CryptoPP - Need help to decrypt String using binary key file

Hi...

I'm just started to work with CryptoPP and I need to decrypt a std::string (text)
I have a private binary key file for the decryption + encrypted file (encrypted with that key)
I didn't succeed to decrypt it, I always got exception

terminate called after throwing an instance of 'CryptoPP::DefaultDecryptor::KeyBadErr'
  what():  DefaultDecryptor: cannot decrypt message with this passphrase
Abort

Could anyone knows how to do that?

I have tried to use some methods in "test.cc":

String DecryptString(const char *instr, const char *passPhrase);

void DecryptFile(const char *in, const char *out, const char *passPhrase);


But it doesn't do what I need:
The first method only takes text not file descriptor
and the second method throw the exception above

I need your help

Thanks in advanced