What Is the Difference Between Public Key Cryptography and Public Key Infrastructure?

Public key cryptography is the use of an asymmetric algorithm. Thus, the terms asymmetric algorithm and public key cryptography are interchangeable and mean the same thing. Examples of asymmetric algorithms are RSA, elliptic curve cryptosystem (ECC), Diffie-Hellman, El Gamal, LUC, and Knapsack. These algorithms are used to create public/private key pairs, perform key exchange or agreement, and generate and verify digital signatures. Note that Diffie-Hellman can only perform key agreement and cannot generate or verify digital signatures.

Public key infrastructure (PKI) is a different animal. It is not an algorithm, a protocol, or an application—it is an infrastructure based on public key cryptography. Let’s look at why we even need PKIs today. When Erika needs to send Tanya a symmetric key securely, she must obtain Tanya’s public key. Erika could get the key from a public repository that holds public keys for many individuals, but if Lance has switched out Tanya’s public key and inserted his own, when Erika acquires a key she thinks is Tanya’s, she actually receives Lance’s key and Erika has no idea.