One ciphertext, multiple keys, multiple messages I recently came across following idea, does anyone know a name and if it is actually used?


  A wants to send B a message (A and B know both have a common secret
  key) and C wants to intercept it (without the knowledge of the key)
  
  Now we assume they have an encryption method that let's you define two
  distinct messages for two distinct keys that both are encoded in the
  same cyphertext.
  
  With this method A can send the secret message "it is raining
  tomorrow" encrypted with the secret key, but A can also include the
  message "it is not raining tomorrow" with another key.
  
  If C manages to intercept the message and manages to find one or both of the
  keys, they still don't know anything, as there are two contradicting
  messages. But since B knows which key was used, they can use the
  correct message.


If such an method exists, wouldn't it very secure, or are there any problems that come with it? The only problem I can think of is obiously the size of the cyphertext, and depending on the complexity of the messages you might have to find a lot of messages to completely "diffuse" the meaning.

Sorry for the probably not very fitting tags, I did not find any better ones, as this is just a relativel vague question.
This functionality is usually called deniable encryption. It is feasible for (innocuous/incriminating) pairs of a few messages.

Wikipedia: 


  Deniable encryption makes it impossible to prove the existence of the plaintext message without the proper encryption key. This may be done by allowing an encrypted message to be decrypted to different sensible plaintexts, depending on the key used. This allows the sender to have plausible deniability if compelled to give up his or her encryption key. The notion of "deniable encryption" was used by Julian Assange and Ralf Weinmann in the Rubberhose filesystem and explored in detail in a paper by Ran Canetti, Cynthia Dwork, Moni Naor, and Rafail Ostrovsky in 1996.


There are related questions and answers, e.g., is-there-an-encryption-decryption-algorithm-that-can-give-two-different-outputs, and deniable-encryption-from-simple-primitives as well as a few others.