Added an SmsMessage class, and code for parsing a PDU to an SMS message.

sms_message.cc

+// Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "sms_message.h"
+
+#include <glog/logging.h>
+
+#include "utilities.h"
+
+static const uint8_t INTL_E164_NUMBER_FORMAT = 0x91;
+static const uint8_t SMSC_TIMESTAMP_LEN = 7;
+static const size_t  MIN_PDU_LEN = 7 + SMSC_TIMESTAMP_LEN;
+
+static char NibbleToChar(uint8_t nibble) {
+  switch (nibble) {
+    case 0: return '0';
+    case 1: return '1';
+    case 2: return '2';
+    case 3: return '3';
+    case 4: return '4';
+    case 5: return '5';
+    case 6: return '6';
+    case 7: return '7';
+    case 8: return '8';
+    case 9: return '9';
+    case 10: return '*';
+    case 11: return '#';
+    case 12: return 'a';
+    case 13: return 'b';
+    case 14: return 'c';
+    case 0xff: return '\0';  // padding nibble
+  }
+  return '\0';
+}
+
+// Convert an array of octets into a BCD string. Each octet consists
+// of two nibbles which are converted to hex characters. Those hex
+// characters are the digits of the BCD string. The lower nibble is
+// the more significant digit.
+static std::string SemiOctetsToBcdString(const uint8_t* octets,
+                                         int num_octets) {
+  std::string bcd;
+
+  for (int i = 0; i < num_octets; ++i) {
+    char first = NibbleToChar(octets[i] & 0xf);
+    char second = NibbleToChar((octets[i] >> 4) & 0xf);
+
+    if (first != '\0')
+      bcd += first;
+    if (second != '\0')
+      bcd += second;
+  }
+  return bcd;
+}
+
+SmsMessage* SmsMessage::CreateMessage(const uint8_t* pdu, size_t pdu_len) {
+  utilities::DumpHex(pdu, pdu_len);
+
+  // Make sure the PDU is of a valid size
+  if (pdu_len < MIN_PDU_LEN) {
+    LOG(INFO) << "PDU too short: " << pdu_len << " vs. " << MIN_PDU_LEN;
+    return NULL;
+  }
+
+  // Format of message:
+  //
+  //  1 octet  - length of SMSC information in octets, including type field
+  //  1 octet  - type of address of SMSC (value 0x91 is international E.164)
+  //  variable - SMSC address
+  //  1 octet  - first octet of SMS-DELIVER (value = 0x04)
+  //  1 octet  - length of sender address in decimal digits (semi-octets)
+  //  1 octet  - type of sender address (value 0x91 is international E.164)
+  //  variable - sender address
+  //  1 octet  - protocol identifier (value = 0)
+  //  1 octet  - data coding scheme (value = 0)
+  //  7 octets - SMSC timestamp
+  //  1 octet  - user data length in septets
+  //  variable - user data (body of message)
+
+  // Do a bunch of validity tests first so we can bail out early
+  // if we're not able to handle the PDU. We first check the validity
+  // of all length fields and make sure the PDU length is consistent
+  // with those values.
+
+  uint8_t smsc_addr_num_octets = pdu[0];
+  uint8_t variable_length_items = smsc_addr_num_octets;
+
+  if (pdu_len < variable_length_items + MIN_PDU_LEN) {
+    LOG(INFO) << "PDU too short: " << pdu_len << " vs. "
+              << variable_length_items + MIN_PDU_LEN;
+    return NULL;
+  }
+
+  // where in the PDU the actual SMS protocol message begins
+  uint8_t msg_start_offset = 1 + smsc_addr_num_octets;
+  uint8_t sender_addr_num_digits = pdu[msg_start_offset + 1];
+  // round the sender address length up to an even number of semi-octets,
+  // and thus an integral number of octets
+  uint8_t sender_addr_num_octets = (sender_addr_num_digits + 1) >> 1;
+  variable_length_items += sender_addr_num_octets;
+  if (pdu_len < variable_length_items + MIN_PDU_LEN) {
+    LOG(INFO) << "PDU too short: " << pdu_len << " vs. "
+              << variable_length_items + MIN_PDU_LEN;
+    return NULL;
+  }
+
+  uint8_t tp_pid_offset = msg_start_offset + 3 + sender_addr_num_octets;
+  uint8_t user_data_offset = tp_pid_offset + 2 + SMSC_TIMESTAMP_LEN;
+  uint8_t user_data_num_septets = pdu[user_data_offset];
+  variable_length_items += (7 * (user_data_num_septets + 1 )) / 8;
+
+  if (pdu_len < variable_length_items + MIN_PDU_LEN) {
+    LOG(INFO) << "PDU too short: " << pdu_len << " vs. "
+              << variable_length_items + MIN_PDU_LEN;
+    return NULL;
+  }
+
+  // wow do some validity checks on the values of several fields in the PDU
+
+  // smsc number format must be international, E.164
+  if (pdu[1] != INTL_E164_NUMBER_FORMAT) {
+    LOG(INFO) << "Invalid SMSC address format: " << std::hex << (int)pdu[1]
+              << " vs. " << std::hex << INTL_E164_NUMBER_FORMAT;
+    return NULL;
+  }
+  // we only handle SMS-DELIVER messages, with more-messages-to-send false
+  if ((pdu[msg_start_offset] & 0x07) != 0x04) {
+    LOG(INFO) << "Unhandled message type: " << std::hex
+              << (int)pdu[msg_start_offset] << " vs. 0x04";
+    return NULL;
+  }
+  // we only handle the basic protocol identifier
+  if (pdu[tp_pid_offset] != 0) {
+    LOG(INFO) << "Unhandled protocol identifier: " << std::hex
+              << (int)pdu[tp_pid_offset] << " vs. 0x00";
+    return NULL;
+  }
+  // for data coding scheme, we only handle the default alphabet, i.e. GSM7
+  if (pdu[tp_pid_offset+1] != 0) {
+    LOG(INFO) << "Unhandled data coding scheme: " << std::hex
+              << (int)pdu[tp_pid_offset+1] << " vs. 0x00";
+    return NULL;
+  }
+
+  std::string smsc_addr = SemiOctetsToBcdString(&pdu[2],
+                                                smsc_addr_num_octets-1);
+  std::string sender_addr = SemiOctetsToBcdString(&pdu[msg_start_offset+3],
+                                                  sender_addr_num_octets);
+  std::string sc_timestamp = SemiOctetsToBcdString(&pdu[tp_pid_offset+2],
+                                                   SMSC_TIMESTAMP_LEN-1);
+  std::string msg_text = utilities::Gsm7ToUtf8String(&pdu[user_data_offset]);
+
+  // The last two semi-octets of the timestamp indicate an offset from
+  // GMT, and are handled differently than the first 12 semi-octets.
+  uint8_t toff_octet = pdu[tp_pid_offset+1+SMSC_TIMESTAMP_LEN];
+  sc_timestamp += (toff_octet & 0x8) ? '-' : '+';
+  uint8_t offset_in_hours =
+      ((toff_octet & 0x7) << 4 | (toff_octet & 0xf0) >> 4) / 4;
+  sc_timestamp += (char)((offset_in_hours / 10) + '0');
+  sc_timestamp += (char)((offset_in_hours % 10) + '0');
+
+  return new SmsMessage(smsc_addr, sender_addr, sc_timestamp, msg_text);
+}