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Beyond eCK: Perfect Forward Secrecy Under Actor Compromise and Ephemeral-key Reveal

Cas J. F. Cremers, Michèle Feltz

Published Jan 1, 2015• 0 citations
Publisher

Abstract

Abstract. We show that it is possible to achieve perfect forward secrecy in two-message or one-round key exchange (KE) protocols even in the presence of very strong active adversaries that can reveal random values of sessions and compromise long-term secret keys of parties. We provide two new game-based security models for KE protocols with increasing security guarantees, namely, eCKw and eCK-PFS. The eCKw model is a slightly stronger variant of the extended Canetti-Krawczyk (eCK) security model. The eCK-PFS model captures perfect forward secrecy in the presence of eCKw adversaries. We propose a security-strengthening transformation (i. e., a compiler) from eCKw to eCK-PFS that can be applied to protocols that only achieve security in a weaker model than eCKw, which we call eCKpassive. We show that, given a two-message Diffie-Hellman type protocol secure in eCKpassive, our transformation yields a two-message protocol that is secure in eCK-PFS. We demonstrate how our transformation can be applied to concrete KE protocols. In particular, our methodology allows us to prove the security of the first known one-round protocol that achieves perfect forward secrecy under actor compromise and ephemeral-key reveal. 1

DOI
10.3929/ethz-b-000073097

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Abstract. We show that it is possible to achieve perfect forward secrecy in two-message or one-round key exchange (KE) protocols even in the presence of very strong active adversaries that can reveal random values of sessions and compromise long-term secret keys of parties. We provide two new game-based security models for KE protocols with increasing security guarantees, namely, eCKw and eCK-PFS. The eCKw model is a slightly stronger variant of the extended Canetti-Krawczyk (eCK) security model. The eCK-PFS model captures perfect forward secrecy in the presence of eCKw adversaries. We propose a security-strengthening transformation (i. e., a compiler) from eCKw to eCK-PFS that can be applied to protocols that only achieve security in a weaker model than eCKw, which we call eCKpassive. We show that, given a two-message Diffie-Hellman type protocol secure in eCKpassive, our transformation yields a two-message protocol that is secure in eCK-PFS. We demonstrate how our transformation can be applied to concrete KE protocols. In particular, our methodology allows us to prove the security of the first known one-round protocol that achieves perfect forward secrecy under actor compromise and ephemeral-key reveal. 1
We propose two security models for one-round group key exchange (ORGKE), which are called as g-eCKw and g-eCK-PFS. The g-eCK-PFS is a stronger variant of g- eCKw, which particularly formulates perfect forward se- crecy for ORGKE. A new tripartite ORGKE is proposed to provide g-eCKw security without random oracles under standard assumptions, that is also more e_cient than its predecessor by Li and Yang on CANS'13. We also show how to transform (compile) a g-eCKw secure protocol to achieve g-eCK-PFS security. In particular, our result enables us to prove the security of the first ORGKE protocol that achieves perfect forward secrecy without random oracles in a strong security model allowing adversary to compromise critical information of session participants such as long-term or ephemeral private key.
Recently, LaMacchia, Lauter and Mityagin proposed the extended Canetti-Krawczyk (eCK) model for Authenticated Key Exchange (AKE) protocols that covers many attacks on existing models. An ID-based AKE protocol with Perfect Forward Secrecy (PFS) (respectively Master Perfect Forward Secrecy (MPFS)) ensures that revelation of the static keys of the parties (respectively the master secret key of the private key generator), must not compromise even a single bit of the session keys of the past sessions between the parties. Currently, to the best of our knowledge, there is no ID-based eCK secure single round AKE protocol with either PFS or MPFS. In our preliminary version [18] we have proposed, without proofs, ID-based eCK secure single round AKE protocols with PFS and MPFS in the random oracle model. To achieve this, we also have constructed ID-based eCK secure single round AKE protocols, one without Master Forward Secrecy (MFS) and one with MFS, almost at the same computational cost as the existing efficient ID-based eCK Secure Single Round AKE protocols. In this full version, we provide proofs to show that all of our protocols are secure under the Gap Bilinear Diffie-Hellman (GBDH) problem.
Abstract We examine the recently introduced CF and CF‐perfect forward secrecy (PFS) models for two‐message authenticated key exchange (TMAKE) by Cremers et al., where the difference between CF and CF‐PFS model is that the CF formulates the weak PFS (wPFS), whereas the CF‐PFS formulates the PFS. The CF model is claimed by Cremers et al. to be strictly stronger the previous extended Canetti‐Krawczyk (eCK) model. However, we notice that the implication relations among CF, CF‐PFS, eCK and eCK‐PFS model have not been completely studied. Based on TMAKE, we particularly show that CF model and eCK model imply each other under random oracle model. Moreover, we provide a new result on the generic security strengthening transformation (compiler) for building CF‐PFS‐secure TMAKE protocols. In contrast to a previous work, we show that it is possible to apply the transformation to all CF‐secure AKE protocols including all eCK‐secure TMAKE protocols in the random oracle model, without restricting to a small specific class of Diffie–Hellman key based protocols. Copyright © 2015 John Wiley & Sons, Ltd.
Abstract. This paper investigates the two-pass authenticated key exchange protocol in the enhanced Canetti-Krawczyk (eCK) with perfect forward security. Currently, there exist no authenticated key exchange protocols which are provably secure in eCK model and meanwhile achieve perfect forward security against active adversary in one round. We propose a new two-pass authenticated key exchange protocol which enjoys following desirable properties. First, our protocol is shown secure in the eCK model under the gap Diffie-Hellman (GDH) assumption. Moreover, our protocol does not use the NAXOS transformation, the drawback of which will be discussed in the introduction. Second, under the same assumption, we prove that our protocol achieves perfect forward security against active adversary in one round. To the best of our knowledge, our proposal is first two-pass (one round) AKE protocol provably secure in the eCK model and achieving perfect forward security against active adversary.
The extended Canetti-Krawczyk(eCK) model proposed by LaMacchia et al.is currently regarded as the strongest security model for authenticated key exchange(AKE) protocols.In the eCK model,the adversary can make queries on the test session,and reveal both ephemeral private keys,both long-term keys,or one of each from the two different parties.In this paper,we analyze two AKE protocols designed in the eCK model based on the definition of eCK model.We propose ephemeral keys compromised attacks and long-term keys compromised attacks on one of them,and also propose ephemeral keys compromised attacks on the other one.The results of analysis show that both of them are insecure in the eCK model.
In 2007, LaMacchia et al.proposed the extended Canetti-Krawczyk (eCK) model for authenticated key exchange (AKE) protocols. In this paper, we first modify the eCK model by adding a new query to make the adversary can reveal all ephemeral secret information. Then we propose a new efficient and strongly secure AKE protocol in the asymmetric setting, called E-NETS (enhanced NETS) protocol, and prove its security in the modified eCK (meCK) model under the random oracle assumption and the gap Diffie-Hellman assumption.