Symmetric encryption vulnerable when encrypting and decrypting are the same?
The short answer to your question is yes, but the devil is in the details of "(i.e.  can be considered a cryptographically secure source of randomness)".  For one thing, the "entropy stream" or any portion of it must never be reused within the same (periodicity) or any &amp; all other input data streams.  For another, it is in principle impossible to prove randomness.  OTOH, "cryptographic security" is more tractable.

Interestingly (at least to me) there is another binary primitive that has the same reflexive property as XOR: complementation, or double subtraction.  Consider this C fragment:
    Output_byte = (512 - Input_byte - Key_byte) &amp; 0xff;

There are other (faster) ways of doing it using logical inverse &amp; the properties of two's complement representation, but it's still nowhere near as fast as XOR. It does, however, have the same self-inverse property that XOR does.