If it were possible to compute digits of pi from an arbitrary location (to generate the stream) in constant time, then it would be an excellent cipher. Unfortunately, this isn't possible.

Maurer suggested using the surface of the moon for a source of random information instead: http://download.springer.com/static/pdf/752/chp%253A10.1007%252F3-540-46877-3_33.pdf?auth66=1379955068_ec70c1616cecd9bfba9100a7bcb17cbc&ext=.pdf

If it were possible to compute digits of pi from an arbitrary location (to generate the stream) in constant time, then it would be an excellent cipher. Unfortunately, this isn't possible.

Maurer suggested using the surface of the moon for a source of random information instead: link