SSHChain is Free Software
For SSHChain to work, you have to give it permission to access your Keychain, or at least your SSH identity information. Although it is easy to click that Allow button, it isn't something you should do lightly, particularly if you don't know the people who made the software you're granting permission to.
To give you some reason to trust SSHChain, it is being released as open source, licensed under the GNU General Public License. See the Source Code page to download the source and project files, and for building instructions.