[12 points] This problem is about the security services each protocol provides. Let us consider four security services: Confidentiality (C), Integrity (I), sender Authentication (A), and Non-Repudiation (NR). Notes: assume that the public key (and its private key) cannot be forged and is authentic. So, if a signature of a message can be verified via the corresponding public key, the sender will not be able to deny having sent the message. Suppose the following notations are used: k1,k2 : keys shared between a sender S and a receiver R;Ek(x) : (symmetric or asymmetric) encryption of x under k; SIGk(x) : signature on x under k;Xpri : private key of entity X;Xpub: public key of entity X;H : a public secure cryptographic hash function such as SHA1;PRNGs: a binary stream from a pseudo-random number generator seeded with s; : simple concatenation; and M : the message. For each of the following protocols, use C,I,A, and/or NR to represent the service(s) a protocol provides. If a protocol can not provide any service, use "None". (a) S generates a random session key sk and sends ESpub(sk)ERpub(sk)(M PRNGsk ) to R. (b) S sends y=Ek1(xH(k2x)) to R. (c) S sends y=ERpub(xSIGSpri(H(x))) to R. (d) S generates a new symmetric key sk and sends y=ESpub(sk)ERpub(sk)SIGSpri(sk)Esk(x) to R