Cryptology ePrint Archive: Report 2014/144

How to Securely Release Unverified Plaintext in Authenticated Encryption

Elena Andreeva and Andrey Bogdanov and Atul Luykx and Bart Mennink and Nicky Mouha and Kan Yasuda

Abstract: Scenarios in which authenticated encryption schemes output decrypted plaintext before successful verification raise many security issues. These situations are sometimes unavoidable in practice, such as when devices have insufficient memory to store an entire plaintext, or when a decrypted plaintext needs early processing due to real-time requirements. We introduce the first formalization of the releasing unverified plaintext (RUP) setting. To achieve privacy, we propose using plaintext awareness (PA) along with IND-CPA. An authenticated encryption scheme is PA if it has a plaintext extractor, which tries to fool adversaries by mimicking the decryption oracle without the secret key. Releasing unverified plaintext then becomes harmless as it is infeasible to distinguish the decryption oracle from the plaintext extractor. We introduce two notions of plaintext awareness in the symmetric-key setting, PA1 and PA2, and show that they expose a new layer of security between IND-CPA and IND-CCA. To achieve integrity of ciphertexts, INT-CTXT in the RUP setting is required, which we refer to as INT-RUP. These new security notions are used to make a classification of symmetric-key schemes in the RUP setting. Furthermore, we re-analyze existing authenticated encryption schemes, and provide solutions to fix insecure schemes.

Category / Keywords: secret-key cryptography / Symmetric-key Cryptography, Authenticated Encryption, Releasing Unverified Plaintext, Plaintext Awareness, Plaintext Extractor, CAESAR Competition

Date: received 25 Feb 2014, last revised 28 May 2014

Contact author: Nicky Mouha at esat kuleuven be

Available format(s): PDF | BibTeX Citation

Note: Updated abstract.

Version: 20140528:094938 (All versions of this report)

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