Re: [bitcoindev] New Proposal：String Substring Search in Bitcoin Script - OP_ISSUBSTR

[Javier Mateos <javierpmateos@gmail.com>]

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  The solution of splitting the string and using OP_CAT only works if the 
exact position of the substring is known. How would a case be handled where 
the substring could be in any position?  

El miércoles, 19 de marzo de 2025 a las 22:02:05 UTC-3, Vojtěch Strnad 
escribió:

> Hi Weichu,
>
> You can implement this game by having the user supply in the initial stack 
> the two parts of their public key with the middle "goodluck" removed, and 
> inserting the "goodluck" as part of the script:
>
>    - script: "goodluck" OP_SWAP OP_CAT OP_CAT OP_CHECKSIG
>    - initial stack: signature pubkey_left pubkey_right
>
> Hope this helps.
>
> Vojtěch
> On Thursday, March 20, 2025 at 1:06:41 AM UTC+1 weichu deng wrote:
>
>> Hi Rijndael,
>>
>>  
>>
>> Thanks for your example 
>>
>> [witness: foobar  foo] bar CAT EQ
>>
>>  
>>
>> Yes, the unfixed string can be checked against a target substring in your 
>> example. However, if the target substring is located in the middle of the 
>> unfixed string, how to check it? In other words, how to have the same 
>> function as “foobar ob ISSUBSTR” with CAT if “foobar” is unfixed?
>>
>>  
>>
>> For example, suppose that a lucky draw game has the rule: if anyone has a 
>> publicKey which includes a special substring "goodluck", he/she will be 
>> awarded. 
>>
>> This game can be easily implemented with OP_ISSUBSTR as follow.
>>
>> - LockScript: OP_DUP goodluck OP_ISSUBSTR...
>>
>> - UnlockScript: signature publicKey
>>
>> How to implement it with OP_CAT?
>>
>>  
>>
>> *Regards*
>>
>> Weichu deng
>>
>> weich...@stu2024.jnu.edu.cn
>>
>>  
>>
>> 在2025年3月19日星期三 UTC+8 10:28:25<Rijndael> 写道：
>>
>>> Stack elements in Taproot are limited to 520 bytes. The current proposal 
>>> for re-activating OP_CAT includes this restriction: creating a string 
>>> longer than 520 bytes with CAT will cause the script to fail.
>>>
>>> With either CAT or ISSUBSTR, you can either commit to the substrings or 
>>> provide them at spend-time as witness data (and allow them to be unfixed in 
>>> the script).
>>>
>>> Fixed: FOOBAR BAR ISSUBSTR == FOOBAR FOO BAR CAT EQ
>>> Variable: [witness: FOOBAR] BAR ISSUBSTR == [witness: FOOBAR FOO] BAR 
>>> CAT EQ
>>>
>>>
>>> rijndael 
>>>
>>>
>>> On Mar 18, 2025, at 11:32 AM, weichu deng <weich...@stu2024.jnu.edu.cn> 
>>> wrote:
>>>
>>> Hi, Peter Todd
>>> Thanks for your feedback. I agree that "Bitcoin scripts are about 
>>> validation. Not computation."
>>> String search and concatenation are equivalent in some cases, such as in 
>>> the example you provided.
>>> However, it is still necessary to introduce the OP_ISSUBSTR operation 
>>> separately.
>>> One example is converting a non-deterministic signature to a 
>>> deterministic one.
>>> Another case is when the substring in question is located in the middle 
>>> of the checked string.
>>> CAT cannot replace ISSUBSTR for the following reasons:
>>>
>>>    1. The security of CAT is still controversial. It can easily 
>>>    generate overly long strings, potentially causing a stack overflow. 
>>>    Additionally, whether OP_CAT will be restored is still under discussion.
>>>    2. The other substring (bar) must be known in advance.
>>>    
>>>
>>> With respect,
>>>
>>> Weichu Deng
>>>
>>> weich...@stu2024.jnu.edu.cn
>>> 在2025年3月18日星期二 UTC+8 01:01:16<Peter Todd> 写道：
>>>
>>> On Mon, Mar 17, 2025 at 09:14:05AM -0700, weichu deng wrote:
>>> > 
>>> > 
>>> > Dear fellow Bitcoin developers,
>>> > 
>>> > 
>>> > 
>>> > I am pleased to present a new BIP proposal. This proposal introduces a 
>>> new 
>>> > opcode for Bitcoin scripts: OP_ISSUBSTR.
>>> > 
>>> > 
>>> > *Abstract*
>>> > 
>>> > This BIP introduces two string opcodes, OP_ISSUBSTR and 
>>> OP_ISSUBSTRVERIFY 
>>> > (similar to the relationship between OP_EQUAL and OP_EQUALVERIFY), to 
>>> > determine whether one string is a substring of another. As these 
>>> opcodes do 
>>> > not alter any blockchain state, they are secure.
>>>
>>> Bitcoin scripts are about validation. Not computation.
>>>
>>> This means that substring search and concatenation are equivalent. For
>>> every script that validates a substring search, you can instead
>>> concatenate the substring with the rest of the string, and validate
>>> equality instead.
>>>
>>> Basically speaking:
>>>
>>> foobar foo IsSubStr
>>>
>>> is equivalent to:
>>>
>>> foobar foo bar Cat Equal
>>>
>>> A real-world example would be more complex. But I hope that illustrates
>>> my point sufficiently.
>>>
>>> -- 
>>> https://petertodd.org 'peter'[:-1]@petertodd.org
>>>
>>>
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>>> .
>>>
>>>
>>>

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