Things are slowly starting to come together, as they should be considering this was the second to last episode of the season!

Tyrell is alive, maybe, or at least he is in Elliot’s imagination. Elliot lets Mr. Robot take the lead this episode and he revealed what he wanted from Elliot’s apartment: a takeout menu. More info on this below.

We now know that the Dark Army may have been behind the deaths of Elliot’s dad and Angela’s mom, but we still don’t know why the Dark Army is interested in the Washington Township plant.

The Chinese seem to have everyone and everything under control. Dom, Mr. Price and perhaps Tyrell and Elliot (?) want to regain control.

At Elliot’s apartment, Mr. Robot takes a Red Wheelbarrow BBQ and starts to decode a message that was hidden in the menu.

Stefanie: What is this called, when you have a number that corresponds to a letter in the alphabet?

Jaromir Horejsi, senior malware analyst: It’s called substitution cipher and it is the encoding of a plain text into a cipher text. One character is substituted by another one according to a fixed rule. Cryptanalysts can crack substitution ciphers by frequency counts, e.g. in English, the most frequent letter is E, so they find the most frequent letter in a substitution cipher text and try substituting the most frequent letter in the cipher with the letter E, then they continue until the text is decoded.

Stefanie: What is an ROT-13 cipher?

Jaromir: ROT-13 is a simple substitution, where each letter is replaced by the following 13th letter in English alphabet, e.g. A is substituted with N. Because the English alphabet has 26 letters, applying the ROT-13 cipher twice gives you the original plain text, so ROT-13 (‘A’) = N, ROT-13 (‘N’) = A, therefore ROT-13(ROT-13(‘A’)) = ‘A’.

Stefanie: What are Perrin and Erdos-Woods numbers? What are they normally used for?

Jaromir: Perrin numbers are defined by the following formula:

P(n) = P(n − 2) + P(n − 3) for n > 2,

with initial values

P(0) = 3, P(1) = 0, P(2) = 2.

You start with 3 numbers (3, 0, 2), to calculate the rest, you take number on position 3-2 = 1 and position 3-3 = 0, which is 3 + 0 = 3.

There are an infinite number of Erdős–Woods numbers, the smallest of which is 16. The definition says, that a positive number A exists, such that  A has non-trivial factors, the number (A+16) has some non-trivial common factor numbers and all the consecutive numbers between A and (A+16) share at least one non-trivial factor with either one of these endpoints (A, A+16).

Stefanie: Why do you think Tyrell used this method?

Jaromir: Because Tyrell knew that Elliot (and/or Mr. Robot) is smart enough to decode it ;-).

I hope we find out if Darlene and Cisco are alive in next week’s episode and maybe we will also learn where Trenton and Mobley have been hiding? It would also be great to know what “Stage 2” is! Let’s see where Sam Esmail leaves us hanging this season. Till next week and, as usual, let us know what you thought of the episode in the comment section below!