SUSE 12 zypper can't install unsigned package non interactively

I have a problem with SUSE 12 zypper. I need to install an unsigned package with zypper in non-interactive mode from script. Unfortunately, zypper version is 1.13 and there is no option for flag --allow-unsigned-rpm. So I've been trying to "pipe" output of the printf command into zypper, but having no luck so far. Here is an example of what I am doing: printf "y\ni\n" | sudo zypper install UNSIGNED_RPM_PATH_HERE What am I doing wrong? This pipe works for every other script I tried to write myself.