mbedtls_mpi_gcd: fix the case B==0
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index e6258fd..5b9c8cb 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2282,6 +2282,16 @@
lz = mbedtls_mpi_lsb( &TA );
lzt = mbedtls_mpi_lsb( &TB );
+ /* The loop below gives the correct result when A==0 but not when B==0.
+ * So have a special case for B==0. Leverage the fact that we just
+ * calculated the lsb and lsb(B)==0 iff B is odd or 0 to make the test
+ * slightly more efficient than cmp_int(). */
+ if( lzt == 0 && mbedtls_mpi_get_bit( &TB, 0 ) == 0 )
+ {
+ ret = mbedtls_mpi_copy( G, A );
+ goto cleanup;
+ }
+
if( lzt < lz )
lz = lzt;