Douglas Sinclair asked about the earlier discussion re the use of radiation to generate true random numbers for one time pads. The problem, as I recall, was with the quantity of bits needed. OTP's eat bits like crazy. People have talked about filling CD-ROM's or other optical media with hundreds of megabytes or even gigabytes of random numbers. Now, the problem is how long it will take to produce that much random data. A few bits per second won't be fast enough. Suppose you wanted to produce 100 megabytes per day (which would take over a week to create a gigabyte). That requires about 10,000 random bits per second. Now, your detector is not going to be 100% efficient. Only a certain fraction of the emitted particles are going to be detected. So you will need more decays than this, possibly many more. Also, relying on a half-life calculation in which we wait a certain time interval, and see if there is a decay or not, won't be that accurate. If your time is off a little, it could bias the results. Tim May posted the best (IMO) fix for this. You collect bits in pairs; discard 00 and 11; for each 01 output a 0, for each 10 output a 1. This way even if there is a bias where, say, 60% of the bits are 0's and 40% are 1's you still get 50-50 0's and 1's out. This means you get about 1 output bit for each 4 inputs, so you have to increase the necessary decay rate by a factor of 4. So, the needed particle emission rate is 40,000 divided by the efficiency of your detector. Perhaps Douglas could get some efficiency figures from his father, and judge whether this rate of radiation emission would be safe. Hal Finney 74076.1041@compuserve.com {.