decrypt_c.c

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

void printBN(char *msg, BIGNUM *a)
{
    char *number_str_a = BN_bn2hex(a);
    printf("%s %s\n", msg, number_str_a);
    OPENSSL_free(number_str_a);
}

int main()
{
    // init
    BN_CTX *ctx = BN_CTX_new();
    BIGNUM *n = BN_new();
    // BIGNUM *e = BN_new();
    BIGNUM *M = BN_new();
    BIGNUM *d = BN_new();
    BIGNUM *C = BN_new();

    // assign values
    BN_hex2bn(&n, "DCBFFE3E51F62E09CE7032E2677A78946A849DC4CDDE3A4D0CB81629242FB1A5");
    BN_hex2bn(&d, "74D806F9F3A62BAE331FFE3F0A68AFE35B3D2E4794148AACBC26AA381CD7D30D");
    BN_hex2bn(&C, "8C0F971DF2F3672B28811407E2DABBE1DA0FEBBBDFC7DCB67396567EA1E2493F");

    // decrypt C: C^d mod n
    BN_mod_exp(M, C, d, n, ctx);
    printBN("Decryption result:", M);

    // clear sensitive data
    BN_clear_free(n);
    BN_clear_free(d);
    BN_clear_free(M);
    BN_clear_free(C);

    return 0;
}