Beter error detection in encoding LoRa RX frames

RPi-LoRa-KISS-TNC/KissHelper.py

@@ -29,6 +29,7 @@
 # Changes by PE1RXF
 #
 # 2022-01-23: - in encode_address() added correct handling of has_been_repeated flag '*'
+# 2022-01-28: - in encode_kiss() and encode_address(): better exeption handling for corrupted or mal-formatted APRS frames
 #
 
 import struct
@@ -56,12 +57,23 @@ def encode_address(s, final):
         # print("Message has been repeated")
         ssid = ssid[:-1]
         encoded_ssid |= 0b10000000
-        
-    encoded_ssid |= (int(ssid) << 1) | 0b01100000 | (0b00000001 if final else 0)
-
+    
+    # If SSID was not pressent (and we added the default -0 to it), the has_been_repeated flag could be at the end of the call, so check that as well
+    # Also, there is a lot of bad software around (including this code) and ignorance of the specifications (are there any specs for LoRa APRS?), so always check for the has_been_repeated flag
+    if call[-1] == 42:
+        call = call[:-1]
+        encoded_ssid |= 0b10000000
+    
+    # SSID should now be one or two postions long and contain a number (idealy between 0 and 15).
+    if len(ssid) == 1 and ssid[0] > 47 and ssid[0] < 58:
+        encoded_ssid |= (int(ssid) << 1) | 0b01100000 | (0b00000001 if final else 0)
+    elif len(ssid) == 2 and ssid[0] > 47 and ssid[0] < 58 and ssid[1] > 47 and ssid[2] < 58:
+        encoded_ssid |= (int(ssid) << 1) | 0b01100000 | (0b00000001 if final else 0)
+    else:
+        return None
+    
     return encoded_call + [encoded_ssid]
 
-
 def decode_address(data, cursor):
     (a1, a2, a3, a4, a5, a6, a7) = struct.unpack("<BBBBBBB", data[cursor:cursor + 7])
     hrr = a7 >> 5
@@ -74,22 +86,56 @@ def decode_address(data, cursor):
         call = addr
     return (call, hrr, ext)
 
-
+########################################################################
+# Encode string from LoRa radio to AX.25 over KISS
+#
+# We must make no assumptions as the incomming frame could be carbage.
+# So make sure we think of everthing in order to prevent crashes.
+#
+# The original code from Thomas Kottek did a good job encoding propper APRS frames.
+# But when the frames where not what they should be, the program could crash.
+#
+########################################################################
 def encode_kiss(frame):
-    # Ugly frame disassembling
+
+    # First check: do we have a semi column (seperator path field and data field)
+    # Note that we could still be wrong: for example when the field seperator is corrupted and we now find a semi column from, lets say, an internet address in the data field...
     if not b":" in frame:
         return None
+    
+    # Split the frame in a path field and a data field
     path = frame.split(b":")[0]
+    data_field = frame[frame.find(b":") + 1:]
+
+    # The source address is always followed by a greather than sign, so lets see if its there.
+    # There is always a change that there is another greather than sign because the frame could be corrupted...
+    if not b">" in path:
+        return None
+
+    # Split the path into a source address and a digi-path array (because digis should be seperated by commas, but again, corruption....)
     src_addr = path.split(b">")[0]
     digis = path[path.find(b">") + 1:].split(b",")
+    
     # destination address
-    packet = encode_address(digis.pop(0).upper(), False)
+    return_value = encode_address(digis.pop(0).upper(), False)
+    if return_value is None:
+        return None
+    packet = return_value
+        
     # source address
-    packet += encode_address(path.split(b">")[0].upper(), len(digis) == 0)
+    return_value = encode_address(src_addr.upper(), len(digis) == 0)
+    if return_value is None:
+        return None
+    packet += return_value
+    
     # digipeaters
     for digi in digis:
         final_addr = digis.index(digi) == len(digis) - 1
-        packet += encode_address(digi.upper(), final_addr)
+        return_value = encode_address(digi.upper(), final_addr)
+        if return_value is None:
+            return None
+        packet += return_value  
+
     # control field
     packet += [0x03]  # This is an UI frame
     # protocol ID