A confusing error-message when a private key of incorrect length is used

[barakman]

When using a private-key of incorrect length, for example, a private key of 20 bytes instead of 32 bytes:

w3.eth.account.from_key("0x1234567812345678123456781234567812345678")

The following exception is thrown:

ValueError: The private key must be exactly 32 bytes long, instead of 42 bytes.

The problem here is that while the length of a legal private key is indeed 32 bytes, it nevertheless requires 64 ASCII characters from the set [0-9]|[a-f] in order to represent it as a hexadecimal string.

This is because despite the fact that an ASCII character consists of 8 bits, each ASCII character in the set [0-9]|[a-f] actually represents 4 bits when used within a string designated for representing a hexadecimal value.

Let alone, the fact that we also need to prefix that string with two additional characters, namely, "0x".

Hence, in the example above, the illegal private key represents a 20-byte value, using a string of 42 ASCII characters.

There are two possible error messages which would imply the problem in a clear manner:

1. The private key must be string of exactly 2+64 hexadecimal characters, but 2+40 were provided
2. The private key must be hexadecimal string which represents exactly 32 bytes, but 20 were provided

Instead, you throw an error-message which is kind of a hybrid of the two options above.

The related code is in file account.py, lines 797-803:

        try:
            return self._keys.PrivateKey(HexBytes(key))
        except ValidationError as original_exception:
            raise ValueError(
                "The private key must be exactly 32 bytes long, instead of "
                f"{len(key)} bytes."
            ) from original_exception

You probably want to split that up into two try/catch clauses:

• First, check if the call to HexBytes succeeds. If not, then the input itself is not a hexadecimal string, and you can throw an error-message which describes that problem in a very accurate manner.
• Then, check if the call to PrivateKey succeeds. If not, then the input string represents an incorrect number of bytes, and again - you can throw an error-message which describes that problem in a very accurate manner.

Thanks

[kclowes]

This is a confusing error message. Thanks for the report!

[sanjaysudagani]

I can reproduce this and I am preparing a small fix to improve the incorrect-length private key error path, along with a regression test.