Question

One of most important use of public-key cryptosystem is to encrypt secret keys for distribution. In the following scheme, B can generate a session key Ks and use A's public PUA to encrypt the session and send to A. Is this scheme secure? What kind of attack this scheme is subject to? Describe how the attack occurs.

----- PUA || IDA ----->
[A] [B]
<----- E(PUA, Ks) -----

Answer 1

The scheme is not secure and is subject to a man-in-the-middle attack.

Step-by-step explanation:

Is the scheme secure?

The scheme described is not secure. While encrypting the session key Ks with A's public key PUA ensures confidentiality during transmission, it does not provide authentication or integrity. An attacker could intercept the encrypted session key and replace it with their own, compromising the security of the communication.

What kind of attack is the scheme subject to?

This scheme is subject to a man-in-the-middle attack. In this attack, an adversary intercepts the communication between A and B and impersonates both parties to establish separate encrypted sessions with each. The attacker can then decrypt and read the messages exchanged between A and B.

How does the attack occur?

The attack occurs when the encrypted session key E(PUA, Ks) is intercepted by the attacker. The attacker can replace the intercepted encrypted session key with their own encrypted session key, E(PUA, Ks'), before forwarding it to A. A will unknowingly use the attacker's session key, enabling the attacker to decrypt and read the messages exchanged between A and B.