As nouns the difference between legitimacy and genuineness
is that legitimacy is the quality of being legitimate or valid; validity while genuineness is the quality of being genuine; authenticity.
legitimacy
English
Noun
(head)
the quality of being legitimate or valid; validity
Synonyms
* legitimateness (much less common)
genuineness
English
Noun
(-)
The quality of being genuine; authenticity.