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Question

I am struggling with a client certificate problem and hope somebody here can help me. I'm developing a client/server pair using boost asio but I'll try to be unspecific. I'm on windows and using openssl 1.0.1e

Basically, I want to have client authentication by using client certificates. The server shall only accept clients that have a certificate signed by my own CA. So I have setup a self signed CA. This has issued two more certificates. One for the client and one for the server. Both signed by the CA. I have done that quite a few times now and I am confident that I got it.

My server side also works fine. It requests client certificates and if I'm using s_client and give those certs everything works. Also if I'm using a browser and have my root CA installed as trusted and then import the client certs.

The only thing that I can't get to work is the libssl client. It always fails during the handshake and as far as I can see it will not send the client certficate:

$ openssl.exe s_server -servername localhost -bugs -CAfile myca.crt -cert server.crt
 -cert2 server.crt -key private/server.key -key2 private/server.key -accept 8887 -www 
 -state -Verify 5
verify depth is 5, must return a certificate
Setting secondary ctx parameters
Using default temp DH parameters
Using default temp ECDH parameters
ACCEPT
SSL_accept:before/accept initialization
SSL_accept:SSLv3 read client hello A
SSL_accept:SSLv3 write server hello A
SSL_accept:SSLv3 write certificate A
SSL_accept:SSLv3 write key exchange A
SSL_accept:SSLv3 write certificate request A
SSL_accept:SSLv3 flush data
SSL3 alert read:warning:no certificate
SSL3 alert write:fatal:handshake failure
SSL_accept:error in SSLv3 read client certificate B
SSL_accept:error in SSLv3 read client certificate B
2675716:error:140890C7:SSL routines:SSL3_GET_CLIENT_CERTIFICATE:peer did not return a
certificate:s3_srvr.c:3193:
ACCEPT

I'm using this s_server as debugging tool but against my real server the same thing occurs. s_client will work fine with the same certificates. Also, if I disable "-Verify" in the server the connection works. So it really seems just the client refusing to send it's certficate. What can be the reason for that?

Since I'm using boost asio as an SSL wrapper the code looks like this:

m_ssl_context.set_verify_mode( asio::ssl::context::verify_peer );
m_ssl_context.load_verify_file( "myca.crt" );
m_ssl_context.use_certificate_file( "testclient.crt", asio::ssl::context::pem );
m_ssl_context.use_private_key_file( "testclient.key", asio::ssl::context::pem );

I have also tried to bypass asio and access the SSL context directly by saying:

SSL_CTX *ctx = m_ssl_context.impl();
SSL *ssl = m_ssl_socket.impl()->ssl;
int res = 0;
res = SSL_CTX_use_certificate_chain_file(ctx, "myca.crt");
if (res <= 0) {
    // handle error
}
res = SSL_CTX_use_certificate_file(ctx, "testclient.crt", SSL_FILETYPE_PEM);
if (res <= 0) {
    // handle error
}
res = SSL_CTX_use_PrivateKey_file(ctx, "testclient.key", SSL_FILETYPE_PEM);
if (res <= 0) {
    // handle error
}

I can't see any difference in behavior. It should be mentioned that I am using a very old boost 1.43 asio which I cannot update but I suppose all relevant calls go more or less directly to OpenSSL anyway and the server works fine with that version so I think I can rule that out.

If I start forcing client and server to specific versions, the error messages change but it never works and still always works with the s_client test. Currently it is set to TLSv1

If I switch it to TLSv1 for example there is more chatter between client and server and eventually I get the error:

...
SSL_accept:SSLv3 read client key exchange A
<<< TLS 1.0 ChangeCipherSpec [length 0001]
    01
<<< TLS 1.0 Handshake [length 0010], Finished
    14 00 00 0c f4 71 28 4d ab e3 dd f2 46 e8 8b ed
>>> TLS 1.0 Alert [length 0002], fatal unexpected_message
    02 0a
SSL3 alert write:fatal:unexpected_message
SSL_accept:failed in SSLv3 read certificate verify B
2675716:error:140880AE:SSL routines:SSL3_GET_CERT_VERIFY:missing verify 
message:s3_srvr.c:2951:
2675716:error:140940E5:SSL routines:SSL3_READ_BYTES:ssl handshake failure:s3_pkt.c:989:
ACCEPT

I have found an older bug entry posted on the openssl mailing list that refereed to this. Apparently a wrong CRLF in the handshake that has been fixed two yrs ago. Or has it?

I have been debugging this for almost a week now and I'm really stuck. Does anyone have a suggestion on what to try? I'm out of ideas...

Cheers, Stephan

PS: Here is what the above s_server debug out would be with s_client and the same certficate:

$ openssl s_client -CAfile ca.crt -cert testclient.crt -key private/testclient.key -verify 2 -connect myhost:8887

ACCEPT
SSL_accept:before/accept initialization
SSL_accept:SSLv3 read client hello A
SSL_accept:SSLv3 write server hello A
SSL_accept:SSLv3 write certificate A
SSL_accept:SSLv3 write key exchange A
SSL_accept:SSLv3 write certificate request A
SSL_accept:SSLv3 flush data
depth=1 C = DE, // further info
verify return:1
depth=0 C = DE, // further info
verify return:1
SSL_accept:SSLv3 read client certificate A
SSL_accept:SSLv3 read client key exchange A
SSL_accept:SSLv3 read certificate verify A
SSL_accept:SSLv3 read finished A
SSL_accept:SSLv3 write session ticket A
SSL_accept:SSLv3 write change cipher spec A
SSL_accept:SSLv3 write finished A
SSL_accept:SSLv3 flush data
ACCEPT

... handshake completes and data is transferred.

Answer 1

All right, after much suffering, the answer has been found by Dave Thompson of OpenSSL.

The reason was that my ssl code called all those functions on the OpenSSL context after the socket object (SSL*) was created from it. Which means all those functions did practically nothing or the wrong thing.

All I had to do was either:

1. Call SSL_use_certificate_file

res = SSL_use_certificate_file(ssl, "testclient.crt", SSL_FILETYPE_PEM);
if (res <= 0) {
    // handle error
}
res = SSL_use_PrivateKey_file(ssl, "testclient.key", SSL_FILETYPE_PEM);
if (res <= 0) {
    // handle error
}

(notice the missing CTX )

2. Call the CTX functions

Call the CTX functions upon the context before the socket was created. As asio seemingly encourages to create the context and socket right afterwards (as I did in the initializer list) the calls were all but useless.

The SSL context (in lib OpenSSL or asio alike) encapsulates the SSL usage and each socket created from it will share it's properties.

Thank you guys for your suggestions.

Answer 2

You should not use both SSL_CTX_use_certificate_chain_file() and SSL_CTX_use_certificate_file(), as SSL_CTX_use_certificate_chain_file() tries to load a chain including the client certificate, not just the CA chain. From SSL_CTX_use_certificate(3) :

SSL_CTX_use_certificate_chain_file() loads a certificate chain from file into ctx. The certificates must be in PEM format and must be sorted starting with the subject's certificate (actual client or server certificate), followed by intermediate CA certificates if applicable, and ending at the highest level (root) CA.

I think you should be fine using only SSL_CTX_use_certificate_file() and SSL_CTX_use_PrivateKey_file(), as the client does not care much for the CA chain anyway.

Answer 3

I think you need to call SSL_CTX_set_client_CA_list on the server side. This sets a list of certificate authorities to be sent together with the client certificate request.

The client will not send its certificate, even if one was requested, if the certificate does not match that CA list sent by the server.