International Association for Cryptologic Research

International Association
for Cryptologic Research

IACR News item: 02 June 2015

Takanori Isobe, Kyoji Shibutani
ePrint Report ePrint Report
We propose new generic key recovery attacks on Feistel-type block ciphers. The

proposed attack is based on the all subkeys recovery approach presented in SAC 2012, which

determines all subkeys instead of the master key. This enables us to construct a key recovery

attack without taking into account a key scheduling function. With our advanced techniques,

we apply several key recovery attacks to Feistel-type block ciphers. For instance, we show

8-, 9- and 11-round key recovery attacks on n-bit Feistel ciphers with 2n-bit key employing

random keyed F-functions, random F-functions, and SP-type F-functions, respectively.

Moreover, thanks to the meet-in-the-middle approach, our attack leads to low-data complexity.

To demonstrate the usefulness of our approach, we show a key recovery attack on the

8-round reduced CAST-128, which is the best attack with respect to the number of attacked

rounds. Since our approach derives the lower bounds on the numbers of rounds to be secure

under the single secret key setting, it can be considered that we unveil the limitation of

designing an efficient block cipher by a Feistel scheme such as a low-latency cipher.

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