Three requirements for One Time Pads are:

1.  Truly Random Key
   
2.  Key as long as the message
   
3.  Key is never used again

Assuming the three part computer system I described above (for operational security) the only problem is the first.  Turns out it's only possible to derive an algorithm capable of computing a pseudo-random number, at some point every algorithm cycles back to the beginning. Thus, any practical implementation is not mathematically 'complete' but it doesn't really matter.  Practical systems use a pseudo-random seed value - say the current barometric pressure divided by the current temperature times the second through ninth numbers in the mantissa of the current time - fed into a Good Enough pseudo-random number generator for the key.  

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre