Background
Since March 6th, 2015, a YouTube channel called Noc +10 has been uploading puzzle videos about once a week. Each video begins with the same introduction, followed by some form of clues which can be decoded to point to a hidden video's YouTube ID.
The hidden videos have had a nautical theme, possibly related to Naval Operation Concept 2010, but haven't presented any clear puzzles, aside from whatever meta puzzle or story is being told.
This question provides a writeup of the first five videos, and it's primary answer solves the fifth to reveal the sixth.
After that, three very strange videos that broke the pattern were posted, but they contained no apparent puzzles. I choose to ignore them for the time being.
7. Fourth contact - March 30, 2015
0101 0101 0000 0111
The seventh video, which is numbered sequentially in binary with all the others, begins with the NOC +10 logo and the same music playing in the background. At the halfway point, it switches to displaying the text ISL 4 9 xEFTvZ while playing a new song, identified as "Air On the G string", August Wilhelmj's arrangement of a piece by J. S. Bach.
As before, we can expect the text displayed to decode to a eleven-alphanumeric-character YouTube video ID.
According to Wikipedia, the song is most notable for Wilhelmj's arrangement, which allowed him to play the entire piece on just the G string of his violin. I suspect that that means that the letter G is a major factor in the decoding, although I'm not sure how.
I've tried shifting the letters by 'G' (like with the Caesar cipher in the first video), as well as several operations on the ASCII values involving G, but nothing has led to an active video ID.
Can you crack the last video's code and find the ID of the next one?
Other links:
Answer
The mystery has finally been solved!
The answer is:
Explanation:
The encrypted text is actually "lSL 4 9 xEFTvZ" and not "ISL 4 9 xEFTvZ" (the difference is in the first letter, which is lower-case L instead of upper-case i).
The code is then easily decrypted shifting back the first block of letters by 4, the second block by 9 and keeping intact the two numbers.
The result is:
hOH49oVWKmQ
As you can see, the main difficulty was in the interpretation of the first character (absolutely impossible to tell from the video). The cipher itself wasn't too hard (just two Gronsfeld ciphers).
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