An algorithm using a block cipher as a primitive to provide a service is a:

1. A block cipher alone only transforms one block. 2. Something must wrap it to handle real messages. 3. A mode of operation uses a block cipher as a cryptographic primitive to provide a service. 4. So the answer is a mode of operation. _Source: NIST SP 800-175B Rev. 1, 'Guideline for Using Cryptographic Standards in the Federal Government'_