Cryptography Concepts Quiz

Choose the incorrect option(s) from the following:

Which of the following is/are true for the hash-function based commitment scheme?

Choose the correct option(s) from the following:

There exists a server A, who wants to broadcast messages to n recipients B1, . . . , Bn in an authentic manner. We are not interested in the privacy, but rather authenticity and integrity, where each user should be assured that the messages it receives are indeed sent by A. For this, the users decide to use a MAC, where server A shares a secret MAC key k with B1, . . . , Bn and each message from the server to the receivers is accompanied by a MAC tag, computed with key k.

Let Π = (Gen, Enc, Dec) be a symmetric-key encryption scheme with message-space and ciphertext space M and C = {0, 1} ` respectively. Then consider a variant Π0 = (Gen, Enc’, Dec’) with message-space and ciphertext space M and C 0 = {0, 1} `+1 respectively, where Enc0 k (m) = Enck(m)||0 and Dec’k(c) = Deck(c[0, . . . , ` − 1]). Here c ∈ {0, 1} `+1 and c[0, . . . , ` − 1] denotes the first ` bits of c.

Choose the right option(s) for the authenticate-then-encrypt approach which is instantiated with a CPA-secure encryption scheme and an SCMA-secure MAC:

In the key-exchange problem, the goal of the sender and receiver is to:

Which of the following is/are sufficient to build an authenticated encryption scheme?

Let sender and receiver have a pre-shared, random and private AES key k. Then consider the following method of authen- ticating messages of size which is a multiple of 64 bits: to authenticate a message m ∈ {{0, 1} 64} ≤` containing ` blocks m1, . . . , m` each of size 64 bits, the tag-generation algorithm outputs t = (t1, . . . , t`) as the tag, where ti = AESk(mi || < ` >). Here < ` > denotes a 64-bit representation of the integer `, the number of blocks in m. Accordingly, the tag- verification algorithm performs the corresponding verification steps. Identify the false statement(s) from the following.

Consider a secure PRF Fk : {0, 1} n → {0, 1} n, using which we construct a keyed function F 0 k : {0, 1} 2n → {0, 1} 2n, where F 0 k (x1||x2) def = Fk(x1)||Fk(Fk(x2)) and x = x1||x2, where x1, x2 ∈ {0, 1} n.