Secure Programming Cookbook for C and C++

10.7. Verifying an SSL Peer’s Certificate

Problem

You are using OpenSSL to support SSL-enabled communication between a client and a server. You want to instruct OpenSSL to verify the certificate received from the peer.

Solution

Every SSL connection has an SSL object, which in turn has an SSL_CTX object, and that object, in turn, has an X509_STORE object. OpenSSL uses the X509_STORE object as a container for any certificates and CRLs required to verify another certificate. OpenSSL creates an X509_STORE_CTX object and calls X509_verify_cert( ) for you, but not by default.

OpenSSL’s default behavior is to not verify peer certificates, which is the worst default behavior that any SSL implementation could possibly provide. By not verifying certificates in an SSL connection, the strength of the security provided by SSL is severely reduced, to the point where the two parties in the conversation might as well be using nothing more than a symmetric cipher with keys exchanged in the clear. Without verifying certificates, you will have security against passive eavesdroppers, but that is all. With a small amount of effort, anyone could hijack the TCP connection before the SSL session is established and act as a man-in-the-middle.

Discussion

To have OpenSSL verify a peer’s certificate, you must issue a call to SSL_CTX_set_verify( ) . SSL_CTX_set_verify( ) accepts a bitmask of flags that tell OpenSSL how to deal with certificates. Depending on whether the SSL_CTX object is being used as a client ...