Sending Confidential Messages

Sending Confidential Messages

Using Public Key Cryptography to send a confidential message

Because the keys are mutually exclusive but related to each other mathematically using a one-way function, any message that is encrypted with a public key can only be decrypted with the corresponding other half of the key pair, the private key.

Therefore, as long as the key holder keeps the private key secure, there exists a method of transmitting a message with confidentiality. The sender encrypts the message with the public key of the receiver. This ensures that only the receiver with the private key would be able to open or read the message, providing confidentiality.

Please note, all asymmetric algorithms run very slowly; this would only be useful for very small messages (i.e. 64kb). In practice, we need a hybrid approach.

Open Message

Public key cryptography can be used to achieve | other results. Assume, for example, that message confidentiality is not our goal.

Disclosure of the message is not important, but rather it may be very important to verify the identity of the sender. This goal can also be achieved using asymmetric key cryptography. In this case, the sender of the message would encrypt the message with their own private key. This would ensure that the only key that would be able to decrypt the information is the sender's public key. Because the public key is not kept secret, this method does not ensure message confidentiality. However, because the message was encrypted using the sender's private key, it offers us a way to prove that it was actually encrypted by the sender, because they must have used their own private key. The sender, at this point, cannot deny having sent the message.

Confidential Messages with Proof of Origin (Digital signature)

What if the goal is to provide confidentiality of the message and to prove the source of it? Asymmetric key cryptography can address this as well; however, it requires two encrypting steps and two decrypting steps.

In this scenario, encryption is done first using the sender's private key. A second encrypting step is now necessary. The message is encrypted again using the receiver's public key. This will ensure that only the recipient will be able to decrypt the message because as we have said before, we have to assume that their private key is kept confidential. At the other end, the following is necessary.

First, the receiver will need to decrypt using their own private key and then decrypt again using the sender's public key. This series of steps achieves two services, it proves the message came from the actual sender, and also it provides confidentiality of the message. Therefore, by encrypting a message with the private key of the sender and the public key of the receiver, the ability exists to send a message that is confidential and also has proof of origin.

Proof of Origin Operation

Confidentiality Operation