CRC Implementation with Socket - CRC Generator

#include <stdio.h>

#include <string.h>

char *getcrcmessage(char *input, char *key)

{

int i, j, keylen, msglen;

char temp[30], quot[100], rem[30], key1[30];

keylen = strlen(key);

msglen = strlen(input);

strcpy(key1, key);

for (i = 0; i < keylen; i++)

{

input[msglen + i] = '0';

}

for (i = 0; i < keylen; i++)

temp[i] = input[i];

for (i = 0; i < msglen; i++)

{

quot[i] = temp[0];

if (quot[i] == '0')

for (j = 0; j < keylen; j++)

key[j] = '0';

else

for (j = 0; j < keylen; j++)

key[j] = key1[j];

for (j = keylen - 1; j > 0; j--)

{

if (temp[j] == key[j])

rem[j - 1] = '0';

else

rem[j - 1] = '1';

}

rem[keylen - 1] = input[i + keylen];

strcpy(temp, rem);

}

strcpy(rem, temp);

input[i] = '\0';

rem[keylen - 1] = '\0';

quot[msglen] = '\0';

char *response = strcat(input, rem);

printf(" ------------------------ CRC calculation --------------------");

printf("\nQuotient is: %s \n", quot);

printf("\nRemainder is : %s \n", rem);

printf("\nFinal data is: %s\n", response);

printf("--------------\n");

response[msglen+keylen-1] = '\0';

return response;

}

int verifycrc(char *input, char *key)

{

int i, j, keylen, msglen;

char temp[30], quot[100], rem[30], key1[30];

keylen = strlen(key);

msglen = strlen(input);

strcpy(key1, key);

for (i = 0; i < keylen; i++)

temp[i] = input[i];

for (i = 0; i < msglen - keylen; i++)

{

quot[i] = temp[0];

if (quot[i] == '0')

for (j = 0; j < keylen; j++)

key[j] = '0';

else

for (j = 0; j < keylen; j++)

key[j] = key1[j];

for (j = keylen - 1; j > 0; j--)

{

if (temp[j] == key[j])

rem[j - 1] = '0';

else

rem[j - 1] = '1';

}

rem[keylen - 1] = input[i + keylen];

strcpy(temp, rem);

}

strcpy(rem, temp);

input[i] = '\0';

rem[keylen] = '\0';

quot[msglen] = '\0';

printf(" ------------------------ CRC Verification --------------------");

printf("\nQuotient is: %s \n", quot);

printf("\nRemainder is : %s \n", rem);

printf("--------------\n");

if (strcmp(rem, "0000") == 0)

{

printf("\nChecksum Success\n");

return 1;

}

printf("Checksum Failure\n");

return -1;

}