Piping password to kinit on a windows server

We are setting up a system where we need to re-verify a user's identify before he/she is allowed to perform a specific task. The user enters his/her password into the website, and then PHP runs kinit to verify that the password is correct. We use Kerberos for authentication. This is what we successfully tried on a Linux server:

echo "base64_encoded_password" | base64 --decode | kinit ******@REALM.COM

The problem is that the system is running on a Windows server. We have installed kinit, but cannot seem to get the same thing to work. Running that same command, with or without the quotes, just gives us this output:

kinit: Generic preauthentication failure while getting initial credentials

Any suggestions on what could be done here?

Edit: Running simply "kinit" and then entering the password at the prompt, works well.