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Table 4.3
Primitive Operations of RC5
a + b
Integer addition modulo 2 w
a - b
Integer subtraction modulo 2 w
a b
Bitwise XOR of w -bit words
a * b
Integer multiplication modulo 2 w
a <<< b
Rotate the w -bit word a to the left by the amount
given by the least significant l g w bits of b
a >>> b
Rotate the w -bit word a to the right by the amount
given by the least significant l g w bits of b
A i-1
B i-1
<<<
<<<
S 2i
+
S 2i+1
+
A i
B i
Figure 4.6
RC5 w/r/b symmetric block cipher diagram.
expansion has already been performed so that S [0], S [1], …, S [ t - 1]
have been computed. The steps of the encryption algorithm can be
summarized as follows:
A = A+S [0];
B = B+S [1];
For i = 1 to r do
A = ((A B) <<< B) +S [2i];
B = ((B A) <<< A) +S [2i+1];
End
In each round of RC5, both registers A and B are updated as shown
in Figure 4.6.
The decryption step can be sum-
4.5.6.2 RC5 Decryption Algorithm
marized as follows:
For i = r downto 1 do
B = ((B - S [2i+1]) >>> A) A;
A = ((A - S [2]) >>> B) B;
 
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