When I create a new application on Facebook, I meet the problem. Facebook asks me to provide the Key Hash. But it does not show the guidance about how to generate. After Googling, I have found this works for me. Before executing the following command, you need install openssl
keytool -exportcert -alias your-alias-value -keystore your-keystore-path | openssl sha1 -binary | openssl base64
Replace your-alias-value and your-keystore-path with the real data.