I don't understand how the Bible passage (Matthew 7.7) let Alan Turing to identify the Russian spy on the team in The Imitation Game. Can anyone explain (in some detail?)
TL,DR: There was an encoded message sent from a spy in Bletchley Park to the Russians. Matthew 7:7 was used as the encrypting code. The only way to decrypt this message is to know that the key used to create the cipher was Matt 7:7. Which was information that was related to Turing in the bar by Hugh. When Turing was back in the hut he saw Alan's Bible was bookmarked at Matthew 7:7. that is how he knew Alan was the spy.
Matthew 7:7 was used as the key to encode a message from Bletchley Park to the Russians in a Vigenère cipher. I will try to explain how the Vigenère cipher works so you get the idea:
The Vigenère cipher uses a plaint-text message, a key and this grid:
Notice that there is a line between the top row and the main grid and the left vertical side and the main grid. These lines are the axes (plural of axis) of the grid.
When creating the encryption you write the plain-text message with the key underneath it, in this example I will use "The eagle has landed we have a man on the moon" as the phrase I want to encrypt, and Matt7:7: "Ask and it will be given to you; seek and you will find; knock and the door will be opened to you." (but we don't need the full phrase) as the key encryption key.
Here's where the grid comes in. We use the horizontal axis for the plain text and the vertical axis for the key. The first letter of the plaintext is "T" and the first letter in the key is "A" Going out the horizontal axis to T and down the vertical axis to a A you get the result of "T" for our first encrypted letter.
The second plain-text letter is "h" and the second letter of our key is "s". Going out the horizontal axis to the "h" and down to the vertical axis to the "s" gives us our second letter of the encrypted code "Z".
Following this out you get an encrypted message that is:
Tzo enjtx did wbrjmy ar aots u eer yn gkc aikv.
group the letters by 5 and make it all CAPS (like the Germans did with Enigma) and it is:
TZOEN JTXDI DWBRJ MYATA OTSUE ERYNG KCAIK V
The only way to decrypt this message is to know that the key used to create the cipher was Matt 7:7. Which was information that was related to Turing in the bar by Hugh. When Turing was back in the hut he saw Alan's Bible was bookmarked at Matthew 7:7. that is how he knew Alan was the spy.
To decrypt the cipher, you write the key under the cipher and go down the vertical axis of the grid, then straight out on it's line until you get to the letter of the cipher, then look up at the horizontal axis of the top line of the grid, for the corresponding letter for the plain-text message.