Is this a question or a bug report?

The first 3 lines of NewTripleDESCipher are:

if len(key) != 24 {
	return nil, KeySizeError(len(key))
}

so you'll need to handle any key-length-related error that the function returns.

We don't use the issue tracker for questions, so if you have further questions please see the Questions wiki page; it has a list of good places for asking them.

Closing, since this appears to be not-a-bug-report.