How to verify incomming message against a *.cer certificate

I am consuming a web service that responds to me with signed messages. In a file of type * .cer I have the public key corresponding to the private key with which the messages were signed. When I try to create an entry in keystores or trustores it always asks me for the password, but the * .cer file has no password. Can I validate the signature of this type of message? How should I proceed if the * .cer file has no password?
Thanks.