A digital signature is a well-known primitive that provides integrity, non-repudiation and authenticity of messages in cryptographic protocols. In an ordinary digital signature scheme, the signer creates a signature which is publicly verifiable by everyone. Although this public verifiability of a traditional digital signature is useful in many applications, however there are also many scenarios in which the signer must be authenticated without disturbing his/her privacy. In this thesis, we concentrated on two concepts of the privacy of the signer in digital signatures: confidentiality and non-transferability which are satisfied by signcryption and designated verifier signature schemes respectively. Consequently, we solved some open problems and provided some contributions in these two fields. It is important to note that we proved the security of all of our proposed schemes in the standard model (i.e. without the ideal assumptions).
