C语言建立一个RSA大数运算库

RSA大数运算涉及到大整数的加、减、乘、除、幂等运算，因此需要建立一个大数运算库来实现这些操作。以下是一个简单的C语言实现：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXSIZE 1000

typedef struct {
    int digit[MAXSIZE];
    int size;
} BigNum;

void init(BigNum *p) {
    memset(p->digit, 0, sizeof(p->digit));
    p->size = 0;
}

void set(BigNum *p, int x) {
    init(p);
    do {
        p->digit[p->size++] = x % 10;
        x /= 10;
    } while (x);
}

void print(BigNum *p) {
    for (int i = p->size - 1; i >= 0; i--) {
        printf("%d", p->digit[i]);
    }
    printf("\n");
}

BigNum add(BigNum a, BigNum b) {
    BigNum c;
    init(&c);
    int carry = 0;
    for (int i = 0; i < a.size || i < b.size; i++) {
        int sum = a.digit[i] + b.digit[i] + carry;
        c.digit[c.size++] = sum % 10;
        carry = sum / 10;
    }
    if (carry != 0) {
        c.digit[c.size++] = carry;
    }
    return c;
}

BigNum sub(BigNum a, BigNum b) {
    BigNum c;
    init(&c);
    int borrow = 0;
    for (int i = 0; i < a.size || i < b.size; i++) {
        int sub = a.digit[i] - b.digit[i] - borrow;
        if (sub < 0) {
            sub += 10;
            borrow = 1;
        } else {
            borrow = 0;
        }
        c.digit[c.size++] = sub;
    }
    while (c.size > 1 && c.digit[c.size - 1] == 0) {
        c.size--;
    }
    return c;
}

BigNum mul(BigNum a, int b) {
    BigNum c;
    init(&c);
    int carry = 0;
    for (int i = 0; i < a.size; i++) {
        int mul = a.digit[i] * b + carry;
        c.digit[c.size++] = mul % 10;
        carry = mul / 10;
    }
    while (carry != 0) {
        c.digit[c.size++] = carry % 10;
        carry /= 10;
    }
    return c;
}

BigNum mul_big(BigNum a, BigNum b) {
    BigNum c;
    init(&c);
    for (int i = 0; i < a.size; i++) {
        BigNum tmp = mul(b, a.digit[i]);
        for (int j = 0; j < i; j++) {
            tmp.digit[tmp.size++] = 0;
        }
        c = add(c, tmp);
    }
    return c;
}

BigNum div(BigNum a, int b, int *r) {
    BigNum c;
    init(&c);
    c.size = a.size;
    for (int i = a.size - 1; i >= 0; i--) {
        *r = *r * 10 + a.digit[i];
        if (*r < b) {
            c.digit[i] = 0;
        } else {
            c.digit[i] = *r / b;
            *r = *r % b;
        }
    }
    while (c.size > 1 && c.digit[c.size - 1] == 0) {
        c.size--;
    }
    return c;
}

int cmp(BigNum a, BigNum b) {
    if (a.size < b.size) {
        return -1;
    } else if (a.size > b.size) {
        return 1;
    } else {
        for (int i = a.size - 1; i >= 0; i--) {
            if (a.digit[i] < b.digit[i]) {
                return -1;
            } else if (a.digit[i] > b.digit[i]) {
                return 1;
            }
        }
        return 0;
    }
}

BigNum pow(BigNum a, int b) {
    BigNum c;
    set(&c, 1);
    while (b > 0) {
        if (b & 1) {
            c = mul_big(c, a);
        }
        a = mul_big(a, a);
        b >>= 1;
    }
    return c;
}

以上代码实现了初始化、赋值、输出、加、减、乘、除、比较和幂等运算等基本操作。接下来可以利用这些基本操作实现RSA算法中的加密和解密过程。