Biginteger in Iphone and related funtions

Question

I have a RSA Exponent key value which is supposed to be a biginteger but i have it in NSString/NSdata with full value in(UTF8 encoded)

as Part of RSA encryption , i need to do the following in the Iphone Env

1. I need to find the bit length of the above exponent value

2. I need to do arithmatic operations on exponent and modulus values including PowMod

3. so which data type i can use (uint64_t or NSNUmber or NSUinteger) for arithmatic operations as well as holding the bigint result value.

4. do i need to go for a specfic bigint implementation, can i able to manage with the above existing iphone data types for bigint ?

5. those external bigint implementations expect to port openssl or gmp lib to Iphone ?

Thanks