Programmatically verify certificate chain using OpenSSL API

Question

This is very similar to other questions but the ones I've looked at either don't have an answer or don't quite ask the same question. I have a self-signed CA certificate, and two other certificates that are signed with that CA certificate. I'm fairly sure the certificates are correct, because 'openssl verify' works:

$ openssl verify -CAfile ca.pem server.pem
server.pem: OK

(The above is from memory, I don't have them in front of me, so it may be slightly off).

Now I want to verify the certificates programatically. I have a utility function with pseudocode below:

int verify_cert(X509 *cert, X509 *cacert)
{
     int ret;
     X509_STORE *store;
     X509_STORE_CTX *ctx;

     store = X509_STORE_new();
     X590_STORE_add_cert(store, cacert);

     ctx = X509_STORE_CTX_new();
     X509_STORE_CTX_init(ctx, store, cert, NULL);

     ret = X509_verify_cert(ctx);

     /* check for errors and clean up */
}

My problem is that the above code always returns 'failed to find issuer certificate'. What have I done wrong? I believe I am creating a new store, adding the cacert, creating a new context, and adding the child cert to be verified to the context with a pointer to the store which contains the CA. I'm pretty obviously doing something wrong, but I'm unsure what.

Any ideas?

Update: I'm aware I can save these certs to disk and use something like X509_LOOKUP_file or something similar. I'm looking for a solution that doesn't touch the disk unnecessarily.

Answer 1

I think, you can use "X509_STORE_set_verify_cb" to add a callback to identify the actual error:

static int  verify_cb(int ok, X509_STORE_CTX *ctx)
{
    if (!ok)
    {
        /* check the error code and current cert*/
        X509 *currentCert = X509_STORE_CTX_get_current_cert(ctx);
        int certError = X509_STORE_CTX_get_error(ctx);
        int depth = X509_STORE_CTX_get_error_depth(ctx);
        printCert(currentCert);
        printf("Error depth %d, certError %d", depth, certError)
    }

    return(ok);
}

int verify_cert(X509 *cert, X509 *cacert)
{
     int ret;
     X509_STORE *store;
     X509_STORE_CTX *ctx;

     store = X509_STORE_new();
     X509_STORE_set_verify_cb(store, verify_cb);
     X590_STORE_add_cert(store, cacert);

     ctx = X509_STORE_CTX_new();
     X509_STORE_CTX_init(ctx, store, cert, NULL);

     ret = X590_verify_cert(ctx);

     /* check for errors and clean up */
}

Unless we know the error code it is difficult to guess the actual problem. The code otherwise looks OK.

Answer 2

I encountered this problem myself and started off with code very close to the OP. My certificate chain included 3 certificates: Certificate 1 (root-ca) Issuer: root-ca Subject: root-ca Certificate 2 (signing-ca) Issuer: root-ca Subject: signing-ca Certificate 3 (device) Issuer: signing-ca Subject: device

I wanted to verify the device certificate. My ca.pem equivalent (wrt OP) contained the root-ca and signing-ca.

The X509_verify_cert function needs the entire certificate chain all the way to the root (root-ca & signing-ca) in the X509_store.

Below is my code that works for me. Checks on return values were omitted to lean the code down.

int getIssuerCert(X509_STORE *x509_store){
    STACK_OF(X509_INFO) *inf;
    X509_INFO *itmp;
    BIO *in;
    int i, count = 0;

    in = BIO_new(BIO_s_mem());
    BIO_write(in, issuerCertStr, strlen(issuerCertStr)); //string containing root-ca & signing-ca
    inf = PEM_X509_INFO_read_bio(in, NULL, NULL, NULL);
    if(in != NULL) BIO_free(in);
    for(i = 0; i < sk_X509_INFO_num(inf); i++) {
        itmp = sk_X509_INFO_value(inf, i);
        if(itmp->x509) {
            X509_STORE_add_cert(x509_store, itmp->x509);
            count++;
        }
        if(itmp->crl) {
            X509_STORE_add_crl(x509_store, itmp->crl);
            count++;
        }
    }
    sk_X509_INFO_pop_free(inf, X509_INFO_free);
    return 0;
}


int verify_cert(){
    int ret = 0;
    X509 *devCert = NULL;
    X509_STORE *x509_store = NULL;
    X509_STORE_CTX *x509_store_ctx = NULL;

    OpenSSL_add_all_algorithms();
    devCert = getDeviceCert(); //  Returns X509 pointer

    x509_store = X509_STORE_new();
    X509_STORE_set_verify_cb(x509_store, verify_cb);
    X509_STORE_set_flags(x509_store, 0);

    x509_store_ctx = X509_STORE_CTX_new();

    X509_STORE_CTX_init(x509_store_ctx, x509_store, devCert, NULL)

    X509_STORE_CTX_set_purpose(x509_store_ctx, X509_PURPOSE_ANY);
    ret = X509_verify_cert(x509_store_ctx);

    if(x509_store_ctx != NULL) X509_STORE_CTX_free(x509_store_ctx);
    if(x509_store != NULL) X509_STORE_free(x509_store);
    if(devCert != NULL) X509_free(devCert);
    EVP_cleanup();
    return ret;
}

I didn't need to create any lookup methods. The key for me was looping through my certificates from my string in memory so I had all the certificates I need to complete the chain. The string is equivalent to what I would have fed into openssl verify for the option -CAfile.

Also, make sure your X509 pointers are not null when the are used.

Answer 3

A possible answer (don't have the rep points to add a comment, sorry): the manpage for SSL_CTX_load_verify_locations(3) says,

When building its own certificate chain, an OpenSSL client/server will try to fill in
missing certificates from CAfile/CApath, if the certificate chain was not explicitly
specified (see SSL_CTX_add_extra_chain_cert(3), SSL_CTX_use_certificate(3).

(Failure to match parens theirs, not mine.)

Which seems to mean that, as an alternative to SSL_CTX_load_verify_locations(3), it should be possible to use SSL_CTX_add_extra_chain_cert(3) or SSL_CTX_use_certificate(3) -- both of which take a X509 * arg. Thus obviating the need for Mr Ed's solution as seen above.

Answer 4

Please take a look at SSL_CTX_load_verify_locations () function: http://www.openssl.org/docs/ssl/SSL_CTX_load_verify_locations.html

SSL_CTX_load_verify_locations() specifies the locations for ctx, at which CA certificates for verification purposes are located. The certificates available via CAfile and CApath are trusted.

You can generate a CA certificate file containing both ca.pem server.pem:

 #!/bin/sh
 rm CAfile.pem
 for i in ca.pem server.pem ; do
   openssl x509 -in $i -text >> CAfile.pem
 done

And then set CAfile variable to point to CAfile.pem file.

Hope it helps !

Answer 5

See the official source code: apps/verify.c

static int check(X509_STORE *ctx, const char *file,
                 STACK_OF(X509) *uchain, STACK_OF(X509) *tchain,
                 STACK_OF(X509_CRL) *crls, int show_chain);

You can see how to output 'OK' here:

    if (i > 0 && X509_STORE_CTX_get_error(csc) == X509_V_OK) {
        printf("%s: OK\n", (file == NULL) ? "stdin" : file);

The function dependence can be found in apps/apps.c