If there is a TCP unencrypted packet sent to and encrypted data streaming APP what would happen?

Question

If I am using a encryption between two computers and the encryption key is predetermined (know hand shake) and an attacker can not and does not know the encryption key. If I create a TCP connection between computer A and B. If the attacker intercepts the connection and sends TCP data which is not encrypted back to one of the computers with the correct sequence number etc what would happen to that TCP packet?

Would it get modified with the decryption key and the then the application will try to run the code making error because it does not make any sense Or what what will be the result?

Would it just packet be accepted and run by the application? Or what would happen?

Answer 1

Would it get modified with the decryption key and the then the application will try to run the code making error because it does not make any sense Or what what will be the result?

There is nothing inherent in TCP which would make an application run any code send by the other side. TCP is just for data transfer. But of course there could be an application protocol on top of TCP which is used to send code to the other side of the connection and this other side could then execute the code.

The behavior then depends on this application protocol and how it deals with encrypted data. If it uses for example TLS as encryption layer and tampering with existing packets or injection of new packets will be noticed. But if you design your own application protocol and ignore such kind of attacks you might create a protocol which is vulnerable to such injections.