Is there any existing obfuscation scheme that makes cipher text indistinguishable from plain text?

Question

Suppose a totalitarian government (in the name of anti-terrorism / protection of intellectual property):

1. has outlawed encryption itself - encryption is only approved for cases where the state has reviewed the design and made sure it can decrypt/inspect the message, and made any unapproved encryption a criminal offense
2. has total control over anything in and out of the network at ISP-level, as well as anything that passes through web services

How could two citizens Alice and Bob, using approved (and monitored) instant messaging service to set up a secure line of communication, conceal the fact that the communication is encrypted, i.e. to make it indistinguishable from unencrypted data, or at least, make it computationally- or financially-infeasible to distinguish it from plain text?

For example, no one would assume the following message to be encrypted:

• Across the Great Wall, we can reach every corner in the world.

But it would be assumed that the following is:

• WZ2A805Wq3rzpiuzE+ZCulgDrn76pVRW5PVUJ4DDadFQD4P9PsTeegbo5CAkqI4yZrO//p
  sYT+ZQkqZ6IrSGng==

• 599D80F34E56AB7AF3A62BB313E642BA
  5803AE7EFAA55456E4F5542780C369D1
  500F83FD3EC4DE7A06E8E42024A88E32
  66B3BFFE9B184FE65092A67A22B4869E

For the purpose of this question, we assume the following technical details:

1. the IM service is text-only, binary data is not allowed (in an IM setting, sending primarily small binary fragments back and forth would probably raise suspicion anyway)
2. communication between Alice and the IM service, Bob and the IM service, are both end-to-end encrypted. A government agent Eve has a copy of the decryption key the IM service used
3. proof that the message is encrypted is not required. I.e. Eve does not need to know the plain text or the algorithm used to produce the cipher text. She only needs to tell, with a reasonably-low false-positive rate, if a message is the result of an encryption
4. the endpoint is secure, no backdoor or malware on the computer/router, etc.

I'd like to know if there are any reliable research on this, is it feasible or not, and if feasible, any existing protocol or algorithm developed for this?

_{Eve, in case you are watching, I'm asking this for academic purposes only. 😄}

Answer 1

As was mentioned, what you're asking for is a form of steganography,which may or may not use encrypted content.

The classic approach derives from talking around the subject (Crimes and Spies) as in:

"The Apple crates are Golden Delicious. There are 15 crates."

A more technical approach I've seen is Spam Mimmic

Spam Mimmic encode: Bob, meet me at the secret lair at 1900.

Spam Mimmic decode:

Dear Friend , We know you are interested in receiving red-hot news ! This is a one time mailing there is no need to request removal if you won't want any more . This mail is being sent in compliance with Senate bill 1626 ; Title 1 , Section 307 ! THIS IS NOT MULTI-LEVEL MARKETING ! Why work for somebody else when you can become rich inside 31 DAYS ! Have you ever noticed the baby boomers are more demanding than their parents plus how many people you know are on the Internet ! Well, now is your chance to capitalize on this . WE will help YOU use credit cards on your website plus increase customer response by 120% ! You can begin at absolutely no cost to you ! But don't believe us . Mr Anderson who resides in Montana tried us and says "My only problem now is where to park all my cars" ! We are licensed to operate in all states . For the sake of your family order now ! Sign up a friend and you'll get a discount of 10% . Thanks . Dear Internet user , This letter was specially selected to be sent to you ! If you are not interested in our publications and wish to be removed from our lists, simply do NOT respond and ignore this mail . This mail is being sent in compliance with Senate bill 1620 ; Title 9 , Section 303 . This is not a get rich scheme . Why work for somebody else when you can become rich within 92 WEEKS . Have you ever noticed most everyone has a cellphone plus nearly every commercial on television has a .com on in it ! Well, now is your chance to capitalize on this ! We will help you decrease perceived waiting time by 120% and process your orders within seconds . You can begin at absolutely no cost to you . But don't believe us . Ms Ames of Mississippi tried us and says "Now I'm rich, Rich, RICH" . We are licensed to operate in all states . We urge you to contact us today for your own future financial well-being . Sign up a friend and your friend will be rich too . God Bless ! Dear Web surfer ; This letter was specially selected to be sent to you ! If you are not interested in our publications and wish to be removed from our lists, simply do NOT respond and ignore this mail . This mail is being sent in compliance with Senate bill 1619 ; Title 3 ; Section 301 ! THIS IS NOT MULTI-LEVEL MARKETING . Why work for somebody else when you can become rich in 41 DAYS ! Have you ever noticed people love convenience plus most everyone has a cellphone . Well, now is your chance to capitalize on this ! We will help you deliver goods right to the customer's doorstep and deliver goods right to the customer's doorstep . You can begin at absolutely no cost to you . But don't believe us . Mrs Simpson of Montana tried us and says "My only problem now is where to park all my cars" ! This offer is 100% legal ! DO NOT DELAY - order today . Sign up a friend and you'll get a discount of 50% ! Warmest regards !

It's rather wordy and not practical for heavy use, but it gives you an idea. In general auto generating believable text is very difficult, hence the use of SPAM as it tends to blend in with other unbelievable text.