Programming

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

#include <openssl/pem.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>


int 
main(int argc, char *argv[])
{
    FILE *fp = NULL;
    char *filename;
    struct stat fstat;

    if (argc != 2) {
        fprintf(stdout, "ERROR - No filename provided\n");
        exit(1);
    }

    filename = argv[1];

    if ((stat(filename, &fstat) != 0) || (fp = fopen(filename, "r")) == NULL) {
        fprintf(stdout, "ERROR - File could not be opened\n");
        exit(1);
    }

    // parse certificate
    X509 *cert = PEM_read_X509(fp, NULL, NULL, NULL);
    if (!cert) {
        fprintf(stderr, "ERROR - Unable to parse certificate\n");
        fclose(fp);
        exit(1);
    }
    
    // version number
    int version = ((int) X509_get_version(cert)) + 1;
    printf("Version: %d\n", version);

    // serial number
    ASN1_INTEGER *serial = X509_get_serialNumber(cert);
    
    BIGNUM *bn = ASN1_INTEGER_to_BN(serial, NULL);
    if (!bn) {
        fprintf(stderr, "ERROR - Unable to convert ASN1INTEGER to BN\n");
        exit(1);
    }
    
    char *tmp = BN_bn2dec(bn);
    if (!tmp) {
        fprintf(stderr, "ERROR - Unable to convert BN to decimal string\n");
        BN_free(bn);
        exit(1);
    }
    
    printf("Serial Number: %s\n", tmp);

    BN_free(bn);
    OPENSSL_free(tmp);

    // subject
    tmp = X509_NAME_oneline(X509_get_subject_name(cert), NULL, 0);
    printf("Subject: %s\n", tmp);
    OPENSSL_free(tmp);

    // issuer
    tmp = X509_NAME_oneline(X509_get_issuer_name(cert), NULL, 0);
    printf("Issuer: %s\n", tmp);
    OPENSSL_free(tmp);

    // signature algorithm
    int pkey_nid = OBJ_obj2nid(cert->cert_info->key->algor->algorithm);
    if (pkey_nid == NID_undef) {
        fprintf(stderr, "ERROR - Signature algorithm name\n");
        exit(1);
    }

    const char* buf  = OBJ_nid2ln(pkey_nid);
    printf("Algorithm: %s\n", buf);

    X509_free(cert);
    fclose(fp);
}