Many thanks to /u/pieter888 for their excellent reddit posts covering the progress in the ARG. Also, huge thanks to /u/Lucario995 and Discord user Q101 for their fantastic docs covering the tweet and message translations!
The Organisations Atwitter
@ApertureSciInc, related to the organization of the same name from the Portal series, has existed as far back as 2016-03-28.
Black Mesa Research Facility
The more interesting @BlackMesaRF, related to the organization of the same name from the Half-Life series, has existed as far back as 2016-03-13. It has a pinned tweet promising to answer all questions asked to it, and although this is not quite true, many questions have been answered in this way.
On 2016-04-11, @BlackMesaRF tweeted the following messages:
This last tweet turned out to be valid Base64, and was solved by /u/pieter888. The tweet decoded to "@conxact," leading to a third Twitter account.
All Your Base64 Are Belong To Us
@conxact continued to post similar tweets, some with no encryption and others using Base64 or ASCII85, from 2016-04-12. These tweets and their conversions are covered on the Archives page. Only tweets relevant to the progression of the ARG will be covered here.
On 2016-04-25, @conxact tweeted this, using the ASCII85/qwerty method of enciphering;
This decoded to 0yMLeJqxWhM98QpS, which was actually the code for a Discord server entitled xxxxxx.
Discord user xxxxx joined the server between 2016-04-26 and 2016-04-28 to chat. Initially, xxxxx's name was in ASCII85, but this later changed to plain text. Conversations with @conxact and @ApertureSciInc also occurred, and are located on the archive page.
On 2016-05-01, xxxxx returned to the server and gave the following message, which is yet to be deciphered:
|u lC rraha enafsqsntku.geii erlrI le ia mnlsya leoyp.t|
A clue was later given as an @conxact tweet which converted to 5 3 and translated to te:
A few questions were answered by xxxxx on Discord, with this piece of information also given:
|If you cannot solve that small puzzle, then the message itself will become irrelevant.|
Aperture Science advised this: A coworker of mine has told me that it looks like a strange version of the 'bible code', if that helps. Another tweet was later tweeted by @conxact, converting to begin after 3:
<~@V'1dDBNA"FCfJ81B~> This was followed by yet another, this time deciphering to every fifth:
<~AThX*Gp$5Aoq~> The puzzle was eventually solved by Discord users carnifex, whizzer0, and butterfingerbatman by taking every fifth character after the third character:
|Change in plans. I may require your skill set after all.|
This led to another set of messages:
These decoded to key 5-16, Szazyxrptarqalaebifxoyrgqtvwdtbewbgx, and grid respectively. Szazyxrptarqalaebifxoyrgqtvwdtbewbgx was decoded in a Bifid cipher with the key maysixten, resulting in:
Your services will be invaluable in the war
Later that evening, another code was given:
This was found to be a 6-rail railfence, and decoded to:
You are improving... I am impressed.
Three days later, another message arrived:
This message was a Keyed Vigenere cipher, with a passkey of FINAL and an alphabet of MESABCDFGHIJKLNOPQRTUVWXYZ, and decoded to:
Your training is complete.