Applicability of theoretical attack procedure in actual attack to ChaCha cipher

hiren_garai

6/17/23, 7:26 AM

I was studying the theoretical attacks on ChaCha cipher here (See section 3). There is one special attack procedure which require key-IV(Initial Vector) pairs. These key-IV pairs are special in the sense that they produce high bias (minimum difference after one round in the differential attack) so that it is helpful in further attack (Finding the PNBs and all other things).

My question is " In actual attack, the attacker only has the option to choose the IVs, so it is quite convenient for the attacker to choose the IVs from the key-IV pair. But the key has a very less chance of being paired with the particular IV, and hence the further attack might not be successful, as it might produce more difference and hence the bias might be decreased and overall attack might not be successful (in the sense that the complexity is not improved). So how does the study of that attack model helps in the actual attack ?"

Any insight will be helpful. Thank you.

0 + 0

stream-cipher

chacha

fgrieu

6/17/23, 7:54 AM

Per the abstract, the linked attack assumes ChaCha reduced to 7 rounds _and_ has $2^{239}$ complexity. Thus being impractical for the further reason noted in the question is moot.

hiren_garai

6/17/23, 8:53 AM

If we assume that we have system and resources with which we can carry the attack, still my question remains the same. Why the chosen IV without the paired key will work in the attack ? The IV that the attacker will put in the actual attack without knowing the key( Master key) has a high chance of not being the suitable IV for the master key (giving minimum difference, high bias), so the implication of the attack and getting success is very less I think. Where is the justification of studying the attack ?

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Applicability of theoretical attack procedure in actual attack to ChaCha cipher

TH: การประยุกต์ใช้ขั้นตอนการโจมตีตามทฤษฎีในการโจมตีจริงกับรหัส ChaCha

RO: Aplicabilitatea procedurii de atac teoretic în atacul real la cifrul ChaCha

RU: Применимость теоретической процедуры атаки при реальной атаке на шифр ChaCha

VI: Khả năng áp dụng quy trình tấn công lý thuyết trong tấn công thực tế vào mật mã ChaCha

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.