Hi!
I have more or less finished writing a manual page for SSL_CTX_set_verify(),
but I have not been able to completely understand the usage of
SSL_CTX_set_verify_depth().
If I understand it correctly, the verify_depth only influence the depth
up to which OpenSSL tries to collect CA certificates itself?! (x509_vfy.c)
At least it never issues a X509_V_ERR_CERT_CHAIN_TOO_LONG itself...
Steve, probably you know best :-)
Anyway, I have appended a draft copy of what is there by now.
Best regards,
Lutz
--
Lutz Jaenicke [EMAIL PROTECTED]
BTU Cottbus http://www.aet.TU-Cottbus.DE/personen/jaenicke/
Lehrstuhl Allgemeine Elektrotechnik Tel. +49 355 69-4129
Universitaetsplatz 3-4, D-03044 Cottbus Fax. +49 355 69-4153
=pod
=head1 NAME
SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth -
set peer certificate verification parameters
=head1 SYNOPSIS
#include <openssl/ssl.h>
void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
int (*verify_callback)(int, X509_STORE_CTX *));
void SSL_set_verify(SSL *s, int mode,
int (*verify_callback)(int, X509_STORE_CTX *));
void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth);
void SSL_set_verify_depth(SSL *s, int depth);
int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);
=head1 DESCRIPTION
SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode>
and specifies the verification B<verify_callback> function to be used. If no
callback function shall be specified, the NULL pointer can be used for
B<verify_callback>.
SSL_CTX_set_verify() sets the verification flags for B<ssl> to be B<mode>
and specifies the verification B<verify_callback> function to be used. If no
callback function shall be specified, the NULL pointer can be used for
B<verify_callback>. In this case last B<verify_callback> set specifically for
this B<ssl> remains. If no special B<callback> was set before, the default
callback for the underlying B<ctx> is used, that was valid at the the time
B<ssl> was created with L<SSL_new(3)|SSL_new(3)>.
SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
that shall be allowed for B<ctx>. (See the BUGS section.)
SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
that shall be allowed for B<ssl>. (See the BUGS section.)
=head1 NOTES
The verification of certificates can be controlled by a set of logically
or'ed B<mode> flags:
=over 4
=item SSL_VERIFY_NONE
B<Server mode:> the server will not send a client certificate request to the
client, so the client will not send a certificate.
B<Client mode:> if not using an anonymous cipher (by default disabled), the
server will send a certificate which will be checked. The result of the
certificate verification process can be checked after the TLS/SSL handshake
using the L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function.
The handshake will be continued regardless of the verification result.
=item SSL_VERIFY_PEER
B<Server mode:> the server sends a client certificate request to the client.
The certificate returned (if any) is checked. If the verification process
fails as indicated by B<verify_callback>, the TLS/SSL handshake is
immediately terminated with an alert message containing the reason for
the verification failure.
The behaviour can be controlled by the additional
SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags.
B<Client mode:> the server certificate is verified. If the verification process
fails as indicated by B<verify_callback>, the TLS/SSL handshake is
immediately terminated with an alert message containing the reason for
the verification failure. If no server certificate is sent, because an
anonymous cipher is used, SSL_VERIFY_PEER is ignored.
=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT
B<Server mode:> if the client did not return a certificate, the TLS/SSL
handshake is immediately terminated with a "handshake failure" alert.
This flag must be used together with SSL_VERIFY_PEER.
B<Client mode:> ignored
=item SSL_VERIFY_CLIENT_ONCE
B<Server mode:> only request a client certificate on the initial TLS/SSL
handshake. Do not ask for a client certificate again in case of a
renegotiation. This flag must be used together with SSL_VERIFY_PEER.
B<Client mode:> ignored
=back
Exactly on of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be
set at any time.
The B<verify_callback> function is used to control the behaviour when the
SSL_VERIFY_PEER flag is set. It must be supplied by the application and
receives two arguments: B<preverify_ok> indicates, whether the verification of
the certificate in question was passed (preverify_ok=1) or not
(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
for the certificate chain verification.
The certificate chain is checked starting with the deepest nesting level
(the root CA certificate) and worked upward to the peer's certificate.
At each level signatures and issuer attributes are checked. Whenever
a verification error is found, the error number is stored in B<x509_ctx>
and B<verify_callback> is called with B<preverify_ok>=0. By applying
X509_CTX_store_* functions B<verify_callback> can locate the certificate
in question and perform additional steps (see EXAMPLES). If no error is
found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
before advancing to the next level.
The return value of B<verify_callback> controls the strategy of the further
verification process. If B<verify_callback> returns 0, the verification
process is immediately stopped with "verification failed" state. If
SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
the verification process is continued. If B<verify_callback> always returns
1, the TLS/SSL handshake will never be terminated because of this application
experiencing a verification failure. The calling process can however
retrieve the error code of the last verification error using
L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its
own error storage managed by B<verify_callback>.
=head1 BUGS
In client mode, it is not checked whether the SSL_VERIFY_PEER flag
is set, but whether SSL_VERIFY_NONE is not set. This can lead to
unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not
used excluding each other.
The certificate verification process does not enforce the certificate
chain depth limit. It must be realized by the B<verify_callback> supplied
by the application.
=head1 RETURN VALUES
The SSL*_set_verify*() functions do not provide diagnostic information.
=head1 EXAMPLES
The following code sequence realizes an example B<verify_callback> function
that will always continue the TLS/SSL handshake regardless of verification
failure, if wished. The callback realizes a verification depth limit.
All verification errors are printed, informations about the certificate chain
are printed on request.
The example is realized for a server that does allow but not require client
certificates.
...
int verbose_mode;
int verify_depth;
int always_continue;
...
static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
{
char buf[256];
X509 *err_cert;
int err, depth;
err_cert = X509_STORE_CTX_get_current_cert(ctx);
err = X509_STORE_CTX_get_error(ctx);
depth = X509_STORE_CTX_get_error_depth(ctx);
X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);
if (preverify_ok && (depth > verify_depth)) {
preverify_ok = 0;
err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
X509_STORE_CTX_set_error(ctx, err);
}
if (!preverify_ok) {
printf("verify error:num=%d:%s:depth=%d:%s\n", err,
X509_verify_cert_error_string(err), depth, buf);
}
else if (verbose_mode)
{
printf("depth=%d:%s\n", depth, buf);
}
/*
* At this point, err contains the last verification error. We can use
* it for something special
*/
if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)
{
X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256);
printf("issuer= %s\n", buf);
}
if (always_continue)
return 1;
else
return preverify_ok;
}
...
SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
verify_callback);
...
SSL_accept(ssl); /* check of success left out for clarity */
if (peer = SSL_get_peer_certificate(ssl))
{
if (SSL_get_verify_result(ssl) == X_509_OK)
{
/* The client sent a certificate which verified OK */
}
}
=head1 SEE ALSO
L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>,
L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>,
L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>
=cut
