You explained your problem well, however I'm curious as to why you want it to be random looking.
One method to make it 'look' random could be to use a
ROT13 inspired scheme. The very determined adversary would be able to figure it out, but for the random geek looking at the hexdump, he would be none the wiser.
Since the english alphabet has 26 letters, you could roll your own rotation scheme.
Start by doing something like:
A = M
B = X
C = H
etc..
Do this until all letters are mapped to another. You should now have 13 pairs. Since the mapping is fairly random, if someone hexdumped the data, instead of ABC, they'd see MXH. If there's considerable amounts of letters in the data, it would be possible to reverse engineer your mapping by using a
frequency table and some experimentation.
Another perhaps easier method would be to try different encryption methods and view the hex of the data and see if any of those satisfy your requirements. Of course, if you chose to go with a rotation/mapping scheme, you'd have to reverse it before you could decrypt the data with normal methods.
But it would be a good programming exercise. First you'd have to read in the file, then do the substitutions. For example 'S' = '53', so that would need to be replaced with the letter you map it to. Let's say you mapped it to 'G', then the hexcode would be '47'. The hedump would now show '47' instead of '53'.
Such a wrapper could be done in any programming language.
When trying to decrypt, it should always succeed, but return bogus data on wrong password.
When you use openssl to decrypt, you will receive standard messages from that program. That behaviour could however be changed locally if you got the source code, then changed it slightly to achieve the result you are after. However, this would naturally only be in effect on your local machine, and not on anyone elses, as they'd not have the modified openssl program.
However, if some geek where to get your encrypted data-file that you already had obfuscated through your rotation scheme and tried to do something like "echo 'U2FsdGVkX19lqSi+Z8UoUmfnUqX7QJf+vQwwlyQ0tL4=' |openssl aes-256-cbc -salt -a -d", they would probably get an error as openssl most likely would not recognize the datafile as an encrypted file. I have not tested it, so I might be wrong, but that's what seems logical.
If you want stealth, ie. hiding data without anybody being able to see it, you might want to try stenography, that means altering the content of an image to contain data. Any non-technical expert browsing through your files, would only see your nice vacation images, and not see your elaborate idea for a new energy device that would free the world.
![Grin](https://bitcointalk.org/Smileys/default/grin.gif)
If it's easy or not to detect images with hidden data in it, I don't know. This would warrant more research on your part.
There are most likely many other methods as well, that would be far more complicated to detect and that could deter even a very resourceful adversary, however what I just described should fool most geeks.