Abstract
The SoDark cipher is used to protect transmitted frames in the second and third generation automatic link establishment (ALE) standards for high frequency (HF) radios. The cipher is primarily meant to prevent unauthorized linking and attacks on the availability of HF radio networks. This paper represents the first known security analysis of the cipher used by the second generation ALE protocol—the de facto world standard—and presents a related-tweak attack on the full eight round version of the algorithm. Under certain conditions, collisions of intermediate states several rounds into the cipher can be detected from the ciphertext with high probability. This enables testing against the intermediate states using only parts of the key. The best attack is a chosen-ciphertext attack which can recover the secret key in about an hour with 100% probability, using 29 chosen ciphertexts.
Highlights
This paper presents related-tweak attacks on the full eight round SoDark cipher used to protect frames in the second generation (2G) automatic link establishment (ALE) standard for high frequency (HF) radios
The attacks presented in this paper break the full eight-round SoDark used in 2G ALE in practice
5 Conclusions and Future Developments. Both data and time complexities of the attacks on eight-round SoDark presented in this paper are such that they are feasible in practice, as demonstrated in the previous
Summary
This paper presents related-tweak attacks on the full eight round SoDark cipher used to protect frames in the second generation (2G) automatic link establishment (ALE) standard for high frequency (HF) radios. SoDark is a 24-bit block cipher with a 56-bit key and a 64-bit tweak [DoD17]. The cryptanalysis presented here may be relevant for the third generation (3G) ALE standard which uses the same algorithm, but with sixteen rounds instead of the eight used in 2G ALE
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