pyln: Improve zbase32 encoding / decoding

- Adds bitarray filling so mesages of any length can be encoded, instead of forcing the encoding to be of messages with length multiple of 5.
- Adds checks for encoding / decoding and raises expections if the inputs are not as expected.
- Flags functions that are supposed to be internal as "private".

contrib/pyln-proto/pyln/proto/zbase32.py

@@ -23,34 +23,68 @@
 ]
 
 
-def bitarray_to_u5(barr):
-    assert len(barr) % 5 == 0
+def _message_to_bitarray(message):
+    barr = bitstring.ConstBitStream(message)
+    padding_len = 5 - (len(barr) % 5)
+    if padding_len < 5:
+        # The bitarray length has to be multiple of 5. If not, it is right-padded with zeros.
+        barr = bitstring.ConstBitStream(bin="{}{}".format(barr.bin, '0' * padding_len))
+    return barr
+
+
+def _bitarray_to_message(barr):
+    padding_len = len(barr) % 8
+    if padding_len > 0:
+        return bitstring.Bits(bin=barr.bin[:-padding_len]).bytes
+    else:
+        return barr.bytes
+
+
+def _bitarray_to_u5(barr):
     ret = []
-    s = bitstring.ConstBitStream(barr)
-    while s.pos != s.len:
-        ret.append(s.read(5).uint)
+    while barr.pos != barr.len:
+        ret.append(barr.read(5).uint)
     return ret
 
 
-def u5_to_bitarray(arr):
+def _u5_to_bitarray(arr):
     ret = bitstring.BitArray()
     for a in arr:
         ret += bitstring.pack("uint:5", a)
     return ret
 
 
-def encode(b):
-    uint5s = bitarray_to_u5(b)
+def is_zbase32_encoded(message):
+    if isinstance(message, str):
+        message = message.encode("ASCII")
+    elif not isinstance(message, bytes):
+        raise TypeError("message must be string or bytes")
+    return set(message).issubset(zbase32_chars)
+
+
+def encode(message):
+    if isinstance(message, str):
+        message = message.encode('ASCII')
+    elif not isinstance(message, bytes):
+        raise TypeError("message must be string or bytes")
+
+    barr = _message_to_bitarray(message)
+    uint5s = _bitarray_to_u5(barr)
     res = [zbase32_chars[c] for c in uint5s]
     return bytes(res)
 
 
-def decode(b):
-    if isinstance(b, str):
-        b = b.encode('ASCII')
+def decode(message):
+    if isinstance(message, str):
+        message = message.encode('ASCII')
+    elif not isinstance(message, bytes):
+        raise TypeError("message must be string or bytes")
+
+    if not is_zbase32_encoded(message):
+        raise ValueError("message is not zbase32 encoded")
 
     uint5s = []
-    for c in b:
+    for c in message:
         uint5s.append(zbase32_revchars[c])
-    dec = u5_to_bitarray(uint5s)
-    return dec.bytes
+    dec = _u5_to_bitarray(uint5s)
+    return _bitarray_to_message(dec)