diff --git a/include/polarssl/dhm.h b/include/polarssl/dhm.h
index f63eefd..e80c45e 100644
--- a/include/polarssl/dhm.h
+++ b/include/polarssl/dhm.h
@@ -64,7 +64,7 @@
  * \brief          Setup and write the ServerKeyExchange parameters
  *
  * \param ctx      DHM context
- * \param x_size   private value size in bits
+ * \param x_size   private value size in bytes
  * \param output   destination buffer
  * \param olen     number of chars written
  * \param f_rng    RNG function
diff --git a/library/dhm.c b/library/dhm.c
index 1f17909..904b592 100644
--- a/library/dhm.c
+++ b/library/dhm.c
@@ -99,20 +99,22 @@
     unsigned char *p;
 
     /*
-     * generate X and calculate GX = G^X mod P
+     * Generate X as large as possible ( < P )
      */
     n = x_size / sizeof( t_int );
     MPI_CHK( mpi_grow( &ctx->X, n ) );
     MPI_CHK( mpi_lset( &ctx->X, 0 ) );
 
-    n = x_size - 1;
     p = (unsigned char *) ctx->X.p;
-    for( i = 0; i < n; i++ )
+    for( i = 0; i < x_size - 1; i++ )
         *p++ = (unsigned char) f_rng( p_rng );
 
     while( mpi_cmp_mpi( &ctx->X, &ctx->P ) >= 0 )
            mpi_shift_r( &ctx->X, 1 );
 
+    /*
+     * Calculate GX = G^X mod P
+     */
     MPI_CHK( mpi_exp_mod( &ctx->GX, &ctx->G, &ctx->X,
                           &ctx->P , &ctx->RP ) );