The first implementation of identity-based signatures and an email-address based public-key infrastructure (PKI) was developed by Adi Shamir in 1984,[1] which allowed users to verify digital signatures using only public information such as the user's identifier.
Under Shamir's scheme, a trusted third party would deliver the private key to the user after verification of the user's identity, with verification essentially the same as that required for issuing a certificate in a typical PKI.
However, he was unable to come up with a concrete solution, and identity-based encryption remained an open problem for many years.
Identity-based systems allow any party to generate a public key from a known identity value, such as an ASCII string.
[8] This limitation may be overcome by including a time component (e.g. the current month) in the identity.