One of the major paradoxes of Zen, especially in medieval Japan, involved the importance of what can only be called enlightenment certification which was a legal matter that required a legal document. Given that the authentic transmission in Zen is considered to be a "Mind to Mind transmission" (J., ishin denshin) which is wholly spiritual, that it should have to be further instantiated by a legal document is certainly paradoxical.
Adding to the seriousness of this paradox there is no record of the Buddha, himself, being transmitted by another. When we look through the Pali canon to shed some light on certification it strikes us that the Buddha’s own charisma was more likely the Buddha’s imprimatur—not a piece of paper. And even this may not have been very effective.
According to the Ariyapariyesana Sutta (M.i.160), when challenged by an Ajivika ascetic named Upaka as to the basis of his recent enlightenment, the Buddha replies with some bravado, “By knowing for myself, whom should I point to? For me there is no teacher—one like me does not exist in the world with its gods. No one equals me." However, Upaka is not impressed with the Buddha’s words who, shaking his head, walked away saying, "May it be so, my friend."
Turning to the realpolitik of certification, to be able to obtain such a certificate, technically called an “inheritance certificate”, often meant the difference between being a heir to a thriving religious business, in a manner of speaking, or being a nobody. Faced with this, some monks, desperate I suppose, were not adverse to using underhanded methods to obtain a certificate—including forgery. Supposedly, even the certificate (J., Shisho) given to Dogen by his teacher Ju-ching turns out to have been a medieval forgery (Steven Heine, Did Dogen Go To China?, 260).
As modern scholars deconstruct the religious institution of Zen, in effect deflating its once powerful myths, modern Zennists may have no choice but to look at Zen the way they might view the Tao To Ching of Lao-tzu which is understood to be “philosophical Taoism” which is not in the same ballpark as “religious Taoism”.