mbedtls DER formatted certificate chain verification

I am trying to verify a certificate chain using mbedTLS version 2.16.2. The certificate chain in DER format as individual files. The PKI is simple as three levels (1) Root_CA (2) Intermediate_CA (3) Device_CERT

I could able to verify the certificates using openssl verify --- command ; but with below program, the certificate verification fails with Error: -0x2700; flag: 8

Am I verifying the cert chain (DER formatted files) the correct way?

void verify_cert_chain_driver( void )
{
   int32_t r = -1; uint32_t flags = 0;

   mbedtls_x509_crt ca, chain;

   do{

      r = parse_from_file( &ca, "x509-root-ca.der" );
      if( EXIT_SUCCESS != r )    break;
      
      r = parse_from_file( &chain, "x509-intermediate-ca.der" );
      if( EXIT_SUCCESS != r )    break;

      r = parse_from_file( &chain, "x509-dev-cert.der" );
      if( EXIT_SUCCESS != r )    break;

      r = mbedtls_x509_crt_verify( &chain, &ca, NULL, NULL, &flags, NULL, NULL );
      if( EXIT_SUCCESS != r )    break;

      printf( "Verify OK\r\n" );
   }while(0);

   if( 0 != r )  printf( "Error: 0x%04x; flag: %u\r\n", r, flags );
}

Solution

the variables ca and chain are not initialized, you should call mbedtls_x509_crt_init for them
parse_from_file should probably be mbedtls_x509_crt_parse_file.
to build the complete chain, parsing should be done in the right order, starting with the device certificate and then the intermediate one
for more detailed errors mbedtls_x509_crt_verify_info can be called if the return value indicates an error
mbedtls_x509_crt_verify provides the possibility for a custom callback: this verification callback is called for each certificate in the chain and can be used to output additional information

The sample code for cert_app.c provides a nice verification callback called my_verify as well as a snippet to extract better error information via mbedtls_x509_crt_verify_info.

If you then apply the above points, your code would look something like this:

#include <stdio.h>
#include <stdint.h>
#include "mbedtls/platform.h"
#include "mbedtls/entropy.h"
#include "mbedtls/ssl.h"

//taken from https://github.com/Mbed-TLS/mbedtls/blob/development/programs/x509/cert_app.c
static int my_verify(void *data, mbedtls_x509_crt *crt, int depth, uint32_t *flags) {
    char buf[1024];
    ((void) data);

    mbedtls_printf("\nVerify requested for (Depth %d):\n", depth);
    mbedtls_x509_crt_info(buf, sizeof(buf) - 1, "", crt);
    mbedtls_printf("%s", buf);

    if ((*flags) == 0)
        mbedtls_printf("  This certificate has no flags\n");
    else {
        mbedtls_x509_crt_verify_info(buf, sizeof(buf), "  ! ", *flags);
        mbedtls_printf("%s\n", buf);
    }

    return (0);
}


void verify_cert_chain_driver(void) {
    int32_t r;
    uint32_t flags = 0;

    mbedtls_x509_crt ca, chain;

    mbedtls_x509_crt_init(&ca);
    mbedtls_x509_crt_init(&chain);
    do {

        r = mbedtls_x509_crt_parse_file(&ca, "x509-root-ca.der");
        if (EXIT_SUCCESS != r) break;

        r = mbedtls_x509_crt_parse_file(&chain, "x509-dev-cert.der");
        if (EXIT_SUCCESS != r) break;

        r = mbedtls_x509_crt_parse_file(&chain, "x509-intermediate-ca.der");
        if (EXIT_SUCCESS != r) break;

        //see https://github.com/Mbed-TLS/mbedtls/blob/development/programs/x509/cert_app.c
        if ((r = mbedtls_x509_crt_verify(&chain, &ca, NULL, NULL, &flags,
                                         my_verify, NULL)) != 0) {
            char vrfy_buf[512];
            mbedtls_printf(" failed\n");
            mbedtls_x509_crt_verify_info(vrfy_buf, sizeof(vrfy_buf), "  ! ", flags);
            mbedtls_printf("%s\n", vrfy_buf);
        } else
            mbedtls_printf(" Verify OK\n");

    } while (0);

    if (0 != r) mbedtls_printf("Error: 0x%04x; flag: %u\n", r, flags);
}

int main(void) {
    verify_cert_chain_driver();
    return 0;
}

An error message from mbedtls_x509_crt_verify_info could e.g. look like this:

 failed
  ! The certificate is not correctly signed by the trusted CA

A successful test would output something like this:


Verify requested for (Depth 2):
cert. version     : 3
serial number     : 08:3B:E0:56:90:42:46:B1:A1:75:6A:C9:59:91:C7:4A
issuer name       : C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert Global Root CA
subject name      : C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert Global Root CA
issued  on        : 2006-11-10 00:00:00
expires on        : 2031-11-10 00:00:00
signed using      : RSA with SHA1
RSA key size      : 2048 bits
basic constraints : CA=true
key usage         : Digital Signature, Key Cert Sign, CRL Sign
  This certificate has no flags

Verify requested for (Depth 1):
cert. version     : 3
serial number     : 02:79:AC:45:8B:C1:B2:45:AB:F9:80:53:CD:2C:9B:B1
issuer name       : C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert Global Root CA
subject name      : C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G1
issued  on        : 2017-11-27 12:46:10
expires on        : 2027-11-27 12:46:10
signed using      : RSA with SHA-256
RSA key size      : 2048 bits
basic constraints : CA=true, max_pathlen=0
key usage         : Digital Signature, Key Cert Sign, CRL Sign
ext key usage     : TLS Web Server Authentication, TLS Web Client Authentication
certificate policies : ???, ???
  This certificate has no flags

Verify requested for (Depth 0):
cert. version     : 3
serial number     : 0C:B1:71:BD:2C:AD:62:1A:68:9C:7A:2D:9C:F4:35:03
issuer name       : C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G1
subject name      : CN=software7.com
issued  on        : 2022-01-04 00:00:00
expires on        : 2023-01-04 23:59:59
signed using      : RSA with SHA-256
RSA key size      : 2048 bits
basic constraints : CA=false
subject alt name  :
    dNSName : software7.com
    dNSName : www.software7.com
key usage         : Digital Signature, Key Encipherment
ext key usage     : TLS Web Server Authentication, TLS Web Client Authentication
certificate policies : ???
  This certificate has no flags
 Verify OK