Aug2016 cs Q30

0. The plain text message BAHI encrypted with RSA algorithm using e = 3, d = 7 and n = 33 and the characters of the message are encoded using the values 00 to 25 for letters A to Z. Suppose character by character encryption was implemented. Then, the Cipher Text message is _____.

Answer
Comment
Array

Option : B
Explanation :
RSA algorithm using e = 3, d = 7 and n = 33 and text message BAHI(using the values 00 to 25 → A(00), B(01), C(02),............). for
B → [m = c^d mod n]
i.e. m = 1⁷ mod 33
m = 1 mod 33.
m = 1 → B
A → 0⁷ mod 33
m = 0 mod 33.
m = 0 → A
m = 7⁷ mod 33
m = 13 → N
m = 8⁷ mod 33
m = 17 → R
Message will be BANR (using the values 00 to 25 → A(00), B(01), C(02),............). No option is matching. But if we use the values 01 to 26 → A(01), B(02), C(03)....... Then
B → [m = c^d mod n]
i.e. m = 2⁷ mod 33
m = 8 mod 33.
m = 8 → H
A → 1⁷ mod 33
m = 1 mod 33.
m = 1 → A
m = 8⁷ mod 33
m = 17 → Q
m = 9⁷ mod 33
m = 3 → C
Message will be HAQC.
So, option (B) is correct.

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