Improve sign/key_tries handling
(Unrelated to restartable work, just noticed while staring at the code.)
Checking at the end is inefficient as we might give up when we just generated
a valid signature or key.
diff --git a/library/ecp.c b/library/ecp.c
index 0f1cbfc..9b586ee 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -2531,24 +2531,23 @@
* - keep the leftmost nbits bits of the generated octet string;
* - try until result is in the desired range.
* This also avoids any biais, which is especially important for ECDSA.
+ *
+ * Each try has at worst a probability 1/2 of failing (the msb has
+ * a probability 1/2 of being 0, and then the result will be < N),
+ * so after 30 tries failure probability is a most 2**(-30).
+ *
+ * For most curves, 1 try is enough with overwhelming probability,
+ * since N starts with a lot of 1s in binary, but some curves
+ * such as secp224k1 are actually very close to the worst case.
*/
do
{
+ if( ++count > 30 )
+ return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
+
MBEDTLS_MPI_CHK( f_rng( p_rng, rnd, n_size ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_read_binary( d, rnd, n_size ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, 8 * n_size - grp->nbits ) );
-
- /*
- * Each try has at worst a probability 1/2 of failing (the msb has
- * a probability 1/2 of being 0, and then the result will be < N),
- * so after 30 tries failure probability is a most 2**(-30).
- *
- * For most curves, 1 try is enough with overwhelming probability,
- * since N starts with a lot of 1s in binary, but some curves
- * such as secp224k1 are actually very close to the worst case.
- */
- if( ++count > 30 )
- return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
}
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 ||
mbedtls_mpi_cmp_mpi( d, &grp->N ) >= 0 );