Convergent generates a DEK depending only on ptextHash.

Copy copies the data at ref from src to dst.

DeriveKey uses the blake2b XOF to fill out.

DeriveStream returns a cryptographically secure psuedorandom stream derived from a high entropy secret, and arbitrary additional bytes.

SaltedConvergent uses salt to generate convergent keys for each blob.

KeyFunc produces a key for a given blob.