Alice and Bob are employees of two different financial investment companies. Alice works for the S&M investment funds that operates in Melbourne and Bob works for MK superfunds that operates in Sydney. Alice and Bob are both familiar with modern cryptograhpic tools and know how to use the tools to perform symmetric and asymmetric encryption algorithms as well as generating hash and message authentication codes. During a conference organised by both companies Alice and Bob have met and exchanged their public keys. Using the provided notation for each of the following scenarios define a protocol that achieves the required goals.

Notation: M is the message (file), H () a cryptographical strong hash function, E () as encryption algorithm corresponding to the specified keys (symmetric for secret key, asymmetric for public /private key), M AC() is a Message Authentication Code algorithm, K xy is a secret key shared between entities X and Y (symmetric algorithm), PU _x is the public key and PR _x the private key of entity X (symmetric algorithm), II is concatenation function, X → Y specifies X sends to Y.

For each of the scenarios that follows the example you need to:

• define the protocol in one line using the provided notation;
• and then explain clearly why it satisfies the requirements of the given scenario (for instance why an attacker will not succeed and how receiver can verify a whether the required property is achieved).

Example: Alice must send a message over a public network to Bob where only the confidentiality of the message is required and Alice and Bob have already a shared secret K _AB .

Protocol: A → B : E (K _AB , M)

Explanation: Since Alice and Bob have already a shared secret K _AB Alice can use a symmetric encryption algorithm to transform the message to ciphertext. To recover the original plaintext message the knowledge of the secret key is required hence and attacker such as Even who does not know the valui of the Shared secret key K _AB cannot recover the original meesage from the captured ciphertext. Bob can recover the message using the decryption algorithm and the shared secret key.

Notes:

• When the scenario asks for efficiency it means you will receive no or partial mark if your proposed protocol is not the most efficient approach that satisfies the requirements.
• You can also use “->” in the place of →, and “K_XY”, “PR_X” notation in place of K _XY , PU _X and PR _X respectively (to simplify the use of your chosen word processor).
• Use concaternation II when the protocol is comprised of multiple parts, for example M II H (M) represents the message and the hash of the message.
• Use brackets to specify the scope of a function, for example E(K _AB , M)IIH(M) and E(K _AB , MIIH(M)) are two different protocols possibly providing different services.
• You can use both symmetric and asymmetric algorithms in the same protocol when necessary.

Scenarios: Alice need to send a IGB (GigaByte) file to Bob over a public network.

1. Explain how Alice could efficiently send the file when it contains public information that must not be tampered with and Bob must know Alice is the only entity who could have sent the file.
2. Explain how Alice could efficiently send the file when it contains confidential information and Bob must know Alice is the only entity who could have sent the file, if Alice and Bob already have shared secret.
3. Explain how Alice could efficiently send the file that needs to be confidential to Bob when they have not a shared secret.

Explain how Alice could efficiently send the file when it contains public information that must not be tampered with (only integrity is important) and Alice and Bob already have a shared secret.