源程序来自Gethub的Simple implementation of the RSA algorithm。

程序中有关类型转换代码略做修改，并且已经能够编译运行。

程序如下：

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>#define ACCURACY 5
#define SINGLE_MAX 10000
#define EXPONENT_MAX 1000
#define BUF_SIZE 1024/*** Computes a^b mod c*/
int modpow(long long a, long long b, int c) {int res = 1;while(b > 0) {/* Need long multiplication else this will overflow... */if(b & 1) {res = (res * a) % c;}b = b >> 1;a = (a * a) % c; /* Same deal here */}return res;
}/*** Computes the Jacobi symbol, (a, n)*/
int jacobi(int a, int n) {int twos, temp;int mult = 1;while(a > 1 && a != n) {a = a % n;if(a <= 1 || a == n) break;twos = 0;while(a % 2 == 0 && ++twos) a /= 2; /* Factor out multiples of 2 */if(twos > 0 && twos % 2 == 1) mult *= (n % 8 == 1 || n % 8 == 7) * 2 - 1;if(a <= 1 || a == n) break;if(n % 4 != 1 && a % 4 != 1) mult *= -1; /* Coefficient for flipping */temp = a;a = n;n = temp;}if(a == 0) return 0;else if(a == 1) return mult;else return 0; /* a == n => gcd(a, n) != 1 */
}/*** Check whether a is a Euler witness for n*/
int solovayPrime(int a, int n) {int x = jacobi(a, n);if(x == -1) x = n - 1;return x != 0 && modpow(a, (n - 1)/2, n) == x;
}/*** Test if n is probably prime, using accuracy of k (k solovay tests)*/
int probablePrime(int n, int k) {if(n == 2) return 1;else if(n % 2 == 0 || n == 1) return 0;while(k-- > 0) {if(!solovayPrime(rand() % (n - 2) + 2, n)) return 0;}return 1;
}/*** Find a random (probable) prime between 3 and n - 1, this distribution is* nowhere near uniform, see prime gaps*/
int randPrime(int n) {int prime = rand() % n;n += n % 2; /* n needs to be even so modulo wrapping preserves oddness */prime += 1 - prime % 2;while(1) {if(probablePrime(prime, ACCURACY)) return prime;prime = (prime + 2) % n;}
}/*** Compute gcd(a, b)*/
int gcd(int a, int b) {int temp;while(b != 0) {temp = b;b = a % b;a = temp;}return a;
}/*** Find a random exponent x between 3 and n - 1 such that gcd(x, phi) = 1,* this distribution is similarly nowhere near uniform*/
int randExponent(int phi, int n) {int e = rand() % n;while(1) {if(gcd(e, phi) == 1) return e;e = (e + 1) % n;if(e <= 2) e = 3;}
}/*** Compute n^-1 mod m by extended euclidian method*/
int inverse(int n, int modulus) {int a = n, b = modulus;int x = 0, y = 1, x0 = 1, y0 = 0, q, temp;while(b != 0) {q = a / b;temp = a % b;a = b;b = temp;temp = x; x = x0 - q * x; x0 = temp;temp = y; y = y0 - q * y; y0 = temp;}if(x0 < 0) x0 += modulus;return x0;
}/*** Read the file fd into an array of bytes ready for encryption.* The array will be padded with zeros until it divides the number of* bytes encrypted per block. Returns the number of bytes read.*/
int readFile(FILE* fd, char** buffer, int bytes) {int len = 0, cap = BUF_SIZE, r;char buf[BUF_SIZE];*buffer = (char *)malloc(BUF_SIZE * sizeof(char));while((r = fread(buf, sizeof(char), BUF_SIZE, fd)) > 0) {if(len + r >= cap) {cap *= 2;*buffer = (char *)realloc(*buffer, cap);}memcpy(&(*buffer)[len], buf, r);len += r;}/* Pad the last block with zeros to signal end of cryptogram. An additional block is added if there is no room */if(len + bytes - len % bytes > cap) *buffer = (char *)realloc(*buffer, len + bytes - len % bytes);do {(*buffer)[len] = '\0';len++;}while(len % bytes != 0);return len;
}/*** Encode the message m using public exponent and modulus, c = m^e mod n*/
int encode(int m, int e, int n) {return modpow(m, e, n);
}/*** Decode cryptogram c using private exponent and public modulus, m = c^d mod n*/
int decode(int c, int d, int n) {return modpow(c, d, n);
}/*** Encode the message of given length, using the public key (exponent, modulus)* The resulting array will be of size len/bytes, each index being the encryption* of "bytes" consecutive characters, given by m = (m1 + m2*128 + m3*128^2 + ..),* encoded = m^exponent mod modulus*/
int* encodeMessage(int len, int bytes, char* message, int exponent, int modulus) {int *encoded = (int *)malloc((len/bytes) * sizeof(int));int x, i, j;for(i = 0; i < len; i += bytes) {x = 0;for(j = 0; j < bytes; j++) x += message[i + j] * (1 << (7 * j));encoded[i/bytes] = encode(x, exponent, modulus);
#ifndef MEASUREprintf("%d ", encoded[i/bytes]);
#endif}return encoded;
}/*** Decode the cryptogram of given length, using the private key (exponent, modulus)* Each encrypted packet should represent "bytes" characters as per encodeMessage.* The returned message will be of size len * bytes.*/
int* decodeMessage(int len, int bytes, int* cryptogram, int exponent, int modulus) {int *decoded = (int *)malloc(len * bytes * sizeof(int));int x, i, j;for(i = 0; i < len; i++) {x = decode(cryptogram[i], exponent, modulus);for(j = 0; j < bytes; j++) {decoded[i*bytes + j] = (x >> (7 * j)) % 128;
#ifndef MEASUREif(decoded[i*bytes + j] != '\0') printf("%c", decoded[i*bytes + j]);
#endif}}return decoded;
}/*** Main method to demostrate the system. Sets up primes p, q, and proceeds to encode and* decode the message given in "text.txt"*/
int main(void) {int p, q, n, phi, e, d, bytes, len;int *encoded, *decoded;char *buffer;FILE *f;srand(time(NULL));while(1) {p = randPrime(SINGLE_MAX);printf("Got first prime factor, p = %d ... ", p);getchar();q = randPrime(SINGLE_MAX);printf("Got second prime factor, q = %d ... ", q);getchar();n = p * q;printf("Got modulus, n = pq = %d ... ", n);if(n < 128) {printf("Modulus is less than 128, cannot encode single bytes. Trying again ... ");getchar();}else break;}if(n >> 21) bytes = 3;else if(n >> 14) bytes = 2;else bytes = 1;getchar();phi = (p - 1) * (q - 1);printf("Got totient, phi = %d ... ", phi);getchar();e = randExponent(phi, EXPONENT_MAX);printf("Chose public exponent, e = %d\nPublic key is (%d, %d) ... ", e, e, n);getchar();d = inverse(e, phi);printf("Calculated private exponent, d = %d\nPrivate key is (%d, %d) ... ", d, d, n);getchar();printf("Opening file \"text.txt\" for reading\n");f = fopen("text.txt", "r");if(f == NULL) {printf("Failed to open file \"text.txt\". Does it exist?\n");return EXIT_FAILURE;}len = readFile(f, &buffer, bytes); /* len will be a multiple of bytes, to send whole chunks */fclose(f);printf("File \"text.txt\" read successfully, %d bytes read. Encoding byte stream in chunks of %d bytes ... ", len, bytes);getchar();encoded = encodeMessage(len, bytes, buffer, e, n);printf("\nEncoding finished successfully ... ");getchar();printf("Decoding encoded message ... ");getchar();decoded = decodeMessage(len/bytes, bytes, encoded, d, n);printf("\nFinished RSA demonstration!\n");free(encoded);free(decoded);free(buffer);return EXIT_SUCCESS;
}