How can the HC128 streamcipher be used to decrypt a ciphered message in smaller parts? - encryption

Question

I use the HC128 streamcipher in an application where the sender
applies the HC128 cipher on the message, slices the ciphered data
into (fixed size) smaller parts and sends them to the receiver.
The receiver gets these encrypted data chunks but it cannot wait until all parts
arrive to decrypt the whole message. Instead it should decrypt
the individual message parts and use them before all parts arrive.
I tried to realize this concept but it did not work: when I decrypted
the message parts I did not get the original message. Do I misuse
the HC128 streamcipher or is there a missing step? How can I
achieve the expected behavior?
The HC128 source.
#include <iostream>
#include <ecrypt-sync.h>
int main()
{
ECRYPT_ctx sender_ctx;
ECRYPT_ctx receiver_ctx;
unsigned char key[32];
unsigned char iv[32];
unsigned char plaintext[100];
unsigned char ciphertext[100];
unsigned char plaintext_receiver[100];
// SENDER
// ------
// Key and initial value for the example.
for (int i = 0; i < 16; i++)
key[i] = iv[i] = i;
// Initialize sender's encryptor.
ECRYPT_keysetup(&sender_ctx, key, 128, 128);
ECRYPT_ivsetup(&sender_ctx, iv);
// The message to send.
for (int i = 0; i < 24; i++)
plaintext[i] = i;
// Encrypt the message. The ciphertext will be sent in two parts for this example.
ECRYPT_encrypt_bytes(&sender_ctx, plaintext, ciphertext, 24);
// RECEIVER
// --------
// Initialize receiver's decryptor.
ECRYPT_keysetup(&receiver_ctx, key, 128, 128);
ECRYPT_ivsetup(&receiver_ctx, iv);
// First data chunk has received.
ECRYPT_decrypt_bytes(&receiver_ctx, ciphertext, plaintext_receiver, 12);
// Second data chunk has received.
ECRYPT_decrypt_bytes(&receiver_ctx, ciphertext + 12, plaintext_receiver + 12, 12);
for (int i = 0; i < 24; i += 1)
{
if (plaintext[i] != plaintext_receiver[i])
{
std::cout << "Error at index: " << i << "\n";
return -1;
}
}
std::cout << "Everything OK!" << "\n";
return 0;
}