Some Instant- and Practical-Time Related-Key Attacks on KTANTAN32/48/64

The hardware-attractive block cipher family KTANTAN was studied by Bogdanov and Rechberger who identified flaws in the key schedule and gave a meet-in-the-middle attack. We revisit their result before investigating how to exploit the weakest key bits. We then develop several related-key attacks, e.g., one on KTANTAN32 which finds 28 key bits in time equivalent to 23.0 calls to the full KTANTAN32 encryption. The main result is a related-key attack requiring 228.44 time (half a minute on a current CPU) to recover the full 80-bit key. For KTANTAN48, we find three key bits in the time of one encryption, and give several other attacks, including full key recovery. For KTANTAN64, the attacks are only slightly more expensive, requiring 210.71 time to find 38 key bits, and 232.28 for the entire key. For all attacks, the requirements on related-key material are modest as in the forward and backward directions, we only need to flip a single key bit. All attacks succeed with probability one...

• Type: Journal
• Year: 2011
• Where: IACR
• Authors: Martin Ågren
• Keywords: Bogdanov, IACR 2011, Key Bit, Key Bits