A BN_CTX is a structure that holds BIGNUM temporary variables used by
library functions. Since dynamic memory allocation to create BIGNUMs
is rather expensive when used in conjunction with repeated subroutine
calls, the BN_CTX structure is used.
BN_CTX_new() allocates and initializes a BN_CTX
structure. BN_CTX_init() initializes an existing uninitialized
BN_CTX.
BN_CTX_free() frees the components of the BN_CTX, and if it was
created by BN_CTX_new(), also the structure itself.
If BN_CTX_start(3) has been used on the BN_CTX,
BN_CTX_end(3) must be called before the BN_CTX
may be freed by BN_CTX_free().
RETURN VALUES
BN_CTX_new() returns a pointer to the BN_CTX. If the allocation fails,
it returns NULL and sets an error code that can be obtained by
ERR_get_error(3).
BN_CTX_init() and BN_CTX_free() have no return values.