BN_CTX_start, BN_CTX_get, BN_CTX_end — use temporary BIGNUM variables
BIGNUM *BN_CTX_get(BN_CTX *ctx);
These functions are used to obtain temporary BIGNUM variables
from a BN_CTX (which can been created by using BN_CTX_new(3))
in order to save the overhead of repeatedly creating and freeing BIGNUMs
in functions that are called from inside a loop.
A function must call BN_CTX_start() first. Then, BN_CTX_get()
may be called repeatedly to obtain temporary BIGNUMs.
All BN_CTX_get() calls must be made before calling any other functions
that use the ctx as an argument.
Finally, BN_CTX_end() must be called before returning from
the function. When BN_CTX_end() is called, the BIGNUM pointers
obtained from BN_CTX_get() become invalid.
BN_CTX_start() and BN_CTX_end() return no values.
BN_CTX_get() returns a pointer to the BIGNUM,
or NULL on error. Once BN_CTX_get() has failed,
the subsequent calls will return NULL as well,
so it is sufficient to check the return value of the last BN_CTX_get()
call. In case of an error, an error code is set, which can be obtained
BN_CTX_start(), BN_CTX_get() and BN_CTX_end() were added in