Why not use symmetric encryption?

Technology CommunityCategory: CryptographyWhy not use symmetric encryption?
VietMX Staff asked 3 years ago

Ok, most of us communicate most of the time with people we already know. Let’s say, Alice and Bob probably have physical contact, too. Symmetric encryption therefore is appropriate and simpler than asymmetric – one key instead of four for Alice and Bob. But how do you share the secret key? You tell them in person, read it over the phone, use a One Time Secret or Diffie-Hellman. Why not to use symmetric encryption? Have I missed something?

Arguments to mention:

  1. It’s 21 century. You talk to machines more often than you talk to persons. In particular every time you visit an “https://” Web site. How would you, precisely, read a key over the phone with a server to keep it secret from 3-rd party? Public key will help in that case.
  2. Let’s imagine you have 1000 friends. If you talk to 1000 persons and use pre-shared symmetric keys, then you have to remember 1000 secret keys. Secret key storage can be hard. With asymmetric encryption you just have to remember only public keys, and that is easy because public keys are public, so they can be “remembered” by being published somewhere, where everybody can see them.
  3. Another example. Suppose you have a symmetric key K. Alice, Bob, and Eve share this key K so that they can all trade encrypted messages. However, Eve gives a copy of K to Mallory (who has malicious intent) without telling anyone. Now the entire network has been compromised, and Mallory can send and read messages as any other member.
  4. If instead, Alice, Bob, and Eve each had their own private keys AB and E respectively (with public keys A+B+ and E+), then when Mallory gets ahold of E, all she can do is read messages intended for Eve, and send messages that it looks like Eve encrypted. We go from a situation in which Eve has compromised the entire network, to a situation where Eve’s poor choice only affects messages to/from her.
  5. Even if Alice and Bob could have key K1, Bob and Eve could have key K2, Alice and Eve can have key K3. Then if Mallory gets K3, she can still only intercept Alice/Eve communication, nothing else. Group messages can be handled by sending copies encrypted separately. However, then you have to keep track of n private keys instead of (n-1) public keys and 1 private.