Passing a key to AES decryption in Crypto++ - c++

Question

I have searched a lot for this issue but did not find any solution. In my current project, I have to work on encrypting images with a sender receiver form. So i have to generate a key in the sender part to encrypt the file, and i have to use the same key (which is passed as an argument to the main) to get the original data, to continue program execution.
I save the key on a text file:
void GetKeyAndIv() {
// Initialize the key and IV
prng.GenerateBlock( key, key.size() );
prng.GenerateBlock(iv, iv.size());
};
/*********************Begin of the Function***********************/
//Function encrypt a file (original file) and store the result in another file (encrypted_file)
void Encrypt(std::string original_file, std::string encrypted_file_hex,string encrypted_file,string binary) {
ofstream out;
out.open("Key.txt");
out.clear();
out<<"key = "<< key<<endl;
out<<"iv = "<< iv<<endl;
string cipher, encoded;
//Getting the encryptor ready
CBC_Mode< CryptoPP::AES >::Encryption e;
e.SetKeyWithIV( key, key.size(), iv );
try
{
ifstream infile(original_file.c_str(), ios::binary);
ifstream::pos_type size = infile.seekg(0, std::ios_base::end).tellg();
infile.seekg(0, std::ios_base::beg);
//read the original file and print it
string temp;
temp.resize(size);
infile.read((char*)temp.data(), temp.size());
infile.close();
// Encryption
CryptoPP::StringSource ss( temp, true,
new CryptoPP::StreamTransformationFilter( e,
new CryptoPP::StringSink( cipher )//,
//CryptoPP::BlockPaddingSchemeDef::NO_PADDING
) // StreamTransformationFilter
); // StringSource
std::ofstream outfile1(encrypted_file.c_str(),ios::out | ios::binary);
outfile1.write(cipher.c_str() , cipher.size());
}
catch( const CryptoPP::Exception& e )
{
cout <<"Encryption Error:\n" <<e.what() << endl;
system("pause");
exit(1);
}
Then i pass it to the client side using the following code:
int main(int argc, char* argv[])
{
.....
string s1=argv[7];
SecByteBlock b1(reinterpret_cast<const byte*>(&s1[0]), s1.size());
string s2=argv[8];
SecByteBlock iv1(reinterpret_cast<const byte*>(&s2[0]), s2.size());
.....
}
I got an error while trying to decrypt the file, using the following code
void Decrypt(std::string encrypted_file,SecByteBlock key,SecByteBlock iv,string decrypted_file) {
string recovered;
try
{
// Read the encrypted file contents to a string as binary data.
std::ifstream infile(encrypted_file.c_str(), std::ios::binary);
const std::string cipher_text((std::istreambuf_iterator<char>(infile)),
std::istreambuf_iterator<char>());
infile.close();
CBC_Mode< CryptoPP::AES >::Decryption d;
d.SetKeyWithIV( key, key.size(), iv );
Decryption Error:
StreamTransformationFilter: invalid PKCS #7 block padding found
Which means i have different key during decryption process. Why this happened, and if anyone can help solving this issue.

Answer 1

It happen if the key used for decryption in not the same as the key that has been used for encryption.
During decryption, in PKCS#7 mode, just after decrypting the last block of 16 bytes, there is a check of padding bytes in order to know the original length of the message (which is not necessary a multiple of 16 bytes) : the last byte should be 0x01, or the last two bytes should be equal to 0x02, or the last three bytes should be equal to 0x03, ... When the decryption key is not the same as the encryption key, the padding bytes are not decrypted correctly and this implies a PKCS#7 block padding error when decrypting.

Answer 2

I change the CBC_Mode to another modes instead, ODB_Mode work for me