Digital signatures use a private and public key pair, authenticating both parties. The integrity of the contents exchanged is controlled through the hashing mechanism that is signed by the private key of the exchanging party. A digital hash in itself helps in ensuring integrity of the contents, but not nonrepudiation. Symmetric encryption wouldn’t help in nonrepudiation since the keys are always shared between parties. Strong passwords only ensure authentication to the system and cannot be used for nonrepudiation involving two or more parties.