6.3. Entropy Pool Techniques

Many modern pseudo-random number sources, such as those described in Sections 7.1.2 and 7.1.3 utilize the technique of maintaining a "pool" of bits and providing operations for strongly mixing input with some randomness into the pool and extracting pseudo-random bits from the pool. This is illustrated in the figure below.

         +--------+    +------+    +---------+
     --->| Mix In |--->| POOL |--->| Extract |--->
         |  Bits  |    |      |    |   Bits  |
         +--------+    +------+    +---------+
                           ^           V
                           |           |
                           +-----------+

Bits to be fed into the pool can come from any of the various hardware, environmental, or user input sources discussed above. It is also common to save the state of the pool on system shutdown and to restore it on re-starting, when stable storage is available.

Care must be taken that enough entropy has been added to the pool to support particular output uses desired. See [RSA_BULL1] for similar suggestions.