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I really love working these out on my own and then looking at how others handled the problem to see new approaches.
My first take at this was a little clunky but worked by generating a row for each letter in the original message and then applying the rotation, then using the crosstab to put the rows back into a column.
After looking at some of the other responses I realized that the regexp -tokenize was a more efficient approach with summary to pull everything back together.
Here is my solution, used Join+Union (to bring back spaces & punctuation etc) rather than Find & Replace used in other solutions. Preferred to use a Letter mapping rather than Char to Int as you also need to account for the Unicode integer.
These cryptography challenges are so much fun! This one reminded me of having a "secret language" with my friends in middle school, so we could write notes in class (it was just a simple substitution cipher).