Cryptology ePrint Archive: Report 2012/154

The Joint Signature and Encryption Revisited

Laila El Aimani

Abstract: We study the Sign\_then\_Encrypt, Commit\_then\_Encrypt\_and\_Sign, and Encrypt\_then\_Sign paradigms in the context of three cryptographic primitives, namely designated confirmer signatures, signcryption, and verifiably encrypted signatures. Our study identifies weaknesses in those paradigms which impose the use of expensive encryption (as a building block) in order to meet a reasonable security level. Next, we propose some optimizations which annihilate the found weaknesses and allow consequently cheap encryption without compromising the overall security. Our optimizations further enjoy verifiability, a property profoundly needed in many real-life applications of the studied primitives.

Category / Keywords: public-key cryptography / Sign\_then\_Encrypt, Commit\_then\_Encrypt\_and\_Sign, Encrypt\_then\_Sign, (public) verifiability, designated confirmer signatures, signcryption, verifiably encrypted signatures, zero knowledge proofs.

Date: received 23 Mar 2012

Contact author: laila elaimani at yahoo fr

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Version: 20120323:201628 (All versions of this report)

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