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

void caesar_shift(char *s, int shift)
{
    int delta = ((shift % 26) + 26) % 26;
    for (int i = 0; s[i] != '\0'; i++) {
        unsigned char c = (unsigned char)s[i];
        if (isupper(c)) {
            s[i] = 'A' + (c - 'A' + delta) % 26;
        } else if (islower(c)) {
            s[i] = 'a' + (c - 'a' + delta) % 26;
        }
    }
}

int simple_hash(const char *s)
{
    int h = 0;
    for (int i = 0; s[i] != '\0'; i++) {
        h = (h + (unsigned char)s[i]) % 256;
    }
    return h;
}

int main(void)
{
    char buf[64] = "Transfer funds now";
    int h = simple_hash(buf);
    printf("plaintext: %s\n", buf);
    printf("hash: %d\n", h);
    caesar_shift(buf, 3);
    printf("ciphertext: %s\n", buf);
    caesar_shift(buf, -3);
    printf("decrypted: %s hash_match=%s\n", buf, simple_hash(buf) == h ? "yes" : "no");
    caesar_shift(buf, 3);
    buf[4] = 'X';
    caesar_shift(buf, -3);
    printf("tampered decrypt: %s hash_match=%s\n", buf, simple_hash(buf) == h ? "yes" : "no");
    return 0;
}