What type of algorithm uses the same key to encryp and decrypt data?

A symmetric algorithm uses one key for both encryption and decryption. Examples include AES, DES, and 3DES. Because both parties must possess the same secret key, secure key exchange is the primary challenge of symmetric cryptography.

An asymmetric algorithm uses a mathematically linked key pair - a public key to encrypt and a private key to decrypt, which is the opposite of symmetric behavior.

Public Key Infrastructure (PKI) is a framework for managing digital certificates and key pairs, not a specific algorithm type defined by single-key usage.

IPSec is a protocol suite used to secure IP communications at the network layer and is not a classification of encryption algorithm based on key usage.