1990s: new concepts

Key derivation function (KDF)

Key stretching (password stretching) was formally defined and studied
J. Kelsey, B. Schneier, C. Hall, and D. Wagner, "Secure Applications of Low-Entropy Keys", 1997
A related concept became known as "strengthening" (throw salt away), but stretching ended up winning

Passphrase
In PGP, S/Key, SSH, encrypted filesystems