PKI must use asymmetric encryption because it is managing the keys in many cases. This implies the use of public and private key pairs, which is asymmetric.
Mostly for performance - symmetric encryption is much much faster (order of magnitudes) than asymmetric encryption.
If you are using an encrypted channel, then at some point everyone uses symmetric encryption. It is fast (compared to asymmetric).The first part of an encrypted conversation will probably use asymmetric encryption to provide the shared private key that is later on used for the bulk part of data conversations on an encrypted channel.
You can actually use both; the problem with symmetric encryption is how to get the shared key to someone else in a secure manner.The problem with asymmetric encryption is that it is too slow to be used to general communications.So, using both can solve the problems of drawbacks to just using one or the other.
Symmetric cryptography uses the same secret (private) key to encrypt and decrypt its data whereas asymmetric uses both a public and private key. Symmetric requires that the secret key be known by the party encrypting the data and the party decrypting the data. Asymmetric allows for distribution of your public key to anyone with which they can encrypt the data they want to send securely and then it can only be decoded by the person having the private key. This eliminates the need of having to give someone the secret key (as with symmetric encryption) and risk having it compromised. The issue with asymmetric is that it is about 1000 times slower than symmetric encryption which makes it impractical when trying to encrypt large amounts of data. Also to get the same security strength as symmetric, asymmetric must use strong a stronger key than symmetric. If you do a quick search on the differences between symmetric and asymmetric you can find many more explanations. This just a quick overview.
Smart cards typically use a combination of symmetric and asymmetric encryption. Common algorithms include AES (Advanced Encryption Standard) for symmetric encryption and RSA or ECC (Elliptic Curve Cryptography) for asymmetric encryption. These encryption methods help secure the data stored on the card and facilitate secure communication between the card and external systems. Additionally, smart cards often implement secure key management practices to protect sensitive cryptographic keys.
From another user: Symmetric cryptography uses the same secret (private) key to encrypt and decrypt its data whereas asymmetric uses both a public and private key. Symmetric requires that the secret key be known by the party encrypting the data and the party decrypting the data. Asymmetric allows for distribution of your public key to anyone with which they can encrypt the data they want to send securely and then it can only be decoded by the person having the private key. This eliminates the need of having to give someone the secret key (as with symmetric encryption) and risk having it compromised. The issue with asymmetric is that it is about 1000 times slower than symmetric encryption which makes it impractical when trying to encrypt large amounts of data. Also to get the same security strength as symmetric, asymmetric must use strong a stronger key than symmetric.
use/ application
The most common hybrid system is based on the Diffie-Hellman key exchange, which is a method for exchanging private keys using public key encryption. Diffie-Hellman key exchange uses asymmetric encryption to exchange session keys. These are limited-use symmetric keys for temporary communications; they allow two entities to conduct quick, efficient, secure communications based on symmetric encryption, which is more efficient than asymmetric encryption for sending messages. Diffie_Hellman provides the foundation for subsequent developments in public key encryption. It protects data from exposure to third parties, which is sometimes a problem when keys are exchanged out-of-land.
Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption.
Asymmetric encryption employs the use of public/private key pairs.
A VPN primarily uses link encryption, which secures the data transmitted between your device and the VPN server. This encryption protects the data from interception during transit over the internet. However, it does not provide end-to-end encryption, meaning that once the data reaches the VPN server, it may be decrypted before being sent to its final destination, potentially exposing it to further risks.
PGP (Pretty Good Privacy) encryption works by using a combination of symmetric and asymmetric encryption techniques. When a user wants to send a secure message, PGP generates a random symmetric key to encrypt the message. This symmetric key is then encrypted using the recipient's public key before sending the message. The recipient can use their private key to decrypt the symmetric key, which can then be used to decrypt the original message.