2.3 Perhaps the simplest "serious" symmetric block encryption algorithm is the Tiny Encryption Algorithm (TEA). TEA operates on 64-bit blocks of plaintext using a 128-bit key. The plaintext is divided into two 32-bit blocks (L0, R0), and the key is divided into four 32-bit blocks (K0,K1,K2,K3). Encryption involves repeated application of a pair of rounds, defined as follows for rounds i and i 1: Li Ri 1 Ri Li 1 F(Ri 1, K0, K1, δ i) Li 1 Ri Ri 1 Li F(Ri, K2, K3, δ i 1) where F is defined as F(M, Kj, Kk, δ i) ((M 4) Kj) ((M 5) Kk) (M δi) and where the logical shift of x by y bits is denoted by x y, the logical right shift of x by y bits is denoted by x y, and δ i is a sequence of predetermined constants. a. Comment on the significance and benefit of using the sequence of constants. b. Illustrate the operation of TEA using a block diagram or flow chart type of depiction. c. If only one pair of rounds is used, then the ciphertext consists of the 64-bit block (L2, R2). For this case, express the decryption algorithm in terms of equations. d. Repeat part ( c) using an illustration similar to that used for part (b). | |
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