DRBG algorithm and ReturnedBits

[Bala Duvvuri]

Hi,

If we check the DRBG specifications - 
http://csrc.nist.gov/groups/STM/cavp/documents/drbg/DRBGVS.pdf

For cases with prediction resistance enabled, each trial consists of the 
following functions called in sequence:

(1) instantiate drbg
(2) generate ReturnedBitsLen random bits, do not print
(3) generate ReturnedBitsLen random bits, print out
(4) uninstantiate

Also in fips_drbgvs.c

      if (gen == 2)
         {
         OutputValue("ReturnedBits", randout, randoutlen,
                           out, 0);
         FIPS_drbg_free(dctx);
         dctx = NULL;
         gen = 0;
         }


What is reason to call the FIPS_drbg_generate twice?

Why do we need to call generate function twice?

thanks,
Bala
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