mpi2bn
unsigned char * mpi2bn(unsigned char **pptr, size_t *sz);
BIGNUM * mpi2bn(unsigned char **pptr);
key->key->n = mpi2bn(&ptr, &key->key->nlen);
key->key->e = mpi2bn(&ptr, &key->key->elen);
rsa->n = mpi2bn(&ptr);
rsa->e = mpi2bn(&ptr);