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I wrote it in one hour for internal corporate usage.

• win32 exe: x509conv.zip

cross-compiled:

mingw32-gcc -static -I /usr/local/mingw32/include/ -L/usr/local/mingw32/lib \
  -o x509conv.exe x509conv.c \
  -lssl -lcrypto -lgdi32

x509conv.c

/*
 * x509conv.c
 *
 * Copyright 2004-2019 Oleg Borodin  <borodin@unix7.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.
 */
 
 
#include <stdio.h>
#include <string.h>
#include <openssl/x509.h>
#include <openssl/pem.h>
#include <openssl/err.h>
#include <openssl/x509.h>
#include <unistd.h>
 
void qexit(int code) {
    printf("# press enter key to continue\n");
    getchar();
    exit(code);
}
 
void readwrite_cert(char *ifilename, char *cn, size_t cn_size) {
    FILE* pemfile = fopen(ifilename, "r");
    if (pemfile == NULL) {
        printf("# cannot open pem file %s, exit.\n", ifilename);
        qexit(1);
    }
 
    X509* x509 = PEM_read_X509(pemfile, NULL, NULL, NULL);
    printf("# open pem file %s\n", ifilename);
 
    if (x509 == NULL) {
        printf("# cannot find certificate in %s, exit.\n", ifilename);
        qexit(1);
    }
 
    char* subj = X509_NAME_oneline(X509_get_subject_name(x509), 0, 0);
    if (subj) {
        printf("# found subject: %s\n", subj);
        OPENSSL_free(subj);
    }
 
    X509_NAME *subject_name = X509_get_subject_name(x509);
    int nid_email = OBJ_txt2nid("CN");
 
    X509_NAME_get_text_by_NID(subject_name, nid_email, cn, cn_size);
    printf("# cn is %s\n", cn);
 
    const int filename_maxsize = 256;
    char cert_filename[filename_maxsize];
    memset(cert_filename, 0, sizeof(cert_filename));
 
    strcat(cert_filename, cn);
    strcat(cert_filename, ".crt");
 
    FILE* certfile = fopen(cert_filename, "w");
    if (certfile == NULL) {
        printf("# cannot write certificate, exit.\n");
        qexit(1);
    }
    PEM_write_X509(certfile, x509);
    printf("# wrote cert to %s\n", cert_filename);
    fclose(certfile);
 
    X509_free(x509);
    fclose(pemfile);
}
 
void readwrite_prikey(char *ifilename, char *cn) {
 
    FILE *pemfile = fopen(ifilename, "r");
 
    if (pemfile == NULL) {
        printf("# cannot open pem file %s, exit.\n", ifilename);
        qexit(1);
    }
 
    RSA *prikey = PEM_read_RSAPrivateKey(pemfile, NULL, NULL, NULL);
    if (prikey == NULL) {
        printf("# cannot find key in file %s, exit.\n", ifilename);
        qexit(1);
    }
 
    printf("# read pem file %s\n", ifilename);
 
    const int filename_maxsize = 256;
    char prikey_filename[filename_maxsize];
    memset(prikey_filename, 0, sizeof(prikey_filename));
 
    strcat(prikey_filename, cn);
    strcat(prikey_filename, ".key");
 
    FILE *keyfile = fopen(prikey_filename, "w");
    if (keyfile != NULL) {
        PEM_write_RSAPrivateKey(keyfile, prikey, NULL, NULL, 0, 0, NULL);
        printf("# wrote key to %s\n", prikey_filename);
    }
}
 
 
int main(int argc, char** argv) {
 
    if(argc == 1) {
        printf("# no command line arguments, baby, I exit.\n");
        qexit(1);
    }
 
    char *filename = argv[1];
    if (strlen(filename) < 1) {
        printf("# zero file name, exit.\n");
        qexit(1);
    }
 
    const size_t cn_size = 256;
    char cn[cn_size];
    memset(cn, 0, cn_size);
 
    readwrite_cert(filename, cn, cn_size);
    readwrite_prikey(filename, cn);
 
    qexit(0);
    return 0;
 
}

freebsd native.

$ cc -Wall -o x509conv x509conv.c -lssl -lcrypto
$ ./x509conv a.pem

# open pem file a.pem
# found subject: /C=RU/O=Lazurit/CN=WRT Mosaic01
# cn is WRT Mosaic01
# wrote cert to WRT Mosaic01.crt
# read pem file a.pem
# wrote key to WRT Mosaic01.key

$ ls -l *.crt *.key *.pem
-rw-r--r--  1 ziggi  wheel  1497 19 Mar 17:09 a.crt
-rw-r--r--  1 ziggi  wheel  1497 19 Mar 17:53 WRT Mosaic01.crt
-rw-r--r--  1 ziggi  wheel  1675 19 Mar 17:53 WRT Mosaic01.key