Large integer implementation

Why do many large integer libraries store their data as a string of symbols instead of a block of integers native to the system? It seems to me that this would make bitwise operations rather difficult to implement.

I don't know how "many" many is, but most of the implementations I've seen tend to store bignums as some sort of packed bit arrays. Most C-based ones do provide string input as C doesn't support bignums natively and has no way of modifying the compiler to read something in as a static bignum. In languages with native bignum support (for example Common Lisp), bignums tend to be stored and inputted in saner manners (packed bit arrays).

The only implementation I've seen that is so stupid is FGInt, which sometimes uses temporary strings like "101010101" for 341.

>>2
You are right, I only looked for libraries in C and found several using char arrays. I also found a tutorial and their algorithms were only usable on strings.

I've implemented bitwise operators and schoolbook addition with carry / subtraction with borrowing so far.

Do you know how I could implement large integer operations such as modulo on blocks of 64bits?

Stop trying to implement RSA and get someone else to do your homework for you.

GMP uses an array of machine words for speed reasons. GNU bc uses a char array of decimal digits.

To not answer your question, see The Art of Computer Programming, vol. 2: Seminumerical Algorithms by Donald Knuth.

I find this page informative: http://www.codeproject.com/KB/cs/biginteger.aspx

>>5
There's plenty of decent bignum libs for C, and most provide some sort of powmod function (a**b mod n) which is what you usually need for fast RSA encryption/decryption.