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Internet Security - Dang Thanh Binh's Page

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92 INTERNET SECURITY<br />

3.3.4 Decryption<br />

RC5 decryption is given in the pseudocode as shown below.<br />

For i = r downto1do<br />

B = ((B − S[2i + 1]) >>> A) ⊕ A<br />

A = ((A − S[2i]) >>> B) ⊕ B<br />

B = B − S[1]<br />

A = A − S[0]<br />

The decryption routine is easily derived from the encryption routine. The RC5 encryption/decryption<br />

algorithms are illustrated as shown in Figures 3.10 and 3.11, respectively.<br />

Example 3.15 Consider the decryption problem of RC5-32/12/16. To decrypt the ciphertext<br />

obtained in Example 3.14, the output of round 11 is inputted into two 32-bit<br />

A B<br />

S[0] S[1]<br />

S[2i]<br />

A B<br />

Figure 3.10 RC5 encryption algorithm.<br />

S[2i + 1]<br />

Repeat for<br />

i rounds

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