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Alice would like to communicate with Bob using RSA procedures. She thinks of two numbers $p = 3$, $q = 13$, and calculates $n = p · q = 39$.

Note: Only RSA trapdoor function can be used.

Bob would like to send a locked message to Alice $(A = 1, B = 2, ..., Z = 26)$ and sends each of them as separate message per letter. How to calculate encrypted message for $D$ $A$ $D$ ?

Can someone give me tip how to do it?

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    $\begingroup$ Since we don’t tend to act as a task-solving service, I hope you don’t mind me asking: What have you tried? And where exactly did you get stuck while trying? $\endgroup$ – e-sushi Dec 29 '16 at 23:52
  • $\begingroup$ I'm not asking someone to do everything just to give me begining tip. $\endgroup$ – Alena Dec 30 '16 at 10:08
  • $\begingroup$ Tip: Pick a public exponent and apply the RSA function letter-by-letter. $\endgroup$ – SEJPM Jan 3 '17 at 14:14
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You need to know public exponent e for starters. Also the letters need to be encoded or it would literally be impossible to encrypt A as it is with RSA because any exponent to 1 is still 1, and thus no mod truncation.

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