decipherment vs secret writing

decipherment and secret writing both are nouns.

Word	Noun	Adjective	Verb	Adverb
decipherment	Yes	No	No	No
secret writing	Yes	No	No	No

As nouns, secret writing is a hypernym of decipherment; that is, secret writing is a word with a broader meaning than decipherment:

decipherment: the activity of making clear or converting from code into plain text
secret writing: act of writing in code or cipher

Other hypernyms of decipherment include coding, cryptography, steganography.

decipherment (noun)	secret writing (noun)
the activity of making clear or converting from code into plain text	act of writing in code or cipher
	a piece of writing in code or cipher