Question

Two parties, A and B, need to perform encryption. They have one symmetric key between them. An intruder acquires knowledge of the key. What is the impact on secrecy of previously exchanged messages? What would be the impact on secrecy of future messages? Is there any harm in A sending a plaintext message to B saying the key is no longer secret? Should A send that message encrypted with the now-exposed key? Explain your answers.

(Computer Security II)

Answer 1

Symmetric-key algorithms are algorithms that use the same cryptographic keys for both encryption of plain text and decryption of cipher text. It is also called secret key algorithm as  key used is kept secret by the systems engaged in both the encryption and decryption processes.

One of the main disadvantage of symmetric key cryptography is that it uses only one key .

If the intruder acquires the key, then it can be a possibility that he can decrypt the message .

Also, he can alter the key, encrypt the message and send it to the destination system (engaged in the Symmetric cryptgraphic algorithm). There will be no secrecy in future communication between the system

Now, if A knows that  the key is already exposed then A should encrypt the message with a new encryption key rather just sending it in a plain text as this will provide confidentiality again.

The main disadvantage of this technique is that it provides confidentiality but not integrity as we do not know the sender is valid or not.