Quadral 18: Sealed with a Kiss Very Hard

One word:

  1. It is used in law to show authorization or approval.
  2. It is used in biology to distinguish or classify an organism.
  3. It is used in digital security to verify integrity or source.
  4. (Twist) Must suggest a counterpart, a match, or something that's returned in kind.

AI will judge if your word fits—and if it's better than the AI's answer.

Need a hint?

The AI answer is also known by the name of a U.S. founding father.