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I'm partway through watching The Imitation Game, the biopic about Alan Turing. In the scenes depicting Turing's childhood, Turing and Christopher exchange notes/letters, which they first encrypt to prevent others reading them.

The first message is passed to Turing by Christopher in class, before it's confiscated and thrown away by the class teacher, only for Turing to fish it out from the bin and then decipher it, as shown below:

First encoded message, the note passed to Turing by Christopher in class

The second is a short letter Turing is shown writing in his dorm room, before then encrypting it:

Second encoded message, the short letter to Christopher that Turing composes

What methods are these two encrypted with, and therefore, how would they be solved? I realise they're probably stupidly simple, but you know, I'm dumb. :)

Also, as an optional bonus, there's the "Under 10 Minutes" crossword puzzle that the older Turing devises to recruit new Bletchley Park agents. It seems to be just a straight crossword, but it looks impossible to solve given that the handwriting of the clues is really tough to make out.

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  • What codes are you actually talking about here. It's a little unclear what you're even asking for. Can you be little more specific please?
    – Napoleon Wilson
    Commented Sep 18, 2016 at 13:14
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    The new question is indeed much more coherent and clear. Thanks for the updates. +1
    – Napoleon Wilson
    Commented Sep 18, 2016 at 18:32
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    The first one is a simple letter substitution puzzle where each letter is substituted for another (in this case, E is I, S is W etc.), but the second one's too short, so it's probably more complicated.
    – Walt
    Commented Sep 18, 2016 at 18:58
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    They both look like substitution ciphers. Note that it in the second note Q=O twice.
    – dbugger
    Commented Sep 18, 2016 at 19:14
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    Nice question :-)
    – Panther
    Commented Apr 3, 2017 at 4:48

1 Answer 1

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These are both substitution ciphers. The first message is a simple Caesar cipher, where each letter is substituted with the letter a fixed number of positions away in the alphabet. (S maps to W, E maps to I, Y maps to C, and so on. These substitutions are all 4 positions away.)

ABCDEFGHIJKLMNOPQRSTUVWXYZ - plain text alphabet
EFGHIJKLMNOPQRSTUVWXYZABCD - cipher text alphabet

This kind of message is pretty easy to crack with letter frequency analysis. Since the original message has a lot of Es (the most frequently used letter of the alphabet), it would be pretty easy to guess that the "key" is a shift by 4 letters.

The second message is a bit harder to crack. First, the cipher doesn't appear to use a fixed shift for every substitution. (I maps to P, a shift of 7. L maps to Z, a shift of 14.) Second, this message is much shorter, making it impossible to use letter frequency to guess any of the letters. We can use the message and its encrypted form to create a partial cipher alphabet.

ABCDEFGHIJKLMNOPQRSTUVWXYZ - plain text alphabet
    E   P  Z  Q     RA  T  - cipher text alphabet

The shifts appear to be random. The recipient would probably require the cipher alphabet in order to decode this message. Given the weight of the message at that time, I'm not sure young Alan Turing wanted it to be decoded at all. (I haven't seen the film since it was in theaters, so I don't remember if this message was ever delivered or decoded.)

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  • Don't forget a one time pad code.
    – cde
    Commented Sep 19, 2016 at 0:07
  • Well, as @cde says, it could be a one-time pad code. Or it could be a Vignere cypher - a substitution cipher where the offset changes per letter, using a code-word as a basis. See en.wikipedia.org/wiki/Vigen%C3%A8re_cipher . This cipher is pretty hard to crack with minimal ciphertext to work with though.
    – iandotkelly ♦
    Commented Sep 19, 2016 at 21:35
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    Since both Os in the "I LOVE YOU" are encrypted to Qs and no two different letters code to the same letter, it's probably not a Vignere-style cypher, although it still might be.
    – Christian
    Commented Oct 7, 2017 at 14:21

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