pool, this would not be generally useful.</li>
<li>Consumers MUST ignore constituent addresses with Typecodes they do not recognize.</li>
<li>Consumers MUST reject unified payment addresses in which the same Typecode appears more than once, or that include both P2SH and P2PKH transparent addresses, or that contain only a transparent address.</li>
<li>Producers SHOULD order the constituent addresses in the same order as the list of address types above. However, consumers MUST NOT assume this ordering, and it does not affect which address should be used by a consumer.</li>
<li>There MUST NOT be additional bytes at the end of the raw encoding that cannot be interpreted as specified above.</li>
<li>A wallet MAY allow its user(s) to configure which Receiver Types it can send to. It MUST NOT allow the user(s) to change the order of the Priority List used to choose the Receiver Type.</li>
<p>Security goal (<strong>near second preimage resistance</strong>):</p>
<ul>
<li>An adversary is given
<spanclass="math">\(q\)</span>
Unified Addresses, generated honestly.</li>
<li>The attack goal is to produce a “partially colliding” valid Unified Address that:
<olsuffix=")"type="a">
<li>has a string encoding matching that of <em>one of</em> the input addresses on some subset of characters (for concreteness, consider the first
<spanclass="math">\(n\)</span>
and last
<spanclass="math">\(m\)</span>
characters, up to some bound on
<spanclass="math">\(n+m\)</span>
);</li>
<li>is controlled by the adversary (for concreteness, the adversary knows <em>at least one</em> of the private keys of the constituent addresses).</li>
<li>In this variant, part b) above is replaced by the meaning of the new address being “usefully” different than the address it is based on, even though the adversary does not know any of the private keys. For example, if it were possible to delete a shielded constituent address from a UA leaving only a transparent address, that would be a significant malleability attack.</li>
<p>There is a generic brute force attack against near second preimage resistance. The adversary generates UAs at random with known keys, until one has an encoding that partially collides with one of the
<spanclass="math">\(q\)</span>
target addresses. It may be possible to improve on this attack by making use of properties of checksums, etc.</p>
<p>The generic attack puts an upper bound on the achievable security: if it takes work
<spanclass="math">\(w\)</span>
to produce and verify a UA, and the size of the character set is
<spanclass="math">\(c\)</span>
, then the generic attack costs
<spanclass="math">\(\sim \frac{w \cdot
c^{n+m}}{q}\)</span>
.</p>
<p>There is also a generic brute force attack against nonmalleability. The adversary modifies the target address slightly and computes the corresponding decoding, then repeats until the decoding is valid and also useful to the adversary (e.g. it would lead to the Sender using a transparent address). With
<spanclass="math">\(w\)</span>
defined as above, the cost is
<spanclass="math">\(w/p\)</span>
where
<spanclass="math">\(p\)</span>
is the probability that a random decoding is of the required form.</p>
<figcaption>Diagram of 4-round unkeyed Feistel construction</figcaption>
</figure>
<p>(In practice the lengths
<spanclass="math">\(\ell_L\)</span>
and
<spanclass="math">\(\ell_R\)</span>
will be roughly the same until
<spanclass="math">\(\ell_M\)</span>
is larger than
<spanclass="math">\(128\)</span>
bytes.)</p>
</section>
<sectionid="usage-for-unified-addresses"><h4><spanclass="section-heading">Usage for Unified Addresses</span><spanclass="section-anchor"><arel="bookmark"href="#usage-for-unified-addresses"><imgwidth="24"height="24"src="assets/images/section-anchor.png"alt=""></a></span></h4>
<p>The producer of a unified address appends 16 zero bytes to the encoding of the sequence of (Typecode, length, addr), then applies
<spanclass="math">\(\mathsf{F4Jumble}\)</span>
before encoding the result with Bech32m.</p>
<p>The consumer rejects any Bech32m-decoded byte sequence that is less than 22 bytes or greater than 16448 bytes; otherwise it applies
. It rejects any result that does not end in 16 zero bytes, before stripping these 16 bytes and parsing the result.</p>
<p>(22 bytes is the minimum size of a valid encoded address sequence, corresponding to just a transparent address, and 16448 bytes is the largest input/output size supported by
, then the adversary is able to introduce a controlled
<spanclass="math">\(\oplus\)</span>
-difference
<spanclass="math">\(a \oplus a' = y \oplus y'\)</span>
, but the other three pieces
<spanclass="math">\((b, c, d)\)</span>
are all randomized, which is sufficient;</li>
<li>if
<spanclass="math">\(y' = y\)</span>
but
<spanclass="math">\(x' \neq x\)</span>
, then the adversary is able to introduce a controlled
<spanclass="math">\(\oplus\)</span>
-difference
<spanclass="math">\(d \oplus d' = x \oplus x'\)</span>
, but the other three pieces
<spanclass="math">\((a, b, c)\)</span>
are all randomized, which is sufficient;</li>
<li>if
<spanclass="math">\(x' \neq x\)</span>
and
<spanclass="math">\(y' \neq y\)</span>
, all four pieces are randomized.</li>
</ul>
<p>Note that the size of each piece is at least 11 bytes. TODO: analyze whether this is sufficient when using 4 rounds.</p>
<p>It would be possible to make an attack more expensive by making the work done by an address producer more expensive. (This wouldn't necessarily have to increase the work done by the consumer.) However, given that addresses may need to be produced on constrained computing platforms, this was not considered to be beneficial overall.</p>
<p>The cost is dominated by 4 BLAKE2b compressions for
<spanclass="math">\(\ell_M \leq 128\)</span>
bytes. A UA containing a transparent address, a Sapling address, and an Orchard address, would have
<spanclass="math">\(\ell_M = 112\)</span>
bytes. The restriction to a single address with a given Typecode (and at most one transparent address) means that this is also the maximum length as of NU5 activation.</p>
<p>For longer UAs (when other Typecodes are added), the cost increases to 6 BLAKE2b compressions for
<p>BLAKE2b, with personalization and variable output length, is the only external dependency. TODO: would it be useful to remove the requirement for variable output length?</p>
<p>[Eliminating Random Permutation Oracles in the Even–Mansour Cipher](<ahref="https://www.iacr.org/cryptodb/data/paper.php?pubkey=218">https://www.iacr.org/cryptodb/data/paper.php?pubkey=218</a>)</p>
<ul>
<li>This paper argues that a 4-round unkeyed Feistel is sufficient to replace a random permutation in the Even–Mansour cipher construction.</li>
</ul>
<p>[On the Round Security of Symmetric-Key Cryptographic Primitives](<ahref="https://www.iacr.org/archive/crypto2000/18800377/18800377.pdf">https://www.iacr.org/archive/crypto2000/18800377/18800377.pdf</a>)</p>
<p>The authors would like to thank Benjamin Winston, Zooko Wilcox, Francisco Gindre, Marshall Gaucher, Joseph Van Geffen, Brad Miller, Deirdre Connolly, and Teor for discussions on the subject of Unified Addresses.</p>
