아핀암호 C코드

#pragma warning (disable: 4996)
#include<stdio.h>
#include<string.h>
void affine_cipher_Decryption(char *cleartext, char *encryptext, int len, int multiplicative_key, int additive_key);
void affine_cipher_Encryption(char *cleartext, char *encryptext, int len, int multiplicative_key, int additive_key);
int gcd_returninverse(int a, int b);

int main()//�"���"
{
 char cleartext[80] = "hello";
 char encryptext[80];
 puts("�"문자열을 입력하세요"");
 //fgets(cleartext, 80 , stdin);
 affine_cipher_Encryption(cleartext, encryptext, strlen(cleartext), 7,2);
 affine_cipher_Decryption(cleartext, encryptext, strlen(cleartext), 7,2);
 printf("%s\n", encryptext);
 printf("%s\n", cleartext);

 system("pause");
 return 0;
}

void affine_cipher_Encryption(char *cleartext, char *encryptext, int len, int multiplicative_key,int additive_key)//Decryption]
{
 int inverse_key = gcd_returninverse(26, multiplicative_key);
 if (inverse_key == 0)
 {
  printf("key가 26과 서로소가 아니라서 역원이 존재하지않습니다\n");
  return;
 }
 for (int i = 0; i <= len; i++)
 {

  if (cleartext[i] >= 'a' && cleartext[i] <= 'z')
  {
   encryptext[i] = ((cleartext[i] - 'a')*multiplicative_key+ additive_key) % 26 + 'A';
  }
  else
   encryptext[i] = cleartext[i];

 }
}// A(65)Z(90) -a(97) 급간32

void affine_cipher_Decryption(char *cleartext, char *encryptext, int len, int multiplicative_key, int additive_key)//Decryption]
{
 int inverse_key = gcd_returninverse(26, multiplicative_key);
 if (inverse_key == 0)
 {
  printf("key가 26과 서로소가 아니라서 역원이 존재하지않습니다");
  return;
 }
 
 while (inverse_key < 0)
 {
  inverse_key += 26;
 }
 for (int i = 0; i <= len; i++)
 {
  if (encryptext[i] >= 'A' && encryptext[i] <= 'Z')
  {
   cleartext[i] = (inverse_key * (encryptext[i] - 'A' +26 - additive_key)) % 26 + 'a';
  }
  else
   cleartext[i] = encryptext[i];
 }
}// A(65)Z(90) -a(97) 급간32

int gcd_returninverse(int a, int b)
{
 int r;
 int q, t1 = 0, t2 = 1, t;
 while (b)
 {
  r = a % b;
  q = a / b;
  a = b;
  b = r;
  t = t1 - t2 * q;
  t1 = t2;
  t2 = t;
 }
 if (a != 1)
  return 0;
 return t1;
}