Recently on Oct. 14, my friend's account smallK233 was banned. All his submissions in DeepInfra Round 1 (Codeforces Round 1126, Div. 1 + Div. 2) became "Skipped". I think this is a mistake, though I have no evidence which can directly prove that he didn't cheat.

In this round we both excelled, I've got +100 and he got +127 (Of course we didn't share our ideas. One can easily find that our ideas in D, E, F all differed a lot by comparing our submissions). On 14, he opened his computer to participate in the educational round, to find that he was blocked. After that, he checked his email and found nothing special.

Firstly, we are both OIers and ICPCers from China, and have known each other for four years. In fact his OI level is higher than me, while his rating is lower. There are various reasons for this. For example the contest time in China (usually 22:35 to 0:35) is not very convenient for him, and also, he is not so good at ad-hoc problems as me. His performance in this round reflects his true skill level.

Recently the rapid growth of AI has brought challenges to Codeforces, as well as all other CP platforms. I surely understand that cheating undermines the integrity of the competitions and ruins the experience for the entire community. In order to prevent users from cheating, it's understandable that Codeforces has a hidden system to detect AI use and block accounts. But to be honest, after looking at his code (in this round), I can't find any evidence that he used AI or someone else's code. Everything seems normal.

Yes, his submission time may be a little strange. He spent 30 mins on C, 40 mins on E, while 17 mins on D and F each. He said that his first idea on E turned out to be wrong, wasting much time on E. In F, he just observed some examples and guessed a conclusion, and then immediately implemented and submitted this. He said, "I'm not so serious about my rating, so I'm not serious about that 50 points too. And fortunately it got accepted." However, no matter what he explains, I think that if the system will block an account just because the strange submission time, the system may be too strict, isn't it? Here we should know that he had not received any "Skipped" before this round. And also it's known that CP problem solving always contains randomness and instability.

I don't even know whether the reason for blocking is cheating.

Secondly, he had been noticed for his extremely weak password. But there's no evidence that someone else had stolen his account and done something bad.

Thirdly, he told me that he used a proxy during the contest. Thus, Codeforces might find that his IP was from Japan instead of China. Will that be a cause of account blocking?

Lastly, in https://codeforces.me/blog/entry/134759?#comment-1205861, MikeMirzayanov says that using secondary accounts to participate is unacceptable and violates the rules. But this account has a history of 7 years, which is surely his primary account. He also said that he didn't have any secondary account. There are not many submissions in this account, and it suddenly performed very well, but I think this is surely not enough for proving it is a secondary account, because the owner can have had a lot of training on other platforms.

So why is the account blocked? I finally think this is a mistake, and appeal for a review of this ban. MikeMirzayanov could you please just spend a little time reading this topic and review this blocking? If there's actually evidence that can prove his cheating, then blocking is OK.

Something I want to say:

I have been active in Codeforces for more than 2 years. During the past 2 years, I've practiced about 1000 problems, participated in 41 rated contests, and of course never cheated. I used to consider being blocked very far from me, until this blocking happened. Being unable to find any reason for this blocking, I have even begun to worry about my own account, "Will I be blocked if I perform very well in some contest next time? Or will I be blocked if I solve F quickly but get stuck in D?" What should I do?

If you've find any clues or possible reasons for this blocking, feel free to contact me.

Besides, my friend has already become disappointed with Codeforces, and is not willing to make an appeal. He just created a new account silently. But I think it's unfair, and that's why I'm writing this topic.