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 ...
Alice encrypts her data with HER secret key. Then she sends the encrypted data to Bob who should encrypt it with HIS secret key. Caroline should then eventually get this double encrypted data. ...