from ecp_curves.c:993
#define INIT( b ) \
int ret; \
signed char c = 0, cc; \
uint32_t cur; \
size_t i = 0, bits = (b); \
mbedtls_mpi C; \
mbedtls_mpi_uint Cp[ (b) / 8 / sizeof( mbedtls_mpi_uint) + 1 ]; \
\
C.s = 1; \
C.n = (b) / 8 / sizeof( mbedtls_mpi_uint) + 1; \
C.p = Cp; \
memset( Cp, 0, C.n * sizeof( mbedtls_mpi_uint ) ); \
\
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( N, (b) * 2 / 8 / \
sizeof( mbedtls_mpi_uint ) ) ); \
LOAD32;