How to verify a X509 certificate in C

Question

I have a certificate in X509 format. this a input parameters in a function. What I would like to do is to verify the validity of the certificate. How can it be done?

Answer 1

I am here just to post my answer as I found it with the above comments.

I had no certificate chain, so in the work I'm doing I only have a certificate generated by me programatically. I wanted to check the validity of it, so I created the following function, which checks the certificate against itself in other to verify the validity of it.

void check_certificate_validaty(X509* certificate)
{
    int status;
    X509_STORE_CTX *ctx;
    ctx = X509_STORE_CTX_new();
    X509_STORE *store = X509_STORE_new();

    X509_STORE_add_cert(store, certificate);

    X509_STORE_CTX_init(ctx, store, certificate, NULL);

    status = X509_verify_cert(ctx);
    if(status == 1)
    {
        printf("Certificate verified ok\n");
    }else
    {
        printf("%s\n", X509_verify_cert_error_string(ctx->error));
    }
}

Hope this helps someone :)

Answer 2

To verify a certificate signature, you need the public key of an issuer certificate. This issuer certificate's signature is verified with another issuing certificate (or trusted root certificate). Thus if a certificate's signature verifies all the way up a chain to a trusted root, then that certificate is considered trusted.

Self-signed certificates' signatures are verified using their own public key, like the example below:

int verify_cert(const char* pem_c_str)
{
    BIO *bio_mem = BIO_new(BIO_s_mem());
    BIO_puts(bio_mem, pem_c_str);
    X509 * x509 = PEM_read_bio_X509(bio_mem, NULL, NULL, NULL);

    EVP_PKEY *pkey=X509_get_pubkey(x509);
    int r= X509_verify(x509, pkey);
    EVP_PKEY_free(pkey);

    BIO_free(bio_mem);
    X509_free(x509);
    return r;
}

from: http://www.zedwood.com/article/openssl-c-verify-self-signed-certificate-signature

Answer 3

See the documentation here.

You need to create a certificate store using X509_STORE_CTX_new. Then add certificate chain using X509_STORE_CTX_set_chain. Add trusted root certificate using X509_STORE_CTX_trusted_stack. Finally add certificate to be verified using X509_STORE_CTX_set_cert.

After that call X509_verify_cert.

I hope this will help you to start on this.