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In Pretty Good Privacy, it is said that Sender sends compressed message and encrypts the message with the private key of himself. And then Receiver will decrypt that message with the public key of the Sender.

So, my question is that Public key is known to everyone, so how it is secured?

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So, In this scenario confidentiality do matters that's why in later scenario of Pretty Good Privacy,

Confidentiality is observed with one-time session key. So, Receiver decrypts the shared session key with the help of his private key. Then, uses this session key to encrypt the rest of the message. Then, matches the hash of the message with the digest created by the Sender.

Confidentiality with One-Time Session key

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Sender sends compressed message and encrypts the message with the private key of himself and then Receiver will decrypt that message with the public key of the Sender.

This explanation of PGP's asymmetric encryption is completely wrong. In simple terms, to get a message to Receiver, Sender will encrypt it with the public key of Receiver ... and then Receiver will decrypt it with their own private key.

Detail:

  • When Sender wants to get an encrypted message to Receiver, a PGP program will symmetrically encrypt the message with a random session key and then for each intended recipient, a copy of that session key is encrypted with the recipient's public key.
  • Optionally, a copy of the session key is also encrypted with a simple passphrase.
  • The final PGP message encapsulates all that together (as well as signature information, potentially).
  • When Receiver gets the message, they must use their private key to decrypt it.

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