Untitled

#include <stdio.h>
#include <openssl/bn.h>

void initialize_bn(BIGNUM **bn, const char *value) {
    *bn = BN_new();
    BN_dec2bn(bn, value);
}

void print_bn(const char *label, BIGNUM *bn) {
    char *value = BN_bn2dec(bn);
    printf("%s: %s\n", label, value);
    OPENSSL_free(value);
}

int main() {
    BN_CTX *ctx = BN_CTX_new();

    BIGNUM *n, *e, *M, *d, *C;
    initialize_bn(&n, "6887");
    initialize_bn(&e, "143");
    initialize_bn(&M, "1234");

    // Compute d: The modular inverse of e modulo phi(n)
    // For simplicity, and since n = 6887 is a product of two primes (83 and 83),
    // phi(n) = (p-1)(q-1) = 82*82 = 6724
    BIGNUM *phi_n;
    initialize_bn(&phi_n, "6724");
    d = BN_mod_inverse(NULL, e, phi_n, ctx);

    // Compute C = M^e mod n
    C = BN_new();
    BN_mod_exp(C, M, e, n, ctx);

    print_bn("Decryption key (d)", d);
    print_bn("Ciphertext (C)", C);

    // Cleanup
    BN_free(n); BN_free(e); BN_free(M); BN_free(d); BN_free(C); BN_free(phi_n);
    BN_CTX_free(ctx);
    return 0;
}