# New Attacks on IDEA with at Least 6 Rounds

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## Abstract

IDEA is a 64-bit block cipher with 128-bit keys which was introduced by Lai and Massey in 1991. The best previously published attack on IDEA could only handle 6 of its 8.5-rounds. In this paper, we combine a highly optimized meet-in-the-middle attack with a keyless version of the Biryukov–Demirci relation to obtain a greatly improved attack on 6-round IDEA which requires only two known plaintexts, and the first key recovery attacks on versions of IDEA with 6.5 to 8.5 rounds.

## Key words

IDEA Cryptanalysis Biryukov–Demirci relation Zero-in-the-Middle attack## Notes

### Acknowledgements

The authors thank Willi Meier and the anonymous referees for their constructive and helpful comments.

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© International Association for Cryptologic Research 2013