Paper 2023/1786

CASE: A New Frontier in Public-Key Authenticated Encryption

Shashank Agrawal, Coinbase
Shweta Agrawal, Indian Institute of Technology Madras
Manoj Prabhakaran, Indian Institute of Technology Bombay
Rajeev Raghunath, Indian Institute of Technology Bombay
Jayesh Singla, Indian Institute of Technology Bombay

We introduce a new cryptographic primitive, called Completely Anonymous Signed Encryption (CASE). CASE is a public-key authenticated encryption primitive, that offers anonymity for senders as well as receivers. A "case-packet" should appear, without a (decryption) key for opening it, to be a blackbox that reveals no information at all about its contents. To decase a case-packet fully - so that the message is retrieved and authenticated - a verifcation key is also required. Defining security for this primitive is subtle. We present a relatively simple Chosen Objects Attack (COA) security definition. Validating this definition, we show that it implies a comprehensive indistinguishability-preservation definition in the real-ideal paradigm. To obtain the latter definition, we extend the Cryptographic Agents framework of [2, 3] to allow maliciously created objects. We also provide a novel and practical construction for COA-secure CASE under standard assumptions in public-key cryptography, and in the standard model. We believe CASE can be a staple in future cryptographic libraries, thanks to its robust security guarantees and efficient instantiations based on standard assumptions.

Available format(s)
Public-key cryptography
Publication info
authenticated public key encryptionchosen objects attack security (COA)agents frameworkquasi-deterministic pke
Contact author(s)
sagrawal @ protonmail ch
shweta a @ gmail com
mp @ cse iitb ac in
mrrajeev @ cse iitb ac in
jayeshs @ cse iitb ac in
2023-11-27: last of 2 revisions
2023-11-19: received
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Creative Commons Attribution


      author = {Shashank Agrawal and Shweta Agrawal and Manoj Prabhakaran and Rajeev Raghunath and Jayesh Singla},
      title = {{CASE}: A New Frontier in Public-Key Authenticated Encryption},
      howpublished = {Cryptology ePrint Archive, Paper 2023/1786},
      year = {2023},
      note = {\url{}},
      url = {}
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