In cryptography, signcryption is a public-key primitive that simultaneously performs the functions of both digital signature and encryption.
Encryption and digital signature are two fundamental cryptographic tools that can guarantee the confidentiality, integrity, and non-repudiation. Until 1997, they were viewed as important but distinct building blocks of various cryptographic systems. In public key schemes, a traditional method is to digitally sign a message then followed by an encryption (signature-then-encryption) that can have two problems: Low efficiency and high cost of such summation, and the case that any arbitrary scheme cannot guarantee security. Signcryption is a relatively new cryptographic technique that is supposed to perform the functions of digital signature and encryption in a single logical step and can effectively decrease the computational costs and communication overheads in comparison with the traditional signature-then-encryption schemes.
Signcryption provides the properties of both digital signatures and encryption schemes in a way that is more efficient than signing and encrypting separately. This means that at least some aspect of its efficiency (for example the computation time) is better than any hybrid of digital signature and encryption schemes, under a particular model of security. Note that sometimes hybrid encryption can be employed instead of simple encryption, and a single session-key reused for several encryptions to achieve better overall efficiency across many signature-encryptions than a signcryption scheme but the session-key reuse causes the system to lose security under even the relatively weak CPA model. This is the reason why a random session key is used for each message in a hybrid encryption scheme but for a given level of security (i.e., a given model, say CPA), a signcryption scheme should be more efficient than any simple signature-hybrid encryption combination.