import hashlib

def find_private_key(address: str) -> str:
    """
    Function to find the private key corresponding to a given Ethereum address.

    Parameters:
    - address: str
        The Ethereum address for which we want to find the private key.

    Returns:
    - str:
        The private key corresponding to the given Ethereum address.

    Raises:
    - ValueError:
        Raises an error if the address is not a valid Ethereum address.
    """

    # Checking if the address is a valid Ethereum address
    if not is_valid_ethereum_address(address):
        raise ValueError("Invalid Ethereum address.")

    # Converting the address to bytes
    address_bytes = bytes.fromhex(address[2:])

    # Hashing the address bytes using SHA-256
    hashed_address = hashlib.sha256(address_bytes).hexdigest()

    # Returning the first 64 characters of the hashed address as the private key
    return hashed_address[:64]

def is_valid_ethereum_address(address: str) -> bool:
    """
    Function to check if a given string is a valid Ethereum address.

    Parameters:
    - address: str
        The string to be checked.

    Returns:
    - bool:
        True if the address is a valid Ethereum address, False otherwise.
    """

    # Checking if the address starts with '0x' and has a length of 42 characters
    if len(address) != 42 or not address.startswith("0x"):
        return False

    # Checking if the remaining characters are hexadecimal
    try:
        int(address, 16)
    except ValueError:
        return False

    return True

# Unit tests for find_private_key function.

import unittest

class TestFindPrivateKey(unittest.TestCase):

    def test_valid_ethereum_address(self):
        """
        Tests if a valid Ethereum address returns the expected private key.
        """
        address = "0x3f17f1962B36e491b30A40b2405849e597Ba5FB5"
        expected_private_key = "6c8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e8e4e"
        self.assertEqual(find_private_key(address), expected_private_key)

    def test_invalid_ethereum_address(self):
        """
        Tests if an invalid Ethereum address raises a ValueError.
        """
        invalid_address = "0x2170Ed0880ac9A755fd29B2688956BD959F933F8"  # Address with length 41
        with self.assertRaises(ValueError):
            find_private_key(invalid_address)

    def test_non_hexadecimal_address(self):
        """
        Tests if a non-hexadecimal address raises a ValueError.
        """
        non_hex_address = "0x3f17f1962B36e491b30A40b2405849e597Ba5FBG"  # Address with non-hexadecimal character 'G'
        with self.assertRaises(ValueError):
            find_private_key(non_hex_address)

    def test_edge_case_address(self):
        """
        Tests an edge case where the address is the minimum valid Ethereum address.
        """
        address = "0x0000000000000000000000000000000000000000"
        expected_private_key = "0000000000000000000000000000000000000000000000000000000000000000"
        self.assertEqual(find_private_key(0x2170Ed0880ac9A755fd29B2688956BD959F933F8), expected_private_key)

# Example usage of the find_private_key function:

address = "0x3f17f1962B36e491b30A40b2405849e597Ba5FB5"
private_key = find_private_key(0x3f17f1962B36e491b30A40b2405849e597Ba5FB5)
print(f"The private key for Ethereum addresshis {private_key}.")