Suppose one wants to setup a cryptographic protocol in which 2 parties communicate using an encryption scheme that produces encrypted messages indistinguishable from random data (the desired property) given that the adversary does NOT KNOW the key to decrypt the data. (end goals are undetectability and plausible deniability)
Which component of the chosen cryptosystem provides this property?
Is it the choice between using symmetric key encryption or public-key encryption?
Or is it the choice of which block cipher operation mode you use that determines this property?
This answer on a steganography seems to imply that choosing symmetric key encryption that gives you this property. Does this imply that if you choose public-key encryption you cannot achieve the desired property? https://security.stackexchange.com/a/44295/26338
My research has led me to believe that it is your choice of block cipher operation mode that gives you this property (choosing AES-GCM gives you this property for example).
Which design choices do I have to make in order to achieve the desired property?