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I understand the concept of the hybrid-cryptosystem that OpenPGP uses by generating a random symmetric key, using that to encrypt the message before encrypting itself with the recipient's public key.

However, in this paper describing OpenPGP, it describes the process the other way: encrypting the symmetric key, before using that encrypted key to encrypt the message. Is this correct?

https://tools.ietf.org/html/rfc4880#section-2.1

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The step explained in the paper is correct. The confusion you probably are facing is with the key exchange step I guess.

Let me explain it further.

Let A be the sender and B, the recipient.

  • A generates the message - M
  • A generates a pseudorandom session key - K Key for symmetric encryption
  • A encrypts the message M with session key K - E(K,M) Message encrypted with symmetric encryption
  • A encrypts the session key K with public key of B - E(KBPUB, K) K encrypted with public key of B so that B can retrieve K with it's private key.
  • A appends the results of step 3 and 4 and sends it to B - E(K,M) || E(KBPUB, K)

  • B receives E(K,M) || E(KBPUB, K)

  • B retrieves K from E(KBPUB, K) -> K = E[KBPRIV, E(KBPUB, K)] Retrieving K by decrypting it with private key of B.
  • B uses K to retrieve M from E(K,M) -> M = D[K, E(K,M)] Decrypting the symmetrically encrypted message.

Your confusion is in the 3rd and 4th steps. Te order of 3rd and 4th steps doesn't matter as you see from the above.

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    I think the confusion was from me thinking that it was using the encrypted symmetric key to encrypt the message, but instead they both use the original session key instead – user4191887 Nov 7 '16 at 21:04

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