Unexpected bytes when reading a file - encryption

Question

So i have been working on an encryption algorithm and i wanted to see if it'd successfuly encrypt a text file (i want the program to be versatile so it's reading bytes).
I run the program, encryption works (amount of letter match the amount of letters in plain text) fine BUT whenever i try to read the encrypted file it adds a random byte. \xc3\x81 to be exact.
For example:
if plain text is "noob" decrypted text is, for some reason, Ánoob. "Á" being the random byte.
Here's the code,
.
.
.
# I tried avoiding the byte by adding the statements about 193
# I couldn't add some of the
# While loops are there to make sure the byte is within supported ascii range
def encryption_algorithm(self):
NBYTE = b''
for file in self.file_path_generator(): # self.file_path_generator() just gets the list of files
if file.name != os.path.basename(__file__):
read = self.access_files(file, None ,"rb", False, False) # same thing as "with open("filename", "rb") as f:"
for byte in read:
if byte % 5 == 0 and byte != 193:
byte = (byte + 30)
while byte > 255:
byte -= 255
NBYTE += bytes(chr(byte), "utf-8")
elif byte % 5 == 4 and byte != 193:
byte = (byte + 2)
while byte > 255:
byte -= 255
NBYTE += bytes(chr(byte), "utf-8")
elif byte == 193:
NBYTE += bytes(chr(byte), "utf-8")
else:
byte = (byte + 1)
while byte > 255:
byte -= 255
NBYTE += bytes(chr(byte), "utf-8")
self.access_files(file, NBYTE, "wb", False, False)
def decryption_algorithm(self):
NBYTE = b''
for file in self.file_path_generator():
if file.name != os.path.basename(__file__):
read = self.access_files(file, None ,"rb", False, False)
for byte in read:
if byte % 5 == 0 and byte != 193:
byte = (byte - 30)
while byte < 0:
byte += 255
NBYTE += bytes(chr(byte), "utf-8")
elif byte % 5 == 1 and byte != 193:
byte = (byte - 2)
while byte < 0:
byte += 255
NBYTE += bytes(chr(byte), "utf-8")
elif byte == 193:
NBYTE += (chr(byte), "utf-8")
else:
byte = (byte - 1)
while byte < 0:
byte += 255
NBYTE += bytes(chr(byte), "utf-8")
self.access_files(file, NBYTE, "wb", False, False)
Note: Replacing the "Á" with None does work for text files but not image files.
Note2: I'd add the full encrypted text aswell but some characters are not supported. It was something like "?ppc"

Answer 1

utf-8 can use more than one byte.
https://en.wikipedia.org/wiki/UTF-8#Description
with your example noob. n=0x6e when you add 30.
The result is 140 (0x8c).
And it will need 2 bytes to be encoded in utf-8 .
you can try:
NBYTE = b''
byte=140
NBYTE += bytes(chr(byte), "utf-8")
print(NBYTE)
you will see that the result will be:
b'\xc2\x8c'