@axapaxa provided a good answer explaining why it was off-topic, but I'll add me two cents and dive into the reasons why I "locked" the Q&A.
Actually, I would have put the question "on hold" as off-topic because (quote):
Requests for analyzing ciphertext or reviewing full cryptographic designs are off-topic, as the results are rarely useful to anyone else and/or would be too long for this site.
Note that I actively interpreted the NET stuff as benign and assumed that to be your way of describing your home-made algo, and not to be part of a "is my NET code correct/secure/whatever" kind of question. Nevertheless, your question clearly stumbled over the off-topic reason provided above - which is why I locked it.
Actually, the lock mesage already indicates why locking was the more constructive thing to do (quote):
This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. This question and its answers are frozen and cannot be changed.
But I'll gladly explain why it was the more constructive choice and what the main, most important reasons were for me to "lock" it instead of putting it "on hold":
- Putting it "on hold" would stil allow people to downvote your question, which already showed
-3 when I locked it. Locking a Q&A prevents further voting... and thereby saves you from losing even more reputation due to further "off topic" based downvotes.
- Putting it "on hold" would put the Q&A at risk of potential deletion. Locking it based on "historical significance" prevents deletion, or - at least - makes people think twice before killing the Q&A at a later time.
- Your question received two usefull answers, of which you already accepted one at the time of locking. Since that answer obviously answered your question to your satisfaction, locking it keeps the Q&A around for your personal, future reference. (remember #2)
All in all, the only really negative part of locking things in your specific case is that locking the Q&A keeps an off-topic Q&A around. That's why the bolded lock-message explains to other users that the question is not on-topic, so they can't argue "hey, someone asked an alike question so why can't I do the same"; which would only open the door to alike, off-topic questions.
Hope that helps you understand why - for you - my choice to lock this particular Q&A was the most constructive and gentle way to handle your off-topic question. In case of doubt or if something is unclear, please feel invited to drop a line via our chat. I'm always open to help by explaining whatever part(s) of my wording may be confusing.
Last but not least: since you state you explicitly wanted to avoid clashing into off-topicness and checked major parts of the help center, it might well be that our help center is missing some information somewhere. (Nothing is perfect.) If you think that's the case, you are definitely more than welcome to suggest/request additions and/or edits to our help center by simply posting an according meta Q.