#include <stdio.h>
#include <string.h>
#include <math.h>
#include <time.h>
#define PRIME_MAX 200 // 生成素数范围
#define EXPONENT_MAX 200 // 生成指数e范围
#define Element_Max 127 // 加密单元的最大值，这里为一个char, 即1Byte
char str_read[100]="hello world !" // 待加密的原文
int str_encrypt[100] // 加密后的内容
char str_decrypt[100] // 解密出来的内容
int str_read_len // str_read 的长度
int prime1, prime2 // 随机生成的两个质数
int mod, eular // 模数和欧拉数
int pubKey, priKey // 公钥指数和私钥指数
// 生成随机素数，实际应用中，这两个质数越大，就越难破解。
int randPrime()
{
int prime, prime2, i
next:
prime = rand() % PRIME_MAX // 随机产生数
if (prime <= 1) goto next // 不是质数，生成下一个随机数
if (prime == 2 || prime == 3) return prime
prime2 = prime / 2 // prime>=4, prime2 的平方必定大于 prime , 因此只检查小于等于prime2的数
for (i = 2i <= prime2i++) // 判断是否为素数
{
if (i * i >prime) return prime
if (prime % i == 0) goto next // 不是质数，生成下一个随机数
}
}
// 欧几里德算法，判断a,b互质
int gcd(int a, int b)
{
int temp
while (b != 0) {
temp = b
b = a % b
a = temp
}
return a
}
//生成公钥指数，条件是 1<e <欧拉数，且与欧拉数互质。
int randExponent()
{
int e
while (1)
{
e = rand() % eularif (e <EXPONENT_MAX) break
}
while (1)
{
if (gcd(e, eular) == 1) return ee = (e + 1) % eularif (e == 0 || e >EXPONENT_MAX) e = 2
}
}
//生成私钥指数
int inverse()
{
int d, x
while (1)
{
d = rand() % eular
x = pubKey * d % eular
if (x == 1)
{
return d
}
}
}
//加密函数
void jiami()
{
str_read_len = strlen(str_read) //从参数表示的地址往后找，找到第一个'\0'，即串尾。计算'\0'至首地址的“距离”，即隔了几个字符，从而得出长度。
printf("密文是：")
for (int i = 0i <str_read_leni++)
{
int C = 1int a = str_read[i], b = a % mod
for (int j = 0j <pubKeyj++) //实现加密
{
C = (C*b) % mod
}
str_encrypt[i] = C
printf("%d ", str_encrypt[i])
}
printf("\n")
}
//解密函数
void jiemi()
{
int i=0 for (i = 0i <str_read_leni++)
{
int C = 1int a = str_encrypt[i], b=a%mod
for (int j = 0j <priKeyj++)
{
C = (C * b) % mod
}
str_decrypt[i] = C
}
str_decrypt[i] = '\0'printf("解密文是：%s \n", str_decrypt)
}
int main()
{
srand(time(NULL))
while (1)
{
prime1 = randPrime()prime2 = randPrime()printf("随机产生两个素数：prime1 = %d , prime2 = %d ", prime1, prime2)
mod = prime1 * prime2printf("模数：mod = prime1 * prime2 = %d \n", mod)if (mod >Element_Max) break// 模数要大于每个加密单元的值
}
eular = (prime1 - 1) * (prime2 - 1) printf("欧拉数：eular=(prime1-1)*(prime2-1) = %d \n", eular)
pubKey = randExponent()printf("公钥指数：pubKey = %d\n", pubKey)
priKey = inverse()printf("私钥指数：priKey = %d\n私钥为 (%d, %d)\n", priKey, priKey, mod)
jiami()jiemi()
return 0
}
文件分为文本文件和二进制文件。加密方法也略有不同。1、文本文件
加密的主要是文本的内容，最简单的方法就是修改文档的内容，比如1.txt中的文件内容：
abcd
只要给每一个字符+1，就可以实现加密。文件内容即会变为
bcde
2、二进制文件加密
二进制文件加密也就是对应用程序加密，需要理解可执行文件格式，比如Windows平台的Exe文件它是PE结构，Linux上的可执行文件是ELF结构，要对这样的程序进行加密，实际上是开发一种叫做“壳”的程序，这种程序的开发，需要将扎实的底层基础，同时也需要对软件加密解密有细致的理解，比如流行的vmprotect、z壳以及早些年的upx壳、aspack等等。
3、无论哪种加密都牵涉到文件操作的问题，使用C语言进行文件操作时，极少使用C标准库中的I/O函数，大多数使用操作系统提供的内存文件映射相关的API函数，有兴趣，可以搜索相关的资料。
根据你的需要，修改了之前的代码。
#include <stdio.h>#include <string.h>
#include <stdlib.h>
#include <time.h>
const unsigned int MAX_KEY_LENGTH = 1000
int encode(char const *datafile, char const *keyfill)
int decode(char const *datafile, char const *keyfile)
int loadKey(char const *keyfile, int *keys, unsigned int size)
int saveKey(char const *keyfile, int *keys, unsigned int size)
int generateKey(int *keys, unsigned int size)
int main(int argc, char const *argv[])
{
char datafile[] = "encrypted.txt"
char keyfile[] = "key.txt"
int retcode, choice, loop = 1
char ch[5] = {'\0'}
while(1)
{
printf("1. Encryption.\n")
printf("2. Decryption.\n")
printf("3. Exit.\n")
printf("Selection (1,2,3):")
fgets(ch, sizeof(ch), stdin)
sscanf(ch, "%d", &choice)
switch(choice)
{
case 1:
retcode = encode(datafile, keyfile)
if (retcode != 0) printf("error, %d\0", retcode)
break
case 2:
retcode = decode(datafile, keyfile)
if (retcode != 0) printf("error, %d\0", retcode)
break
case 3:
loop = 0
break
default:
break
}
if (0 == loop) break
}
return 0
}
int generateKey(int *keys, unsigned int size)
{
char str[]="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz,./\"'<>?"
size_t str_len = sizeof(str)/sizeof(str[0])
int i
srand(time(NULL))
for (i = 0 i < size ++i)
keys[i] = str[rand() % str_len]
return 0
}
int loadKey(char const *keyfile, int *keys, unsigned int size)
{
int i = 0
FILE *pfile
int retcode = 0
pfile = fopen(keyfile, "r")
if (pfile == NULL) return -1
while ( !feof(pfile) ) {
if (i < size)
fscanf(pfile, "%d ", &keys[i++])
else
break
}
fclose(pfile)
return i
}
int saveKey(char const *keyfile, int *keys, unsigned int size)
{
FILE *pfile
int i
pfile = fopen(keyfile, "w")
if (pfile == NULL) return -1
for(i = 0 i < size ++i) {
fprintf(pfile, "%d ", keys[i])
}
fclose(pfile)
return 0
}
int encode(char const *datafile, char const *keyfile)
{
char original[MAX_KEY_LENGTH] = {'\0'}
char encrypted[MAX_KEY_LENGTH] = {'\0'}
int i, size
int keys[MAX_KEY_LENGTH]
FILE *pdatafile, *pkeyfile
pkeyfile = fopen(keyfile, "w")
if (NULL == pkeyfile) return -1
fclose(pkeyfile)
puts(" input message:")
gets(original)
size = strlen(original)
if (0 != generateKey(keys, size)) return -2
if (0 != saveKey(keyfile, keys, size) ) return -3
pdatafile = fopen(datafile, "w")
if (NULL == pdatafile) return -4
for (i = 0 i < size ++i) {
encrypted[i] = original[i] + keys[i]
fputc(encrypted[i], pdatafile)
fputc(encrypted[i], stdout)
}
printf("\n")
fclose(pdatafile)
return 0
}
int decode(char const *datafile, char const *keyfile)
{
FILE *pdatafile, *pkeyfile
int keys[MAX_KEY_LENGTH] = {0}
char original[MAX_KEY_LENGTH] = {'\0'}
char encrypted[MAX_KEY_LENGTH] = {'\0'}
int i, size
pkeyfile = fopen(keyfile, "r")
if (NULL == pkeyfile) return -1
fclose(pkeyfile)
pdatafile = fopen(datafile, "r")
if (NULL == pdatafile) return -2
fscanf(pdatafile,"%s",encrypted)
fclose(pdatafile)
size = loadKey(keyfile, keys, MAX_KEY_LENGTH)
if (size < 1) return -3
for (i = 0 i < strlen(encrypted) ++i) {
original[i] = encrypted[i]-keys[i]
fputc(original[i], stdout)
}
printf("\n")
return 0
}
运行结果：
1. Encryption.
2. Decryption.
3. Exit.
Selection (1,2,3):1
input message:
this is A test!
╓┐»╞Lñ╗ù|t▄╬╢╒è
1. Encryption.
2. Decryption.
3. Exit.
Selection (1,2,3):2
this is A test!
1. Encryption.
2. Decryption.
3. Exit.
Selection (1,2,3):3
