on 2.3

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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 (Lo, Ro), and the key is

divided into four 32-bit blocks (Ko, K₁, K2, K3). Encryption involves repeated appli-

cation of a pair of rounds, defined as follows for rounds i and i+1:

L₁ = Ri-1

R₁ = Li-F(R-1, Ko, K₁, 8;)

Lit1 = R₁

Ri+1= LiF(R₁, K2, K3, Si+1)=

where F is defined as

F(M, Kj, Kk, 8;)

((M<<< 4) EK;) ((M » 5) EK) → (M= 8;)

Jugni tol

and where the logical shift of x by y bits is denoted by x <

of x by y bits is denoted by x >> y, and 8; 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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