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

A symmetric algorithm such as AES or DES uses one secret key that both the sender and receiver must possess. The same key is applied during encryption to produce ciphertext and during decryption to recover the original plaintext, requiring secure key distribution between communicating parties.

An asymmetric algorithm uses two mathematically linked keys - a public key for encryption and a private key for decryption (or vice versa for digital signatures), not the same key for both operations.

Public Key Infrastructure (PKI) is a framework of policies, certificates, and certificate authorities for managing asymmetric keys, not a type of encryption algorithm itself.

IPsec is a protocol suite for authenticating and encrypting IP communications at the network layer; it can use symmetric or asymmetric algorithms but is not a key type or algorithm category.