Extracted text: Recall that a protocol specification is said to be non-executable when the principals cannot construct the messages that the protocol requires them to send. Consider the following variants of the Needham-Schroeder Public-Key Protocol and identify the ones that are executable AND do not suffer from the man-in-the-middle attack. A - B: {N1, N2, A}x, B → A : {N1, N2, N3, B}x, A → B: {N1, N2}x, a. b. A - B: {N1, A}x, В - А: (NI,N2, В}к. A → B: {N1, N2}x, c. A - B: {N1, A}x, B → A : {NI, A, N2, N3, B}K, A → B : {N2, N3}x, d. A - B: {N1, A}x, B → A : {N2, N1, B}K, A → B: {N2}x, e. A - B: {NI, A}x, B → A : {N1, N2, B}x, O f. A- B: {NI, B}x, B → A : {NI, B,N2}x, A - B: {N2}x,