I hope it's not too late to answer this thread.
You can try "LibTomMath" which is opensource and free (the author give away this project as public domain). It works out of the box without any configuration, just put all bn_*.c and tommath*.h to your Xcode project and you are ready to go.
#import "tommath.h"
mp_int number1, number2, number3;
mp_init(&number1);
mp_init(&number2);
mp_init(&number3);
mp_read_radix(&number1, "0a120edfff558c98a73015d5d67e8990", 16);
mp_read_radix(&number2, "12e6f45d698c7b7009a841c1348d6ff4", 16);
mp_mul(&number1, &number2, &number3);
char output[1000];
mp_toradix(&number3, output, 16);
NSLog(@"number3:%s", output);
mp_div(&number3, &number1, &number2, NULL);
mp_toradix(&number2, output, 16);
NSLog(@"number2:%s", output);
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