PDU Decoding Error

[mwfong-csl]

gsmmodem/pdu.py:_decodeUserData () unconditionally skips the first character of any PDU that contains UDH fields.

def _decodeUserData(byteIter, userDataLen, dataCoding, udhPresent):
    """ Decodes PDU user data (UDHI (if present) and message text) """
    result = {}
    if udhPresent:
        # User Data Header is present
        result['udh'] = []
        udhLen = next(byteIter)
        ieLenRead = 0
        # Parse and store UDH fields
        while ieLenRead < udhLen:
            ie = InformationElement.decode(byteIter)
            ieLenRead += len(ie)
            result['udh'].append(ie)
        del ieLenRead
        if dataCoding == 0x00: # GSM-7
            # Since we are using 7-bit data, "fill bits" may have been added to make the UDH end on a septet boundary
            shift = ((udhLen + 1) * 8) % 7 # "fill bits" needed to make the UDH end on a septet boundary
            # Simulate another "shift" in the unpackSeptets algorithm in order to ignore the fill bits
            prevOctet = next(byteIter)
            shift += 1
    :
    :

should be

def _decodeUserData(byteIter, userDataLen, dataCoding, udhPresent):
    """ Decodes PDU user data (UDHI (if present) and message text) """
    result = {}
    if udhPresent:
        # User Data Header is present
        result['udh'] = []
        udhLen = next(byteIter)
        ieLenRead = 0
        # Parse and store UDH fields
        while ieLenRead < udhLen:
            ie = InformationElement.decode(byteIter)
            ieLenRead += len(ie)
            result['udh'].append(ie)
        del ieLenRead
        if dataCoding == 0x00: # GSM-7
            # Since we are using 7-bit data, "fill bits" may have been added to make the UDH end on a septet boundary
            shift = ((udhLen + 1) * 8) % 7 # "fill bits" needed to make the UDH end on a septet boundary
            # Simulate another "shift" in the unpackSeptets algorithm in order to ignore the fill bits
            if shift:
                prevOctet = next(byteIter)
                shift += 1
            else:
                prevOctet = 0
    :
    :

This online SMS PDU decoder correctly decodes the following PDU, but _decodeUserData () does not:

07912160130350F66003C916F100009170914115628AA0060804931802015474D8BD9E83CC6F39482C4FBBCF69F719947FD7E52038FAED2E83E86F50B34C97BF41E23C88DA6ABEC569763904B2A6E7693A084DA7C375AF57BB4C97BFE1E3B92BFD7E09A3EDF3710A0ABBC9A073990EA2A3CBA066BE59A6CBDFA0301C0E9ABF41F9771D340EBB41EDB03B7C2E83F2EFBA1C141E8FDF75371D14769341EDF0BA0C1AA3C3

Test code:

import gsmmodem.pdu as pdu

_smsIn = [
    '+CMGL: 0,0,,155',
    '07912160130350F66003C916F100009170914115628AA0060804931802015474D8BD9E83CC6F39482C4FBBCF69F719947FD7E52038FAED2E83E86F50B34C97BF41E23C88DA6ABEC569763904B2A6E7693A084DA7C375AF57BB4C97BFE1E3B92BFD7E09A3EDF3710A0ABBC9A073990EA2A3CBA066BE59A6CBDFA0301C0E9ABF41F9771D340EBB41EDB03B7C2E83F2EFBA1C141E8FDF75371D14769341EDF0BA0C1AA3C3',
    '+CMGL: 1,0,,48',
    '07912160130350F66003C916F100009170914115728A2506080493180202EE73790E92A7CF683AC82C7FB741F9775D0E4287DDE479991E02',
    'OK'
]

for _i in range (1, len (_smsIn) - 1, 2):
    print (pdu.decodeSmsPdu (_smsIn[_i]))

Results:

#
# Original gsmmodem/pdu.py:_decodeUserData ()
#
% python3 pduDecodeBug.py 
{'smsc': '+12063130056', 'tpdu_length': 155, 'type': 'SMS-DELIVER', 'number': '611', 'protocol_id': 0, 'time': datetime.datetime(2019, 7, 19, 14, 51, 26, tzinfo=<gsmmodem.pdu.SmsPduTzInfo object at 0x10517fcc0>), 'udh': [<gsmmodem.pdu.Concatenation object at 0x1046e3ef0>], 'text': 'hanks for bringing your phone to Metro by T-Mobile! Visit http://metropcs.io/BQmgGS and get the MyMetro app so you can manage your account and make cha'}
{'smsc': '+12063130056', 'tpdu_length': 48, 'type': 'SMS-DELIVER', 'number': '611', 'protocol_id': 0, 'time': datetime.datetime(2019, 7, 19, 14, 51, 27, tzinfo=<gsmmodem.pdu.SmsPduTzInfo object at 0x104700240>), 'udh': [<gsmmodem.pdu.Concatenation object at 0x1046e3f60>], 'text': 'ges right from your handset!'}

#
# Corrected gsmmodem/pdu.py:_decodeUserData ()
#
% python3.7 pduDecodeBug.py 
{'smsc': '+12063130056', 'tpdu_length': 155, 'type': 'SMS-DELIVER', 'number': '611', 'protocol_id': 0, 'time': datetime.datetime(2019, 7, 19, 14, 51, 26, tzinfo=<gsmmodem.pdu.SmsPduTzInfo object at 0x10fcc4390>), 'udh': [<gsmmodem.pdu.Concatenation object at 0x10fccf470>], 'text': 'Thanks for bringing your phone to Metro by T-Mobile! Visit http://metropcs.io/BQmgGS and get the MyMetro app so you can manage your account and make cha'}
{'smsc': '+12063130056', 'tpdu_length': 48, 'type': 'SMS-DELIVER', 'number': '611', 'protocol_id': 0, 'time': datetime.datetime(2019, 7, 19, 14, 51, 27, tzinfo=<gsmmodem.pdu.SmsPduTzInfo object at 0x10fb4a7b8>), 'udh': [<gsmmodem.pdu.Concatenation object at 0x10fb4a6d8>], 'text': 'nges right from your handset!'}

[JimJty]

I ran into this issue as well. I'm testing the fix for my service, but looks to be working so far.