Let me ask you a question: about how long might one expect the key to be? Also, is it possible that the file might be found on one of Wikileak's servers?
I have a suspicion that what you may be looking for is the "stone," or the actual data. Early on, there was a rumor that the "key" was the password that would give access to one of Julian Assange's "insurance" drops. He's made at least nine so far–the latest I know of was a 64.31 GB drop on Christmas of last year:
https://steemit.com/conspiracy/@vaerospace/wikileaks-drops-65gb-torrent-link-weiners-emails
There were many rumors that Julian Assange was passing data to visitors via thumb drive (64 GB obviously seems a coincidental fit). There were a couple of suspects which are relatively easy to find with a search; for instance, Nigel Farage visited Assange on March 9th, 2017; on January 19, he was accused of helping:
https://www.theguardian.com/politics/2018/jan/19/trump-russia-inquiry-is-told-nigel-farage-may-have-given-julian-assange-data
Trump has met with Nigel Farage several times since and before Farage's visit with Assange. This is all tied to research I did quite awhile ago–which was ignored; I'm sure you understand the frustration. Anyway, some anon found a book called "Expand Your Thinking," which listed a series of mind-mapping techniques. I started to focus on the bracketed text, going on the assumption that the last relevant date was December 15. The reasoning was because of this unique post on that day:
>Follow the crumbs.
>You have it all.
>SEC Conf will be analyzed.
>Dark [10].
>Enjoy the show!
>Q
I'd set about trying to figure out what the key was using the methods in that book and the cryptography methods listed on Q's link to the C_A's website for kids. On March 6th, I posted pic related:
https://8ch.net/qresearch/res/565493.html#q565535
(You can find a link to some of the "Expand Your Thinking" texts by following one of the responses to my post).
The next day, Q posted other pic related. Most anons figured he was speaking to JA, but I saw something different–if you're thinking in terms of shifting letters, "L-6" becomes "F"…because L is the 12th letter of the alphabet, and F is the 6th. The significance of that is that the very first content in braces is "[F]" (post #14, see last pic related).
I spent some time working on it, and could probably put together a program to spit out all possible combinations of braced content fairly quickly…but there is still too much uncertainty. How do we know what is a subpart of what? What is the length of the key? Would our part be in used in combination with another part?
There have been several "pre-commitments" already (see link at end, plus search "NSA pizzagate tweet", plus look up Assange's mysterious New Year's tweet). Even if you stop at Dec. 15th, there are already 54 separate instances of content in braces. Without knowing the length you need, whether clues should be interpreted as letters (such as …+1), knowing how to automate a program to make attempts, and being fairly busy irl, it just seemed out of reach. Maybe with your knowledge and that of others, we can winnow down the possibilities.
I'll keep checking back; hopefully we can get this figured out. I think that, with the clues Q has given, we're a lot closer than we've ever been.
If you'd like more background on Wikileak's files, you can read up here (it's two years old, but covers things that aren't well-known):
https://steemit.com/wikileaks/@ausbitbank/how-to-download-the-wikileaks-archives-insurance-files-email-leaks-for-safe-keeping