First 16 characters of decryption is garbage, the rest is decrypted correctly!

Question

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Hello,

I'm performing an encryption/decryption functions within a client/server application.

the server sends an encrypted value to the client.
NOTE: The part where I copy the encrypted value into char * have solved the problem of sending the value to the client with the first character missing.

C#

const char * Enc_Message2 = AES_Encrypt(Message2, Shared_Session_Key_S, I_V);
	cout<<"\n\nENCRYPTED MESSAGE2 {S,nb,ns}:";
	cout<<"\n============================="<<"\r\n"<<Enc_Message2;
	//cout<<"\n\nSIZE OF ENCRYPTED MESSAGE2: "<<sizeof(Enc_Message2);
	//cout<<"\n\nSIZE OF ENCRYPTED MESSAGE2 {S,nb,ns}: "<<sizeof(Enc_Message2);

	int e = strlen(Enc_Message2);
	char * enc_msg2 = new char[e+1];
	strcpy(enc_msg2, Enc_Message2);

	s5 =clientSock->SendString(enc_msg2);
	 if (s5 != true) {
		std::cout << clientSock->lastErrorText() << "\r\n";
		return;	}

Next, the client receives the value correctly and starts decrypting it, but the first 16 characters are garbage, and the rest of the decrypted value is correct.

C++

const char * Recieved_Enc_Msg2 = 0;
	 Recieved_Enc_Msg2 = socket.receiveString();
		if (Recieved_Enc_Msg2 == 0 ) {
		std::cout << socket.lastErrorText();
		return;
	}
	std::cout<<"\n\nRECIEVED MESSAGE2: "<<"\r\n"<<Recieved_Enc_Msg2;

	 const char * Dec_Msg2 = AES_Decrypt(Recieved_Enc_Msg2, s_sessionKey, iv);
	 cout<<"\n\nDECRYPTED MESSAGE2: "<<"\r\n"<<Dec_Msg2;

the following is the AES Decryption process:

C#

nst char * Client_Side::AES_Decrypt(const char * Enc_Msg2, const char * Client_sessionKey, const char * i_v)
  {
	CkCrypt2 decrypt;

	 bool success = decrypt.UnlockComponent("****************");
	if (success != true) {
		std::cout << decrypt.lastErrorText() << "\r\n";
		//return;
	}

	//  Decrypt something...
	decrypt.put_CryptAlgorithm("aes");
	decrypt.put_KeyLength(128);
	decrypt.put_CipherMode("cbc");
	decrypt.SetEncodedKey(Client_sessionKey,"base64");
	decrypt.SetEncodedIV(i_v,"base64");
	decrypt.put_EncodingMode("base64");

	//  Use an IV that is the MD5 hash of the session key...
/*	const char *iv = 0;
	iv = decrypt.hashStringENC(shared_secret_key_S);

	//  AES uses a 16-byte IV:
	cout << "Initialization Vector:" << "\r\n";
	cout << iv << "\r\n";   */


	const char * Decrypted_Message2 ;
	//Decrypted_Message1 = decrypt.decryptEncoded(rec_enc_msg1);
	Decrypted_Message2 = decrypt.decryptStringENC(Enc_Msg2);

	cout <<"\n\nLength"<< strlen(Decrypted_Message2);
	return Decrypted_Message2;
 }

What I have tried:

1- tried copying the const char * into a new char *, didn't work.

Posted 15-Dec-16 14:56pm

raniam

Updated 15-Dec-16 21:42pm

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Jochen Arndt · Answer 1 · 2016-12-15T21:42:00

You are returning a pointer to the decrypted message which is no longer valid:

const char * Client_Side::AES_Decrypt(/*...*/)
{
    CkCrypt2 decrypt;
    // ...
    const char * Decrypted_Message2 ;
    //Decrypted_Message1 = decrypt.decryptEncoded(rec_enc_msg1);
    Decrypted_Message2 = decrypt.decryptStringENC(Enc_Msg2);
    // ...
    return Decrypted_Message2;
}

CkCrypt2 will allocate an internal buffer for the decrypted string which is returned by the decrypt function. When returning from your function, the CkCrypt2 instance decrypt goes out of scope, the destructur is called, and the buffer is released. So the value returned by your function is still pointing to the buffer address but the memory content is undefined.

Also your network transfer is at least suspicious:

How does socket.receiveString() detects the end of a received string?
Is clientSock->SendString() sending a NULL terminated string or is there some kind of protocol used that contains the string length?