Friday, October 10, 2014

Plaintext Awareness and Signed ElGamal

For the first study group of the academic year, David Bernhard spoke on recent developments in the area of CCA-secure public key encryption. After introducing the security goals of the area and some key previous results, he presented a paper by Yannick Seurin and Joana Treger, from CT-RSA 2013 [0], which aims to provide such a scheme, which is a variant of Schnorr-signed ElGamal encryption...

ElGamal Encryption

We begin with a recap of the ElGamal encryption scheme. ElGamal is a public-key cryptosystem whose security is derived from the security of the discrete logarithm problem (DLP). Very briefly, it uses a publicly known cyclic group $G$ (in which the DLP must be hard!) of prime order $p$, with a chosen generator $g$. The user setting up the scheme picks a secret variable $x$, and sets their public key to be $h:=g^x$. To encrypt a message $m$ under this public key, one sends sends the pair $(g^y,mh^y)$, which the initial user can decrypt since they know $x$ and $p$.

ElGamal encryption is IND-CPA secure if (and only if) the Discrete Diffie-Hellman (DDH) assumption holds[1]. However, its ciphertexts are malleable and thus the scheme cannot be IND-CCA2. Indeed, trying to extend it to such a scheme turned out not to be as simple as one might think. Various papers made progress in this direction, taking two rather different directions. One branch aimed to create a hybrid scheme (e.g. DHIES [2]) and reduces security to a stronger assumption on the group and that of the symmetric primitive used. The method we will look at is the alternative...


Plaintext Awareness

A scheme is said to be plaintext aware in the Random Oracle Model (ROM-PA) if an adversary cannot generate a valid ciphertext without already knowing the plaintext of it. That is, it encapsulates the behaviour of a good scheme that prevents the adversary from generating valid ciphertexts other than by encrypting plaintexts herself (or doing something she knows to be equivalent to this). The technicalities of this definition are rather complex, but roughly mean that that given a ciphertext and the list of random oracle queries made to generate it, one can deduce the underlying plaintext.

Now, the key result for us is that if a scheme is both IND-CPA and ROM-PA secure, then it is IND-CCA2 secure [3].


Making ElGamal ROM-PA

Various earlier results had considered designing a plaintext aware scheme by combining ElGamal with a Schnorr signature [1,4], forming a scheme this paper refers to as SS-EG. Whilst SS-EG inherits IND-CPA security from ElGamal, unfortunately it does not add plaintext awareness. The key contribution of this paper[0] was to observe that actually SS-EG is in some sense "very close", and define a very similar scheme using Chaum-Pedersen commitments rather than the Schnorr signatures. Denoted CPS-EG, the only real difference between the schemes is a slight modification to the variables in the ciphertext and the addition of two more variables to the Random Oracle query when generating the commitment. It is the second of these differences that is criticial, because the extra information supplied to the random oracle (information that an extractor is given direct access to) is sufficient to make the scheme ROM-PA.

Making ElGamal IND-CCA2

At this point, one may hope that the scheme is indeed IND-CCA2, but there remains one final twist. There are various different ways one may transmit the required data for a signature, of which one traditional method is the Fiat-Shamir scheme, consisting of a challenge commitment and appropriate response (i.e. a non-interactive proof of knowledge). However, there are also alternative representations that are more efficiently packaged, leading to more concise ciphertexts. The particular representation chosen in the original scheme in fact failed to inherit IND-CPA security from the underlying encryption scheme [5].

Returning to the (less concise) Fiat-Shamir scheme ensures the final version of CPS-EG is indeed an IND-CCA2 secure public key encryption scheme.


[0] A Robust and Plaintext-Aware Variant of Signed ElGamal Encryption
Yannick Seurin and Joana Treger
CT-RSA 2013

[1] On the Security of ElGamal Based Encryption.
Yiannis Tsiounis and Moti Yung
PKC '98

[2] The Oracle Diffie-Hellman Assumptions and an Analysis of DHIES
M. Abdalla, M. Bellare and P. Rogaway
CT-RSA '01

[3] Relations among notions of security for public-key encryption schemes
M. Bellare, A. Desai, D. Pointcheval and P. Rogaway
Crypto '98

[4] A practical mix
Markus Jakobsson
Eurocrypt '98

[5] Cited by authors as 'personal communication'
Bertram Poettering
Jan 2013

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