reading it again I'm no longer sure we can say account label isn't security perpose: entire point of this challange is those account-specific hostname CNAMEed to some delegated dns server for acme perpose (like https://github.com/joohoi/acme-dns). and when clients are using 3rd part DNS hosting service for that most trivial attack method from delegated DNS server for such dns server would be trying to create an account that using same validation domain. While I'm think while current way is safe enough as it needs createing more than 2^40 accounts to expecting some collison, I don't think we can say it's non-security and should mention this lable need some collision resistence.

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