wmbusmeters/src/wmbus_utils.cc

/*
 Copyright (C) 2018-2020 Fredrik Öhrström

 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/>.
*/


#include"aes.h"
#include"util.h"
#include"wmbus.h"

#include<assert.h>
#include<memory.h>

bool decrypt_ELL_AES_CTR(Telegram *t, vector<uchar> &frame, vector<uchar>::iterator &pos, vector<uchar> &aeskey)
{
    if (aeskey.size() == 0) return true;

    vector<uchar> encrypted_bytes;
    vector<uchar> decrypted_bytes;
    encrypted_bytes.insert(encrypted_bytes.end(), pos, frame.end());
    frame.erase(pos, frame.end());
    debugPayload("(ELL) decrypting", encrypted_bytes);

    uchar iv[16];
    int i=0;
    // M-field
    iv[i++] = t->dll_mfct_b[0]; iv[i++] = t->dll_mfct_b[1];
    // A-field
    for (int j=0; j<6; ++j) { iv[i++] = t->dll_a[j]; }
    // CC-field
    iv[i++] = t->ell_cc;
    // SN-field
    for (int j=0; j<4; ++j) { iv[i++] = t->ell_sn_b[j]; }
    // FN
    iv[i++] = 0; iv[i++] = 0;
    // BC
    iv[i++] = 0;

    vector<uchar> ivv(iv, iv+16);
    string s = bin2hex(ivv);
    debug("(ELL) IV %s\n", s.c_str());

    int block = 0;
    for (size_t offset = 0; offset < encrypted_bytes.size(); offset += 16)
    {
        size_t block_size = 16;
        if (offset + block_size > encrypted_bytes.size())
        {
            block_size = encrypted_bytes.size() - offset;
        }

        assert(block_size > 0 && block_size <= 16);

        // Generate the pseudo-random bits from the IV and the key.
        uchar xordata[16];
        AES_ECB_encrypt(iv, &aeskey[0], xordata, 16);

        // Xor the data with the pseudo-random bits to decrypt into tmp.
        uchar tmp[block_size];
        xorit(xordata, &encrypted_bytes[offset], tmp, block_size);

        debug("(ELL) block %d block_size %d offset %zu\n", block, block_size, offset);
        block++;

        vector<uchar> tmpv(tmp, tmp+block_size);
        debugPayload("(ELL) decrypted", tmpv);

        decrypted_bytes.insert(decrypted_bytes.end(), tmpv.begin(), tmpv.end());

        incrementIV(iv, sizeof(iv));
    }
    debugPayload("(ELL) decrypted", decrypted_bytes);
    frame.insert(frame.end(), decrypted_bytes.begin(), decrypted_bytes.end());
    return true;
}

string frameTypeKamstrupC1(int ft) {
    if (ft == 0x78) return "long frame";
    if (ft == 0x79) return "short frame";
    return "?";
}

bool decrypt_TPL_AES_CBC_IV(Telegram *t, vector<uchar> &frame, vector<uchar>::iterator &pos, vector<uchar> &aeskey)
{
    if (aeskey.size() == 0) return true;

    vector<uchar> buffer;
    buffer.insert(buffer.end(), pos, frame.end());
    frame.erase(pos, frame.end());
    debugPayload("(TPL) AES CBC IV decrypting", buffer);

    size_t len = buffer.size();

    if (t->tpl_num_encr_blocks)
    {
        len = t->tpl_num_encr_blocks*16;
    }

    debug("(TPL) num encrypted blocks %zu (%d bytes and remaining unencrypted %zu bytes)\n",
          t->tpl_num_encr_blocks, len, buffer.size()-len);

    if (buffer.size() < len)
    {
        warning("(TPL) warning: decryption received less bytes than expected for decryption! "
                "Got %zu bytes but expected at least %zu bytes since num encr blocks was %d.\n",
                buffer.size(), len,
                t->tpl_num_encr_blocks);
        len = buffer.size();
    }

    // The content should be a multiple of 16 since we are using AES CBC mode.
    if (len % 16 != 0)
    {
        warning("(TPL) warning: decryption received non-multiple of 16 bytes! "
                "Got %zu bytes shrinking message to %zu bytes.\n",
                len, len - len % 16);
        len -= len % 16;
        assert (len % 16 == 0);
    }

    uchar iv[16];
    int i=0;
    // If there is a tpl_id, then use it, else use ddl_id.
    if (t->tpl_id_found)
    {
        // M-field
        iv[i++] = t->tpl_mfct_b[0]; iv[i++] = t->tpl_mfct_b[1];

        // A-field
        for (int j=0; j<6; ++j) { iv[i++] = t->tpl_a[j]; }
    }
    else
    {
        // M-field
        iv[i++] = t->dll_mfct_b[0]; iv[i++] = t->dll_mfct_b[1];

        // A-field
        for (int j=0; j<6; ++j) { iv[i++] = t->dll_a[j]; }
    }

    // ACC
    for (int j=0; j<8; ++j) { iv[i++] = t->tpl_acc; }

    vector<uchar> ivv(iv, iv+16);
    string s = bin2hex(ivv);
    debug("(TPL) IV %s\n", s.c_str());

    uchar buffer_data[buffer.size()];
    memcpy(buffer_data, &buffer[0], buffer.size());
    uchar decrypted_data[buffer.size()];
    AES_CBC_decrypt_buffer(decrypted_data, buffer_data, len, &aeskey[0], iv);

    frame.insert(frame.end(), decrypted_data, decrypted_data+len);
    debugPayload("(TPL) decrypted ", frame, pos);

    if (len < buffer.size())
    {
        frame.insert(frame.end(), buffer_data+len, buffer_data+buffer.size());
        debugPayload("(TPL) appended  ", frame, pos);
    }
    return true;
}

bool decrypt_TPL_AES_CBC_NO_IV(Telegram *t, vector<uchar> &frame, vector<uchar>::iterator &pos, vector<uchar> &aeskey)
{
    if (aeskey.size() == 0) return true;

    vector<uchar> buffer;
    buffer.insert(buffer.end(), pos, frame.end());
    frame.erase(pos, frame.end());
    debugPayload("(TPL) AES CBC NO IV decrypting", buffer);

    size_t len = buffer.size();

    if (t->tpl_num_encr_blocks)
    {
        len = t->tpl_num_encr_blocks*16;
    }

    debug("(TPL) num encrypted blocks %d (%d bytes and remaining unencrypted %d bytes)\n",
          t->tpl_num_encr_blocks, len, buffer.size()-len);

    if (buffer.size() < len)
    {
        warning("(TPL) warning: decryption received less bytes than expected for decryption! "
                "Got %zu bytes but expected at least %zu bytes since num encr blocks was %d.\n",
                buffer.size(), len,
                t->tpl_num_encr_blocks);
        len = buffer.size();
    }

    // The content should be a multiple of 16 since we are using AES CBC mode.
    if (len % 16 != 0)
    {
        warning("(TPL) warning: decryption received non-multiple of 16 bytes! "
                "Got %zu bytes shrinking message to %zu bytes.\n",
                len, len - len % 16);
        len -= len % 16;
        assert (len % 16 == 0);
    }

    uchar iv[16];
    memset(iv, 0, sizeof(iv));

    vector<uchar> ivv(iv, iv+16);
    string s = bin2hex(ivv);
    debug("(TPL) IV %s\n", s.c_str());

    uchar buffer_data[buffer.size()];
    memcpy(buffer_data, &buffer[0], buffer.size());
    uchar decrypted_data[buffer.size()];
    AES_CBC_decrypt_buffer(decrypted_data, buffer_data, buffer.size(), &aeskey[0], iv);

    frame.insert(frame.end(), decrypted_data, decrypted_data+buffer.size());
    debugPayload("(TPL) decrypted ", frame, pos);

    if (len < buffer.size())
    {
        frame.insert(frame.end(), buffer_data+len, buffer_data+buffer.size());
        debugPayload("(TPL) appended  ", frame, pos);
    }

    return true;
}