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add examples for TxRefs with Outpoints; fix some typos and wording
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danpape authored and veleslavs committed Jun 13, 2019
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== Introduction ==

=== Abstract ===
This document proposes a convenient human useable format, '''"TxRef"''', as a standard way to refer to a transaction position within the Bitcoin Blockchain, and optionally a particular outpoint index with the referred transaction. The primary purpose of this format is to allow users to refer to a confirmed transaction (and optionally an outpoint index within) in a standard reliable and concise way.
This document proposes a convenient human useable format, '''"TxRef"''', as a standard way to refer to a transaction position within the Bitcoin Blockchain, and optionally a particular outpoint index within the referred transaction. The primary purpose of this format is to allow users to refer to a confirmed transaction (and optionally an outpoint index within) in a standard, reliable, and concise way.

''Please note: Unlike TxID where there is strong cryptographic link between the ID and the actual transaction. TxRef only provide a weak link to a particular transaction. TxRef locates an offset within a blockchain for a transaction, that may - or may not - point to an actual transaction, which fact may change with reorganisations. We recommend that TxRef's should be not used for positions within the blockchain having a maturity less than 100 blocks.''
''Please note: Unlike TxID where there is strong cryptographic link between the ID and the actual transaction, TxRef only provides a weak link to a particular transaction. TxRef locates an offset within a blockchain for a transaction, that may - or may not - point to an actual transaction, which in fact may change with reorganisations. We recommend that TxRef's should be not used for positions within the blockchain having a maturity less than 100 blocks.''

=== Copyright ===

This BIP is licensed under the 2-clause BSD license.

=== Motivation ===
Since the first version of Bitcoin, TxID's (Transaction Identifiers) that are a core part of the consensus protocol, have been routinely used to identify individual transactions between users.
Since the first version of Bitcoin, TxID's (Transaction Identifiers) have been a core part of the consensus protocol and have been routinely used to identify individual transactions between users.

However, for many use-cases they have practical limitations:
* TxIDs are expensive for full nodes to lookup (requiring either a linear scan of the blockchain, or an expensive TxID index).
* TxIDs require third-party service for SPV wallets to lookup.
* TxIDs require third-party services for SPV wallets to lookup.
* TxIDs are very long HEX encoded values (64 characters long).
For transactions that have been embedded in the blockchain, it is possible to reference them not by their TxID, but by their location within the blockchain itself. The encoding can be made friendly for occasional human transcription. In this document, we propose a standard for doing this.

=== Examples ===
These examples are for Bitcoin Transactions.
* Genesis Coinbase Transaction: <tt>tx1:rqqq-qqqq-qmhu-qhp</tt>
* Genesis Coinbase Transaction (Transaction #0 of Block #0): <tt>tx1:rqqq-qqqq-qmhu-qhp</tt>
* Transaction #2205 of Block #466793: <tt>tx1:rjk0-uqay-zsrw-hqe</tt>
== Specification ==
Expand Down Expand Up @@ -69,12 +69,12 @@ TxRef uses standard Bech32<ref name=":0">'''Why use Bech32 Encoding for Confirme
* Separator: '''"1"'''.
* Data Part.
Please node: other specifcations, such as [https://w3c-ccg.github.io/did-spec/ Decentralized Identifier syntax] , have implicitly encoded the information contained within the HRP elsewhere. In this case they may choose not include the HRP as specified here.
Please note: other specifications, such as [https://w3c-ccg.github.io/did-spec/ the Decentralized Identifiers spec], have implicitly encoded the information contained within the HRP elsewhere. In this case they may choose to not include the HRP as specified here.

To increase portability and readability additional separators SHOULD be added:

* A Colon<ref>'''Why add a colon here?''' This allows it to conform better with W3C URN/URL standards.</ref> '''":"''' added after '1'.
* Hyphens<ref>'''Why hyphens to the TxRef?''' As TxRef's are short, we expect that they will be quoted via voice or written by hand. The inclusion of hyphens every 4 characters breaks the string and means people don't lose their place so easily.</ref> '''"-"''' added after every 4 characters beyond the colon.
* Hyphens<ref>'''Why hyphens within the TxRef?''' As TxRef's are short, we expect that they will be quoted via voice or written by hand. The inclusion of hyphens every 4 characters breaks up the string and means people don't lose their place so easily.</ref> '''"-"''' added after every 4 characters beyond the colon.
All non-bech32-alphabet characters after the bech32 code separator MUST be ignored/removed when parsing (except for terminating characters).<ref>'''Why strip all non-bech32-alphabet characters?''' We do not wish to expect the users to keep their TxRef's in good unicode form (hyphens, colons, invisible spaces, random unicode characters, etc). We expect them to copy, paste, write by-hand, write in a mix of character sets, etc. Parsers should automatically correct for all sorts of these common errors.
</ref>
Expand Down Expand Up @@ -116,10 +116,10 @@ All non-bech32-alphabet characters after the bech32 code separator MUST be ignor
|1
|"'''-'''"
|}
The Data - Hyphen patten is repeated for the entire length of data, ( a hyphen is inserted after every encoded 20 bits or 4 data characters).
The Data - Hyphen pattern is repeated for the entire length of data, ( a hyphen is inserted after every encoded 20 bits or 4 data characters).
=== Data ===

The 75 or 90 bits of data encoded in the string above are defined in this manner:
Depending on if an optional transaction outpoint is included, there can be 75 or 90 bits of data encoded in the string above. These bits are defined in this manner:

{| class="wikitable"
|+TxRef Binary Format for Bitcoin Mainnet and Bitcoin Testnet:
Expand All @@ -130,7 +130,7 @@ The 75 or 90 bits of data encoded in the string above are defined in this manner
!'''Values'''
!'''Notes'''
|-
|Magic
|Magic Code
|0 – 4
|5
|Chain Namespacing Code
Expand Down Expand Up @@ -161,7 +161,7 @@ The 75 or 90 bits of data encoded in the string above are defined in this manner
|Tx 0 (coinbase) to Tx position 32767
|Max Tx's in block is 16665
|}
If the magic is '''0x4''' or '''0x7''', an optional outpoint is included in the encoding:
If the magic code is '''0x4''' or '''0x7''', an optional outpoint is included in the encoding:

{| class="wikitable"
|+Optional Outpoint Index Encoding:
Expand All @@ -176,7 +176,7 @@ If the magic is '''0x4''' or '''0x7''', an optional outpoint is included in the
|45 – 59
|15
|The index of the Outpoint inside the Tx
|Outpoint 0 to Outpoint Position 32767
|Outpoint 0 to Outpoint Position 32767
|
|}

Expand Down Expand Up @@ -220,19 +220,19 @@ There are no known compatibility issues.
<references />

== Reference implementations ==
C Reference Implementation (supports version 0): https://github.com/jonasschnelli/bitcoin_txref_code
C Reference Implementation (supports magic codes 0x3 and 0x6): https://github.com/jonasschnelli/bitcoin_txref_code

Go Reference Implementation (supports version 0): https://github.com/kulpreet/txref
Go Reference Implementation (supports magic codes 0x3 and 0x6): https://github.com/kulpreet/txref

C++ Reference Implementation (support versions 0 and 1): https://github.com/dcdpr/btcr-DID-method/blob/master/libtxref
C++ Reference Implementation (support magic codes 0x3, 0x4, 0x6, 0x7): https://github.com/dcdpr/btcr-DID-method/

== Appendices ==

=== Test Vectors ===
There are two sets of Test Vectors included here:

* Bech32 Encoding Test Vectors. These are to test if a implementation accepts the encoding, with the correct human readable part, and separator.
* Bitcoin TxRef Test Vectors. These test the full specification, in particular correct values for block height and the transaction index.
* Bitcoin TxRef Test Vectors. These test the full specification, in particular, correct values for block height and the transaction index.
==== Bech32 Encoding (for TxRef). ====
''Please Note: All test vectors are shown to help test if a string is compliant or not. All real-life applications (such as for Bitcoin) should comply with the Bitcoin Test Vectors listed Below.''
Expand All @@ -247,15 +247,22 @@ The following list gives invalid TxRef's and the reason for their invalidity.
* <tt>bc1qw508d6qejxtdg4y5r3zarvary0c5xw7kg3g4ty</tt>: Invalid human-readable part
* <tt>tx1qw508d6qejxtdg4y5r3zarvary0c5xw7kv8f3t5</tt>: Invalid checksum
==== Bitcoin TxRef ====
The following list gives properly encoded Bitcoin TxRef's and the values in hex. (block height, transaction index)
==== Bitcoin TxRef (mainnet and testnet) ====
The following list gives properly encoded Bitcoin mainnet TxRef's and the values in hex. (block height, transaction index)

* <tt>tx1:rqqq-qqqq-qmhu-qhp</tt>: <tt>(0x0, 0x0)</tt>
* <tt>tx1:rqqq-qqll-l8xh-jkg</tt>: <tt>(0x0, 0x7FFF)</tt>
* <tt>tx1:r7ll-llqq-qghq-qr8</tt>: <tt>(0xFFFFFF, 0x0)</tt>
* <tt>tx1:r7ll-llll-l5xt-jzw</tt>: <tt>(0xFFFFFF, 0x7FFF)</tt>
The following list gives valid Bitcoin TxRef's and the values in hex. (block height, transaction index)
The following list gives properly encoded Bitcoin testnet TxRef's and the values in hex. (block height, transaction index)

* <tt>txtest1:xqqq-qqqq-qkla-64l</tt>: <tt>(0x0, 0x0)</tt>
* <tt>txtest1:xqqq-qqll-l2wk-g5k</tt>: <tt>(0x0, 0x7FFF)</tt>
* <tt>txtest1:x7ll-llqq-q9lp-6pe</tt>: <tt>(0xFFFFFF, 0x0)</tt>
* <tt>txtest1:x7ll-llll-lew2-gqs</tt>: <tt>(0xFFFFFF, 0x7FFF)</tt>
The following list gives valid (though strangely formatted) Bitcoin TxRef's and the values in hex. (block height, transaction index)
* <tt>tx1:rjk0-uqay-zsrw-hqe</tt>: <tt>(0x71F69, 0x89D)</tt>
* <tt>TX1RJK0UQAYZSRWHQE</tt>: <tt>(0x71F69, 0x89D)</tt>
* <tt>TX1RJK0--UQaYZSRw----HQE</tt>: <tt>(0x71F69, 0x89D)</tt>
Expand All @@ -268,6 +275,38 @@ The following list gives invalid Bitcoin TxRef's and the reason for their invali
* <tt>tx1:rjk0-u5ng-gghq-fkg7</tt>: Valid Bech32, but 10x5bit packages instead of 8.
* <tt>tx1:rjk0-u5qd-s43z</tt>: Valid Bech32, but 6x5bit packages instead of 8.
==== Bitcoin TxRef with Outpoints (mainnet and testnet) ====
The following list gives properly encoded Bitcoin mainnet TxRef's with Outpoints and the values in hex. (block height, transaction index, TXO index)

* <tt>tx1:yqqq-qqqq-qqqq-ksvh-26</tt>: <tt>(0x0, 0x0, 0x0)</tt>
* <tt>tx1:yqqq-qqll-lqqq-v0h2-2k</tt>: <tt>(0x0, 0x7FFF, 0x0)</tt>
* <tt>tx1:y7ll-llqq-qqqq-a5zy-tc</tt>: <tt>(0xFFFFFF, 0x0, 0x0)</tt>
* <tt>tx1:y7ll-llll-lqqq-8tee-t5</tt>: <tt>(0xFFFFFF, 0x7FFF, 0x0)</tt>
* <tt>tx1:yqqq-qqqq-qpqq-5j9q-nz</tt>: <tt>(0x0, 0x0, 0x1)</tt>
* <tt>tx1:yqqq-qqll-lpqq-wd7a-nw</tt>: <tt>(0x0, 0x7FFF, 0x1)</tt>
* <tt>tx1:y7ll-llqq-qpqq-lktn-jq</tt>: <tt>(0xFFFFFF, 0x0, 0x1)</tt>
* <tt>tx1:y7ll-llll-lpqq-9fsw-jv</tt>: <tt>(0xFFFFFF, 0x7FFF, 0x1)</tt>
* <tt>tx1:yjk0-uqay-zrfq-g2cg-t8</tt>: <tt>(0x71F69, 0x89D, 0x123)</tt>
* <tt>tx1:yjk0-uqay-zu4x-nk6u-pc</tt>: <tt>(0x71F69, 0x89D, 0x1ABC)</tt>
The following list gives properly encoded Bitcoin testnet TxRef's with Outpoints and the values in hex. (block height, transaction index, TXO index)

* <tt>txtest1:8qqq-qqqq-qqqq-cgru-fa</tt>: <tt>(0x0, 0x0, 0x0)</tt>
* <tt>txtest1:8qqq-qqll-lqqq-zhcp-f3</tt>: <tt>(0x0, 0x7FFF, 0x0)</tt>
* <tt>txtest1:87ll-llqq-qqqq-nvd0-gl</tt>: <tt>(0xFFFFFF, 0x0, 0x0)</tt>
* <tt>txtest1:87ll-llll-lqqq-fnkj-gn</tt>: <tt>(0xFFFFFF, 0x7FFF, 0x0)</tt>
* <tt>txtest1:8qqq-qqqq-qpqq-622t-s9</tt>: <tt>(0x0, 0x0, 0x1)</tt>
* <tt>txtest1:8qqq-qqll-lpqq-q43k-sf</tt>: <tt>(0x0, 0x7FFF, 0x1)</tt>
* <tt>txtest1:87ll-llqq-qpqq-3wyc-38</tt>: <tt>(0xFFFFFF, 0x0, 0x1)</tt>
* <tt>txtest1:87ll-llll-lpqq-t3l9-3t</tt>: <tt>(0xFFFFFF, 0x7FFF, 0x1)</tt>
* <tt>txtest1:8jk0-uqay-zrfq-xjhr-gq</tt>: <tt>(0x71F69, 0x89D, 0x123)</tt>
* <tt>txtest1:8jk0-uqay-zu4x-aw4h-zl</tt>: <tt>(0x71F69, 0x89D, 0x1ABC)</tt>

=== Bitcoin TxRef Payload Value Choice: ===
Some calculations showing why we chose these particular bit-length of the block height and transaction index.

Expand All @@ -285,24 +324,5 @@ Some calculations showing why we chose these particular bit-length of the block
*The ''extreme'' smallest Tx is 60 Byte's: Max 16665 tx in a block.
**4B version + 1B tx_in count + 36B previous_output + 1B script length + 0B signature script + 4B sequence + 1B tx_out count + 8B amount + 1B script length + 0B pubkey script + 4B lock_time = 60B
=== Test Vectors (version 1) ===
These test vectors are extended TxRefs (version 1):

==== Bitcoin Extended TxRef ====
The following list gives properly encoded Bitcoin Extended TxRef's and the values in hex. (block height, transaction index, TXO index)

* <tt>tx1:rpqq-qqqq-qqqq-q2ge-ahz</tt>: <tt>(0x0, 0x0, 0x0)</tt>
* <tt>tx1:rpqq-qqql-llqq-qshz-qhw</tt>: <tt>(0x0, 0x7FFF, 0x0)</tt>
* <tt>tx1:rp7l-lllq-qqqq-qpvh-wkq</tt>: <tt>(0xFFFFFF, 0x0, 0x0)</tt>
* <tt>tx1:rp7l-llll-llqq-qmnv-nkv</tt>: <tt>(0xFFFFFF, 0x7FFF, 0x0)</tt>
* <tt>tx1:rpqq-qqqq-qqpq-qg2s-2w6</tt>: <tt>(0x0, 0x0, 0x1)</tt>
* <tt>tx1:rpqq-qqql-llpq-qj4t-hwk</tt>: <tt>(0x0, 0x7FFF, 0x1)</tt>
* <tt>tx1:rp7l-lllq-qqpq-qrw7-e0c</tt>: <tt>(0xFFFFFF, 0x0, 0x1)</tt>
* <tt>tx1:rp7l-llll-llpq-qe39-y05</tt>: <tt>(0xFFFFFF, 0x7FFF, 0x1)</tt>
* <tt>tx1:rpjk-0uqa-yzu4-x0w0-kuq</tt>: <tt>(0x71F69, 0x89D, 0x1ABC)</tt>
* <tt>txtest1:xpjk-0uqa-yzu4-xgrl-pue</tt>: <tt>(0x71F69, 0x89D, 0x1ABC)</tt> (testnet, magic number = 0x6)
== Acknowledgements ==
Special Thanks to Pieter Wuille and Greg Maxwell for Bech32, a wonderful user-facing data encoding.

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