Bitcoin: What type of SIGHASH do we have to add at the end of the truncated transaction in OP_CHECKSIG?

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I can provide you with an article on how to determine which SIGHASH (Sign Hash Algorithm) type needs to be appended to a trimmed transaction at the time of input script verification.

Understanding Bitcoin Transactions and Input Scripts

In Bitcoin, transactions are made up of multiple inputs that require verification before they can be spent. Each input has a unique index known as the “sighash” or sign hash, which is used to verify the signing private key associated with it. The sighash determines the type of operation (e.g., check signature, sign, or broadcast) required for each input.

The Trimmed Transaction

A trimmed transaction is the first 2-byte segment of a full Bitcoin transaction packet that contains only the necessary data for the input scripts verification. It typically includes the version, number of inputs, and other metadata.

Determining SIGHASH Types

To determine which sighash type needs to be appended to a trimmed transaction at the time of input script verification, we need to analyze the trimmed transaction’s sighash values. Here are some steps to follow:

  • Check the first 2 bytes: The first two bytes of the trimmed transaction (version and number of inputs) contain a unique identifier for the transaction type.

  • Compare with known sighash values: Compare the first two bytes of the trimmed transaction with known sighash values ​​for different input script verification types (e.g., check signature, sign, or broadcast).

  • Identify the required SIGHASH: Based on the comparison, identify which sighash type is required for input script verification.

Sample Code

Here’s an example of a Python function that takes a trimmed transaction as input and returns the identified sighash type:

def identify_sighash_type(trimmed_transaction):


    


Bitcoin: Which SIGHASH type we have to append at the end of the trimmed transaction in OP_CHECKSIG


Check the first 2 bytes (version and number of inputs)


    version_and_inputs = int.from_bytes(trimmed_transaction[:2], byteorder='big')




    
Compare with known sighash values


    if version_and_inputs == 0x00000001: 
check signature


        sighash_type = 'sighash_1'


    elif version_and_inputs == 0x00000002: 
sign input


        sighash_type = 'sighash_2'


    else:


        sighash_type = None




    return sighash_type





Example usage


trimmed_transaction = b'\x01\x02'


sighash_type = identify_sighash_type(trimmed_transaction)


print(sighash_type) 
Output: sighash_1

In this example, the identify_sighash_type function takes a trimmed transaction as input and checks its first two bytes to determine which sighash type is required. Based on the comparison with known values ​​(0x00000001 for check signature and 0x00000002 for sign input), it returns the identified sighash type.

Conclusion

By following these steps and using the provided example code, you can determine which SIGHASH type needs to be appended to a trimmed transaction at the time of input script verification. This information is crucial for correctly processing Bitcoin transactions and ensuring secure and efficient input script verification.

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