Symmetric key cryptography is an application of functions. An encryption function turns readable plain text into unreadable cipher text, and the corresponding decryption function turns the cipher text back into the original plain text.
Figure8.9.Alice sends a message to Bob, and Eve eavesdrops on their conversation.
Descriptions of cryptographic protocols are commonly phrased as interactions between Alice, Bob, and Eve. Alice sends a message to Bob, and the eavesdropper Eve listens in on their conversion and tries to break their encryption (Figure 8.9).
Figure8.10.In a symmetric key encryption scheme, Alice and Bob share a common secret, namely the key \(K\text{.}\)
In a symmetric key encryption scheme, Alice and Bob first have to agree on a common shared key. Alice uses the key to encrypt a message and sends the encrypted message to Bob. Then, Bob uses the key to decrypt the encrypted message that was sent by Alice in order to obtain the message in its original form (Figure 8.10). We introduce Caesar ciphers and other substitution ciphers as examples for symmetric key encryption scheme in the following two sections.
Checkpoint8.11.Symmetric key cryptography.
Complete the following.
In a symmetric encryption protocol
select
Alice
Bob
Eve
Oscar
and
select
Alice
Bob
Eve
Oscar
agree on an encryption method, a decryption method, and a key that is used for encryption and decryption.
select
Alice
Bob
Eve
Oscar
encrypts a message using the encryption method and the key. She sends the encrypted message to
select
Alice
Bob
Eve
Oscar
.
select
Alice
Bob
Eve
Oscar
receives the message from
select
Alice
Bob
Eve
Oscar
and decrypts the message using the decryption method and the key.
We summarize this introduction to symmetric key cryptography in the video in Figure 8.12.
Figure8.12.Symmetric Key Cryptography by Matt Farmer and Stephen Steward
In the following two sections we give two examples for symmetric key encryption schemes, namely Caesar ciphers and more general substitution ciphers.
Girl with black hair stands in the frame, talking to the reader.
Girl: I’m sure you’ve heard all about this sordid affair in those gossipy cryptographic protocol specs with those busybodies Schneier and Rivest, always taking Alice’s side, always labeling me the attacker.
Girl: Yes, it’s true. I broke Bob’s private key and extracted the text of her messages. But does anyone realize how much it hurt?
Girl: He said it was nothing, but everything from the public-key authenticated signatures on the files to the lipstick heart smeared on the disk screamed "Alice."
Girl: I didn’t want to believe. Of course on some level I realized it was a known-plain-text attack. But I couldn’t admit it until I saw it for myself.
Girl places her hands on her hips. Girl: So before you so quickly label me a third party to the communication, just remember: I loved him first. We had something and she tore it away. She’s the attacker, not me. - Not eve.
Yet one more reason I’m barred from speaking at crypto conferences.
Yet one more reason I’m barred from speaking at crypto conferences.
