pe1rxf-aprs-server/RPi-LoRa-KISS-TNC/KissHelper.py

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.


# This program provides basic KISS AX.25 APRS frame encoding and decoding.
# Note that only APRS relevant structures are tested. It might not work
# for generic AX.25 frames.
# 11/2019 by Thomas Kottek, OE9TKH
#
# Inspired by:
# * Python script to decode AX.25 from KISS frames over a serial TNC 
#   https://gist.github.com/mumrah/8fe7597edde50855211e27192cce9f88
#
# * Sending a raw AX.25 frame with Python
#   https://thomask.sdf.org/blog/2018/12/15/sending-raw-ax25-python.html
#
# TODO: remove escapes on decoding
#
# 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

KISS_FEND = 0xC0  # Frame start/end marker
KISS_FESC = 0xDB  # Escape character
KISS_TFEND = 0xDC  # If after an escape, means there was an 0xC0 in the source message
KISS_TFESC = 0xDD  # If after an escape, means there was an 0xDB in the source message


# Addresses must be 6 bytes plus the SSID byte, each character shifted left by 1
# If it's the final address in the header, set the low bit to 1
# Ignoring command/response for simple example
def encode_address(s, final):
    if b"-" not in s:
        s = s + b"-0"  # default to SSID 0
    call, ssid = s.split(b'-')
    if len(call) < 6:
        call = call + b" "*(6 - len(call)) # pad with spaces
    encoded_call = [x << 1 for x in call[0:6]]
    
    encoded_ssid = 0b00000000
    # If ssid ends with *, the message has been repeated, so we have to set the 'has_been_repeated' flag and remove the * from the ssid
    if ssid[-1] == 42:
        # print("Message has been repeated")
        ssid = ssid[:-1]
        encoded_ssid |= 0b10000000
    
    # 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) is 1 and ssid[0] > 47 and ssid[0] < 58:
        encoded_ssid |= (int(ssid) << 1) | 0b01100000 | (0b00000001 if final else 0)
    elif len(ssid) is 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
    ssid = (a7 >> 1) & 0xf
    ext = a7 & 0x1
    addr = struct.pack("<BBBBBB", a1 >> 1, a2 >> 1, a3 >> 1, a4 >> 1, a5 >> 1, a6 >> 1)
    if ssid != 0:
        call = addr.strip() + "-{}".format(ssid).encode()
    else:
        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):

    # 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
    return_value = encode_address(digis.pop(0).upper(), False)
    if return_value is None:
        return None
    packet = return_value
        
    # source address
    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
        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
    packet += [0xF0]  # No protocol
    # information field
    packet += frame[frame.find(b":") + 1:]

    # Escape the packet in case either KISS_FEND or KISS_FESC ended up in our stream
    packet_escaped = []
    for x in packet:
        if x == KISS_FEND:
            packet_escaped += [KISS_FESC, KISS_TFEND]
        elif x == KISS_FESC:
            packet_escaped += [KISS_FESC, KISS_TFESC]
        else:
            packet_escaped += [x]

    # Build the frame that we will send to Dire Wolf and turn it into a string
    kiss_cmd = 0x00  # Two nybbles combined - TNC 0, command 0 (send data)
    kiss_frame = [KISS_FEND, kiss_cmd] + packet_escaped + [KISS_FEND]
    try:
        output = bytearray(kiss_frame)
    except ValueError:
        print("Invalid value in frame.")
        return None
    return output


def decode_kiss(frame):
    result = b""
    pos = 0
    if frame[pos] != 0xC0 or frame[len(frame) - 1] != 0xC0:
        print(frame[pos], frame[len(frame) - 1])
        return None
    pos += 1
    pos += 1

    # DST
    (dest_addr, dest_hrr, dest_ext) = decode_address(frame, pos)
    pos += 7
    # print("DST: ", dest_addr)

    # SRC
    (src_addr, src_hrr, src_ext) = decode_address(frame, pos)
    pos += 7
    # print("SRC: ", src_addr)

    result += src_addr.strip()
    # print(type(result), type(dest_addr.strip()))
    result += b">" + dest_addr.strip()

    # REPEATERS
    ext = src_ext
    while ext == 0:
        rpt_addr, rpt_hrr, ext = decode_address(frame, pos)
        # print("RPT: ", rpt_addr)
        pos += 7
        result += b"," + rpt_addr.strip()

    result += b":"

    # CTRL
    # (ctrl,) = struct.unpack("<B", frame[pos])
    ctrl = frame[pos]
    pos += 1
    if (ctrl & 0x3) == 0x3:
        #(pid,) = struct.unpack("<B", frame[pos])
        pid = frame[pos]
        # print("PID="+str(pid))
        pos += 1
        result += frame[pos:len(frame) - 1]
    elif (ctrl & 0x3) == 0x1:
        # decode_sframe(ctrl, frame, pos)
        print("SFRAME")
        return None
    elif (ctrl & 0x1) == 0x0:
        # decode_iframe(ctrl, frame, pos)
        print("IFRAME")
        return None

    return result


class SerialParser():
    '''Simple parser for KISS frames. It handles multiple frames in one packet
  and calls the callback function on each frame'''
    STATE_IDLE = 0
    STATE_FEND = 1
    STATE_DATA = 2
    KISS_FEND = KISS_FEND

    def __init__(self, frame_cb=None):
        self.frame_cb = frame_cb
        self.reset()

    def reset(self):
        self.state = self.STATE_IDLE
        self.cur_frame = bytearray()

    def parse(self, data):
        '''Call parse with a string of one or more characters'''
        for c in data:
            if self.state == self.STATE_IDLE:
                if c == self.KISS_FEND:
                    self.cur_frame.append(c)
                    self.state = self.STATE_FEND
            elif self.state == self.STATE_FEND:
                if c == self.KISS_FEND:
                    self.reset()
                else:
                    self.cur_frame.append(c)
                    self.state = self.STATE_DATA
            elif self.state == self.STATE_DATA:
                self.cur_frame.append(c)
                if c == self.KISS_FEND:
                    # frame complete
                    if self.frame_cb:
                        self.frame_cb(self.cur_frame)
                    self.reset()


if __name__ == "__main__":
    # Playground for testing

    # frame = "\xc0\x00\x82\xa0\xa4\xb0dr`\x9e\x8ar\xa8\x96\x90u\x03\xf0!4725.73NR00939.61E&Experimental LoRa iGate\xc0"
    frame = "\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90q\x03\xf0!4725.51N/00939.86E[322/002/A=001306 Batt=3.99V\xc0"


    # print(decode_kiss(frame))
    # encoded = encode_kiss("OE9TKH-8>APRS,RELAY,BLA:!4725.51N/00939.86E[322/002/A=001306 Batt=3")
    # encoded = encode_kiss("OE9TKH-8>APRS,digi-3,digi-2:!4725.51N/00939.86E[322/002/A=001306 Batt=3")
    # print((decode_kiss(encoded)))

    # print((decode_kiss("\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90t\xae\x92\x88\x8ab@\x03\x03\xf0}OE9GHV-10>APMI06,TCPIP,OE9TKH-10*:@110104z4726.55N/00950.63E&WX3in1 op. Holger U=14.2V,T=8.8C\xc0")))

    def newframe(frame):
        print(repr(frame))


    two_example_frames = "\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90u\x03\xf0}SOTA>APZS16,TCPIP,OE9TKH-10*::OE9TKH-8 :<Ass/Ref> <Freq> <Mode> [call] [comment]{7ba\xc0\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90u\x03\xf0}SOTA>APZS16,TCPIP,OE9TKH-10*::OE9TKH-8 :/mylast{7bb\xc0\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90u\x03\xf0}SOTA>APZS16,TCPIP,OE9TKH-10*::OE9TKH-8 :/last{7bc\xc0\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90u\x03\xf0}SOTA>APZS16,TCPIP,OE9TKH-10*::OE9TKH-8 :/time(/zone){7bd\xc0"
    sp = SerialParser(newframe)
    sp.parse(two_example_frames)