commit a16001e2d9780f392eb4dfe68d8e59354d51f849
author Gilles Peskine <Gilles.Peskine@arm.com> Tue Apr 13 21:55:35 2021 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> Wed Jun 02 21:39:31 2021 +0200
tree ef3ac380775d641a2ee315f4c599532929ca9de2
parent 3d60ece16c7bd7d1fae8df359899344d7a509d18

mpi_fill_random_internal: remove spurious grow() call

Since the internal function mpi_fill_random_internal() assumes that X
has the right size, there is no need to call grow().

To further simplify the function, set the sign outside, and zero out
the non-randomized part directly.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/bignum.c

diff --git a/library/bignum.c b/library/bignum.c
index 244725c..d949fd1 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2397,6 +2397,7 @@
 
 /* Fill X with n_bytes random bytes.
  * X must already have room for those bytes.
+ * The size and sign of X are unchanged.
  * n_bytes must not be 0.
  */
 static int mpi_fill_random_internal(
@@ -2409,9 +2410,9 @@
 
     if( X->n < limbs )
         return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
-    MBEDTLS_MPI_CHK( mbedtls_mpi_lset( X, 0 ) );
-    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, limbs ) );
 
+    memset( X->p, 0, overhead );
+    memset( (unsigned char *) X->p + limbs * ciL, 0, ( X->n - limbs ) * ciL );
     MBEDTLS_MPI_CHK( f_rng( p_rng, (unsigned char *) X->p + overhead, n_bytes ) );
     mpi_bigendian_to_host( X->p, limbs );
 
@@ -2443,6 +2444,7 @@
         mbedtls_mpi_init( X );
         MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, limbs ) );
     }
+    X->s = 1;
     if( size == 0 )
         return( 0 );
 
@@ -2501,6 +2503,7 @@
         mbedtls_mpi_init( X );
         MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, N->n ) );
     }
+    X->s = 1;
 
     /*
      * Match the procedure given in RFC 6979 §3.3 (deterministic ECDSA)