It is an important and hard problem in the area of computer network security to analyze cryptographic protocols. A methodology is presented using a model checke r of formal methods, SMV (symbolic model verifier), to analyze the well known Ne edham-Schroeder Public-Key Protocol. The SMV is used to discover an attack upo n the protocol, which has never been discovered by BAN logic. Finally, the proto col is adapted, and then the SMV is used to show that the new protocol is secure.