Welcome to the resource topic for 2015/397
Title:
Relaxing Full-Codebook Security: A Refined Analysis of Key-Length Extension Schemes
Authors: Peter Gazi, Jooyoung Lee, Yannick Seurin, John Steinberger, Stefano Tessaro
Abstract:We revisit the security (as a pseudorandom permutation) of cascading-based constructions for block-cipher key-length extension. Previous works typically considered the extreme case where the adversary is given the entire codebook of the construction, the only complexity measure being the number q_e of queries to the underlying ideal block cipher, representing adversary’s secret-key-independent computation. Here, we initiate a systematic study of the more natural case of an adversary restricted to adaptively learning a number q_c of plaintext/ciphertext pairs that is less than the entire codebook. For any such q_c, we aim to determine the highest number of block-cipher queries q_e the adversary can issue without being able to successfully distinguish the construction (under a secret key) from a random permutation. More concretely, we show the following results for key-length extension schemes using a block cipher with n-bit blocks and \kappa-bit keys: - Plain cascades of length \ell = 2r+1 are secure whenever q_c q_e^r \ll 2^{r(\kappa+n)}, q_c \ll 2^\ka and q_e \ll 2^{2\ka}. The bound for r = 1 also applies to two-key triple encryption (as used within Triple DES). - The r-round XOR-cascade is secure as long as q_c q_e^r \ll 2^{r(\kappa+n)}, matching an attack by Gazi (CRYPTO 2013). - We fully characterize the security of Gazi and Tessaro’s two-call 2XOR construction (EUROCRYPT 2012) for all values of q_c, and note that the addition of a third whitening step strictly increases security for 2^{n/4} \le q_c \le 2^{3/4n}. We also propose a variant of this construction without re-keying and achieving comparable security levels.
ePrint: https://eprint.iacr.org/2015/397
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