diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index f08703582..c0d0c8246 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -829,14 +829,14 @@ int mbedtls_mpi_mod_int( mbedtls_mpi_uint *r, const mbedtls_mpi *A,
  * \param E        The exponent MPI. This must point to an initialized MPI.
  * \param N        The base for the modular reduction. This must point to an
  *                 initialized MPI.
- * \param _RR      A helper MPI depending solely on \p N which can be used to
+ * \param prec_RR  A helper MPI depending solely on \p N which can be used to
  *                 speed-up multiple modular exponentiations for the same value
  *                 of \p N. This may be \c NULL. If it is not \c NULL, it must
  *                 point to an initialized MPI. If it hasn't been used after
  *                 the call to mbedtls_mpi_init(), this function will compute
- *                 the helper value and store it in \p _RR for reuse on
+ *                 the helper value and store it in \p prec_RR for reuse on
  *                 subsequent calls to this function. Otherwise, the function
- *                 will assume that \p _RR holds the helper value set by a
+ *                 will assume that \p prec_RR holds the helper value set by a
  *                 previous call to mbedtls_mpi_exp_mod(), and reuse it.
  *
  * \return         \c 0 if successful.
@@ -848,7 +848,7 @@ int mbedtls_mpi_mod_int( mbedtls_mpi_uint *r, const mbedtls_mpi *A,
  */
 int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
                          const mbedtls_mpi *E, const mbedtls_mpi *N,
-                         mbedtls_mpi *_RR );
+                         mbedtls_mpi *prec_RR );
 
 /**
  * \brief          Fill an MPI with a number of random bytes.
diff --git a/library/bignum.c b/library/bignum.c
index 75c7439f4..364332664 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2297,7 +2297,7 @@ cleanup:
  */
 int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
                          const mbedtls_mpi *E, const mbedtls_mpi *N,
-                         mbedtls_mpi *_RR )
+                         mbedtls_mpi *prec_RR )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t wbits, wsize, one = 1;
@@ -2365,17 +2365,17 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
     /*
      * If 1st call, pre-compute R^2 mod N
      */
-    if( _RR == NULL || _RR->p == NULL )
+    if( prec_RR == NULL || prec_RR->p == NULL )
     {
         MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &RR, 1 ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_shift_l( &RR, N->n * 2 * biL ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &RR, &RR, N ) );
 
-        if( _RR != NULL )
-            memcpy( _RR, &RR, sizeof( mbedtls_mpi ) );
+        if( prec_RR != NULL )
+            memcpy( prec_RR, &RR, sizeof( mbedtls_mpi ) );
     }
     else
-        memcpy( &RR, _RR, sizeof( mbedtls_mpi ) );
+        memcpy( &RR, prec_RR, sizeof( mbedtls_mpi ) );
 
     /*
      * W[1] = A * R^2 * R^-1 mod N = A * R mod N
@@ -2523,7 +2523,7 @@ cleanup:
     mbedtls_mpi_free( &W[1] ); mbedtls_mpi_free( &T ); mbedtls_mpi_free( &Apos );
     mbedtls_mpi_free( &WW );
 
-    if( _RR == NULL || _RR->p == NULL )
+    if( prec_RR == NULL || prec_RR->p == NULL )
         mbedtls_mpi_free( &RR );
 
     return( ret );