Sender-equivocable encryption schemes secure against chosen-ciphertext attacks revisited

Fehr et al. (2010) proposed the first sender-equivocable encryption scheme secure against chosen-ciphertext attacks (NCCCA) and proved that NC-CCA security implies security against selective opening chosen-ciphertext attacks (SO-CCA). The NC-CCA security proof of the scheme relies on security against substitution attacks of a new primitive, the “crossauthentication code”. However, the security of the cross-authentication code cannot be guaranteed when all the keys used in the code are exposed. Our key observation is that, in the NC-CCA security game, the randomness used in the generation of the challenge ciphertext is exposed to the adversary. Based on this observation, we provide a security analysis of Fehr et al.’s scheme, showing that its NC-CCA security proof is flawed. We also point out that the scheme of Fehr et al. encrypting a single-bit plaintext can be refined to achieve NC-CCA security, free of the cross-authentication code. Furthermore, we propose the notion of “strong cross-authentication code”, apply it to Fehr et al.’s scheme, and show that the new version of the latter achieves NC-CCA security for multi-bit plaintexts

Sender Equivocable Encryption Schemes Secure against Chosen-Ciphertext Attacks Revisited

Abstract. In Eurocrypt 2010, Fehr et al. proposed the first sender equivocable encryption scheme secure against chosen-ciphertext attack (NC-CCA) and proved that NC-CCA security implies security against selective opening chosen-ciphertext attack (SO-CCA). The NC-CCA security proof of the scheme relies on security against substitution attack of a new primitive, “cross-authentication code”. However, the security of cross-authentication code can not be guaranteed when all the keys used in the code are exposed. Our key observation is that in the NC-CCA security game, the randomness used in the generation of the challenge ciphertext is exposed to the adversary. This random information can be used to recover all the keys involved in cross-authentication code, and forge a ciphertext (like a substitution attack of cross-authentication code) that is different from but related to the challenge ciphertext. And the response of decryption oracle, with respect to the forged ciphertext, leaks information. This leaked information can be employed by an adversary to spoil the NC-CCA security proof of Fehr et al.’s scheme encrypting multi-bit plaintext. In this paper, we provide a security analysis of Fehr et al.’s scheme, showing that its NC-CCA security proof is flawed by presenting an attack. We point out that Fehr et al.’s scheme encrypting single-bit plaintext can be refined to achieve NC-CCA security, free of cross-authentication code. We introduce the strong notion of cross-authentication code, apply it to Fehr et al.’s scheme, and show that the new version of Fehr et al.’s scheme achieves NC-CCA security for multi-bit plaintext

Sender-equivocable encryption schemes secure against chosen-ciphertext attacks revisited

Fehr et al. (2010) proposed the first sender-equivocable encryption scheme secure against chosen-ciphertext attacks (NCCCA) and proved that NC-CCA security implies security against selective opening chosen-ciphertext attacks (SO-CCA). The NC-CCA security proof of the scheme relies on security against substitution attacks of a new primitive, the “cross-authentication code”. However, the security of the cross-authentication code cannot be guaranteed when all the keys used in the code are exposed. Our key observation is that, in the NC-CCA security game, the randomness used in the generation of the challenge ciphertext is exposed to the adversary. Based on this observation, we provide a security analysis of Fehr et al.'s scheme, showing that its NC-CCA security proof is flawed. We also point out that the scheme of Fehr et al. encrypting a single-bit plaintext can be refined to achieve NC-CCA security, free of the cross-authentication code. Furthermore, we propose the notion of “strong cross-authentication code”, apply it to Fehr et al. 's scheme, and show that the new version of the latter achieves NC-CCA security for multi-bit plaintexts

Constructing and Understanding Chosen Ciphertext Security via Puncturable Key Encapsulation Mechanisms

In this paper, we introduce and study a new cryptographic primitive that we call puncturable key encapsulation mechanism (PKEM), which is a special class of KEMs that satisfy some functional and security requirements that, combined together, imply chosen ciphertext security (CCA security). The purpose of introducing this primitive is to capture certain common patterns in the security proofs of the several existing CCA secure public key encryption (PKE) schemes and KEMs based on general cryptographic primitives which (explicitly or implicitly) use the ideas and techniques of the Dolev-Dwork-Naor (DDN) construction (STOC\u2791), and break down the proofs into smaller steps, so that each small step is easier to work with/verify/understand than directly tackling CCA security. To see the usefulness of PKEM, we show (1) how several existing constructions of CCA secure PKE/KEM constructed based on general cryptographic primitives can be captured as a PKEM, which enables us to understand these constructions via a unified framework, (2) its connection to detectable CCA security (Hohenberger et al. EUROCRYPT\u2712), and (3) a new security proof for a KEM-analogue of the DDN construction from a set of assumptions: sender non-committing encryption (SNCE) and non-interactive witness indistinguishable proofs. Then, as our main technical result, we show how to construct a PKEM satisfying our requirements (and thus a CCA secure KEM) from a new set of general cryptographic primitives: SNCE and symmetric key encryption secure for key-dependent messages (KDM secure SKE). Our construction realizes the decrypt-then-re-encrypt -style validity check of a ciphertext which is powerful but in general has a problem of the circularity between a plaintext and a randomness.We show how SNCE and KDM secure SKE can be used together to overcome the circularity. We believe that the connection among three seemingly unrelated notions of encryption primitives, i.e. CCA security, the sender non-committing property, and KDM security, to be of theoretical interest

Critical Perspectives on Provable Security: Fifteen Years of Another Look Papers

We give an overview of our critiques of “proofs” of security and a guide to our papers on the subject that have appeared over the past decade and a half. We also provide numerous additional examples and a few updates and errata