Entropy.c

// This file was extracted from the TCG Published
// Trusted Platform Module Library
// Part 4: Supporting Routines
// Family "2.0"
// Level 00 Revision 01.16
// October 30, 2014

#include <stdlib.h>
#include <stdint.h>
#include <memory.h>
#include "TpmBuildSwitches.h"
//
//
//          Local values
//
//     This is the last 32-bits of hardware entropy produced. We have to check to see that two consecutive 32-
//     bit values are not the same because (according to FIPS 140-2, annex C
//           “If each call to a RNG produces blocks of n bits (where n > 15), the first n-bit block generated after
//           power-up, initialization, or reset shall not be used, but shall be saved for comparison with the next n-
//           bit block to be generated. Each subsequent generation of an n-bit block shall be compared with the
//           previously generated block. The test shall fail if any two compared n-bit blocks are equal.”
//
extern uint32_t               lastEntropy;
extern int                    firstValue;
//
//
//          _plat__GetEntropy()
//
//     This function is used to get available hardware entropy. In a hardware implementation of this function,
//     there would be no call to the system to get entropy. If the caller does not ask for any entropy, then this is
//     a startup indication and firstValue should be reset.
//
//     Return Value                       Meaning
//
//     <0                                 hardware failure of the entropy generator, this is sticky
//     >= 0                               the returned amount of entropy (bytes)
//
LIB_EXPORT int32_t
_plat__GetEntropy(
      unsigned char            *entropy,                  // output buffer
      uint32_t                  amount                    // amount requested
)
{
      uint32_t                rndNum;

      if(amount == 0)
      {
          firstValue = 1;
          return 0;
      }
      // Only provide entropy 32 bits at a time to test the ability
      // of the caller to deal with partial results.
      rndNum = random();  //TODO(vbendeb): compare to rand_s case
      if(firstValue)
              firstValue = 0;

      lastEntropy = rndNum;
      if(amount > sizeof(rndNum))
              amount = sizeof(rndNum);
      memcpy(entropy, &rndNum, amount);

   return (int32_t)amount;
}