Why would you want to use a digital envelope? As I understand it, a plain-text message is encrypted using a secret key and then the key is encrypted with the receiver's public key. The encrypted message and the encrypted key is sent as the digital envelope. Now, what I don't understand is why one just don't encrypt the plain-text message with the public key in the first place? The literature I have been reading states that the secret key used to encrypt the message is a one-time key, so wouldn't this render the digital envelope unnecessary? I would understand the purpose of a digital envelope if it was to encrypt the secret key for continued communication, but according to the literature, this does not seem to be the case.

  • $\begingroup$ The technique you describe is also known as "hybrid encryption". See crypto.stackexchange.com/questions/31234/… (and also crypto.stackexchange.com/questions/14/… and crypto.stackexchange.com/questions/5782/…) for reasons to use it. That said, I think this a good duplicate question, since none of the earlier ones mention the term "digital envelope". $\endgroup$ – Ilmari Karonen Jan 7 '17 at 19:37
  • $\begingroup$ A search for digital envelopes did not yield any results. While it is true that the answer to why hybrid encryption is used can be found in other questions, the concept of digital envelopes is never mentioned, as Ilmari Karonen stated as well. I don't know if this is enogh to consider the question a non-duplicate, but it sure would have helped me if this question was around since I didn't know the term hybrid encryption. $\endgroup$ – Safe Jan 7 '17 at 21:44

why one just don't encrypt the plain-text message with the public key in the first place?

If you want to send more than one message, the client and server need a shared secret that they can both use to encrypt and decrypt with.

Speed is very important. Encrypting with a symmetric key is much faster than using a set of asymmetric keys when the plaintext becomes longer.

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