9, 15, 15, 16, 25, 6, 11, 2, 9, 6, 11, 2, 21, 1, 14, 9, 19, 23, 25, 17, 19, 18, 1
Look up n-bonacci
Starting numbers are different
Message length is the same as encoded length
Hint 4 (slightly changed):
All the numbers are in a certain range. What is the modulo for that range?
This cipher has no key.
Hint 6 (basically gives it away)
n-bonacci starting numbers are the message.
Hint 7 (literally just describes the encryption algorithm)
Start with the letter values of the message, and repeat as many times as there are letters in the message: Change the current letter values to the values except the first, and then the sum of the values, including the first, mod 26. In code:
for _ in range(len(message)): message = message[1:] + sum(message) % 26
I will add more if it is not solved soon.
Hints are in no particular order, but some might be less helpful than others.
This will probably be very tedious without computers, so they are allowed. The message is all lowercase letters, with no spaces, but it is more than one word.