"Padless" One-time-Pad encryption I have just read about the perfect security of an OTP encryption and what came to my mind was that what if the Pad used for encryption/decryption did not have to be transported separately from the message, but instead together with it?

And what if it was computed using an algorithm from the message itself (from all the characters in an 1:1 way so that it had the same size) providing a random-looking pad for encrypting the message, and after encrypting the message would be stored in an interwoven way in the message itself, expanding the file size by a factor of 2. The encrypted file itself would have double size of the original one but no-one would be able to find out what is the message and what is the key since it would be interwoven using a secret algorithm, perhaps the same one as was used for generating the pad.

Would this be a good encryption comparable to real OTP with the pad transferred and generated separately? Did I just invent the wheel, or is this a breakthrough thing (I mean storing the pad inside the file in a secret way preventing others to extract it)?

I know calculating the pad from the message itself would perhaps break the real randomness so this could be avoided and the pad simply taken from a real random key number generator.
Your "secret algorithm" is now effectively the key.

If the algorithm stays fixed for all messages, you now no longer have a one-time pad. So let's make your algorithm be tweakable by, say, accepting as input a string that describes its exact operation. Now we have the benefit of being able to publicize the "secret algorithm", since the actual secret details are contained in this input string. Except now that input string is the key, and now you have to find a way to transmit it... which is the problem you were trying to solve in the first place.