This question is a bit different from other questions here, but I think it is suitable to correctly understand the terminology of cryptography.
Consider the following two sets of terms:
- Encryption scheme, signature scheme
- Identification scheme, commitment scheme, secret-sharing scheme
The latter are actually "protocols", while we usually do not consider encryption/signature as (interactive) "protocols". Moreover, the word "scheme" is not usually applied to terms like "hash functions", "psudorandom generators", "one-way functions", etc.
What does the term "scheme" mean? When is it meaningful to apply this term to a cryptographic construct? How does it relate to "protocols"?