steganography

https://www.techtarget.com/searchsecurity/definition/steganography

Steganography is the practice of concealing a message, image, or file within another message, image, or file. 
The goal of steganography is to hide the existence of the hidden message or content, 
so that it is not detected or recognized by anyone other than the intended recipient.

There are several different ways that steganography can be implemented. One common method is to embed the hidden message or content
within an image or audio file by altering the least significant bits (LSBs) of the file's pixels or samples.
For example, an image might be modified to contain a hidden message by altering the LSBs of certain pixels in a way that 
is not noticeable to the human eye but can be detected and interpreted by someone who knows the encoding scheme.

Another method of steganography is to use a cover text or image as a "mask" for the hidden message or content. 
In this case, the hidden message is encoded as a sequence of characters that are interspersed with the cover text or pixels
in a way that is not noticeable to the casual observer.

Steganography can be used for a variety of purposes, such as to protect the confidentiality of sensitive information, 
to hide the true nature of a message or file, or to bypass censorship or content filters. However,
it is important to note that steganography is not a foolproof method of hiding or protecting information,
as sophisticated forensic techniques and software can be used to detect and extract hidden content.