Prof Bill Buchanan OBE
Nov 14, 2021
4 min read
Last we a student took one of our cryptography test and answered a question saying that Alice will use her public key to encrypt for Bob, and then Bob uses his private key to decrypt. Well, I marked it wrong … and for good reason. But there is a method that does this, and it is named…
When Alice encrypts a message using her private key before sending it to Bob What will Bob need to decrypt Alice's message?
If Bob wants to send Alice a secret message, he encrypts it with Alice's public key. Then no one but Alice can decrypt it. Only her private key can undo the encryption, and no one can figure out the private key from the public key.
How does Alice guarantee to Bob that it was her who sent the message and that the message wasn't modified during transmission without encrypting the entire message?
Generally, for signature purposes Alice will encrypt not her entire message but simply a secure hash of it, and then send both the message and the RSA-encrypted hash. Bob decrypts the encrypted hash, and the signature checks out if the result matches the hash of the corresponding message.
When Alice sends a message to Bob what key will she use to encrypt the message?
If Alice sends Bob a message and that message is encrypted with two keys simultaneously: a symmetric key (Ks) and Bob's public key. The symmetric key (Ks) is also sent to Bob, encrypted with the private key of Alice.
How does the private key decrypt the message?
Private key encryption systems use a single key that is shared between the sender and the receiver. Both must have the key; the sender encrypts the message by using the key, and the receiver decrypts the message with the same key.