certifier; authenticator
A person or entity that certifies or authenticates something, often in official, legal, or technical contexts.
この書類には認証者の署名が必要です。
This document requires the signature of a certifier.
彼は公的な認証者として任命された。
He was appointed as an official certifier.
Compound of 認証 (ninshō, 'certification') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the components are transparent.