Abstract
We propose a new type of meet-in-the-middle attack that splits a cryptographic primitive in parallel to the execution flow of the operations. The result of the division are two primitives that have smaller input sizes and thus require lower attack complexities. The sub-primitives are not completely independent, but mutually depend on a certain number of bits. When the number of such bits is relatively small, we show a technique based on three classical meet-in-the-middle attacks that can recover the secret key of the cipher faster than an exhaustive search. We apply our findings to the lightweight block cipher Klein and show attacks on 10/11/13 rounds of Klein-64/-80/-96. We note that our approach works in the known-plaintext attack model and requires only one or two pairs of known plaintexts.
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Notes
We refer to the proposal of the low-data attack [3] for the importance of practical data complexity for analyzing ciphers.
Actually, we are looking for a division that minimizes the number of guessed bits from the other half.
Note that a simple exhaustive key search requires \(2^{\frac {k}{2} + r\cdot b_{2}}\) encryption queries, hence when \(r\cdot b_{2} > \frac {k}{2}\) the exhaustive search of the subcipher has a higher complexity than the exhaustive search of the cipher. Therefore we present more advanced techniques for recovering the key of the subcipher.
We assume the subciphers are invertible.
This number is in fact the average number of possible k-bit keys that encrypt n-bit plaintext P to ciphertext C, i.e. with our approach we find all the possible keys for the cipher.
One nibble is 4 bits.
This property, only for the key schedule, already has been pointed out in [1]
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The work in this paper is supported by the Singapore National Research Foundation Fellowship 2012 (NRF-NRFF2012-06).
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Nikolić, I., Wang, L. & Wu, S. The parallel-cut meet-in-the-middle attack. Cryptogr. Commun. 7, 331–345 (2015). https://doi.org/10.1007/s12095-014-0118-1
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DOI: https://doi.org/10.1007/s12095-014-0118-1