def crypt(text,key):
    CodeList=[ord(char) for char in text]
    KeyList=[ord(char) for char in key]
    NewKeyList=KeyList[:]
    while len(NewKeyList)<len(CodeList): NewKeyList+=KeyList
    while len(NewKeyList)>len(CodeList): del NewKeyList[-1] 
    EncryptedCodeList=[]; char=0
    for char in range(0,len(CodeList)): EncryptedCodeList.append(CodeList[char]^NewKeyList[char])
    EncryptedList=[chr(char) for char in EncryptedCodeList ]
    EncryptedText="".join(EncryptedList)
    return EncryptedText

>>> padding(b'a' * 15)
'aaaaaaaaaaaaaaa_'
>>> padding(b'b' * 15)
'bbbbbbbbbbbbbbb_'
>>> padding(b'abc' * 5)
'abcabcabcabcabc_'
>>> padding(b'a' * 15)