decryption vs secret writing

decryption and secret writing both are nouns.

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

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

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

Other hypernyms of decryption include coding, cryptography, steganography.

decryption (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