mbedtls_mpi_read_string: make an empty bignum for an empty string
In mbedtls_mpi_read_string, if the string is empty, return an empty bignum
rather than a bignum with one limb with the value 0.
Both representations are correct, so this is not, in principle, a
user-visible change. The change does leak however through
mbedtls_mpi_write_string in base 16 (but not in other bases), as it writes a
bignum with 0 limbs as "" but a bignum with the value 0 and at least one
limb as "00".
This change makes it possible to construct an empty bignum through
mbedtls_mpi_read_string, which is especially useful to construct test
cases (a common use of mbedtls_mpi_read_string, as most formats use in
production encode numbers in binary, to be read with mbedtls_mpi_read_binary
or mbedtls_mpi_read_binary_le).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 418fbf2..d2284a4 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -512,6 +512,12 @@
mbedtls_mpi_init( &T );
+ if( s[0] == 0 )
+ {
+ mbedtls_mpi_free( X );
+ return( 0 );
+ }
+
if( s[0] == '-' )
{
++s;