The Joint Signature and Encryption Revisited

TL;DR

This paper critically examines common paradigms in cryptography involving signatures and encryption, identifies their weaknesses, and proposes optimized methods that enable secure and efficient cryptographic primitives with verifiability.

Contribution

It introduces optimizations that eliminate weaknesses in existing paradigms, allowing for secure, cost-effective encryption with verifiability in designated confirmer signatures and signcryption.

Findings

Identified weaknesses in Sign_then_Encrypt, Commit_then_Encrypt_and_Sign, and Encrypt_then_Sign paradigms

Proposed optimizations enable cheaper encryption without security loss

Enhanced verifiability in cryptographic primitives

Abstract

We study the Sign_then_Encrypt, Commit_then_Encrypt_and_Sign, and Encrypt_then_Sign paradigms in the context of two cryptographic primitives, namely designated confirmer signatures and signcryption. 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.