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I have a series of messages (limited to only a-z and A-Z) that are encoded such that the sum is taken of every two ASCII values and concatenated. For instance, the message hello would be decomposed into the pairs he, ll, and o which would then become 104 + 101, 108 + 108, and 111 which finally is output as 205216111. Each pair is guaranteed to be 3 digits by zero-padding.

Is it possible to trivially decode this final number back into the plaintext? The words are in plain English and each word is capitalized. So far I have attempted to enumerate all possible permutations and identify words from a dictionary list but this has proven to be rather tedious with short messages alone.

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    $\begingroup$ "this has proven to be rather tedious with short messages alone" - surely, you have a computer doing the searching, don't you? $\endgroup$
    – poncho
    Apr 13 '16 at 2:32
  • $\begingroup$ @poncho Of course, It is just that the possible pairs are many in number and while a dictionary attack can identify words it cannot discern sentences: small articles often end up making a word soup that makes a totally automated approach nigh impossible $\endgroup$
    – J. Doe
    Apr 13 '16 at 10:02
  • $\begingroup$ Sounds alot like a Trifid cipher. en.wikipedia.org/wiki/Trifid_cipher $\endgroup$
    – user33402
    Apr 13 '16 at 10:27
  • $\begingroup$ Attacking this is very similar to attacking a many-time-pad. One advanced attack example: crypto.stackexchange.com/questions/59/… $\endgroup$ Apr 13 '16 at 10:55
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It would be impossible i think. But if you consider evolving the encrypion system such as changing pair like : "he","el","ll","lo","o" it would be way to easy to decrypt.

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