It is easy to encrypt as specified with the aid of a spreadsheet but the encryption will always be a symmetrical string. Given either first half or last half of solution, it would be easy to determine the other half with a spreadsheet. The problem arises that every letter in the encrypted can be any one of at least 13 different combinations. If you do not know one, you cannot positively identify the other. Bottom line, this cannot be decoded by any algorithm. With sufficient hints and basic word composition rules, we could eventually get the answer but it would be an elaborate maze of guesses, especially since no spaces are provided for word division.
Each character can be decoded into one of 26. Even using a dictionary, the biggest problem is not speed (though it is a major issue anyway), but how to recognise words. The most commonly used dictionary has 250000 words, which is too many. If a search algorithm accepts words of any length, then the search would start off creating many trivial words like BA or CA, words that are virtually never used; there is no easy way to distinguish MEANINGFUL words. This might be faintly plausible if I could find a dictionary of common words only.
Okay, letter by letter is getting me nowhere fast. I think that a brute force with a dictionary could probably break this pretty quickly though, but I do not have access to that other than manually, which would be pretty slow. I can find a word if it is in there somewhere, so I will ask for a word hint please.