Adding dummy bytes to ciphertext I was wondering if interleaving the resulting crypted data with dummy bytes placed at random positions but known in advance by the peers could improve security (no matter the algorithm and strength used, this is just baby step theory here) or do the contrary (and why).
E.g.:
normal: kIolmspJgheydhNud
with dummies: kI9oklmdspJgfheydshNufd
Update:
after Thomas answer, I realized that my question was not clear where the dummy bytes would go.
My mistake.
So I am interested in adding dummies in the ciphertext. After all encryption of the real text occurred. The net result is to make the ciphertext longer and not logical anymore.
If you're using a real encryption scheme then no it cannot weaken or strengthen the system because the encryption scheme's security is supposed to be independent the actual data being encrypted. The plaintext could be random bytes or all zeroes for all it cares, it will be just as secure.

All this modification achieves is lower your encryption scheme's capacity (number of plaintext bytes you can transfer in a given number of ciphertext bytes) and performance (number of plaintext bytes encrypted by second) as well as complicating the algorithm even further for no reason.

This is, of course, if the adversary only has access to the bytes over the wire. Your implementation of this modification may well leak sensitive information through a side channel (don't know how, but it's possible since you need to add code to implement this, and this code may have bugs and security vulnerabilities, as with any code.

Basically, it doesn't really do anything useful and has only cons, so don't do it.