1) Let's say $n$ parties hold additive shares of a value $a$. The parties want to compute the value $2a$.
  (a) How would the parties coordinate to achieve this?
  (b) Let's the n-th player becomes malicious, and wants to change the result to be $2a + 100$. How could she achieve this?

2) SPDZ uses multiplication triples to implement multiplication gates. Let's say we have two parties A and B that want to generate a multiplication triple. The only primitive they have on hand is a additively homomorphic encryption scheme `Enc`. Enc supports adding ciphertexts, multiplying ciphertexts with a plaintext, and adding a plaintext to a ciphertext. A and B come up with the following protocol:

  ```
  1. A generates a public key pk for Enc, samples a random value $a$, and sends $Enc(a) to B. 
  2. B $Enc(a)$ with its own random value b, and then adds a random mask $r$, thus obtaining a ciphertext $Enc(ab + r)$. 
  3. B sends this back to A, who decrypts it to obtain $ab + r$. 
  4. A and B thus hold shares of $a$, $b$ and $c = ab$.
  ```

  Is this protocol secure? If yes, give a one-sentence explanation. If not, describe a one-sentence attack.