# Calculating characters from the sum of ASCII pairs

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.

• "this has proven to be rather tedious with short messages alone" - surely, you have a computer doing the searching, don't you? Apr 13 '16 at 2:32
• @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 Apr 13 '16 at 10:02
• Sounds alot like a Trifid cipher. en.wikipedia.org/wiki/Trifid_cipher
– user33402
Apr 13 '16 at 10:27
• Attacking this is very similar to attacking a many-time-pad. One advanced attack example: crypto.stackexchange.com/questions/59/… Apr 13 '16 at 10:55