/* terrible signature verification code */

#include <openssl/evp.h>
#include <openssl/pem.h>
#include <openssl/bio.h>
#include <openssl/ec.h>
#include <openssl/err.h>

#define SIGNATURE_FILE      "sig.dat"
#define DATA_FILE           "data.dat"
#define CERTIFICATE_FILE    "cert.pem"

int load_cert(const char *file, EVP_PKEY **key)
{
    if (file == NULL || key == NULL)
        return 0;

    FILE *fh = fopen(file, "r");
    if (fh == NULL){
        perror("Can't open certificate file");
        return 0;
    }

    X509 *x509 = X509_new();
    if (! PEM_read_X509(fh, &x509, NULL, NULL)){
        fprintf(stderr, "ERROR: PEM_read_X509\n");
        return 0;
    }

    *key = X509_get_pubkey(x509);
    if (*key == NULL){
        fprintf(stderr, "ERROR: X509_get_pubkey\n");
        return 0;
    }
    if (fh) fclose(fh);
    if (x509) X509_free(x509);
    return 1;
}

int load_data(const char *file, unsigned char **data, size_t *sz)
{

    FILE *fh = fopen(file, "rb");
    if (fh == NULL){
        perror("Can't open data file.");
        return -1;
    }

    fseek(fh, 0, SEEK_END);
    long len = ftell(fh);
    rewind(fh);

    unsigned char *buf = malloc(len);
    if (buf == NULL){
        perror("Can't read data file into memory");
        fclose(fh);
        return -1;
    }

    long nbytes = fread(buf, 1, len, fh);
    *data = buf;
    *sz = (size_t) len;

    if(fh) fclose(fh);
    return nbytes;
}

int verify_signature(EVP_PKEY *pubkey,
        unsigned char *sig,
        size_t sig_len,
        unsigned char *data,
        size_t data_len)
{
    if (pubkey == NULL || sig == NULL || data == NULL){
        return -1;
    }

    /* Assumes ecdsa_sha1 for brevity */
    const EVP_MD *dgst = EVP_get_digestbyname("ecdsa-with-SHA1");
    EVP_MD_CTX ctx;
    EVP_MD_CTX_init(&ctx);

    int rc = 0;
    if (! (rc = EVP_VerifyInit(&ctx, dgst)))
        return rc;

    if (! (rc = EVP_VerifyUpdate(&ctx, data, data_len)))
        return rc;

    rc = EVP_VerifyFinal(&ctx, sig, sig_len, pubkey);

    printf("Signature verification: ");
    switch (rc){
        case 0:
            printf("FAILED\n");
            break;
        case 1:
            printf("SUCCEEDED\n");
            break;
        default:
            printf("ERROR\n");
    }
    return rc;
}

int main(int argc, char *argv[])
{
    unsigned char *sig = NULL;
    size_t sig_len;

    unsigned char *data = NULL;
    size_t data_len;

    EVP_PKEY *pubkey = NULL;


    if (load_data(SIGNATURE_FILE, &sig, &sig_len) > 0 &&
        load_data(DATA_FILE, &data, &data_len) > 0 &&
        load_cert(CERTIFICATE_FILE, &pubkey)){

        verify_signature(pubkey, sig, sig_len, data, data_len);
    } else {
        printf("Bad input.\n");
    }
    return 0;
}