title: OpenSSL how to load CA cert with DER format
body: <p>I am new to OpenSSL, so forgive me if I ask anything stupid.</p>
  
  <p>I need to convert an existing server project which uses axTLS to OpenSSL. As the server requires client authentication, I need to load the server cert file, private key file and the CA cert file. All 3 files are in DER format.</p>
  
  <p>For server cert file and private key file, I can use <code>SSL_CTX_use_certificate_file()</code> and <code>SSL_CTX_use_PrivateKey_file()</code> to load a DER format file (ASN1 format). However, I couldn't find any API that loads the CA cert in DER format. <code>SSL_CTX_load_verify_locations()</code> and <code>SSL_load_client_CA_file()</code> seem only work for PEM format.</p>
  
  <p>Any other API/method I can use to work around this issue?</p>
  
  <p>Thank you.</p>