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Public-key digital certificate has been widely used in public-key infrastructure (PKI) to provide user public key authentication. However, the public-key digital certificate itself cannot be used as a security factor to authenticate user. In this paper, we propose the concept of generalized digital certificate (GDC) that can be used to provide user authentication and key agreement. A GDC contains user's public information, such as the information of user's digital driver's license, the information of a digital birth certificate, etc., and a digital signature of the public information signed by a trusted certificate authority (CA). However, the GDC does not contain any user's public key. Since the user does not have any private and public key pair, key management in using GDC is much simpler than using public-key digital certificate. The digital signature of the GDC is used as a secret token of each user that will never be revealed to any verifier. Instead, the owner proves to the verifier that he has the knowledge of the signature by responding to the verifier's challenge. Based on this concept, we propose both discrete logarithm (DL)-based and integer factoring (IF)-based protocols that can achieve user authentication and secret key establishment.