Which is stronger: 

asymmetric or 

symmetric keys? 

Which is Stronger: Asymmetric or Symmetric Keys?

Since asymmetric keys are bigger than symmetric keys, data that is encrypted asymmetrically is tougher to crack than data that is symmetrically encrypted. However, this does not mean that asymmetric keys are better. Rather than being compared by their size, these keys should be compared by the following properties: computational burden and ease of distribution.

Symmetric keys are smaller than asymmetric, so they require less computational burden. However, symmetric keys also have a major disadvantage—especially if you use them for securing data transfers. Because the same key is used for symmetric encryption and decryption, both you and the recipient need the key. If you can walk over and tell your recipient the key, this isn’t a huge deal. However, if you have to send the key to a user halfway around the world (a more likely scenario) you need to worry about data security. Asymmetric encryption doesn’t have this problem. As long as you keep your private key secret, no one can decrypt your messages. You can distribute the corresponding public key without worrying who gets it. Anyone who has the public key can encrypt data, but only the person with the private key can decrypt it.

How does TLS/SSL use both asymmetric and symmetric encryption? 

Public Key Infrastructure (PKI) is the set of hardware, software, people, policies, and procedures that are needed to create, manage, distribute, use, store, and revoke digital certificates. PKI is also what binds keys with user identities by means of a Certificate Authority (CA). PKI uses a hybrid cryptosystem and benefits from using both types of encryptions. For example, in TLS/SSL communications, the server’s TLS certificate contains an asymmetric public and private key pair. The session key that the server and the browser create during the SSL Handshake is symmetric.