Ethereum: How to create the part in “scriptSig”?

Creating the sig and PubKey Part: Unlocking Ethereum Transactions

When you receive an Ethereum transaction, it is split into several components, including the script data (scriptSig) and public keys. Understanding how to extract these parts can help you decrypt and verify transactions.

In this article, we will focus on creating a sig and PubKey part in the scriptSig section of a transaction.

The scriptSig Section

The scriptSig section is the first part of the script data in an Ethereum transaction. It contains the unlock script, which determines how to derive the private key from the public keys used by the sender and receiver.

A typical scriptSig section starts with the following format:

0x
...

In your case, assuming you have a private key in a paper wallet and want to receive a transaction from someone, the scriptSig section would look like this:

0x
tx_address
some_key_id

Here:

• 

  is the sender’s public address.

• is a unique identifier for your private key.

• The unlocking_script part is where we will create our signature.

Creating the signature

To create a sig, you need to compute the signature using your private key. In Ethereum, the signing process involves computing the digest of your private key and then comparing it to the expected signature.

Here is an example of how you might create a sig in Solidity, a popular programming language for smart contracts:

pragma solidity ^0.8.0;
contract UnlockingScript {
function createSignature() public payable returns (bytes32) {
// Calculate the private key digest using the keccak-256 hash
bytes32 privateKeyDigest = keccak256(abi.encode_packed(
4, // 4 bytes for address and key ID
msg.sender.address,
msg.value // msg.value is not directly available in Solidity, but we can use it as a placeholder for now.
));
// Create the signature using the private key digest
bytes32 sig = keccak256(abi.encode_packed(
1, // 1 byte for nonce (we'll set this to 0 initially)
privateKeyDigest
));
return sig;
}
}

In this example:

• We compute a private_key_digest using the keccak-256 hashing algorithm with the sender address and value.

• We create a signature by computing an empty digest (using 1 byte for nonce) and comparing it to the expected private key digest.

Verifying the signature

To verify the signature, you can compare it to the expected signature used in the transaction. In this case, we’ll use the same unlocking_script as before:

0x
tx_address
some_key_id

We’ll create a new signature using the private key digest and compare it to the expected signature:

bytes32 sig = 0x...
bool isValidSignature() public view returns (bool) {
return keccak256(abi.encode_packed(
1, // 1 byte for nonce
sig
)) == bytes32("..." // expected private key digest
);
}

If the two signatures match, we can be confident that the signature was generated correctly.

Conclusion

Creating a sig and verifying its accuracy are essential steps to decrypting Ethereum transactions. By understanding how to extract the scriptSig and PubKey parts of a transaction and creating your own signature using your private key, you will be able to verify the authenticity of incoming transactions. Remember to always use safe practices when working with sensitive information, such as storing your private keys securely.