You have said multiple times that the outer wrapper was CC, and the offline, inner wrapper was KC. All they need to do is infer it is CC output, either through online attacks, anomalies in its statistical distribution, block size, etc. Also, certain modes like ECB have watermarking attacks, which inherently reveal that ECB was used.

RC4 is a common stream encryption which can definitely be detected. In contrast AES seems much harder.

It is called a Distinguishing attack. See: https://eprint.iacr.org/2013/176.pdf http://www.security.iitk.ac.in/hack.in/2009/repository/bimal... http://elligator.cr.yp.to/elligator-20130828.pdf http://cr.yp.to/streamciphers/mag/053.pdf http://christina-boura.info/sites/default/files/KeyDifferenc...

The output of KC is not necessarily completely random. It can contain some metadata. Like "encrypted with KC4096bits, initialization vector is 490282348992489, length of ciphertext is 26728 bytes".

When you pass this through a bad cipher it may create a characteristic fingerprint.

I think you confuse cipher with protocol.

To make it clear: for me a cipher is a bijective function on arrays of a predefined length.

What do you propose doing with the initialization vectors then? Afaik, all commonly used encryption algorithms have those, for good reason too.