private key not available for client_cert_cb

Jan Just Keijser janjust at
Mon Jan 11 16:01:56 UTC 2021


On 08/01/21 22:35, George wrote:
> Hi,
>    I have been trying to setup mutual authentication using a smart 
> card but I can't seem to get the OpenSSL Engine to send a response 
> back to the server containing client's certificate from the smart card.
> I'm using the following to configure the certificate and private key:
>     ENGINE_ctrl_cmd(engine, "LOAD_CERT_CTRL", 0, &cert_info, NULL, 0);
>     SSL_CTX_use_certificate(sslContext, cert_info.cert);
>     EVP_PKEY* privateKey = ENGINE_load_private_key(engine, 
> "2b2586c684d69b670c0a805edf514e720f2b757d8e2faa0b3a7ff23d1ccfc7ba", 
> transfer_pin, &cb_data);
>     SSL_CTX_use_PrivateKey(sslContext, privateKey);
> (I have been using the code in 
> as a guide.)
> This seems be successful. However, when I start the mutual 
> authentication with
> SSL_connect(ssl)
> , the mutual authentications handshake fails. I can see the server 
> requesting the certificate from the client and the client sends back 
> an ACK for this message. However, the client does not send the 
> certificate to the server.
> I was looking through the OpenSSL code openssl-1.0.2u\ssl\ssl_rsa.c 
> and noticed something interesting. The comment indicates that the flag 
> *RSA_METHOD_FLAG_NO_CHECK* should be set for smart cards:
> static int ssl_set_pkey(CERT *c, EVP_PKEY *pkey)
> {
>  . . .
> #ifndef OPENSSL_NO_RSA
> *   /***
> **         * Don't check the public/private key, this is mostly for 
> smart**
> **         * cards.**
> **         */*
>         if ((pkey->type == EVP_PKEY_RSA) &&
>             (RSA_flags(pkey->pkey.rsa) & RSA_METHOD_FLAG_NO_CHECK)) ;
>         else
> #endif
> . . .
> }
> However, it is not actually set when I use a debugger to inspect the 
> flag. Does it need to be set? If so, how is this done? I could not 
> find anything related to this in

if you read through the code blob that Michael pointed you to, you will 
find that this flag needs to be set *under certain circumstances* when 
using smartcards. It has to do mostly with the situation where
- private key is on the smart card
- the public key/certificate is NOT on the smart card
- you ask OpenSSL to verify the private key without explicitly providing 
a public key.

I've never run into this issue, but then again, I have not tested very 
often the case where the certificate was not present on the HSM/smart 
card but the private key is.  YMMV.

As for using pksc11helper versus using libp11: that is just a matter of 
taste. I used the engine_pkcs11 + libp11 route for the eap-tls code , 
mostly because it was the first "working" set of tools I found at the time.

You can also take the "pkcs11helper" route, which is what OpenVPN does 
(see  Both methods have pro's and con's.

Do you run into problems if you DO not set the RSA_METHOD_FLAG_NO_CHECK 
flag?  All that flag does is to stop OpenSSL from verifying that a 
public key/cert and private key match/belong together for RSA keys only; 
if your smartcard supports EC keys then this flag will do you no good.



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