Public Key Cryptography
I am a private key and I have no identity and everything between you and me must go through my friend public key
Public Key Infrastructure
I am a private key and I have no identity and everything between you and me must go through my friend public key who has an identity signed by a certification authority
Distributed Ledger Technology
I am a private key and I have no identity and everything between you and me must go through my friend public key and a transaction using distributed ledger technology