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2024 | OriginalPaper | Buchkapitel

Anamorphic Encryption, Revisited

verfasst von : Fabio Banfi, Konstantin Gegier, Martin Hirt, Ueli Maurer, Guilherme Rito

Erschienen in: Advances in Cryptology – EUROCRYPT 2024

Verlag: Springer Nature Switzerland

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An anamorphic encryption scheme allows two parties who share a so-called double key to embed covert messages in ciphertexts of an established PKE scheme. This protects against a dictator that can force the receiver to reveal the secret keys for the PKE scheme, but who is oblivious about the existence of the double key. We identify two limitations of the original model by Persiano, Phan, and Yung (EUROCRYPT 2022). First, in their definition a double key can only be generated once, together with a key-pair. This has the drawback that a receiver who wants to use the anamorphic mode after a dictator comes to power, needs to deploy a new key-pair, a potentially suspicious act. Second, a receiver cannot distinguish whether or not a ciphertext contains a covert message.

In this work we propose a new model that overcomes these limitations. First, we allow to associate multiple double keys to a key-pair, after its deployment. This also enables deniability in case the double key only depends on the public key. Second, we propose a natural robustness notion, which guarantees that anamorphically decrypting a regularly encrypted message results in a special symbol indicating that no covert message is contained, which also eliminates certain attacks.

Finally, to instantiate our new, stronger definition of anamorphic encryption, we provide generic and concrete constructions. Concretely, we show that ElGamal and Cramer-Shoup satisfy a new condition, selective randomness recoverability, which enables robust anamorphic extensions, and we also provide a robust anamorphic extension for RSA-OAEP.

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Nächstes Kapitel Anamorphic Encryption: New Constructions and Homomorphic Realizations

The original work considers a further case, the anamorphic with normal encryption or aneAME, but in our model, since the anamorphic key generation algorithm does not output a key-pair, this case is equivalent to our third case, and hence irrelevant.

We use the term distinguisher rather than adversary because the latter is more general, but our notions are all real-or-ideal.

We identify a parallel between our re-formulation and enhancement of the anamorphic model to the work of Young and Yung [37], who claimed to have done the same for universal re-encryption of Golle et al. [18].

In case of pre-computation, this is true also for the space complexity of \(\textsf{aDec}\).

In practice, the ciphertext might be a bit string, in which case we would instead have \(c=\alpha ( pk ,m,r)\Vert \beta (r)\). Moreover, note that order does not matter, so we could also have \(c=(\beta (r),\alpha ( pk ,m,r))\).

Recall that, even if we did not explicitate it here, we assume that \(\textsf{pp}\) can be obtained from both \( sk \) and \( pk \).

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Titel: Anamorphic Encryption, Revisited
verfasst von: Fabio Banfi
Konstantin Gegier
Martin Hirt
Ueli Maurer
Guilherme Rito
Verlag: Springer Nature Switzerland
Buch: Advances in Cryptology – EUROCRYPT 2024
Print ISBN: 978-3-031-58722-1

Electronic ISBN: 978-3-031-58723-8

Copyright-Jahr: 2024
DOI: https://doi.org/10.1007/978-3-031-58723-8_1

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