diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index 1308557..12435f7 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -539,7 +539,9 @@
  * \return         0 if successful,
  *                 1 if memory allocation failed
  */
-int mpi_fill_random( mpi *X, size_t size, int (*f_rng)(void *), void *p_rng );
+int mpi_fill_random( mpi *X, size_t size,
+                     int (*f_rng)(void *, unsigned char *, size_t),
+                     void *p_rng );
 
 /**
  * \brief          Greatest common divisor: G = gcd(A, B)
@@ -578,7 +580,9 @@
  *                 1 if memory allocation failed,
  *                 POLARSSL_ERR_MPI_NOT_ACCEPTABLE if X is not prime
  */
-int mpi_is_prime( mpi *X, int (*f_rng)(void *), void *p_rng );
+int mpi_is_prime( mpi *X,
+                  int (*f_rng)(void *, unsigned char *, size_t),
+                  void *p_rng );
 
 /**
  * \brief          Prime number generation
@@ -594,7 +598,8 @@
  *                 POLARSSL_ERR_MPI_BAD_INPUT_DATA if nbits is < 3
  */
 int mpi_gen_prime( mpi *X, size_t nbits, int dh_flag,
-                   int (*f_rng)(void *), void *p_rng );
+                   int (*f_rng)(void *, unsigned char *, size_t),
+                   void *p_rng );
 
 /**
  * \brief          Checkup routine