Design of cryptographic protocols, i.e. ways of using algorithms (primitives) to achieve one or more security goals like integrity, confidentiality, authenticity (maybe together with non-security-related goals). If you ask about a specific protocol, tag with its name instead (or additionally, if about its design).
- Wikipedia: Cryptographic protocol
If you want to know about design of cryptographic primitives, see algorithm-design instead.
For example, many protocols involve a data compression algorithm, a secrecy-preserving encryption algorithm, and an authentication algorithm. Questions asking why we apply them in that order -- such as a or b -- should be tagged protocol-design.