from itertools import zip_longest

def grouper(n, iterable, padvalue=None):
"grouper(3, 'abcdefg', 'x') --> ('a','b','c'), ('d','e','f'), ('g','x','x')"
return zip_longest(*[iter(iterable)]*n, fillvalue=padvalue)

def decrypt(s):
def bytehandler(x):
""" handles a pair of hex nibbles i.e. ("A", "0") """
nr = int("".join(x), 16) - int(s[-1], 16)
if nr < 0: nr += 255
return chr(nr)

return "".join(
map(
bytehandler,
grouper(2, s[:-1]) # group all nibbles except the last into pairs
)
)

str1 = "7A6B797A6"
print(decrypt(str1))

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