use temporary BIGNUM variables
BN_CTX_start(BN_CTX *ctx);BIGNUM *
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.