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Q1. I do not understand why e should be public? It may be more secure to keep it private and known only to the sender and receiver.

Q2. I need comments on the following proposed algorithm: Both sender and receiver have their own encryptors and decryptors $(e_1,d_1)$ and $(e_2,d_2)$. Only p is known in public.

i- A message $m$ is sent as $m^{e_1}~mod~p.$

ii- It is returned as $m^{e_1\ast e_2}~mod~p.$

iii- It is sent back as $m^{e_1\ast e_2\ast d_1}=m^{e_2}~mod~p.$

iv It is decrypted as $m^{e_2\ast d_2}~mod~p$

Thank you for your help.

zoli
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e.ahmed
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  • I know that the second algorithm is Shamir 3 pass protocol. But can one compare between it and RSA? Thanks – e.ahmed May 09 '15 at 09:19

1 Answers1

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A1. There is one practical reason I can think of. In public key cryptography you have two basic scenarios:

  1. The first is to use you private key to sign a message and the other end verifies the message using your public key. In this case you already know the receiver so it make sense to provide your public key only to the receiver.
  2. The second is when someone wants to send to you an encrypted message. The sender needs your public key to encrypt the message and the receiver (you) decrypts it using the private key. In this scenario you are not able to know who might want to send you a message (eg: an encrypted email) so you have to publish your public key.
George Gkitsas
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