Efficient Constructions of Signcryption Schemes and Signcryption Composability

In this paper, we investigate simple but efficient constructions of signcryption schemes. Firstly, we show how symmetric primitives can be used to efficiently achieve outsider multi-user security, leading to a signcryption scheme with the currently lowest ciphertext and computational overhead. For the mixed security notions outsider confidentiality/insider unforgeability and insider confidentiality/outsider unforgeability, this approach yields lower ciphertext overhead and a higher level of security, respectively, compared to the current schemes. Secondly, we show a simple optimization to the well known "sign-then-encrypt" and "encrypt-then-sign" approaches to the construction of signcryption schemes by using tag-based encryption. Instantiations with our proposed tag-based schemes yield multi-user insider secure signcryption schemes in the random oracle model which is at least as efficient as any other existing scheme both in terms of ciphertext overhead and computational cost. Furthermore, we show that very efficient standard model signcryption schemes can be constructed using this technique as well. Lastly, we show how signatures and encryption can be combined in a non-black-box manner to achieve higher efficiency than schemes based on the above approach. We refer to signature and encryption schemes which can be combined in this way as signcryption composable, and we show that a number of the most efficient standard model encryption and signature schemes satisfy this, leading to the most efficient standard model signcryption schemes. Since all of our constructions are fairly simple and efficient, they provide a benchmark which can be used to evaluate future signcryption schemes.