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

From: weichu deng <[email-masked]>
Date: Mon, 17 Mar 2025 09:14:05 -0700 (PDT)

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.

*Specification*

These opcodes check if the second string on the stack is a substring of the 
first string. If the opcode is OP_ISSUBSTRVERIFY, it verifies the condition 
and throws an error if false, without retaining the result.

*Execution Process*

   1. Take the two strings at the top of the stack. 
   2. Use standard library functions to compare the two strings. 
   3. Pop the two strings from the stack and push the result onto the stack. 
   4. If the opcode is OP_ISSUBSTRVERIFY, do not push the result. 

*Motivation*

The absence of string operations in Bitcoin scripts restricts its 
applicability. When developers need string operations for applications, 
they must simulate these functions through off-chain preprocessing or 
complex scripts, increasing development difficulty and potentially 
introducing centralized dependencies.

Early Bitcoin versions supported some string operations, such as OP_SUBSTR, 
which extracted a substring of specified position and length from a string, 
replacing the original string. For security reasons, OP_SUBSTR was disabled 
in Bitcoin v0.3.10 and later versions due to a vulnerability 
(CVE-2010-5137) caused by OP_LSHIFT. To prevent similar overflow 
vulnerabilities, Bitcoin disabled several opcodes, including OP_SUBSTR. As 
Bitcoin adoption grows, the limitations of lacking string operations have 
become more evident. Our proposed OP_ISSUBSTR adds string search 
functionality to Bitcoin scripts without changing any state, making it safe.

*Advantages *

   1. *Enhanced Script Functionality and Flexibility* Developers can 
   process string logic on-chain without off-chain reliance. For example, in 
   multi-signature wallets, developers can verify specific signer information 
   or remarks directly in scripts using OP_ISSUBSTR to check transaction 
   comments or signature fields for particular substrings. 
   2. *Support for String Searching* In some scenarios, developers need to 
   verify if parts of a string match a format or contain specific data. For 
   example, checking if a payment transaction's payee name matches a preset 
   value. 
   3. *Conversion of Non-deterministic Algorithms to Deterministic Ones* 
   Some signature algorithms or hash functions produce non-deterministic 
   outputs. OP_ISSUBSTR allows developers to check if these outputs contain 
   known substrings in scripts, converting non-deterministic results to 
   deterministic ones. For example, verifying if a hash value contains a 
   specific hexadecimal sequence (like "0000") to trigger contract logic. 
   4. *Simplified Address Verification Logic* Bitcoin addresses typically 
   start with specific prefixes or suffixes. OP_ISSUBSTR enables direct 
   address format verification in scripts. For example, checking if a 
   transaction target address starts with "bc1" to ensure validity or detect 
   "address pollution" attacks. 
   5. *Integration with Modern Programming Languages* Modern languages 
   widely support string operations. OP_ISSUBSTR makes Bitcoin scripts more 
   aligned with these languages, lowering the barrier for developers. 

We have provided detailed documentation and a reference implementation in 
the BIP draft. You can read the full proposal here: 
https://github.com/Weichu-Deng/bips/blob/OP_ISSUBSTR/bip-yongdong%20wu-OP_ISSUBSTR.md

Thank you for your feedback! 

With respect,

Weichu Deng

 [email-masked]

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