A message is sent from a person to another. The plain message is first encrypted, even with a weak algorithm - say, DES. Then, the encrypted message is encoded with a simple substitution, which is private and remains the same through all the conversation.
Is it feasible for an external attacker to decrypt the message, only knowing the encrypted message and the process (i.e. only that an unknown encryption and later an unknown substitution, both of which remain the same through all the conversation)?
The substitution algorithm, the encryption algorithm and any key the algorithms use are private and known to both the sender and the receiver. The substituted text may be longer than the non-substituted one.
My guess so far is that since frequency analysis can usually not be applied to encryption, a simple substitution makes it infeasible to decrypt the text, provided that the map (knowing what is encoded to what) remains unknown