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Hilariously, the program itself is its own inverse. A little bash to handle the base64 decoding, and out pops the flag.

python shiftcrypt.py "`echo 'CjBPewYGc2gdD3RpMRNfdDcQX3UGGmhpBxZhYhFlfQA=' | base64 -D`" | base64 -D; echo