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A friend dropped this code on me, and gave me no hints besides that it should be a simple enough code to crack, but no luck on my end. (It might be fantasy based was all I got, if the words come out strange)

4.1.2.11, 5.1.17.1, 8.1.28.3

26.2.2.3, 154.2.32.7

378.1.46.6, 4.1.3.8, 386.1.7.2 4.1.2.5, 45.1.1.6

7.9.1 24.1.6.7 4.1.3.8 25.2.2.3

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    $\begingroup$ This is not likely to be decoded unless you provide a little more information. It looks like a book cipher and the numbers could be Page.Column.Paragraph.Letter so the name of the book would be required. Also, the numbers "7.9.1" in the last row are not consistent and may be erroneous. $\endgroup$ – Len Apr 26 '15 at 18:04
  • $\begingroup$ Ah! thanks! He did leave a book here, that makes a lot more sense, I'll get to work on it now. $\endgroup$ – Babaganoosh Apr 26 '15 at 18:24
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    $\begingroup$ @Len, the column is missing, which might suggest that there's only one column on that page. It could also be an accidental omission, of course. $\endgroup$ – Ian MacDonald Apr 27 '15 at 4:22
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    $\begingroup$ Please let us know the book (including the edition), and whether you manage to solve it! $\endgroup$ – IanF1 Apr 27 '15 at 6:33
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As Len said in a comment, these cryptograms look like what's called a book cipher. From Wikipedia:

A book cipher is a cipher in which the key is some aspect of a book or other piece of text. It is typically essential that both correspondents not only have the same book, but the same edition.

Traditionally book ciphers work by replacing words in the plaintext of a message with the location of words from the book being used. In this mode, book ciphers are more properly called codes.

In your example, each string of four numbers will probably give you a word, or perhaps a letter. Let's ignore "7.9.1" for now as it only has 3 numbers rather than 4 and may be some sort of mistake.

  • The first number in each string can get as high as 386, so it's unlikely to be the volume number of a multi-volume work and more likely to be the page number.

  • The second number is always either 1 or 2, so it's probably the number of the relevant column on the given page.

  • The third number can go as high as 46, so it's unlikely to be the paragraph number and more likely to be the line number.

  • The final number tells us the word (possibly the letter, if the message is very short) to look for in the given line.

So for instance "4.1.2.11" denotes the 11th word in the 2nd line of the 1st column on the 4th page of some book. It's impossible to say more than this without knowing what book your friend is referring to (some fantasy novel perhaps?). But you, the OP, know which book it is. So this answer should be enough for you to solve your problem, while also being more generally useful to people trying to solve a similar puzzle.

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