Question

In this problem we will compare the security services that are provided by digital signatures (ds) and message authentication codes (mac). We assume that Oscar is able to observe all messages sent from Alice to Bob and vice versa. Oscar has no knowledge of any keys but the public one in the form of ds. Answer whether and how (i) ds and (ii) mac protect against each of the following attacks and explain your answers. The value auth(x) is computed with a ds or a mac algorithm, respectively. Here, x is just the message (without additional information). (Message integrity) Alice sends a message x - 'Transfer $1000 to Mark' in the clear and also sends auth(x) to Bob. Oscar intercepts the message and replaces 'Mark' with 'Oscar.' Will Bob detect this?

1) Yes, Bob will detect the modification because digital signatures provide message integrity.
2) No, Bob will not detect the modification because digital signatures do not provide message integrity.
3) Yes, Bob will detect the modification because message authentication codes provide message integrity.
4) No, Bob will not detect the modification because message authentication codes do not provide message integrity.

Answer 1

Bob will detect the modification whether a digital signature or a MAC is used because both provide message integrity. Digital signatures use asymmetric keys for signing and validation, while MACs rely on a shared secret key, but both ensure the authenticity of the message.

Step-by-step explanation:

The security services provided by digital signatures (ds) and message authentication codes (mac) are essential in ensuring the integrity and authenticity of messages exchanged between parties. When Alice sends a message x - 'Transfer $1000 to Mark', she includes auth(x), which can be computed either using a ds or a mac algorithm. If Oscar intercepts this message and modifies 'Mark' to 'Oscar', the security outcomes will be different based on whether a ds or a mac is used.

Case for Digital Signatures:

Digital signatures rely on asymmetric cryptography where a private key is used for signing and a public key is used for verification. When Bob receives the modified message with the original auth(x), he will use Alice's public key to verify the signature. If it does not match the altered message, the change will be detected, hence, Bob will detect the modification because digital signatures provide message integrity.

Case for Message Authentication Codes:

On the other hand, MACs use a shared secret key between the sender and receiver. If Oscar does not have access to this key, he cannot generate a valid mac for the modified message. When Bob computes the mac using the shared secret key and it does not match the received auth(x), the alteration will be discovered, thus, Bob will detect the modification because message authentication codes provide message integrity.