Understanding Ethereum Transaction IDs

As a developer building a non-mainstream Ethereum client, you’re probably familiar with the concept of Transaction IDs (TxIDs) on the Ethereum blockchain. In this article, we’ll dive into how these unique identifiers are generated and what happens to them after each block.

Generating a Transaction ID

A transaction ID is an 18-byte hexadecimal string that represents a single Bitcoin transaction. The generation process involves several steps:

• Block Creation: A new block is created on the Ethereum network when a valid transaction is sent to the network.

• Transaction Hashing: Each transaction in a block is hashed using the sha256 algorithm with the block hash as input and the Merkle root of all transactions as additional inputs.

• Transaction ID Generation: The transaction hash is then used to generate a unique 18-byte hexadecimal string known as the Transaction ID (TxID).

The TxID is generated in two parts:

• Block Digest

  : This is the actual hash of the block, which is a combination of the block header and other metadata.

• Merkle Root: This is an additional input to the transaction hashing algorithm that helps ensure uniqueness and integrity.

TxID Generation Algorithm

The Ethereum community has developed a consensus algorithm called “Keccak-256” to generate transaction IDs. The algorithm takes two inputs:

• Block Digest: This is the actual hash of the block.

• Merkle Root: This is an additional input to calculate the Merkle root.

The resulting TxID is generated using a combination of the block hash and the Merkle root, resulting in an 18-byte hexadecimal string.

When does it change?

When each block is created on the Ethereum network, the transaction ID for that block changes. This change occurs because each new block in the chain has its own unique transaction hash and Merkle root.

To illustrate this concept, consider the following example:

• Block 1: A new block is created using a specific block hash (e.g. 0x1234567890abcdef).

• Transaction hashes: Each of the 10 transactions in block 1 has its own unique transaction hash using Keccak-256.

• Merkle roots: The Merkle root for each of these transactions is also generated.

As a result, as we create new blocks on the Ethereum network, they will have their own unique block hashes and Merkle roots, which will lead to changes in the transaction identifiers associated with those blocks.

Example use case:

To demonstrate how this works, let’s consider an example using Python:

import hashlib
import json
def generate_txid(block_hash):

Hash function used for Keccak-256
def hash_function(input_string):
return hashlib.sha256(input_string.encode()).hexdigest()
block_merkle_root = hash_function(str(block_hash))
transaction_hashes = []
for i in range(10): 
assuming 10 transactions
txid = hash_function(json.dumps(i, sort_keys=True)) + block_merkle_root
transaction_hashes.append(txid)
return json.dumps(transaction_hashes), block_merkle_root
block_hash = "0x1234567890abcdef"
txids, merkle_root = generate_txid(block_hash)
print("Transaction Hashes:", txids)
print("Merkle Root:", merkle_root)

In this example, we create a new block hash using the hash_function function. We then use this block hash as input to the transaction hashing algorithm and store the resulting TxID along with the Merkle root.

This process ensures that each new block on the Ethereum network has its own unique Transaction ID and Merkle root, which leads to changes in the Transaction IDs associated with those blocks.

Conclusion

Understanding how transaction identifiers are generated and changed is crucial to building a reliable and efficient Ethereum client outside the mainstream.

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